LBB

Occurrence, Active

Commodities and mineralogy

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

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale A-1
Latitude 63.231
Longitude -144.1
Nearby scientific data Find additional scientific data near this location
Location and accuracy The LBB occurrence is at an elevation of about 6,400 feet approximately 2.5 miles east of Rumble Creek. The occurrence is about 0.6 mile south of peak 7405 and in the NE1/4NE1/4 section 29, T. 17 N., R. 7 E., Copper River Meridian. The location is accurate.

Geologic setting

Geologic description

The LBB massive sulfide occurrence crops out on a steep talus slope as a 1-foot-thick band of fine- to medium-grained, banded, pyrite-rich rock that has a fairly high base metal grade. The host rocks are thin, discontinuous, fine-grained, interbedded felsic tuffs and graphitic sedimentary rocks flanked by bands of metasedimentary rocks. Sericite-quartz schist, quartz-sericite-chlorite schist, and chloritic and graphitic phyllites predominate. Dikes of hornblende basalt, diabase, and andesite cross-cut the area. Intense deformation is locally common, and quartz veins are ubiquitous in the area (J.K. Muntzert and others, unpublished Resource Associates of Alaska Inc. report, 1977).
The LBB sulfide horizon can be traced in scree for about 100 feet along strike. Two types of sulfide textures are noted in cobbles derived from an area of thin interbedded felsic metavolcanic rocks and carbonaceous phyllite. The first is a medium- to coarse-grained, banded pyrite-sphalerite-galena rock with minor chalcopyrite. The other is a fine-grained black pyrite-sphalerite-chalcopyrite-galena rock typically occurring as smaller cobbles topographically higher in the scree than the first type. Six grab samples of float average 1.38 percent copper, 5.54 percent lead, 10.04 percent zinc, and 3.89 ounces of silver per ton (J.K. Muntzert and others, unpublished Resource Associates of Alaska Inc. report, 1977). A sample from subcrop contained 0.1 percent copper, 8.0 percent lead, 20.6 percent zinc, 139 parts per million (ppm) silver, and 0.4 ppm gold (S.S. Dashevsky, written communication, 2003).
The LBB prospect is located within a moderately to steeply southwesterly dipping section of the Drum unit (R.A. Blakestad and others, unpublished Resource Associates of Alaska Inc. report, 1979). The Drum metavolcanic rocks that host this deposit are composed of white to pale-gray-green, rusty weathering, quartz-eye-bearing quartz-sericite(-chlorite-pyrite) schist with minor gray to black carbonaceous phyllite and rare interbeds of chloritic phyllite. The protoliths of the schist are about two-thirds volcanic rocks of rhyodacite and dacite composition and one-third sedimentary rocks. A phyllitic parting is present in the schist in many places (Dashevsky and others, 2003).
Geologic map unit (-144.102248776025, 63.2306234916846)
Mineral deposit model Kuroko massive sulfide (Cox and Singer, 1986; model 28a).
Mineral deposit model number 28a
Age of mineralization The Drum unit, which is part of the metamorphic sequence that includes the rocks at this deposit, has been dated at the Devonian-Mississippian boundary on the basis of one SHRIMP U-Pb zircon age of 359 +/- 6 Ma at the nearby DD South prospect (MH325).

Production and reserves

Workings or exploration One core hole was drilled to a depth of 339 feet near the LBB prospect, but it was collared stratigraphically beneath the target horizon and failed to test the down-dip projection of the massive sulfide (S.S. Dashevsky, written communication, 2003). Surface trenching exposed as much as 1 foot of massive sulfide (S.S. Dashevsky, written communication, 2003).
Indication of production None

Additional comments

The 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.

References

References

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