CHARACTERISATION OF GOLD FROM THE LUBUK MANDI AREA, TERENGGANU, MALAYSIA

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2 I British Geological Survey TECHNICAL REPORT WC/94/21 Overseas Geology Series CHARACTERISATION OF GOLD FROM THE LUBUK MANDI AREA, TERENGGANU, MALAYSIA P J Henney, M T Styles, D J Bland, and P D Wetton A Report prepared for the Overseas Development Administration under the ODABGS Technology Development and Research Programme, Project 92/1 ODA Classafication Subsector: Others Subject: Geoscience Theme: Mineral Resources Project Title: Alluvial Gold Characterisation in Exploration P1anning Reference number: R5549 Bibliographic reference: P J Henney, M T Styles, D J Bland, and P D Wetton Characterisation of gold fiom Lubuk Mandi, Terengganu, Malaysia BGS Technical Report WC/94/2 1 Keywords: Gold, mineralogy, electron microprobe, Malaysia Front cover illustration: Alluvial gold grain from Lubuk Mandi, Malaysia 0 NERC 1994 Keyworth, Nottingham, British Geological Survey, 1994

3 TABLE OF CONTENTS List of plates List of figures List of tables EXECUTIVE SUMMARY 8 1. INTRODUCTION 1.1 Geology of the Lubuk Mandi gold deposit SAMPLE COLLECTION BEDROCK SAMPLE STUDIES Sample descriptions 3.1.I Hand specimens Thin section study Electron probe microanalysis Microchemical mapping of bedrock gold Quantitative electron microprobe analysis Accessory mineralogy Summary ALLUVIAL GOLD GRAINS Gold morphology study for Lubuk Mandi samples 4.1.I SEM characterisation Size analysis Quantitative electron probe micro-analysis Microchemical mapping of alluvial grains Quantitative electron microprobe analysis Inclusions in gold grains Summary

4 5. COMPARISON OF BEDROCK AND ALLUVIAL GOLD Compositional comparison Comparison of mineral inclusions Morphology CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES 28 2

5 Plate captions Plate l(a) Steeply-dipping, low-grade metasediments of Lower Carboniferous age which host the mineralisation at Lubuk Mandi and Bukit Panji. Plate l(b) The main ore zone at Lubuk Mandi. Plate l(c) Sample of drillcore from the main ore zone at Lubuk Mandi showing the close association between stylolitic structures, metasedimentary clasts and mineralisation. Plate l(d) Small-scale extraction of alluvial gold using high pressure water hoses at Lubuk Mandi. Plate 2(a) Dendritic gold on quartz. Sample MA&. Field of view approx 5mm. Plate 2(b) Gold and pyrite associated with metasedimentary clasts and stylolitic structures. Sample MA45 Field of view approx. 5mm. Plate 3(a) Gold at the contact of quartz and carbonate. Sample MA32. Field of view approx 3mm. Plate 3(b) Filigree and platey gold in quartz. Sample MA32. Field of view approx 0.8mm. Plate 3(c) irregular blebby gold in fractures in quartz. Sample MA33. Field of view approx. 1.6mm. Plate 3(d) irregular platey gold in a fracture in quartz. Sample MA32. Field of view approx. 0.8mm. Plate 3(e) Inclusions of galena within gold in quartz. Sample MA32. Field of view approx. 0.3mm. Plate 3(f) Pyrite showing multiple overgrowths. Sample MA44. Field of view approx. 3mm 3

6 Plate 4(a) Large, porous, unabraded alluvial gold grain (approx 2mm). Sample MA13. Plate 4(b) Porous, partially rounded alluvial gold grains (<5mm). Sample MA17. Plate 4(c) Irregular alluvial gold grain with well rounded corners and edges (approx 2mm). Sample MA13. Plate 4(d) Porous alluvial gold grains with well rounded edges and corners (<5mm). Sample MA17. Plate 5(a) SEM photograph showing a rounded, elongate alluvial gold grain MA13.2 showing minor abrasion polishing. Plate 5(b) Alluvial grain from sample MA14 showing a angular, porous habit indicating minor abrasion and the removal and/or dissolution of mineral and gangue inclusions. Plate 5(c) High magnification SEM view of alluvial gold grain surface showing well developed remnant porous structures. Sample MA11.2 Plate 5(d) Alluvial gold grain showing complex, partially rounded surface morphology. Sample MA16.5. Plate 5(e) Spherical structures on the surface of an alluvial gold grain thought to be the product of microbial activity. Sample MA16.5. Plate 5(f) Platey, porous alluvial gold grain with well rounded edges and corners and highly abraded surface. Sample MA

7 Figure Captions Figure 1 Location map for the Bukit Panji and Lubuk Mandi gold- bearing mineralisation (locations 1 and 2). Figure 2(a) X-ray map of alluvial grain in MA14 showing Au distribution. Figure 2(b) X-ray map of alluvial grains in MA14 showing Ag distribution. Figure 3(a) S-curve showing range of Ag contents for bedrock and alluvial gold samples Figure 3(b) Scatter plot of Hg wt% vs. fineness ((Au/Au+Ag)*1000) for alluvial and bedrock gold 5

8 List of Tables Table l(a) EPMA data for bedrock sample MA32 Table l(b) Hg% vs fineness and 'SI plots for bedrock sample MA32 Table 2(a) EPMA data for bedrock sample MA33 Table 2(b) Hg% vs fineness and 'S' plots for bedrock sample MA33 Table 3 EPMA data, Hg% vs fineness and IS' plots for bedrock sample MA34 Table 4 EPMA data and 'SI plots for bedrock sample MA42 Table 5 Shape data and area kurve for alluvial sample MA13 Table 6 Shape data, DMAX and area S-curve for alluvial sample MA14 Table 7 Shape data, DMAX, FSHAPE and area S-curve sample MA16 for alluvial Table 8 Shape data, FSHAPE and area S-curve MA17 for alluvial sample Table 9 EPMA data and 'S' plots for alluvial sample MA13 Table 10 EPMA data and 'S' plots for alluvial sample MA14 Table 11 EPMA data and IS' plots for alluvial sample MA16 Table 12 EPMA data and IS' plots for alluvial sample MA17 Table 13 EPMA data for mineral inclusions from MA13 Table 14 Atomic proportions and identification of mineral inclusions 6

