Sources of metals and fluids in orogenic gold deposits; insights from the Otago and Alpine Schists, New Zealand

The Otago and Alpine schists of the S Island of New Zealand form a young (<200 Ma), lithologically monotonous, metasedimentary belt with exposures ranging from unmetamorphosed graywackes to amphibolite fades rocks. The belt contains abundant orogenic gold deposits, including the 125-t Au Macraes deposit. The schist belt is an ideal geologic setting for investigation of the sources of metals and fluids responsible for formation of metamorphic rock-hosted gold deposits. A large suite of samples representative of the lithologic and metamorphic variation in the Otago and Alpine schists was collected and analysed for a comprehensive suite of elements. The aim was to identify any rock type or metamorphic setting that may be depleted in the suite of ore-forming elements (Au, Ag, As, Sb, Hg, Mo, and W) relative to unmetamorphosed protoliths, perhaps representing the source for the enrichments observed in the Otago ore deposits. Gold, Ag, As, Sb, Hg, Mo, and W were found to have significantly lower concentrations in higher-grade metamorphic rocks compared to unmetamorphosed protolith samples. These were the only elements in a suite of 12 major and 50 trace elements to show systematic depletions with metamorphic grade. Investigation of the trace element chemistry of sulphide minerals indicates that the whole-rock depletions are caused by the disappearance between greenschist and amphibolite facies conditions of pyrite, galena, sphalerite, and cobaltite, the major host phases for the ore-forming elements. More than 95 percent of upper greenschist and amphibolite facies samples are significantly depleted in the ore-forming elements. It is proposed that the orogenic gold deposits in Otago, such as Macraes, were formed directly from metal-rich metamorphic fluid produced during prograde metamorphism at depth.