Unnamed (Near Mount Estelle)

Prospects, Active

Commodities and mineralogy

Main commodities Au; Cu
Other commodities As; Mo; Pb; Sb; Zn
Ore minerals arsenopyrite; chalcopyrite; galena; malachite; molybdenite; native gold; pentlandite; pyrite; pyrrhotite; sphalerite; stibnite
Gangue minerals carbonate; chlorite; limonite; quartz; sericite

Geographic location

Quadrangle map, 1:250,000-scale TY
Quadrangle map, 1:63,360-scale D-8
Latitude 61.8497
Longitude -152.8928
Nearby scientific data Find additional scientific data near this location
Location and accuracy This records includes numerous prospects are in a 10-mile-square area near Mt. Estelle. The coordinates are at about the center of this area that includes parts of T. 20 N., R. 20 W., T. 21 N., R., 19 W., and T. 21 N., R. 20 W.

Geologic setting

Geologic description

Note: This is an early ARDF record that describes work in the general area of Mount Estelle. Exploration work from 2000 to 2008 (Hidefield Gold, Plc., 2008; International Tower Hill Mines, Ltd., 2007; Millrock Resources Inc., 2008) has defined several specific prospects in or near this area within this area or nearby that are described separately, e.g., the Train (T031) and Shoeshine (TY032) prospects. This record is retained for its general geologic information on the geology and mineralization in the area.
The Mount Estelle pluton, dated at 65 to 66 Ma by Reed and Lanphere (1972), is the southern-most Late Cretaceous/early Tertiary composite pluton in 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 of the Kahiltna terrane. The Mount Estelle pluton (Reed and Elliott, 1970) is zoned from a granite core to marginal phases of quartz monzonite, quartz monzodiorite, augite monzodiorite, diorite and lamprophyric mafic and ultramafic rocks (Millholland, 1995; Crowe and others, 1991; Crowe and Millholland, 1990). Xenoliths of the country rocks and of the various intrusive phases occur throughout the pluton. Sheeted joint sets and unusual spherical, onion-skin-like features occur in the core of the pluton. Sparse tourmaline and beryl occur 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 in composition 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 country rock is hornfelsed and locally exhibits red staining and sericite-clay alteration, and pyrite in disseminations and along fractures.
Float and stream-sediment samples taken in the vicinity of the Mount Estelle pluton suggest widespread polymetallic mineralization. Stream sediments consistently contain anomalous gold, copper, arsenic, and silver. Mineralized rock samples 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) shows gold values. 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 inch to more than 15 feet in diameter (Millholland, 1995; Crowe and others, 1991; Crowe and Millholland, 1990). These structures are 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 several sites downstream from the plutons (Mertie, 1919; Cobb, 1973; ARDF records TL001, TL 002, TL003, TL020, TL023, TL051, TL052, and TL053).
There are notable similarities between the Mount Estelle pluton and the Kohlsaat pluton described by Reiners, Nelson, and Nelson (1996). The Kohsaat pluton is a concentrically-zoned body with a biotite-granite-porphyry core that intrudes 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 form 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. The composite Kohlsaat pluton may have formed from depleted mantle melts that were contaminated by crustal components during several stages of mafic magma generation. Also see ARDF record TL052.
Geologic map unit (-152.895023925057, 61.8491042633301)
Mineral deposit model Intrusive-hosted gold-copper deposits.
Age of mineralization Late Cretaceous to Early Tertiary based on 65 to 66 Ma. intrusions that are probably related to the mineralization (Reed and Lanphere, 1972).
Alteration of deposit 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 through 1989).

Production and reserves

Workings or exploration
Prospecting in the area has been conducted by several private companies since the 1970s. From 1980 to 1985, many of the claims were held for their placer potential; however, they were never mined to any great extent because the large glacial boulders in the stream gravels hampered recovery. Work in the area 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.
This is an early ARDF record that describes work in the general area of Mount Estelle and has been retained for its regional geologic and mineral information. Exploration work from 2000 to 2008 (Hidefield Gold, Plc., 2008; International Tower Hill Mines, Ltd., 2007; Millrock Resources Inc., 2008) has defined several prospects in or near that are described separately, e.g., the Train (TY031) and Shoeshine (TY032) prospects.
Indication of production None

References

MRDS Number A010587

References

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.
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.
Hidefield Gold Plc., 2008, South Estelle: http://www.hidefieldgold.com/s/SouthEstelle.asp (as of May 25, 2008).
International Tower Hill Mines Ltd., 2007, ITH further defines mineralization at South Estelle: http://www.ithmines.com/s/NewsReleases.asp?ReportID=267305&_Type=News-Releases&_Title=ITH-Further-Defines-Mineralization-at-South-Estelle (News Release, October 18, 2007).
Millholland, M.A., 1995, Geology and discovery at Mount Estelle: Newsletter of the Alaska Geological Society, v. 24, no. 8, p. 1.
Millrock Resources Inc., 2008, Estelle: http://www.millrockresources.com/index.php/projects/estelle/ (as of May 25, 2008).
Reiners, P.W., Nelson, B.K., and Nelson, S.W., 1996, Evidence for multiple mechanisms of crustal contamination of magma from compositionally zoned plutons of the Alaska Range: Journal of Petrology, v. 37, p. 261-292.
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.
Reporters Madelyn A. Millholland (Anchorage, AK), James Riehle (USGS); D.J. Grybeck (Port Ludlow, WA)
Last report date 6/5/2008