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TitleExploration for porphyry deposits in British Columbia using trace-element chemistry of detrital apatite
AuthorRukhlov, A S; Plouffe, A; Ferbey, T; Mao, M; Spence, J
SourceGAC-MAC-CSPG-CSSS Joint Meeting, Abstracts vol. 39, 2016 p. 80-81
LinksOnline - En ligne
Alt SeriesEarth Sciences Sector, Contribution Series 20150465
PublisherGeological Association of Canada
MeetingMargins through Time - GAC-Mac 2016; Whitehorse, Yukon; CA; June 1-3, 2016
Mediaon-line; digital
File formatpdf
AreaHighland Valley; Gibraltar; Mount Polley
Subjectseconomic geology; geochemistry; surficial geology/geomorphology; mineral deposits; trace element analyses; trace element geochemistry; porphyry deposits; exploration methods; glacial deposits; tills; apatite; detrital minerals; Phanerozoic; Cenozoic; Quaternary
ProgramIntrusion/Porphyry Ore Systems, Targeted Geoscience Initiative (TGI-4)
AbstractApatite grains from tills at the Highland Valley, Gibraltar, and Woodjam calc-alkaline porphyry Cu-Mo±Au and the Mount Polley alkalic porphyry Cu-Au deposits, and mineralized bedrock at the Woodjam Southeast Zone were analyzed by electron microprobe and LA-ICP-MS. The molar Cl/F and Cl/OH ratios of these grains are similar to those of apatites from producing porphyry deposits elsewhere (e.g., Yerrington, Nevada; Bingham, Utah; Henderson, Colorado). Most apatite grains in till from Mount Polley have higher SO3, SiO2, Na2O, Sr, Mg, V, Ba, Zr, Mo, Nb contents and (La/Sm)CN, and lower Mn, Y, and SHREE contents relative to those from the Highland Valley, Gibraltar and Woodjam deposits. Apatite grains in till from Mount Polley also show weak negative Ce anomalies (Ce/Ce*; mostly 0.89-1.03) and moderate to weak negative Eu anomalies (Eu/Eu*; mostly 0.50-0.99). In contrast, those from the Highland Valley, Gibraltar and Woodjam deposits show weak positive Ce anomalies (Ce/Ce* mostly between 1.00 and 1.24) and strong negative Eu anomalies (Eu/Eu* mostly between 0.11 and 0.47). Most apatite grains from tills at Woodjam and a few from other deposits indicate variable degrees of depletion in minor and trace elements and LREEs, with highly variable (Ce/Yb)CN (<0.01-47), (La/Sm)CN (0.06-6.0), (Gd/Yb)CN (0.01-39), Ce/Ce* (0.23-3.8) and elevated Eu/Eu* (up to 1.5), probably reflecting metasomatic alteration of pristine apatite. The systematic differences in abundances of redox-sensitive SO3, Mn, V, As, Ce and Eu between apatite grains from Mount Polley and other porphyry deposits are consistent with a more oxidized alkalic porphyry Cu-Au compared to more reduced calc-alkaline porphyry Cu±Mo±Au systems. The strong negative Eu anomalies, coupled with higher Mn and lower Mg and Sr contents, of apatite grains at the Highland Valley, Gibraltar and Woodjam deposits also indicate more fractionated magmas, lacking significant Fe-Mg minerals, compared to the Mount Polley alkalic porphyry Cu-Au system. Classification of the analyzed apatite grains using discriminant functions correctly identifies the porphyry deposit-types at all four deposits. Unlike detrital apatite grains at other deposits, those from tills at Woodjam mostly show barren-rock affinity or evidence of metasomatic depletion in most minor and trace elements, perhaps indicating derivation mainly from a low-temperature alteration zone rather than from mineralized rocks such as at the Woodjam Southeast Zone. Our results show that apatite trace-element chemistry is diagnostic of specific porphyry deposit-types and thus can be used as an exploration tool for these deposits.
Summary(Plain Language Summary, not published)
The composition of a common phosphate mineral called apatite can give an indication of the source of the mineral: rocks with economic minerals or rocks with no minerals of economic interest. In our study, we demonstrate that the composition of apatite grains found in unconsolidated sediments deposited by glaciers gives an indication of the source rocks which can be intrusive rocks (like granite) that contains copper minerals of economic interest.