THE DISTRIBUTION OF SKARNS IN BRITISH-COLUMBIA AND THE CHEMISTRY AND AGES OF THEIR RELATED PLUTONIC ROCKS

British Columbia contains at least 694 recorded skarns (as defined by Burt, 1977) that range from small occurrences to deposits exceeding 20 million metric tons of ore. Skarns have accounted for nearly 90 perce nt of the iron (magnetite), 80 percent of tile tungsten, 16 percent of the gold, and 12 percent of die copper produced by hard-rock mining i n 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%) lac k significant metallic minerals but represent potential industrial min eral deposits. Three separate plutonic episodes were responsible for m ost of the skarns. The earliest and most significant occurred during t he Early to Middle Jurassic when over half of the skarns developed. La ter important episodes during tile Cretaceous and Eocene-Oligocene acc ounted for 17 and 9 percent, respectively. Skarns in British Columbia are distributed across 19 tectono-stratigraphic terranes. However, the re is a marked spatial and temporal relationship between certain metal lic classes of skarn, their metal production, and the character and or igin of the host terranes. Despite having abundant plutonic rocks, ter ranes of the Coast belt host only 4 percent of the province's skarns a nd these have had negligible metal production. The terranes in the Nor th American basement and craton host 5 percent of the skarns, but thes e have been responsible for all of the W produced. However, terranes d ominated by oceanic island-are rocks contain over 80 percent of the sk arns and have accounted for virtually all the Fe, Cu, Au, Ag, and Zn p roduced. The majority of British Columbia's Fe, Cu, and Au sk,rns deve loped during Early to Middle Jurassic oceanic island-are activity in W rangellia, Quesnellia, and Stikinia. By contrast, nearly all the W ska rns are related to a belt of Cretaceous plutons that intrude sedimenta ry rocks deposited either close to or on the ancestral North American continent. In many terranes: there is no consistent relationship betwe en the number of skarn occurrences and metal production. Wrangellia te sts nearly all the Fe skarns and has accounted for virtually all the 3 5 million metric tons of magnetite produced in British Columbia. Yet i t 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 a nd trace element analyses indicate that the plutonic recks associated with Fe, Cu, Au, and, to a lesser extent, Mo skarns are chemically dis tinct from those associated with W and Sn skarns. Plutons related to t he former group are relatively undifferentiated and Fe rich; their Nb, Y, and Rb contents are characteristic of ''volcanic are granites.'' T hose associated with W and Sn skarns are highly differentiated, Fe poo r, large ion lithophile elements rich, and represent ''within-plate'' intrusions, Copper- and Au-bearing skarns are currently the most econo mically attractive targets for skarn exploration in British Columbia, and this study indicates that island-are assemblages in the Quesnellia . terrane, or in correlative rocks elsewhere, have the best exploratio n potential for these metals.