Groundhog Basin Prospects, Petersburg Mining District, Wrangell City and Bourough, Alaska, USAi

Regional Level Types
Groundhog Basin Prospects	Group of Prospects
Petersburg Mining District	Mining District
Wrangell City and Bourough	Borough
Alaska	State
USA	Country

The Groundhog Basin prospect is covered by 4 patented claims.
Location: The coordinates are near the center of a group of adits and other workings that extend for nearly a mile on the east side of upper Groundhog Basin The prospect is is in the NW1/4, section 7, T. 62 S., R. 86 E. between elevations of 2,000 and 2,500 feet. Gault and others (1953) includes an excellent detailed geologic map of the prospects, as do Still and others (2002).
Geology: The country rocks in the area of the Groundhog Basin base-metal deposits are Tertiary to Cretaceous biotite schist, biotite-garnet-quartz schist, quartzofeldspathic gneiss, and minor marble and calc-silicate gneiss, all metamorphosed from Mesozoic or Paleozoic protoliths (Gault and others, 1953; Brew, 1997; Still and others, 2002). The metamorphic rocks near the prospects are cut by several large Tertiary felsic dikes and sills that are related to a biotite granite stock that is well exposed about a mile northeast of the belt of mineralization that contains this deposit. The granite has been dated at 16.3 Ma. Newberry and Brew (1989) classify this stock as a 'zinnwaldite' or 'tin' granite and they genetically relate this granite to the base metal prospects in Groundhog Basin and also the nearby porphyry molybdenum deposit (PE102) . A large Cretaceous tonalite pluton intrudes the metamorphic rocks less than 3,000 feet west of the Groundhog Basin deposits. The Groundhog Basin deposits were discovered in 1904 and were extensively explored in 1916-1917. Four claims were patented over the mineralization in 1930. In the early 40's, Ventures Ltd. did considerable surface trenching, drove about 450 feet of underground workings from four adits, and drilled three holes, 107 to 335 feet deep. Gault and others (1953) described much of this early work and did considerable geologic mapping in the area. In 1976, Bunker Hill Mining Company optioned this property and a large block of claims around it (Still and others, 2002). They collected surface samples and drilled 24 holes, 25 to 350 feet deep, but dropped the property at the end of the field season. From 1968 through 1981, Groundhog Basin and the surrounding area was optioned by a succession of companies--Humble Oil and Refining Co., El Paso Natural Gas Co. and AMAX Exploration Inc.--but they did little work specifically on the Groundhog Basin deposits. In 1983, Houston Oil and Minerals Exploration Company sampled the deposits. In 1988, Newberry and Brew (1989) studied the core from previous drilling and the company reports on the Groundhog Basin prospects; they were the first to publicly report on the tin content of the mineralization. The deposit has undoubtedly been examined by numerous companies since but there has been no extensive work since about 1990. Four distinct steeply-dipping 'ore beds' have been defined, which collectively extend for nearly a mile and parallel the strike of the metamorphic host rocks. The most extensive and thickest, beds 3 and 4, extend horizontally for about 3,700 feet through a vertical distance of about 1,500 feet (Gault and others, 1953; Still and others, 2002) The ore beds consist of a) masses of ore minerals up to several feet thick, mainly of sphalerite, pyrite, pyrrhotite, galena, chalcopyrite, magnetite, and cubanite(?); and b) layers containing the same assemblage of ore minerals disseminated through the metamorphic host rock. The ore beds are interlayered with steeply-dipping, medium- to high-grade pelitic and quartzofeldspathic schist and gneiss, and locally with banded calc-silicate gneiss. Garnet, pyroxene, and epidote are common near the ore beds in the calcareous rocks and locally form massive skarns. (There is considerable detail available that can be found in Gault and others (1953) and Still and others (2002) on the geometry and sampling of the ore beds that is only summarized here.) Newberry and Brew (1989) identified tin in cassiterite as a major constituent of the ore beds and has classified the deposits as Ag-Sn-Pb-Zn skarns that replace calcareous beds in the schist and gneiss. They genetically tie the skarns to a 16.3 Ma zinnwaldite 'tin' granite that crops out north of the deposits. Near this zinnwaldite granite, massive sulfide samples commonly contain several percent tin and selected samples contain up to 18%. They interpreted the mineral paragenesis as: 1) formation of pervasive albite-zinnwaldite gneiss in the cupola of an evolving granite with formation of pyroxene-garnet in adjacent biotite schist and mafic dikes; 2) deposition of lower temperature