The volcanic and deformation history, geodynamic setting, and metallogenesis of the Chisel Sequence, Snow Lake, Manitoba, Canada

Abstract

The largest Paleoproterozoic VMS district in the world is hosted in the Flin-Flon Glennie Domain of the Trans-Hudson Orogen. Most of the VMS deposits occur within the Flin Flon and Snow Lake arc assemblages of the Flin Flon belt. The Chisel sequence of the ca. 1.89 Ga Snow Lake assemblage is host to six economic VMS deposits; however, its deformation history prior to ca. 1.86 Ga and its ore-forming volcanic environment are poorly constrained. The Photo Lake deposit occurs within the Chisel sequence and exhibits anomalous Au and Cu enrichment when compared to the other Chisel sequence deposits, but the source of this metal enrichment and its stratigraphic position are unknown. Detailed field mapping, including lithofacies and structural mapping, was completed at 1:2,000 scale, with local detailed areas at 1:100 or 1:200 scale. Core from 20 Hudson Bay Exploration and Development (HBED) drill holes were re-logged and an additional 39 drill holes were examined using whole-rock geochemistry and drill logs provided by HBED to trace the extent of units. Eighty-two outcrop samples and 65 drill core samples, including 32 sulfide samples, were analyzed for whole rock major and trace element geochemistry. Eight of the outcrop samples were also analyzed for whole rock Nd, Hf, and Pb isotopes. Scanning electron microscopy provided the semi-quantitative compositions of mineral phases in least altered and altered samples, and quantitative mineral chemical data for pyrite and pyrrhotite were acquired using an electron microprobe. Trace element maps were produced by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for individual pyrite grains in four of the sulfide samples. Detailed field mapping, re-logging of selected drill holes, and whole rock geochemistry completed in this study indicate: 1) the volcanic rocks of the Snow Lake assemblage underwent a folding and thrusting event prior to 1.86 Ga due to intraoceanic accretion or early accretion to the Amisk collage; 2) with the exception of the Threehouse unit, the entire Upper Chisel sequence represents folded and thrust-repeated Lower Chisel sequence strata; 3) the Chisel sequence VMS deposits, including the Photo Lake deposit, formed within a single time-stratigraphic ore interval during a protracted arc-rifting event; 4) Au enrichment in the Photo Lake deposits is syngenetic and can be attributed to a magmatic input; 5) the Anderson VMS deposits formed during widespread arc rifting, whereas the Chisel VMS deposits formed during localized extension above a magma chamber in the extended arc.