Regolith geochemical exploration in the Girilambone District of New South Wales

Transcription

1 University of Wollongong Thesis Collections University of Wollongong Thesis Collection University of Wollongong Year 2005 Regolith geochemical exploration in the Girilambone District of New South Wales Benjamin R. Ackerman University of Wollongong Ackerman,Benjamin R, Regolith geochemical exploration in the Girilambone District of New South Wales, PhD thesis, School of Earth and Environmental Sciences, University of Wollongong, This paper is posted at Research Online.

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3 CHAPTER SEVEN DISCUSSION 7.1 Introduction This chapter summarises the findings of geochemical and mineralogical investigations of the Girilambone North and Tritton study areas and discusses aspects of mineral exploration within the Girilambone district and beyond. Geochemical and mineralogical results from both study areas are brought together in order to identify the trace element dispersion mechanisms and characteristics of mineralised and lithogeochemical signatures in the regolith profile and surficial regolith environments. In particular, surface geochemical exploration data are discussed within the context of landscape/regolith-landform position and features of the regolith profile common to both study areas. A number of secondary issues associated with geochemical exploration techniques and methodologies have been identified during the course of the current study, which will be discussed in relation to data presented in preceding chapters and relevant examples from recent literature. These issues include aspects of geochemical sampling, analysis and data interpretation. Finally, interpretations of this and other studies have allowed the development of geochemical exploration strategies for Girilambone-style mineralisation in the Girilambone district and beyond. 7.2 Lithogeochemical Variation of Girilambone Lithologies: Identifying background geochemical variance Metasediments at Tritton show a crude laminated appearance due to the alternation of psammite and pelite layers, although terms used to describe these rocks in previous literature and company databases are many. Subdivisions of rock types have previously been on the basis of the matrix component e.g. quartzwacke, greywacke, packestone, phyllite etc., which at best creates confusion to the reader or persons attempting to follow literature citings of similar lithologies. Thus, this study prefers the broader classifications afforded by the terms psammite and pelite, which simply refer to metamorphosed sediments of sandstone and siltstone respectively and quartzite, used to describe pervasively silicified rocks of either texture. Where strong deformation 255

4 (schistosity) is evident, the term schist may be added to further describe the physical nature of the rock, although most rocks display at least some degree of deformation. However, for descriptions of rock units in various sections of this study, it has been necessary to retain the various texturally defined rock descriptions of previous workers, although these have been somewhat simplified in the compilation of Appendix 3. In this study, division of these rocks has largely been on a geochemical basis, in order to describe the variable geochemical signature of metasediment lithologies. Lithogeochemical data are commonly used in geochemical mapping to classify rock types, characterisation of tectonic environment and in mineral exploration by providing chemical information of alteration and mineralisation assemblages (Govett, 1983; Taylor & McLennan, 1985; Harris et al., 1999; Harris et al., 2000). Yusta et al. (1998) state that determination of the background geochemical populations are necessary steps in order to isolate anomalies which might be related to mineralisation, the purpose of which forms the principal task of mineral exploration. Furthermore, when the entire data set represents mixtures of multi-element geochemical populations, which is invariably the case, subdivisions should be made in order to identify geochemical variance of each, otherwise levels of geochemical variance may reflect changes in background geology or regolith (Agnew, 1999). The lithogeochemical variation of Girilambone Group lithologies is here considered irrespective of the degree of weathering, in order to establish a background signature for each lithology type, on which anomalies can more easily be recognised. These observations were based upon mineralogical and geochemical analyses of rock chip and drill core samples from the Girilambone North and Tritton study areas and compared with other relevant studies of the Girilambone district. Four main lithology types were identified within the Girilambone North and Tritton study areas, which include chlorite-rich and quartz-rich quartzose metasediments, mafic intrusive rocks and sulfide mineralised rocks. However, discussions of sulfide mineralisation and associated geochemical signatures are presented in a later section of this chapter Quartz- and chlorite-rich metasedimentary lithologies Quartzose metasediments form a series from chlorite-rich lithologies where chlorite alteration has been particularly pervasive and overprinting quartz-muscovite mineral assemblages, which grade to relatively chlorite-poor silicified quartzose metasediments 256

5 and pure quartzite units. Consequently, the lithogeochemical variation observed for quartzose lithologies of the Girilambone North and Tritton study areas in the current study were due primarily to the occurrence of chlorite alteration and intense silicification. Fergusson et al. (2005) similarly describe quartzose metasediment rocks of the Murrawombie open pit (Girilambone Mine), Hartmans open pit and outcrop exposures in the Girilambone district. Chlorite-rich lithologies of the current study largely encompass the quartz-chlorite-sericite schists described by Shields (1996) and Fogarty (1998), which grade towards quartz-rich lithologies, corresponding with quartzite units and silicified schists of the same workers. Chlorite alteration was found to be characterised by relative enrichment of the elements Ba, Br, Cs, Cr, Co, Cu, Fe, Pb, Mo, Ni, K, Rb, Sc, Ta, Th, U, and the REE. Conversely, silicified schists and quartzite of the quartz-rich lithologies, were found to be characterised by a relative increase in the elements Sb, Ca, Hf, Na, W, U and Zr and marked depletion of the elements As, Ba, Br, Cs, Cr, Co, Fe, Mo, Ni, K, Rb, Sc, Ta, Th, Zn and the REE relative to other lithological units. The distribution of REE shows particularly well the range of both quartz- and chlorite-rich quartzose metasediments, which generally display PAAS-like chondrite-normalised REE geochemical patterns. Chlorite-rich lithologies are indicated by increased REE occurrence, with some overlap observed with relatively REE-deficient quartz-rich lithologies. The range of selected trace elements for metasedimentary lithologies are summarised Table 7.1. While apparent local variations were observed between quartz- and chlorite-rich samples in drill hole sections described previously in Chapter Six, the overall geochemistry of each lithology group are quite similar. Figure 7.1 depicts the range of elemental abundances of selected trace and major elements of quartzose metasedimentary samples determined by the present study. In general, the multi-element geochemical pattern of each type is very similar, with the exception being notable enrichment in the elements Fe, K and REE in chlorite-rich samples and Sb, Ba and Ca for quartz-rich samples. Thus, despite the local influence of chloritic alteration, the elemental composition of non-mineralised metasedimentary rocks varies only slightly within the Girilambone district, according with given the fairly homogenous nature of deep marine sediments. Geochemical variance can thus be described and reasonably predicted elsewhere within this region and possibly, more broadly, the greater Ordovician metasediments of the Lachlan Orogen. 257

6 Table 7.1 Range of elemental contents in metasedimentary lithologies of the Tritton (and Girilambone district) study compiled from geochemical analyses of the current study. Elemental abundances are in ppm unless otherwise indicated. Quartz-rich (n=74) Chlorite-rich (n=91) Element Min. Max. Min. Max. Sb As < Ba < < Br < <5 3.1 Ca <0.5% 6.88% <0.5% 1.76% Cs < < Cr Co < Cu Hf Fe 0.35% 4.66% 0.43% 7.66% Pb <5 50 <5 43 K <0.2% 2.99% <0.2% 5.60% Rb < < Sc Na 0.02% 2.80% 0.02% 2.83% Ta < < Th W < U < < Zn Zr ΣREE Mafic lithologies Mafic rocks of the Girilambone district have been described previously by other workers (Shields, 1996; Berthelsen, 1998; Fogarty, 1998) and occur as intermediate to ultramafic intrusive rocks, dykes and sills of gabbroic to doleritic compositions and minor serpentinite schists. Fogarty (1998) indicates a close association of mafic mineralogies with known occurrence of sulfide mineralisation within the Girilambone district and that mineralisation and intense silicification may be related to mafic intrusive bodies. Petrographic examination of mafic rock types in the present study identified rocks of doleritic composition with randomly oriented elongate plagioclase phenocrysts up to 0.5 to 1 mm in length, altered pyroxene and minor anhedral Mg-rich calcite within a fine grained groundmass of albite, pyroxene, epidote, chlorite and calcite. Mafic schists from the Tritton deposits were found to contain abundant disseminated pyrite, which have been previously described by Berthelsen (1998) and found to form the basal unit of Tritton lithologies and host mineralisation of the lower ore zone. Similarly, Fogarty (1998) had described mafic schists as hosts of minor chalcopyrite-pyrite lenses within the Larsens East deposit. The mafic lithologies of the 258

7 ppm Ta Br Na K(%) Fe(%) U Ca(%) Cs Sc Hf Th Sb W Pb ΣREE Zn Cr Co Rb As Zr Ba Quartz-rich Chlorite-rich Figure 7.1 Uniform geochemical variance of (unmineralised) quartz- and chlorite-rich metasedimentary samples of the Girilambone district. Minimum and maximum values are shown; all elements in ppm, except where otherwise indicated. current study incorporate a variety of mafic rock types, although not all have been described. Distinctive geochemical signatures were however observed for these mafic rock types, which showed a marked enrichment of Ca, Cr, Co, Cu, Fe, Ni, Na and Sc, and strong depletion of the (L)REE, Hf, Sc, Rb, Th, K, Th and W relative to other identified lithologies. The chondrite-normalised REE pattern of mafic lithologies was quite distinct and showed significant LREE (La, Ce, Sm) depletion relative to PAAS. A mafic zone was identified in the Larsens East deposit, which showed similar geochemical response, notably anomalous Ba, Co, Cr, Ni, Sc, Na and Zn and LREE depletion relative to PAAS. As has been discussed, mafic mineralogies of the Tritton and Girilambone North study areas are easily identified from other lithologies and mineralisation on the basis of geochemistry, and the subsequent trace element signature of mafic lithologies is well- 259

