Spatial distribution patterns of sulfur isotopes, nodular carbonate, and ore textures in the McArthur River (HYC) Zn-Pb-Ag deposit, Northern Territory, Australia

Abstract

The HYC Zn-Pb-Ag deposit at McArthur River is the largest known and least deformed Australian sedimenthosted stratiform base metal deposit. A study of mineralogical, geochemical, and isotopic zonation through the deposit reveals concentric distribution patterns in (1) the occurrence of nodular carbonate, (2) the δ34S composition of sphalerite, and (3) microscopic ore textures. The correlation of previously documented lateral metal zonation with these other zonation patterns precludes exclusive postsedimentation mineralization and provides some insight into the mechanisms and controls on synsedimentary to early diagenetic mineralization. This mineralization timing is also supported by new carbon and oxygen isotope analyses, recognition of two stages of texturally and isotopically distinct sphalerite, and reinterpretation of complex sulfide textures. Dolomitic carbonate nodules occur in the south, southwest, northwest, and northern fringes of the deposit and are interpreted to have surrounded the high-grade ore lenses prior to structural and erosive truncation. The carbon and oxygen isotope composition of these carbonate nodules is similar to the sedimentary dolomite within the ore lenses. There is no evidence for incorporation of light carbon derived from oxidation of organic material under closed-system conditions and the nodular carbonate probably formed under open-system conditions in communication with seawater. These nodules displace siltstone laminae and, therefore, likely formed in the very shallow subsea-floor environment. In ore lens 3, the δ34S of laminated early sphalerite (sp1) changes from a mean value of 5 per mil in the deposit center to a mean of 1.7 per mil in the extreme fringes. We interpret this to be the result of sulfide precipitation in a restricted marine basin. Later sphalerite (sp2) associated with nodular carbonate has a mean δ34S value of 9.8 per mil, whereas laminated early sphalerite (sp1) has a mean δ34S value of 3.8 per mil. This isotopic separation of the two paragenetic stages of sphalerite is found in immediately adjacent aggregates. Concentration of 34S in late sphalerite (sp2) is likely the result of closed-system conditions in the sediment pore fluid. Textures of laminated sphalerite change from strongly anastomosing in the central part of the deposit to plane laminar and patchy in the deposit fringes. Mass-balance calculations preclude substantial carbonate dissolution and a stylolitic origin for these textures. Instead we propose that rapidly precipitated sphalerite coagulated and trapped pelagic silt and early pyrite (py1) as it was deposited on the sea floor. In contrast, paragenetically late sphalerite (sp2) and pyrite (py2) must be diagenetic as they overprint and are pseudomorphous after the carbonate nodules. We propose a repeating paragenetic sequence of galena/sphalerite (sp1) → pyrite (py1) → nodular carbonate → sphalerite (sp2) → pyrite (py2), which accounts for all the textural complexity and isotopic disequilibrium between sulfide phases. The data presented in this paper suggest that base metal sulfides formed both in the water column and in the uppermost sediment pile. Biological and thermochemical sulfate reduction probably occurred simultaneously in different parts of a complex physicochemical system in which stratification of the marine environment is seen as the primary control on the lateral distribution of the mineralized facies. We propose a stratified water body in which sharp internal chemical gradients separate a surficial oxic layer, an anoxic layer, and a basal hypersaline brine pool. Asymmetric metal zonation across the deposit reflects individual pulses of metalliferous fluid that were introduced into the basin as a bottom-hugging dense current.