ALKALIC PORPHYRY CU-AU DEPOSITS: MAGMATIC-HYDROTHERMAL MINERALIZATION WITH SIMILARITIES TO FE-OXIDE CU-AU DEPOSITS

The Quesnel and Stikine intra-oceanic island arc terranes of British Columbia, Canada, host numerous Cu-Au deposits that are characterized by disseminated mineralization within and immediately adjacent to silica-saturated to silica-undersaturated alkalic intrusive centers hosted by shoshonitic volcanic sequences. These deposits have historically been called alkalic porphyry Cu-Au deposits (AlkPCDs) and include Copper Mountain, Galore Creek and Afton/Ajax in British Columbia, with additional examples elsewhere (e.g., Didipio, Philippines). The group has been suggested as the endmember of a continuum that extends from classic, sub-alkalic porphyry Cu-Mo+/-Au deposits (e.g., Sierrita, Arizona), through transitional, near-alkalic, typically Au-bearing porphyry deposits (e.g., Bingham Canyon, Utah). AlkPCDs are further characterized by well-developed Na-Ca-K alteration, a marked lack of quartz, sericitic alteration and argillic alteration, variable concentrations of hypogene magnetite and/or hematite, a low-sulfur ore assemblage, and formation at shallow to moderate paleodepths.

There are many similarities between AlkPCDs and Fe-oxide Cu-Au deposits that include: 1) strongly developed Na-Ca-K alteration; 2) alteration rich in magnetite and, locally, hematite; 3) low sulfur ore assemblages; and 4) an association with monzonite to diorite intrusions that is at least similar to Fe-oxide Cu-Au deposits found in convergent margin settings (e.g., Candelaria, Chile). Differences between the two deposit types are, however, more numerous and potentially more fundamental. 1) The tectonic setting of AlkPCDs is one of relatively primitive island arcs with a complex accretion history, whereas Fe-oxide Cu-Au deposits are found in oxidized continental rifts or arcs. 2) AlkPCDs, as currently known, are restricted to the Mesozoic and Tertiary, as are major classic porphyry deposits; Fe-oxide Cu-Au deposits, however, span a large portion of earth history that extends to at least 2600 Ma. 3) Fe oxide Cu-Au deposits are largely controlled by crustal scale structures and appear to have formed over a large range of paleodepths, whereas AlkPCDs generally formed at moderate to shallow depths influenced by local structure. 4) AlkPCDs manifest a definite spatial, temporal and genetic association with intrusions, but Fe oxide Cu Au deposits exhibit only a regional association that has hindered assignment of a specific magma type as critical to their formation. 5) The metal assemblage of Fe oxide Cu Au deposits includes Mo, U, REE and others, many of which are consistent with their crustal setting, which are not significantly enriched in AlkPCDs.

AlkPCDs can be confidently linked to classic porphyry deposits, but potential genetic overlaps with Fe oxide Cu-Au deposits are less clear. Similarities suggest that the demonstrably magmatic-hydrothermal origin of AlkPCDs may find at least local application to Fe oxide Cu-Au deposits, but the many fundamental differences between these deposit types implicate distinct ore forming environments and processes which remain to be fully evaluated.