云南富乐铅锌矿床中铜矿物的矿物学特征及地质意义

Mineralogical Characteristics and Geological Significance of Copper Minerals in Fule Pb-Zn Deposit, Yunnan Province, China

MVT型铅锌矿床中矿物组成一般较简单,铜矿物非常少见。云南富乐铅锌矿床是川滇黔MVT型铅锌成矿域中代表性大型铅锌矿床,其赋矿层位为该区最新地层—中二叠统阳新组白云岩,矿体距上覆峨眉山玄武岩不到160 m。通过矿相、扫描电镜及能谱等分析测试,本研究在该矿床中发现了大量铜矿物,主要包括以下四类,即黄铜矿、锌砷黝铜矿、黝铜矿和孔雀石,这些铜的独立矿物常交代闪锌矿和黄铁矿等矿物,形状多为环带状、脉状及不规则状等,部分黄铜矿呈乳滴状分布于闪锌矿颗粒内部或呈他形交代闪锌矿,可能与闪锌矿同时形成,锌砷黝铜矿和黝铜矿呈他形细脉状穿插于闪锌矿或分布于闪锌矿边缘及孔洞中,暗示这些铜矿物形成略晚于铅锌成矿。上述铜矿物常见于中低温热液铅锌矿床,其中锌砷黝铜矿是硫盐矿物中较罕见的矿物,黝铜矿和锌砷黝铜矿的出现指示相对氧化的成矿环境,而孔雀石是在铜矿物的氧化过程中形成的次生矿物。研究表明,本矿床矿石矿物的生成顺序为:黄铁矿→闪锌矿(乳滴状黄铜矿)→方铅矿→黄铜矿→锌砷黝铜矿→黝铜矿→孔雀石,结合矿床产出的地质地球化学特征,云南富乐铅锌矿床中铜可能有两个来源:早期的乳滴状黄铜矿与铅锌矿同期且均来自基底地层——昆阳群;后生铜矿物(黄铜矿、黝铜矿和锌砷黝铜)主要来源于上覆峨眉山玄武岩,这与铅锌主要来源于昆阳群等基底地层有所差异,研究成果为认识川滇黔地区铅锌成矿作用与峨眉山玄武岩关系提供了新的地球化学依据。

The mineral assemblage in the MVT Pb-Zn deposits is generally simple because the copper minerals of this type of deposit are very rare.As one of the largest MVT deposits in the Sichuan-Yunnan-Guizhou(SYG)Pb-Zn metallogenic domain,the Fule Pb-Zn deposit is hosted in the Permian Yangxin Formation dolostone,which is the youngest ore-bearing strata in the area,and the distance between Pb-Zn ore bodies and the Permian Emeishan basalts is less than 160 m.By means of microscopic,Scanning Electron Microscopy(SEM)and Energy Dispersive Spectrum(EDS)analysis,a great number of copper-bearing minerals were found in the deposit,including chalcopyrite,Zn-tennantite,tetrahedrite and malachite.These minerals occur as banded,vein and irregular,and replace sphalerite or pyrite.The research shows that the droplet chalcopyrite is distributed within sphalerite,implying that both of them were formed at the same mineralization stage.The Zn-tennantite and tetrahedrite,occurring as irregular veins,are cross-cutting the sphalerite grain,or distributed at the edge of the sphalerite,indicating that it was formed later than that of Pb-Zn mineralization.The copper minerals mentioned in this study commonly occur in middle-low temperature hydrothermal deposits,in which the Zn-tennantite is a rare sulfosalts.The appearance of Zn-tennantite and tetrahedrite indicates an oxidizing environment,and malachite is a secondary mineral formed at supergene weathering stage.The paragenetic sequence in the deposit is inferred as follows:pyrite→sphalerite(droplet chalcopyrite)→galena→chalcopyrite→Zn-tennantite→tetrahedrite→malachite.Combined with the geologic and geochemical characteristics of this deposit,it is suggested that there are two possible sources of copper.Early-stage copper was derived mainly from the basement of the Kunyang group and the late-stage copper was likely stemmed from the upper Emeishan basalts,which are different from those of Pb and Zn derived mainly from the Kunyang Group in the deposit.The research provids new geological and geochemical evidence for understanding the genetic relationship between the Emeishan basalts and Pb-Zn mineralization in the SYG district.