C. Stewart Eldridge

White Island, New Zealand, volcanic-hydrothermal system represents the geochemical environment of high-sulfidation Cu and Au ore deposition

The White Island volcanic-hydrothermal system, New Zealand, is thought to closely represent the chemical conditions that lead to the formation of high- sulfidation Cu-Au ore deposits. The amounts of Cu and Au produced over a 10 ka period of activity, largely from degassing magma, are calculated to be 106 and 45 t, respectively. Altered andesite blocks ejected from recent vents contain alunite, anhydrite, and pyrite. Their S isotopic composition indicates vein filling at ∼380 °C. At this temperature, Cu and Au are highly soluble in acid solutions, which may explain the depletion of Cu and absence of Au in the ejecta. Mass-balance calculations, however, suggest that Cu and Au are precipitated in cooler zones before the acid solutions discharge at the surface.

Diamond growth history from in situ measurement of Pb and S isotopic compositions of sulfide inclusions

In a continuing effort to understand crust-mantle dynamics, we have determined the S and Pb isotopic compositions of mantle sulfides encapsulated within diamonds from under the Siberian craton and compared these results to those of previously investigated African counterparts. Because diamond inclusions are isolated from exchange with surrounding mantle, they may preserve the history of diamond growth and act as direct tracers of the origins of mantle materials. Study of these inclusions may thus offer the best chance of recognizing global-scale interaction between Earth9s crust and mantle. Although δ 34 S values of the Siberian sulfides do not deviate significantly from the mantle value of 0‰ ± 3‰, Pb isotopic compositions are highly variable. Pb isotopic compositions of sulfides from peridotitic suite diamonds generally plot near the terrestrial Pb growth curve, with model ages ranging between 0 and 2 Ga, whereas sulfides from eclogitic suite diamonds have radiogenic compositions, plotting beyond the growth curve. These results, which are similar to those for sulfides in African diamonds, suggest that the sulfides from eclogitic suite diamonds were derived from a source with an unusually high U/Pb ratio and may indicate a common process (such as subduction of crystal materials into the mantle) operating beneath Africa and Siberia. The absence of extremely radiogenic Pb in sulfides from eclogite xenoliths suggests that the radiogenic material from which eclogitic suite diamonds grew was a transient feature of the mantle, associated with diamond growth. The ultimate origin of this high U/Pb signature, however, remains enigmatic. Large variations in Pb isotopic composition of sulfides from different zones in a single peridotitic suite diamond document (1) crystallization of the diamond9s core near 2.0 Ga, (2) growth of its outer zone in an environment with a high U/Pb ratio similar to the growth environment of eclogitic suite diamonds, and (3) growth of the diamond rim near the time of kimberlite emplacement.

Cyclic variations of sulfur isotopes in Cambrian stratabound Ni-Mo- ( PGE-Au) ores of southern China

Abstract Cyclic variations of δ 34 S values over a range of at least 48‰ in pyrite nodules from stratabound Ni-Mo-PGE(Au) ores of southern China are attributed to biogenic reduction of seawater sulfate in an anoxic, phosphogenic, and metallogenic basin. Cyclic introduction and mixing of normal seawater into typically stagnant basin waters led to extreme variations in δ 34 S values of aqueous sulfide species present at different times. Intermittent venting of metal-laden hydrothermal fluids into such a bacteriogenic sulfide-rich environment resulted in precipitation of metal sulfides as pseudomorphous replacements of organic debris and as sulfide sediments that record large δ 34 S CDT variations from −26 to +22‰. Apatite and silica dominated the replacement of the organic debris when metals were not being introduced into the basin. The combination of abundant organic debris, localized topographic basins for accumulation of the debris, bacterial production of sulfide species, and introduction of metal-bearing hydrothermal fluids provided the environment necessary to form these unusually rich Ni-Mo ores.

Ion microprobe determination of sulfur isotope variations in iron sulfides from the Post/ Betze sediment-hosted disseminated gold deposit, Nevada, USA

Abstract Secondary ion mass spectrometric analyses of ore samples from the Post/Betze sedimenthosted disseminated gold deposit were utilized to constrain elemental distribution of Au and As in iron sulfide phases. Most of the Au was deposited very early in the paragenetic sequence. Although Au and As are covariant in arsenian pyrite, Au apparently was depleted much more rapidly from the hydrothermal solutions than was As. Sensitive high-resolution ion microprobe (SHRIMP) sulfur isotope analyses of iron sulfides from the Post/Betze deposit vary widely from δ 34 S = −29 to 23‰ and provide important information on the origin of sulfur and constraints on depositional mechanisms. Ore solutions had high δ34S values and were most likely derived, at least in part, from thermochemical reduction of lower Paleozoic seawater-derived sulfate, possibly bedded barite. Late-stage ore fluids ( δ 34 S sulfide = −12 to −29‰ ) are extremely depleted in 34S relative to main-stage ore fluids ( δ 34 S sulfide = 16–23‰ ). Although such low δ34S values can be generated hypothetically from the original ore fluids by oxidation (possibly boiling), the stability of pyrite is compromised. Introduction of and mixing with Fe-rich fluids is necessary to deposit pyrite having low isotopic values.