Gilmore Mining District (Texas Mining District), Lemhi County, Idaho, USAi

Regional Level Types
Gilmore Mining District (Texas Mining District)	Mining District
Lemhi County	County
Idaho	State
USA	Country

The Texas Mining District is located along the east flank of the Lemhi Range near the historic mining town of Gilmore. The district is generally accepted to extend from Long Canyon on the south to Deer Creek on the north.

Mining began during the 1880's and most of the early production was shipped to the Nicholia lead smelter located southeast of Gilmore. In 1889, the smelter closed and mining slowed dramatically in the district. A rail line reached the Lemhi Valley in 1910 from Armstead, Montana and major development and production began again. Mining continued, more or less sporadically, from then until the early 1980's.

Mineralization generally consists of lead-silver replacement deposits which formed along northeast-trending, northwest dipping fractures in limestone and dolomite of the Jefferson and Saturday Mountain Formations. The ore contains chiefly cerussite, with lesser amounts of anglesite, smithsonite, hemimoiphite, and cerargyrite, in a gangue of earthy hematite, limonite, and manganese oxide. Only small amounts of the primary sulfide minerals galena, sphalerite, and chalcopyrite are present, even at depth.

Gold is generally scarse with the exception of three mines. The Allie, just outside the Forest, was the only mine in the district with gold as its principal commodity. An estimated 15,720 ounces gold were produced from ore which averaged about 0.5 ounce per ton. Some small pockets contained as much as 25-30 ounces per ton. The Democrat and Hilltop mines, both inside the Forest, produced significant amounts of gold along with large amounts of lead and silver. Estimated production from the two mines is between $ 1 and 2 million. The Pittsburg-Idaho and Latest Out lead-silver mines, both just outside the Forest, accounted for most of the estimated district production of $17-17.5 million (Ruppel and Lopez, 1988, p. 100-108). According to Mitchell (1995c, p. 13), production from the major mines totalled 21,396 ounces gold, 3,401,117 ounces silver, 1,086,249 pounds copper, 146,857,591 pounds lead, and 1,048,685 pounds zinc.

The lead-silver replacement deposits were generally high grade. The average grade of ore mined at the Pittsburg-Idaho was about 27 percent lead, 5-10 percent zinc, 13 ounces of silver per ton, and 0.03 ounce or less of gold per ton. Representative ores from the 200-foot-level in the Latest Out mine contained 35 percent lead, 7 percent zinc, 16 ounces silver per ton, and 0.025 ounce gold per ton (Ruppel and Lopez, 1988, p. 82-108).

TEXAS DISTRICT.

SITUATION. The Texas mining district comprises an irregular area of about one township in the southeast part of the county near the head of Lemhi Valley. It lies immediately north of the Spring Mountain district, Long Canyon being generally taken as the dividing line. To the north and west not even approximate boundaries are recognized. The junction district lies about 18 miles to the north, and the Blue Wing district well beyond the summit of the mountains to the west. On the east the wide valley of Lemhi River, deeply filled with Miocene lake beds, forms at present a natural boundary.

Gilmore (PI. XIV), a mining camp of possibly 500 inhabitants, is the local post office and supply point for the district. It is reached by the Gilmore & Pittsburgh Railroad, which connects via Junction and Bannock Pass with the Oregon Short Line at Armstead, Mont.

The district is primarily important for its lead-silver ores, although one gold vein (Allie) and one silver vein (Silver Moon) are recognized.

HISTORY AND PRODUCTION. Inspired by the great bodies of lead-silver ore at the Viola mine, situated across the Lemhi Valley, prospectors located many claims in the surrounding country in the early eighties. The most promising among these claims occupied a belt about 12 miles in extent along the east face of the Lemhi Range, a little north of west from the Viola deposits. The Texas district comprises the northern portion of the mineral belt thus early discovered and largely staked out. In it prospecting continued, and mining was carried on in a desultory way for a number of years, some ore being hauled to the Nicholia (Viola) smelter; but with the abandonment of that property about 1890, whatever enthusiasm may have prompted work in the Texas district died out and for 10 or 12 years there was little progress.

In 1902 a group of claims, the chief of which now constitute the Pittsburgh-Idaho property, were purchased by F. G. Laver, of Dubois, Pa., for himself and associates. Early development revealed, at a depth of about 200 feet, ore bodies which greatly exceeded the anticipation of the owners. In a short time considerable ore was blocked out and the method of treatment became a problem of prime importance. The old Nicholia smelter, which had afforded ready market in the early days of the district, had long since been dismantled and shipping to the large reduction works in Utah or Montana necessitated a haul of 85 miles by wagon in addition to the railway charges. The alternatives were to erect a local plant or await railway transportation. The wagon haul to Dubois, Idaho, was adopted and continued, during the open season, for four years. The roads were so destructive of wagons, however, that it became almost impossible to keep them in repair, and in the fall of 1906 a traction engine with a train of four steel wagons, each of 15 tons capacity, was put on the road. The cars were not equal to the continued strain even though the route was almost ideal for such transportation, and after a dozen trips this method of haulage was abandoned.