8 defined. Little differentiation however, between mafic samples has been demonstrated thus far, which, if present might be used to suggest differing magma sources or periods of magmatism. Rollinson (1993) describes methods of igneous rock discrimination by way of normalised multi-element diagrams, similar to that of REE patterns examined earlier, although for a wider range of elements. While there exists many sets of normalising values (Rollinson, 1993), in this instance, the chondrite values of Wood et al. (1979) as quoted in Rollinson (1993) were used, as the available elements and ranges of each were most amenable to the current study. Figure 7.2 plots the chondritenormalised multi-element chemical diagram of Tritton mafic samples (n=16). Two prominent groups were identified, with the first group (indicated by filled triangles) characterised by relative enrichment of Rb, K, Th and the LREE and depletion of Lu, Sc, Fe, Cr, Co and Ni and thought to represent a high-th magmatic phase. The converse was true for the second group, which displayed significant enrichment of the elements Cr, Co, Fe, Lu, Ni and Sc and a deficiency in K, Rb, Th and the LREE, which could be described as a relatively Cr- or Ni-rich magmatic phase. Samples plotted as green triangles are from within the regolith profile sampled from BDS007, which may show slightly anomalous geochemistry (i.e. reduced Rb, K and Th) relative to other high-th samples of the same drill hole. All other samples (i.e. not from drill hole BDSM007) are from well below the influence of weathering, and represent different mafic units within the Tritton deposit, of which two distinct groups of chondrite-normalised multi-element geochemical variance have been identified. While this classification is tentative, it is likely that mafic rocks found within the Tritton study area are indicative of at least two phases of magmatism. The identified mafic phases are seen to co-exist within close proximity of one another from sampling of multiple mafic bodies within the Tritton profile. While it was not the expressed aim of this study to evaluate mafic-rock origins, the realisation of a close association of mafic mineralogies with known occurrences of mineralisation and tentative identification of at least two phases of magmatism by the current study, suggests further studies of mafic lithologies could prove worthwhile. In particular, better characterisation of mafic bodies and establishment of magmatic provenance could assist greatly in the understanding of ore genesis. Furthermore, should mineralisation be associated with a particular magmatic phase, identification of this phase could prove to be useful in the search for copper sulfide mineralisation in the Girilambone district. 260

9 Dupuis et al. (2005) identified the likely tectonic setting of mafic rocks from southern Tibet based on, amongst other criteria, the ratios of Th-Hf-Ta, expressing mafic rock samples on ternary plots discriminating tectonic setting first developed by Wood (1980). Although, relatively simple to recreate, unfortunately, a similar approach cannot be adopted in the present study due to the relatively low contents of Ta in mafic samples (below DL for all samples). Other methods of determining mafic provenance generally require whole rock geochemical analyses, or determination for elements such as Ti, which were not generally determined within the current study. It would be however a simple matter to extend the current geochemical suite should the need arise. Figure 7.2 Chondrite-normalised multi-element geochemical patterns of Tritton mafic samples, which have been normalised against chondrite values of Wood (1979). At least two phases of magmatism are inferred (triangles and circles) which are indicative of relatively Th-rich and Cror Ni-rich phases, respectively. 7.3 Girilambone-style Mineralisation: Geochemical characteristics from studies of Girilambone North and Tritton study areas As has been shown, the Girilambone district represents an area of historical mining and mineral exploration significance, and is hosted by the lesser known Girilambone Group metasediments of Ordovician age. Renewed interest in the area has arisen from recent initiatives of various research bodies and government agencies, to encourage mineral exploration in the Cobar basin and Girilambone districts. In addition development of Tritton underground operations and further exploitation of un-mined primary copper resources planned for the Girilambone mines (including Girilambone North) by present 261

10 stakeholder, Tritton Resources Limited, in combination with rising global commodity prices, demonstrates the economic viability of the area for future exploration and production interests. Girilambone-style mineralisation, as it has been coined by various workers, while genetically similar to mineralisation styles of the nearby Cobar mineral field and elsewhere in the Lachlan Orogen, generally manifests as Cu only, Cu-Au or Cu-Au-Ag association. Unlike the (Siluro-Devonian) poly-metallic mineral deposits of Cobar, Girilambone-style mineralisation is thought to be of Ordovician age. While several origins have been inferred for the genesis of mineralisation at Tritton and Girilambone deposits, emplacement of mineralising fluids in shear dilation zones is the favoured genesis of this and other studies. North to northwest structural zones are seen to host mineralisation of Girilambone, Girilambone North and Tritton mineral deposits. Primary mineralisation manifests as massive sulfide (pyrite-chalcopyrite) mineralised bodies, with chalcopyrite replacing earlier pyrite which commonly forms a disseminated halo about known mineralised occurrences. Minor sulfide development in the form of galena and sphalerite (North East deposit; Girilambone North) may accompany chalcopyrite mineralisation. Where primary sulfide mineralisation has been influenced by weathering processes, primary sulfide minerals have been converted to secondary sulfides, carbonates and oxides within the weathering profile. Classic-style supergene oxide zones have formed at Girilambone (Murrawombie pit), with appreciable secondary copper mineralisation developed within the weathered profile of smaller primary massive sulfide deposits of the nearby Girilambone North and Budgerigar deposits. Common also to deposits of the Girilambone district, is the association of mineralisation with intense silicification and mafic occurrences. Other alteration has been suggested to occur as various epidote, jasperoid, carbonate (siderite and calcite), ilmenite, magnetite and micaceous mineral assemblages which have been described by other workers (Fogarty, 1996; Shields, 1996; Berthelsen, 1998; Fogarty, 1998; Hastings, 2001). Despite the obvious differences in economic mineralisation encountered at the Girilambone North (and Girilambone Mine) and Tritton prospects i.e. supergene copper oxides and carbonate minerals versus primary copper sulfide mineralisation respectively, the primary sulfide mineralisation for each are similar. The difference 262

11 between these deposits has been the location of each with respect to the regolith profile, which in the case of Girilambone North deposits, has occurred close to the surface level and within the influence of regolith development. Subsequent supergene processes have mobilised copper and associated elements in the regolith profile and created relatively flat-lying economic mineral deposits, readily exploitable by mining practices and metallurgical processing of the day. Within the Girilambone North study area, Larsens East was observed to have a considerable copper sulfide resource below the extent of mine workings. In fact, this has become the target of current owner/operators, Tritton Resources Limited, which at the time of writing, were in the process of re-developing access to recover the copper-bearing minerals from the primary zone of the Larsens East and Murrawombie open pits. Hartmans on the other hand represents a deposit of largely remobilised copper accumulation derived from either from the adjacent North East or en echelon Larsens East primary mineralised zones (pers. comm. J.M. Fogarty, 2001). There is however, a small massive sulfide body identified at the base of the northern extent of Hartmans workings, recognised by this and a previous study (Pahlow, 1995). Thus, there is considerable evidence for dispersion of copper in the regolith profile, which is generally observed to have dispersed in a northerly direction and transported by a series of north-trending late-stage faults. Dispersion of copper and other trace elements will be discussed in more detail in a later section of this chapter. Geochemical characteristics of primary sulfide mineralisation are summarised in Table 7.2, for the mineral deposits examined in the present study. Primary sulfide mineralisation of the Girilambone North deposits (including both chalcopyrite and chalcocite mineralisation), is characterised by anomalous abundances of the elements Sb, As, Bi, Br, Co, Cu, Au, Fe, Pb, Mo, Se, Ag, Sr, S, Te, Sn and Zn. Tritton copper deposit represents the highest grade primary resource, with copper lodes reaching up to 25-30% Cu, although commonly in the order of 2.5% Cu, with lesser credits of Au (0.25 g/t) and Ag (11 g/t) (Tritton Resources Limited, 2003). Elements also elevated in primary mineralisation of the Tritton copper deposit include Sb, As, Cd, Co, Au, Fe, Pb Mo, Ni, Se, Ag, Te, W and Zn. In comparing the three deposits, Larsens East has a considerably reduced Zn content, a reflection of the absence of sphalerite noted in ore-mineralogical studies of other workers (Pahlow, 1995; Fogarty, 1998), while the copper grade is quite high, reaching 263

12 up to 31.5 % Cu. Hartmans shows significantly reduced Cu, Au, Pb, Mo, Se and Te relative to the other studied deposits, which is most likely indicative of the low overall sulfide mineral content observed in this deposit. Zinc was significantly higher in Hartmans primary sulfides than those of Larsens East, although no sphalerite has been reported. Tritton however, hosts minor sphalerite mineralisation (Berthelsen, 1998), which gives rise to increased Zn in mineralised zones and subsequently explains the increase in Cd, which readily substitutes for Zn in a sphalerite solid solution (Deer et al., 1992). Similarly, the higher levels of As, Sb and Au at Tritton may be indicative of minor tetrahedrite mineralogies observed by Berthelsen (1998), although these are generally anomalous in association with massive sulfides elsewhere. Iron is anomalous in association with mineralisation in all deposits and related to the Fe-abundant phases pyrite (FeS 2 ) and chalcopyrite (CuFeS 2 ). Other elements such as Co, Mn, Ni, Sn and Zn may substitute for Cu or Fe in these minerals, while As and Se (also Bi, Te and V) readily substitute for S (Deer et al., 1992). Table 7.2 Geochemical characteristics of whole rock samples of Girilambone-style sulfide mineralisation with reference to Girilambone North and Tritton mineral deposits (samples from this study only). Chalcocite mineralisation is incorporated in determination of the elemental ranges for the Girilambone North deposits. Measurement units are in ppm unless otherwise indicated. Larsens East (n=15) Hartmans (n=14) Tritton (n=23) Element Min. Max. Min. Max. Min. Max. Sb As Ba Bi Cd < 10 - < Ca <0.5% 0.53% <0.5% 2.31% Co Cu % % 1.50% 21.05% Au <5ppb 417 ppb <5 ppb 157 ppb < 5 ppb 880 ppb Fe 1.74% 36.30% 4.35% 26.80% 0.68% 42% Pb Li < Mo <5 18 < Ni Se < <2 593 Ag < < Sr < S % Te < < Sn Ti W < <2 129 V Zn < % 264

