Metallogeny of the Shilu Fe–Co–Cu deposit, Hainan Island, South China: Constraints from fluid inclusions and stable isotopes

Abstract

The Shilu deposit is a world-class Fe–Co–Cu orebody located in the Changjiang area of the western part of Hainan Island, South China. The distribution of Fe, Co, and Cu orebodies is controlled by strata of the No. 6 Formation in the Shilu Group and the Beiyi synclinorium. Based on a petrological study of the host rocks and their alteration assemblages, and textural and structural features of the ores, four mineralization stages have been identified: (1) the sedimentary ore-forming period; (2) the metamorphic ore-forming period; (3) the hydrothermal mineralization comprising the skarn and quartz–sulfide stage; and (4) the supergene period. The fluid inclusions in sedimentary quartz and/or chert indicate low temperatures (ca. 160°C) and low salinities from 0.7 to 3.1wt.% NaCleq, which corresponds to densities of 0.77 to 0.93g/cm3. CO2-bearing or carbonic inclusions have been interpreted to result from regional metamorphism. Homogenization temperatures of fluid inclusions for the skarn stage have a wide range from 148°C to 497°C and the salinities of the fluid inclusions range from 1.2 to 22.3wt.% NaCleq, which corresponds to densities from 0.56 to 0.94g/cm3. Fluid inclusions of the quartz–sulfide stage yield homogenization temperatures of 151–356°C and salinities from 0.9 to 8.1wt.% NaCleq, which equates to fluid densities from 0.63 to 0.96g/cm3.Sulfur isotopic compositions indicate that sulfur of the sedimentary anhydrite and Co-bearing pyrite, and the quartz–sulfide stage, was derived from seawater sulfate and thermochemical sulfate reduction of dissolved anhydrite at temperatures of 200°C and 300°C, respectively. H and O isotopic compositions of the skarn and quartz–sulfide stage demonstrate that the ore-forming fluids were largely derived from magmatic water, with minor inputs from metamorphic or meteoric water. The Shilu iron ore deposit has an exhalative sedimentary origin, but has been overprinted by regional deformation and metamorphism. The Shilu Co–Cu deposit has a hydrothermal origin and is temporally and genetically associated with Indosinian granitoid rocks.