The Mount Riordan (Crystal Peak) garnet skarn, Hedley District, southern British Columbia

British Columbia Geological Survey, Economic Geology Section, Victoria, BC, Canada

The Mount Riordan (Crystal Peak) skarn, situated 7 km east of the Nickel Plate gold skarn, represents an industrial garnet deposit that has drill-indicated reserves totaling 40 million metric tons averaging 78 percent by volume garnet. Three garnet-rich zones have been outlined. Drilling shows they are at least 70 to 90 m thick although the total true thickness is unknown; one test drill hole encountered massive garnet to a depth of 175 m. Market feasibility studies have been completed, and environmental and Provincial Government mine review studies are in progress to determine whether the proposed annual production of 30,000 to 60,000 metric tons of industrial abrasive garnet should proceed.Sulfides and other metallic minerals in the skarn are rare; they include magnetite, pyrite, pyrrhotite, chalcopyrite, scheelite, and trace powellite. Unlike the Nickel Plate deposit, the Mount Riordan skarn has high garnet/pyroxene ratios (avg <11% by volume pyroxene), does not contain abundant scapolite, and is not enriched in arsenic, cobalt, bismuth, tellurium, or gold.The garnet deposit is believed to be related to the adjacent Mount Riordan stock, a granodioritic to gabbroic body that has yielded a U-Pb zircon age of 194.8 + or - 2.4 Ma (Early Jurassic). The generally low titanium content of the garnet (<0.05 wt % TiO ₂ ) together with rare, remnant breccia textures suggests that the protolith of the skarn was probably massive limestone and calcareous breccia of the Upper Triassic Nicola Group French Mine Formation. Elsewhere in the district, this formation hosts the Good Hope and French gold skarn deposits.Electron microprobe analyses indicate that garnet and clinopyroxene at Mount Riordan have low manganese contents (<1.0 and <1.3 wt % MnO, respectively), similar to those in the nearby gold skarns. Two phases of garnet are recognized. The early phase consists of pink, andraditic garnets (avg Ad ₉₀ mole%) that are isotropic and commonly form cores of larger garnet crystals. The later phase comprises colorless, birefringent, and more grossularitic garnets that range from Ad ₄₅ to Ad ₆₅ mole percent; they are present either as discrete crystals or as overgrowths on older garnets. Clinopyroxenes are intermediate between diopside and hedenbergite, ranging in composition from Hd ₄₁ to Hd ₅₁ mole percent.Garnet and pyroxene compositions suggest that the Mount Riordan skarn developed initially in a relatively oxidized environment that became more reduced as the system evolved; this contrasts with the gold skarns in the district that formed in highly reduced conditions. The dissimilar mineralogy, geochemistry, and oxidation state of the Mount Riordan and Nickel Plate deposits probably reflect differences in their sedimentary protoliths, compositions of the associated intrusions and magmatic fluids, and tectonic settings. The pyroxene-dominant Nickel Plate skarn is associated with reduced, iron-rich quartz diorites and gabbros that intruded a highly reduced siltstone sequence, whereas emplacement of the more oxidized granodioritic to gabbroic Mount Riordan stock into massive limestone produced the garnet-dominant Mount Riordan skarn.Skarns have formerly been considered poor exploration targets for industrial garnet because of their generally low garnet content (<20% by volume) and the poor quality and high inclusion content of the garnet. However, the high grade and quality of the garnet at Mount Riordan should change this perception.

This record provided courtesy of AGI/GeoRef.