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PACRIM 99 Congress

Conference Proceedings

PACRIM 99 Congress

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Stable Isotopes and Ore Fluid Chemistry of Gold Mineralisation at Waitangi Deposit, Thames District, Coromandel Peninsula, North Island, New Zealand

The Thames gold-silver deposits are part of the Hauraki goldfield, situated within the Miocene to Pliocene calc-alkaline Coromandel Volcanic Zone (CVZ). The Waitangi vein system is one of the mineralised ore shoots in the Thames district. The Thames gold deposits are characterised by shallow-level epithermal vein mineralisation with phyllic to advanced argillic alteration assemblages and abundant sulphide minerals including pyrite, arsenopyrite, chalcopyrite, galena and sphalerite. Gold occurs as electrum in association with sulphide minerals. Sulphur isotope analyses of mixed sphalerite-galena and sphalerite-galena-chalcopyrite samples yield a narrow range of 34 SCDT values (-1.0 to 0.9 ; n=5), whereas late spongy pyrite gives unusually light sulphur isotope values (-16.7 to -27.9 ; n=4). Vein quartz from the mineralised zone at Waitangi yields 18OSMOW values from 6.8 to 12.7_x000D_
(n=23). Fluid inclusion studies indicate a homogenisation temperature range of 190 - 296C and salinity range of 0.2 - 3.0 wt per cent NaCl equivalent. Similar temperature and salinity ranges have been recorded for the other gold deposits in the Coromandel Peninsula. PIXE (Proton Induced X-ray Emission) analysis revealed that the Waitangi ore fluids have 1090 - 2700 ppm K (average 1600), 145 - 205 ppm Ca (average 180), 80 - 100 ppm Mn (average 90), and 80 - 200 ppm Fe (average 120) but no detectable Cu, Zn, Br, Rb, Sr, Pb and Bi (only one fluid inclusion contains 28 ppm Zn). The sulphur isotopic values of base metal sulphides from Waitangi suggest that the sulphur was derived from a magmatic source (either directly, or by leaching of igneous sulphides). The calculated oxygen isotopic composition of ore fluids is between -2.1 to 3.8_x000D_
at 250C and -0.1 - 5.8_x000D_
at 300C. Oxygen isotope analyses have revealed significant variations in 18O values with elevation and gold grades. Zones of gold enrichment coincide with a zone of variable oxygen isotope values at the top of the vein system, where boiling and/or fluid mixing most likely occurred. Mixing of ore fluids with convective ground water is a likely mechanism for gold deposition in a shallow environment for the Waitangi vein system and there is also lack of fluid inclusion evidence (eg liquid-rich and vapour-rich inclusions in the same growth zone) for boiling of ore fluids. Oxygen isotope values varying with gold grades within the Waitangi conduits suggest that 18O values may be useful in targeting bonanza grade gold mineralisation in epithermal vein systems.
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  • Stable Isotopes and Ore Fluid Chemistry of Gold Mineralisation at Waitangi Deposit, Thames District, Coromandel Peninsula, North Island, New Zealand
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  • Published: 1998
  • PDF Size: 0.354 Mb.
  • Unique ID: P199904072

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