zinnwaldite-sphalerite-cassiterite veins in the granite and formation of the Ag-Sn-Pb-Zn ore bodies in Groundhog Basin by replacement of calcareous layers in the schist and gneiss; and 3) peripheral sphalerite-galena-fluorite veins as distal, lower temperature manifestation of the granite-related hydrothermal system. Gault and others (1953), document various attempts to define the ore reserves and resources of the deposit and to quantify the size and grade of certain portions of the 'ore beds.' They conclude, however, that there is insufficient information to justify making detailed estimates of the ore reserves in Groundhog Basin. However, in summary, they state, '...it appears reasonably certain that several hundred thousand tons each of solid and disseminated ore are present.' There solid ore contains about 8 percent zinc, 1.5 percent lead, and 1.5 ounces of silver per ton. The disseminated ore contains about 2.5 percent zinc and 1 percent lead. Newberry and Brew (1989) estimated that the deposit contains about 1 million tonnes of ore containing 0.8 percent tin but emphasize the great uncertainty of their estimate. Still and others (2002) note several other estimates of the reserves or resources of the Groundhog Basin deposit. They conclude that the best mineralization is the 'solid ore' in beds 3 and 4 as exposed on the surface, in underground workings, and in drill holes. They estimate that those beds contain about 466,000 tons of 'indicated and inferred resources' with an average grade of 8 percent zinc, 3.5 percent lead, 1 ounce of silver per ton, and 0.39 percent copper. The molybdenite-fluorite deposits that overlap this site are described separately (PE102).
Workings: The Groundhog Basin deposits were discovered in 1904 and were extensively explored in 1916-1917. Four claims were patented over the mineralization in 1930. In the early 40's, Ventures Ltd. did considerable surface trenching, drove about 450 feet of underground workings from four adits, and drilled three holes, 107 to 335 feet deep. Gault and others (1953) described much of this early work and did considerable geologic mapping in the area. In 1976, Bunker Hill Mining Company optioned this property and a large block of claims around it (Still and others, 2002). They collected surface samples and drilled 24 holes, 25 to 350 feet deep, but dropped the property at the end of the field season. From 1968 through 1981, Groundhog Basin and the surrounding area was optioned by a succession of companies--Humble Oil and Refining Co., El Paso Natural Gas Co. and AMAX Exploration Inc.--but they did little work specifically on the Groundhog Basin deposits. In 1983, Houston Oil and Minerals Exploration Company sampled the deposits. In 1988, Newberry and Brew (1989) studied the core from previous drilling and the company reports on the Groundhog Basin prospects; they were the first to publicly report on the tin content of the mineralization. The deposit has undoubtedly been examined by numerous companies since but there has been no extensive work since about 1990.
Age: 16.3 Ma based on a genetic tie to a nearby, zinnwaldite 'tin' granite (Newberry and Brew, 1989).
Alteration: Deposit associated with formation of pyroxene-epidote-garnet skarn in the host rocks.
Reserves: Gault and others (1953), document various attempts to define the ore reserves and resources of the deposit and to quantify the size and grade of certain portions of the 'ore beds.' They conclude, however, that there is insufficient information to justify making detailed estimates of the ore reserves in Groundhog Basin. However, in summary, they state, '...it appears reasonably certain that several hundred thousand tons each of solid and disseminated ore are present.' There solid ore contains about 8 percent zinc, 1.5 percent lead, and 1.5 ounces of silver per ton. The disseminated ore contains about 2.5 percent zinc and 1 percent lead. Newberry and Brew (1989) estimated that the deposit contains about 1 million tonnes of ore containing 0.8 percent tin but emphasize the great uncertainty of their estimate. Still and others (2002) note several other estimates of the reserves or resources of the Groundhog Basin deposit. They conclude that the best mineralization is the 'solid ore' in beds 3 and 4 as exposed on the surface, in underground workings, and in drill holes. They estimate that those beds contain about 466,000 tons of 'indicated and inferred resources' with an average grade of 8 percent zinc, 3.5 percent lead, 1 ounce of silver per ton, and 0.39 percent copper.

Commodities (Major) - Ag, Pb, Sn, Zn; (Minor) - Au, Cu, Mo, Sn
Development Status: No
Deposit Model: Banded Ag-Cu-Sn-Pb-Zn tabular replacement bodies, veins, and stringers.

Other Databases

Link to USGS - Alaska:	PE040

Other Regions, Features and Areas containing this locality

References