Geochemical Characteristics And Metallogenic Model Study Of Laochang VMS Pb-Zn Polymetallic Deposit, Western Yunnan Province

The Laochang Pb-Zn-Ag-Cu volcanic massive sulfide (VMS) deposit, located inthe south of the Changning-Menglian suture zone, Yunnan Province, SW China. Thedeposit comprises three ore clusters including stratiform orebodies occurring withinthe Early Carboniferous volcano-sedimentary sequence (cluster I), the Middle-UpperCarboniferous marine carbonates (cluster III), and the interface between them (clusterII). Early Carboniferous sequence resides on the side of the Laochang caldera hostingmost of the sulfide ores is composed mainly of basaltic to andesitic lavas andvolcaniclastic rocks which were formed by three major volcanic cycles. The verticallystacked massive sulfides characterized the deposit were accumulated mostly at theend of the second and third volcanic cycles.The major orebodies (e.g. the No. I1+2and II1) at Laochang display a typicalVHMS vertical zonation, from bottom to top, of a sulfide stockwork, disseminatedsulfides, massive sulfide lenses, and laminated carbonaceous-sulfidic silicalite withdelicate exhalative-sedimentary textures, and a related well developed metal zonationdefined by a sequence from Fe-Cu to Cu-Zn-Pb to Pb-Zn-Ag. The sulfate andFe-oxide are lacking within and proximal to the massive sulfide lenses, suggestingvery low SO42-concentrations in the hydrothermal solutions during mineralization.Trace elements chemistry of the host andesitic tuff and basalt show clearlyfractionated REE pattern and enriched in LILE (e.g., Rb, Th, U and LREE) and mostHFSE (e.g., Nb, Ta, Hf and Zr), which are similar to those of the oceanic island basalt(OIB) reservoir. Sulfur isotope compositions of the sulfides are concentrated (δ34S‰=–2.1to3.5‰and averaging0.2‰, n=40) and consistent with sulfur derivationfrom leaching of the footwall volcanic rocks and/or degassed directly from magma.Lead isotopic compositions of sulfides are homogenized and composed of18.486-18.684,15.668-15.712, and38.725-39.024for206Pb/204Pb,207Pb/204Pb, and208Pb/204Pb respectively. The extremely enriched values resemble those of the hostvolcanic rocks, suggesting that the lead (ore-forming metals) are derived fromleaching of the underlying rock pile and/or discharged directly from a related magma. All these enriched compositions are similar to those of the global oceanic sediments(GLOSS) and EMII reservoir, implying the possible involvement of upper continentalcrust (UCC) component in the magma chamber. The sedimentary record of the hoststrata and tectonic history of the Changning-Menglian ocean placed Laochang at apelagic environment far from continents.Three important VMS-metallogenic provinces include Northern Qilian,Sanjiangand Altai provinces in China. Cu-Zn deposits of Urals are major types of ChineseVMS deposits, relative to VMS deposits of the world, have three epochs of large scalemineralization of Proterozoic, Palaeozoic and Mesozoic. Chinese VMS depositsformed in a wide range of paleotectonic settings and depositional environments, suchas island-arcs, rifts and ocean ridge hot spots. The Laochang polymetallic depositformed in island-arc hot spots,unlike the four typical classification VMS deposits ofthe world,characterized by Pb-rich and lacked the sulfate and Fe-oxide.Laochangrepresents a new type of volcanic massive sulfide Pb-Zn-Cu deposit.