Abstract:

The Burnt Timber gold deposit is located in the Paleoproterozoic Lynn Lake greenstone belt of the Trans-Hudson Orogen, Manitoba. The deposit is in the Johnson shear zone and is hosted in LREE enriched and depleted tholeiitic volcanic arc basalts. eNd values for these basalts range from 3.3 to 4.7, which is typical of rocks of oceanic island arc affinity of this age. The Johnson shear zone is a regional, belt-parallel structure characterized by an intensification of the regional S2 foliation. At the Burnt Timber deposit, S2 is folded by centimeter- to metre-scale, Z-shaped, F3 chevron folds that have an axial-planar S3 crenulation cleavage. Within the 20 m to 30 m wide interior of the shear zone, S2 and F3 are transposed parallel to S3 and the volcanic rocks are strongly sheared parallel to S3. The presence of a steeply plunging stretching lineation, together with dextral shear sense indicators along both S2 and S3 on horizontal surfaces, suggest that the Johnson shear zone is a dextral transpression zone.<br/>Gold occurs in pyritic, biotitic, chloritic, and carbonatized mafic volcanic rocks along the core of the shear zone. Mylonitic mafic volcanic rocks contain localized high-grade zones (>3 g/t Au) that differ from wider, carbonatized, low-grade zones (<3 g/t Au) by the presence of deformed gold-bearing quartz-pyrite veins and by stronger chlorite and albite alteration. High-grade zones are also associated with an intensification of quartz-pyrite veins that cut across sericitized and carbonatized feldspar porphyry dykes that acted as brittle competent bodies during shearing. Gold deposition was controlled by the channeling of hydrothermal fluid along the core of the shear zone during shearing of the volcanic rocks and brittle fracturing of the porphyry dykes. An altered and mineralized feldspar porphyry intrusion, 1 km west of the Burnt Timber deposit has a U/Pb zircon upper intercept age of 1814 + 15 Ma, providing a maximum age of 1829 Ma for gold mineralization. Unmineralized basalts proximal to the deposit are enriched in biotite and contain anomalous molybdenum values. This enrichment in biotite and molybdenum extends the size of the exploration target beyond the mineralized zones and provides a new exploration tool to aid discovers of new gold deposits along the Johnson shear zone.