Mt. Estelle Prospects, Yentna District, Matanuska-Susitna Borough, Alaska, USA

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Location: Numerous occurrences are within the area from Mt. Estelle on the west to Timber Creek on the south and 2 miles to the east of Mt. Estelle, all within T. 20 N., R. 20 W., T. 21 N., R., 19 W., and T. 21 N., R. 20 W. (U.S. Bureau of Mines, 1998, MAS # 0020840011; Alaska Department of Natural Resources claim records, 1987; MacKevett and Holloway, 1977, p. 76, locality 1; Cobb, 1972, MF 385, localities 1 and 2; and Reed and Elliott, 1970, p. 15, 21-22). The site covers about 10 square miles and the coordinates are at the center of the site.
Geology: The Mount Estelle pluton, dated at 65 to 66 m.y. by Reed and Lanphere (1972), is the southern most Late Cretaceous/early Tertiary composite pluton (Tcp) of the Yentna trend. This trend is described by Reed and Nelson (1980) as a 65-km-long curvilinear belt extending from Mount Estelle in the south to Cascade Creek in the northeast. These plutons primarily intrude Mesozoic marine sedimentary rocks (KJs) of the Kahiltna terrane. The Mount Estelle pluton (Tme; Reed and Elliott, 1970) is zoned from a granite core to marginal phases of quartz monzonite, quartz monzodiorite, augite monzodiore, diorite and lamprophyric mafic and ultramafic rocks (Millholland,1995; Crowe and others, 1991; Crowe and Millholland, 1990). Xenoliths of the KJs country rocks and of the various intrusive phases are noted throughout the pluton. Sheeted joint sets and unusual spherical, onion skin-like features occur primarly within the core of the pluton. Tourmaline and beryl are noted in and adjacent to the pluton. Foley and others (1997) note that the composite plutons at the southwestern end of the Yentna trend are more granitic and probably have not been as deeply eroded as the more mafic and ultramafic bodies to the northeast in the Talkeetna quadrangle. Adjacent to the Mount Estelle pluton the KJs is hornfelsed and locally exhibits red staining and sercite-clay-pyrite alteration as disseminations and fracture coatings. The altered sediments do not contain elevated gold values. Float and stream sediment samples from the Mount Estelle pluton indicate widespread polymetallic mineralization. Stream sediments indicate consistently positive anomalous amounts of gold, copper, arsenic, and silver. Mineralized rock samples, both disseminated and vein type, typically give high values for gold and copper and some contain anomalous amounts of silver, arsenic, molybdenum, bismuth, boron, lead, antimony, tin, zinc and manganese. Cominco American Incorporated's work (unpublished reports, 1985-1989) show gold values ranging from ppm-level to one or more ounces per ton in select samples. Native gold occurs with chalcopyrite, pyrite, arsenopyrite, and pyrrhotite associated with sericite, carbonate and chlorite alteration in sheeted joints, stockwork veinlets, and circular structures that range from 1 inches to more than 15 feet in diameter (Millholland,1995; Crowe and others, 1991; Crowe and Millholland, 1990). These structures are localized in the felsic and intermediate phases of the pluton. Gold associated with pyrrhotite, chalcopyrite, pentlandite and molybdenite also occurs in ultramafic rocks on the south side of the pluton. Mineralization is less common in the sedimentary rocks. In Muddy Creek one zone of coarse galena, pyrrhotite, sphalerite and chalcopyrite replaces carbonate-rich horizons in the hornfelsed sedimentary rocks. Anomalous gold, platinum-group elements, copper, chrome, nickel and arsenic are reported from many of the composite plutons of the Yentna trend (Reed and others, 1978; Reed and Nelson, 1980; Nelson and others 1992). Gold and platinum group element placers have been worked at sites downstream from these bodies (Mertie, 1919; Cobb, 1973). See also ARDF numbers TL001, TL 002, TL003, TL020, TL023, TL051, TL052, TL053. There are notable similarities between the Mount Estelle pluton and the Kohlsaat pluton described by Reiners and others (1997). Kohsaat is a concentrically zoned body with a biotite-granite-porphyry core that intruded seriate and porphyritic intermediate composition rocks. The latter include olivine-, pyroxene-, and biotite-bearing quartz syenite, quartz monzonite, and monzonite. Lamprophyric mafic and ultramafic rocks that range in composition from peridotite to alkali gabbro and monzonite are found as large xenoliths at the northeastern margin of the pluton. The granite- porphyry core is altered to sericite, quartz, carbonate and tourmaline assemblages, with minor enrichments of copper, gold, lead, molybdenum, and zinc. They have postulated that the composite pluton at Kohlsaat formed from depleted mantle melts that were contaminated during several stages of mafic magma generation by crustal components. See also ARDF number TL052.
Workings: Prospecting in the area has been conducted by several private companies since the 1970's. From 1980 to 1985 the ground was held for placer potential, however it was never worked because the large glacial boulders in the stream gravels hampered recovery efforts. Work on the property included: silt and rock geochemistry, mapping, magnetic, VLF, IP and radar geophysical surveys, and diamond drilling. To evaluate the steep, glaciated terrain, technical climbers from Dihedral Exploration were employed.
Age: Late Cretacous to Early Tertiary based on 65 to 66 m.y. intrusions that are probably related to the mineralization (Reed and Lanphere,1972).
Alteration: Alteration is weak and restricted to vein and joint selvages. Carbonate, chlorite, sericite, and quartz are commonly associated with the sulfide and gold mineralization (Millholland,1995; Crowe and others, 1991; Crowe and Millholland, 1990; Cominco American Incorporated, unpublished reports, 1985 -1989).

