Regionally metamorphosed, calc-silicate-hosted deposits of the Brooks Range, northern Alaska

Abstract

Three distinctly different calc-silicate-ore suites in the Brooks Range, northern Alaska, show contrasting geologic and metallogenic associations. Both host rocks and plutonic rocks are of Devonian age and all lithologies were subjected to a major Late Jurassic-Early Cretaceous regional metamorphic event. One calc-silicate suite is associated with peraluminous, high initial 8?Sr/86Sr, presumably S-type granites and is characterized by anomalous Sn contents, stanniferous grandite and subcalcic garnets with crosscutting amphibole-clinozoisite, and a distinctive boron-rich mineral association. These skarns resemble the Sn-bearing skams of the Seward Peninsula, except that they have low cassiterite contents and show only limited greisen alteration. The noneconomic nature of the Brooks Range Sn skarns may be related to relatively deep levels of exposure. The second calc-silicate suite is associated with generally metaluminous, presumably /-type, quartz-sericite-pyrite-altered granite and granodiorite stocks and isolated fault slivers. This suite contains high Cu and Ag, variable Pb and Zn, and low Sn; is characterized by andraditic garnet, diopsidic pyroxene, and bomite-chalcopyrite, cut by epidote, actinolite, and pyrite veinlets; and generally resembles a continental margin, porphyry copper-related skarn. The third calc-silicate suite, the so-called gnurgle gneiss, represents a regionally metamorphosed metalliferous chamical sediment and is not related to igneous-metasomatic processes. This suite is characterized by welldeveloped mineral foliation, generally low metal contents, lack of sulfide veining and retrograde alteration, and combination of metal ratios and calc-silicate mineral compositions not commonly observed in skarn deposits.