Order of sphalerite and galena precipitation: A case study from lead-zinc deposits in southwest China

Abstract

Most of the lead and zinc deposits in Southwest China, are characterized by mineral zoning, which is especially true for the Huize and Zhaotong deposits. The mineral assemblage zoning is consistent for both horizontal and vertical zoning, from the base (center) of the ore body to the top (outermost), the mineral zones are as follows. coarse-grained pyrite and a little puce sphalerite; I-2: brown sphalerite, galena, and ferro-dolomite; I-3: galena, sandy beige and pale yellow sphalerite, and calcite; and I-4: fine-grained pyrite, dolomite, and calcite. Among them, sphalerite is the landmark mineral of different zoning. From I-1 to I-3, the color of sphalerite changes from dark to light, its crystalline size changes from coarse to fine, and its structure changes from disseminated to veinlet. This mineral zoning is seen not only on a microscopic scale, but is also clear on a mesoscopic and macroscopic scale. It is caused by the order of the sphalerite and galena precipitation. We studied the metallic minerals and fluid inclusions using a thermodynamic phase diagram method, such as $$\lg {f_{{{\rm{O}}_2}}} - \lg {f_{{{\rm{S}}_2}}}$$, $${\rm{pH}} - \lg {f_{{{\rm{O}}_2}}}$$, pH−lg[Pb2+] and pH−lg[HS−], discussed the constraints on the order of the sphalerite and galena precipitation in the migration and precipitation process of lead and zinc under different pH values, oxygen fugacity, sulfur fugacity, and ionic activity. We also explain the formation mechanism and propose that the main controlling factor of the order of the sphalerite and galena precipitation is sulfur fugacity.