 |  |  |  | |||||||||||
 | ||||||||||||||
 | | | | | | | | | | | | | | |
The Skarns of Montana
Introduction The geology of Montana is perfect for the formation of the contact deposits known as skarn or tactite. The mountains of the state have an abundance of limestones and other carbonate rocks, and many of these have been intruded by igneous rocks of various composition. It is this combination, an igneous body rich in fluids intruding a carbonate rock, that creates the contact zone that many mineral collectors love—the skarn that typically contains calc-silicate minerals such as epidote, garnet (especially andradite and grossular), diopside, vesuvianite and titanite, as well as quartz, hematite and calcite. Skarns also form in regional metamorphic environments and in other metasomatic environments. It is also fortunate that these bodies also often contain tungsten, titanium, copper, gold or iron. For man’s search for these metals often leads to the development of prospect or mine excavations that expose the mineralized bodies of interest to collectors. Several skarn deposits in Montana have been prospected or mined, and some of these are skarns that have cavities and the minerals of interest. Other skarns, that appear to be devoid of metallic minerals, and thus have not been prospected to any extent, have also been producers of mineral specimens. Brief Descriptions of a few of the Skarns and Their Mineralogy
Bald Mountain This skarn, is well known and has produced specimens for many years, although it has not been as productive in recent years. The skarn is located to the northeast of Toll Mountain, southeast of Butte and is accessible by the Rader Creek road. The skarn forms a layer on the eastern and northeastern side of Bald Mountain. On the northern part of the east side, there are three small pits that have been dug by crystal collectors. The pits expose massive green diopside with cavities that are lined with coarse diopside crystals and filled with blue calcite. The diopside crystals are commonly up to about an inch long and uncommonly up to 2 inches or more. Many of them are twinned and they have a green color, often with dark green terminations. Very fine groups, sometimes with doubly terminated crystals have been collected, although they often are fractured and tend to fall apart. Some fine quality brown to orange-brown grossular crystals have also been collected, up to about an inch across. Apatite crystals have been found but are uncommon. These form white to cream colored crystals up to more than an inch long, occurring in groups with diopside. Epidote and tiny crystals of stilbite have also been reported in the Bald Mountain skarns.
Pat’s Gulch A skarn is exposed in mine workings on Pat’s Gulch, off of Snowshoe Gulch northwest of Elliston, west of Helena. This is a typical skarn with large masses of brown garnet. The best exposures are in small pits and cuts. Like most large garnet masses in skarns, large cavities with fine quality crystals are uncommon, but they do exist. The location is best known for very fine quality epidote crystals with crystals forming coarse groups and radial groups rod-like crystals. However, good quality specimens are rare. Some fine garnet specimens have also been recovered from this location.
Dry Gulch near Helena This is one of the better known skarns in Montana due to the blue spinel crystals that occurred there. This deposit has been mined for specimens and heavily worked by many collectors. The zone that produced good specimens has reportedly been mined out. The skarn is near the top of a small hill a few miles south of Helena on Dry Gulch. The blue spinel occurs as opaque anhedral to euhedral crystals commonly 1/4 to 1/2 inch across, but fine quality crystals to more than an inch across have been found. The spinel is mostly octahedrons that are sharp and with dull but smooth faces. Tiny crystals are sometimes gemmy. Occurring with the spinel are bundles of a bluish black tourmaline which has not been identified. Grossular occurs as white to light green crystals, often rounded, but sometimes sharp and well formed up to more than an inch across. Stilbite occurs as small spherical groups. Vesuvianite is uncommon. Other minerals include small books of clintonite, phlogopite, scapolite, epidote, diopside and calcite.
Scratchgravel & John G Mine Road North of Helena a few miles, is a well known occurrence in a low cut alongside a railroad track. This location is on the east side of the Scratchgravel Hills. Access is by going north out of Helena on Green Meadow Drive to the road to the Scratchgravel landfill, parking at the railroad tracks and walking up the railroad tracks. The exposure is in a low cut in massive green diopside which has cavities with euhedral crystals to about an inch long, although most are much smaller. Most of the cavities are filled with blue calcite which must be etched away to expose the diopside. There also are small crystals of a greenish gray micaceous mineral, possibly clintonite, but this has not been identified. To the north of this, accessible via the John G Mine Road are small workings in more skarns. The prospects in this area are accessible by following the John G Mine road west off the Green Meadow road, and parking on the west side of the railroad tracks. The land is BLM, and the diggings are a short distance north and northwest of the parking area towards and in the foothills. These deposits have produced a few dark brown to black andradite(?), vesuvianite and diopside. The andradites are up to about an inch across and some have a fair luster.
