Lost River-skarn Mine, Port Clarence District, Nome Borough, Alaska, USA
|Latitude & Longitude (WGS84):||65° 28' 26'' North , 167° 9' 21'' West|
|Latitude & Longitude (decimal):||65.4738888889, -167.155833333|
Location: The Lost River Mine area includes the Cassiterite dike exogreisen deposit (TE048), the Lost River Mine skarn deposit (TE049), the Lost River Mine endogreisen deposit (TE050), and the Ida Bell dike exogreisen deposit (TE051). The Lost River skarn deposit is located 0.9 to 1 mile up Cassiterite Creek from its confluence with Lost River. This confluence is 5 miles upstream from the mouth of Lost River on the Bering Sea coast. The deposit is developed adjacent to and 800 feet south of the surface trace of the Cassiterite dike (Dobson, 1982, figure 3). It extends across Cassiterite Creek at an elevation of about 275 feet but most of the deposit (both at the surface and in the subsurface) is east of the creek. This deposit was included as part of locality 8 by Cobb and Sainsbury (1972). References were summarized under the name 'Lost River' by Cobb (1975).
Geology: The Lost River skarn is a roughly equidimensional, 10 million cubic yard volume of intense calc-silicate veining and replacement in Ordovician limestone above the apex of a fine-grained, equigranular, and leucocratic granite cupola. The buried granite cupola is known from drill core (Dobson, 1982, figure 6) and underground workings of the Lost River mine (Sainsbury, 1964, plate 10). The age of the mineralization is assumed to be related to the development of tin systems in the Lost River area and therefore Late Cretaceous, the age of the tin-mineralizing granites there (Hudson and Arth, 1983). Fine-grained, leucocratic granite collected from a Lost River Mine dump has been dated at 80.2 +/- 2.9 my (Hudson and Arth, 1983, p.769). As described by Dobson (1982), the skarn grades outward from a core of intense calc-silicate veining and replacement to a peripheral zone of fluorite-mica veining. A core of early anhydrous skarn, dominated by garnet and idocrase, was subsequently overprinted and enlarged by a hydrous skarn with abundant fluorite, biotite, and hornblende. Less intense veining, mostly fluorite-mica veins but also hydrous skarn veins, extends outward several hundred feet from the center of skarn development. Late-forming hydrothermal breccias overprint the center of the skarn. Tin was introduced with the early anhydrous skarn development where it was primarily incorporated in silicate phases such as andraditic garnet, although some cassiterite and base metal sulfide minerals did accompany later idocrase and garnet veining at this stage. Cassiterite became common as part of the hydrous skarn, which also included fluorite, scheelite, and sulfide minerals such as sphalerite, chalcopyrite, pyrrhotite, and pyrite in addition to the hydrous silicates (biotite and hornblende). Cassiterite and wolframite also accompanied the late fluorite-mica veining. Dobson (1982) points out that destruction of early calc-silicate minerals by hydrous skarn as well as later hydrothermal mica- and clay-matrix breccias appears to have remobilized and redeposited tin as cassiterite, thereby upgrading the recoverable tin content of the skarn as a whole. Extensive diamond drilling of this skarn by Lost River Mining Corporation led to a resource calculation of 23, 527,000 tons grading 16.43% fluorite, 0.26% tin, and 0.04% WO3 that could be mined by open pit methods (WGM, 1972, p. 63). However, the spatial and mineralogic complexity of the deposit documented by Dobson (1982) suggests caution in using this early estimate of tonnage and grade. Dobson (1982) developed a temporal and spatial framework for understanding relations between skarn evolution and development of veining and greisen in the subjacent granite cupola and the superjacent Cassiterite dike exogreisen deposit. In general, it appears that the overall polymetallic and aluminous character, the abundance of fluorine, and the significant potassium enrichment of the skarn reflect evolution of the highly evolved felsic magma in the subjacent granite pluton.
Workings: Some of the underground workings of the Lost River mine encounter parts of the Lost River skarn and it is reasonably well exposed at the surface in outcrops and dozer trenches. However, it is primarily known from extensive diamond drilling (WGM, 1972, p. 63) which includes that of the USBM (22 holes totalling 8,693 feet), USDMEA (several underground holes totalling 1,984 feet), US Steel Corporation (15 holes totalling 5,201 feet) and Lost River Mining Corporation (68 holes totalling 36,949 feet).
Age: The age of the mineralization is assumed to be related to the development of tin systems in the Lost River area and therefore Late Cretaceous, the age of the tin-mineralizing granites there (Hudson and Arth, 1983). Fine-grained, leucocratic granite collected from a Lost River Mine dump has been dated at 80.2 +/- 2.9 my (Hudson and Arth, 1983, p.769).
Alteration: There are several stages and styles of alteration in the Lost River skarn deposit; (1) early anhydrous skarn with abundant garnet and idocrase, (2) hydrous skarn with biotite and hornblende, (3) fluorite-mica veining, (4) mica-matrix breccias, and (5) clay-matrix breccias.