13 Secondary copper mineralisation also shows distinct geochemical variance compared to un-mineralised lithologies and primary mineralisation. Table 7.3 summarises the geochemistry of elements related to primary mineralisation within the oxide and carbonate zones of Girilambone North. Copper mineralisation in the Larsens East oxide zone persists up to 1.25% Cu and (locally) in the Hartmans oxide zone up to 8.8% Cu, although, grades are typically much lower. Thus, Cu probably forms the most important element for geochemical exploration of primary (and secondary) mineralised nearsurface copper deposits. Table 7.3 Geochemistry of the weathered ore zones of the Larsens East and Hartmans prospects, including carbonate, oxide and limonite ore zones. For the Larsens East deposit, samples denoted with * indicate geochemical data derived from only one sample, while samples with **, denote geochemical data derived from 68 samples. All measurements are in ppm unless otherwise indicated. Larsens (n=4) Hartmans (n=71) Element Min. Max. Min. Max. Sb < As** Ba Cr Co** Cu** % % Au <5ppb 343ppb <5ppb 711ppb Hf Fe 2.98% 8.06% 0.50% 54.20% Pb** 0 37 <1 157 Mg* Mn* Mo <5 - < Ni* Se < Ag <5 - < Sr* S* Te <5 - <5 15 Sn* Ti* W V* Zn** Indicators of primary (and secondary) mineralisation can be assessed by examining the variance of elements associated with primary mineralisation in the primary and oxide ore zones of Girilambone North deposits (Tables 7.2 and 7.3), which are depicted in Figures 7.3 and 7.4 for the Larsens East and Hartmans deposits respectively. The Larsens East oxide zone displays a marked depletion of the elements Sb, Co, Cu, Fe, Pb, 265

14 Mo, Se, S, Te and W, while the elements As, Ba, Au, Sr, V and Zn display similar levels of abundance as observed in the primary mineralised zone. Similarly, in the Hartmans weathered ore zones, the elements As, Ba, Cu, Au, Pb, Mo, Ag and Te were strongly depleted relative to primary mineralisation, with comparable or increased occurrence of Co, Se and Zn identified in the weathered ore zones. Thus, while many trace elements associated with primary mineralisation are strongly depleted within the weathering profile (and supergene enriched ore zones), there is evidence that suggests elements such as As, Ba, Co, Au, Se, Sr, V and Zn may form a larger target for geochemical exploration than primary ore zones, albeit for fewer geochemical elements and at a reduced magnitude. The mechanism of dispersion responsible for formation of secondary mineralised zones and geochemical signatures will be discussed in more detail in a later section Primary mineralisation Secondary mineralisation ppm Larsens East Fe Cu S Ti Co As Zn Ba Au Pb Se Sr Mo V Ni W Te Sb Sn Ag Figure 7.3 Range of chemical composition in ppm of the Larsens East primary and secondary mineralised zones. The lower limits of response for selected (below detection) elements are represented as half of the detection limit. 7.4 Geochemical Variance in the Regolith Profile The behaviour of trace elements under the influence of weathering has been examined for the Girilambone North and Tritton study areas. Multi-element geochemical analyses have identified zones of surface enrichment, followed by strong depletion in the upper profile, particularly at Girilambone North, and relative enrichment at the base of the upper saprolite. The dispersion of trace elements within these and less weathered portions of the profile have been observed on a single-element and multi-element basis, in order to identify the multi-element relationships and susceptibility of elements to 266

15 weathering in the regolith environment. The net result, is a better understanding of the regolith processes governing elemental (re)distribution, including potential pathfinder elements and the identification of elemental relationships and mineral hosts. As with most weathered profiles, the dominant products of chemical weathering observed within the Girilambone regolith are iron-oxides and oxyhydroxides (e.g. hematite and goethite) and aluminosilicates (e.g. halloysite, kaolinite). Gibbsite and associated enrichment of Al in the regolith profile, commonly accepted as indicative of prolonged intense weathering, were not encountered. Furthermore, the existence of residual micaceous minerals preserved high in the regolith profile (muscovite, illite), the limited extent of kaolinisation and lack of strongly ferruginised and indurated surface horizons, is indicative of the relative immaturity of these profiles and poor lateritic profile development. The absence of appreciable ferruginous mottling and formation of soil profiles directly overlying weathered saprolite, is indicative of at least partial truncation by erosive stripping since the onset of increased aridity (erosional regime). In comparison to other regions e.g. Yilgarn Craton of Western Australia or tropical climatic zones, regolith development is not yet as advanced, although deep weathering has still developed due to prolonged periods of tectonic quiescence, low relief and falling watertables associated with the onset of aridity since the Middle Miocene. Thus, the regolith profiles observed within the Girilambone district are not unlike the profiles preserved in other semi-arid areas described by Butt & Zeegers (1992), Leah (1996), Cairns et al. (2001) and Chan et al. (2003a; 2003b) Geochemical response formation of enrichment/depletion zones Table 7.4 summarises the behaviour of elements within the Girilambone regolith profiles as identified by the current study. Other investigations elsewhere of geochemical abundances in the regolith profile have yielded similar results, which have been extensively summarised by the works of Butt et al. (2000), and shown in Table 2.1 (Chapter 2). In the Girilambone regolith profile a large number of elements were observed to deplete with increasing influence of weathering (i.e. higher in the weathering profile). The Larsens East profile showed decreased contents of Al, Ca, Li, Mg, Mn, Ni, P, Na, S, Sr, Th, Sn, Ti, W and Zn from the base of the profile to the surface level. Similarly, in the Hartmans profile, Cs, Cr, Cu, Ni, K, Na, Th and U are progressively depleted with increased degree of weathering. Examination of a quartzite 267

16 unit from the surface level through the regolith profile in the Tritton study area revealed a decrease in Sb, As, Cs, Co, Au, Na and Zn content with proximity to the surface level. Surface enrichment was noted for many elements. The Larsens profile displayed a marked enrichment of Br, Hf, Fe, Pb, K, Rb, Sc, Th, U and the REE and the surface enrichment of Cs, Co, Pb, Fe and the REE was noted within the Tritton study area. Possibly the best examples of surface enriched zones were identified within the upper profile of the Hartmans profile. The elements Br, Cu, Au, Hf, Fe, Rb, Sc, Se, Na, Ta, Th and the REE showed significant enrichment from 0-5 m depth. Immediately below the surface enriched zone, a zone of strong depletion was identified, which generally persisted to the base of the upper saprolite, where geochemical response was observed to increase, albeit reduced compared to unweathered rock for many elements. The Larsens profile depletion zone was identified by a strongly reduced geochemical response for the elements Hf, Au, K, Th and the REE. Similarly, strong depletion was noted for Br, Au, Fe, Sm, Sc and the REE in the upper saprolite zone of the Hartmans profile. While a depletion zone was identified within the Tritton profile, it was somewhat less-developed than those of the Girilambone North study area. In most circumstances, distribution of the REE, in particularly the LREE, were most indicative of zones of enrichment and depletion. Other elements such as Cu, Au and Se, while good indicators of enrichment/depletion zones in the vicinity of mineralisation, were generally variable outside the mineralised bodies. Thus, the clearest determination of enrichment and depletion zones was provided by examining the distribution of REE and Fe abundance. Table 7.4 Summary of geochemical characteristics of the Girilambone regolith profiles. Surface enrichment zone (0-5 m) - Girilambone North Upper profile enrichment (0-30 m) - Fe oxide - Tritton only Upper-profile depletion zone (10-45 m) - poorly developed at Tritton Gossan/Ironstone enrichment * Girilambone North only Increased contents in weathering profile Br, Cu, Au, Hf, Fe, Rb, Sc, Se, Na, Ta, Th, REEs Sb, As, Cs, Co, Au, Ni, Na, Zn Br, Au, Hf, Fe, K, Sc, Th, REEs Sb, As, Br, Ce, Co, Cu, Au, Hf, Fe, Pb, Mo, Sc, Se, Ta, Th, W, U, (L)REE (Ba, Cs strongly depleted) Br, Fe, Hf, Sc Decreased contents at weathering front Sb, Al, As, Ca, Cs, Cr, Co, Au, Li, Mg, Mn, Ni, K, Na, Sr, Th, Sn, Ti, W,U, Zn 268

17 Zones of surface and upper profile enrichment have been identified by many workers, mainly in association with ferruginised zones and duricrusts (Leah, 1996; Butt, 1998; Cairns et al., 2001). There exist a number of possibilities for the explanation of this phenomenon, the accumulation of Fe-oxides in this zone not the least. The scavenging properties of ferruginous minerals have been well documented, and will be discussed in more detail in the following section. Another mechanism of surface enrichment considers the reduction in volume upon weathering and retention of some elements in more weathering-resistant, residual or stable weathering products. With the transport of soluble minerals from the profile during weathering, and retention of more resistant minerals, the concentration of retained elements is effectively increased (Figure 7.5). In this way, this process acts in tandem with sequestration of metals by Fe-minerals, as these are commonly the stable weathering products at the surface level. Residual minerals from the incomplete breakdown of parent lithologies, such as muscovite or orthoclase, may be retained in the upper profile (Taylor & Eggleton, 2001). Possibly the most resistant to weathering and most abundant residual mineral identified in the Girilambone profiles (and indeed many others) is quartz, although the geochemical signature of this is usually one of dilution, given the inert nature of this mineral and its low content of trace elements. However, where sulfides such as pyrite are retained in quartz, as is the case with outcropping quartzites and quartz lag at Tritton, total analysis methods such as INAA, are capable of determining the abundance of elements within residual quartz. Other resistate minerals may include zircon, rutile, ilmenite, anatase, magnetite, garnet tourmaline and monazite (Taylor & Eggleton, 2001), although these were not generally recognised within the Girilambone profiles. Strong depletion of metals and other trace elements in the upper profile has been noted by many workers (Butt & Zeegers, 1992; Butt et al., 2000; Taylor & Eggleton, 2001). Depletion is due simply to increased availability of weathering agents (namely water and oxygen), being proximal to the point at which these agents enter the profile. As such the weathering reactions of oxidation, hydrolysis and dissolution occur more rapidly in this zone and weathering products are readily removed due to increased hydraulic gradient and groundwater movement. Furthermore, as weathering initiates from the surface down, these materials represent the oldest regolith materials in the profile, which have been exposed to the longest periods of weathering since exposure to atmospheric conditions. 269

