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 Northwest-trending faults locally cut the mineralized horizon (R.A. Blakestad and others, unpublished Resource Associates of Alaska, Inc. report, 1978).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and Nickel Company concluded that mineralization is associated with and adjacent to these fault structures (E. Hunter, unpublished data, 1996).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). |
