Mineral markers of porphyry processes: regional and local signatures of porphyry prospectivity

Porphyry-style mineralisation occurs chiefly as a consequence of the release of large volumes of metal-bearing aqueous brine during the cooling and crystallization of plutonic and intrusive magmatic systems. In addition to the metals of economic interest, many additional elements are preferentially incorporated into the aqueous fluid during its segregation from the parent magma, or are gained from interactions with proximal county rocks. During its segregation, pooling and ascent, this magmatic-hydrothermal fluid also deposits many of its elemental components; either through direct precipitation of new minerals or by metasomatic interactions with existing minerals. The elemental and mineralogical signatures of each of these fluid generation, mineral precipitation and hydrothermal alteration processes may be recorded within the resultant mineral residues at each stage, and will likely reflect a complex and evolving suite of physicochemical influences that operated during the formation of each ore deposit. Modern analytical methods are increasingly able to interrogate these signatures at smaller scales and lower cost, thus bringing effective and efficient mineral analysis within the scope of even small scale mineral exploration programs. In this presentation we will discuss a suite of mineralogical and mineral chemistry studies performed in the last few years as part of the Government of Canada's Geomapping for Energy and Minerals and Targeted Geoscience Initiative Programs. All are targeted toward increasing both the efficiency and effectiveness of porphyry mineral exploration within Canada. Late Triassic to early Jurassic plutonic rocks of British Columbia are genetically associated with the great majority of Canada's porphyry copper, molybdenum and gold resources and producing mines, forming the 'Copper Pine of the Cordillera'. Within Yukon they host the sole currently producing copper-gold mine in the territory. However, additional exploration activity within Yukon has been largely unsuccessful and, away from Minto mine itself, the metal endowment of Triassic- Jurassic plutonic suites remains uncertain. In order to assess the prospectivity of these rocks and to provide a tool whereby future exploration in the region could be focused, our study examined the Ce4+/Ce3+ composition of igneous zircon samples taken from across the district, as well as comparative samples from the late Cretaceous Casino deposit. Our results indicate that physicochemical conditions which prevailed during formation of the Minto deposit occurred widely across the Yukon Triassic-Jurassic plutonic suites, but that mineralisation may have occurred at much deeper crustal depths than expected for 'typical' porphyry deposits. Within central and southern British Columbia, many known porphyry deposits are partly or entirely covered by Quaternary glacial sediments. However, a suite of indicator minerals in till in areas around these deposits can detect diagnostic mineralogical signatures of porphyry mineralisation many kilometres down-ice of the deposit location. In addition to indicator minerals, chemical analysis of the till can be used to fingerprint the type of mineralisation expected. A combination of till geochemistry and mineralogy might potentially be developed to provide preliminary information on the metal endowment of a buried mineralized body. At the Woodjam porphyry Cu-Au±Mo deposits of central BC, we can demonstrate that mineralogical complexity in bedrock tourmaline minerals - associated with both ore-forming and distal hydrothermal alteration processes - can be detected in surficial materials, providing context and refined detail to the broad exploration target produced. Within porphyry and greisen-style tin-tungsten-base metal systems of the Canadian Appalachian district, zones of high indium content are significant exploration targets. However, despite apparent bulk mineralogy and base metal grade similarities between indium-rich and -poor ore, these zones do not occur evenly throughout the region. As a part of a detailed inventory and mineralogical assessment of indium distribution within southern New Brunswick and Nova Scotia we have identified distinct mineralogical fingerprints associated with indium enrichment.