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The Skarns of Idaho
Introduction Fortunately for the mineral collector, there are several skarns in Idaho of which a few produce some fine mineral specimens. Along with the minerals commonly found in skarn deposits, such as andradite, grossular, epidote, diopside, quartz, hematite and magnetite; some of the skarns in Idaho produce less common minerals, such as ilvaite, hedenbergite and ludwigite.
Brief Descriptions of the Skarns and Their Mineralogy
Seven Devils Mining District One of the largest skarn areas in the state is at the Seven Devils Mining District in western Adams County. The mining district overlooks the spectacular Hells Canyon a few miles north of Cuprum. There are several distinct skarn bodies in the district which have been prospected and mined for copper and tungsten. The district is abandoned, and the mines are becoming slumped and overgrown, limiting exposure of the skarns. The skarns of the district are dominated by garnet, mostly brown andradite, with some grossular. In some areas, especially around the Peacock mines, lustrous crystals of brown andradite have been collected. These garnet bodies also contain epidote, and during mining operations, spectacular lustrous crystals to several inches in length were produced (or at least mined and sent to the crusher with the ore… ). Other minerals include powellite (type locality), lindgrenite and copper secondary minerals. There also is some quartz and hematite of interest to the collector.
Iron Mountain This mining district is located northwest of Weiser on upper Fourth of July Creek, off the Mann Creek road. There are a few mine workings in the skarns, including one pit. Brown grossular forms large masses in the pit and these masses have a few cavities. Most of the cavities are irregular and filled with loose powdery brown Fe-Mn oxides. Some of these cavities also contain small books of chlorite (species not known), small brown grossular crystals and rough, pitted pseudomorphs of chlorite and other minerals after unknown minerals. Larger cavities in the massive garnet are sometimes lined with large grossular crystals, but most of these are shattered and if removed crumble to sand-size pieces. Specimen recovery from this zone is rare. One small magnetite pod in the pit contains a few cavities with magnetite crystals, but most of these are oxidized and have brown surfaces. Those that are not oxidized, have dull surfaces. The uncommon magnetite crystals in these cavities are dodecahedrons. Also in the pit, is a pod of calcite that contains large spheroidal groups of green phlogopite. Although not particularly attractive or recoverable as good crystals, this is an interesting occurrence of this mica.
Spring Mountain District Another interesting skarn is in the Spring Mountain District, southeast of Leadore. The skarn is exposed along Quartzite Canyon, upper Bruce Canyon and lower Lemhi Union Gulch. Most of the skarn is massive diopside with one zone of massive forsterite and small pods of magnetite. The magnetite pods have cavities that are lined with magnetite crystals, but most are filled with calcite. Most of the crystals are only the typical druse lining cavities and not fully developed crystals, but in some areas there are better crystals. Many of the crystals have fractures and tend to fall apart, and all have dull faces. If a collector is willing to use a lot of HCl to dissolve calcite, a few good specimens may be found. One pod of marble contains large crystals of ludwigite, up to a few inches in length. These are quite unusual in that most worldwide ludwigite occurrences are of fine acicular crystals frozen in rock. The crystals are partially altered so that the surfaces are brown; this is a Mn-Fe-B hydroxide, the identity has not been made. If it is removed with oxalic acid or Iron Out, the etched and pitted ludwigite is exposed. The ludwigite also has an abundance of fractures, so the crystals fall apart, and none of them are terminated. Even though the crystals are not terminated and in any recovered group most will show fractures, the specimens are interesting for the large size of the crystals. Ludwigite is also common as fine crystals in the magnetite bodies and in some calcite bodies. This fine acicular ludwigite occurs in radial groups and scattered crystals. Typically it is black, but some of it is altered to a white szaibellyite and other minerals, or altered to the brown unknown. In one area, the ludwigite is in tiny radial groups that are quite fine have a green/olive drab color. Tiny to small octahedrons of black spinel are also present in the skarn, especially with the ludwigite and forsterite. In one area on Lemhi Union Gulch it also occurs with diopside. Most of the spinel is less than 1/8 inch across, but a few crystals are up to about 1/4 inch. It is always black, and the octahedrons have dull surfaces. Forsterite is uncommon as euhedral crystals in cavities in a body of massive forsterite. The crystals are blocky and have a cream to tan color. Occasionally spinel crystals are perched on the forsterite. Diopside forms tiny or small green crystals in the cavities in the massive diopside, and in some areas these are accompanied by tiny spinel crystals.
