Stable isotopes (O, C) and fluid inclusion study of quartz-carbonate veins from the antimony line, Murchison Greenstone Belt

Abstract

In the homogeneously deformed Murchison Greenstone Belt region, fluids circulations were focused along a localized deformation brittle-ductile zone, and the so-called Antimony Line. In order to trace the origin of the fluid, its composition, the temperature of the precipitation along the Antimony Line, we investigate the stable isotope (oxygen and carbon) and fluid inclusion compositions of the quartz-carbonate veins, as well as chemical analyses of the carbonates and ⁴⁰Ar-³⁹Ar dating on the fuchsite associated with the veins.

Three types of fluid inclusions are recognized for the H₂O-CO₂-dominated (plus minor CH₄-N₂) fluid(s). Microthermometric measurements together with the paragenesis of the veins and host rocks indicate that the pressure-temperature of the precipitation is 350 to 450 °C, 200 to 300 MPa, which is an unusually high temperature for an antimony deposit. The carbonates are Fe-Mg-rich and are characterized by flat MREE and HREE patterns, with slight depletions in LREE. The oxygen isotope compositions of the quartz are rather homogeneous at the scale of the Antimony Line (δ¹⁸O_quartz = 10.9-14.3‰). In detail, subtle differences in the δ¹⁸O values for the quartz and carbonate exist depending on the sampling sites. These differences are likely due to minor temperature and/or host rock geochemistry variations along the Antimony Line. Data point to a metamorphic origin for the fluids responsible for the mineralization, which likely escaped from the nearby greenstone lithologies. Finally, we present an overview of the Murchison Greenstone Belt geological evolution by integrating the antimony metallogeny of the Antimony Line, which can be compared to that of an orogenic gold deposit.