From the fall of 1907 until the spring of 1910 the Pittsburgh-Idaho mine was idle, awaiting the completion of the railroad which was being built from Armstead, a station on the Montana branch of the Oregon Short Line, 90 miles south of Butte. During this period the Latest Out mine became active, the ore being hauled to Dubois. In the spring of 1909 several of the smaller properties supplied small amounts of ore to a new smelter which was opened at Hahn, in the Spring Mountain district.

With the extension of the railroad to a point within 9 miles of Gilniore by June, 1910, the camp took on new life and has been increasingly active ever since. In 1910 the railroad was within 1½ miles of the producing mines of the district.

The total production of the Texas district probably falls between $2,000,000 and $2,500,000. Somewhat more than 700 tons of lead bullion and over 100,000 ounces of silver are said to have been derived from ores treated by the old Nicholia smelter between 1885 and 1888. From 1902 until 1908 about 6,270 tons of lead bullion and 325,000 ounces of silver were extracted from ores hauled 85 miles to Dubois, and thence shipped to the smelters in Utah. Between June
and October, 1910, about 1,600 tons of lead and 72,000 ounces of silver were produced from Texas district ores, and for the fiscal year from September 1, 1910, to August 3, 1911, 7,750 tons of lead and 351,500 ounces of silver were produced.

Prior to 1910 the district produced almost no gold, but during the spring of that year a promising gold-bearing lode was discovered on the Martha claim of the Allie group.

PHYSIOGRAPHY. Elevations in the district range from about 7,000 feet in the eastern part to more than 10,500 feet in the mountain areas along the western border. The local topographic features are due to numerous deep canyons, which extend back well toward the crest line of an otherwise even mountain slope, which rises abruptly to a height of almost 4,000 feet above the western margin of the broad valley of Lemhi River.

These mountain valleys are invariably U-shaped and near their junction with the lowland present the irregular topography characteristic of terminal moraines; their headward terminations have striking amphitheater-like forms, in the basins of which little lakes are not uncommon.

There are no important streams in the district. Those which rise in the mountains flow for short distances only, the waters sinking before the mouths of the canyons are reached. In the lowland northeast of Gilmore numerous springs occur at elevations of about 6,800 feet; these give rise to Texas Creek, an important member of the group of streams which by their union constitute Lemhi River. The water supply for Gilmore is derived from a lake at the head of Meadow Lake Gulch, whence it is conveyed to the mines and settlement some 3 miles distant. Although abundant water is thus supplied for domestic purposes, a sufficient supply for concentration on a large scale can be secured only by heavy outlay. Considerable water power can be developed within 20 miles to the north.

ORE DEPOSITS.

DISTRIBUTION. The known deposits of the Texas district occur in a comparatively narrow north-south belt bounded on the east by the Miocene lake beds of the Lemhi Valley and on the west by the quartzite that forms the crest of the range and thence dips eastward, disappearing beneath the limestones which inclose the veins. The mineral locations are mainly along the walls of valleys which cut back into the otherwise regular mountain face, thus exposing the lodes. The mines at Gilmore are situated in such a valley. The Pittsburgh-Idaho mine appears in the south side of this depression near its head, and the Latest Out vein crosses its steep upper end. Several claims, not now operated but showing strong mineralization in places, are situated in Silver Moon and Liberty gulches south of Gilmore, and in Texas and Ulich gulches to the north.

RELATION TO STRUCTURE. In general the lodes strike a few degrees east of north and dip west at angles varying widely but usually of more than 45°. Thus the course of the veins is parallel to the strike of the formations although their dip is generally opposite and steeper. This relation suggests that the fissures which the ores follow were formed when the rocks were folded into their present attitude, for it is apparent that fissures with dip toward the core of an uplift would result from the upbending of a great series of rocks with resistant quartzite at the base and inelastic limestone above.

Intersecting the veins at right angles are fissures, some of which are open and unmineralized; others, though seldom mineralized far from the north-south fissures which seem to have carried the solutions, bear a definite relation to the ore shoots. An apparent exception was noted on the Dorothy claim, where an east-west fissure, well removed from any known north-south ore-bearing vein, is locally mineralized. As some of the east-west fissures are barren and others carry ore, it is thought that the east-west breaks occurred at two distinct periods. In a few places where the open or younger channels cross soft north-south mineral veins, loose ore has worked out along the former for several feet; this, however, is the result of purely mechanical processes. An illustration of such condition is seen at the intersection of the Allie crosscut with the Martha vein.

