Fluid Inclusions, Stable Isotopes and Geochemistry of Porphyry Copper and Epithermal Vein Deposits of the Hauraki Gold - Silver Province, New Zealand

Abstract

Tertiary epithermal Au-Ag-Pb-Zn-Cu vein, and porphyry copper deposits occur in the Hauraki Province. The epithermal deposits were extensively mined for gold and silver in the late 1800’s, and early 1900’s and produced approximately 300 million grams (10 million ounces) of gold and 1,000 million grams (30 million ounces) of silver. They occur in Jurassic greywacke suite rocks, lower Miocene-Pliocene andesites and dacites, and upper Miocene-Pleistocene rhyolites although the deposits in the and esites and dacites produced most of the gold and silver mined. Base metal assemblages of the epithermal deposits are dominated by pyrite, sphalerite, galena and chalcopyrite, whereas acanthite and native gold (electrum) are the most common precious metal minerals. Tellurides (e.g. hessite) and seleniferous - selenide minerals are locally important. Gangue minerals are mainly quartz and calcite. Near neutral or slightly alkaline fluid pH is indicated for the epithermal fluids by the occurrence of sericite and/or adularia in wall rock alteration mineral assemblages. Acidic fluids, forming kaolinite, are characteristic of late stages or near surface environments. Fluid inclusion filling temperatures, and sulphur isotope temperatures from sphalerite-galena pairs, indicate that base metal deposition occurred mainly between 320 and. 280°C, precious metal assemblages predominantly in the range of 280 - 200°C and late stage barite, in some deposits, generally below 200°C. There is fluid inclusion evidence for boiling during mineralisation in some deposits. Apparent salinities of the epithermal fluids, determined from fluid inclusion freezing temperatures, range from 0 - 6.1 eq. wt. % NaCl. No consistent difference in average apparent salinity was recognised between the different types of epithermal deposits, although the highest recorded values were from the base metal deposits. The absence of liquid CO2 in fluid inclusions limits the maximum possible concentration of CO2 to approximately 3 mole %. Extraction and measurement of CO2, from some samples indicates an average concentration of approximately 1 mole %. Corrections for dissolved CO2 required to transform apparent salinities to true salinities indicate that CO2 is the major solute in low salinity inclusions and that its concentration varied widely during mineral deposition in most deposits. Thermodynamic models of the geochemical environments of mineral deposition indicate that the large gold-silver deposits were formed by solutions in which sulphur occurred predominantly in reduced form, whereas many other deposits formed from solutions with approximately equal concentrations of oxidised and reduced aqueous sulphur species. Mineral deposition resulted from several different processes including: changes in fluid pH accompanying reactions with the wall rocks, mixing with other types of fluids, boiling, and variations in the concentration of CO2 in solution. These various processes acted separately in different parts of the hydrothermal system and general deposited characteristic mineral assemblages. Deuterium/hydrogen ratios of water extracted from fluid inclusions indicate that most hydrothermal fluids were originally meteoric water. Sulphur isotope ratios of sulphide and sulphate minerals, in association with the thermodynamic relations of the mineral assemblages, indicate that the sulphur was derived from at least two different sources; sedimentary sulphate and magmatic SO2, the relative importance of each varying from one deposit to another. Two types of hydrothermal systems are postulated for the formation of the epithermal deposits. During andesitic volcanism in the Miocene-early Pliocene, hydrothermal fluid convective cells were generated by heat from near surface small intrusive bodies of magma, whereas during rhyolitic volcanism in the late Miocene-Pleistocene the heat sources were larger plutons at greater depth. Porphyry copper deposits are associated with quartz diorite stocks intruded into Jurassic greywacke suite rocks and Miocene andesites. They are “diorite” model hypabyssal and volcanic types. The major minerals are quartz, pyrite, chalcopyrite and sphalerite. Additional minerals differ between the different deposits and define two contrasting geochemical environments of deposition, one characterised by low fS2, fO2 and ΣS, indicated by the presence of pyrrhotite, and the other of moderate to high fS2, fO2 and ΣS, indicated by the occurrence of bornite, magnetite or hematite. Associated hydrothermal alteration is generally propylitic although limited phyllic and “potassic” (defined by secondary biotite) types also occur in some deposits. Fluid inclusion and sulphur isotope studies of the Miners Head. porphyry copper deposit suggest that copper mineralisation occurred at a temperature of approximately 425°C from fluids with apparent salinities up to.15.5 eq. wt. % NaCl and containing sulphur of magmatic origin, predominantly as H2S.