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

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.