The Paleoproterozoic MacLellan deposit and related Au-Ag occurrences, Lynn Lake greenstone belt, Manitoba: An emerging, structurally-controlled gold camp

Abstract

The Paleoproterozoic Era is important for Au-Ag mineralization, including deposits in the Trans-Hudson Orogen of central Canada. The association between Au-Ag deposits and regional structures that formed during this era has seen markedly less research than Archean counterparts. The MacLellan deposit and related Au-Ag occurrences occur within the northern part of the Paleoproterozoic Lynn Lake greenstone belt in northwest Manitoba. Field relationships, combined with a detailed characterization of the host rocks, alteration, and structure, along with mass balance calculations, indicate that the host rocks were largely derived from a common amphibole-plagioclase schist protolith (originally a porphyritic basalt) that was variably deformed and altered to initially biotite ± quartz during D2 deformation, and subsequently to Cr-bearing chlorite (± carbonate, ± amphibole, ± quartz) during the late stages of D2 and D3. Gold-silver mineralization is associated with these alteration events and consists predominantly of native Au, aurostibite (AuSb2), Au in arsenopyrite, and Ag in galena. These can be separated into two distinct Au mineralizing events: 1) early disseminated arsenopyrite aligned with D2 fabrics that contains structurally-bound Au; and 2) late fracture-filling native Au, aurostibite, and Au-rich rims and fractures on and within arsenopyrite. The highest concentrations of Au and Ag occur where E-W trending dextral D2 shear zones intersect NNE-SSW trending brittle faults (D4), suggesting late remobilization of precious metals by hydrothermal fluids into dilatant zones. The structural control on formation and distribution of Au-Ag deposits along the MacLellan Shear Zone is similar to that observed along the regionally significant Johnson Shear Zone, which occurs in the southern portion of the Lynn Lake greenstone belt, suggesting similar processes operated within both portions of the belt. Integrating detailed geologic and structural mapping, petrography, geochemistry, and alteration, demonstrates that these Au-Ag occurrences formed as a result of structurally-controlled mineralizing systems associated with major shear zones, rather than being primarily syngenetic in origin (i.e., seafloor exhalative), as previously suggested. These major Paleoproterozoic shear zones display similar characteristics and relationships to well-documented structural ‘breaks’ in Archean terranes (e.g., Porcupine-Destor Deformation Zone and Larder Lake-Cadillac Deformation Zone), suggesting that, at least in terms of the Lynn Lake region, structural and mineral deposit forming processes may have occurred on comparable spatial scales to those in Archean systems.