9 from MA13 Table 15 EPMA data for mineral inclusions from MA14 Table 16 Atomic proportions and identification of mineral inclusions from MA14 Table 17 EPMA data for mineral inclusions from MA16 Table 18 Atomic proportions and identification of mineral inclusions from MA16 Table 19 EPMA data for mineral inclusions from MA17 Table 20 Atomic proportions and identification of mineral inclusions from MA17 7

10 EXECUTIVE SUMMARY This study of gold from the Lubuk Mandi and Bukit Panji area in Terengganu, forms part of a wider study of gold from Malaysia and elsewhere carried out under ODA/BGS Technology Development and Research (TDR) project,"alluvial Gold Characterisation in Exploration Planning" (Project 92/1, R5549). The TDR Programme is a contribution to the British Government programme of aid to the developing countries. In Terengganu, gold is found in quartz veins in Carboniferous shales and phyllites, a style of mineralisation found in several areas in Malaysia. Bedrock samples were collected largely from drillcores with a few surface samples. Alluvial samples were mainly from an area of alluvial mining 1-2km from the deposit. Laboratory studies were made to characterise the bedrock and alluvial gold and to check which features of the alluvial gold were inherited from the bedrock source and which had been changed. Studies of size and shape of alluvial gold show, as expected, that the gold is probably derived from a single source. The low degree of deformation is consistent with a local provenance from nearby bedrock mineralisation. The bulk compositions of the bedrock and alluvial gold are very similar, particularly the limited range of Ag contents, around 10%, for the majority of the grains. The minor variations observed in the Ag contents of a small number of alluvial grains (including the presence of gold - rich rims and the formation of silver - rich tracks) can be attributed to weathering processes and minor recrystallisation of gold grains in the surface environment. These data demonstrate that the alluvial gold is derived from the bedrock mineralisation. Microscopic mineral inclusions in alluvial gold grains are dominantly galena with lesser pyrite, arsenopyrite and minor chalcopyrite. These minerals occur in the sulphide mineralisation associated with the bedrock gold and as rare inclusions in the bedrock gold. No 'exotic' mineral species have been found as inclusions in the alluvial gold; all are known from in the bedrock mineralisation. Studies of gold from these two deposits confirm that the alluvial gold preserves many of the mineralogical, compositional and morphological features of its bedrock source. This demonstrates that detaded studies of alluvial gold can be used to characterise the bedrock source and provide important constraints on gold exploration programmes. 8

11 1. INTRODUCTION This report contains the results of work carried out on ODA/BGS TDR project 9211, R5549 entitled Gold Characterisation as a Guide to Exploration Planning. The study involves sample collection and laboratory study of gold from various developing countries. The aim of the project is to characterise gold grains from both bedrock and associated placer deposits to establish whether variations in bedrock gold are inherited by alluvial gold and may be used to fingerprint primary sources from different geological environments. This characterisation covers the study of morphological features such as shape, size, growth patterns, the presence of inclusions or heterogeneites within grains, as well as detailed microchemical analyses (including both point analyses and chemical mapping). The work on samples from Malaysia was conducted in association with the British Geological Survey (BGS) contribution to the Overseas Development Administration (ODA) joint Technical Co-operation programme involving the Geological Survey of Malaysia (GSM). The results from the Lubuk Mandi and nearby Bukit Panji (Figure 1) deposits are described in this report. 1.1 GEOLOGY OF THE LUBUK MANDI GOLD DEPOSIT A full description of the geology of the Lubuk Mandi deposit is given by Gunn et al. (1993a) from which this summary has been derived. The geology of the Bukit Panji deposit is described in Gunn et al. (1993b). The Lubuk Mandi area is underlain by a sequence of well bedded, shallow marine sediments which have experienced low grade (greenschist) regional metamorphism. They are composed of shale, phyllite, siltstone and sandstones with the shales and phyllites being finely laminated and rich in carbonaceous material (Plate la). These sediments have been assigned to the Sungai Perlis Beds and are thought to be of Lower Carboniferous age based on palaeontological evidence. The regional strike is to the north-north west with moderate to steep dips in an easterly direction although these vary reflecting the presence of local asymmetric and overturned folds. Exposures of intrusive granitic rocks occur about lokm to the south west and highly altered mafic dykes have been intersected in boreholes at Lubuk Mandi. Landsat TM images of this part of Terengganu have revealed several circular structures, 1-3 km in diameter, two of which are present over the Panji and Lubuk Mandi areas and may relate to sub-surface intrusions. Polyphase quartz vein systems cut the metasediments with an early phase of narrow, 9

12 impersistent, often highly strained bedding parallel veins, including a zone up to 10m wide, of intense veining and brittle deformation containing the Au mineralisation (Plate lb). This zone has been traced for approximately 1 km along strike. The ore body may be composed of either a single quartz vein 4 3 m in width or, more frequently, as several thinner veins separated by screens of fractured and brecciated country rock. Late quartz veins discordant to the fabric in the metasediments are also developed, particularly associated with areas of brittle deformation and faulting. The ore body shows evidence from boreholes of pinching and swelling along strike and is also disrupted by late faulting, particularly in the north of the area (Gunn et al. 1993a). The gold is occasionally observed in hand specimens of vein material and is most often found associated with the margins of metasedimentary clasts and streaks and associated with stylolite structures (Plate lc). It occurs as irregular or equant grains up to 1.5mm in size and often shows a spatial association with pyrite or arsenopyrite. Less commonly it occurs as delicate dendritic growths in vugs and as irregular grains filling fractures within quartz veins. The mineralised veins contain sparse sulpide mineralisation. Early pyrite and arsenopyrite are succeeded by minor galena and sphalerite accompanying the introduction of gold. The gold has been worked locally on a small scale from overburden and streams close to Lubuk Mandi. Mining of the bedrock deposit by open pit methods has recently commenced (Plate 1 d). 2. SAMPLE COLLECTION The sample collection in Malaysia was carried out by Dr M.T. Styles with Mr A. G. Gunn in 1992 and was supplemented by other samples collected by Mr Gunn as part of the TC project (see GSM-BGS Gold Sub-Programme 93/2 and Appendices 1 and 2 for more details). The locations are shown on the geological map of Malaysia (Fig. 1) and a list of samples and localities is given in Appendix 1. The Lubuk Mandi deposit is located on the east coast of mainland Malaysia, about 15 km south of the town of Kuala Terengganu. A total of 8 bedrock samples, mainly from drill core samples donated by the exploration section of the Terengganu State Economic Development Corporation, and 5 alluvial gold samples were studied. The bedrock specimens were first visually examined with a binocular microscope in order to describe the rock types and to decide which samples were best suited for further study. Some samples with fine grained visible gold were not suitable for mounting for electron microprobe study. 10

