Susan E. Ho

The source of ore fluids and solutes in Archaean lode-gold deposits of Western Australia

Abstract The ore fluids which produced Archaean lode-gold deposits in sub-amphibolite facies rocks of the Yilgarn Block, Western Australia, are characteristically low- to moderate-salinity H 2 O-CO 2 -CH 4 fluids. Wall-rock alteration and ore deposition were from a generally reducing, near-neutral fluid at 200–350°C and 1–3 kbar, although more oxidising fluids produced some assemblages in the giant Golden Mile deposit. Gold was transported as reduced-sulphur complexes. The ore fluid composition and stable and radiogenic isotope data indicate multiple sources for the fluid and solute. Deep plumbing systems are invoked, with the fluids being either metamorphic fluids that derived solutes from the lower and upper crusts, or fluids exsolved from deep-seated granitoids formed by anatexis of such crust.

Archean cratons, diamond and platinum: Evidence for coupled long-lived crust-mantle systems

Diamondiferous intrusions and magmatic Pt-Pd deposits are both concentrated on, or adjacent to, the oldest cratons, those with >3.0 Ga high-grade gneiss terranes and/or greenstone belts. Given the old age (>3.0 Ga) of peridotitic inclusions both in diamonds and in kimberlites, diamonds presumably grew in mantle depleted in basaltic major elements near the base of thickened lithosphere and below early sialic nuclei. Most genetic models for magmatic Pt-Pd deposits require a Pt-Pd–enriched, high–Mg-Si melt generated from analogously depleted mantle. The depleted mantle was most likely formed by removal of basaltic melts that contributed to Archean intracratonic greenstone belts. Extensive melting also decreased density and increased rigidity beneath ancient cratons, favoring stabilization and preservation of thick Archean continental lithosphere. Overall, these considerations suggest that localized, thick sialic crust and rigid lithosphere developed before 3.0 Ga, forming enduring, coupled crust-mantle systems below ancient sialic nuclei, the sites of selectively preserved greenstone belts. The data confirm that Early Archean terranes are highly prospective for post-Archean magmatic ore deposits.

Criteria for determining initial lead isotopic compositions of pyrite in Archaean lode-gold deposits: a case study at Victory, Kambalda, Western Australia

Abstract The initial Pb isotopic compositions of ore-associated sulphides are important in isotopic tracer studies. Studies of Archaean lode-gold deposits from Western Australia indicate that the Pb isotopic data distribution for ore-associated galenas and pyrites is commonly linear, and determination of the initial Pb ratio relies on selecting the least-radiogenic composition. This detailed Pb isotopic study of sulphides from the 32 Ore Zone, which transects a variety of rocks, in the Victory mine (mineralisation dated at 2627 Ma) has resolved the origin of the Pb isotopic heterogeneities which yield linear arrays. It has led to the development of criteria for sample selection which more efficiently and effectively target pyrite samples which best preserve the initial Pb ratio of the sulphide and hence the ore fluid. The most important controls on the displacement of the measured Pb ratio of pyrite from the initial Pb ratio are, in order of importance: (1) Pb content of pyrite; (2) pyrite abundance; (3) proximity of pyrite to ore-fluid channelway (i.e. vein); and (4) host rock. The host rock is important because the Th, U and Pb contents and presence of minor UTh-bearing phases influence the amount and nature of any post-formation Pb represented in the pyrite data. The first three parameters control the magnitude of that influence. A high Pb content, high pyrite abundance and setting in a vein or at a vein margin produce a Pb isotopic composition which is more likely to be dominated or buffered by fluid Pb (i.e. more accurately reflects the initial Pb isotopic ratio). Conversely, pyrite with a low Pb content, low abundance and siting distal from veining is likely to be more susceptible to post-formation modification by contributions from the host rock, resulting in a more radiogenic Pb isotopic composition.