Big Five Prospect, Kasaan Peninsula, Prince of Wales Island, Ketchikan Mining District, Prince of Wales-Hyder Census Area, Alaska, USAi

Regional Level Types
Big Five Prospect	Prospect
Kasaan Peninsula	Peninsula
Prince of Wales Island	Island
Ketchikan Mining District	Mining District
Prince of Wales-Hyder Census Area	Census Area
Alaska	State
USA	Country

Location: As described by Warner and others (1961), the Big Five prospect is about 4,600 feet N65W of Tolstoi Mountain at an elevation of about 250 feet. However, the contours on their map do not coincide well with the modern USGS 1:63,360-scale topographic map. For this record, the prospect is in the SE1/4 section 5, T, 72 S., R. 85 E. The geology and workings in the area are shown on plate 16 of Warner and others (1961).
Geology: The Big Five is a small, old prospect northwest of Tolstoi Mountain. The deposit was explored by a 40-foot adit with a 15-foot winze, probably before 1908. There is no record of production and little evidence of more recent work. The deposit is in impure limestone near the contact of a northwest-trending diorite dike (Warner and others, 1961, plate 16). Near the diorite, the limestone has been altered to tactite. The deposit consists of small pods and stringers of pyrrhotite with magnetite and chalcopyrite. The Big Five prospect is one of many copper-iron deposits on the Kasaan Peninsula having similar geology and origin (Warner and others, 1961; Eberlein and others, 1983; Brew, 1996). The rocks on the peninsula consist mainly of andesite ('greenstone' in much of the older literature) interbedded with about 25 percent sedimentary rocks comprising approximately equal amounts of limestone or marble, calcareous mudstone and sandstone, and graywacke and conglomerate. These units are part of the Luck Creek Breccia of Silurian and Devonian age, but many of the sedimentary units are similar to and probably grade into rocks of the Silurian and Ordovician, Descon Formation. The bedded rocks are intruded by a profusion of Silurian or Ordovician dikes, sills, and irregular masses of porphyritic gabbro, basalt, andesite, diorite, dacite, and granodiorite. Near some of the deposits, these intrusions may make up 20 percent or more of the outcrop and usually are associated with the development of tactite and alteration of the greenstone. The area subsequently was intruded by several large Silurian or Ordovician plutons; they are mainly granodiorite but locally are diorite and gabbro. The ore deposits are typically small and of irregular shape; often the ore bodies form lenses or mantos. Some of the deposits conform to the layering in the greenstone and sedimentary rocks. The principal ore minerals are chalcopyrite, pyrite, and magnetite; hematite is often present and a little molybdenite occurs in some deposits. Most of the deposits are associated with tactite or skarn with varying amounts of actinolite, calcite, chlorite, garnet, diopside, epidote, and hornblende. There was significant by-product silver and gold in the ore that was mined in the past, and the gold values in some deposits are high enough to have encouraged exploration in recent years. Marble is more common in the deposits in the western part of the peninsula, where the gold values are generally higher as well (Wright and Wright, 1908; Wright, 1915; Warner and others, 1961; Myers, 1985; Bond, 1993; Maas and others, 1995). Early interpretations of the ore deposits on the Kasaan Peninsula emphasize their contact metamorphic origin and their probable Mesozoic age (for example, Warner and others, 1961). However, recent radiometric dating and mapping indicate that the deposits formed in a Silurian or Ordovician, arc-related environment characterized by deposition of andesite and submarine sedimentary rocks that were intruded by swarms of dikes of varying composition, mineralized, and then intruded by large granodiorite plutons (Hedderly-Smith, 1999 [Inventory]). The copper deposits of the Kasaan Peninsula were known to the Russians and the first claim was staked in 1867. Most of the production and development occurred from about 1900 to 1918, especially from 1905 to 1907, when copper prices soared and a smelter was built at Hadley on the north side of the Kasaan Peninsula. After World War I, copper supply exceeded demand, prices fell, and there has been no further copper production since 1918 (Wright, 1915; Warner and others, 1961; Roppel, 1991; Maas and others, 1995). However, because of the intense and widespread mineralization on the peninsula, the area has repeatedly been re-examined for copper, iron, and gold, notably during WW II (Warner and others, 1961) and in the last several decades.
Workings: The Big Five is a small, old prospect northwest of Tolstoi Mountain. The deposit was explored by a 40-foot adit with a 15-foot winze, probably before 1908.
Age: The deposit formed in a Silurian or Ordovician, submarine arc-related environment characterized by the deposition of volcanic and sedimentary rocks, the intrusion of swarms of dikes of diverse composition, and the emplacement of several large plutons.
Alteration: Limestone is altered to tactite near diorite body.
Production: There is no record of production.

Commodities (Major) - Cu, Fe
Development Status: None
Deposit Model: Cu-Fe skarn (Cox and Singer, 1986; model 18d).

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Link to USGS - Alaska:	CR065

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References