GLADSTONE HILL EPITHERMAL SYSTEM, WAIHI, N.Z.

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1 195 Proc. 1lth New Zealand Geothermal Workshop 1989 GLADSTONE HILL EPITHERMAL SYSTEM, WAIHI, N.Z. Gordon Epithermal Mineralisation Research Unit, Geology Department University of Auckland, Private Bag, Auckland, New Zealand Abstract The small Gladstone Hill epithennal vein system is hosted by hydrothermally altered pyroxene andesites of Miocene age. The alteration is characterised by a propylitic assemblage which increases in intensity towards veins which comprise Fluid inclusion data obtained on surface samples shallow indicates depositing fluids were of low apparent equivalent) salinity at temperatures that reached in sectionsof the area below the top of Gladstone Hill), dropping to below in the shallowest The top of Gladstone Hill was then below the water-table. Late deposited calcite and sulphate produced mineral assemblages occur, the latter being particularly associated with a brecciated zone. Both assemblages probably related to the collapse of the geothermal system responsiblefor the mineralisation. Introduction The epithermal Gladstone Hill system is located in the town of Waihi, approximately south-east of Auckland City. It was a minor producer of gold between and 1905 with tonnes milled for recovery of 71. oz. of bullion. Mining was out on three north-east striking veins, the of which was followed for 213m from an adit and a deep shaft (Morgan, 1924). Recent exploration began due to renewed interest in the Coromandel in general and the re-opened Martha Mine (-10 mt at 2. Au) (Brathwaiteet al., 198) in particular. Exploration at Gladstone initially consisted of sampling outcrop and rehabilitation of old workings. Following this programme, 27 diamond drillholes were put down for a total length of over most cores were assayed leaving skeleton remainder (McOnie, 1988). This study is basedon examinationof outcropand drillcore as part of the author's thesis. Selected samples were crushed and analysed by X-ray fluorescence for major and trace elements, while a smaller number of these were analysed for selected trace elements by the neutron activation method. All the analysed rocks were thin-sectioned and analysed by powder X-ray diffraction together with other rocks from the area. Homogenisation temperature and freezing point depression measurements were made on fluid inclusions in several double polished thin sections. Geological Setting Waihi is in the southern part of the Hauraki Goldfield a 200 km long mineralised zone containing 47 separate epithermal gold-silver deposits (Christie and Brathwaite, 198) associated with a sequence of Miocene to Pliocene volcanic rocks (Skinner, 198). The goldfield is aligned along a trending basement high of block faulted Mesozoic greywacke. Overlying the greywackeis a thick sequence of andesiteand dacite flows and tuffs of the Coromandel Group, associated with westward dipping subduction between the Australian and Pacific plates Ballance, 197). Intrusive rocks associated with this but are exposed north of Waihi. Over the lavas are mainly rhyolitic rocks of the Whitianga Group. The overall trend of the volcanic rocks follows the NNW trend in the basement greywacke. Faulting is common with dominant and NW directions and displacement. Major structures identified include a series of rhyolite filled grabens on the eastern side of the peninsula (Hochstein, and possible eroded caldera structures (Skinner, 198). Waihi is thought to lie near the northern boundary of a graben possibly associated with a buried caldera and near a major ENE fault (Brathwaite et al., 198). Figure Geology Pacific Ocean Geological map and structural interpretation of the Peninsula, after Hochstein, 1980; and The basement greywacke does not outcrop in the Waihi and the altered Coromandel Group andesites hosting the mineralisation are the oldest rocks in the immediate area (Fig.2). Fresh rock from Gladstone Hill is porphyritic andesite containing phenocrysts of plagioclase, hypersthene and augite with minor amounts of opaques. The fine-grained groundmass some pyroxene but is dominated by plagioclase laths. Other rocks described in the area have similarmineralogy but higher silica contents 1980) due to the presence of primary quartz phenocrysts. Andesites in Waihi have been divided into two units based on the presence of primary quartz phenocrysts in Martha Hill and their reported lack in Union Hill and Hill samples (Brathwaite, 1980). The present study' however, indicates that primary quartz occur in both areas but they much less abundant at Gladstone Hill with -8 per thin section for Martha Hill (Brathwaite, 1980) and 1-2 section on Gladstone Hill.