13 3. BEDROCK SAMPLE STUDIES. Gold was found in all but one bedrock sample but not all gold-bearing samples could be sectioned in such a way as to ensure that the gold was preserved on the polished thin section, and therefore only four samples were selected for detailed mineralogical and electron microprobe studies. 3.1 Sample descriptions 3.1.I Hand specimens All the samples comprise sections of drillcore (MA 32,4245) or surface rock (MA 33-25,46) and are of the quartz veins forming the ore zone. The majority of the samples are composed of several varieties of milky white to translucent vein quartz, commonly with included clasts of metasediment. Gold occurs in a variety of habits ranging from equant flat plates up to 1.5mm in size, through to irregular, dendritic coatings on quartz grains (Plates 2 a-b). Both the gold and associated sulphides show an association with metasedimentary clasts. The gold is also found at the contact between different generations of quartz (Plates 2 c-d) Thin section study The essential mineralogy of all the samples is similar. Gold occurs in a variety of habits within, or at the margins of, quartz veins and their contact with late stage epidote- geothite-carbonate bearing veins. Sulphide minerals, in particular pyrite and arsenopyrite, are not intimately associated with the gold in the veins but are generally restricted to an earlier sulphidic phase of mineralisation in a separate generation of veins and veinlets. Sulphide mineral inclusions, mainly galena, occasionally occur within some of the gold grains. The gold which forms at the margins of the quartz veins, particularly at the contact with epidote-geothi te-carbonate bearing veins, is characteristically flat and plate-like in morphology ( Sample MA32; Plate 3a) orientated parallel to the margins of the quartz vein. Gold deposited within the quartz vein generally forms more rounded or irregularly shaped grains, occasionally developing complex dendritic forms infilling fractures within the quartz (Samples MA32,33; Plates 3b-c). The gold also occurs in some samples as "platey" crystals with inclusions of the host vein quartz and Fe-rich carbonate the most common component (MA32, Plate 3d). Galena occurs as rare, small, often <5 micron, rounded inclusions within the gold grains indicating that some sulphides crystallised at the same time as the gold was precipitated (Sample MA 32, Plate 3e). Significantly there is no evidence for inclusions of gold within the main sulphide phases, pyrite and arsenopyrite, although some of the larger euhedral-subhedral sulphide crystals are heavily cracked and contain rounded 11

14 inclusions of the host quartz. The large pyrite crystals also show evidence of multiple growth phases with clearly distinguishable overgrowths and edge-parallel inclusion trails (Sample MA 44, Plate Electron probe microanalysis Polished thin sections containing gold were studied using a fully automated computercontrolled Cameca SX50 electron microprobe. A combination of quantitative point analyses for accurate chemical analyses of the gold grains and microchemical maps to investigate spatial variation in composition was used. Microchemical maps are made by making point analyses on a grid of 256x256 points with in this case a distance of 2 pm between points, achieved by moving the specimen stage. Mapping of gold grains was carried out at 20 kv excitation potential, 50 na beam current, and a counting time per point of 30 ms for major elements and 50 ms for minor elements. The data collected are X-ray counts with no correction for matrix effects, but as the matrix is essentially the same within each grain this produces little error. The data are presented as multicolour maps, each showing the concentration of a particular element according to a rainbow scale where red indicates the highest concentration. The scale is relative for each element and a particular colour does not indicate a particular concentration Microchemical mapping of bedrock gold Microchemical mapping is a qualitative technique, but it is powerful when combined with quantitative analyses. It is particularly valuable for determining compositional variations within grains such as growth patterns or inclusions. Selected alluvial grains from sample MA 14 were mapped to illustrate some of the chemical and morphological feature of the alluvial gold. The results are described in the section on alluvial grains. None of the bedrock samples appeared to have any internal variation based on optical microscopy, and microchemical maps were not made for these samples Quantitative electron microprobe analysis Quantitative analyses have been made on over 50 gold grains and associated minerals using the wavelength dispersive system under operating conditions of 30 kv high voltage and 20 na beam current. Sixteen elements were analysed for each point. The in-house standards used were mostly pure metals apart from: pyrite for S, arsenopyrite for As, HgTe alloy for Hg and Te, and PbF2 for Pb. A variant of a standard S-curve has been produced by plotting the values in ascending order of silver content. Different populations are shown by changes in slope. The gold analyses from each sample are plotted as an S-curve of the silver content as this element is the main chemical "impurity" in gold. The results of the gold analyses and S-curve for each sample are 12

15 presented in Tables 1 4. Sample MA32 Fifty points were analysed from a large number of gold grains in this sample (Tables la-b). Ag contents and fineness values (Au/Au+Ag) for the majority of points cover a narrow range ( Ag; ) with only one sample showing an anomalously low Ag content and fineness value (5.58 Ag%; ).The S-curve plot of Ag is flat with no significant inflections apart from one low value. This is thought to reflect an Au rich rim on one of the grains. Although most trace element contents are generally very low, close to the calculated detection limits, Hg ( %) and As ( %) contents show some variation, possibly due to micro inclusions of arsenopyrite (highest As correlates with highest Fe and S) and minor Hg in solid solution. There is no correlation between fineness and Hg content. Sample MA33 Thirty three points were analysed from grains in this sample (Tables 2a-b). Ag contents and fineness values again cover a restricted range of values ( Ag%; ). The S-curve plot of Ag% is also flat with no inflections. Hg ( %) and As contents ( %) are similar to those in MA32 and are thought to have a similar origin, although Fe contents show no correlation with As. Sample MA34 Only sixteen points were analysed from this sample due to its lower gold content (Table 3). Ag contents show a wider range than MA32 and 33 ( %) but the S-curve plot shows that the majority of the points have Ag contents between 8-9 wt% with a pronounced tail of 3 samples with values between 9-13 wt%. Similarly, although fineness values ( ) show a wide range, the majority of the samples cluster between with one low value at 869. The mean Ag content of grains in this sample is 2-3wt% lower than those for MA 32 and 33, with correspondingly higher fineness values. Hg values ( %) are comparable with those in MA32 and 33 as are As values ( ). This sample is from Panji, a few kilometres from the main Lubuk Mandi deposit, and therefore may be expected to show some primary differences in bedrock gold characteristics from the other samples. Sample MA42 Again only 16 points were analysed from this sample (Table 4). Ag contents are comparable with MA34 ( %) but an S-curve plot shows a more restricted 13

