Ore-forming processes of the Baisong carbonate-hosted Pb-Zn deposit, SW China: Constraints from in-situ sphalerite trace element and sulfide S-isotopic compositions

Abstract

The western Hunan-eastern Guizhou (WHEG) Pb-Zn ore belt in the southwestern Yangtze block is a main Pb-Zn province in China. The metallogenic characteristics of this ore belt were widely reported, but the ore-forming process is still poorly understood. The lower Cambrian carbonate-hosted Baisong deposit is a representative Pb-Zn deposit in the southern WHEG belt. Sphalerite trace elements and sulfur isotopic compositions from the Baisong Pb-Zn deposit could reveal the ore-forming fluid source and metallogenic processes. Four types of sphalerites have been distinguished (black colloidal sphalerite (Sp) I, light-colored sector Sp II, red zoned Sp III and brown zoned Sp IV, indicating four sulfide mineralization stages from the early ore phase. Sp I (66-117℃, δ34S: +24.8 to + 27.8‰) has the highest Cd-Ga-Ag-Cu contents among the four sphalerite types, and commonly overgrow with marcasite. This suggests that Sp I was formed from acidic fluids under high precipitation rate. The trace elements and sulfur isotopic compositions of Sp II (80-128℃, δ34S: +26.5 to + 28‰) are similar to Sp I. Sp II was likely formed by a decrease in oversaturation and precipitation rate with fluid pH increase. Sp III (127-164℃, δ34S: +31 to + 32.5‰), commonly overgrow on pyrite (Py I), has higher Mn-Fe-Ge but lower Cd-Cu-Ga-Ag contents and heavier sulfur isotopes than Sp II. Changes in precipitation environment and fluid compositions are likely major factors in forming Sp III (temperature increase, pH increase and more reduced). The forming of Sp III and Py I likely consumed the Fe and Ge in the fluid, depriving those elements from the later-formed Sp IV (97-141℃, δ34S: +25.7 to + 29.8‰). The sulfide δ34S value (+22.5 to + 46.3‰) implies that the S2- at Baisong was sourced from seawater sulfates via thermochemical sulfate reduction (TSR). Sphalerite trace element features reflect that the Baisong is an MVT deposit.