Although the deposits are but rarely offset by faults (all small), slickensides and crushing within the ore are common, implying that movement since the ore deposition has largely followed the original lines of weakness. The faults which cut the veins follow the beds in such a way as to indicate a settling toward the Lemhi Valley of successively overlying strata. The largest offset of this kind is in the west vein on the 400-foot level of the Pittsburgh-Idaho mine, where a displacement of 10 feet is recorded.

The ore deposits, although in some places extending out along bedding planes and in others abruptly evading some rock not as susceptible to dissolution as its neighbor, are on the whole to be considered as tabular bodies and classed as veins.... In the Jumbo mine the ore is clearly of replacement origin, narrow vertical stringers serving apparently as feeders to larger masses which extend out along the bedding). The Latest Out ore body is decidedly tabular in outline, but isolated bunches of ore within the walls adjacent to the vein are common.

ORES. The deposits are predominantly lead-silver, only two exceptions being known. One of these is the ore shoot in the Martha vein, which carries no lead, but averages about $12 a ton in gold; the other is the Silver Moon vein, which has produced silver almost exclusively. Copper rarely exceeds a fraction of 1 per cent. Zinc is present in nearly all the deposits, but has not been found to exceed 9 per cent in broad averages.

The structure of the ore is greatly obscured by the extensive oxidation which prevails throughout the present workings. As exposed the ore is a mass of earthy carbonate heavily stained with iron and manganese and usually showing a faint metallic luster. Two types of lead-silver ore may at present be distinguished oxidized ore consisting of lead carbonate, iron oxide, and other minerals, and primary ore made up of galena, pyrite, and zinc blende.

All the development is well within the zone of oxidation, so that such primary ore as is found occurs in spots which have escaped the influence of surface waters. In the Latest Out mine pockets of sulphide are found as bunches isolated in the limestone adjacent to the veins, and occasionally as cores within large blocks of secondary ore. In the Pittsburgh-Idaho mine, near the bottom of the east vein, where downward-percolating waters are checked by a cross seam of clay, some stopes afford galena and pyrite. In the Jumbo mine oxidation is less advanced than elsewhere, probably on account of little fissuring in connection with the deposit. The primary ore thus exposed is made up of galena, quartz, pyrite, and zinc blende, decreasingly important in the order named.

Probably more than 85 per cent of the total ore exposed in the lead-silver deposits is composed of minerals resulting from the oxidation and carbonation of the group just enumerated. Cerusite and iron oxide are by far the most conspicuous. Anglesite is frequently seen as a narrow band around a core of galena.

Smithsonite is common as botryoidal linings of small cavities and as stringers along joints. Calamine occurs as needle-like crystals extending from the sides of vugs otherwise lined by smithsonite. Manganese oxide is omnipresent as stains within the ore and less frequently as dendrites on the inclosing limestone. Pyromorphite is rare, cerargyrite probably very common but in exceedingly small grains, malachite unusual, and minium very exceptional.

AGE AND GENESIS. The deposits of the Texas mining district are of late Cretaceous or early Eocene age, although local evidence does not confine the limits so closely. The veins are obviously younger than the Carboniferous limestone, which is mineralized in places. They are younger than the quartz diorite dikes, for one of the latter cut the ore as shown in the upper workings of the Latest Out mine. The dikes in turn are older than the present topographic features, for they cut across the mountain tops, as near the head of Meadow Lake. As the present topographic features are post-Eocene, it follows from local evidence that the deposits are post-Carboniferous and pre-Oligocene. From broader considerations, however, it is thought that the deposits are late Cretaceous or early Eocene.

Alteration has been so complete that the ore itself retains few of those evidences of origin which are generally recorded in mineralogic structure and relations. That the deposits were formed at a depth of 2,000 feet or more is evident from their relations to the topography. The general features of the deposits indicate that both replacement and fissure filling have taken place. In the Pittsburgh-Idaho mine the east fissure continues downward after the ore has given out. In the Latest Out mine also the vein walls continue in many places after the ore has entirely ceased.

The veins are clearly older than the quartz diorite porphyry dikes which cut them. It is, however, interesting that throughout the Texas district and the Spring Mountain district to the south, these dikes are never far removed from the ore deposits, possibly indicating that the two have a common source. Broader considerations lead to the belief that the ores of the Texas district are genetically related to an underlying granitic or monzonitic mass corresponding to the great batholith extensively exposed in central Idaho to the west, probably in the Wood River district to the south, and at many places along the Beaverhead Mountains to the east.