Mesozoic gold mineralization in Hainan Province of South China: Genetic types, geological characteristics and geodynamic settings

Abstract

Numerous gold deposits and occurrences in Hainan Province of South China, predominantly of Mesozoic age account for more than 143t of proven gold reserves. The Au mineralization occurs either along a group of NE-, NNW- and WNW-trending shear zones or within NW- to NNW-trending intraformational detachment faults which are closely related to folding, shearing and transpressional to transtensional deformation. Combined with the Mesozoic tectonics and associated magmatism, the ore geology, fluid inclusion geochemistry, C-H-O-S-(Pb) isotopes and geochronology consistently indicate that there are at least two gold deposit-types in Hainan Island, i.e., orogenic-type and intrusion-related.The predominant orogenic gold mineralization, which produces more than 95% of gold metal reserves in Hainan Island, formed in the Early Mesozoic (ca. 228–224Ma). This ore deposit-type, represented by the Baolun, Gezhen, and Wangxia deposits, is generally hosted by metamorphosed volcaniclastic sedimentary rocks of the Paleo- to Mesoproterozoic, the Silurian and the Permian ages, and is closely associated with brittle-ductile shearing. These deposits, with native gold as main gold occurrence and low sulfide abundance (<5% by volume), are derived from the CO2-rich (mainly 4.8–16.8mol%), near neutral (pH=∼7), low-salinity (generally 3–10.5wt.% NaCleq.), and intermediate-temperature ore fluids. In combination with the paleomagnetism, sedimentation, and petrographical and geochemical features of the Late Paleozoic to Early Mesozoic granitoids, the orogenic gold mineralization is considered to be formed in a post-collisional tectonic setting, in response to the Indosinian orogeny in South China triggered by the closure of the Paleotethys Ocean. Abundant Bi-As-Te-Mo-S phases, which grew synchronously with the gold minerals, suggest a possible involvement of magmatic fluids, especially for the large-scale, high-grade Baolun deposit.The Fuwen Au-dominated Au-Ag deposit, which is hosted by Lower Cretaceous continental clastic rocks, was interpreted as an intrusion-related deposit and most likely formed in the Late Cretaceous. This deposit is endowed with high gold grade (average 28–95g/t Au), extremely high sulfide abundance (>50% by volume), Au-Ag-Cu-Pb-Zn metal association, and small amounts of alteration minerals (pyrite, quartz, sericite, chlorite and calcite). The rare isotopic data of O-H-S-Pb, and the ore occurrences mainly in the intraformational detachment fault zones and subordinately within the Late Cretaceous adakite-like granitoids (zircon U-Pb age of ca. 100Ma), suggest that the Fuwen deposit had an intimate genetic link to the arc-related extension-type magmatism, due to asthenospheric upwelling triggered by the slab roll-back of the subducted Paleo-Pacific plate beneath the South China Block.