Production: Production from the Lost River Mine has been from the Cassiterite dike exogreisen deposit (TE048).
Reserves: Extensive diamond drilling of this skarn by Lost River Mining Corporation led to a resource calculation of 23, 527,000 tons grading 16.43% fluorite, 0.26% tin, and 0.04% WO3 that could be mined by open pit methods and 1,275,000 tons of 11.66% fluorite, 0.15% tin, and 0.01% WO3 that would need to be mined by underground methods (WGM, 1972, p. 63). However, the spatial and mineralogic complexity of the deposit documented by Dobson (1982) suggests caution in using this early estimate of tonnage and grade.
Commodities (Major) - Fluorite, Sn, W; (Minor) - Ag, Cu, Pb
Development Status: None
Deposit Model: Tin-bearing skarn (Cox and Singer, 1986; model 14b)
9 valid minerals.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org
443.8 - 485.4 Ma
Age: Ordovician (443.8 - 485.4 Ma)
Description: Brooks Range, Chukotka, Arctic Shelf, Brooks Range; Seward, western Chukotka
Comments: Orogen, fold-thrust belt, folded region; Wilson & Hults, unpublished compilation, 2007-08
Reference: J.C. Harrison, M.R. St-Onge, O.V. Petrov, S.I. Strelnikov, B.G. Lopatin, F.H. Wilson, S. Tella, D. Paul, T. Lynds, S.P. Shokalsky, C.K. Hults, S. Bergman, H.F. Jepsen, and A. Solli. Geological map of the Arctic. doi:10.4095/287868. Geological Survey of Canada Map 2159A. 
|Middle Ordovician - Early Ordovician|
458.4 - 485.4 Ma
|Argillaceous limestone and limestone|
Age: Ordovician (458.4 - 485.4 Ma)
Description: Thin-bedded, argillaceous, silty and (or) dolomitic limestone, lesser massive micritic limestone, and local chert; rocks are light gray to medium gray and weather light gray to orange to tan. The unit is widely exposed in and adjacent to the York Mountains in the western and central Teller quadrangle, and is at least 350 m thick (Dumoulin and Harris, 1994). Like unit Ol, it contains 8- to 15-m-thick shallowing-upward cycles (Vandervoort, 1985) and locally abundant trace fossils, but Oal is less fossiliferous than Ol and includes quartzose grainstone and ripple marks not seen in Ol. Common rock types in Oal are dolomitic, locally argillaceous lime mudstone and grainstone made up mainly of peloids and intraclasts with lesser bioclasts and ooids. Mud-supported strata are bioturbated, with bedding-plane feeding trails and subvertical burrows. Grain-supported rocks are planar- to cross-bedded with locally well developed oscillation and current ripples. Some grainstones contain 10 to 40 percent fine-sand- to silt-size non-carbonate grains, mainly quartz and lesser feldspar, with trace amounts of pyroxene, zircon, and leucoxene (Sainsbury, 1969b). Most exposures of Oal are fault-bounded, and its original depositional relations with other units in the York Mountains are uncertain. Sainsbury (1969b) reported that Oal conformably underlies Ol, but megafossil and conodont data suggest that the upper part of Oal is coeval with much of Ol. Oal is chiefly of Early Ordovician (Tremadoc-early Arenig) age; the tightest ages are based on conodonts (Table A-1). The oldest conodonts represent the Ro. manitouensis Zone, and are older than any definitively dated faunas known from Ol. Younger collections in Oal, however, include those of Mac. dianae Zone age and overlap ages determined for Ol. Sparse megafossils in Oal include brachiopods, gastropods, and trilobite fragments (Sainsbury, 1969b); echinoderm debris, calcareous sponge spicules, and possible calcispheres were noted in thin sections. Various types of stromatolites occur locally and form biostromes as much as 5 m thick (Sainsbury, 1969b; Vandervoort, 1985).Lithologic and fossil data indicate that Oal was deposited in a range of subtidal to supratidal settings within a deepening-upward regime (Dumoulin and Harris, 1994); overall, Oal appears to have formed in somewhat shallower and more agitated water than Ol. Conodonts in Oal are scarcer and less diverse than in Ol, likely because Oal accumulated more rapidly and (or) suffered more terrigenous input. Conodont assemblages in Oal are mainly cosmopolitan but include a few Laurentian (North American) and Siberian endemic forms (Dumoulin and Harris, 1994; Dumoulin and others, 2002; J.E. Repetski, written commun., 2008). Some rocks presently included in O
Reference: Wilson, F.H., Hults, C.P., Mull, C.G, and Karl, S.M. (compilers). Geologic map of Alaska. doi: 10.3133/sim3340. U.S. Geological Survey Scientific Investigations Map 3340, pamphlet 196.