The SB massive sulfide occurrence is a somewhat enigmatic occurrence that consists of numerous banded, pyrite-rich massive sulfide boulders as much as 15 feet in diameter. This occurrence is about 2.5 miles downstream from the DW-LP massive sulfide zone in an area of thick overburden. The boulders are concentrated in an area approximately 1,400 feet north-south by 500 to 800 feet east-west (J.K. Muntzert and others, unpublished Resource Associates of Alaska, Inc. report, 1977).
The SB boulders were systematically sampled in 1977. The average of 10 samples is 0.26 percent copper, 1.03 percent lead, 4.4 percent zinc, and 1.39 ounces of silver per ton; the samples were not analyzed for gold (J.K. Muntzert and others, unpublished Resource Associates of Alaska, Inc. report, 1977). Sampling by American Copper and Nickel Company in 1994 gave similar values of base metals and silver, but the modern assay techniques indicated the presence of gold. Samples averaged 0.4 percent copper, 1.7 percent lead, 4.5 percent zinc, 63 parts per million (ppm) silver, and 1.7 ppm gold (Dashevsky and others, 2003).
Considerable geophysical, geochemical, and geological work was done in the 1970s to locate the source of the sulfide boulders. The current hypothesis is that these sulfide boulders are glacial erratics from the DW-LP massive sulfide horizon that were rafted down the creek valley on ice (E. Hunter, unpublished data, 1998).The rocks around the SB occurrence are part of the Tushtena Pass unit; it consists of medium- to coarse-grained, calcareous, quartz-sericite-chlorite-schist, with local carbonate interbeds. Tushtena Pass rocks are typically green to gray, foliated, schistose to blocky, laminated to medium-bedded, quartz-eye bearing, quartz-rich lithologies with subordinate muscovite, sericite, and chlorite.