16 range of values ( %) with a noticeable tail at low values and a single high value (9.12%). Fineness values ( ) also cover a relatively restricted range of values, smaller than that observed in MA34. No Hg was recorded from any of the points analysed and As values are low (O.OS-O.l%) Accessory mineralogy Although identifiable mineral inclusions are rare in the Lubuk Mandi bedrock gold, the early phases of mineralisation and hydrothennal activity in the deposit included the deposition of an early quartz-bearing sulphide assemblage dominated by pyrite and arsenopyrite (this study; Gunn et al. 1993a,b). However in the samples containing gold, sulphide minerals are not abundant, as the gold is usually associated with late stage carbonate veining. Reflected light studies coupled with qualitative energydispersive x-ray analysis show that the main sulphide minerals in the quartz veins are pyrite and arsenopyrite, with minor galena and sphalerite more closely associated with the gold Summary The bedrock gold samples from Lubuk Mandi are characterised by Ag contents between 8 and 12% with fineness values in the range MA32 and 33 have higher mean Ag contents than MA34 and 42 and this may reflect some compositional variation within the deposit. Trace element contents are generally low in all samples although there is some variation in Hg, As and Fe content above the detection limit (ca %). This may be due to (i) Hg solid solution in gold and (ii) micro-inclusions of arsenopyrite and possibly pyrite within the gold. 4. ALLUVIAL GOLD GRAINS Studies of their chemical composition and included minerals were carried out using the CAMECA SX50 electron microprobe. The samples were hand-picked under a binocular microscope to separate the gold from other heavy minerals present in the heavy mineral concentrate. The grains were placed on double sided sellotape that was mounted on a glass slide and the slide then carbon coated so that the first stage of examination, by scanning electron microscopy (SEM), could be carried out. Grains <10 microns in size were mounted directly onto epoxy resin on a glass slide and were not studied by SEM. Samples of alluvial gold from seven localities were examined using optical petrography, SEM and image analysis techniques (measurements of size and shape of the grains). Some of the differing characteristics and morphologies of the alluvial grains are illustrated in Plates 4(a-d). 14

17 4.1 Gold morphology study for Lubuk Mandi samples The alluvial gold sampled for EPMA was briefly characterised morphologically by the use of two methods. Visual morphology of the grains in each sample was studied by SEM techniques utilising a Cambridge Stereoscan 250 instrument. Dimensional morphology characterisation was produced using a semi-automated Image Analysis program, developed for this project, and was performed using a Kontron IBAS image analysis system. 4.1.I SEM characterisation. Characterisation of individual grains in the samples was performed on two levels, the first being a macro morphological description of the grain shape and general appearance as illustrated in Plates 4(a-d). The second method used was examination of the grain surfaces at high magnification to identify micro scale morphological features. These observations have been summarised to provide a typical bulk sample morphological definition with features illustrated by the use of SEM photomicrographs (Plates 5a-f). Sample MA13. The grains present in this sample show rounded forms with a low level of high energy deformation. Sample morphology tends towards flat, elongate grains with irregular surfaces and minor areas of abrasion polishing. A high proportion of the sample population shows embedded mineral grains. Examination at high magnification shows these minerals to be both syngenetic, being entrapped during gold formation, and secondary, becoming embedded during formation of the alluvial deposit. At high magnification the surfaces of the grains show areas of remnant porous or sponge-like structures with minor rounding and deformation effects. The grains typically show numerous areas of micro-scale plastic deformation but little polishing or abrasion. This typical morphology is shown in Plate 5(a). Sample MA14. The gold present in this sample shows a more complex morphology than that from sample MA13; an example is shown in Plate 5(b). These complex forms result from displacement of the syngenetic mineral grains originally trapped within the gold. Displacement has occurred during transport of the gold from the bedrock to the alluvial environment from which it was sampled. The gold shows a low level of deformation with typical grain surfaces showing only low-energy plastic deformation of micro features and no abrasion or folding of the gold. High magnification examination of the grains shows areas of well preserved remnant porous structures. Plate 5(c) shows a typical area of the porous structure at high magnification. This may be a relict of 15

18 primary dendritic growth structures present in the bedrock deposit. Sample MA1 6. Gold from this sample showed well preserved original morphology with very little deformation. The grains have a highly complex but rounded surface morphology, resulting from the complex geometry of the fractures in which the bedrock gold was formed (Plate 5d). Minimal deformation is visible in the form of minor sub-rounding of vertices and folding of edges. The original grain surfaces are well defined with a porous or sponge-like structure predominating. Many of the samples show a coating of Fe-Mn oxides, a common secondary growth feature occurring in the alluvial or eluvial environment. This coating ranges from individual crystals to small areas of amorphous material though on such a fine scale that identification was not possible. This sample also contained one grain in which possible microbial forms were found on the grain surface, shown in Plate 5(e). Analysis of these areas by energy dispersive X-ray analysis showed aluminium present in trace element concentrations not seen on other areas of the grain surface. This may be due to aluminium being present within the microbe structure or possibly due to deposition of the metal on to surface of the gold by microbial action. Sample MAl7. This sample shows a variety of grain morphologies. The grains are typically rounded and tend towards flake proportions as illustrated in Plate 5(f). Some grains from the sample show significant levels of abrasion and polishing with some deformation of the grain edges by folding. The surfaces of other grains show little evidence of abrasion or deformation. Complex, delicate sponge-like forms seen previously in other samples at high magnification predominate. Summary. The alluvial gold shows a generally elongate morphology with highly irregular edge profiles. The grains have complex shapes reflecting their original forms within the bedrock deposit but show no evidence of negative crystal imprints or crystalline gold morphologies. For this sample suite, given the low level of deformation seen in the gold, the absence of good crystalline forms may be interpreted as indicating that the gangue minerals in gold bearing veins did not show good crystal forms or that the gold was largely precipitated in irregular fractures and cracks rather than along grain boundaries. The low level of deformation in the samples is typical of alluvial gold in close proximity to the bedrock source. The complex shapes, lack of heavy abrasion and irregular 16

