“Fuchsite” and Chlorite in Carbonated Komatiites at Kerr-Addison, Ontario, Canada: Genesis of Listvenites in Orogenic Gold Settings

Abstract The empirical association between Cr-bearing green micas (i.e., muscovite var. fuchsite and mariposite) and gold deposits has been known for more than a century, but details regarding the genetic relationship between the two remain unknown. Using the 513 t Au Kerr-Addison deposit (Virginiatown, Ontario, Canada) as a case study, this contribution seeks to (1) characterize and model the progressive hydration and carbonation of komatiites using petrography and thermodynamic modeling; (2) investigate the distal to proximal variations in mineral chemistry of spinel-, chlorite-, and dioctahedral mica-group minerals using scanning electron microscopy–energy dispersive spectroscopy and shortwave infrared reflectance spectroscopy; and (3) evaluate how Cr is mobilized from precursor minerals and ultimately incorporated into muscovite using modal analysis and scanning electron microscopy–energy dispersive spectroscopy. Results show: The observed alteration assemblages of komatiitic protoliths (i.e., hydrated komatiite, regional carbonation, talc-chlorite, carbonate-chlorite, and carbonate-muscovite facies), as well as observed reaction textures (tremolite → dolomite, talc → magnesite + quartz, and chlorite → muscovite) are compatible with thermodynamic modeling of progressively carbonated komatiites in the presence of K+ (in the Na2O–CaO–K2O–MgO–Al2O3–SiO2–H2O–CO2, i.e., NCKMASHC, composition space). Komatiitic protoliths exhibit different initial alteration assemblages compared to harzburgitic protoliths because komatiites have higher bulk-rock SiO2 contents than harzburgites. This highlights the importance of host-rock control on observed alteration mineral assemblages, as well as mechanisms for gold deposition (e.g., sulfidation versus dolomitization).Exploration implications for mineral chemistry. All chlorite-group minerals are clinochlore in composition and display a distal to proximal zonation of increasing Al content via Tschermak substitution. This zonation correlates with an increasing shift of the ∼2250 nm peak position in shortwave infrared reflectance spectra. The trend is consistent with the progressive carbonation of clinochlore under increasing log fCO2 conditions. Muscovite shows an inconsequential increase in Al content toward proximal zones, a feature which may be advantageous for global-scale comparisons between different deposits and environments.Source and pathway for chromium. Two sources of Cr are present: silicates (olivine and clinopyroxene, ∼55%) and spinel-group minerals (magnesiochromite to chromite, ∼45%). Both break down early in the alteration sequence (within the hydration, regional carbonation, and talc-chlorite facies), releasing Cr to form clinochlore. With increasing log fCO2, clinochlore is replaced by muscovite, which inherits its Cr contents. Results indicate that (1) muscovite var. fuchsite is dominantly derived from the carbonation of Cr-bearing clinochlore under high log fCO2conditions (>2.3–2.7 log units) in the presence of K+; (2) Al content in clinochlore can be used as a proxy for fluid–rock reactions, and the trend may potentially be mapped on the deposit scale using shortwave infrared reflectance spectroscopy; and (3) Cr is mineralogically mobile, being sourced from primary silicates and oxides, retained in clinochlore, and finally inherited in muscovite in proximal settings.