Mallard Duck Bay

Occurrences, Active?

Alternative names

Calcite
Marshinlak

Commodities and mineralogy

Main commodities Ag; Au; Cu; Mo; Pb; Zn
Other commodities As
Ore minerals arsenopyrite; chalcopyrite; galena; gold; pyrite; sphalerite
Gangue minerals calcite; quartz; siderite

Geographic location

Quadrangle map, 1:250,000-scale CG
Quadrangle map, 1:63,360-scale A-2
Latitude 56.231
Longitude -158.511
Nearby scientific data Find additional scientific data near this location
Location and accuracy This site represents an area southeast of Chignik Lagoon at the head of Mallard Duck Bay (Berg and Cobb, 1967, locality 2; Cobb, 1972: MF-374, locality 2; MacKevett and Holloway, 1977, locality 2; Cox and others, 1981, locality 8; Nokleberg and others, 1987, locality AP 11). The map site is plotted at an elevation of about 1000 feet, about 0.2 miles southeast of the head of Mallard Duck Bay. The location is accurate to within 1/4 mile.

Geologic setting

Geologic description

This site represents a large propylitically altered zone covering an area 6 miles by 2 1/2 miles at the head of Mallard Duck Bay (Wilson and Cox, 1983). Elevation of the zone varies from sea level to 600 meters. The altered zone is associated with widespread introduction of pyrite and with low-order drainage anomalies (60 ppm copper, 15 ppm molybdenum, 0.3 ppm silver).
Rock types include diorite stocks up to 2000 feet in diameter and a system of northeast-trending quartz-diorite dikes which are up to 1/2 mile in length and up to several hundred feet in thickness. A system of andesite dikes trends N 30 to 40 W and dips vertically across a 4000-foot wide zone. All of these intrusives cut Eocene-Oligocene volcanics of the Meshik Formation which consists of andesite and basalt flows, volcaniclastic breccias, volcaniclastic sediments, and tuffaceous units.
Within the large area of propylitized rock is a zone of intense argillic alteration which covers an area of approximately 2 miles by 1/2 mile. This is referred to as the central argillic zone (Anderson and others 1979). The argillic alteration appears to be related to the northwest-trending andesite dikes because it forms envelopes up to 100 feet wide on either side of the dikes. The altered areas exhibit some silicification and introduction of disseminated pyrite. They also contain coarse stockworks of fractures, some of which contain quartz, pyrite, and rarely molybdenite.
Molybdenum mineralization occurs primarily in a stockwork that lies between two parallel andesite dikes outside of the central argillic zone. The mineralized rock is limonitic and exihibits silicification and minor argillization. As the dikes are approached the alteration increases and the molybdenum mineralization decreases (Anderson and others, 1979). The molybdenum zone has been traced for 100 feet but the total length is unknown. Fields (1977) notes molybdenum values ranging from 2 to 99 ppm in rock samples.
Systems of northwest-trending polymetallic veins are present in the propylitized volcanics. The Calcite and Mallard Duck vein systems occur in an area 1/2 to 1 mile southwest of the central argillic zone. Individual veins are up to 800 feet long and 10 feet wide. Other veins are known to extend along this trend for over 4 miles northwest to the beach exposures at Mallard Duck Bay. The Marshinlak vein system is located approximately 2 miles north of the central argillic zone and has been traced intermittently for over 6500 feet in a northwest direction. These veins are up to 15 feet in thickness. Minor strike trends include northeast, north-south, and east-west. The veins vary in composition from quartz to quartz-calcite to quartz-siderite and may carry up to 5 percent sulfides, including arsenopyrite, chalcopyrite, galena, pyrite, and sphalerite. Some veins exhibit cockscomb features and some show evidence of repetitive pulsing of quartz and sulfides.
Metal values in the Calcite and Mallard Duck systems range as follows: 3360 ppm to 4.2 percent copper, less than 0.02 ppm to 3.3 ppm gold, 1100 ppm to 4.45 percent lead, 20 ppm to 21 ounces silver ounces per ton, 1100 ppm to 22.7 percent zinc (Butherus and others, 1981). Trace amounts of antimony and mercury were also detected. Copper, lead, and zinc values obtained in the Marshinlak system were all less than 100 ppm. Gold values ranged from 0.48 ppm to 0.163 ounce per ton and silver values from 3.4 ppm to 0.54 ounce per ton.
As recently as 2013 at least, Millrock Resources Inc has made an agreement with Bristol Bay Native Corporation, the mineral land owner, to explore for minerals. By 2013, a regional aeromagnetic survey was completed. There appears to be good correlation between magnetic highs and known porphyry occurrences. Historic work includes reconnaissance surface sampling, regional scale mapping by Full Metal Minerals, an IP survey of one line, and a ground magnetic survey of one line (Beischer, 2013).
Geologic map unit (-158.513035070879, 56.2302189643943)
Mineral deposit model Porphyry copper; porphyry copper-molybdenum; polymetallic veins (Cox and Singer, 1986; models 17, 21a, 22c).
Mineral deposit model number 17, 21a, 22c
Age of mineralization The probable age of mineralization is estimated at 21-27 million years (Cox and others, 1981).
Alteration of deposit Propylitically altered volcanics cover an area of approximately 6 by 2 1/2 miles. Within this altered zone argillic alteration forms envelopes up to 100 feet wide peripheral to northwest-trending andesite dikes, resulting in a central argillic zone which covers an area of approximately 2 by 1/2 miles.

Production and reserves

Workings or exploration
Gold claims were staked in the early part of the century. Bear Creek Mining Company mapped and sampled this zone in 1975 and 1976. Resource Associates of Alaska did detailed mapping and sampling in 1979, 1981, and 1982.
As recently as 2013 at least, Millrock Resources Inc. has made an agreement with Bristol Bay Native Corporation, the mineral land owner, to explore for minerals. By 2013, a regional aeromagnetic survey was completed. There appears to be good correlation between magnetic highs and known porphyry occurrences. Historic work includes reconnaissance surface sampling, regional scale mapping by Full Metal Minerals, an IP survey of one line, and a ground magnetic survey of one line (Beischer, 2013). In 2014, Millrock completed a geophysical survey or high-resolution airborne magnetic and radiometric data (Millrock Resources Inc., 2014).
Indication of production None

Additional comments

This site is on land patented by or interim conveyed to the Bristol Bay Native Corporation.

References

MRDS Number A012449

References

Millrock Resources Inc., 2014, Millrock Completes Geophysical Surveys at Alaska Peninsula Project, Alaska, Press Release dated July 7, 2014: http://www.millrockresources.com/news/millrock-completes-geophysical-surveys-at-alaska-peninsula-project-alaska (as of July 14, 2014).
Reporters S.H. Pilcher (Anchorage); V.C. Zinno (Alaska Earth Sciences, Inc.)
Last report date 7/15/2014