1% Figure 2. Geological map of Waihi showing mineralised vein systems in the area (veins projected to surface). The Martha mine is the western system and Gladstone Hill is the system.

2 1% Figure 2. Geological map of Waihi showing mineralised vein systems in the area (veins projected to surface). The Martha mine is the western system and Gladstone Hill is the system. Note the division of andesites into pyroxene and quartz-pyroxene members is not necessary. Chalcedonic silica outcrops are the silicifiedandesite from this paper. From et The unaltered Black Hill hornblende and biotite bearing dacite (Fig.2) unconformably overlies the altered pyroxene andesites as flows over a steeply-dipping fossil weathering surface 1984). Whitianga Group rocks are represented by spheruliticand flow-banded rhyolite and pyroclastic breccias to the east of Black Hill and extensive ignimbrites over an erosion surface which cuts the older volcanics. Alluvial and eluvial material overlies this unit and are in turn blanketed by late Quaternary ash (Brathwaite et al, 198). Structure The Gladstone Hill system is separated from Union Hill by a fault (geophysically determined) which has expression as a swampy depression. The contact between the system and Black Hill in the south-east is a steeply dipping weathered horizon, possibly also fault-controlled (McOnie, 1989). The system has an unknown boundary to the north-east and is bounded by the Ohinemuri River in the south-west. All the major structures in the area strike between north and east (Table 1). Veins on Gladstone Hill have dips from near vertical to steep south-east. Veins from Winner Hill however all dip moderately steeply to the west or north-west. A brecciated zone passing beneath both hills appears to follow the same trend with a steep south-east dip under Gladstone Hill changingto a steep northwest dip under Winner Hill (geophysically determined) (McOnie, 1989). Table 1. Summary of FEATURE Gladstone no. 1 drive) Gladstone chalcedony Winner no. 1 (last Winner no. 3 veins Gladitone Hill) (under Winner Hill) Field Relations Gladstone and Winner Hills. AVERAGE STRIKE AND DIP Erosion has been extensive in the area since the geothermal system waned, as shown by the presence of alluvial material containing sulphides on the top of Gladstone Hill. Quaternary ashes have blanketed the present topography and are up to several metres thick. As a result, exposed outcrops are few, and float is dominated by the most enduring rocks (generally the silicified varieties) except where exposed by the activities of man. However, overall patterns can be determined to provide an interpretive map of the features of Gladstone and Winner Hills (Fig.3). W W to to outcrop : Both Gladstone and Winner Hills have similar patterns of float and outcrop. Rocks encountered in the north-west portion of the area include vein quartz up to 2m across (Fig.3) consisting of banded fine-grained and chalcedonic silica and rarer quartz calcite. The exposureof this material is controlled by the presence of waste from mining which has brought weakly altered material to the surface. Further south, (up the hills) silicified andesite is more common (Fig.3) and veins are finer grained (more chalcedonic). Material immediately NW of the top of Gladstone Hill comprises only completely silicified andesite with shaped voids. Occasional chalcedony veins occur and some samples contain a white clay in their voids. On Winner Hill the silicified andesite more commonly contains veins of poorly crystalline quartz and there is a coating of sugary quartz in the voids. The crests of both hills are made up of blocks of silicified andesite as described above, but discontinuouschalcedonyveins are more common than in those rocks. An outcrop on Gladstone Hill contains a face where chalcedony vein are surrounded by brown coloured silicified rock, while the body of the rock is grey. Immediately to the of this outcrop, the brown silica is more prominent as a filling and fluidising material in the rocks, often with a fine layering texture. These breccias (Fig.3) contain chalcedony and silicified andesite as clasts in a of brown silica. Other breccias are present as float. These contain fine vein material present clasts in an opaline, white brown silica matrix, often with the vein material forming sedimentary-like layering. The south-east edge of the prospect comprises silicified andesite and breccia float as described above. In addition, an old water race cut into Winner Hill (Fig.3) exposes highly oxidised and unsilicified andesite containing thin (lcm) quartz and quartz pseudomorphing calcite veins. At the very base of the south-eastof Winner Hill fine pyritic material occurs and this pyritic chert is brecciated in two phases. The first produced fractures now filled by clear quartz followed by a later more complete fluidisation with brown silica as a matrix. This material is reported to occur below this area in an adit now collapsed 1984). Underground The walls of the adits on both hills are over 80 old, and as a result are well covered in oxidised clays. However it is possible to see the nature of the veins and some of the wallrock. Veining is generally widespread with thin (-lcm) fine quartz veins and wider veins. These are usually massive or very poorly layered when seen underground but slabbing shows the main vein in Gladstone no. 1 drive to be crudely layered with thick bands of fine quartz, quartz pseudomorphing calcite and finely brecciated material. In Winner no. 3 drive the main vein was 2m thick and strongly layered with alternating fine quartz and quartzafter-calcite.the wallrock is commonly a white or tan clay and quartzmixture. A representative cross-section (Fig.4) shows the major components of Gladstone Hill. An important feature is that the Gladstone vein, over thick in Gladstone no. 1 drive, does not extend to any depth with the same width, but probably correlates with numerous smaller veins seen in cores. In DDH UW calcite is only Seen as a hand specimen mineral below RL. Samples in this hole are generally intensely altered porphyritic andesites but some tuffs occur. Beneath the crest of Gladstone Hill drilling has shown a zone of brecciated veining and hyrothermal with an apparent steep SE dip flanked by highly altered andesite. At greater depth this andesite is less altered so fresh andesite is encountered. This could imply that bars of less altered andesite exist between sites of major veining or the andesite here was less permeable. Silicified andesite exposed the top of Gladstone Hill was shown to be shallow. Alteration Four main alteration types are recognised in the study area (Table 2): (1) widespread propylitic alteration (2) silicification associated with veining (3) later calcitedepositing event (4) an acid sulphate event

