STRONTIUM ISOTOPE SYSTEMATICS OF HYDROTHERMAL MINERALS FROM EPIGENETIC ARCHEAN GOLD DEPOSITS IN THE YILGARN BLOCK, WESTERN AUSTRALIA

Strontium isotope compositions are presented for samples of hydrothermal scheelite and tourmaline from one granitoid-hosted and 14 greenstone-hosted Archean gold deposits in the Yilgarn block, Western Australia. The direct measurement of the Sr-87/Sr-86 ratios of Rb-poor minerals such as scheelite and tourmaline, which show an intimate paragenetic relationship to gold, allows an accurate estimate of the local strontium isotope composition of the Archean gold-bearing hydrothermal fluids. The population of greenstone-hosted gold deposits is subdivided according to gangue mineralogy into gold skarns with garnet-pyroxene-biotite and amphibole-biotite-calcite wall-rock alteration, which formed in the lower parts of the greenstone belts at depths of 10 to 15 km, and medium-temperature gold deposits with biotite or sericite-ankerite-albite wall-rock alteration, which formed in the upper parts of the belts at depths of 3 to 7 km. The Sr-87/Sr-86 ratios of the fluids vary from 0.7020 to 0.7035 in skarns with garnet-pyroxene-biotite alteration, from 0.7020 to 0.7043 in skarn-quartz orebodies with amphibole-biotite-calcite alteration, and from 0.7014 to 0.7028 in the medium-temperature gold deposits. The variations in the strontium isotope composition of scheelite from the same mining district are interpreted to reflect mixing between radiogenic strontium, introduced by the hydrothermal fluid, and nonradiogenic strontium released from the wall rocks during interaction with this fluid. The greenstone host rocks of all deposits, mainly metamorphosed ultramafic or mafic volcanic rocks and their intrusive equivalents, are characterized by the low initial Sr-87/Sr-86 ratio (0.7007-0.7012) and low Rb/Sr ratio of the Archean mantle. On the other hand, the initial Sr-87/Sr-86 ratios of granodiorite and granite plutons in the batholiths, which underlie and border the greenstone belts, vary from 0.7023 to 0.7044. The strontium isotope data indicate that the deep-seated gold skarns formed predominantly from fluids which evolved from or interacted with anatectic magmas in the granitoid batholiths. The data do not as clearly constrain the origin of the fluids in the higher level, medium-temperature hydrothermal systems. They are, however, inconsistent with models involving predominantly metamorphic fluids generated by the devolatilization of the lower parts of the greenstone belts.