Midnight Hill; North Midnight Prospects, Black Mining District, Yukon-Koyukuk Census Area, Alaska, USAi

Regional Level Types
Midnight Hill; North Midnight Prospects	Group of Prospects
Black Mining District	Mining District
Yukon-Koyukuk Census Area	Census Area
Alaska	State
USA	Country

These prospects are on Doyon, Ltd. conveyed land. For more information contact Doyon, Ltd., Fairbanks, Alaska.
Location: The approximate center of the area of these prospects is about one mile west of the top of Midnight Hill, in the SW1/4 section 11, T.14 N., R.31.E., of the Fairbanks Meridian. The prospects occur along an east-west trending gossan, about 250 feet wide, that extends for about 900 feet . Another, smaller, gossan is located approximately 1000 feet to the northeast (Bright, 1989). The location is accurate within 500 feet. These prospects are located on Doyon, Ltd. conveyed land.
Geology: Bedrock in the area consists chiefly of Carboniferous to Permian, grayish black shale and argillite, and dark-gray limestone (Brabb, 1970). Upper Proterozoic to Lower Paleozoic, Tindir Group carbonate, quartzite, argillite, and subordinate volcanic rocks also occur (Dover, 1994). Iron-stained, gossanous, soil and rock outline a prominent, north-trending fault zone that separates Precambrian dark gray limestone and dolomite to the west from Paleozoic quartzite to the east (Doyon Limited, 1987; North Star Exploration, 2002). A limestone block in the middle of the fault zone may either be in place, or, more likely, a fault-displaced fragment. The fault zone is approximately 130 meters wide and occupies a saddle between prominent outcrops of quartzite to the east and more subdued and lower-relief outcrops of limestone and dolomite to the west. The quartzite is massive and blocky; bedding and most fractures strike approximately north and dip 80 to 85 W. Other sets of fractures strike N35W to N55W, and dip 65 to 70 SW; and strike N50E and dip 77 NW. There are minor quartz veinlets in the massive quartzite. Minor iron-oxide staining occurs along some fractures. The dark gray limestone and dolomite unit strikes about N77W and dips 26 NE. This carbonate unit is fine-grained and has a distinctive phyllitic texture probably developed during shearing associated with the faulting. Minor karst development and fossils occur near the base of the unit (Doyon Limited, 1987). Minor black sphalerite occurs on joint surfaces; minor cerussite, smithsonite and malachite are locally present. The bedding steepens adjacent to the fault (North Star Exploration, 2002). Partly oxidized pyrite cubes occur in an east-west-trending, limestone-hosted gossan, 800 feet long by 250 feet wide. The gossan is typically spongy, brick red to black limonite after siderite, and it also contains minor boxworks and smithsonite (Doyon Limited, 1987). These prospects were discovered in 1976 during follow-up of a color anomaly identified in a petroleum exploration program carried out by Louisiana Land and Exploration. In 1976 and 1978, WGM Inc. conducted soil, silt, and rock sampling, and magnetic and electromagnetic surveys over the area (Doyon Limited, 1987). They collected approximately 500 silt, soil, and rock-chip samples. Grab samples of rocks contain up to 4.4 percent zinc, 587 ppm lead, 34 ppm silver, and 0.16 percent copper. Soil samples contain up to 6,900 ppm zinc. In 1978, WGM, Inc. drilled three BQ holes totaling 800 feet to test the gossan, but the holes did not intersect significant mineralization. The highest zinc assay in the core was 1240 ppm. Geophysical surveys did not indicate a large target at depth (Jirik and others, 1979). In the mid-1970's, the U.S. Geological Survey (1997) conducted a water geochemical survey in the Black River quadrangle. Their highest zinc anomaly is in Bull Creek, about three miles west of Midnight Hill, which drains this prospect area. Water at that sample site contains up to 1,584 ppb zinc, along with elevated barium, cobalt, chromium, iron, magnesium, manganese, and vanadium.
Workings: These prospects were discovered in 1976 during follow-up of a color anomaly identified in a petroleum exploration program carried out by Louisiana Land and Exploration. In 1976 and 1978, WGM, Inc. conducted soil, silt, and rock sampling, and magnetic and electromagnetic surveys over the area (Doyon Limited, 1987). They collected approximately 500 silt, soil, and rock-chip samples. Grab samples of rocks contain up to 4.4 percent zinc, 587 ppm lead, 34 ppm silver, and 0.16 percent copper. Soil samples contain up to 6,900 ppm zinc. In 1978, WGM, Inc. drilled three BQ holes totaling 800 feet to test the gossan, but the holes did not intersect significant mineralization. The highest zinc assay in the core was 1240 ppm. Geophysical surveys did not indicate a large target at depth (Jirik and others, 1979).
Alteration: Partly oxidized pyrite cubes occur in an east-west-trending, limestone-hosted gossan, 800 feet long by 250 feet wide (Doyon Limited, 1987). Typical gossan is spongy, brick red to black limonite after siderite. Small gossans also occur along the north-trending fault zone. Local oxidation of copper, lead, and zinc minerals.

Commodities (Major) - Zn; (Minor) - Ag, Cu, Pb
Development Status: No
Deposit Model: Appalachian-type zinc deposit in carbonate rocks (Cox and Singer, 1986; model 3

Other Databases

Link to USGS - Alaska:	BR005

Other Regions, Features and Areas containing this locality

References