19 surfaces all point to limited low energy deformation processes acting for a relatively short period of time. If the gold had travelled any appreciable distance from the source, (e.g.>5km), then the grain forms would be expected to show more mechanical rounding and folding and a less extensive range of shapes with high levels of deformation of micro-scale structures Size analysis. Measurement of the size characteristics of the samples was performed using a petrological microscope coupled to a video camera to capture images for input to the image analyser. The samples were mounted on microscope slides and the microscope operated in transmitted light mode to provide silhouettes of the grains for image analysis purposes. The grains were mounted such that their two major axes lay in the image plane of the microscope so as to present the largest possible cross-section to the analyser. This was done to maximise the validity of data obtained by a 2-dimensional technique operating upon 3-dimensional subjects. The image analysis routine includes individual grain identification, allowing size and shape data to be cross-referenced to results obtained by EPMA. It is then possible to explore correlations between sample populations suggested by chemical and morphological techniques. Dh4AX and DMIN are the maximum and minimum diameters of the grain respectively, automatically selected from 32 measurements of the grain diameter made at an angular resolution of 5.7". FSHAPE is a simple aspect ratio for the grain arrived at by division of the minimum diameter by the maximum diameter of the grain. Objects with low values of FSHAPE have elongate shapes whilst high values signify objects which are equant. The function FCIRCLE is a measure of the circularity of the grain, defined by the equation FCIRCLE = &Area Perimeter2 where a circle returns the value 1. In addition to indicating the circularity of the object, FCIRCLE is also a function of the edge roughness. FCIRCLE plotted against FSHAPE provides a basic morphological characterisation. For example consider a set of grains where the aspect ratio (FSHAPE) remains approximately constant but the value of FCIRCLE decreases. This behaviour represents a population where the general shape of the grains remains the same but the irregularity of their perimeters is increasing. The size characterisation was made by measuring area, DMAX, DMIN and FSHAPE for each grain in the sample. Additionally the perimeter and the function FCIRCLE were measured in sample MA 14. These parameters are segregated into primary 17

20 measurements which are taken directly from the grain and derived measurements, calculated from the primary measurements. Analysis of the data obtained was made by plotting the data sets in the form of tables and generating S-curves by plotting the data in ascending value order for each of the samples (Tables 5-8). On these plots different populations having a tendency to plot as lines of differing slope and thus can be discriminated. Sample MA13. The grains from this sample are small in size and show an area range of mm2 and average area of 0.329mm2. The S-curves generated, of which the area S-curve shown in Table 5 is typical, show a split into two major populations. The minor steps and inflections may further sub-divide these, but given the low numbers of grains measured this cannot be confirmed. The bulk of the grains fall in the first population having areas up to 0.447mm2. The second population contains three grains with areas greater than 0.491mm2. The FSHAPE data obtained indicate the morphology of the grains to be generally elongate. The maximum and minimum values obtained, and respectively, show that the shapes of the grains vary considerably. The S-curve for these data in Table 5 shows a complex set of populations present with four main groupings on the curve. These groupings show no apparent correlation with those present in the primary data curves. This lack of correlation shows that the size of the grains is not a controlling factor in their geometry for this sample. Taking into account the low levels of deformation seen in the gold for MA 13 the geometry of the grains must be dependent upon their original geometry established during formation in the bedrock. Sample MA14 The grains present in this sample show the smallest average size seen in this study. The range of sizes present is mm2 with an average grain area of 0.087mm2. This range is split into two populations, a low area population in the range mm2 and a higher area population in the range mm2. This population distinction is supported by similar trends seen in the remaining primary measurement data sets. The shape factor data for this sample, FCIRCLE and FSHAPE, show differing trends to those described above. The FSHAPE data show a single population present, plotting as a single line with values ranging from to This describes all grains as elongate to differing degrees. FCIRCLE values generated range from to showing the grain forms to be far from circular. 18

21 Combination of these two shape factors in a scatter plot allows a simple shape description as shown in Table 6. This plot shows the grains to be elongate in nature to varying degrees and to have very irregular perimeters. The plot suggests the presence of two shape populations within the sample. The bulk of the grains constitute a population with FCIRCLE between 0.4 and 0.8 approximately and with a similar range in FSHAPE values. Four remaining grains lie in the region where FCIRCLE<0.35, forming a population which features highly irregular perimeters. Sample MA16. The gold present in this sample contains very fine to intermediate grain sizes ranging in area from of 0.007mm2 to 1.115mm2. The average grain area for the sample is 0.307mm2, less than that seen for sample MA 13 in which the grain size range was significantly smaller. This indicates that despite its large size range the sample is dominated by smaller gold grains. The area s-curve for MA 16 in Table 7 shows this clearly. The majority of the grains in the sample have areas of less than 0.428mm2 with only three grains exceeding this area. This population split in the area data is also seen in the DMIN data but the values for DMAX show very different trends. The DMAX data show two populations but splits the grains into 19 grains with low DMAX values and 10 with significantly higher values. The FSHAPE data for the sample show no indication of agreement with either of the trends seen in the primary measurement data. FSHAPE values range from to showing a transition from highly elongate to nearer equant grains with a possible shape population boundary at FS HA PE=O Sample MAl7. This sample contains the largest grains and the largest grain size range seen in this study (Table 8). The area data ranges from 0.103mm2 to 4.938mm2 with an average grain area of 0.828mm2. S-curves for the primary measurements show similar trends each with three populations present. The area data show size populations of with areas< 1.Omm2, a second population having areas of mm2 and a third population with areas above 2.0mm2. The FSHAPE data for this sample show a range from to with the bulk of the grains having values between and A single grain lies outside this group, grain MA 17.5, having FSHAPEO This indicates that this grain is extremely elongate in form, approaching needle-like proportions. Summary. Size characterisation of the gold from these samples shows a wide range of size populations present both within and between samples. The range of sizes found and the 19