18 Pb ThCo REE Pb Volume reduction during weathering REE Se Th Se Pb Pb Co Cs Th REE Rb REE Se Th REE Rb REE Pb Pb ThCo Relative enrichment Pb Th Se Co Cs Figure 7.5 Schematic diagram showing the process of relative surface enrichment in the upper regolith profile by retention of elements in resistate or stable minerals following reduction in profile volume due to weathering. Strong leaching observed at Girilambone North is due primarily to the influence of sulfide weathering, and in particular, pyrite, which forms acidic solutions upon oxidation (Williams, 1990). The resultant acidic groundwater solutions, subsequently accelerate dissolution of common rock-forming minerals e.g. plagioclase, and increase the mobility of metals in solution. Thus, acid-soluble metals and other ions are readily transported away from the sulfide mineralisation source in acidic groundwaters, and precipitate from solution when acidity decreases. The increase in ph necessary for precipitation of metals is most likely caused by a combination of decreased concentration of H + away from the sulfide weathering source, groundwater mixing (and thus dilution of acidic solutions) and buffering by wallrocks and regolith through which the acidic solutions permeate. The Girilambone North deposits typically exhibit anomalous geochemical signatures within the extent of vacuum and RAB-drilling programs at the northern extent of known mineralised bodies. In this instance, northtrending structural features have acted as a corridor for permeation of acidic metalbearing solutions, which have subsequently deposited metals from solution as groundwaters have become less acidic away from the weathering sulfide source. Deeper in the profile, supergene-enriched zones of Cu minerals exist, which are also elongate in a northerly direction. However, supergene-enriched minerals and their characteristic geochemical signatures, which occur much closer to the primary mineralised zones, are not generally at a level exploitable by the shallow drilling of vacuum- (<5 m) or RABdrilling (18-20 m) programs. Thus, a displaced broad near-surface anomalous zone, much greater in extent than the primary mineralisation from which it originated, has 270

19 developed due to strong leaching of the regolith profile overlying primary (and secondary) mineralised zones. Figure 7.6 depicts schematically the development of strongly leached depleted zones and remobilised geochemical anomalies in the vicinity of weathering sulfide mineralisation. Figure 7.6 Schematic representation of strongly leached depletion zones formed from migration of acidic groundwaters in the vicinity of weathering sulfide mineralisation, in (a) plan and (b) cross-sectional (Figures not to scale) Mineralogical controls of trace element occurrence Quartzose metasedimentary rocks of the Girilambone district commonly display a quartz-chlorite-muscovite mineral assemblage, which under the influence of weathering, has developed weekly kaolinised and moderately ferruginised regolith zones. Residual quartz, mica (muscovite), illite, sericite and chlorite and various Fe-minerals persist in the regolith profile and weathered outcrop. Elsewhere in the Cobar district, Cairns et al. (2001) investigated the mineralogical controls of base-metal cations and suggested important host minerals in the regolith profile, by observing the correlation between elements and major mineral phases. A similar approach was adopted herein, which determined bivariate correlations for major mineral phases and element distributions from the Girilambone North and Tritton study areas. Figure 7.7 depicts diagrammatically the correlation of each analyte element with 271

20 identified mineral phases (determined by XRD). Mineral abundances were arbitrarily assigned a value for major (25%), minor (12.5%), trace (5%) occurrence by volume and mineral absence (0%) and a correlation matrix subsequently derived for all analyte elements. The results of this analysis provide a broad indication of the mineralogical control on elemental abundance. Albite shows strong positive correlations for Ba, Cr, albite Sb As Ba Br Ce Cs Cr Co Eu Au Hf Fe La Lu Mo K Rb S Sc Se Na Ta Tb Th W U Yb Zn Al Bi Ca Cu Pb Li Mg Mn Ni P Ag Sr S Te Sn Ti V Zn chlorite Sb As Ba Br Ce Cs Cr Co Eu Au Hf Fe La Lu Mo K Rb S Sc Se Na Ta Tb Th W U Yb Zn Al Bi Ca Cu Pb Li Mg Mn Ni P Ag Sr S Te Sn Ti V Zn mica Sb As Ba Br Ce Cs Cr Co Eu Au Hf Fe La Lu Mo K Rb S Sc Se Na Ta Tb Th W U Yb Zn Al Bi Ca Cu Pb Li Mg Mn Ni P Ag Sr S Te Sn Ti V Zn kaolinite Sb As Ba Br Ce Cs Cr Co Eu Au Hf Fe La Lu Mo K Rb S Sc Se Na Ta Tb Th W U Yb Zn Al Bi Ca Cu Pb Li Mg Mn Ni P Ag Sr S Te Sn Ti V Zn pyrite Sb As Ba Br Ce Cs Cr Co Eu Au Hf Fe La Lu Mo K Rb S Sc Se Na Ta Tb Th W U Yb Zn Al Bi Ca Cu Pb Li Mg Mn Ni P Ag Sr S Te Sn Ti V Zn Larsens East Tritton Hartmans Figure 7.7 Bivariate correlation coefficient plots (r 2 values) for selected elements with mineral phases identified by XRD for Larsens East, Hartmans and Tritton profile samples. Mineralogical controls are considered irrespective of degree of weathering. 272

21 Hf, Ni, Sc, Na, W, and Zn. Similarly, chlorite was found to be strongly positively correlated with the elements Sb, Al, As, Cr, Co, Fe, Li, Mg, Mn, Ni, Sc, Na, Ti, Zn and the REE. Iron oxides (hematite and goethite) on the other hand showed high correlation with the elements As, Br, Fe, Mo, Cu, Sr and V. Muscovite, while being almost ubiquitous in all samples, showed a high degree of correlation with the elements Al, Ba, Cs, Hf, Li, Mg, K, Rb, Na, Ta, Th, Ti, U and V. The occurrence of kaolinite was limited, although it did show an increased correlation with the elements Co, Hf, Se, Th and Ni in the Larsens East weathered profile. Lastly, pyrite identified in the Larsens East profile correlated highly with the elements Sb, As, Bi, Cr, Co, Cu, Au, Fe, Pb, Mo, Sc, Se, Ag, Te and Zn. These mineral/element relationships are consistent with major mineral phases and gross lithogeochemical differences observed for the Girilambone North and Tritton study areas. Koons et al. (1980) studied the association of trace elements with Fe and Mn oxides during the early stages of rock weathering, by determining the linear correlation of trace elements with Fe and Mn contents. In this study, analysis of the Fe correlations of Hartmans and Larsens East profiles identified strong positive correlations for Sb, As, Bi, Br, Co, Cu, Au, Pb, Mo, Se, Sr, Ta and U (Table 7.5). Manganese was only determined for Larsens East samples. Of note is the low correlation between Fe and Mn, and low abundance of Mn in the Larsens East profile weathered samples ( ppm), which suggests Mn-oxides were not present in any appreciable amount. With respect to Fe, similar results were obtained by Koons et al. (1980), who showed a close relationship between Fe oxides and As, Co, Cr, Mn, Sc, Th, U, Zn and the HREE during the early stages of rock weathering, and Butt & Smith (1992) identified ferruginous regolith to retain As, Bi, Co, Cu, Pb, Mn, Ni, and Zn. Cairns et al. (2001) identified important host minerals for a range of base metal cations, suggesting that Fe oxides and oxyhydroxides (goethite and to a lesser extent hematite) and intergrown minerals of the jarosite and alunite group, readily retain Sb, As, Bi, Cu, Pb and Zn as substituted or adsorbed cations. Taylor and Eggleton (2001) summarise studies of the retention of base metal and other cations in Fe-oxides and oxyhydroxides, which examine the development of these minerals in the regolith profile. While goethite and hematite may be formed directly from weathering of Fe-bearing minerals, they commonly progressively develop as an early precipitate of ferrihydrite, which has a large surface area and is capable of adsorbing many trace elements. Upon conversion to goethite 273

22 and/or hematite, adsorbed cations may be retained in the crystal structure of the newly formed mineral. Alternatively, the scavenging capabilities of goethite have been well documented, whereby soluble transitional and other metals may be adsorbed on the precipitated goethite or hematite crystals, the process of which is largely ph-dependent. Thornber (1992) showed the experimentally derived sequence of selected transitional metal adsorption to goethite which progressed from Cu at ph 4.5, to Mn at ph 7.5 (Table 7.6). Furthermore, hematite generally showed a lesser ability to scavenge metal ions, particularly Cu and Ag (Taylor & Eggleton, 2001), a phenomenon also reflected in the work of Cairns et al. (2001). Given the strong correlation observed for iron minerals with Ag, Cu, Mo and Pb in the Larsens East profile, one could conclude that the dominant Fe phase responsible for metal scavenging is more than likely goethite, and that metal uptake more than likely occurred in a highly acidic environment (ph<5). Table 7.5 Bivariate correlations (r 2 values) of Fe with other analyte elements within the Larsens East (n=73) and Hartmans (n=182) regolith profiles. Bold type indicates significant bivariate correlations (α=0.01). Asterix denotes samples analysed by ICP-MS/AES methods (n=17). Larsens East - Fe (n=73) Hartmans - Fe (n=182) Ag* La Ta As Ce Cu* Sc Ca* Au Zn Mo* Cr V* Co La Pb* Te W Sb Cr As Yb S* Mo Yb Bi* Co Rb Se Lu Se Lu Mn* U Eu Sr* Zn Cs Cu Zr Mo Sn* Mg* Ta Na Sb Na Ti* Br Ca Br Th K Sc Ag U Hf Zr* Te W Sm Zn* Al* Sm Ba Au Ba Ba* Hf Rb Ce Ni* Li* Th Cs Eu P* Tb K Table 7.6 Sequence of adsorption ph for selected metals to goethite. After Thornber (1992) as quoted by Taylor & Eggleton (2001). Cu 2+ Pb 2+ Zn 2+ Co 2+ Ni 2+ Cd 2+ Mn 2+ ph

