The Genetic Significance of Sulfur Isotopic Analyses from the Jardine-Crevasse Mountain Mining District, Southwestern Montana

Abstract

The Jardine-Crevasse Mountain area contains the only known economic metal deposits associated with Precambrian rocks in Montana. Previous field investigations indicate that (1) the ores are probably magmatic hydrothermal, (2) the Crevasse Mountain granite was the source of the ore solutions, and (3) the emplacement of the granite ended the Precambrian history in the area. The gold-bearing sulfide ores and sulfurbearing phases from the country rocks and from ore-alteration products have been investigated by sulfur isotopic analyses in order to determine the genesis of the ores and the other sulfur-bearing phases. Ore sulfides range from + 1.0 to + 4.4 permil (8 S34), disseminated sulfides in the country rocks from + 1.6 to + 3.3 permil (8 S34), and alteration products from the ores show: melanterite + 3.7 permil (8 S34) and native sulfur + 4.7 permil (8 S34). The data are consistent with a magmatic hydrothermal origin for both the ore sulfides and the disseminated sulfides, and the higher 8 S34 values for the alteration products indicate that post-ore events have only been of local importance. Introduction The gold-arsenic-tungsten ores and their host and source rocks in the Jardine-Crevasse Mountain area, Southwestern Montana have been investigated by sulfur isotopic variations and by petrographic and mineragraphic studies. The area lies along the north boundary of Yellowstone National Park in Park County, Montana. This area is in the extreme southwest part of the Beartooth-Absaroka uplift. Some of the geology and the sample locations are shown in Figure 1. Early investigations indicated that the ores of the area were not typical of other deposits in Montana as they are the only ones not related to any Tertiary event(s). Seager (1944) has published the only comprehensive study of the area to date and Brown (1965) has recently substantiated many of Seager's earlier conclusions as well as investigating the area by isotopic means. This content downloaded from 207.46.13.159 on Sat, 22 Oct 2016 04:46:00 UTC All use subject to http://about.jstor.org/terms Genetic SigniJc J.X£IE CMSE ..:, ,. ' p , , , ',/01,; ...... / ApAs A ,,,; :, i 10N a X ; v QU 2ZJ.iRY ';' ( _t= = 1 1 ALi j A; tg os ( tY s * <s;) = k aX,, HI Extrusives