Transport and Precipitation of Gold in Phanerozoic Metamorphic Terranes from Chemical Modeling of Fluid-Rock Interaction

摘要

This study reports results of mass transfer calculations using chemical modeling software (HCh) to determine chemical parameters that may have had a significant effect on gold deposition in turbidite-hosted Phanerozoic metamorphic terranes. The chemical system modeled was Al-As-Au-C-Ca-Cl-Cu-Fe-H-K-Mg-N-Na-O-S-Sb-Si to simulate fluid-rock interaction, gas partitioning, and mineral precipitation in veins. Each modeling ran takes into consideration both (1) the composition of the vein fluid and the minerals predicted to precipitate in the vein and (2) the composition of the fluids during fluid-rock interaction and the predicted alteration assemblage of the host rocks. Results of the modeling are in good agreement with observed mineral assemblages in variably endowed Paleozoic orogenic gold provinces (central Victoria and northeast Tasmania, Australia; Buller terrane, New Zealand; Meguma terrane, Canada; Sierra de Rinconada, Argentina), and illustrate that gold is precipitated efficiently and over a wide temperature range (400 deg-200 deg C) from low-salinity, mixed aqueous-carbonic fluids. The modeling also allowed us to vary the critical components of the fluid (e.g., confining pressure that affects phase separation, SIGMA CO_2, SIGMA S, f_(O_2), and pH) to investigate how each of these parameters influences the amount of gold predicted to precipitate in the vein and the associated mineral assemblage. With the exception of the low SIGMA S fluid, the modeling scenarios predict the precipitation of gold mainly in the vein due to desulfidation processes. Carbon dioxide and other gases in the fluid play an important role: they limit the f_(O_2) to a range where elevated gold concentrations can be maintained and transported in the fluid and also have an important effect on fluid immiscibility. We also investigated what effect a range of possible source rock compositions (i.e., granite, turbidites, greenstones, auriferous exhalative interflow sediments) have on gold solubility in fluids equilibrated with these rocks and hence, how rock compositions may influence ore transport and deposition. The results indicate that, at high temperatures, fluids in equilibrium with auriferous exhalative interflow sediments or greenstones will contain the highest gold concentrations. This outcome suggests that tectonic settings that enable preconcentration of gold in sulfide-rich siliciclastic or exhalative interflow sediments might provide more favorable conditions for the formation of well-endowed gold provinces.