Introduction Southern Ruby Range (map area B) In response to requests from the Bureau of Land Management (BLM) and the U.S. Forest Service (USFS), A number of small, mafic and ultramafic bodies of apparent Archean age intrude Archean rocks of the the U.S. Geological Survey (USGS) conducted a mineral resource assessment in the Dillon BLM Resource Area Ruby Range metamorphic complex (Desmarais, 1978, 1981). Compositionally, the bodies include locally in Beaverhead and Madison Counties, southwestern Montana. These agencies use mineral resource data in serpentinized metaperidotite (harzburgite) and metapyroxenite. The bodies tend to be aligned parallel to the o o o 113 15' 112 30' 111 15' o 45' 30' 45' o creating and updating land-use management plans for federal lands for the reasonably foreseeable future. regional metamorphic foliation (Garihan, 1979). Multiple phases of ultramafic rocks have been described for 45 30' 45 30' Mineral resources that have not been developed in the past may be developed in the future, based on changing the Ruby Range bodies, and primary cumulate textures may have been obscured by metamorphism. The Ruby Table 2. Chemical analyses of metaultramafic rocks in the study area. commodity demands and market conditions. Therefore, federal land managers need geologic information on Range ultramafic rocks have been prospected for nickel and copper (Sinkler, 1942; Geach, 1972) and the Samples 95JH101 and 95JH102 are grab samples from two pyroxene-rich 6 known mineral occurrences as well as on areas that are permissive for the occurrence of undiscovered mineral contact zones have been explored for vermiculite (Berg, 1995, 1997). ultramafic bodies near the Elk Creek vermiculite deposit. Sample 96JH67 is resources. This map was prepared to provide this type of geologic information for mineral deposits that can be The fault-bounded Wolf Creek pluton of Heinrich (1960) is a northeast-trending series of exposures of a grab sample of a moderately serpentinized, pyroxene-rich ultramafic body 3 associated with ultramafic rocks. Areas of exposed Precambrian ultramafic rocks are labeled with uppercase weathered peridotite bodies, also described by Sinkler (1942) and Desmarais (1978, 1981). Heinrich (1960) mapped by Hadley (1969). Refer to Hammarstrom and Van Gosen (1998) letters (A-F). Sources of geologic maps used to compile this map are shown on the smaller index map ("Index to interpreted the numerous, small ultramafic bodies in the area as one intrusion, cut by faults. Desmarais (1978) 5 for sample locations, analytical techniques, and complete analyses. Geologic Mapping"); lowercase letters (a-m) on the index map are keyed to the reference list. and Berg (1995) interpret the small bodies as individual pods emplaced before Precambrian folding, although [Total iron reported as Fe2O3; LOI, loss on ignition; N.D., not determined; the nature of the emplacement (tectonic emplacement of serpentinite or magmatic intrusion) is uncertain. ppm, parts per million; ppb, parts per billion] 2 4 Location of the study area Sinkler (1942) described the occurrence of nickel associated with the ultramafic body, which was later called the “Wolf Creek pluton” by Heinrich (1960), as the “Dillon nickel prospect”, and suggested that it Southern Ruby Range Virginia City area represented a source of low-grade nickel silicate-bearing rocks. The pluton was prospected on the basis of its The study area includes the eastern part of Beaverhead County, Montana, and the western part of (map area B) (map area F) similarity to the Jake Creek pluton to the south (map area A), and consequently, nickel was discovered at Wolf Madison County, Montana. The study area (see Index map), designated the "Gravelly's Landscape Analysis Sample 95JH101 95JH102 96JH67 Unit" by the regional land managers of the BLM and USFS, is defined as follows: (1) The western boundary is Creek in 1938. Heinrich (1960) showed a number of nickel and chromite prospects on his geologic map of the __________________________________________________ U.S. Interstate Highway 15 from Monida to Dillon, and State Highway 41 from Dillon to Twin Bridges; (2) The pluton (table 1, sites 1, 7, 9, 10, and 12), but none of these have ever been developed. Nickel is present in northern boundary is State Highway 287 from Twin Bridges to Ennis; (3) The eastern boundary is the Madison relatively small concentrations as the supergene nickel arsenate mineral, annabergite. No primary sulfide Major elements, in weight percent River from Ennis to Raynolds Pass and State Highway 87 from Raynolds Pass south to the Montana-Idaho minerals have been recognized in the ultramafic rocks, and the nickel is probably derived from olivine or other border; and (4) The southern boundary is the Montana-Idaho border (the Continental Divide) between State silicates. Sinkler (1942) reported