Farlin - Birch Creek District Farlin is the townsite that was the center of activity in the Birch Creek Mining District, a small mining district west of the Apex exit on I-15, north of Dillon. Access is easy via a well maintained county and forest road. The mines of the district are mostly located in one area on the north side of a narrow canyon near the site of the smelter. Several workings here have dumps with oxidized ores showing azurite and malachite. A few outcrops near the workings show massive garnet with epidote, quartz and hematite. These have not been very productive, but a few good specimens of quartz, epidote and brown garnets have been collected. There are a few workings in outlying areas and one of these is at a skarn that produced a few fine specimens of epidote and quartz Japan law twins. Apparently these were from one cavity and extensive prospecting by collectors has failed to produce any more good specimens.
Ivanhoe Mine This skarn is another one exposed in a mine. The location is west of the Glen exit off I-15, north of the Apex exit. The pit sits on the side of the canyon a mile above Brownes Lake in a narrow canyon in the mountains. It is accessible by a walk up the old mine road. There is not much to collect at this mine. The skarn is mostly massive garnet and unidentified very fine grained rock, showing layers in limestone. It was mined in the 1950s during the tungsten boom years. The brown garnet mass has small cavities with crystals that are typically less than 1/4 inch across. Most do not make attractive specimens, being tightly packed linings of the cavities. One area in one of the garnet masses shows malachite staining and some potential for tiny fuzzy malachite. Some areas of massive diopside also have small cavities with crystals up to about 1/4 inch. There is potential here for small specimens. Quartz also occurs at the mine in small crystals. The one mineral that is noteworthy here, but rare, is hematite as small floater roses. At least one piece of calcite was found that contained an abundance of hematite roses from microscopic to about 1/4 inch across. These were freed by dissolving the calcite in HCl. The small hematite specimens are lustrous and well formed.
Calvert Hill tungsten mine (Red Button claims) This mine, located on Calvert Hill, southwest of Wise River, is accessible by a Forest Service road that is in fairly good condition. It consist of two pits, a small one, and a larger one that is nearly filled with water. Steep sides on the large pit prohibit collecting around most of it, but the smaller pit is accessible. There also is a large dump with boulders up to the size of a car. The Calvert Hill skarn is an interesting deposit with several minerals available, including two that aren’t commonly seen in skarns. It is best known for the aquamarine that occurs there; this is unusual, beryl is rarely reported in skarns. Also, it has produced some very fine quality smoky quartz crystals. Garnets are moderately common in sharp crystals that are various shades of brown and reddish brown. The dodecahedral crystals are often lustrous and up to more than an inch across. Epidote is a common mineral at the deposit, but most of it is frozen in quartz. Sometimes the epidote can be freed from the enclosing white or smoky quartz mass, and fine quality crystals are sometimes found in cavities. The epidote can be of large size, up to more than 6 inches in length and an inch across, but most good crystals are under 3 inches long. Some of it is lustrous dark green, and smaller crystals may be transparent. The aquamarine occurs frozen in quartz, white calcite or brown siderite. Often it occurs in clusters of crystals varying from an inch or more in length and only 1 or 2 mm across, up to 2 inches or more in length and over 1/2 inch across. Crystals have been found that are 6 inches in length. Most of it is fractured, but a few very fine quality crystals have been recovered. Small clusters have been etched out of the siderite or calcite enclosing them. Smoky quartz is commons as masses, often filling in the space around epidote crystals. There also are high quality smoky quartz crystals 1-3 inches in length, and a few have been found more than 6 inches in length. Sometimes they are gem quality, and a few have been found with epidote inclusions. Tungsten occurs as scheelite which forms microscopic to large masses, and this mine was also mined during the tungsten boom years of the 1950s. Other minerals include tiny micaceous groups of an unidentified mineral. Hematite has been found as small roses up to around 1/4 inch across. Rutile has also been found as tiny prisms epitactic on the hematite roses. Other minerals include small flakes of molybdenite, pyrrhotite and actinolite.
Burnt Creek south of Norris This occurrence is little known among collectors of Montana minerals. It is located on private land a few miles south of Norris at the intersection of sections 34 and 35, T. 3 S., R. 1 W. with sections 1 and 2, T. 4 S., R. 1 W. The deposit is exposed in low hills incised by small intermittent drainages. The skarns were prospected and are best exposed in the few small pits and shafts dug in the skarn. This deposit shows good potential for lustrous brown garnet (andradite?) up to about an inch across. One outcrop consists of an abundance of these crystals, mostly around 1/2 inch intergrown with an unidentified green fibrous mineral. Some of the garnets are complete, but most show contacts where intergrown. At other workings, quartz is exposed, and the dumps show small quartz crystals, sometimes with rutile inclusions. A 11/2 inch goethite pseudomorph of pyrite was also found at this location. | ||
| | |