3 197 KEY Silicified Andesite Veined Silicified Andesite Breccia Tracks All at -45 _ Figure 3. Interpreted geology of A-A is line of for Fig. adits (eg. Winner no.3) an water-race marked. For both = drillhole sample location Figure 4. Cross-section of Gladstone Hi on outcrop, underground an interpretation. Gladstone no. 1 drive 15 GLADSTONE HILL Silicified Andesite and Chalcedony Veins Gladstone Veins (mined) x 0 Limit of Calcite Occurrence Veining Hydrothermal Breccia Altered Andesite

4 198 (1) Initial alteration involved replacement of first hypersthene then augite by chlorite +/- opaques. The chlorite occurs as a fibrous mineral in cracks and as low birefringence material replacing the body of the pyroxenes. Chlorite also occurs as veins crosscutting all primary and secondary minerals. Plagioclase is generally fresh or weakly altered with birefringent minerals in cracks or 'dusty' centres. More rarely, the centre of the plagioclase is completely replaced by illite. The groundmass is generally very weakly altered with most plagioclase being unaltered, but the groundmass in some samples has chlorite and opaques derived from alteration of groundmass pyroxene. (2) More intense alteration is typically associated with quartz and adularia veining or the appearance of these minerals in the groundmass. Pyroxenes have all altered to chlorite +/- opaques with plagioclase being replaced by illite and adularia; adularia appearing as a remnant core, itself being partly replaced by illite. Quartz, illite, pyrite and adularia are vein constituents and usually also appear in the groundmass. The adularia in this case, however, is not alteringto illite indicating that it postdates the illitic alteration of the plagiocase phenocrysts. Adularia encloses euhedral quartz in some samples but the reverse is true in other samples so the deposition of these minerals probably occured in one overall event. In one example of this alteration type chlorite is replaced by illite and small crystals of epidote, the only occurence of this mineral seen in this area. (3) Calcite is present in only a few samples, all deep in DDH but where seen it post dates other alteration. All samples have a of propylitic alteration with adularia, illite and chlorite +/- opaques and in some places vein adularia and quartz. The calcite occurs as thin veins with phenocrysts being replaced only adjacent to veins in sampleswhere this alteration type is weakly developed and as complete replacement of many phenocrysts and widespread veinlets where strongly developed. In samples with quartz veining, calcite encloses cuhedral vug fill quartz while in one sample a vein filling sequence of adularia, quartz then calcite was seen. (4) The acid sulphate alteration is best developed in the breccia zone beneath the crest of Gladstone Hill. This alteration is characterised by extreme silicification of the groundmass, usually together with extensive quartz veining, common pyrite and ubiquitous kaolinite. Alunite and possibly gypsum are rare constituents. This alteration type is commonly seen as coatings on late fracturesas and clay veins. Figure 5. Observed alteration sequence Calcite Quartz Chlorite + Pyroxene Plagioclase + Chlorite Adularia + Adularia + Illite t Opaques Illite + Chlorite Opaques + Illite Opaques : Fresh : Weak + Adularia Illite : Silicification + Chlorite Opaquea Quartz + Kaolinite Alunite) Table 2. Mineralogy and alteration : : Calcite : Acid Px Chl Ill Ad Cal Kaol Fresh P P P - - Weak P P Weak P P P,S S Weak - P P P.S S.V Silicified S - S Silicified S,V S,V S,V Silicified S,V S.V S.V S,V - Silicified S,V - S.V V Silicified S.V S S - Calcite S.V Calcite P,S S S S S.V Calcite S.V - S S - S,V S S S Acid s,v s Acid s Acid