23 7.4.3 Fe-accumulation - ferruginisation, gossans and ironstones Zones of ferruginisation have been recognised in Hartmans and Larsens East profile sections. Within the ore zone of Hartmans, so called limonitic ore has developed above the position of the primary chalcopyrite mineralisation and surrounding pyritic halo, with strong ferruginisation still evident in pit walls at the southern end of mine workings. The Larsens East profile is characterised by several small zones of intense ferruginisation, which occur high in the profile and are seemingly unconnected to weathering sulfides at depth. Invariably, company databases have described these zones as gossanous, with little concern given to the genetic link to weathering sulfide bodies. Eggleton (2001) defines a gossan as the weathering expression of rocks that contain substantial sulfide mineralisation which commonly exhibit a box-work fabric derived from that of their sulfide precursors, whereas, ironstone is described as highly ferruginised weathered material consisting mainly of iron oxides and oxyhydroxides which need not be related to the weathering of sulfide mineralisation. Thus, it would seem that much of the so called gossan, may best be termed ironstone until the genetic link between these Fe-accumulations and weathering sulfides can be established. Taylor & Thornber (1992) state the need to distinguish between ironstones and gossans, which can readily be determined on the basis of geochemistry and mineralogy. While the geochemistry of gossans may vary depending on the style of parent mineralisation (Taylor & Thornber, 1992), Scott et al. (2001), in a study of volcanogenic massive sulfide derived gossans in the eastern Lachlan Fold Belt, suggested the accumulation of Pb to be common to all gossans. This notion is largely supported by earlier work of Taylor & Thornber (1992), which lists Pb as being anomalous in all but a few cases of some 33 gossanous occurrences in various climatic and geomorphological regimes of Australia. The abundance of Pb in gossans of Pb-poor mineralised systems, such as those of the Girilambone district, is however more tentative. Observations of the Hartmans regolith profile in the present study, did not identify appreciable Pb content (<41 ppm) in primary mineralised zones. However, elevated Pb was identified in the limonitic ore zone (up to 157 ppm) at the southern extent of mine workings, and near-surface ferruginous zones of drill hole HARC085 ( ppm). Similarly, despite the Pb-poor Larsens East primary mineralisation ( ppm), Pb in near-surface ferruginous accumulations ( ppm) was identified above a Pb- 275

24 deficient background (<50 ppm) of metasedimentary rocks (Table 7.1). Many other ferruginous zones listed as gossan did not however indicate increased or anomalous Pb abundance and are most likely indicative of barren ironstone material. Gossanous textured ferruginous zones of the Larsens East profile displayed elevated Sb, As, Cr, Co, Au, Pb, Se and the LREE. Similarly, ferruginous zones of the Hartmans profile was characterised by elevated abundance of the elements Co, Au, Hf, Fe, Mo, Rb, Sc, Ta, W, U and the REE. Ferruginous zones of Hartmans and Larsens East were largely distinguished on the basis of Fe abundance, Fe-As association and presence of other metals e.g. Sb, Cr, Co, Au, Mo, which were typically observed in gossanous samples. Siliceous gossanous zones observed in the Hartmans profile were also characterised by increased W response. Thus, tentative classification of ferruginous accumulations is possible on the basis of geochemistry, although only where characteristics of wall-rocks and mineralised samples are known. Even so, gossan zones were generally only intercepted by drilling and only rarely identified at the surface level, occurrences of which have largely been tested by previous exploration efforts within the Girilambone district. Thus, while further identification of ferruginous materials could be obtained from detailed mineralogical studies (Robertson et al., 2001a; Taylor & Eggleton, 2001) or Pb-isotope investigations (Gulson, 1984, 1986), additional investigations in this regard are not warranted. An estimation of the degree of weathering of these profiles can be attained from examination of the distribution of Fe and related minerals in the regolith profile. A lack of appreciable ferruginous duricrust at present surface levels suggests incomplete attainment of a lateritic profile, although deep weathering (up to 110 m) suggests appreciable weathering has occurred. According to Scott et al. (2001), gossans of immature profiles commonly display elevated Cu and Zn responses due to the relatively shorter exposure, or less intense nature of regolith development, while in a mature profiles, these relatively mobile elements would be expected to be leached from highly weathered rocks. Accordingly, the upper profiles of both Larsens East and Hartmans did not display significant abundance of either Co or Zn. Deeper in the Hartmans profile however, gossanous zones at the southern extent of mine workings did show appreciable Cu abundance, although the observed distribution of Cu was more than likely due to the close proximity to secondary Cu-mineralisation. 276

25 A useful indicator of Fe accumulation has been suggested by Brown et al. (2003) as the ratio of Fe to Sc, from which relative and absolute Fe accumulations can be determined. They state that the ionic radius of Fe (III) and Sc (III) are almost identical and Sc readily substitutes for Fe (III) in primary minerals and secondary oxy-hydroxides and oxide minerals. The tight Fe-Sc relationship is however not generally observed where Fe (II) exists, thus, the possibility of the Fe/Sc ratio as an indicator of Fe-accumulation due to weathering was recognised. A similar approach adopted here identifies the distribution of Fe and Sc in Tritton drilling samples, which clearly shows separation between the lithology groups on the basis of Fe/Sc ratio (Figure 7.8). Chlorite- and quartz-rich lithologies generally show a conformable Fe/Sc ratio, which surprisingly seemed unaffected by the influence of weathering which may be considered to approximate the parent rock Fe/Sc ratio (approximately 2830). Data points plotting away from the parent rock Fe/Sc ratio most likely represent samples with ferrous iron content. Iron is observed to be strongly enriched in mineralised samples relative to Sc, and while Fe is elevated in mafic samples, Sc is relatively enriched. Thus, the Fe/Sc ratio can also be used an indicator of lithology type, particularly for the identification of mineralised and mafic samples, which display significantly different Fe/Sc ratios to that of metasedimentary rocks Quartz-rich Chlorite-rich Mafic Mineralisation Unclassified Fe (%) Sc (ppm) Figure 7.8 Lithological discrimination of Tritton drilling samples on the basis of Fe/Sc ratios. Quartz- and chlorite-rich samples approximate parent rock Fe/Sc ratio (approximately 2830). 277

26 As discussed previously in Chapter Two, a limitation of INAA is the inability to determine valence states of analyte elements. In this instance, the Fe/Sc ratio can be used to identify the limits of oxidative processes, by inferring the valence state of Fe in unknown samples. The plot of Fe/Sc for Hartmans and Larsens East generally show significant departure from the Fe/Sc ratio of (relatively) unweathered metasedimentary rocks observed at Tritton (Figure 7.9). An increase in Fe accumulation is observed with increasing degree of weathering. Samples from the base of the Girilambone North profiles where the influence of weathering is reduced, plot close to the approximate parent rock Fe/Sc ratio (2140). Fe (%) Extremeley weathered (n=6) Highly weathered (n=74) Moderately weathered (n=110) Slightly weathered (n=52) Sc (ppm) Figure 7.9 Plot depicting the Fe/Sc ratio of regolith samples from the Girilambone North study area. The lower limit of the Fe/Sc ratio is shown as a red dashed line, which represents the approximate Fe/Sc ratio (approx. 2140) of parent rock material, above which Fe accumulation is indicated Geochemical distribution of copper and associated elements in the regolith profile The geochemical response of primary and secondary mineralisation has been detailed in a previous section of this chapter. This section discusses the distribution of mineralisation-related elements, including Cu, within the regolith profile of Girilambone North. Juxtaposition of Girilambone North profiles and the Tritton regolith profile allows comparisons of geochemical dispersion within the vicinity of weathering sulfide ores and in barren weathered profiles respectively. At Girilambone North, the 278

27 dispersion of Cu and related elements has been negligible below the base of weathering. In the regolith profile, the dominant process of Cu and other element dispersion principally relates to oxidation and dissolution reactions of minerals, which are otherwise stable in the primary environment, and release of soluble ions in oxygenated groundwater solutions. The exposure of primary sulfide minerals at the base of weathering has developed a secondary copper mineralised body, which grades from chalcocite at the weathering front to copper-oxide and copper-carbonate mineral assemblages and sparse native copper higher in the profile. The formation of secondary copper minerals is due to (seasonal) fluctuations in the groundwater table and related supergene processes, which have been described in earlier sections of this study and by various workers for selected mineral deposits (Lewis, 1975; Williams, 1990; Butt, 1998). The dispersion of Cu and related elements is somewhat more subtle in the Tritton weathered profile and related to the up-dip and up-plunge extensions of mineralisation. In this way, the geochemical signatures mainly represent alteration signatures associated with the introduction of mineralising fluids, rather than regolith-related processes, and will be discussed in more detail in the following section. Examination of the Tritton regolith profile does however aid the understanding of regolith profile development and subsequently, strategies for geochemical exploration in the Girilambone district. In general speaking, primary Girilambone-style mineralisation is characterised by elevated levels of the elements Sb, As, Bi, Br, Co, Cu, Au, Fe, Pb, Mo, Se, Ag, Sr, S, Te, Sn and Zn. While many of these elements were found to be in low abundance in comparison with host lithologies, mafic intrusive rocks or even average crustal abundances e.g. Sb, Mo, Ag and Se (Table 7.2), the anomalous occurrence of these elements is confined to the known occurrence of primary mineralisation. Thus, their occurrence elsewhere in the regolith profile, given their low abundance even when anomalous, is likely to be minimal, at best. Nevertheless, at Hartmans, directly overlying the extent of primary mineralisation at the southern end of the prospect, anomalous Br, Au, Fe, Se and to a lesser extent Zn, were identified within the zone of surface enrichment, despite being strongly leached within the underlying depletion zone or in the case of Se and Zn, which displayed only low-level anomalism, being poorly represented elsewhere in the profile. Areas further to the north within the same sampling section, while demonstrating a prominent surface enrichment zone by the elevated occurrence of Fe, K, Sc and the REE did not accommodate mineralisation elements. 279

