Geology, Geochemistry and Genetic Aspects of Kuroko-type Volcanogenic Massive Sulfide Deposits in Sanjiang Region, Southwestern China
Yidun Island Arc in Sanjiang Region is one of the important Kuroko-type volcano-genie massive sulfide deposits (VMS) districts in southwestern China. Tectonically, the region is situated in the eastern portion of the Tethys orogenic belt, and is a late Triassic extensional island-arc. The island-arc underwent earlier arc construction, intra-arc rifting, later arc construction and back-arc spreading during its evolution and formed a complete trench-arc-basin system, i.e., the Ganzi-Litang suture zone, forearc basin, outer arc, intra-arc rifting zone, inner arc and back-arc basin from east to west across the island-arc.
Kuroko-type VMS deposits and occurrences discovered so far are limited to within the intra-arc rifting zone of the Yidun Island Arc and tend to occur in groups or clusters distributed in separate fault-bound basins; e.g., Zengke, Gacun and Xiangcheng basins, in which submarine volcanic rocks, especially bimodal volcanic suites, were extensively developed. The VMS deposits formed in the waning stages of volcanic eruption in the late Triassic and are hosted in the uppermost section of acidic volcanic rocks (rhyolitic-dacitic) within a bimodal suite. The orebodies are associated with these host rocks: rhyolitic-dacitic pyroclastic rocks and exhalites and chemical sedimentary rocks.
All VMS deposits in Sanjiang are of the Zn-Pb-Cu type. The architecture of the deposits is twofold, consisting of concordant lenses and layers of massive sulfides and an underlying discordant stockwork. All deposits show metal zoning. The hydrothermal alteration related to mineralization is strong and asymmetrical, and is found dominantly within the stockwork zone and host rocks surrounding the ores. There are two alteration patterns: (1) chloritization — serialization — silicification from outer sides to core of the alteration pipe; and (2) silicification + serialization — silicification + carbonate from ihe lower lo upper part of the alteration pipe.
The mineralization temperatures of ore-forming fluids vary in the range 200°C to 335°C. Thermal evolution of the fluids displays temperature profiles that first increase and Ihen decrease wilh progression of mineralization processes. The majority of black-ore minerals (Groups I and II) formed during intensifying slages of hydrothermal activity, and the majority of semi-yellow-ore minerals (Group III) formed during a thermal maximum. There followed Ihe formation of fine-grained black-ore minerals (Group IV), which interacted with barite layers during deasasing temperature phases.
Oxygen and hydrogen isotope studies show lhat the sources of ore-forming fluids for the Kuroko-type VMS deposits are two-fold: seawater and magmatic hydrothermal solutions, represented by the Kuroko deposils in Japan and Kuroko-type VMS deposils in Sanjiang, respectively. In the latter case the hydrothermal solutions reacted with host rocks and mixed with cold seawater percolating downward, and circulated through volcanic rocks resulting in development of alteration zones and formation of ore minerals in Kuroko-type VMS deposits.