Third Of July Prospect, Port Clarence Mining District, Nome Census Area, Alaska, USAi

Regional Level Types
Third Of July Prospect	Prospect
Port Clarence Mining District	Mining District
Nome Census Area	Census Area
Alaska	State
USA	Country

Location: The Third of July prospect is located at an elevation of 1,250 feet on the south side of an Ordovician limestone ridge that separates the headwaters of Anderson Creek and an unnamed 13-mile long stream that flows south to Brevig Lagoon. Anderson Creek, with headwaters on the southeast slopes of Brooks Mountain, flows east to the Don River. The prospect is on the south side of a prominent saddle (1,350 feet elevation) developed at the head of north and south draining gulches. The pass between Anderson Creek headwaters and the unnamed stream to the south, at 640 feet elevation, is 0.75 miles west of the prospect. This locality was not identified by Cobb and Sainsbury (1972) or Cobb (1975).
Geology: This prospect is a 500 x 500 foot area of prophyritic granite intrusion in Ordovician limestone. Sainsbury (1969, plate 1) mapped the location of the granite and noted the presence of beryllium-bearing minerals here. Anaconda Minerals Company completed detailed mapping, sampling, and gravity and magnetic surveys in the prospect area during parts of the summers of 1982 and 1983 (Hudson and Wyman, 1983). This work indicates that the exposed granite is a small part of a larger granite body at depth. The exposed intrusion is porphyritic biotite granite with quartz and feldspar phenocrysts in an aplitic groundmass. Thin, 1 to 2.5 inch wide aplite dikes, some with disseminated purple fluorite, cut the porphyritic granite. The age of the mineralization is assumed to be similar to that of tin systems in the Lost River area and therefore Late Cretaceous, the age of the tin-mineralizing granites there (Hudson and Arth, 1983). Ordovician limestone intruded by the granite has been thermally recrystallized over large areas. Thin quartz-fluorite, calcite-fluorite, or fluorite veins and veinlet stockworks are widespread and common within 300 feet of the granite contact. Minor skarn, developed locally along the limestone-granite contact, includes light green quartz, fluorite, epidote, calcite bands with purple fluorite veins and some sulfide minerals. Sulfide minerals include pyrite and pyrrhotite in altered limestone and granite. Endoskarn is locally developed where remnant quartz phenocrysts are present within light to dark green epidote-fluorite-sericite assemblages. Intense silicification of limestone is also locally present in the contact zone. The porphyritic granite is altered to greisen along structural zones and fractures. These altered rocks typically contain vuggy clots of tourmaline crystals in a light colored, aphanitic matrix (quartz and mica?). Vuggy quartz-tourmaline veins up to one-foot wide cut granite and greisen. The exposed granite is similar to precursor granites (Hudson and Reed, 1997, figure 3) and not the fine-grained, equigranular, and leucocratic granite more directly associated with tin metallization elsewhere on Seward Peninsula (Hudson and Arth, 1983). A mineralizing granite phase could be present in the subsurface of this area. Most cassiterite mineralization is associated with altered rocks within granite. Gossan fragments have up to 0.2% tin but only weakly anomalous lead and zinc (100 to 500 ppm). Arsenic, tungsten, and base metals are generally present at low levels; elevated tin, fluorine, and boron characterize the metasomatism here. Greisen samples contain up to 0.4% tin, 5,800 ppm fluorine, and greater than 20,000 ppm boron.
Workings: Surface mapping and sampling and a few shallow hand-dug surface pits have been completed here. An airborne magnetic survey and a gravity survey have been completed over the prospect (McDermott in Hudson and Wyman, 1983, p. 68-71).
Age: The age of the mineralization is assumed to be similar to that of tin systems in the Lost River area and therefore Late Cretaceous, the age of the tin-mineralizing granites there (Hudson and Arth, 1983).
Alteration: Thermal recrystallization of limestone, minor skarn in limestone, minor endoskarn in granite, fluorite-bearing veins and veinlets in both limestone and granite, and local greisenization of granite are all present in the prospect area.
Reserves: Not defined

Commodities (Major) - Sn
Development Status: None
Deposit Model: Tin greisen in granite. (Cox and Singer, 1986; model 15c)

Other Databases

Link to USGS - Alaska:	TE054

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References