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Minerals of the Sweet Home mine
Mineralogical Record, Jul/Aug 1998 by Murphy, Jack A, Hurlbut, James F
INTRODUCTION
This article describes the minerals from the Sweet Home mine, with emphasis on the crystallized specimens discovered during mining operations between 1991 and 1996. For details on electron microprobe analysis, petrographic identifications, fluid inclusion salinities, and geochronology, see other papers in this issue (Wenrich and Aumente-Modreski; Wenrich; Reynolds; and Misantoni et al.). Minerals reported and confirmed from the mine are discussed in the following pages; they are listed and cross referenced as to source in Table 6. Many other minerals have been reported from the mine, but have not been confirmed.
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The mineral associations that typically occur in Sweet Home mine specimens are discussed, but mineral paragenesis is not. The species described are from the copper-antimony-tungsten-silverbase-metal sulfide veins in Precambrian granite-gneiss host rock.
About a dozen metallic vein minerals are known, comprising a suite of common base-metal sulfides and one main sulfosalt, tetrahedrite-tennantite. The occurrence of hubnerite adds to the variety of minerals found; one sparse tin mineral (mawsonite) has also been found. Many of the uncommon to rare sulfides and sulfosalts known elsewhere in Cu-As-S veins have not, as yet, been identified at the Sweet Home mine. Spionkopite has been identified by electron microprobe (Wenrich and Aumente-Modreski, this issue), and another sulfide, tentatively identified as eclarite or miharaite, is under investigation. Minerals that are part of the massive ore assemblage identified by petrographic techniques and described by Honea (1992) and Wenrich and Aumente-Modreski (this issue), in general have also been identified as euhedral crystals in hand specimens, with the exception of stromeyerite, which has not yet been seen in hand specimens. Gangue minerals represented in the veins are common species, and, except for the rhodochrosite which occurs in superior large crystals, calcite, quartz and associated species occur in smaller, well-formed crystals.
What is noteworthy about the minerals from the Sweet Home mine is not so much the frequency of the occurrence of a particular species, as the excellence of the crystals. Superb specimens with impressive euhedral crystals have been recovered, many with three to four different species in association. Many such specimens, which were carefully collected during the mining operation led by Bryan Lees from 1991 to 1997, currently enhance collections around the world.
PREVIOUS STUDIES
Sweet Home mine minerals have long been recognized and reported in the literature, but few, if any, details about them were published. One of the earliest mineral reports is by Endlich (1878), a geologist on the F. V. Hayden expedition to Colorado in 1876. Eight entries in his Catalogue of Minerals Found in Colorado are: cuprite, fluorite, jamesonite, melanterite, rhodochrosite, tetrahedrite, tennantite and zinkenite. The reports were reiterated by Smith (1883). The Alma locality was not mentioned by Kunz (1887), who described superb transparent crystals of rhodochrosite from the John Reed mine at Alicante, Lake County, Colorado. The Sweet Home mine and the Tanner Boy mine were mentioned together by Dana (1898), but rhodochrosite was reported only from the Tanner Boy.
The first comprehensive regional geologic studies of the Alma district (Patton and others, 1912; Butler, 1912) gave little information about the Sweet Home mine, and the mine is not mentioned in Henderson (1926). The early belief that argentiferous galena was the source for silver in the mine (galena has since been shown to be only one minor mineral of several that is the source for silver, Wenrich and Aumente-Modreski, this issue) was reinforced by Patton and others (1912); they also listed rhodochrosite, cuprite, enargite, bornite, azurite, malachite, and fluorite from the mine.
The Sweet Home mine rhodochrosite crystals have overshadowed reports of the other species from the mine (Caplan, 1936). Little information is found in standard mineralogical references. For example, two noteworthy species known from the mine, tetrahedrite/tennantite and hunerite, are not listed in Palache and others (1944, 1951), or by Eckel (1961). Roots (1951a,b) reports collecting rhodochrosite and pyrite at the Sweet Home mine.
Selected mineralized areas of the Alma district have been described in the geological literature (Butler and Singewald, 1940; Singewald, 1932, 1947; Singewald and Butler, 1931a,b, 1933, 1941; Vanderwilt, 1947; Pierson and Singewald, 1953; and Del Rio, 1960). However, none of these detail the Sweet Home mine.
A few articles appeared in the popular literature after World War II (Roots, 1951a,b; Ingle, 1958; Miller, 1971), but the mine was closed during most of this time. The first extraordinary rhodochrosite discoveries were made in the 1960's (Bancroft, 1973, 1984). Some geological investigations of the district were conducted (Corn, 1957); however, much of the emphasis on minerals and mining in this part of Colorado was associated with the Climax molybdenum mine, 5 miles to the northwest in Lake County. When the Climax mine was operating, good specimens of pyrite, fluorite and rhodochrosite were occasionally available (Kosnar and Miller, 1976). Mineral specimens were not readily available from the Sweet Home mine until 1978, when the mine was reopened by Richard Kosnar (Kosnar, 1979a,b).
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