nickel concentrations of 0.003% Ni in gabbro, 0.16% in peridotite, and 0.4% SiO2 48.1 51.7 40.0 in serpentine, and assay values up to 1.25% Ni in grab samples from the Nickel Mountain Claim Group (likely TiO 0.35 0.26 0.11 STUDY AREA Highway 87 and Monida. This study area includes rugged mountains of the Blacktail, Centennial, and Henrys 2 Lake Mountains and the Gravelly, Greenhorn, Ruby and Snowcrest Ranges, as well as the intervening basins. site 9 of table 1). The only sulfide minerals reported are pyrrhotite, pyrite, chalcopyrite and minor amounts of Al2O3 10.4 5.0 2.3 the nickel sulfide mineral pentlandite (along with the iron-oxide magnetite) in gabbro exposed at Ruby Nickel Fe2O3 10.1 11.3 10.7 Mineral deposit types that can be associated with ultramafic and mafic rocks Claim No. 6 on the southeastern side of the ultramafic “body”. Heinrich (1960) mapped a northwest-trending MnO 0.17 0.16 0.13 diabase dike, rather than a gabbro in this area, and the sulfide location appears to be in the area of the Nickel MgO 23.0 26.7 35.1 C Metallic minerals Lode Claims (table 1, site 9). Desmarais (1981) did not mention sulfide minerals in his detailed petrologic study CaO 5.6 3.4 1.1 E of the Ruby Range ultramafic rocks, but did confirm the presence of nickel in silicate and oxide minerals (up to Na O 0.67 0.15 0.04 0.5% NiO in olivine; also present in orthopyroxene, clinohumite, and spinel). Mineral occurrences such as the 2 Ultramafic rocks are igneous rocks that are distinctly dark in color and include pyroxenites, peridotites, and Dillon nickel prospect were not economic at the time of their discovery, and are unlikely to ever be sought as a K2O 0.14 0.06 <0.01 a host of specialized rock types named on the basis of their mineralogy. Ultramafic rocks lack quartz and alkali 15' F 15' source for nickel. P2O5 0.03 0.05 0.02 feldspar and are essentially composed of ferromagnesian silicate minerals, sulfide and metallic oxide minerals. Berg (1995) reported gold (8 ppb or less), platinum (5 ppb), and palladium (all <2 ppb) values for four LOI 0.8 0.6 9.2 Mafic igneous rocks include, among others, gabbros and their extrusive equivalents, basalts; they have a samples of ultramafic rocks from the Wolf Creek pluton in the area of the Elk Creek vermiculite deposit. Total 99.5 99.5 98.7 relatively low silica content (about 45 to 52 weight percent), and are comprised of dense minerals such as D Hammarstrom and Van Gosen (1998) reported on the major and trace element chemistry of two pyroxene-rich S <0.003 <0.003 N.D. pyroxene, amphibole, olivine, plagioclase, and dark mica with little or no quartz. In some ultramafic and mafic ultramafic rocks from the same area (table 2) as well as samples of biotite-rich schist and amphibolite that __________________________________________________ rocks, metallic ore minerals are present in sufficient concentrations to form ore deposits that are mined for developed at the metasomatic reaction contact that localized the vermiculite deposit. Chromium concentrations copper, nickel, chromium, cobalt, gold, iron, titanium, vanadium, and (or) platinum-group elements (PGE). Selected trace elements (units) are typical (about 2,000 ppm Cr) for ultramafic rocks, but nickel concentrations are relatively low (<1,000 ppm) Copper, nickel, chromium, and PGE have been mined from mafic and ultramafic rocks in Montana, most for ultramafic rocks (Hawkes and Webb, 1962). Heinrich and Rabbitt (1960) noted that anomalous chromium notably from mines developed in the Stillwater Complex near Nye, Montana, in the Beartooth uplift area. The Cr (ppm) 2,260 3,340 4,240 concentrations can be obtained for the Wolf Creek pluton even though no significant amounts of chromite are Stillwater Complex is a Late Archean (2,700 Ma) ultramafic to mafic layered igneous intrusion that hosts Ni (ppm) 815 926 1,795 present, because a chromium spinel mineral occurs throughout the peridotite; chromite is present as an essentially all of the identified lode resources of PGE and more than half of the identified chromium resources of Co (ppm) 85 93 117 accessory mineral on a local scale. Low-level geochemical signatures in NURE stream sediments from this area the U.S., along with copper, nickel and other metals (Zientek, 1993, and references therein). Many layered Cu (ppm) 11 99 7 include iron, chromium, nickel, titanium, vanadium, and cobalt. intrusions consist of cyclic units of different ultramafic to mafic rock types, such as peridotite, gabbro, and The Elk Creek vermiculite deposit was mapped and sampled by Berg (1995) and studied by Desmarais Au (ppb) 1 1 4 anorthosite; layers enriched in disseminated sulfide minerals or chromite can be present throughout the (1978, 1981). The vermiculite deposit formed from weathering of biotite in a very coarse-grained biotite schist. Pt (ppb) 7.6 12.2 13.4 intrusion.