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TitleMineral chemistry of greenrock alteration associated with the porphyry Cu-Mo mineralization at the Gibraltar deposit, south central British Columbia, Canada
AuthorKobylinski, C H; Hattori, K; Plouffe, AORCID logo; Smith, S
SourceWhitehorse 2016, Geological Association of Canada, Mineralogical Association of Canada, abstracts; 2016 p. 1 Open Access logo Open Access
LinksOnline - En ligne
Alt SeriesEarth Sciences Sector, Contribution Series 20150489
PublisherGeological Association of Canada
Mediaon-line; digital
File formatpdf
ProvinceBritish Columbia
AreaQuesnel Terrane
Subjectsigneous and metamorphic petrology; alteration; mineralization; copper; molybdenum; porphyry deposits; epidote; chloritization; metamorphism; Gibraltar deposit; Granite Mountain batholith; Triassic
ProgramTargeted Geoscience Initiative (TGI-4) Intrusion/Porphyry Ore Systems
AbstractThe Gibraltar Cu-Mo porphyry deposit with the geological reserves (past production plus reserves) of 2.8 Mt Cu is hosted by in the late Triassic Granite Mountain batholith of late Triassic age in the Quesnellia tTerrane. The batholith, which has the surface exposure of 15 km x 10 km, is mostly composed of tonalite with minor variation in modal abundances of felsic and mafic minerals. Igneous minerals are plagioclase, quartz, biotite, hornblende, with minor amounts of titanite, zircon, apatite and magnetite. Bulk rocks show a typical subduction-related geochemical signature with negative anomalies of high field strength elements, such as Nb, Ta and Ti, with minor positive and negative Eu anomalies (Eu/Eu*=0.09-1.2). The mineralization is accompanied by extensive alteration in the tonalite, which produced illite, quartz, rutile, titanite, magnetite, apatite, chlorite and epidote. The high-temperature potassic alteration zone was likely present in the centre of the current mining operation, but the alteration has been obscured by the replacement of secondary biotite by chlorite. Among alteration minerals, epidote and chlorite are ubiquitous in the tonalitic rocks in and around the mine, producing green rock alteration. Epidote group minerals primarily replace plagioclase. Chlorite is also common after hornblende and biotite, and a mixture of titanite and rutile replaces biotite and hornblende. Epidote minerals are light grass green in hand specimens and also thin sections. They show a large compositional variation even within one sample from Al-rich (high clinozoisite component) to Fe-rich epidote, suggesting multiple alteration events or reflecting varying fluid composition within a hydrothermal system. Some are yellowish in thin sections and they contain significant allanite component with high La2O3 (up 9.54 wt%) and Ce2O3 (up to 14.7 wt%). These Ce-La-rich epidote grains contain higher concentrations of other REEs, Mn, Y and Th than Fe-rich epidote. These LREE-rich epidotes form isolated grains within chlorite aggregates in epidote-rich tonalities. In contrast, Fe-rich, REE-poor epidote is abundant and preferentially replaces plagioclase. Igneous titanite (up to 2 mm) is low in Al2O3 (~1.2 wt%) whereas hydrothermal titanite contains high Al2O3 ranging from 2 to 6 wt%. Hydrothermal titanite is more abundant than its igneous counterpart, and shows a wide compositional variation of Mg and Fe. Rocks with greenrock alteration show high abundances of U, Pb and K in bulk rocks compared to epidote-poor samples. The enrichment of U and Pb in bulk rocks is likely due to their incorporation in epidote as minor components.
Summary(Plain Language Summary, not published)
The composition of specific minerals in rocks that contain economic amount of copper and molybdenum were determined with the objective of identifying the same minerals with the same composition in glacial sediments. In other words, the source of the minerals with a specific composition in glacial sediments can be related to a specific bedrock source that contain copper and molybdenum. Results of this research has implication for mineral exploration.

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