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The amount and character of the tin resources in the Seward Peninsula tin granite belt in western Alaska are directly related to the depth of erosion of the plutonic system. Plutons that have little or no outcrop (little erosion), such as Kougarok or Lost River, are the site of lode tin deposits, whereas placer tin deposits are associated with somewhat eroded plutons (Cape Mountain, Ear Mountain) with a modest outcrop area. The largest pluton in the tin belt, the Oonatut Complex, is a deeply eroded pluton with little tin in either lode or placer deposits.

Textural units in the granite plutons also vary with depth in the plutons. Late-stage, fractionated (Differentiation Index = sum of normative quartz + albite + orthoclase = 91.5 to 96.6) equigranular biotite granite is found at high levels in the plutons, and earlier, less fractionated (Differentiation Index = 71.8 to 92.5), seriate and porphyritic biotite granite is more abundant at greater depth. Fluids associated with the porphyritic and seriate granites were H2O-NaCl–rich and produced metal-poor, idocrase-scapolite-diopside–rich skarns in the carbonate host rocks. Fluids evolved during the late-stage crystallization of the equigranular granite were enriched in incompatible components and produced tin-bearing greisen in granite and hedenbergitic pyroxene-garnet-tourmaline-axinite-cassiterite skarns in marble.

Mineral assemblages and chemistry reflect magma fractionation and record a two-stage evolution for the granite and associated skarns in the tin belt. Biotite compositions show progressive Fe enrichment (Fe/(Fe + Mg) = 0.69 to 0.99) with increasing fractionation of the enclosing granite. Biotite-bearing pegmatite and aplite dikes are associated with the early-crystallizing seriate and porphyritic units of some plutons. Biotite compositions in these dikes are very similar to those in the seriate and porphyritic granites, indicating the H2O-rich fluids associated with the dikes were magmatic in origin.

Tourmaline is found as an interstitial phase in the equigranular granites; it is Fe-rich, similar to the coexisting biotite, suggesting a magmatic origin for this tourmaline. Later, secondary tourmaline is Ca-Mg–rich, and is associated with white mica alteration of biotite in granite and with sulfide deposition in skarns.

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