22 average values for each sample are summarised below. Sample Minimum size Maximum size Average size (mm2> (mm2) (mm2> MA MA MA MA The use of S-curves based on area, DMAX and DMIN to indicate possible differing grain populations within the samples produces results with variable degrees of correlation between the different measurements. For example in sample MA 14 the primary measurement s-curves all show the same general trends and the same population groupings. Sample MA 16 shows widely differing populations suggested by the DMAX data when compared to those seen in the area and DMIN data sets. This discrepancy arises from the complex shape of the grains analysed. The technique used to establish the areas of the grains in this study relies upon counting the total number of pisels within an object and scaling this into real units appropriately. If the grain is very irregular, or has a series of holes within it, then its geometry is such that the area becomes a complex function dependent only partially or not at all upon the values obtained for DMIN and DMAX. In such circumstances the shapes of the S-curves and the trends seen may vary considerably, hence producing apparently different segregations of the population within a single sample. Shape description of the grains using the FSHAPE function show the grain morphology within each sample to range from strongly elongate to near equant in shape. This variation in morphology is independent of the size of the grain for this sample suite. To help establish the reasons for this variation, SEM observations of the grains must be taken into account. SEM showed the samples to consist of grains with irregular perimeters and highly detailed surface structures with little or no deformation present. The absence of large scale deformation indicates that the gold morphology has not been significantly modified since leaving the bedrock environment. Hence the FSHAPE variation and the changes in morphology it describes are due to the variation in the initial bedrock gold morphology. The use of the function FCIRCLE in conjunction with FSHAPE for sample MA14 20

23 carries the shape description further and gives some information on the nature of the grain perimeters. The plot generated shows sample MA 14 to contain two shape populations, a small population of very irregularly edged grains of differing overall shape where FCIRCLE is low for all values of FSHAPE and a less irregular population also of widely varying overall geometry. 4.2 Quantitative electron probe micro-analysis An analysis was made in the core of each gold grain and, if a rim or other feature such as a white patch could be distinguished optically once the grain had been carbon coated, this was also analysed. Any sulphide, selenide and telluride mineral inclusions were also analysed. Silicate inclusions, principally quartz but also orthoclase and rutile, were observed and their identities checked by qualitative analysis but these inclusions were not quantitatively analysed Microchemical mapping of alluvial grains X-ray maps of selected alluvial grains from sample MA14 are shown in Figures 2(a-b) with the highest values represented by yellow and red. In this example these show that, although the grain is not compositionally zoned from core to rim, there are two other distinctive features. There is clear evidence for the development of low temperature Au rich rims and for the presence of numerous Ag-rich linear structures within the grain (Croen et al. 1990). These linear features are thought to mark the outline of sub-grain boundaries in the original bedrock gold which have been preserved in the alluvial gold grain. The high-ag tracks are thought to represent orginal primary sub-grain boundaries with the gold grain which were preferentially leached out and replaced by low-ag gold by low-temperature secondary processes in the soil/stream environment (Croen et al. 1990) Quantitative electron microprobe analysis The results of the gold analyses for each sample are presented in Tables The gold analyses from each sample are plotted as an S-curve of the silver content. The S-curve is produced by plotting the values in ascending order of silver content, similar to the grain size S-curves; different populations are shown by changes in slope. The alluvial gold grains have been analysed for a range of elements and the only major constituent apart from gold is silver. Values for most other elements fall close to the limit of detection and hence must be treated with caution. The exception to this is mercury, but as mercury is widely used by alluvial miners the chances of man-made contamination are high. No systematic pattern of minor element variation has been detected. The analyses of the inclusions are presented in two parts. Tables 13, 15, 17 & 19 show the 21

24 raw analyses and Tables 14, 16, 18 & 20 shows the atomic proportions and a possible identification for each mineral. Sample MA1 3 Twenty nine alluvial grains were extracted and analysed from this sample (Table 9). Ag abundance's range from 8.53 to 13.4% with a smooth S-curve plot with a minor upwards inflection at high Ag values. Most trace element abundance's are below detection limits although minor amounts of Hg ( %) and As ( %) occur. Several grains also show detectable Te ( %) and one grain also contains a small amount of Bi (0.16%). These is no correlation between As and Fe in this sample. Sample MA1 4 Sixteen grains were analysed from this sample, with Ag contents ( %) extending to slightly higher values than MA13 (Table 10). The S-curve cumulative frequency plot is again relatively smooth with minor inflections, particularly at high Ag values. As (O.OS-o.14%), Te (O.OS-o.09%) and Hg ( %) values are similar to values in MA 13. No Bi or Fe was recorded in any of these grains. Sample MA1 6 Twenty nine grains were analysed from this sample, with Ag contents ( %) covering a similar range to MA 13 except for one sample which is very Ag poor with only 1.1% (Table 11). The S-curve cumulative frequency curve is also smooth except for a tail comprising the one low Ag sample. The low Ag sample most probably reflects formation of a late stage Au rich rim on the sample. Hg contents ( %) show a much wider range than As ( %) or Te ( %) with two points from the rim of one grain giving values up to 26 wt% Hg. This is thought to relate to the amalgamation of Au with Hg caused by the release into the alluvium of Hg used by alluvial miners for the extraction of fine gold from the sediment. Sample MA1 7 Forty four grains were studied from this sample, with Ag contents ( %), similar to the range observed in MA 16 (Table 12). However, the S-curve cumulative frequency plot shows a clear inflection, with separation into two distinct populations, one ranging from c.24 % Ag, the other from c. &13%Ag. Trace element contents are similar to the other alluvial sample with As ( %), Te ( %) and Hg( %). There is no significant correlation between trace element and Ag content although some of the lowest Ag values do correspond with low As and high Hg. The discrimination of a distinct population of low Ag, high Au compositions is 22

