The Woxi W–Sb–Au deposit in Hunan, South China: An example of Late Proterozoic sedimentary exhalative (SEDEX) mineralization

Abstract

The Woxi W–Sb–Au deposit in Hunan, South China is hosted by Proterozoic metasedimentary rocks, a turbiditic sequence of graywacke, siltstone, sandy slate and slate. The deposit has two principal styles of mineralization: stratiform and stockwork. The stratiform ores, which account for about 75% of the total proven ore reserves, are composed predominantly of interbedded, massive to laminated Au-bearing quartz, stibnite, scheelite, pyrite, and silty clays. Fine layering, at the macro- to microscale, is the most striking feature of the ores, with individual mm- to cm-scale bands traceable over distances of several meters to a few hundred meters along strike. Structures indicative of soft-sediment deformation due to slumping and synsedimentary sliding are commonly observed in the banded ores and the barren quartz–carbonate beds in the adjacent host rocks. The stockwork mineralization accounts for about 25% of the total ore reserves and is restricted to a 3–10m-thick zone immediately beneath the stratiform ores. It is characterized by numerous quartz+pyrite±scheelite stringer veins that are either subparallel or near perpendicular to the overlying stratiform ore layers. Alteration at Woxi occurs as semiconformable, tabular blankets asymmetrically enveloping the orebodies along their entire length, and, is dominated by a quartz+sericite+chlorite±carbonate assemblage. The footwall alteration is generally more intense and somewhat thicker than the hanging-wall alteration. Geochemical data show that, with distance above and below the mineralized horizons, contents of W, Au, As, and Sb systematically decrease, whereas concentrations of other trace elements such as Hf, Sc, Th, Ta, Y, Zr, Nb and REEs gradually increase. This may reflect a decreasing hydrothermal input in the sediments and an increasing dominance of detrital and seawater-derived components. The chondrite-normalized REE patterns of both bulk ores and individual minerals are comparable to those of many SEDEX-type deposits and associated exhalites, implying similar mineralizing fluids for these deposits. Sulfides from the ore have negative and variable δ34S values (from −12.3‰ to −1.2‰), suggesting a dominant sulfur contribution of bacterial sulfate reduction. A syngenetic (SEDEX) model is thus proposed for the formation of the Woxi deposit. Convective seawater circulated through the sediment pile and episodically vented metalliferous hydrothermal fluids onto the sea floor. Each exhalative episode was preceded by a period dominated by chemical and turbiditic sedimentation (footwall alteration blanket), followed by a peak release of hot metalliferous brines (stratiform ores and associated stringers), and terminated by a waning influx of hydrothermal fluids accompanying burial by later turbidites (hanging-wall alteration blanket).