Geologic descriptionLode mercury mineralization was identified on Arsenic Creek sometime between 1910 and 1920 (Rutledge, 1948). In 1947, the U. S. Bureau of Mines completed 1,500 feet of dozer trenching and 1,440 feet of hand trenching in the area of known lode mineralization (Rutledge, 1948). Three separate small but high-grade lode deposits were identified over a distance of 200 feet on the south side of the creek. These deposits are along or within 50 feet of the creek. The larger deposit (Deposit 1 of Rutledge, 1948, p. 5) is a sheared and fractured zone that is primarily developed in sandstone. The enclosing shears strike N 30 to 35 E, approximately parallel to bedding, but dip 70 northwest, opposite to bedding. Mineralization in the sheared zone includes realgar, cinnabar, and quartz in vein stockworks and seams along fractures and bedding. Eight samples collected over the 25-foot wide zone ranged in grade from 0.6 to 35.4 pounds of mercury per ton; a weighted average for these samples is 3.4 pounds of Hg per ton. Rutledge (1948, p. 5) concluded that the mineralized zone had a rhombohedral, pipe-like form and could not be traced along strike. Another lode deposit, 100 feet upstream, is similar in form but smaller. Five samples from this deposit ranged in grade from 1.6 to 44.6 pounds of Hg per ton; the higher grades were from thin sulfide-rich zones, including silicified shale, only a few inches thick. The third lode deposit, 200 feet east of the first, includes small lenses of white and gray quartz with enclosed cinnabar along a vertical N 30 E-striking fault zone. A composite sample of three, 3- to 6-inch-thick high grade lenses contained 45.8 pounds of Hg per ton. Arsenic, up to 18 percent or more, is present in all samples. In the larger deposits, realgar is the most abundant sulfide mineral. Orpiment is locally present. A realgar- and orpiment-rich sample contained 0.05 ppm Au (Frost, 1990, table 1, sample 7TF003). Although specific lode deposits could not be traced along strike, dozer trenches revealed thin mercury-bearing zones over about 700 feet of length; 0.5- to 1.6-foot-wide samples from these trenches contained 0.2 to 3.4 pounds of Hg per ton and 0.05 to 0.1 percent As. The bedrock in Arsenic Creek is sandstone and shale of the mid-Cretaceous Kuskokwim Group locally intruded by mafic dikes or sills (Rutledge, 1948; Hoare and Cobb, 1977; Box and others, 1993). |