Commodities (Major) - Au, Cu; (Minor) - As, Mo, Pb, Sb, Zn
Development Status: None
Deposit Model: Intrusive-hosted gold-copper

References

Cobb, E.H., 1972, Metallic mineral resources map of the Tyonek quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-385, 1 sheet, scale 1:250,000. Crowe, D.E., and Millholland, M.A., 1990, High-grade gold mineralization associated with high salinity hydrothermal fluids, Mt. Estelle pluton, central Alaska Range [abs.]: Geological Society of America, Abstracts with Programs, v. 22, p. A41. Crowe, D.E., Millholland, M.A., and Brown, P.E., 1991, Precious and base metal mineralization associated with high-salinity hydrothermal fluids in the Mount Estelle pluton, south-central Alaska: Economic Geology, v. 86, p. 1103-1109. Foley, J.Y., Light, T.D., Nelson, S.W., and Harris, R.A., 1997, Mineral occurrences associated with mafic-ultramafic and related alkaline complexes in Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 396-449. Green, C.B., Bundtzen, T.K., Peterson, R.J., Seward, A.F., Deagan, J.R., and Burton, J.E., 1989, Alaska's mineral industry, 1988: Alaska Division of Geological and Geophysical Surveys Special Report 43, 79 p. MacKevett, E.M., Jr., and Holloway, C.D., 1977, Map showing metalliferous and selected non-metalliferous mineral deposits in the eastern part of southern Alaska: U.S. Geological Survey Open-File Report 77-169-A, 99 p., 1 sheet, scale 1:1,000,000. Millholland, M.A., 1995, Geology and discovery at Mount Estelle: Newsletter of the Alaska Geological Society, v. 24, no. 8, p. 1. Reed, B.L., and Elliott, R.L., 1970, Reconnaissance geologic map, analyses of bedrock and stream sediment samples, and an aeromagnetic map of parts of the southern Alaska Range: U.S. Geological Survey Open-file Report 70-271, 24 p., 4 sheets, scale 1:250,000. Reed, B.L., and Lanphere, M.A., 1972, Generalized geologic map of the Alaska-Aleutian range batholith showing potassium-argon ages of the plutonic rocks: U.S. Geological Survey Miscellaneous Field Studies Map MF-372, 2 sheets, scale 1:1,000,000. Reed, B.L., and Nelson, S.W., 1980, Geologic map of the Talkeetna quadrangle: U.S. Geological Survey Miscellaneous Investigations Map I-1174, 1 sheet, scale 1:250,000. U.S. Bureau of Mines, 1998, Minerals Availability System/Minerals Industry Location System (MAS/MILS), Talkeetna quadrangle: Worldwide Web URL http://imcg.wr.usgs.gov/data.html. [Link dead. Jul 2011]

Mineral List

Arsenopyrite Chalcopyrite 'Chlorite Group' Galena

Gold Limonite Malachite Molybdenite

Muscovite var: Sericite Pentlandite Pyrite Pyrrhotite

Quartz Sphalerite Stibnite

15 entries listed. 12 valid minerals.