A comprehensive genetic model for the world’s largest Sb deposit (Xikuangshan, China)

The Mesozoic (160−130 Ma), fault-controlled Xikuangshan Sb deposit within Devonian limestone strata of Hunan Province, Southern China is the world’s largest Sb deposit containing a proven reserve of ∼2.5 m Sb. Although mined for over a century, its genesis remains poorly understood. Here we use new He-Ar isotope data of hydrothermal stibnite and both new and existing C-O-Sr-Nd isotopes of hydrothermal calcite with known stages to decipher its genesis and the major constraints on mineralization intensity. The 3He/4He and 40Ar/36Ar ratios of fluid inclusions trapped in stibnite are from 0.01 to 0.04 Ra (Ra: atmospheric 3He/4He ratio) and 304−1077, respectively, indicating the ore-forming fluids at Xikuangshan were dominated by air-saturated meteoric groundwater after interaction with crustal rocks. Ore-stage calcite C and O isotopes indicated that most CO2 in the fluids was acquired from marine carbonate rocks by dissolution; whereas Sr and Nd isotopes differed from deposited Devonian country rocks but were similar to the underlying regional Proterozoic clastic rocks in the region. Calcite from early and late stages showed a strong positive correlation between δ18O and 87Sr/86Sr, consistent with the mixing between the circulating groundwater and compounds released from the Proterozoic rocks due to extensive fluid-rock interaction. The 3He/Q ratios of the fluid inclusions are low, varying from 4.3 to 18.5 × 10−15 cm3 standard temperature and pressure (STP) J−1, indicating deep-seated magma could have provided heat by conduction but no volatiles into the ore-forming fluids. Based on these new results, we suggest that deep-seated granitic magma heated the down-going meteoric groundwater along fault zones, after which the groundwater extensively interacted with and extracted Sb from the Proterozoic Sb-rich rocks to form Sb-rich fluids. The Sb-rich fluids then ascended through regional faults and deposited Sb as stibnites at favorable structural traps to form the Xikuangshan Sb deposit. This study highlights that extensive water-rock interaction is essential to form the deposit, and more intensive water-rock interaction at an early stage allowed for early-stage mineralization yielding higher Sb reserves (>80%) at Xikuangshan.