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Dobson, D.C., 1984

Geology and geochemical evolution of the Lost River, Alaska, tin deposit

Bibliographic Reference

Dobson, D.C., 1984, Geology and geochemical evolution of the Lost River, Alaska, tin deposit: Palo Alto, California, Stanford University, Ph.D. dissertation, 237 p., illust., map, folded maps in pockets.


Sn-W-Be-F and base metal mineralization at Lost River occurs in stockwork vein skarn, greisen, and breccia in and above the cupola of a late Cretaceous granite that intruded Ordovician limestone within 3 km of the surface. The mineralized intrusion was a highly evolved granite, derived from sialic crust, which formed primarily by volatile-related fractionation mechanisms. Alteration in limestone commenced with skarn assemblages characterized by: (1) andradite; (2) fluorite + idocrase + magnetite; and (3) idocrase + subcalcic garnet +/- fluorite, hornblende. Quartz + topaz + tourmaline + cassiterite + sulfide greisen in granite was contemporaneous with fluorite + biotite + hornblende + sulfide + cassiterite veins in skarn. These were followed by mica-rich greisens in the cupola and margarite + muscovite + plagioclase alteration in carbonates and skarn. Two breccia types cut all earlier assemblages. The first consists of skarn, limestone and igneous rock fragments in a white mica matrix. Kaolinite-matrix breccias are the last hydrothermal event noted. Fluid inclusion studies suggest early skarns formed from fluids at 350-400 degrees C with salinities declining from 15-18 wt% NaCl to 9-15 wt%. Boiling fluids then caused greisenization of granite; concomitantly, residual fluids with enhanced salinity (18-21% NaCl) caused biotite + hornblende deposition in skarn. Pressures indicated by inclusions are 250-400 bars. Phase equilibria suggest fluids were characterized by very low XCO2, moderate to low fO2 and fS2 and high fHF/fH2O). Declining temperatures, and increasing FO2, led to the late formation of margarite + plagioclase in limestone and skarn. Clay alteration probably occurred at T less than 250 degrees C. Data from Lost River and other Sn deposits suggest that sudden pressure variation, fluid boiling, may be a common prerequisite of Sn mineralization. Thus, the search for Sn deposits can probably be restricted to shallow settings where pressure fluctuation can occur via fracturing, and fluid communication with the surface. In the case of Sn skarns, shallow levels are critical because early silicates (e.g. andradite) may contain significant Sn. The deposition of recoverable Sn is dependent on destruction of those phases by retrograde alteration, which is best accomplished at shallow levels.

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