28 Thus, the lateral extent of the primary mineralised signature (other than Cu) is limited to the near-surface horizons above the known locations of primary mineralisation only. Conversely, mineralisation-related elements in the Tritton profile were not seen to persist to surface or near-surface levels and were generally leached within the upper saprolite zones. Thus, in the absence of strong depletion and surface enrichment zones (developed in close proximity to weathering sulfide bodies), the background concentrations of mineralisation elements generally showed a depletion in the upper regolith profile. Exceptions to this were observed for the elements Co and Pb, which were closely associated with Cs, Fe and REE enrichment and increased ferruginisation in the upper profile of the Tritton study area. Elsewhere in the study areas, ferruginous accumulations have been shown to host increased contents of Sb, As, Cr, Au and Se, although these elements were not found in any great abundance in the Tritton regolith profile. Sulfur (in primary sulfide minerals) was strongly leached at the weathering front, with sulfur in non-mineralisation related minerals found to decrease markedly with increasing degree of weathering in the Larsens East profile (S not analysed for any other profile). Earlier discussions of oxide mineralised zones identified partial retention of As, Ba, Au, Sr, V and Zn, which may form additional target elements for mineralisation of this kind, although Cu is present in far greater abundance (Table 7.3) and thus more amenable to detection. These results reflect the susceptibility of sulfide minerals to weathering, whereby retention of these elements in the moderate to extremely weathered portions of the profile is generally due to the presence of Fe-oxide hosts. Butt (2000) suggests that S is in fact the most susceptible element to weathering and associated sulfide elements (Cd, Co, Cu, Mo, Ni and Zn) are generally depleted in the regolith profile, except where appreciable Fe-oxide development has served to retain them. Due to the relative abundance of Cu in the Girilambone North profiles, Cu dispersion forms a special case worth considering. Despite strong leaching in the upper profile, minor Cu (generally in excess of 100 ppm) was encountered within the surface-enriched zones for the length of the Hartmans and Larsens East sampling sections. The extensive flat-lying secondary mineralised zones have formed from northward migration of groundwaters, carrying, almost exclusively, copper, in solution. Some regions which 280

29 display native copper mineralisation are also associated with elevated to anomalous Se, Ag and Te, although generally, the oxidised ore zones do not display significant trace element enrichment. Recent (pre-mining) studies of groundwater geochemistry in the vicinity of the Girilambone and Girilambone North deposits by Elvy (1998), have shown that copper in the present groundwater conditions is stable as secondary copper (carbonate/phosphate) minerals as a malachite-pseudomalachite mineral assemblage. Thus, copper in the present groundwater conditions is relatively immobile and dispersion from copper mineralised bodies in the regolith profile is minimal. Nevertheless, a north-trending hydromorphic dispersion plume has developed up to several hundred metres to the north of known mineralised occurrences at Girilambone North, creating a remobilised copper anomaly, which has precipitated Cu, more than likely in association with Fe-minerals, at the extent of acidic groundwater movement. Thus, regional structural features such as north-trending late-stage faults identified by Fergusson et al. (2005), have acted as fluid conduits for permeating groundwater solutions and carried acid-soluble copper in solution away from the mineralised zones. 7.5 The Surficial Regolith Environment Studies of the surface geochemical environment allowed assessment of geochemical surveys undertaken at the Girilambone North and Tritton study areas as part of this and previous studies. At Tritton, this largely entailed regolith-landform mapping and site investigations, to identify the regolith-landform characterises and potential sampling media for geochemical exploration purposes. At the Girilambone North study area, regolith-landform mapping was not possible due to recent mining activities, however close inspection of the regolith profile and soils developed therein was possible within the open-pits, mining of which has ceased. Mechanisms of trace element dispersion have been recognised in these environments, which are largely mechanical given the present semi-arid climate and erosional regime identified in the study area. When combined with investigations of the regolith profile, previously described, dispersion mechanisms of the surficial regolith environment have allowed development of geochemical dispersion models for Girilambone North and Tritton study areas and elsewhere in the Girilambone district. Interpretation of surface geochemical exploration data have been aided significantly by the understanding of surficial regolith development and dispersion processes. 281

30 7.5.1 Regolith-landform setting Landscapes of the Girilambone district predominantly occupy an erosional regime, with low-relief topographic highs (and some residual high stands), undulating low rises and broad low-energy drainage networks. Outcrop within the study areas is typically highly weathered and exposed only in ephemeral drainages, although less-weathered outcrop is found on low-rises and are consistent with more resistant lithologies such as quartzite or silicified metasedimentary rocks. Given the close association of mineralisation with intense silicification throughout the Girilambone district, it is not surprising to observe mineral occurrences in association with topographic highs e.g. quartzite ridges. Thus, studies of the regolith-landform settings about known occurrences of mineralisation may not necessarily be indicative of the greater Girilambone district. In fact, much of the area to the north and west of the Girilambone district contains widespread sheetwash colluvium deposition with intervening alluvial tracts, infilling palaeo-valleys with up to 40 m of sediment (Chan et al., 2003a). However, these landforms were not encountered within the extent of current investigations. Dominant processes shaping the landscape are due to mechanical processes, whereby material from residual plains and topographic high points, is gradually eroded and redeposited in drainage channels and topographic low points in the landscape. Erosive stripping has at least partially truncated moderately developed lateritic regolith profiles, which in the absence of appreciable ferruginisation or protective duricrust surface layers, have commonly been eroded to the level of upper saprolite (e.g. Tritton). In the vicinity of shallow sulfide mineralisation, regolith development has been somewhat more advanced, with ferruginisation of surface layers and formation (albeit weakly) of mottled clay horizons in the upper profile. At Tritton, mainly residual soils occupy the Tritton landscape, with some localised colluvial material originating from residual rises and plains, accumulating at topographic low points and drainages in the area. Red-brown silty clay loams and sandy loams have developed as a result of bedrock weathering and largely occur as either residual soils, or shallow, (<2m) locally derived colluvial soil mantles, which thicken away from topographic highs. Further characterisation of residual soils has come from investigations of suitable soil size fractions and sample preparation methods (Appendix 1) and analysis of several soil profiles. At the soil/weathered bedrock 282

31 interface, an extensive angular quartz gravel layer forms a prominent horizon, inferred to represent the upper extent of residual soil material. At these depths, which formed the common sampling depths in soil surveys of the current study, weathered saprolite and minor ferruginous clays and mottling was commonly encountered. While slightly increased soil ph was observed in deeper portions of the profile, soils were generally quite neutral. The slight rise in soil ph with depth is most likely due to the accumulations of alkaline salts, although no regolith carbonate accumulations, which might otherwise explain the rise in ph with soil depth, were encountered during this study. While other workers have suggested that aeolian material may influence an appreciable (0.5 to 3 m) component of soil horizons (Chan et al., 2003a; Tate et al., 2003, 2004), based on the characteristics of aeolian materials identified by these studies, the current investigation has shown probable aeolian input within only the top 0.5 m of soils within the Tritton study area. Primarily the outcome of regolith-landform studies has been to identify likely regolith sampling media and areas of transported and residual regolith at the surface. Furthermore, the characterisation of regolith-landform settings has allowed comparisons to be made with conceptual models of geochemical dispersion proposed by other workers (Butt & Smith, 1980; Butt & Zeegers, 1992; Butt et al., 2000). Geochemical dispersion mechanisms within the Girilambone study areas will be considered in a following discussion section of this chapter Surface geochemical response to mineralisation A similar geochemical response to primary mineralisation was encountered at both Girilambone North and Tritton study areas, albeit generally less pronounced at the latter deposit. Levels of anomalism have been discussed previously in Chapters Five and Six. The mechanism of geochemical dispersion and subsequent formation of geochemical response for each deposit are however, subtly different, and will be discussed herein. Table 7.7 summarises the anomalous geochemical response to mineralisation of the Girilambone deposits investigated in the present study. Both Girilambone North and Tritton deposits are associated with north-trending corridors of elevated multi-element contents. Although less well-defined at the Tritton study area, these multi-element zones correspond to the surface location of extensive 283

32 quartzite ridges, which at various locations throughout the Girilambone district, are spatially associated with mineralised occurrences. Fogarty (1996; 1998) describes these geochemical zones of regionally anomalous geochemistry as the Rockdale anomaly, which extends for many kilometres and has been the focus of most recent exploration efforts. Anomalism broadly associated with these geochemical zones are best defined at the Girilambone North deposits, which exhibits increased response of As, Bi, Cu, Au, Mn, Se, Ag and Zn in various sampling media, although the most notable are broad above-background Cu, Au (and Ni) zones. At the prospect scale, Larsens East was defined by coincident anomalism for the elements As, Ba, Cu, Au, Mo, Ag and Te from soil (Regoleach) sampling surveys of previous investigations. Similarly, vacuumdrilling geochemical traverses over the position of Hartmans mineralisation display locally anomalous concentrations of As, Cu, Au, Pb, Mo and Ni. Table 7.7 Summary table of anomalous geochemical responses identified in the Girilambone district study areas of the present study. Anomaly Sampling media Anomalous geochemical response Girilambone North study area Multi-element geochem. corridor all As, Bi, Cu, Au, Mn, Se, Ag, Zn N-trending dispersion halo RAB/vacuum Cu Local - Larsens East soil (Regoleach) As, Ba, Cu, Cu (total), Au, Mo, Ag, Te - Hartmans vacuum As, Cu, Au, Pb, Mo, Ni - Hartmans W (mafic?) vacuum Cu, Co, Mo, Ni, Zn Tritton study area Quartzite ridge all As, Cu, Pb Budgerigar - historic workings vacuum As, Cu, Au, Pb, Mo Budgerigar - down-slope dispersion vacuum Au, Zn Mafic - general (PC1) vacuum Co, Ni, Zn Tritton - up-plunge vacuum As, Cu, Pb (subtle multi-element) - up-plunge soil Sb, As, Br, W, U - up-dip soil Sb, As, Bi, Cd, Cu, Au, (Mo), S, Zn (subtle) - down-slope from up-dip soil Cd, Co, Cu, Zn - decrease from point source - mafic soil Co, Pb, Mo, Ni, Se, Ag, Zn In the Tritton study area, the geochemical response of historic workings of the Budgerigar deposit, are prominently identified in vacuum- and RAB-drilling geochemical surveys, by the anomalous response of As, Cu, Au, Pb and Mo. Downslope dispersion of Au and Zn are recognised extending to the north-east of mine 284

