滇西云县-景谷火山弧带官房铜矿床成因:流体包裹体及年代学证据

官房铜矿位于云县-景谷火山弧带北段,矿体主要呈浸染状、网脉状赋存于小定西组杏仁状玄武岩、玄武质角砾岩及其断裂破碎带中.本文采用LA-ICP-MS方法,首次获得小定西组玄武岩锆石U-Pb年龄为234.3±0.8Ma,表明赋矿地层小定西组形成于中三叠世,而不是前人一直认为的晚三叠世.通过矿石中石英、方解石的流体包裹体形貌特征、均一温度和成分的研究,并结合矿区矿石矿物特征,认为官房铜矿至少存在两期不同的成矿作用,即中三叠世火山-次火山热液成矿和后期(可能为新生代)地下水热液叠加改造.结合云县-景谷火山弧的形成演化过程,认为区域中三叠统小定西组玄武岩和芒怀组流纹岩构成了“双峰”式火山岩组合,官房铜矿产于中三叠世裂谷盆地的基性火山岩中,受海陆交互相的古地理环境制约,早期成矿具有VHMS型成矿特征,由于缺乏火山热液成矿流体聚集的“卤水池”,仅发育典型VHMS矿床下部火山通道相中的脉状-网脉状-角砾状矿体,缺少上部厚大的层状矿体,并经历了后期地下水热液的叠加改造.

Genesis of the Guanfang copper deposit in the Yunxian-Jinggu volcanic arc, western Yunnan: Evidences from fluid inclusions and geochronology

The Guanfang copper deposit is located in the north part of Yunxian-Jinggu volcanic arc. Disseminated Cu ores and veinlets are hosted in the Xiaodingxi Formation almond-shaped basalt, basaltic breccia and associated fractures. We first report a precise zircon U-Pb age of 234.3±0.8Ma of the basalt from the Xiaodingxi Formation. This age indicates that the Xiaodingxi Formation is Middle Triassic, different with previously accepted of Late Triassic. Petrography, microthermometry and Laster Raman spectra studies on fluid inclusions in quartz and calcite indicate that at least two mineralizing episodes occurred in the Guanfang Cu deposit: the volcanic-subvolcanic hydrothermal mineralization during Middle Triassic and the groundwater hydrothermal (possibly genozoic) overprinting. In combination with the evolution history of the Yunxian-jinggu volcanic arc, we conclude that the Middle Triassic Xiaodingxi basalt and Manghuai rhyolite formed bimodal volcanic rock assemblage. The Guanfang Cu deposit occurs in the mafic volcanic rocks in rift basin with marine-continent-alternating sedimentary conditions. It has VHMS features in the early mineralization stage. Due to lack "brine pool", the place that hydrothermal ore-forming fluids accumulation, the Guangfang Cu deposit only occurs veinlet, networks and breccia bodies in volcanic path at the deposit bottom but no massive stratiformed ore bodies at the surface. The deposit overprinted by groundwater hydrothermal fluids at the late mineralization stage.