Evaporite-bearing orogenic belts produce ligand-rich and diverse metamorphic fluids

Abstract

Detailed petrologic and chemical investigation of mid-amphibolite facies calcareous, scapolite-rich metasedimentary rocks from the Mount Isa region in northern Australia is used to explore changing fluid chemistry with prograde metamorphism. The presence of widespread scapolite with Cl- and variably SO4-rich compositions in upper amphibolite facies rocks makes it unavoidable that the regional metamorphic fluids were locally highly saline and oxidised, and that high salinities persisted throughout metamorphism. Electron microprobe analyses and chemical maps of individual scapolite grains show zoning in Cl and S, likely to reflect buffering of the metamorphic fluid by scapolite during progressive metamorphism. The zoning in Cl and S demonstrates that scapolite has the potential to record changes in fluid chemistry during metamorphism. The variation in scapolite composition between samples, in combination with whole rock geochemistry, shows that different layers within this heterogenous rock package generated fluids of different chemistries. Interaction between scapolite-bearing rocks and externally-derived magmatic or metamorphic fluids that are out of equilibrium drives scapolite breakdown, releasing Cl to the fluid. In the Mount Isa region, metamorphic fluid production was enhanced by periods of magmatism, which promoted development of a regionally extensive and unusually saline fluid system that was active at multiple stages over a 250 million-year period. The highly saline and oxidised fluids formed through interaction with scapolite are well suited to transporting a broad range of metals, and may explain the diverse range of syn-orogenic mineral deposits in the Mount Isa Inlier. Metamorphic belts with large volumes of evaporitic material are ideal for generating a broad spectrum of syn-orogenic hydrothermal ore deposit types - including Fe oxide Cu-Au, Fe sulphide Cu-Au, Mo-Re and U-REE, but lacking the Au-only deposits found in typical orogenic belts. Unlike regions hosting traditional orogenic gold deposits, belts containing evaporitic sequences can preserve Cl-rich minerals such as scapolite in the metamorphosed source region, allowing them to remain active as ore forming systems through relatively high-grade metamorphism and multiple stages of tectonism. Periods of supercontinent breakup, such as the Mesoproterozoic, may have resulted in the formation of large, intracontinental basins well suited to the development of widespread evaporitic sequences. This, in combination with overprinting orogenesis and high temperature magmatism, may have provided the ingredients for widespread ore deposit formation at a global scale.