Abstract:

The A.M. breccia is part of the Giant Copper deposit of southern British Columbia. Located approximately 150 km west of Vancouver, this brownfield property has high-grade Cu and Ag mineralization confined to tourmaline breccia pipes. The A.M. breccia, the property's most well-defined tourmaline breccia pipe, has an inferred resource of 30.2 Mt at 0.72% Cu. The A.M. breccia shares similarities with other tourmaline breccia pipes, most notably the tourmaline breccia pipes of Chile and Peru. Additionally, similarities exist with tourmaline breccia pipes associated with PCDs in the Cascadia arc. Although tourmaline breccia pipes are known features in selective PCDs, they can also form in Sn-W intrusion-related deposits. The potential for mineralized tourmaline breccia pipes to form in various intrusion-related ore systems requires a way to differentiate between the different deposit types.

Quantifying the chemical properties and redox conditions of the intrusive suites responsible for breccia pipe formation and ore-bearing fluids can more reliably establish porphyry affiliation. An LA-ICPMS U-Pb zircon date of 24.9 ± 0.2 Ma (2σ) for the Invermay diorite, the intrusion responsible for breccia pipe formation, is consistent with Cascadia suite porphyry-type mineralization and associated breccia pipes. Trace element concentrations of Invermay zircon, on average, have fertility indicators for redox state, water content, and fractionation that correlate well with known PCDs.

Regardless of deposit association, tourmaline is a common alteration mineral in tourmaline breccia pipes. Due to its chemical diversity, thermal and mechanical stability, and boron concentrations, it is an excellent mineral reservoir capable of recording the chemistry of magmatic-hydrothermal processes. Chemical and isotopic characteristics of hydrothermal tourmaline at the A.M. breccia have major-element compositions aligned with tourmaline compositions from PCDs globally. Tourmaline from PCDs is alkali-rich with elevated Fe, Mg, Ti, and Ca, reflecting a metaluminous composition. Sn-W intrusion-related tourmaline is alkali- to X-site vacant-rich and contains high concentrations of Al, F, Si, Mn, and X-site vacancies compared to PCD tourmaline and reflect a peraluminous composition. Boron isotopes for A.M. tourmaline range from -8.1‰ to 5.6‰, indicating an unaltered MORB boron source and I-type affinity. The chemical and isotopic characteristics strongly suggest that the A.M. breccia is PCD-related and that the Invermay diorite has the zircon fertility characteristics of a PCD. The application of this study provides new quantitative criteria for determining the deposit affinity of tourmaline breccia pipes.

Tourmaline sub-species are dependent on their spatial location within the A.M. breccia. Tourmaline outside of the pipe contains higher concentrations of Mg, whereas tourmaline preferentially incorporates Fe within the pipe. These chemical variations are currently indistinguishable in hand specimens. Spectral reflectance data were collected from 587 tourmaline grains to determine if discerning chemical changes in tourmaline can be made field-based and thus more cost-effective. Spectral reflectance differentiates tourmaline associated with ore and ore-bearing breccia textures from tourmaline occurring distal to the pipe contact or within barren tourmaline breccia pipes. This grouping suggests that tourmaline sub-species can be inferred by spectral reflectance, enhancing the efficiency of tourmaline as a mineral vector.

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Dr. Dan Marshall