Geologic descriptionPolymetallic, sulfide-bearing veins, veinlets, replacements, and fracture fillings in Ordovician limestone are locally present over about 1,200 feet of the east-west trending Rapid River fault zone. Sainsbury (1969, 1972) maps the Rapid River fault as a 12-mile long, east-west trending thrust fault in the southern part of the York Mountains although stratigraphic relations across the fault suggest normal displacement. Fluorite and beryllium-bearing mineralization has apparently developed peripheral to the sulfide veins. Lamprophyre dikes and a small plug are present in the mineralized area. Sulfide vein mineralogy is complex. Knopf (1908, p. 57-58) described a 1-foot wide zone of stringer veinlets containing wolframite, stannite, and galena with topaz and fluorite. Steidtmann and Cathcart (1922) described fractured and kaolinized dike rocks, some with disseminated tourmaline and fluorite, cemented with thin seams of galena, pyrite, and chalcopyrite and in places with 3-inch wide vertical stibnite-bearing veins. Sainsbury (1965; 1969, p. 64) described a 1-foot wide diamond drill intercept of semi-massive sulfides containing stannite, pyrite, arsenopyrite, and galena. Grades reported for sulfide-rich samples include trace to 0.03 opt gold, 4.2 to 25.6 opt silver, 0.5 to 9.1 % Pb, 0.48 to 1.53% Cu, about 3% Zn, 0.3 to 1.6% Sn, up to 3.2% WO3 and 3% Sb (Berg and Cobb, 1967, p. 132). Fluorite, chrysoberyl, white mica, and tourmaline are present in replacements of limestone and dolomite peripheral to the sulfide-bearing veins. One sample from the Bessie-Maple adit dump contained 0.39% BeO and 59% fluorite (Sainsbury, 1963, p. 8). Samples from three short USBM diamond drill holes had up to 0.79% BeO and 75% fluorite (Mulligan, 1965). Five inclined diamond drill holes completed by Lost River Mining Corporation through the near vertical fluorite mineralization had average intersections of 60 feet grading 34% fluorite (WGM, 1972, p. 54). Three of these five holes also intersected sulfide mineralization. These intersections were: (1) 10 feet of 0.18% Sn, 0.11% Pb, 4.9% Zn, 0.15% Cu, and 1.34 opt Ag; (2) 4.5 feet of 0.22% lead, 1.89% Zn, and 1 opt Ag; and (3) 2 feet of 0.27% Pb and 2.17% Zn (WGM, 1972, p. 72). Another diamond drill hole drilled vertically at a location north of the main Bessie-Maple prospect encountered 46 feet of 21.2 % fluorite, 0.23% Pb, 0.38 % Cu, and 1.3 opt Ag in the uppermost part of the hole (WGM, 1972, p. 72-73). |
| Alteration of deposit |
The limestone is commonly dolomitized but the relation of this alteration to sulfide and fluorite mineralization is not clear. Lamprophyre dikes are kaolinized and locally contain disseminated tourmaline and fluorite. Fluorite veining and replacement is in effect a type of alteration here that can be thought of as distal alteration to more intense, tin metallization at depth. Mass balance calculations show significant SiO2, Al22O3, alkali, and fluorine enrichment with this type of alteration (Sainsbury, 1968, p. 1567). |