Mackay Mining District The Mackay Mining District, a few miles west of Mackay is a copper district that was developed in skarns. There are many mines in the district in the skarns, limestone and intrusive rock. Minerals present include massive garnet, epidote and magnetite. There are a few cavities, and a few small crystals of garnet, magnetite and epidote are known. Other minerals, including malachite, hemimorphite, chrysocolla and azurite occur, but few specimens have been collected due to the scarcity of cavities. A new mineral was described from one on the occurrences and named custerite. This was later proven to be the rare mineral cuspidine.
South Mountain This mining district in the southwest corner of the state, accessible from Jordan Valley, Oregon is a copper-lead-zinc district. It is fairly well known for ilvaite and hedenbergite crystals. It has also produced good specimens of black sphalerite with pyrrhotite and quartz. The ilvaite is typically up to about 1 inch long and sometimes larger. Crystal faces are somewhat dull, but sharp and well formed. Hedenbergite is common, and may be the most abundant mineral making up the mass of the skarn deposits. Hedenbergite does sometimes form distinct crystalline groups or individuals, but most of it is compact radial masses of coarse crystals. Tiny reddish orange garnets are uncommon, but do occur in some areas on terminations in vuggy hedenbergite masses. Other minerals include chalcopyrite in the ore zones and secondary minerals including pyromorphite, aurichalcite and hemimorphite.
Springfield Mine An unusual skarn is at the Springfield tungsten mine about 12 miles from the Johnson Creek Road, 14 miles southeast of Yellow Pine in west central Idaho. Access is difficult, the upper half of the access road is rough, rocky and steep. The deposit was mined for the scheelite that occurs in pyrrhotite. The skarn is dominantly pyrrhotite, marble and small amounts of calc-silicate minerals. Most of the “usual” skarn minerals are absent or present in small amounts only. There is some quartz as small colorless crystals, small milky crystals associated with pyrite and malformed milky crystals in masses. Reddish garnets occur as shattered and pitted euhedral crystals in the contacts of a coarse white marble body. The contacts sometimes have layers of grains and masses of a black prismatic mineral (a tourmaline?), an unidentified green anhedral to subhedal mineral (probably diopside) and small masses of garnet. There also is a white acicular mineral in layers and masses. This skarn has a small potential for specimens of quartz and garnet, and some of the other minerals.
Vulcan Mine An old gold mine northeast of Lakeview on the east side of Lake Pend Oreille in northern Idaho is partially in a skarn. The main mine working is in the adjoining diorite intrusive, but on the upper road, a large skarn is exposed in roadcuts. Two small prospects are exposed in the cuts. The skarn is dominantly diopside and garnet. Locally it is vuggy with small crystals of diopside and orange-brown garnets. Crystals are mostly under 1/4 inch for both. In some areas, tiny crystals of colorless to orangish scheelite occurs on the diopside and garnet. The scheelite crystals are crude octahedrons. A little pyrite is also present as crumbly masses or tiny crystals.
Deep Creek, Warren Wagon Road Several miles north of McCall on the road to Warren along the west side of Payette Lake the road was cut through a small skarn at Deep Creek. Epidote was reported in crystals to over an inch in the fresh cut, but only traces are visible now. There are tiny crystals of spinel octahedrons of green and purple colors. Tiny books of an orange micaceous mineral are also present. In the area, there are other reported occurrences with epidote, but these are not well documented. To the west, in the granite lake area, small zones in the metamorphic rock (gneiss and migmatite textures) contain reddish brown garnets and massive epidote, but not in skarns. | ||
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