Sediment-hosted gold deposits in Guizhou, China: products of wall-rock sulfidation by deep crustal fluids

Sediment-hosted gold deposits in Guizhou, China, are hosted in late Paleozoic and early Mesozoic sedimentary rocks along the southwest margin of the Precambrian Yangtze craton. They have characteristics similar to Carlin-type gold deposits in Nevada and are notably enriched in As, Sb, Hg, and Tl. The Shuiyindong and Yata deposits consist of disseminated, strata-bound sulfides in Permian bioclastic limestone and fault-controlled mineralization in Middle Triassic calcareous clastic rocks, respectively. Mineralization in both deposits consists of barren milky quartz veins, disseminated gold-bearing arsenian pyrite and arsenopyrite, stibnite, realgar, and orpiment. The barren milky quartz veins occur in the ore-controlling structures with an envelope of gold mineralization in the host rock consisting of disseminated gold-bearing arsenian pyrite and arsenopyrite and replacement-style quartz veinlets. Later drusy quartz, stibnite, realgar, and orpiment fill fractures and vugs on the periphery of gold mineralization. Petrography, microthermometry, laser Raman spectroscopy, and laser ablation inductively coupled plasma mass spectrometric (LA-ICP-MS) analyses of fluid inclusions are used to characterize the chemical evolution of ore fluids at Shuiyindong and Yata.

Early barren milky quartz veins trapped aqueous fluid with moderate salinity (6.0 wt % NaCl equiv) and minor CO2 (<2.4 mol %) at a temperature of 230° ± 30°C. This fluid contains measurable B, Na, K, Cs, Sr, As, Sb, and Au (3.8 ± 0.5–5.7 ± 2.3 µg/g) determined by LA-ICP-MS and is interpreted to approximate the mineralizing fluid responsible for precipitation of early barren quartz veins. Low-salinity (0.9–2.3 wt % NaCl equiv), CO2-bearing (6.3–8.4 mol %), aqueous fluids are recorded in replacement-style quartz veinlets. They formed gold-bearing arsenian pyrite and arsenopyrite and quartz veinlets at a temperature of 210° ± 20°C and pressures of 450 to 1,150 bars, corresponding to depths between 1.7 and 4.3 km under lithostatic conditions. Fluid inclusions in late stibnite-orpiment-realgar-quartz veins contain high CO2 (58–64 mol %) and N2 (19.2–23.7 mol %) with trace CH4 (up to 1.6 mol %). The increasing content of CO2 and decrease in the concentrations of Au and As in aqueous-carbonic inclusions are interpreted to result from carbonate dissolution at the level of the deposits during gold mineralization. Covariance of Au, As, Sb, and Sr concentrations in the fluids is interpreted to reflect deposition of As and Sb sulfides and gold concurrent with carbonate crystallization. Iron is below detection limit (~400 µg/g) in all fluid inclusion types, suggesting that the ore fluids were Fe poor but possibly sulfur rich, possibly explaining their exceptionally high gold contents. Iron in sulfide minerals was probably derived from dissolution of ferroan minerals in the host rocks, sulfidized by H2S-rich fluids to precipitate arsenian pyrite, arsenopyrite, and gold. Pressure fluctuation induced by faulting resulted in local fluid immiscibility and led to late deposition of realgar, orpiment, stibnite, and calcite.

Hydrogen and oxygen isotope compositions of ore fluids (δD(H2O) = –35 to –68‰, δ18O(H2O) = 4–16.5‰) indicate a metamorphic origin, possibly related to crustal thickening and prograde metamorphism during the late Yanshanian orogeny.