The Otago and Alpine schists of the South Island of New Zealand form a young (<200 Ma), lithologically monotonous, metasedimentary belt with exposures ranging from unmetamorphosed graywackes to amphibolite facies rocks. The belt contains abundant orogenic gold deposits, including the 125-t Au Macraes deposit. As such, 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 analyzed 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 sulfide 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. Such regional-scale depletions require pervasive, grain-boundary fluid flow throughout these rocks. The leaching is most likely to have been caused by metamorphic fluid produced by dehydration reactions at the greenschist-amphibolite boundary.The suite of elements depleted in mid-to high-grade Otago and Alpine schists is almost identical to those enriched in the orogenic gold deposits in Otago. Furthermore, the vertical zonation in depletions is similar to the vertical zonation in enrichments that occurs in the Otago deposits. Mass-balance calculations suggest that 2 metric tons (t) Au and 24,000 t As was leached from 1 km 3 of amphibolite facies rock and that the Macraes deposit could have been formed by leaching of a 5-× 5-× 5-km cube of amphibolite facies rock. We propose that the orogenic gold deposits in Otago, such as Macraes, were formed directly from metal-rich metamorphic fluid produced during prograde metamorphism at depth. The contribution of other fluid and metal sources in the formation of these deposits, such as magmatic fluids, cannot be ruled out, but there is no direct evidence to support their involvement. Infiltration of meteoric water, such as occurs currently in the Southern Alps of New Zealand may have contributed to the formation of the late-stage deposits that formed at shallow level during uplift of the Otago schists.

Sulfide minerals in the Otago and Alpine schists, New Zealand, a metasedimentary belt exposed from unmetamorphosed greywackes up to amphibolite facies, underwent systematic changes in abundance, composition and texture during prograde metamorphism. In unmetamorphosed rocks, the most common sulfide mineral is framboidal pyrite, which contains abundant As (up to 14000 ppm), Co (up to 4000 ppm), Cu (up to 14000 ppm), Ni (up to 1100 ppm) and, locally, Ag (up to 270 ppm), Au (up to 90 ppm), and Sb (up to 240 ppm). Chalcopyrite, sphalerite, and galena also occur as isolated grains. Chalcopyrite and sphalerite contain few trace elements, whereas galena contains significant Se (up to 1600 ppm) and locally abundant Hg (up to 600 ppm). The distribution of these trace and minor elements is extremely heterogeneous. In subgreenschist-facies rocks, pyrrhotite replaces pyrite, and there is a clear textural change from framboidal pyrite to composite grains of pyrrhotite, sphalerite, chalcopyrite, galena and cobaltite. Pyrrhotite contains Co (average values 1100 ± 490 ppm), Cu (up to 17000 ppm), and Ni (up to 11000 ppm). Antimony and Hg are above detection in rare individual grains, but none of the other trace and minor elements sought are detectable. Sphalerite, galena and cobaltite increase in proportion in subgreenschist-facies rocks, and also contain higher concentrations of Ag (up to 1480 ppm in galena), Au (up to 230 ppm in galena and 110 ppm in cobaltite), As (38 ± 6 wt. % in cobaltite), Co (26 ± 4 wt. % in cobaltite), Hg (up to 4500 ppm in galena and 1100 ppm in sphalerite), and Sb (up to 1280 ppm in cobaltite and 770 ppm in galena). Pyrite, sphalerite, galena, and cobaltite become less abundant from subgreenschist-to amphibolite-facies rocks. In amphibolite-facies rocks, only pyrrhotite, chalcopyrite and trace amounts of galena and molybdenite occur, and none of these minerals contain detectable levels of Ag, Au, As, or Hg. Mass-balance calculations between sulfide minerals and whole rocks show that sulfides are important host minerals for S, Cu, and As, but host a minor proportion of Fe, Zn, and Pb. As pyrite, sphalerite, galena, and cobaltite become less abundant at higher metamorphic grade, Pb, Zn and Co are retained in the rock incorporated in other minerals, whereas As, Sb, Hg, Au, and Ag are removed from the rock, most likely by metamorphic devolatilization.

A new method for analyzing gold at ultralow concentrations (<10 pg/g) in geological samples has been developed involving HF-aqua regia acid digestion, chromatographic separation of Au from matrix elements using DIBK supported on an inert resin, and analysis by inductively coupled plasma-mass spectrometry (ICPMS). This method has an analytical detection limit of 2 parts per trillion (pg/g), significantly lower than most routinely used methods developed for analysis of ore samples with Au concentrations considerably higher than average crustal abundance ( approximately 2 ng/g). Such methods commonly have detection limits in the low nanogram per gram range. Many areas of geological research including ore genesis, crustal mobility and redistribution, planetary differentiation, and plume volcanism require quantitative analysis of geological materials with much lower Au concentrations. We present a rapid, easy to use method where Au is separated from matrix elements onto extractant primed chromatographic resin and analyzed by quadrupole ICPMS. The method is suitable for the relatively rapid analysis of a large number of samples and is reliable over a wide range of concentrations from picogram to microgram per gram level. Analysis of four different geostandards, GXR1, GXR4, CH-3, and SARM 7, yields concentrations within error of the published concentrations with accuracies of >95%.

Ultramafic portions of ophiolitic fragments in the Arabian-Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Despite the extent of the alteration, little is known about the processes that caused it, the source of the CO 2 , or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) ) low salinity fluid, with trapping conditions of 270°C to 300°C and 0.7 to 1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO 2 -bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Audeposits. This is the first time that Au has been observed to be remobilized from rocks during the lizardite-antigorite transition.

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