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
Mount Polley Mine is an alkaline, silica-under saturated Cu-Au porphyry deposit owned by Imperial Metals Ltd. It is located 8 km southwest of Likely, within the Interior Plateau of British Columbia. This region was glaciated several times during the Quaternary and is predominantly covered by till. This makes it an ideal site to develop drift prospecting exploration methods for detecting buried mineralization. Glaciers are dynamic, eroding bedrock, and transporting and depositing the resulting debris as till in the direction of ice movement. If a glacier erodes mineralized bedrock, the resulting till will be enriched in elements and minerals derived from the mineralization. Porphyry mineralization and the associated alteration zones are spatially extensive, which may result in large dispersal trains in till. The objective of this research is to characterize the geochemical and mineralogical composition of till in the Mount Polley region in order to identify key indicators of porphyry mineralization in till, benefiting future exploration for porphyry deposits in areas of thick drift cover. This research is funded by the Geological Survey of Canada's Targeted Geoscience Initiative (TGI-4) program. Dominant ice flow in the Mount Polley region is to the northwest with an earlier west-southwestward flow. During the 2012 and 2013 field seasons, 87 till samples were collected up-ice, overlying and down-ice from the deposit based on these flow measurements. Elements and minerals associated with the mineralization show elevated content near the deposit, which progressively decreases towards the northwest (or down-ice direction). These include copper, silver (in the 2 ?m fraction) and gold (in the 63 ?m fraction), up to 1548 ppm, 503 ppb and 90.2 ppb, respectively. Gold grain content of up to 87 grains per 10 kg is observed in the silt-fine sand fraction. In the heavy mineral concentrates (s.g > 3.2), apatite (up to 0.5%), chalcopyrite (up to 98 grains per 10 kg), and epidote (up to 90%), observed in the 0.25 - 0.5 mm fraction. Gold grain, chalcopyrite, and epidote are excellant indicators of Mount Polley mineralization. However, apatite may not be a suitable indicator of Mount Polley mineralization in till.
Analysis of over 10 000 water-well records has been used to produce new depth-to-bedrock maps for areas around five cities on the central Interior Plateau of central British Columbia: 100 Mile House, Prince George, Quesnel, Vanderhoof, and Williams Lake. Hitherto, exploration for mineral and hydrocarbon resources has been hampered by a lack of basic knowledge of the thickness of Neogene and Quaternary lithologies. Interpretation of these new maps provides first-order constraints on the localization of thick drift in pre-Late Wisconsinan bedrock paleovalleys, some of which are now buried. Basalt lavas of the Chilcotin Group are restricted to erosional remnants of previously extensive sheets emplaced onto an older peneplain. Our results confirm that the Neogene and Quaternary cover is primarily controlled by paleotopography and is generally thin and patchy across much of the region. Increased understanding of the three-dimensional distribution of cover produces a corresponding increase in the utility of geological, geochemical, and geophysical exploration techniques, and a reduction in the risk for future mineral exploration activities, especially when combined with more sophisticated data sets.
An airborne electromagnetic (AEM) survey using the Fugro RESOLVE™ system was conducted over a known, buried sand and gravel deposit in northeast British Columbia, Canada to determine its effectiveness for mapping similar deposits in the area. The rarity of gravel deposits in this region provides a significant economic constraint for developing oil and gas infrastructure. Topographic relief is minimal and the area is overlain by organic materials, clay-rich tills, and glaciolacustrine sediments. Traditional exploration methods, such as airphoto interpretation, are therefore ineffective for locating these buried deposits. The buried deposit appears as a resistivity high on the apparent resistivity maps, particularly the [Formula: see text] map. The resistivity high covers a larger area than that defined initially by test pitting. Trenching within this extended zone verified the presence of sand and gravel. Other resistive features in the region were identified by the survey and one of these was confirmed by recent test pitting to be a large sand and gravel occurrence. No surface indications of sand and gravel were evident over any of these occurrences. The cross sections indicated these sand and gravel occurrences have resistivity values greater than [Formula: see text], often greater than [Formula: see text], whereas the surrounding till has resistivity values between 5 and [Formula: see text]. The airborne survey proved to be a highly successful method of locating sand and gravel deposits in this area of British Columbia that could be widely used in other plains regions with similar geological settings.
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