|Quadrangle map, 1:250,000-scale||MH|
|Quadrangle map, 1:63,360-scale||B-1|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||The SC East occurrence is centered around a small vegetation kill zone at an elevation of about 4,600 feet on the west side of what is informally called Fourth of July Creek, a north-flowing tributary to the Robertson River. The occurrence is 3.1 miles southeast of the confluence of the Robertson River and Rumble Creek and in the NW1/4 section 10, T. 17 N., R. 7 E., Copper River Meridian. The location is accurate.|
At the SC East occurrence, high-grade precious-metal-rich sulfide float is exposed in a vegetation kill zone developed on a kame terrace of Pleistocene or Holocene glacial sediments (J. Beget, written communication, 1997). The float is partly buried by more recent talus and rock glacier deposits derived from cliffs and slopes directly above the terrace. The high-grade boulders are interpreted to have been transported by a small alpine glacier and were probably eroded from bedrock above and to the west of the terrace in an area now covered by an active rock glacier (S.S. Dashevsky, written communication, 2003).
High-grade mineralized boulders consistently assayed more than 25 percent combined copper, lead, and zinc, more than 200 grams of silver per ton, and 11.3 grams of gold per ton. The origin of the boulders is not known; five holes have been drilled in the area in an attempt to locate their source (www.grayd.com). Various metamorphic and mafic intrusive rocks occur in the kame terrace with the high-grade boulders, including abundant float of siliceous pyritic mudstone, commonly oxidized to a sponge-like texture. This mudstone has been traced to a source in outcrop that has been intersected in drilling, but no high-grade sulfide mineralization has been found in place (S.S. Dashevsky, written communication, 2003).
The SC East prospect is in the lower Lagoon unit. The basal part of the unit consists of banded, medium- to coarse-grained quartz-sericite (-chlorite) schist and carbonaceous schist; the upper part of the unit is finer grained schist and phyllite. Protoliths for the basal section are immature sediments or wackes, mudstone, quartz arenite, and lesser calcareous arenite, and carbonate units. Thin, gray to white and pale-green interbedded metavolcanic rocks of the lower Lagoon unit typically are rhyolite and rhyodacite but include rare andesite and basalt.A prominent graphitic member in the lower sequence hosts the SC East and Trio occurrences (MH333) and serves as a stratigraphic marker near the lower contact. It can be traced in float and by electromagnetic surveys as an extensive low-resistivity zone. A less prominent, but distinctive chloritoid-kyanite assemblage within the graphitic member forms a discontinuous but identifiable horizon that is spatially related to volcanogenic massive sulfide occurrences in lower Lagoon unit for 3 miles along strike. This mineral assemblage potentially delineates a metamorphosed advanced-argillic alteration zone that has been associated with volcanogenic massive-sulfide-deposit environments elsewhere (Dashevsky and others, 2003).
|Geologic map unit||(-144.061851813771, 63.2700257410992)|
|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).|
|Workings or exploration||
In 1995, the geologic follow-up on a prominent conductor found by an airborne EM survey by American Copper and Nickel Company led to discovery of very high grade precious-metal-rich sulfide boulders that consistently assay more than 25 percent combined copper, lead, and zinc, more than 200 grams of silver per ton, and 11.3 grams of gold per ton (S.S. Dashevsky and others, 1996, unpublished report of American Copper and Nickel Company, Inc.). Extensive soil sampling, geologic mapping, and magnetic and electromagnetic surveys were completed in 1995 and 1996 and extended the area of mineralization beyond the initial discovery site.Four holes were drilled above the boulders in 1996, but none of them intersected massive sulfide mineralization. Mapping of the Quaternary geology in 1997 determined that the mineralized boulders did not come directly downslope but are lying in a kame terrace and were transported to the location by a small alpine glacier. One additional core hole was drilled in 1998 on the basis of the projected origin of the mineralization and a geophysical conductor from a previous survey. The hole intersected quartz-sericite-pyrite altered rocks but did not locate the source of the high-grade massive sulfide boulders (See www.grayd.com.).
|Indication of production||None|
Additional commentsThe unpublished data that is cited can be seen by contacting Grayd Resources Inc. in Vancouver, B.C., Canada (www.grayd.com), or Northern Associates Inc. in Fairbanks, Alaska.
|Reporters||W.T. Ellis (Alaska Earth Sciences) and A.W. Wyatt and S.S. Dashevsky (Northern Associates Inc.)|
|Last report date||3/20/2003|