KEY: Qtr-quartz. Plag-plagioclase, Px-pyroxene. Ill-illite. Ad-adularia, Cal-calcite. Kaol-kaolinite P-primary. S-secondary. V-vein 0-rare Rock Chemistry The major and trace-element chemistry of the rocks can be correlated with the alteration types. The weak alteration of pyroxene to chlorite and minor plagioclase alteration is essentially isochemical with few elements showing enrichment or depletion relative to the fresh rock (Appendix 1). This indicates that the alteration is a re-equilibration of the constituents of the rock in response to increasing temperature, rather than involving any major fluid flux. The silicification style alteration shows marked enrichment and depletion of many elements. The flux of alkali and associated elements shows this alteration type well. Total alkali contents fall with respect to the fresh and weakly altered samples but the potassium contents are markedly higher in the silicified samples Calcium and are depleted in these rocks with the alterationof plagioclase feldspar but potassium is added in the form of illite and adularia Chemical changes associated with the calcite depositing event seems to be minor. The change in chemistry with respect to the fresh sample can be ascribed to the alteration type before introduction of calcite. Therefore samples that have an advanced propylitic alteration plot about half-way between fresh and silicified samples while samples where calcite post-dates silicification have the same chemical characteristics as the silicified material. The acid-sulphate alteration is characterised by marked or total depletion of most elements, however sample is enriched in some elements gold because it has extremely abundant pyrite. One sample has many characteristics half-way between silicification and acid-sulphate alteration. This sample contains a vein of which indicates this sample has a weakly developed acid-sulphate overprint. Deduced Fluid Temperature Fluid Inclusions The deepest sample from the area records a homogenisation temperature! of while shallower samples have lower temperatures around The inclusions indicate apparent salinities of the depositing fluid are low, of the order of 0.8 NaCl equivalent (ignoring possible effects of C02 content which would only reduce the apparent salinity further (Hedenquistand Henley, 1985)). Mineralogy Temperatures deduced from minerals are in the same order of magnitude as the fluid inclusions. For example, samples from DDH using illite peak location as an indicator, give temperatures that range from to greater than The sample with small epidote crystals indicates temperatures about were reached Discussion The Gladstone Hill system has many similarities to geothermal systems (Browne, 1984). the features seen in this fossil system with the modem equivalents allows the environment for its formation to be more accurately deduced. The initial alteration involved an isochemical re-adjustment of the country rock with a gradation from fresh rock to intense propylitic assemblages nearer to the veins. The veins themselves are surrounded by quartz-adularia alteration envelopes. The fluids responsible for this alteration were low salinity and probably had low gas content since calcite is rare. Only samples that had direct chemical interaction with this fluid (silicification style alteration) contain detectable gold The temperatures reached in the rocks as recorded by the mineralogy for this phase are as high as Boiling-point for depth relations (Haas, 1971) indicate the top of Gladstone Hill was below the piezometric water surface at this time. The alteration assemblage (quartz, adularia, illite and chlorite) is typical of that produced from alkali-chloride fluids in modem systems.

