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TitleTill geochemistry and mineralogy at Gibraltar and Mount Polley Cu porphyry deposits
AuthorPlouffe, AORCID logo; Ferbey, T; Hashmi, S; Ward, B C
SourceIndicator minerals in till and stream sediments of the Canadian Cordillera; by Ferbey, T; Plouffe, AORCID logo; Hickin, A; Bond, J; 2016 p. 134-154
Alt SeriesEarth Sciences Sector, Contribution Series 20160066
PublisherGeological Association of Canada
MeetingGeological Association of Canada - Mineralogical Association of Canada, annual meeting; Whitehorse, Yukon; CA; May 30 - June 3, 2016
ProvinceBritish Columbia
NTS92I; 92P; 93A; 93B/01; 93B/02; 93B/07; 93B/08; 93B/09; 93B/10; 93B/15; 93B/16
AreaWilliams Lake; Woodjam; Gibraltar; Highland Valley; Mount Polley
Lat/Long WENS-123.0000 -120.0000 53.0000 50.0000
Subjectseconomic geology; surficial geology/geomorphology; metallic minerals; mineralogy; porphyry deposits; porphyry copper; mineral exploration; glacial deposits; tills; till analyses; till geochemistry; drift prospecting; drift deposits; ice flow; ice movement directions; glacial history; Gibraltar Mine; Highland Valley Mine; Mount Polley Mine; Woodjam Mine; Cenozoic; Quaternary
ProgramTargeted Geoscience Initiative (TGI-4) Intrusion/Porphyry Ore Systems
Released2016 01 01
AbstractThe Gibraltar (Cu-Mo) and Mount Polley (Cu-Au) porphyry deposits are hosted in Late Triassic intermediate to felsic intrusive rocks of the Quesnel Terrane in south central British Columbia. Both deposits occur near the top of a mountain and were in large part covered by glacial sediments prior to mining. At both sites, the glacial sediment cover is dominantly till that has variable surface morphology. Where till is thin (< 2 m on average), it follows the bedrock topography and where it is thick (> 2 m on average) it has a rolling to glacially fluted surface expression. As part of the Targeted Geoscience Initiative-4 program, the Geological Survey of Canada and the British Columbia Geological Survey carried out a regional till geochemistry and mineralogy sampling survey around these two deposits.
Interpretation of the till composition (geochemistry and mineralogy) takes into account mineralized bedrock sources and ice-flow history. During the last glaciation, three ice-flow movements affected the Gibraltar deposit region (towards the SE, SW and NW) and two affected the Mount Polley area (towards the SW and NW). At both deposits, the mineralization is reflected in the till composition with high concentrations of multiple elements near and at least 2 km down-ice of mineralization. For example, Cu in the clay-sized fraction (<0.002 mm) occurs in high concentrations (>380 ppm) near and down-ice of Gibraltar and Mount Polley. High concentrations of Mo (> 3 ppm), Zn (>280 ppm), and Ag (>335 ppb) are present down-ice of Gibraltar. At Mt. Polley in the clay-sized fraction of till, Ag (> 340 ppb), Hg (>490 ppb) and Zn (>170 ppm) occurred down-ice (NW) of Mount Polley, with >15 ppb Au in the silt plus clay fraction (<0.063 mm).
Mineral grains of chalcopyrite (0.25-0.5 mm; specific gravity >3.2), one of the dominant Cu ore minerals in both deposits, occur in greater abundance in till near and up to 10 km down-ice of the deposits. Molybdenite is absent from till at Gibraltar probably because this mineral has a low hardness that limited preservation due to comminution during glacial transport. Also, it could be that molybdenite was not abundant enough in the mineralized bedrock that was exposed to glacial erosion. Gold grains were dispersed at least 5 km down-ice (NW) from Mount Polley Cu-Au mineralization. Certain minerals related to hydrothermal alteration processes associated with mineralization are also abundant in till down-ice from porphyry mineralization. The abundance of epidote group minerals in till at Mount Polley defines a palimpsest dispersal train with two lobes extending over 2 km in the direction of the two ice-flow movements: SW and NW.
Results from this survey demonstrate that porphyry mineralization that was exposed to glacial erosion is reflected in the till geochemistry (multi-elements) and mineralogy (ore and alteration minerals). Future research is required on the chemical composition and physical characteristics of porphyry indicator minerals to discriminate their source in relation to barren versus mineralized source rocks.
Summary(Plain Language Summary, not published)
In this presentation we will demonstrate that zones in bedrock that are naturally enriched in copper and that are covered by glacial sediments can be detected by analyzing the mineralogy and geochemistry of the glacial sediments. Some of these zones are currently being mine for copper, molybdenum and gold. This mineral exploration method is being transferred to the industry at this conference.

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