Chemical and boron isotopic compositions of tourmaline from the Archean Big Bell and Mount Gibson gold deposits, Murchison Province, Yilgarn Craton, Western Australia

Abstract

Tourmaline is a commonly found gangue mineral in Archean greenstone belt lode-gold deposits worldwide. In this paper, we report major, trace, and rare-earth element (REE) and boron isotopic compositions of tourmaline from two major amphibolite-grade Archean gold deposits (Big Bell and Mount Gibson; Murchison Province, Yilgarn Craton, Western Australia), in an attempt to better understand the hydrothermal gold formation processes that were also responsible for tourmalinization. Tourmalines in these two major deposits all belong to the dravite–schorl series, and tend to be Mg-rich where closely associated with gold and sulfide ores. The ore-related tourmalines also contain high concentrations of ore metals such as Au, Ag, Pb, and Cu. In the Big Bell deposit, tourmalines occur along the foliation in lode schists, and show low Fe/(Fe+Mg) ratios of 0.11–0.30, TiO 2 contents of 0.15–0.73 wt.%, and negligible MnO (<0.05 wt.%). This type of tourmaline has δ 11B values of −17.7‰ to −15.0‰. In the Mount Gibson deposit, the tourmalines closely associated with the sulfide ores are the most Mg-rich, with low Fe/(Fe+Mg) ratios of 0.19–0.22, while tourmalines from garnet-bearing biotite–muscovite schists that overly the lode-gold orebody are more Fe-rich (Fe/(Fe+Mg)=0.30–0.63). Tourmalines from an amphibolite schist are Mg-rich, with Fe/(Fe+Mg) ratios (0.27–0.34) that lie between the above two groups of tourmalines. The compositional trend from Fe-rich tourmalines in the sulfide-poor schists to Mg-rich in the sulfide ores at Mount Gibson is similar to that observed in massive sulfide ore deposits worldwide. Alternatively, the Mg-rich nature of the sulfide-related tourmalines may be due to metamorphic reactions that are accompanied by Fe incorporation into sulfides, leaving coexisting silicates more Mg-rich. The REE geochemistry of the tourmalines from the Mount Gibson ore zone schists displays strong HREE enrichments and positive Eu anomalies with Eu/Eu* ratios up to 2.4, which are similar to tourmalines associated with massive sulfide deposits. The ore-related tourmalines disseminated in the host schists show δ 11B values of −18.6‰ to −17.0‰, whereas one coarse-grained tourmaline vein has a lower δ 11B value of −21.9‰ to −21.4‰. These data support hypotheses which suggest that the schists that host the gold mineralization are the metamorphosed equivalents of a base metal-rich, hydrothermally altered, seafloor horizon that is typical of VMS-style mineralization, with the boron possibly derived from leaching of footwall sedimentary and volcanic rocks during hydrothermal circulation.