Petrogenesis of the Ear Mountain tin granite, Seward Peninsula, Alaska

Abstract

Ear Mountain is part of a belt of Cretaceous tin on the Seward Peninsula of western Alaska. Porphyritic and seriate biotite quartz monzonite and quartz syenite represent early pluton units. These were followed by equigranular and fine-grained two-mica tourmaline-bearing granite. Late units are enriched in B, F, and Na/K and are depleted in Ti, A greisen alteration assemblage of tourmaline-quartz-cassiterite-white mica + or - magnetite + or - pyrite + or - pyrrhotite + or - rutile + or - arsenopyrite is found in the granite and skarns surrounding the pluton.Granite compositions approximate the minimum in the synthetic granite system at a pressure of 0.5 kb. Compositions of fine-grained granite are shifted toward the F-enriched minimum in the synthetic system. Temperature estimates based on feldspar geothermometry range from 624 degrees to 501 degrees C and are close to the solidus temperature in the F- and B-enriched systems. Coexisting Fe-Ti oxide mineral data give temperature estimates of 625 degrees to 675 degrees C and indicate cooling along a buffered path intermediate between the quartz-fayalite-magnetite and nickel-nickel-oxide curves. Biotite compositions (Fe/Fe + Mg = 0.69-0.99) combined with the feldspar geothermometry also indicate that crystallization of the pluton followed a buffer near the nickel-nickel-oxide curve.The tin contents of Ear Mountain granites largely correlate with degree of greisen alteration. Unaltered granites contain less than 80 ppm Sn, whereas thoroughly altered granites contain over 950 ppm Sn. Tin concentrations in unaltered granitic rocks increase progressively from porphyritic to seriate to some equigranular rocks, indicating a progressive concentration of Sn with fractional crystallization. Most unaltered fine-grained granites contain low Sn values, interpreted as results from Sn lost to the evolved vapor phase.The moderately reducing, weakly peraluminous and low calcium conditions during crystallization of the Ear Mountain pluton allowed for concentration of Sn into the melt. High B and F contents in the melt promoted prolonged fractionation of the system and increased concentration of Sn. Evolution of a vapor phase removed Sn from the granitic melt and reaction of this vapor with the granite and the skarn produced the Sn-enriched greisen alteration. There is no evidence that Sn was remobilized from Sn-enriched biotites into greisen.Greisen development is not extensive at present levels of erosion. Abundant cassiterite in placers surrounding the pluton suggest much of the greisen zone has been removed by erosion. The apical parts of other tin granites on the Seward Peninsula, such as Lost River and Kougarok, have well-developed greisen zones. Ear Mountain probably resembled these systems prior to erosion.