5 199 A H N to LO Silica Si 21 a I I I Gold Silica I I 18 a Silica I 21 0 Ca Figure. Whole rock chemistry. For location of samples see and for alteration type see Table 2. Note samples and 21 are vein samples. Total alkalis +Na + K)vs. silica. Potassium vs. silica. Ca, Na, K ternary diagram. Acid altered samples not included because alkali content is too low to produce meaningful ratios. Gold vs. silica Later events in the system were the appearanceof a calcite alteration assemblage and brecciation associated with a quartzkaolin deposition. The relation between these two events is unclear but they both post-date the quartz-adularia deposition event. No detectable gold is associated with the calcite alteration but a pyrite bearing sample from the quartz-kaolin alteration assemblage has a higher gold content The quartz-kaolin alteration gives rocks a characteristic brown colouration that is also seen in the matrix of those that have been extensively brecciated. The analogues of the calcite and quartz-kaolin alteration are bicarbonate and acid-sulphatewaters respectively. Both these water types can be found in the near-surface environment (Bmwne, 1984) and their presence at depth may be related to the collapse of the geothermal system. Acknowledgements The author thanks Gold Resources for support and access to the area and I also my supervisors Dr. Nicholson and Dr.P. Browne for assistance in my research, and Donald Clarke for editorial assistance. References Ballance, P.F. (197): Evolution of the Upper Cenozoic Magmatic An: and Plate Boundary in Northern New Zealand. Earth and Planetary Science Letters, 28: Brathwaite, R.L. 980): Petrography of Altered Rocks from the Waihi Mines. New Zealand Geological Survey, DSIR Report M80. Brathwaite Henderson, S. (198): The Martha Hill Gold-Silver Deposit in Proceedings of Symposium 5, International Volcanological Congress, University of Auckland. Browne, P.R.L. (1984): Hydrothermal Alteration, Unpublished course notes, Geothemial Institute. Christie, Brathwaite, (198): Epithermal Gold-Silver and Porphyry Copper Deposits of the Hauraki Goldfield - A Review. in Guide to the Active Epithermal (Geothermal) Systems and Precious Metal Deposits of New Zealand. Monograph Series on Mineral Deposits, p Haas, (1971): The Effect of Salinity on the Maximum Thermal Gradient of a Hydrothermal System at Hydrostatic Pressure. Economic Geology P.M and Associates, (1984): Union Hill Progress Report, Unpublished company report. Hedenquist, Henley, (1985): Effects of on Freezing Point Depression Measurements of Fluid Inclusions -- Evidence From Active Systems and Application to Epithennal Studies. Economic Geology, p Hochstein, (1980): Hauraki Rift in M.P Hochstein and T.M Hunt (eds) Guide to the Geophysics of the Volcanic and Geothermal Areas of the North Island, New Zealand. Royal Society of New Zealand, Miscellaneous Series, no.3: p.28-3 McOnie, A; (1988): Comparison Between Union Hill and Martha Hill, Unpublished company report. McOnie, (1989): Results of CSAMT Survey at Union Hill, Unpublished company report. Morgan, (1924): The Geology and Mines of the Waihi Hauraki Goldfield. New Zealand Geological Survey Bulletin 2. Skinner, (198): Neogene Volcanism of the Hauraki Volcanic Region of New Zealand A Review. Royal Society of New Zealand, Late Cenozoic Volcanism in New Zealand, Bulletin 23,

6 200 Appendix 1. Selected chemical analyses. Element: Alteration Fresh Weak Silicified Calcite Acid Total ll Nb Sr 208 Rb 45 Th Pb 13 Aa 94 cu 9 Ni 4 Cr Ba 392 La Sb Eu Hf LU I Sm sc Th Yb Note: X-Ray Fluorescence, NAA- Neutron Activation On Ignition. XRF analyses performed at Auckland University; NAA at, Sydney

WAMUNYU EDWARD MUREITHI I13/2358/2007

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