33 workings. This mineralisation lies within a north-trending silicified ridge, which broadly defined the extent of the Rockdale anomaly within the Tritton study area. Re-analysis of vacuum-drilling geochemical data derived from previous investigations, identified a subtle multi-element geochemical response, defined by the combined geochemical variance of As, Cu and Pb at the up-plunge extension of known mineralisation of the nearby blind Tritton copper deposit. The previously un-described association of these elements was identified by principal component analysis, and is thought to have been overlooked due to the magnitude of geochemical response identified at the nearby Budgerigar deposit, within the same geochemical survey extent. This area occupies a topographic high point within the study area and the possibility of geochemical dispersion from the nearby Budgerigar deposit is not probable given the current topographic and drainage systems (although both deposits are on the same N-trending ridge, a topographic rise and drainage gully separates the positions of anomalous geochemistry). Coincident anomalism at this same location was identified from analysis of a geochemical survey of residual soils of the current study. In the up-plunge extension of mineralisation, Sb, As, Br and W, otherwise found to be associated with primary mineralisation (Table 7.2) were identified in anomalous proportions. There is a possibility however, that soil geochemical anomalism identified in this area, may be due to the presence of outcropping quartzite, which also has elevated contents of these elements. However, although not immediately attributable to the up-plunge extension of mineralisation, responses of Cu, Au and Zn on soil traverses do increase down-slope with distance away from this zone, and accumulation in topographic low points (drainage depressions), which may indicate mechanical dispersion processes have acted to remove these elements from a point source. The geochemical signature identified at the up-dip extension of mineralisation is however, more convincing. Coincident anomalism of the elements Sb, As, Bi, Cd, Co, Cu, Au, Mo and Zn was observed in various soil geochemical surveys of this and previous investigations at the western extent of known mineralisation at Tritton and decreasing Cd, Co, Cu and Zn were recognised in moving down-slope from the mineralised up-dip point source. Given the association of mafic bodies with primary sulfide mineralisation in the Girilambone district (Fogarty, 1998), the geochemical response of mafic bodies forms a suite of interest for geochemical exploration in this region. An earlier section of this chapter has summarised the geochemical characteristics of mafic bodies, which 285

34 commonly display elevated geochemical abundances of Ca, Cr, Co, Cu, Fe, Ni, Na and Sc at Tritton, and anomalous Ba, Co, Cr, Ni, Sc, Na and Zn within the Girilambone North profiles. Elsewhere in the Girilambone district, Chan et al. (2003b) found mafic dykes to be enriched in Cr, Co, Cu and Zn. In the surficial regolith environment, mafic signatures at Tritton have been recognised by the anomalous occurrence of Co, Pb, Mo, Ni, Se, Ag in soil samples in a traverse directly over the known extent of mineralisation. A detailed account of the Tritton copper deposit of Berthelsen (1998) describe a mafic body which has intruded late-stage structural planes dividing the upper zones of Tritton mineralisation. In an unpublished review of soil geochemical surveys conducted at Tritton, Rutherford (1998) suggested there may be a mafic signature within this vicinity. Subsequent favourable responses for Co, Cu and Mo identified within the current study add further credence to the assignment of a mafic lithogeochemical signature in soils above Tritton mineralisation. At Girilambone North, Pahlow (1995) mapped zones of mafic lithologies to the west of known mineralised occurrences, which coincide with extensive zones of Ni dispersion and localised Co, Cu, Pb and Mn, identified in previous geochemical exploration data by the current study. 7.6 Geochemical Dispersion in the Regolith There are, in general, two processes of geochemical dispersion, these being hydromorphic and mechanical dispersion. Geochemical dispersion can further be considered in three-dimensions, or for the purpose of modelling separate processes, within the surficial regolith environment and within the regolith profile. In the regolith profile, the transport of geochemical signatures is predominantly by hydromorphic processes, although at the macro- to local-scale, weathering products may be moved by gravity through voids and open pore spaces, such that weathering-resistant particles may be re-worked though a weathering profile by mechanical means. Conversely, within the surficial regolith, geochemical dispersion is chiefly by mechanical processes. The mobility of elements in the regolith environment in solution or as neo-formed weathering products is largely due to hydromorphic dispersion, or when bound or incorporated in stable phases, to mechanical dispersion mechanisms (Taylor & Eggleton, 2001). Other mechanisms, namely bioturbation and transport by vegetation root up-take, have been considered by other workers (Cohen et al., 1996; Taylor & Eggleton, 2001; Kebede, 2004), although these were thought to have only minor 286

35 contribution to the geochemical distributions investigated in the present study and hence are not further contemplated. While the regolith development is fairly similar throughout the Girilambone district, subtle differences are recognised, particularly in the preservation of near-surface ferruginous zones and development of strongly leached zones, within the vicinity of highly weathered sulfide mineral deposits. So too are the dispersion of potential elements of interest different for each of these regolith-landform situations. This section of discussion considers the dispersion of geochemical elements within these surficial and profile components of the regolith and provides models of geochemical dispersion for each of the study areas, derived from investigations of the current study. Many of the principles governing geochemical dispersion in the regolith discussed here, are borne from conceptual models of geochemical dispersion conceived by Butt & Smith (1980) and Butt & Zeegers (1992), which have been extensively considered by these same workers and many others (Butt, 1998; Butt et al., 2000; Smith et al., 2000; Taylor & Eggleton, 2001; Anand & Paine, 2002), which although not so rigidly applied, are later incorporated in a geochemical exploration model for the Girilambone district Geochemical dispersion in the surficial regolith environment While hydromorphic dispersion may occur, the dominant action of geochemical dispersion in the surficial regolith environment is mechanical transport of sediments or particles by wind or water, and ultimately gravity, to lower portions of the landscape. This is particularly so in semi-arid terrains (Butt, 1987), where low rainfall maintains a reduced groundwater table and provides insufficient residence time of near-surface groundwaters for dissolution of minerals and elements and their transport in an aqueous phase. Periodic rainfall may dislodge surficial sediments, carrying them in suspension to drainages or as down-slope sheet-wash. The net effect is migration of material away from topographically higher portions of the landscape, and with it, any geochemical signatures bound to or incorporated in weathering-resistant particulate matter or minerals. On a regional scale within the Girilambone and greater Cobar districts, physical transport mechanisms have developed extensive drainage networks, filled with gravels and lags of various origins, commonly covered by extensive colluvial fill and derived 287

36 from erosive stripping of palaeo weathering surfaces (Alipour et al., 1997; Chan et al., 2003a; Chan et al., 2003b; Dehaan & Taylor, 2004). Pahlow (1995) indicates that Larsens and Hartmans mineral deposits occupied topographic highpoints in the landscape prior to mining activities, and generally manifested as north-trending silicified ridges of quartzose lithologies, similar to observations of the Tritton study area of the present study. On a local scale, geochemical dispersion by mechanical means has manifested as accumulations of sediments in local drainage networks and down-slope migration of geochemical signatures from topographically higher point sources. At Girilambone North, the geochemical signature of primary mineralisation, while often not encountered over the extent of known mineralisation, was identified at lower points in the landscape flanking topographic highs. This was evident for a number of elements, including Co, Cu, Mg and Te, which may have been retained in coarse fractions of resistant ferruginous regolith and migrated down-slope under the action of mechanical dispersive processes. Similarly, isolated Au anomalies down-slope to the north-east of the up-dip extension of Tritton mineralisation, and decreasing Co, Cu and Zn response down-slope of the up-dip geochemical signature of the same deposit, are most likely indicative of mechanical dispersion away from topographically higher point source. Furthermore, the limit of geochemical signatures being confined to flanking slopes for these elements, more than likely implies that they are retained in coarse particulate fractions, possibly ferruginous materials, which do not necessarily propagate to the lowest points of the landscape, e.g. alluvial tracts. Figure 7.10 depicts the formation of mechanically displaced geochemical anomalism, or reducing geochemical response, in the surficial regolith away from a topographically higher point source. Hydromorphic dispersion of more mobile elements may be present within the study areas. At Girilambone North, extensive southward dispersion of Ni in soils, which broadens away from anomalous Ni zone to the west of current mine workings, is indicative of aqueous transport. While there would undoubtedly be some mechanical component, the dispersion plume of up to 3 km seems too great for mechanical dispersion alone in this low-relief landscape. Similarly, elevation of the elements such as Zn, traditionally thought to be relatively mobile in the regolith environment, at the eastern extent of soil sampling lines at Tritton, is most likely indicative of hydromorphic dispersion. However, similar increases in Th, which is generally regarded as immobile 288

37 (Taylor & Eggleton, 2001), may indicate transport of geochemical signatures as particulate matter rather than in solution. residual plain quartzite outcrop colluvial slope/plain alluvial tract soil /saprolite geochemical signature point source m saprolite quartzite 1 o mineralisation Mechanical dispersion Hydromorphic dispersion soil/saprolite interface weathering front Figure 7.10 Schematic diagram depicting the formation of mechanically dispersed surficial geochemical signatures from a topographically higher point source. The geochemical signature decreases or is translocated down-slope from the point source. Some analyte elements of soil surveys (particularly Regoleach surveys) displayed a geochemical response inverse in magnitude to topographic variation (Figure 7.11), which could possibly be explained by either or both, dispersion mechanisms. In the absence of geochemical enrichment source e.g. mineralisation, these responses may merely indicate accumulation of background signatures in topographically low portions of the landscape. In the case of mechanical dispersion, loosely-bound or selectively incorporated metal ions, which are the target of partial/selective leach digestions, could be transported as fine particulate matter e.g. clays, by sheet-wash or surface runoff towards low points in the landscape. However, if this were true i.e. the geochemical signature is retained by fine particulate matter, one might assume that topographic high points would be just as likely to accommodate such signatures due to the action of aeolian accession, particularly on the windward side of structural traps in semi-arid to arid areas. So too is the notion of these geochemical responses being transported mechanically as coarser fractions, discounted by the perception that coarser 289

