Occurrence, Inactive

Commodities and mineralogy

Main commodities Pb; Zn
Other commodities Ag; Cu
Ore minerals chalcopyrite; galena; pyrite; sphalerite
Gangue minerals calcite; chlorite; quartz; sericite

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale B-1
Latitude 63.2803
Longitude -144.1323
Nearby scientific data Find additional scientific data near this location
Location and accuracy The MB occurrence is 1.6 miles south of the confluence of Rumble Creek and the Robertson River and in the NE1/4SE1/4 section 6, T. 17 N., R. 7 E., Copper River Meridian. The location is accurate.

Geologic setting

Geologic description

The MB sulfide occurrence consists of disseminated pyrite, sphalerite, galena, and chalcopyrite in a medium- to coarse-grained quartzose calcareous arenite. The mineralized horizon crops out at the MB discovery area and again approximately 1,400 feet to the east. Four grab samples from the MB area contained an average of 0.25 percent copper, 2.78 percent lead, 2.82 percent zinc, 2.58 ounces of silver per ton, and traces of gold. The rocks above and below the mineralized layer or unit include interbedded discontinuous arenite, graphitic siltstone, and quartzite. Thin, interbedded felsic metavolcanic rocks occur sporadically through the section. Several gabbroic sills 12 to 15 feet thick are present in the more graphitic horizons above and below the mineralized zone. Northwest-trending faults locally cut the mineralized horizon (R.A. Blakestad and others, unpublished Resource Associates of Alaska, Inc. report, 1978). Subsequent examinations by American Copper and Nickel Company concluded that mineralization is associated with and adjacent to these fault structures (E. Hunter, unpublished data, 1996).
The MB occurrence is in the lower Lagoon unit, which has a basal section of banded medium- to coarse-grained quartz-sericite(-chlorite) schist and carbonaceous schist; the upper section consists of fine-grained schist and phyllite. The protoliths of the basal section are immature sediments or wackes, mudstone, quartz arenite, and lesser calcareous arenite and carbonates. The lower Lagoon unit typically has interbeds of thin, gray to white and pale-green, interbedded felsic metavolcanic rocks. The metavolcanic rocks were originally mainly rhyolite and rhyodacite with rare andesite and basalt. A graphitic layer near the lower contact is a prominent stratigraphic marker horizon that can be traced in float and by electromagnetic surveys as a low-resistivity zone. A less prominent but distinctive chlorotoid-kyanite-bearing horizon that can be traced for 3 miles along strike coincides with several volcanogenic massive sulfide deposits. This distinctive mineral assemblage may have originally been an advanced-argillite alteration zone. Such zones have been associated with high-sulfidation volcanogenic massive sulfide belts elsewhere in the world (Dashevsky and others, 2003).
Geologic map unit (-144.134551749959, 63.2799247920217)
Mineral deposit model Kuroko massive sulfide (Cox and Singer, 1986; model 28a).
Mineral deposit model number 28a
Age of mineralization The lower Lagoon unit, which is part of the metamorphic sequence that includes the rocks at this deposit, has been dated as Devonian on the basis of one SHRIMP U-Pb zircon age of 372 +/-6 Ma at the LZ East prospect (MH328) (Dashevsky and others, 2003).
Alteration of deposit Intense carbonate and mariposite alteration is associated with mineralization; peripheral alteration is silicification and carbonization (Lange and others, 1993). A distinctive chlorotoid-kyanite assemblage is associated with mineralization in basal Lagoon unit.

Production and reserves

Workings or exploration The MB sulfide zone was discovered in 1976 near the confluence of Rumble Creek and the Robertson River. Geologic mapping, geochemical and geophysical surveys began in 1977. During 1978, 1:6,000-scale geologic mapping was completed. Talus and vegetation severely limit outcrop correlation (Rodney A. Blakestad and others, unpublished Resource Associates of Alaska, Inc. report, 1978).
Indication of production None

Additional comments

Unpublished data cited is available for viewing by contacting Grayd Resources Inc. in Vancouver, B. C., Canada (www.grayd.com), or Northern Associates Inc. in Fairbanks, Alaska.



Lange, I. M., Nokleberg, W.J., Newkirk, S. R., Aleinikoff, J.N., Church, S.E., and Krouse, R.H., 1993, Devonian volcanogenic massive sulfide deposits and occurrences, southern Yukon-Tanana terrane, eastern Alaska Range, Alaska: Economic Geology, v. 88, p. 344-376.
Reporters W.T. Ellis (Alaska Earth Sciences) and A.W. Wyatt and S.S. Dashevsky (Northern Associates Inc.)
Last report date 3/20/2003