25 thought to reflect the presence of late stage, low temperature Au rich rims and zones in and around the margins of the alluvial grains Inclusions in gold grains Analysis of inclusions in the gold grains presented problems due to their small size, often less than 5 microns in diameter. When the electron beam of the microprobe hits the sample it penetrates into the sample to a depth of several microns and generates X- rays from a volume that can be larger than the grain being studied. This was often the case with these inclusions and the results given in Tables 13, 15, 17 & 19 show the analyses as collected, including contamination from the surrounding gold. It has been assumed that all of the gold is contamination and from these analyses a corrected analysis has been calculated using the gold: silver ratio determined in the core analysis of the same grain. The appropriate amount of silver has been subtracted from that measured and any remaining amount has been shown in the column labelled Ag. res. (Tables 14, 16, 18 & 20). Atomic proportions of the inclusion analyses have been calculated and are given in Tables 14, 16, 18 & 20. These tables also include possible mineral identifications derived from the atomic proportions. Whilst in many cases the identification was obvious, in other cases it was not. Wherever possible, "common" minerals or mixtures have been assigned to the inclusion analyses. Sample MA13 Thirty one points were analysed from sulphide inclusions in grains in this sample. The most abundant inclusion phase present was galena (19) followed by pyrite (7), chalcopyrite (3) and arsenopyrite (2). Sample MA14 Only eight points were recorded from inclusions in grains in this sample with pyrite (5 points) the most abundant phase followed by galena (2) and arsenopyrite (1). There is no chalcopyrite recorded. Sample MA1 6 Forty two points were analysed from grains in this sample. Galena (21 points) is the most abundant sample followed by pyrite (15) and minor arsenopyrite (6). No chalcopyrite is recorded as inclusions in this sample. Sample MA1 7 Twenty eight points were analysed from grains in this sample with arsenopyrite (1 1 23

26 points) the most abundant phase followed by galena (10) and pyrite (5). Minor chalcopyrite (2 points) is also present in this sample Summary Ag contents in alluvial gold grains range from 1.1 up to 15wt% with the highest and lowest values representing the presence of late stage or secondary Ag-rich tracks and Au-rich rims within and around the grains.the vast majority fall in a very narrow range. The presence of high Hg values in rims on some of the alluvial grains demonstrates the influence of anthropogenic activity, in the form of alluvial mining and Au extraction, in modifying the composition of some of the grains. Galena is the most abundant sulphide inclusion comprising 25 to 61% of the population, although pyrite (1550%) and arsenopyrite (1040%) are also common and appear to be more abundant than in the bedrock gold. 5. COMPARISON OF BEDROCK AND ALLUVIAL GOLD 5.1 Comparison of gold compositions The silver contents of the bedrock and alluvial gold samples are compared by use of cumulative frequency S-curves, illustrated in Figure 3(a). The most striking feature is the close similarity in both the shape and values of the S-curves for both the bedrock and alluvial gold. Both groups have a broad, relatively flat trend with an extensive population of ca points with Ag between 10 and 12.5 %. Differences are apparent at low (~10%) and high (>12.5%) values of Ag. In particular the alluvial samples show a minor population of ca. 10 grains with Ag contents below 5% Ag together with a steepening trend between 10% and ca. 7.5%. This latter feature coincides with an abrupt step in bedrock gold Ag content from 10% down to ca 8%. One bedrock gold point has a Ag content ca. 5.5%. The alluvial gold also shows a minor population (ca. 6 points) with Ag contents > 12.5 %, up to 15% which is not duplicated in the bedrock gold. These minor discrepancies in Ag content between bedrock and alluvial samples can be explained by the presence of late stage Au rich rims and the presence of Ag rich tracks in the alluvial grains as observed in the X-ray maps of MA 14. The step to slightly lower Ag contents (8-10%) in the bedrock samples may reflect some minor Ag-Au heterogeneity in the bedrock source grains. It is clear that the majority of analyses from both alluvial and bedrock gold are very similar. A plot of Hg% against fineness ((Au/Au+Ag)* 1000) in Figure 3(b) shows the Au rich nature of some of the points from the Au rich rims of the alluvial grains. Some of the alluvial samples also show a slight enrichment in Hg content most probably reflecting anthropogenic Hg 24

27 contamination in the stream environment as observed in other areas (Callahan et al., 1994). 5.2 Comparison of mineral inclusions The main mineral inclusion found within the bedrock gold is galena, which is also the most common within the alluvial gold samples. Pyrite, arsenopyrite and chalcopyrite are also found in the alluvial grains but have not yet been observed as inclusions in the bedrock samples. This may be the result of sampling statistics in that the alluvial grains represent a much greater cross section of the gold at Lubuk Mandi than the bedrock samples studied and thus show a greater range of inclusion types. Furthermore, no mineral species have been found as inclusions in the alluvial gold which are not present elsewhere within the Lubuk Mandi deposit (Gunn et al. 1993bj. 5.3 Morphology As detailed in section 4.1, the alluvial grains have a range of morphologies compatible with the variety of grain shapes found in the bedrock gold and show a lack of deformation consistent with local derivation, proximal to the Lubuk Mandi deposit. 6. CONCLUSIONS 1. Samples of bedrock gold were collected from the Lubuk Mandi and Panji deposits. Complementary samples of alluvial gold were collected from sites up to 1 km away from the primary source. This provided good material to test which features of alluvial gold are inherited from the bedrock source and which are altered during weathering and erosion in the tropical rain forest environment of Peninsular Malaysia. 2. Alluvial gold grains show a generally elongate morphology with highly irregular edge profiles and complex shapes reflecting their original forms within the bedrock deposit. The low level of deformation seen in the samples is typical of alluvial gold in close proximity to the bedrock source. 3. The bedrock gold from Lubuk Mandi is characterised by Ag contents between 8 and 12% (fineness values ). Some samples have higher mean Ag contents which may reflect some compositional variation within the deposit. Trace element contents are generally low, although there is some variation in Hg, As and Fe content above the detection limit (c. O.OS-O.l%j. This may be thought to be due to (i) Hg solid solution in gold and (iij micro inclusions of arsenopyrite and possibly pyrite within the gold. 25

