Johnson

Prospect, Inactive

Alternative names

Johnson River

Commodities and mineralogy

Main commodities Au; Cu; Pb; Zn
Ore minerals chalcopyrite; galena; gold; pyrite; sphalerite
Gangue minerals anhydrite; barite; chlorite; sericite

Geographic location

Quadrangle map, 1:250,000-scale KN
Quadrangle map, 1:63,360-scale A-8
Latitude 60.121
Longitude -152.954
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Johnson River prospect is located at an elevation of 1,500 feet on the northwest side of Hill 3050 overlooking the mouth of the Johnson Glacier, it is in the NW1/4 of section 6, T. 1 S., R. 21 W., of the Seward Meridian. This location is accurate to within 300 feet.

Geologic setting

Geologic description

The Johnson River prospect occurs within the lower Jurassic Talkeetna Formation, an approximately 2,575 meter thick sequence of volcanic and volcaniclastic rocks (Determan and Hartsock, 1966). The volcanic section near the prospect is structurally bounded on the west by the Bruin Bay Fault, a major fault system traceable for over 500 kilometers. On the east, the volcanics are unconformably overlain by upper Jurassic marine sediments (Determan and Hartsock, 1965). The Talkeetna Formation is intruded by rocks of the Aleutian range -Talkeetna Mountain plutonic belt approximately 23 km. southwest of the prospect. These intrusives which consist dominantly of coarse-grained quartz diorites and quartz monzonites have been dated between 175 and 145 m.y. (Steefel, 1987).
At the Johnson River Prospect, the Talkeetna Formation has been divided into three major groups (Steefel, 1987). The lowest group consist of purple and green andesite flows and breccia which locally show well-developed pillows. The middle group that host the mineralization, consists of coarse-grain felsic breccias, reworked volcaniclastic rocks, crystal tuffs, and dacite flows. The upper group consists of polymictic andesitic breccias and conglomerates with minor andesitic flows. Informally, the middle group has been named the Johnson unit (Steefel, 1987). Numerous structures within the unit including turbidites and reverse graded bedding indicate a subaqueous origin.
The mineralization and alteration at Johnson River can be subdivided into two stages which are mineralogically, chemically, and temporally distinct. Stage one is an early stage of nodular anhydrite and pyrite accompanied by magnesisum-chlorite, sericite, montmorillonite, and calcite. This stage is either geochemically barren or weakly anomalous in base metals and gold. (Steefel, 1987). Stage two which includes all the high-grade mineralization consists of quartz-sulfide mineralization accompanied by iron-chlorite, sericite, barite, carbonate, and vein-type anhydrite. Sulfide mineralization consists of early pale-colored sphalerite, later dark sphalerite, galena, chalcopyrite, pyrite, and native gold. The highest grade gold concentration is associated with chalcopyrite (Steefel, 1987). Overall the stage two, quartz-sulfide mineralization occurs in a fractured controlled, steeply plunging, pipe-like stockwork zone that measures 160 meters long by 50 meters wide and extends to a depth of at least 250 meters (Bill Ellis, personal communication, 1999).
Geologic map unit (-152.956231471807, 60.1203901456956)
Mineral deposit model Kuroko massive sulfide (Cox and Singer, 1986; model 28a).
Mineral deposit model number 28a
Age of mineralization Early Jurassic.
Alteration of deposit The alteration at Johnson River can be divide into two main events. An early stage of nodular anhydrite and fine-grained disseminated pyrite is accompanied by Mg-chlorite, sericite, montmorillonite and or calcite. This alteration is either barren or weakly anomalous in base metal mineralization. The later alteration consists of silicification and sulfidation accompanied by iron-chlorite, sericite, barite, carbonate, and vein anhydrite. This stage alteration includes all the mineralization at Johnson River (Steefel, 1987).

Production and reserves

Workings or exploration The deposit was discovered in 1975 by RAA as part of a regional exploration program. Anaconda Minerals acquired the prospect in 1981, and did detailed mapping, geophysics, geochemistry, and drilled 44 core drill holes (Bill Ellis, personal communication, 1999). This work defined a 160 meter long by 50 meters wide and 250 meters deep fault bounded, quartz-sulfide ore body. Additional drilling was done by Howard Keck in 1990 and 1991. As of 1997, the prospect contained a drilled out reserve (at $50.00 per ton cutoff) of 1,099,580 tons that contain 0.32 ounces of gold per ton, 0.24 ounces of silver per ton, 0.76 percent copper, 1.17 percent lead and 8.73 percent zinc (Swainbank and others, 1997).
Indication of production None
Reserve estimates As of 1997, the prospect contained a drilled out reserve at ($50.00 per ton cutoff ) of 1,099,580 tons that contain 0.32 ounces of gold per ton, 0.24 ounces of silver per ton, 0.76 percent copper, 1.17 percent lead and 8.73 percent zinc (Swainbank and others, 1997).

Additional comments

This prospect is owned by Cook Region Native Association.

References