The use of combined CMg isotope compositions of carbonates from orogenic SbAu deposits as a tracer of fluid interaction with sea-floor altered crust

The Mesoarchaean Murchison Greenstone Belt is composed of a strongly deformed volcano-sedimentary succession metamorphosed up to amphibolite facies and surrounded by metaluminous and peraluminous granitoids in the north of the Kaapvaal Craton of southern Africa. A circa 40 km-long shear-zone of strongly carbonatised rocks (the Antimony Line, or Sb-Line) oriented along the main trend of the greenstone belt (WSW–ENE) hosts important Sb deposits with accessory Au. These commodities mostly occur as stibnite and native gold hosted by quartz‑carbonate rocks at the centre of the Sb-Line, underlining the importance of CO₂-rich fluid flow for mineralisation during deformation and metamorphism.

In this study, we compare the elemental and stable isotope (C-O-Mg) composition of carbonates from the Sb-Line with regionally distributed carbonates hosted by the volcano-sedimentary succession distal to mineralisation. The carbonates of the Sb-Line and regional rocks have overlapping major element compositions. Both magnesite and dolomite in most Sb-Line samples have marked light-REE-depleted patterns with variable positive Eu anomalies. Carbon isotope ratios define two clusters, with marked δ¹³C peaks at ca. -5 ‰ for Sb-Line rocks and ca. -2 ‰ for regional rocks, implying separate C-sources. The peak at ca. -2 ‰ likely represents early carbonatisation through sea-floor alteration, whereas the first peak at ca. -5 ‰ is indicative of deep CO₂ (mantle, or magma-derived) introduced during tectonic activity of the Sb-Line. Magnesium isotope ratios of regional rocks reveal limited fractionation (bulk δ²⁶Mg = −0.27 ± 0.10 ‰) that overlap with mantle values, but some carbonate-bearing veins present ²⁶Mg-depleted compositions (bulk δ²⁶Mg = −1.91 to −0.40 ‰). Sb-Line carbonated rocks have more fractionated Mg isotope compositions (bulk δ²⁶Mg = −0.8 to 0.0 ‰) and carbonates display marked negative ²⁶Mg values (δ²⁶Mg from −1.46 to −0.31 ‰). We interpret these results in terms of preferential remobilisation of isotopically light Mg during fluid-rock interaction and dissolution of carbonate in the host-rocks. The lack of correlation between δ¹³C and δ²⁶Mg indicates decoupling of these isotopic systems, implying contribution from isotopically distinct sources of C and Mg to the mineralised zone.

Similar to other structurally controlled AuSb mineralisation in Archaean greenstone belts, metal transport and ore deposition in the Murchison Greenstone Belt was closely related to the deep cycle of carbon, linking C-draw-down during sea-floor alteration, carbonate re-mobilisation during metamorphism and CO₂ degassing from deep sources along major crustal discontinuities. In contrast to models for orogenic AuSb deposits invoking a purely intra-crustal, metamorphic origin of mineralising fluids, our results underline the importance of deep-sourced fluids originating from an external source from the volcano-sedimentary succession.