28 4. Ag contents in alluvial gold range from 1.1 up to 15wt% but with the majority in the range 10-12%. The highest values are due to the presence of late stage or secondary Ag-rich tracks along subgrain boundaries within the gold. The lowest Ag values are found in Au-rich rims and films within and around the grains showing the deposition of pure secondary gold in the near surface environment. The presence of high Hg values in rims on some of the alluvial grains indicates the influence of alluvial mining where mercury is used for gold extraction. Some mercury has been released from these workings and has modified the composition of some of the alluvial grains. 5. Comparison of composition of alluvial and bedrock gold shows that these are very similar. The only exception is a small population of Au-rich rims and Ag-rich zones which can be explained by post-depositional processes. 6. Studies of the micro-inclusions in alluvial gold show that galena is the most abundant inclusion comprising 47% of the population. brite (29%) and arsenopyrite (20%) are also common. Minor chalcopyrite also occurs (4%). Galena is only a minor component of the bedrock sulphide assemblage that is dominated by arsenopyrite and pyrite. Galena is however intimately associated with gold and possibly a major part of the actual gold mineralisation in contrast to the earlier formed iron-arsenic sulphides. 7. At Lubuk Mandi and Bukit Panji the mineralogical, chemical and morphological characteristics of the bedrock gold are largely preserved through the weathering cycle and are inherited by the alluvial gold recovered from sites close to the bedrock deposits. 8. Further transport in the alluvial system will modify the shape of gold grains and possibly cause some change in composition to the outer rims. Many features such as internal composition and micro-inclusions that are protected by the gold are unlikely to be substantially changed. Therefore the characteristics of alluvial gold from more distant sites can still be used to characterise the bedrock source. 9. The composition of gold and minerals associated with the gold mineralisation can be identified from the alluvial gold grains. Firstly this information can be used to deduce if more than one source is represented in the alluvial gold population. Secondly, in broad terms, the type of gold deposit can be recognised from the included minerals and from this possible source areas in the major catchment can be identified from known geology. In the case of the Lubuk Mandi situation this is a shear-zone type deposit and structural features will be a major factor in the control of gold mineralisation. 26

29 7. ACKNOWLEDGEMENTS We would like to thank Mr A G Gunn and the ODA GSM/BGS gold project for assistance in planning, conduct and discussions connected with this work. We thank the Terengganu SEDC and chief geologist Mr Che Ghani Ambak for their cooperation and access to drillcores. We thank GSM for logistical support and advice during the field sampling programme. 27

30 8. REFERENCES Callahan, J.E., Miller, J.W. & Craig, J.R Mercury pollution as a result of gold extraction in North Carolina, U.S.A. Applied Geochemistry. v.9, p Groen, J.C., Craig, J.R. & Rimstidt, J.D Gold-rich rim formation on electrum grains in placers. Canadian Mineralogist.v.28, p A.G Gunn, Zulkipli Che Kasim, Sukri Gharzali, Ho Choon Seng, Dzazali Ayub, Paul Ponar Sinjeng & Chow Chong. 1993a. Primary gold mineralisation near Bukit Panji, Rusila, Terengganu, Malaysia. Geological Survey of Malaysia - British Geological Survey Gold Sub-Programme Report 93/2 pp63. A.G Gunn, Paul Ponar Sinjeng & Zulkipli Che Kasim. 1993b. Geochemical and mineralogical studies of primary gold mineralisation at Lubuk Mandi, Rusila, Terengganu, Malaysia. Geological Survey of Malaysia - British Geological Survey Gold Sub-Programme Report 93/6 pp28. 28

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34

35

36

37

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42 Figure 1 Location map for the Lubuk Mandi and Bukit Panji Au bearing mineralisation (locations 1 and 2).

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46 Table l(a) EPMA data for bedrock sample MA32

47 Ag 'S' curve I Number I Hg% vs Fineness I Fineness 1- Table l(b) Hg% vs fineness and IS' plots for bedrock sample MA32

48 Table 2(a) EPMA data for bedrock sample MA33

49 Ag 'S' curve I lo,oo mm m mm mm ~8 m m mm m mm m m Q , Number Hg% vs Fineness I" 0.10 a Fineness Table 2(b) Hg% vs fineness and 'SI plots for bedrock sample MA33

50 ~ 8.00 Ag 'SI curve H m m m "' J I Number Hg% vs Fineness m f # oo Flneness Table 3 EPMA data, Hg%vs Fineness and 'SI plots for bedrock sample MA34

51 Ag 'S' curve P I I L Number Table 4 EPMA data and 'SI plots for bedrock sample MA42

52 1MA I I ~ I MA A E E U E E ~ I Table 5 Shape data and area 'SI curve for alluvial sample MA13

53 ~ NAME ]AREA ~DMAX IDMIN IFSHAPE ~ERM IFCIRCLE MA14.5 MA14.6 MA14.7 r U - m 0.10 g count 20 - I I O.OO I T count 0 I 0.8 c n I p = \ ; ;, FCRCLE Table 6 Shape data, DMAX and area 'S' curves for alluvial sample MA14

54 )NAME IAREA IDMM IDMN IF SHAPE^ MA16.11 I MA I0.293l MA l MA16.14 MA16.16 MA16.17 MA16.18 MA1 6.1 Q MA16.20 MA MA MA MA MA MA MA A A16.38 I A l I E 0.8 U t' Table 7 Shape data, DMAX, FSHAPE and area 'S' curves for alluvial sample MA16

55 NAME I AREA I DMAX I DMlN I FSHAPE MA I T E $ 3.00 U) Y m 2.00 T I I Table 8 Shape data, FSHAPE and area 'S' curves for alluvial sample MA17

56 Silver "S" Curve l4 T.. 4t Number Table 9 EPMA data and 'S' plots for alluvial sample MA13

57 0 I Silver "S" Curve Number Table 10 EPMA data and 'S' plots for alluvial sample MA14

58 Silver "S" Curve I Table 11 EPMA data and 'SI plots for alluvial sample MA16

59 ~~ Silver "S" Curve I 15 T I Number Table 12 EPMA data and 'S' plots for alluvial sample MA17

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61

62 n? ir t mo, 00 2 cw B 0..I v) 3 z I.I L 0 cw?& ;a f. - L ~n -- LUII > I m b. m m so I v) 3 i z..i I E 'E cw 0 E 0 c, Q U.I.I % c, 5 2 a e Q v) t- cu-: Q 5! 8 O W C D c<c Z Z Z aaa X I S

63