Discriminating Between Primary and Secondary Au Events in a Paragenetically Complex Archean Lode-Gold Deposit, Wawa Gold Corridor, Ontario, Canada

Abstract

The Wawa gold corridor, located in the Michipicoten greenstone belt of the Superior province, Canada, comprises Au-bearing shear zones that crosscut the 2745 Ma Jubilee stock and that evolved during protracted deformation (D1-D3). Numerous generations of sulfide minerals crystallized before, during, and after these deformation events, and gold is associated with D1 arsenopyrite, D2 pyrite, and Bi-Te phases and chalcopyrite in assemblages that crosscut D3 veins. Observations of porosity and inclusions in D1 arsenopyrite and D2 pyrite suggest these sulfides underwent coupled dissolution-reprecipitation reactions. By coupling these textural observations with trace element analysis by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), it is evident that such replacement reactions generated gold from Au previously in solid solution. Conversely, textural relationships among paragenetically late gold and Bi-Te minerals are consistent with the precipitation of these phases from Bi-rich polymetallic melts. Mass balance calculations involving comparisons of the mass of Au in sulfides and the total mass of Au in their associated host rocks indicate that only D1 arsenopyrite contained enough Au to account for whole-rock Au content and overall deposit grade. Although D1 arsenopyrite is less volumetrically abundant than the various pyrite generations present in the deposit, it is often replaced by the later pyrite types, which is compatible with higher initial volumes of arsenopyrite than what is presently observed. It is concluded that the D1 Au + arsenopyrite event was the principal Au-mineralizing event in the Wawa gold corridor and that the other gold-bearing assemblages (i.e., gold + D2 pyrite, gold + Bi-Te phases + chalcopyrite) largely represent secondary mobilization of this primary enrichment. Given that LA-ICP-MS sulfide chemistry is regularly used in orogenic Au research, the approach outlined herein to assess the relative impact of distinct Au- and sulfide-mineralizing events could easily be applied to the study of other Au deposits in which complex hydrothermal parageneses are recognized.