The Fort Fraser and Manson River map areas were last glaciated during the Late Wisconsinan Fraser Glaciation. Quaternary sediments predating this event are rare. The oldest deposits identified are oxidized sand and gravel, which locally contain placer gold. Stratigraphic evidence of a pre-Fraser glaciation was found at one site. Sediments of the Olympia Nonglacial Interval have been identified at three sites. Paleoecological reconstruction indicates climatic conditions cooler than at present for at least part of that nonglacial interval. The mountainous regions were the first to be glaciated at the onset of Fraser Glaciation. Valley glaciers advanced from three major accumulation centres: the Skeena, Coast, and Cariboo mountains. In easterly draining valleys, sand and gravel accumulated on outwash plains in front of advancing glaciers. In most westerly draining valleys, the drainage was blocked by advancing ice and glacial lakes were formed. The pattern of ice advance was reconstructed primarily from crosscutting relationships of glacial erosion marks measured on bedrock outcrops. Ice from the Coast and Skeena mountains generally flowed easterly, but was deflected to the north and northeast in the eastern part of the study area, where it coalesced with an ice lobe derived from the Cariboo Mountains. Deglaciation proceeded from east to west along an irregular front controlled by topography. Numerous glacial lakes developed behind decaying ice. Postglacial sediments include colluvium, alluvium, organic, eolian, and anthropogenic deposits. Postglacial aggradation in valleys was followed by incision of valley fills and establishment of the modern drainage.
We investigated if the composition of hydrothermal ore-related magnetite in till could be used to locate porphyry deposits in terrain where glacial overburden overlies rocks that host porphyry Cu-Au mineralization. We test this hypothesis using 20 till samples collected in an ~900 km 2 area surrounding the Mount Polley porphyry Cu-Au deposit, in south-central British Columbia, Canada. At least 100 magnetite grains were randomly selected from the magnetic fraction of each till sample. Nineteen trace elements in ~50 magnetite grains in each sample were measured by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). The large beam or raster size used for laser ablation (to 100 µm) homogenizes any heterogeneous trace element distributions in magnetite that result from oxy-exsolution and/or and dissolution/re-precipitation, avoiding this issue with the few micron size of an electron beam. Linear discriminant analysis (LDA) performed on a compilation of magnetite compositions measured by LA-ICP-MS from worldwide porphyry deposits and intrusive igneous rocks define the chemical signature (Mg, Al, Ti, V, Mn, Co, Ni) of hydrothermal magnetite exclusive to porphyry systems. Application of our LDA models to 985 magnetite compositions we measured in the till samples surrounding Mount Polley showed anomalous amounts of hydrothermal magnetite grains in till up to 2.5 km west-southwest and 4 km northwest of
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
The extent and variation of Cu, Hg, Ni, and Pb loading in soil profiles (humus, B-horizon, C-horizon) were examined in the vicinity of four Canadian industrial sources of airborne metal particulates located in different ecozones and geological terrains: the Cu smelter at Rouyn-Noranda, Québec; the Cu-Zn smelter at Flin Flon, Manitoba; the Pb–Zn smelter at Trail, British Columbia; and the inactive Pinchi Lake Hg Mine, British Columbia. Three major controlling factors on the metal concentrations in soils have been assessed: (1) distance from the anthropogenic point source, (2) organic matter content, and (3) geology of the substrate. Distance from source largely controls smelter-related metal concentrations in humus, with concentrations decreasing with increasing distance according to specific parameters for each element and at each location. In the B-horizon, variations in organic matter content and substrate geology are important controls on metal concentrations, except in areas close to the source where sub-surface contamination was recognized for certain metals. The metal content of the C-horizon is predominantly determined by the nature and composition of the substrate. Variations in humus/C-horizon and B-horizon/C-horizon metal ratios are useful to help distinguish anthropogenic from natural sources of metal enrichments in soils.
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