Stratigraphy, Structure and Geochronology of Archean Lode Gold Deposits in the Southeastern Rice Lake Greenstone Belt, Southeast Manitoba

Abstract

The Central Manitoba mine trend is one of the most important lode gold camps in the Rice Lake greenstone-granitoid belt of the western Uchi Subprovince within the western Superior Province, Manitoba, Canada. Neoarchean host rocks consist of a south-facing volcano-sedimentary succession (2.75–2.73 Ga) intruded by voluminous gabbroic sills and tonalitic-granodioritic plutons (2.73–2.72 Ga), as well as late aplite dikes (2.73–2.72 Ga) and quartz-feldspar porphyry dikes (2.73–2.71 Ga). Five generations of deformation structures have been recognized through detailed geological mapping. The entire succession was folded during early deformation prior to rare late aplite dike emplacement. All fault-fill veins and extension veins cut all lithologic units, and are structurally governed by late conjugate shear zones. Main gold mineralization occurs within fault-fill veins hosted by west-trending steeply-dipping dextral brittle-ductile and ductile shear zones, which occur along or across contacts of metabasalt, metagreywacke and metagabbro or entirely within metagabbro. Microstructural and paragenetic analyses on main gold-bearing veins have revealed that gold is intimately associated with quartz, pyrrhotite and tellurobismuthite. Main gold introduction is interpreted to have taken place contemporaneously with pyrrhotite and tellurobismuthite deposition early during dextral shearing.

The Ogama-Rockland gold deposit consists of shear zone-associated quartz veins hosted by the Ross River pluton, a ca. 2728–2724 Ma tonalitic-granodioritic intrusion in supracrustal rocks (< ca. 2745– 2731 Ma) of the Bidou assemblage within the Rice Lake greenstone belt. The plutonic host rocks contain NE-to-ENE–trending, steep, early foliation that is overprinted by late conjugate sets of W-to-NW–trending dextral and N-to-NE–trending sinistral shear zones. Early gold introduction associated with folded veins and low grade planar extension veins occurred before shearing. Main gold mineralization and remobilization are associated with highly auriferous NW-trending steep shear veins. They were likely emplaced late during dextral shearing. Most of the native gold occurs in fractures and grain boundaries of quartz, pyrite and chalcopyrite-bornite in shear veins, which reflects a strong structural and mechanical control on the microscopic scale.

Early gold mineralization associated with low grade planar veins at the Ogama-Rockland deposit is constrained at ca. 2728 Ma, based on U-Pb zircon ages from two phases of host tonalite (2728.7 ± 0.7 Ma; 2728.2 ± 0.6 Ma) and one aplite dike (2727.9 ± 1.6 Ma) that cuts the early low grade veins. A Re-Os molybdenite age of 2727.4 ± 4.8 Ma was obtained from one vein that cuts aplite dikes and sheeted veins, which agrees well with the U-Pb zircon ages of the dated tonalite phases and the aplite dike. The dated tonalite phases, aplite dike and the dated vein are all cut by late ductile or brittle-ductile shear zones, indicating their emplacement, and possibly early gold mineralization, occurred before dextral shearing. The close time association and high Re concentration in the dated molybdenite suggest that granitoid magmas were the source of mineralizing fluids for early gold mineralization. High grade gold mineralization in dextral shear-zone–hosted veins represents a later gold introduction and/or remobilization during dextral shearing.