38 fragments (particularly in low-relief landscapes) travel lesser distances down-slope, and would thus not necessarily account for the observed geochemical response. As a result, hydromorphic dispersion is favoured (although not exclusively), for the formation of these topographically inverse signatures, whereby weakly-bound metal ions may migrate to lower portions of the landscape in solution, either by sheet-wash and surface run-off, or by hydrous transport at the soil/saprolite interface (Figure 7.11). It is however unclear as to which of these latter processes i.e. runoff versus sub-soil surface water migration, would allow the greater dispersion. Regoleach response saprolite soil/saprolite interface Mechanical dispersion Hydromorphic dispersion m Figure 7.11 Schematic diagram representing possible formation processes of inverse topographic geochemical response identified from Regoleach soil surveys of the current study Geochemical dispersion in the regolith profile The controls of element mobility in solution have been shown by many workers to be dependent on ph and redox conditions (Brookins, 1988; Thornber, 1992; Trescases, 1992), and discussed in some detail in Chapter Two. In considering the ph regime of groundwater solutions and element solubilities, cations are generally more soluble in acidic solutions, and anions, in alkaline solutions (Thornber, 1992). Although element mobilities are largely ph-eh dependent, further controls of element solubility are reasonably well defined, whereby larger ions with a smaller charge (low field strength) are generally more mobile, than smaller ions with a greater charge (high filed strength) (Rollinson, 1993). Where groundwater flow is appreciable, or continually replenished by meteoric waters (rainfall), hydromorphic dispersion is greatest, whereby dissolved 290

39 species are transported towards lower hydraulic gradients in the weathering profile. Hydromorphic dispersion is principally responsible for the formation of enriched copper zones within the Girilambone North copper deposits. In its simplest form, hydromorphic dispersion associated with downward percolating oxidative surface waters, has reprecipitated copper minerals at the top of the (palaeo) watertable, where redox conditions change from oxidising to reducing and given rise to a classic supergene profile. When the effects of lateral (and vertical) groundwater movement are considered, extensive dispersion of ore and related trace elements in the direction of groundwater flow can be achieved, and creation of a broad, albeit reduced magnitude, geochemical halo, larger than the extent of mineralisation itself, may form. As has been discussed previously, greater leaching of upper regolith horizons and development of depletion zones may be heightened in the presence of weathering sulfide minerals. The resultant acidic solutions readily mobilise metal ions and accelerate weathering of common rock-forming minerals, which at the limits of alkaline conditions, are reprecipitated and there will remain stable until further changes in redox conditions eventuate. Furthermore, metals are readily retained by Fe-oxides and oxyhydroxides (particularly goethite) higher in the profile, which may lead to a broad surface expression in surface iron accumulations and saprolite, the extent of which commonly narrows with depth (Figure 7.12a). Thus, geochemical sampling targeting deeper portions of such a geochemical system, would generally locate a much narrower target than higher in the profile. The realisation also that this geochemical anomaly may be remobilised from a source due to strong leaching and groundwater movement must be made for correct interpretation of this anomaly. However, in the absence of surface enrichment associated with ferruginous surface layers e.g. truncated regolith profile such as at Tritton, the metal response would likely be greatest deeper in the profile, below the extent of weathering and possibly aligned more closely with mineralisation (Figure 7.12b) Girilambone mineral deposits Models of geochemical dispersion about the Girilambone mineral deposits investigated as part of the current study have been created. These models largely summarise the observed geochemical response to mineralisation of elements in soil, regolith, unweathered rock and mineralisation (both primary and secondary) discussed thus far. Geochemical patterns pertain to two main regolith-landform situations, within an 291

40 extent of surface ferruginisation 2 o Cu dispersion (Cu >250 ppm) saprolite (a) saprolite (b) 2 o Cu dispersion (Cu >250 ppm) vacuum-drilling extent RAB-drilling extent m Figure 7.12 Anomaly widths of sub-surface geochemical response identified by shallow and deeper regolith sampling (drilling) for (a) regolith profiles with moderately ferruginised surface horizons; and (b) partially truncated regolith profile, with soils directly overlying upper saprolite horizons i.e. no appreciable ferruginised surface layers. Anomaly width of shallow and deeper drilling indicated for each level of sampling. 292

41 otherwise low-relief erosional regime. These regolith-landform units are invariably separated on the basis of presence or absence of surface ferruginised layers, the development of which have been hitherto discussed at length. Characteristics of geochemical dispersion patterns relating to these deposits have not specifically been described previously and in the case of the Tritton copper deposit, at all. N F Open-pit RAB Cu Vacuum Cu Soil Cu Increased Zn Increased Pb Mechanical Hydromorphic 1 o mineralisation 2 o mineralisation Fault F m F Figure 7.13 Geochemical dispersion patterns observed for the Girilambone North study area, in plan view. The Girilambone North deposits are characterised by intense weathering, in particular Hartmans, which displays strong ferruginisation in the surface layers and in close proximity to weathered sulfide mineralisation. Figure 7.13 depicts, in plan, the nature of dispersed geochemical anomalies of the Girilambone North study area. A north-trending sub-surface dispersion is recognised from elongate anomalous zones in RAB- and vacuum-drilling geochemical surveys, which coincide with structural zones of the same orientation. North-trending structural zones have undoubtedly acted as fluid conduits, allowing migration of acidic groundwaters, which in the vicinity of weathering sulfide mineralisation have caused strong leaching and moderate kaolinisation in the upper profile. Greatest dispersion was observed on the north side of the Hartmans mine workings, by where an anomalous Cu zone was identified up to several hundred metres 293

42 in length, broadest in the upper regolith profile and narrowing towards approximately 20 m depth. Copper response from within the immediate vicinity of the Hartmans mine workings was however negligible in shallow drilling, implying strong leaching of upper regolith in the vicinity of the Hartmans primary mineralisation. Anomalous Cu zones of the North East and Larsens East deposits show little dispersion, despite being of higher Cu grade, possibly indicating a lesser degree of weathering or groundwater flow. Alternatively, the dispersion plume could manifest from strong leaching of all (or at least Hartmans and Larsens) deposits, which lie en-echelon to a major late-stage fault structure. Geochemical anomalies developed in soil have a subtle multi-element character, and are generally located directly above primary mineralised zones. Upper ferruginised zones also contain geochemical signatures indicative of primary mineralisation, and have similar spatial extents as observed for soil anomalies. Mafic mineralogies mapped by other workers prior to mining activities, have characteristic geochemical signatures, and in the study area, are mainly indicated by anomalous Ni content. Nickel in soils, is observed to form a large plume, extending from south from topographically higher areas to the west of the Girilambone North mining area. Figure 7.14 depicts a model of Cu (and related trace element) dispersion associated with weathering sulfides in the regolith profile. While it is based specifically on the Hartmans deposit, similar patterns of copper dispersion (at least in the vertical dimension), were observed for Larsens East, albeit with reduced lateral (northerly) dispersion. At the weathering front, chalcocite enrichment is observed, with copper carbonates and oxides observed higher in the profile. Intense ferruginisation associated with weathering of the primary pyritic halo, has resulted in a ferruginous oxide ore zone (limonitic ore). Evidence of regolith development, in keeping with dispersion models indicated above, can be seen in the Hartmans open-pit walls (Figure 7.15). At the southern end of the pit (left), strong ferruginisation (A) persists from near the base of current workings to the surface level and is largely controlled by a crenulation cleavage (S 3 ). A sub-vertical zone of kaolinisation and Fe-oxide development (B) is formed in weathered saprolite (C), the base of which is quite sharp and undulating, with underlying saprock (D) and represents the outer margin of strong leaching associated with acidic weathering solutions. Surface ferruginisation is absent in the central portion of the pit, with weathered saprolite directly overlain by ferruginous soil horizons, although does persist 294

43 south 1 o mineralisation signature N north 2 o Cu dispersion (Cu >250 ppm) surface extent of depletion zone strongly leached saprolite 2 o mineralisation 50 m Cu- limonitic Cu-oxide Cu > 250 ppm Cu-carbonate chalcocite pyrite halo 0 saprolite 1 o mineralisation saprock unweathered rock direction of groundwater flow Mechanical dispersion Hydromorphic dispersion 250 m m 0 Figure 7.14 Schematic model of Cu dispersion in the Hartmans mineralised system, displaying northerly migration of Cu. Strong depletion zones in the upper profile have leached Cu in close proximity to weathering sulfides (acid conditions), and re-deposited it under the influence of decreasing ph groundwaters, moving away (northwards) from the primary mineralised zones. Upper ferruginous zones and soils exhibit subtle, multi-element anomalism in close proximity to primary mineralised zones. 295

44 towards the northern-portion of the pit. Relict bedding (S 1 ) is evident in less weathered portions, shown by alternating bands of psammite and pelite (darker in colour), although these features are not generally preserved in the saprolite. The northerly dispersion is indicated by the elongate north-trending ferruginous zone in the central portion of the photograph. south A A north A C B D Figure 7.15 Regolith zones developed in the western wall of Hartmans open-pit. The base of the open-pit (water level) is approximately 75 m below land surface at the southern end. White dashed lines indicate boundaries of ferruginous (A), kaolinitic (B), saprolite (non-ferruginous) (C) and saprock (D) weathering zones described in the text. The distribution of mineralisation-related elements at Tritton is somewhat more restricted than that observed for the Girilambone North study area, with the exception of course of the geochemical anomalies associated with the Budgerigar mineralisation. Compared to the large anomalous zones identified at Budgerigar, the up-plunge extent of Tritton mineralisation displays subtle multi-element anomalism in As, Cu and Pb (vacuum survey) and coincident anomalism in Sb, As and Au (displaced) in soil samples traversing this area (Figure 7.16a). The up-dip extension of mineralisation shows anomalous Cu and other elements (Figure 7.16b). Both anomalous zones occupy elevated portions of the landscape in association with quartzite, which is seen to outcrop at the up-plunge anomalous zone. The geochemical response to mineralisation at Tritton is thought to represent that of primary alteration, associated with permeation of mineralising fluids, rather than a secondary re-mobilised signature as commonly observed at Girilambone North. Geochemical dispersion in this area is confined mainly to mechanical dispersion of soils containing mineralised signatures down-slope. Thus, the surface expression of Tritton mineralisation is limited, due to the depth of primary 296