Sediment-Hosted Gold Deposits in Guizhou, China: Products of Wall-Rock Sulfidation by Deep Crustal Fluids

Abstract

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, USA and are notably enriched in As, Sb, Hg and Tl. The Shuiyindong and Yata deposits consist of disseminated, stratabound 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 mole %) at a temperature of 230 ± 30°C. This fluid contains measurable B, Na, K, Cs, Sr, As, Sb and Au (3.8 ± 0.5 to 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 to 2.3 wt.% NaCl equiv.), CO2-bearing (6.3 to 8.4 mole %), 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 1150 bar, corresponding to depths between 1.7 and 4.3 km under lithostatic conditions. Fluid inclusions in late stibnite-orpimentrealgar-quartz veins contain high CO2 (58 to 64 mole %) and N2 (19.2 to 23.7 mole %) with trace CH4 (up to 1.6 mole %). 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 l...

The relation between Cu/Au ratio and formation depth of porphyry-style Cu–Au ± Mo deposits

“…Epithermal gold deposits can form where the liquid derived from contracted vapor starts to boil again or where it mixes with shallow groundwater (e.g., Hedenquist et al 1998;Ronacher et al 2004;Pudack et al 2009). Carlintype gold deposit may be formed where the gold-rich contracted vapor encounters reducing or Fe-rich sedimentary rocks (Kesler et al 2003;Heinrich 2005;Su et al 2009). …”

Geochemistry of Hydrothermal Gold Deposits

“…Both gold deposit types are inferred to have formed after peak deformation, metamorphism, and magmatism (c.245-220 Ma) during the final stages of contraction (Mao et al, 2002), transpression (Vielreicher et al, 2002), or postcollisional uplift and extension of the Triassic Qinling collisional orogen. Field relationships and unconventional isotopic dates suggest the deposits are Cretaceous in age (Hu et al, 2002;Su et al, 2009aSu et al, , 2009b, although a new Re-Os isochron date of 193AE 13 Ma on arsenian pyrite and a 40 Ar-39 Ar date of 194.6 AE 2 Ma on sericite from Jinfeng both indicate an older age (Chen et al, 2007;Gu et al, 2010). Field relationships and unconventional isotopic dates suggest the deposits are Cretaceous in age (Hu et al, 2002;Su et al, 2009aSu et al, , 2009b, although a new Re-Os isochron date of 193AE 13 Ma on arsenian pyrite and a 40 Ar-39 Ar date of 194.6 AE 2 Ma on sericite from Jinfeng both indicate an older age (Chen et al, 2007;Gu et al, 2010).…”

Gold-transporting hydrothermal fluids in the Earth's crust: an introduction

“…In these gold-only deposit types, Au is in most cases not only the only commodity of economic interest in the ore, but we can infer from the available multi-element compositional data on fluid inclusions that it is the only commodity of interest transported in the hydrothermal fluids of these deposits (e.g. Yardley et al 1993;Su et al 2009). Gold-only hydrothermal deposits are thus environments in which gold is very effectively fractionated from other metals, presumably as a consequence of gold's unique chemical properties, such as high electronegativity and the tendency to form bonds with strong covalent character (e.g.…”