Till geochemistry and mineralogy: vectoring towards Cu porphyry deposits in British Columbia, Canada

Abstract

Regional till sampling was completed near four Cu porphyry mineralized zones in south-central British Columbia, Canada: Highland Valley Copper (Cu-Mo), Gibraltar (Cu-Mo), and Mount Polley (Cu-Au-Ag) deposits, and the Woodjam (Cu-Au-Mo) prospect. At all sites, Cu concentrations in the clay-sized fraction and chalcopyrite grains (0.25 – 0.5 mm; >3.2 specific gravity) are found in greater abundance in till near and down-ice from mineralized zones compared to surrounding background regions. At Mount Polley, the abundance of gold grains in till defines a dispersal train extending at least 3 km down-ice (SW and NW) from mineralization. At three sites out of four, epidote in till heavy mineral concentrates occurs in greater percentage near and down-ice from mineralized zones compared to background regions suggesting that this mineral could be an indicator of propylitic alteration associated with porphyry mineralization. The distribution pattern of Cu concentrations and chalcopyrite grains in till is controlled by the distribution of Cu-porphyry mineral occurrences in bedrock and the direction of ice-flow movements which prevailed during the last glaciation. By comparing study sites, there is a positive relationship between the areal extent of bedrock mineralization that was exposed to glacial erosion and the absolute values of Cu concentrations and chalcopyrite grain counts in till. In the Woodjam region where the till is thick (>10 m), eight samples with background Cu concentrations in the clay-sized fraction of till contain >4 grains of chalcopyrite/10 kg which is indicative of mineralization. This study demonstrates that a combination of till geochemistry and mineralogy is an efficient method for mineral exploration for Cu porphyry deposits covered by variable amounts of glacial sediments. Supplementary material: The full data sets on till geochemistry and mineralogy are available at: https://doi.org/10.6084/m9.figshare.c.3291503

Exploration Potential of Fine-Fraction Heavy Mineral Concentrates from Till Using Automated Mineralogy: A Case Study from the Izok Lake Cu–Zn–Pb–Ag VMS Deposit, Nunavut, Canada

“…This suggests that till sample 09-MPB-060 is within the hydrothermal alteration halo of the Izok Lake deposit, which is supported by the extent of hydrothermal alteration described in Morrison [22], who describes a late-stage, widespread zone of calcic alteration containing epidote surrounding the deposit. Epidote has been previously identified as an important indicator in porphyry terranes [8], where epidote abundance and trace element composition can be used in tandem to identify hydrothermal carbonate and propylitic alteration halos and assess ore fertility in porphyry systems. However, the chemical changes associated with carbonate and propylitic alteration lead to bulk-rock compositions similar to those of calc-silicate rocks of sedimentary or metasomatic origin [59] and, therefore, epidote abundance will need to be identified along with other indicator minerals to serve as a vector to VMS mineralization.…”

“…Care must be taken to ensure that accurate regional background abundance is established outside of wide-spread carbonate and propylitic alteration halos. Future work should investigate the use of trace-element compositional analysis of epidote grains to assess terrane fertility, after the work of Plouffe et al [8] on porphyry systems. Fe-oxide minerals can indicate the incorporation of gossanous material into till [72] or the weathering of sulfide grains during transport or following deposition.…”

“…Indicator mineral methods applied to sediment samples are important exploration tools in glaciated terrain for diamonds [1] and gold [2][3][4][5]. More recently, their potential to aid porphyry Cu [6][7][8], magmatic Ni-Cu-PGE [5,9], carbonate-hosted Pb-Zn [10] and volcanogenic massive sulfide (VMS) exploration [11] has been demonstrated.…”

Automated Gold Grain Counting. Part 1: Why Counts Matter!

“…Important discoveries, such as Douay deposit in northeastern Québec or Rainy River mine in Western Ontario [6,7] are abundant. Glacial sediment geochemistry and indicator mineral methods have proven to be efficient in drift prospecting for a wide variety of commodities [9][10][11][12][13]18] and particularly diamonds [14][15][16]. Nonetheless, these methods are prominently used for gold exploration as it represents the most efficient grassroots method, and also because the bulk of investment in mineral exploration is dedicated for gold (i.e., in 2017, 65% of exploration spending in Canada [17]).…”

Rare metal indicator minerals in bedrock and till at the Strange Lake peralkaline complex, Quebec and Labrador, Canada

“…Indicator mineral methods are used in exploration programs and government heavy mineral surveys to evaluate the potential of a region or large property to host a variety of commodities, including diamonds (McClenaghan and Kjarsgaard 2007, and references therein), gold (McClenaghan and Cabri 2011;Averill 2013Averill , 2017, Ni-Cu-PGE (Averill 2001(Averill , 2011McClenaghan and Cabri 2011), porphyry Cu (Averill 2011;Kelley et al 2011;Plouffe et al 2016), Mississippi Valley-Type Pb-Zn (Paulen et al 2011;, VMS-hosted Cu-Pb-Zn (McClenaghan et al 2015a, 2015b, and granite-hosted Sn andW (McClenaghan et al 2016, 2017a). However, indicator mineral methods have not been widely studied or applied to rare metal exploration in glaciated terrain.…”