兰坪盆地白秧坪银铜多金属矿集区中银、钴、铋、镍的赋存状态与成因意义

Occurrence of Ag,Co,Bi and Ni elements and its genetic significance in the Baiyangping silver-copper polymetallic metallogenic concentration area,Lanping basin,southwestern China

白秧坪银铜多金属矿集区位于兰坪盆地北部。矿集区可分为东、西两个成矿带。赋矿地层主要为上三叠统三合洞组碳酸盐岩、第三系始新统保相寺组碎屑岩和下白垩统景星组碎屑岩。矿体主要以脉状、网脉状及透镜状形式产出。作者通过显微镜观察、电子探针和扫描分析等综合分析技术,确认白秧坪银铜多金属矿集区中矿物组成相当丰富,已鉴定出的矿物超过50种,既有大量硫化物、硫盐、氧化物、硫酸盐、碳酸盐,又有自然金属及金属互化物、卤化物等。除常见矿物为黄铁矿、毒砂、白铁矿、黄铜矿、方铅矿、闪锌矿、黝铜矿、砷黝铜矿、铜蓝、斑铜矿、辉铜矿、雌黄、菱铁矿、方解石、铁白云石、重晶石、天青石和石英外,作者还鉴定出一些银、钴、铋、镍、砷、锑的矿物,如自然铋、辉铋矿、辉银矿、辉砷钴矿、硫钴镍矿、硫铜铋矿、硫铋铜矿、辉砷镍矿、车轮矿、硫砷铜矿、单斜硫砷铅矿、灰硫砷铅矿等。矿石中矿物种类较多,组成较复杂,存在Co,Bi,Ni等元素的矿物,构成白秧坪银铜多金属矿集区的一大特色。在兰坪盆地白秧坪银铜多金属矿集区各矿段内,除了Cu、Pb、Zn构成工业矿体外,矿石中Ag、Co、Ni、Bi及As、Sb、Ba等元素的含量也相当高,可作为Cu-Pb-Zn-Ag-Co-Ni-Bi矿石来综合开发利用。白秧坪银铜多金属矿集区中Ag、Co、Ni、Bi等元素富集条件为低温、中低盐度,形成压力较小的浅成环境;成矿流体是一种富含CO2的Ca2+-Na+-SO24-Cl-类型、由大气降水演化而成的盆地热卤水。成矿物质主要来源于含有基性火山岩的兰坪盆地基底变质岩系。

The Baiyangping silver-copper polymetallic ore concentration area,occurring in the north segment of the Lanping basin,southwestern China,can be divided into two ore zones. The eastern ore zone consists of Dongzhiyan,Yanzidong,Huishan,Heishan,Xinchangshan,and Huachangshan ore blocks,whereas the western ore zone is composed of Baiyangping,Fulongchang and Wudichang ore blocks. They exist mainly in the limestone and dolomite of the Upper Triassic Sanhedong Formation,sandstone and fine-grained sandstone of Lower Cretaceous Jingxing Formation,and sandstone of the Eocene Baoxiangsi Formation. The orebodies mostly occur in veined,network and lensticular forms. Examination of ores by optical microscope,electron probe,X-ray surface scanning,has revealed that the deposits are quite complex in mineralogy. Up to now more than 50 minerals have been identified. There include not only sulfides and sulfosalt of Cu,Pb,Zn,As and Sb-bearing minerals,a variety of oxides,sulfate,and carbonate minerals but also native metals,intermetallic compounds and halide. Among the most common minerals are pyrite,arsenopyrite,marcasite,chalcopyrite,galena,sphalerite,tetrahedrite,tennantite,coveline,bornite,chalcocite,orpiment,siderite,calcite,ankerite,barite,celestite,and quartz. It is important that the authors have identified some minerals of Ag,Co,Ni,Bi,As,and Sb such as native bismuth,bismuthinite,argenite,cobaltine,siegenite,emplectite,wittichenite,gersdorffite,bournonite,enargite,dufrenysite,and jordanite. The diversity of mineral species in the ores and the extensive occurrence of Co,Bi,Ni-bearing minerals are the outstanding features of this type of the silver-copper polymetallic ore concentration area. The separate orebodies of Cu,Pb,and Zn can be delineated locally in the silver-copper polymetallic ore concentration area. What is particularly important is that in their study of the ore deposits,the authors found a strong enrichment of Ag,Co,Bi,Ni,As,Sb,and Ba. Moreover,Ag or Co ore bodies can be delineated locally. As a result,the commercial value of the deposits is expected to be greatly enhanced. The critical conditions for the enrichment of Ag,Co,Ni,and Bi as seen in the ore concentration area at Baiyangping are:(1) the mineralization should take place at a rather shallow depth in the crust,with lower temperature,medium-lower salinity,and lower pressures; (2) ore-forming fluid is Ca2+-Na+-SO2-4-Cl-type basin thermal brine derived from paleo-meteoric water; and (3) the ore-forming elements in deposit may also derive from basic volcanic rock of the basin basement.