DEPOSITION AND SPECIATION OF GOLD AT THE EARLY CRETACEOUS EPITHERMAL GOLD DEPOSITS IN THE DONGKENG VOLCANIC BASIN, SOUTH CHINA: IN-SITU TRACE ELEMENT COMPOSITION CONSTRAINTS

Auriferous pyrite is common to epithermal gold deposits worldwide. Here to provide a better understanding of the mineralization we discuss pyrite trace element compositions to constrain the speciation and deposition mechanism of gold in epithermal gold deposits.

The Dongji and Maluntou deposits are the two largest epithermal gold deposits in the Early Cretaceous Dongkeng volcanic basin, South China, that are characterized by multiple stages of mineralization. Four types of pyrite are recognized and associated with gold and other sulfides: Py1 is euhedral to subhedral and disseminated within early thick quartz veins; Py2 is cubic grains that is surrounded by Py3; Py3 occurs as fine-grained aggregates, in sulfide-rich quartz veins and sulfide fine veins, possessing a porous texture and irregular edges; and Py4 occurs as coarse grains in late sulfide-poor quartz veins. All four generations of pyrite are unzoned.

Detailed in-situ LA–ICP–MS analysis results revealed a positive correlation of As and Au, with Au/As ratios below the solubility limit of gold for arsenian pyrite, indicating that gold is present as Au¹⁺ (solid solution) in a structurally bound Au complex in studied pyrites. Gold in ultimately associated with Py3 (mean Au concentration of 2.7 ppm), in contrast to Py1, Py2 and Py4 that have very low Au concentrations of 0.09, 0.05, and 0.2 ppm. However, given that Py3 is about 15–25% of the ore veins, only ~16% of the gold can be contributed by invisible gold in Py3 to the ~4.3 g/t gold in the orebodies. Instead, over 84% of gold is present as visible gold (e.g., electrum, native gold and submicrometer gold inclusions) in fractures and voids of Py3, and/or other minerals. Remobilization reactions are effective in modifying the speciation of gold (e.g., formation of visible gold from invisible gold) and upgrading Au grades. However, our geochemical data and texture evidence of Py3 indicate that this mechanism did not occur. As such, gold (both invisible and visible) present here is most likely related to a decrease in H₂S produced by fluid boiling. In addition, sulfidation reactions may only make a minor contribution. Although the role of fluid boiling in generating large goldfields is yet to be demonstrated, this study proposes that its effect on gold deposition may have been underestimated within many epithermal gold deposits.