The distribution of skarns in British Columbia and the chemistry and ages of their related plutonic rocks

Abstract

British Columbia contains at least 694 recorded skarns (as defined by Burr, 1977) that range from small occurrences to deposits exceeding 20 million metric tons of ore. Skarns have accounted for nearly 90 percent of the iron (magnetite), 80 percent of the tungsten, 16 percent of the gold, and 12 percent of the copper produced by hard-rock mining in the province. Copper skarns make up almost half of the occurrences; the other skarn classes, in decreasing abundance, are Fe (21%), Pb-Zn (11%), W (7%), Au (4%), Mo (3%), and Sn (0.4%). A small group (2%) lack significant metallic minerals but represent potential industrial mineral deposits.Three separate plutonic episodes were responsible for most of the skarns. The earliest and most significant occurred during the Early to Middle Jurassic when over half of the skarns developed. Later important episodes during the Cretaceous and Eocene-Oligocene accounted for 17 and 9 percent, respectively.Skarns in British Columbia are distributed across 19 tectono-stratigraphic terranes. However, there is a marked spatial and temporal relationship between certain metallic classes of skarn, their metal production, and the character and origin of the host terranes. Despite having abundant plutonic rocks, terranes of the Coast belt host only 4 percent of the province's skarns and these have had negligible metal production. The terranes in the North American basement and craton host 5 percent of the skarns, but these have been responsible for all of the W produced. However, terranes dominated by oceanic island-arc rocks contain over 80 percent of the skarns and have accounted for virtually all the Fe, Cu, Au, Ag, and Zn produced.The majority of British Columbia's Fe, Cu, and Au skarns developed during Early to Middle Jurassic oceanic island-arc activity in Wrangellia, Quesnellia, and Stikinia. By contrast, nearly all the W skarns are related to a belt of Cretaceous plutons that intrude sedimentary rocks deposited either close to or on the ancestral North American continent.In many terranes, there is no consistent relationship between the number of skarn occurrences and metal production. Wrangellia hosts nearly all the Fe skarns and has accounted for virtually all the 35 million metric tons of magnetite produced in British Columbia. Yet it also has the largest number of Cu skarns of any terrane but has had only one-fifth the skarn copper production of Quesnellia.Both major and trace element analyses indicate that the plutonic rocks associated with Fe, Cu, Au, and, to a lesser extent, Mo skarns are chemically distinct from those associated with W and Sn skarns. Plutons related to the former group are relatively undifferentiated and Fe rich; their Nb, Y, and Rb contents are characteristic of volcanic arc granites. Those associated with W and Sn skarns are highly differentiated, Fe poor, large ion lithophile elements rich, and represent within-plate intrusions.Copper- and Au-bearing skarns are currently the most economically attractive targets for skarn exploration in British Columbia, and this study indicates that island-arc assemblages in the Quesnellia terrane, or in correlative rocks elsewhere, have the best exploration potential for these metals.