Controls on volcanogenic massive sulphide deposits in the Kamiskotia area, Timmins, ON

Abstract

The Kamiskotia area is a volcanogenic massive sulphide (VMS) district in the Abitibi greenstone belt. Past-producing VMS deposits in this area are hosted by the Neo-Archean Blake River assemblage and share several similarities (e.g., comparable bimodal host lithologies, alteration signatures, deposit morphology, stratigraphic position, and ore assemblages) to suggest further mineralization potential. However, no significant deposit has been uncovered since the initial discoveries of the past-producing mines in the 1920s and traditional prospecting is inhibited by relatively low outcrop exposures. This study seeks to describe in detail the geochemical characteristics of >10,000 lithogeochemical data points and to determine if the volcanic units in the study area have geochemical signatures characteristic of VMS mineralization. This study also uses mineral prospectivity mapping (MPM) to identify potential exploration targets. Random forest (RF) was used to integrate predictor maps from lithologic, structural, geophysical, and geochemical data prepared in both continuous and binary surface map formats. Lithogeochemical analyses reveal two general mafic groups (1 and 2) and four rhyolite groups (1– 4) for most assemblages, based on TiO2, P2O5, and Zr plots. The mafic to intermediate rocks are predominantly tholeiitic to transitional basalts and basaltic andesites with MORB-like magma affinities based on the TiO2/Yb proxy. The Th/Nb ratio suggests an interaction of the mafic magmas with existing hydrated crust, resulting in crustal contamination, except for Tisdale mafic samples. Mafic rocks with an evolved signature (i.e., P2O5 > 0.3 wt %, TiO2 > 2.2 wt %, high rare earth element concentrations, and relatively flat chondrite normalized patterns) were also distinguished from their primitive counterparts, except for Tisdale mafic samples. Felsic samples are mainly tholeiitic to transitional rhyolite-dacites, with more fractionated rare-earth element patterns. The rhyolites are predominantly of the fertile high-silica-Zr, FII-FIII type. These geochemical signatures are associated with fertile bimodal-mafic VMS systems. Computed mass changes and alteration indices show that mafic samples and rhyolites from Upper and Lower Blake River assemblages have the greatest degrees of hydrothermal alteration. The probability map from continuous predictor maps (continuous MPM) and binary predictor maps (binary MPM) showed high overall classification accuracies (i.e., > 85 %), success rates of classification and prediction, and area under the curve (AUC) on efficiency curves. The success rates and AUCs obtained were higher for the binary MPM than the continuous MPM, suggesting that binary predictor maps outperform continuous maps. The binary MPM was, therefore, selected as the best performer. Ten areas with probabilities greater than 90 % were highlighted as the most prospective areas, out of which six were interpreted as new potential targets away from past-producing mines that may be prime for follow-up. RF ranks predictor maps from subvolcanic-synvolcanic intrusions and faults, mafic and felsic volcanic lithologies, Bouguer gravity and its derivatives, high-Zr rhyolites, evolved mafic rocks, Cu, Zn, and chloritization indices as the most important parameters to consider for follow-up studies in these areas. The results underscore the usefulness of RF MPM in integrating multiple geoscience datasets to map VMS prospectivity and exhibit the potential for new discoveries in the Kamiskotia area. Based on the presence of (i) contaminated, MORB, and Fe-Ti mafic signatures, (ii) high-Zr, high-silica, and FIII rhyolites with WPG signatures, (iii) high alteration indices (AI and CCPI) with corresponding high mass change intensities, and (iv) high RF probability areas, it is likely that the Upper Blake River, Lower Blake River, and Upper Kidd-Munro assemblages have the best potentials for VMS followed by Lower Kidd-Munro and Deloro. The Tisdale assemblage in the study area may have the lowest potential.