Geology and alteration of the Lost River tin-tungsten-fluorine deposit, Alaska

Abstract

Tin mineralization at Lost River occurs in skarn, greisen, and breccia associated with the intrusion of a Late Cretaceous granite into a thick section of massive Ordovician limestone. In addition to Sn, significant amounts of F, W, Be, Zn, Pb, Cu, and Ag were introduced during alteration.Textural, compositional, and isotope data indicate that the intrusion was a highly differentiated residual melt derived from the fusion of sialic crustal materials. The stock was emplaced at a fairly shallow depth (2-4 km) in what appears to have been a stable cratonic setting.Stockwork skarn veining commenced following the intrusion and crystallization of the rim of the granite; the earliest stages are relatively anhydrous and are dominated by multiple generations of idocrase and garnet. Earliest andraditic garnetitc is cut by a distinctive layered fiuorite-magnetite-idocrase vein skarn. A second generation of garnet-idocrase cuts the magnetite-bearing assemblages. In these early skarns, tin is found largely within silicate lattices. Andradite contains up to 6 weight percent SnO 2 , and both idocrase and grossular contain 0.2 to 0.6 weight percent SnO 2 .Greisen alteration of the intrusive rocks is characterized by the destruction of igneous textures and the replacement of feldspars and biotite by fine-grained quartz with topaz, tourmaline, cassiterite, and sulfides. Greisenization was approximately contemporaneous with a hydrous stage of alteration in the limestone which is characterized by assemblages composed of fluorite-biotite + or - hornblende with sulfides and cassiterite. Late retrograde alteration of both the early and hydrous skarns resulted in the local deposition of chlorite-carbonate-dominated assemblages. Tin liberated by the destruction of early skarn minerals during hydrous and retrograde alteration was commonly redeposited in situ as cassiterite.Mica alteration of the greisen and the formation of muscovite veins in the limestone and skarn were closely followed by the development of a small mica matrix breccia pipe centered over the apex of the granite cupola. During the waning stages of hydrothermal activity, cooling of the circulating fluids led to the formation of solution breccias consisting entirely of fluorite, kaolinitic clay, and Fe, W, and Sn oxides along previously fractured zones.Beryllium mineralization hosted by fluorite-white mica veins is widely distributed throughout the mine area, and at least in part it predates the alteration associated with the Lost River stock. A common spatial relationship with pregranite dikes suggests a tie between fluorite-white mica veining and igneous activity, but the veins do not appear to have been generated by the same processes which produced the skarn and greisen within and above the granite cupola.The juxtaposition of skarn and greisen at Lost River makes it an excellent location for the study of the evolution of alteration processes associated with felsic intrusive rocks. The fact that skarn alteration in part precedes greisenization of igneous rocks is in accordance with observations made by Soviet authors (see Scherba, 1968). Tin is introduced in the hydrothermal system prior to, and along with, the deposition of sulfide minerals, but the majority of cassiterite occurs with sulfides. The mobilization and redeposition of tin liberated from early skarn minerals by later alteration stages is undoubtedly an important process in the formation of economic tin skarns. Low total amounts of Fe and S, and very high F, can probably be attributed to the highly differentiated nature of the Lost River granite. The abundance of F in the volatile phase is probably largely responsible for the abundance of F-bearing minerals, such as idocrase and micas, and the scarcity of pyroxene in the skarn.