四川会理天宝山矿床深部新发现铜矿与铅锌矿的成因关系探讨

The genetic relationship between Cu-and Zndominant mineralization in the Tianbaoshan deposit,Southwest China

天宝山矿床是川滇黔接壤铅锌矿集区内的代表性铅锌矿床之一,赋存于上震旦统灯影组白云岩中。近年来,该矿床深部发现了以铜为主的矿化,甚至形成铜矿体。目前,对铜矿成因及其与铅锌矿的关系尚不清晰。本文报道新发现铜矿的矿物学和同位素地球化学资料,以期揭示铜矿的成矿物质来源,结合铅锌矿的相关研究,探讨铜矿形成机制及其与铅锌矿的成因关系。镜下观察和扫描电镜分析显示,铜矿石中主要金属矿物为黄铜矿和银黝铜矿,其次为方铅矿和毒砂,含少量闪锌矿和黄铁矿;铅锌矿石中闪锌矿是主要金属矿物,方铅矿和黄铁矿次之,含少量黄铜矿和深红银矿。铜矿石中闪锌矿主要呈半自形-他形粒状,与黄铜矿共生或被其包裹,方铅矿主要呈细脉状充填在黄铜矿、银黝铜矿和毒砂的裂隙中或呈他形粒状分布在这些矿物中;铅锌矿石中黄铜矿主要呈浸染状分布于闪锌矿之中。两类矿石金属矿物的组构特征,显示铜矿物与铅锌矿物具有密切的共生、穿插和包裹关系,应属同期成矿。同位素地球化学数据显示,铜矿石中黄铜矿的δ34SCDT值为3.9‰~4.2‰(均值为4.1‰,n=3),铅锌矿石闪锌矿的δ34SCDT值为3.3‰~3.9‰(均值为3.5‰,n=3),十分相近,暗示它们具有相似的S源,应均属赋矿地层海相蒸发岩中硫酸盐热化学还原作用的产物。铜矿石中黄铜矿的^(206)Pb/^(204)Pb=18.441~18.476(均值为18.461,n=3),^(207)Pb/^(204)Pb=15.731~15.751(均值为15.741,n=3),^(208)Pb/^(204)Pb=38.809~38.873(均值为38.849,n=3),μ=9.72~9.76;铅锌矿石中方铅矿的^(206)Pb/^(204)Pb=18.442~18.480(均值为18.455,n=3),^(207)Pb/^(204)Pb=15.746~15.763(均值为15.752,n=3),^(208)Pb/^(204)Pb=38.793~38.892(均值为38.840,n=3),μ=9.75~9.78。两者具有相近的Pb同位素组成且其壳源特征明显,表明它们的成矿金属来源相似,均来自上地壳,与赋矿沉积岩有关。综上,矿物学和同位素地球化学证据表明,天宝山矿床深部新发现铜矿与铅锌矿具有明显的同期共生关系和相似的成矿物质来源,是同一成矿热液体系不同阶段演化的产物。天宝山铜铅锌矿床与MVT矿床的成矿特征不同,暗示其成矿作用(环境)特殊,可能与矿床所处的地质背景有关,其成因认识对川滇黔接壤区同类型矿床深部找铜矿具有重要的指导意义。

The Tianbaoshan, a typical Zn-Pb deposit in the Siehuan-Yunnan-Guizhou （SYG） Pb-Zn metallogenic province, is hosted by dolostone of Upper Sinian Dengying Formation. Recently, a Cu-dominant mineralization was discovered in the deep of the Tianbaoshan deposit and even formed Cu ore body. However, the origin of the Cu ores and the genetic relationship between Cu- and Zn-dominant mineralization are still unclear. This paper presents the mineralogical and isotope geochemical data of the Cu ores, integrates with those of Zn-Pb ores, in order to revealing the sources of ore-forming mineralized elements, and discussing its ore genesis and the genetic relationship with Zn-Pb ores. The microscopic observation and scanning electron microscope （SEM） analysis indicate that Cu ores are mainly composed of chalcopyrite and freibergite, followed by galena and arsenopyrite, with a small number of sphalerite and pyrite. Sphalerite is the main sulfide mineral in Pb-Zn ores, followed by galena and pyrite, with a little of chalcopyrite and pyrargyrite. In the Cu ores, sphalerite occurs as subhedral-xenomorphic granular and coexists with chalcopyrite or enclosed by ehaleopyrite grain, while galena fills into the fracture of chalcopyrite, freibergite and arsenopyrite as veinlet or occurs as xenomorphic granular that was enclosed by those minerals. In the Pb-Zn ores, chalcopyrite presents as disseminated structure and distributes in sphalerite. The mineralogy, textures and structures of metal minerals in Cu- and Zn-dominant ores suggest that chaleopyrite has closely symbiotic, inserted and enclosed relationship with sphalerite and galena, and they should be formed at the same time. δ34ScDT values of chalcopyrite from Cu-dominant ores range from 3.9‰to 4. 2‰ （mean 4. 1‰, n = 3 ） , similar to those of sphalerite from Zn-dominant ores （3.3‰ to 3. 9‰, mean 3. 5‰, n =3）, indicating that the sources of reduced sulfur for the two types of ores are similar and all are mainly related to the sulfate in evaporites within ore-hosting strata. 206pb/204Ph, 207 pb/204 Pb and 208 pb/204 Pb values of chalcopyrite from Cu-dominant ores range from 18. 441 to 18.476 （mean 18.461, n =3）, 15. 731 to 15. 751 （mean 15. 741, n =3）, and 38. 809 to 38. 873 （mean 38. 849, n =3） with μ values range from 9. 72 to 9. 76, similar to those of galena from Zn-dominant ores （206pb/ 204 Pb = 18. 442 - 18. 480 （ mean 18.455, n = 3 ）, 207 pb/204 Pb = 15. 746 - 15. 763 （ mean 15.752, n = 3 ）, 208 pb/204 pb = 38. 79338. 892 （ mean 38. 840, n = 3）, and μ = 9. 75 - 9.78 ）. This reveals a clearly crustal source of Pb, and Cu-dominant ores have the same metal sources to the Zn-dominant ores, which are all related to upper ernst （the ore-hosting sedimentary rocks）. Studies on mineralogy, textures and structures, S and Pb isotope geochemistry demonstrate that the sources of newly found Cu ores and Pb-Zn ores are the same, and the two types of ores were belonged to one hydrothermal system and they were the produces formed at the same age with different ore-forming stages. The ore genesis of the Tianbaoshan Cu-Pb-Zn deposit is absolutely different from MVT deposit, suggesting a special mineralization （environment） related to local geological setting. This paper has vital guiding significance for Cu ore prospecting in the deep of the Pb-Zn deposits in the SYG province, western Yangtze Block, SW China.