Annual report of the mining industry of Idaho,
Robert Bell, Inspector of Mines, 1904

Extracted from google books


The Pearl district is situated twenty miles west of Idaho City near the Canyon County line and about the same distance north from the State capital. This is at present the most productive quartz camp in Boise County.

The Pearl district is in sharp contrast with the Basin. The same weathered and eroded hills are here, but they are smooth and grassy and devoid of timber.

This district shows a remarkable succession of well defined fissure veins in metamorphic granite. These veins are in close association, but not generally in contact, with a system of parallel dikes of diorite and porphyry. The mineralized belt of this important district extends from a mile west of Pearl to the Kentuck and Osborne mines, five miles further northeast, where it crosses the Payette River. It is six miles long by two miles wide and is continued in several good ore exposures beyond the river.

The veins of this district strike from due east and west to northeast and southwest. They are fissures of the true fissure type. They are later than the diorite and porphyry dike and are often split up in passing through them. There is another set of dikes and necks of rhyolite and basalt of recent tertiary date in the vicinity of the mines, as well as some limited areas of lake bed sediments, Payette sandstones and shales.

The best developed mines in the district are the Checkmate, the Lincoln and the Kentuck, all three of which are equipped with small mills. The Kentuck plant was only gotten into commission in December, 1903, and did not cut much figure in the year's output. The Checkmate mill of ten stamps ran only four and one-half months, when the property changed hands and has since been undergoing extensive development. The Lincoln mill is a Chilean mill of one hundred fifty tons capacity and was only run on day shift, making an average daily run of about eight hours during seven months of the year, its operations being bullion, and since the electric power has been introThe total production of these two plants, run irregularly as they were, amounted to nearly $90,000 dollars in gold bullion, and since the electric power has been tinroduced from the big Payette River Power Company's plant near by, they are unlikely to be hampered from lack of power in the future, and their present extensive ore reserves warrant the anticipation that the combined output of the district for 1904 will be as near $300,000 as it was $100,000 in 1903.

The ores of this district resemble those of Gilpin County, Colorado, very much. They consist of oxidized quartz and granitic gangue near the surface, which changes at a comparativly shallow depth to a mixture of iron pyrite, arsenical pyrites together with zinc blend and a small percentage of galena and copper sulphides. The increase in the galena in these ores is a sure indication of increase of gold value and is eagerly anticipated by the miners.

The proportion of values are about ninety-five per cent gold and five per cent silver on the average, and they range from eight dollars to twelve dollars per ton for mill dirt, and forty dollars to one hundred fifty dollars for smelting ore. About twenty-five per cent of the value is saved as free gold on copper plates, and of the balance ninety per cent is saved by concentration on Wilfley tables. The concentrates run about the same as the smelting ore, and are shipped to the smelters at Salt Lake for treatment, except the new plant at the Kentuck group, near the river, the product of which is shipped to the White Knob Copper Company's smelter at Mackay.

The Checkmate mine is developed to a depth of five hundred feet on the dip of the vein and has made a total output of over $500,000. This mine was purchased in August by the Gold Dollar Mining & Smelting Company, who also own the Dewey or Levan group adjoining it at the north, which gives the company a very extensive tract of territory right in the heart of the camp that is traversed by half a dozen well known fissures.

During the year the four hundred-foot vertical shaft on the old Checkmate vein was sunk one hundred feet deeper and a cross-cut started north and south from the fifth level. The cross-cut to the south struck the Bayhorse vein at a point six hundred feet deep below its apex on the dip, and one hundred fifty feet out from the shaft, and just about where it was anticipated from the dip shown in the shallow cuts at the surface. This vein was found to be four feet wide with values ranging from five dollars to twenty dollars per ton.

The old Checkmate vein strikes east and west and dips north at an angle of fifty degrees. Its walls are smooth and well defined, excepting local patches where the vein is interrupted with a small diabase dike having nearly the same strike and dip.

The ore in this vein expanded from a shoot sixty feet long at the first level to a succession of shoots that aggregate five hundred feet along at the fifth level and an average width of about five feet and contain average gold values of eight dollars to twelve dollars per ton.

The ore in these shoots in some places reaches twelve feet between walls. It is largely an altered granite gangue threaded with banded lines of metalic sulphides with almost invariably a pay streak of the clean high grade smelting ore, a few inches to a foot or more thick, on one or the other wall.

These ore shoots are easily mined. Holes can often be drilled with a coal auger. The material crushes readily and makes a granular free running pulp which concentrates to excellent advantage. The ten stamp mill on the property crushes forty tons a day and yields a very clean product of concentrates.

With its four main fissures opened from the cross-cut at the fifth level of the Checkmate shaft, and proving a resource of ore in each one of them of equal volume with that of the Checkmate vein above the fifth level, this property is likely to warrant a milling plant of three or four times its present capacity. It is evident that the ore on the surface of these parallel veins are equal in size and in some openings far superior to what the Checkmate was at the surface. There is some strong evidence at hand that they will carry the same value and strength under ground in proportion as the Checkmate has.

The Lincoln mine, a mile west of the Checkmate, has responded in a very gratifying manner to the development work which has been carried on during the past year.

This mine is opened with an incline shaft three hundred thirty feet deep. At two hundred thirty feet deep a drift has been run out east and west ten hundred twenty-five feet, in continuous pay ore all the way. This remarkably continuous ore shoot varies from a foot to fifteen feet wide, while the extent of the shoot to the east has not yet been reached, but still shows a handsome breast of mineral five feet wide containing an average assay value of fourteen dollars per ton.

The average width of the stopes in this mine are from five to six feet. They produce quite a lot of clean smelting ore worth from three to eight ounces gold, together with ten to fifteen ounces silver, per ton, while the average battery assay of the mill dirt sent up has been over nine dollars for the past month.

The Lincoln Company is making half a carload of high grade concentrates a day at present. A new level is being run out at three hundred thirty feet deep and a hoisting plant of larger capacity is on the way. With these improvements completed the property will be able to make a carload of concentrates a day in addition to the crude shipping ore encountered and promises to show a gross earning capacity of over $20,000 a month in the near future.

The ore at the Lincoln is almost identical to that of the Checkmate; in fact, the same holds true of almost all the mines in the district, with probably a little higher silver results here and there.

There are about a dozen properties being actively operated at the present time in the Pearl district and giving employment to fully two hundred men. These mines are already developed in various stages, ranging from that of the Checkmate to a few hundred feet of shallow work. Several of them have shipping records from surface openings, and the description of their vein's ores and operations would be practically a repetition of the conditions described in the Lincoln and Checkmate, only in varying degrees.

The Pearl district gives remarkable evidence of strength and permanency to great depth and in nearly every property where intelligent development has been carried on the values have either increased or the pay ore bodies have expanded all out of proportion to the surface indication.

This extensively fissured zone has been cut by the canyon of the Payette River to a depth of two thousand feet since these fissures were formed and filled, which is not only an argument for permanency of values at depth in this district but is also likely to have an important bearing on the probable permanency at depth of the values in the Basin veins, which are apparently of the same age.

The ores at Pearl are undeniably base and afford an ideal field for the process man. If some cheap chemical method of getting the values can be devised so that the present shipping cost can be cut out the Pearl district would likely develop a resource of gold ore that would prove a worthy comparison in value and volume to the Little Kingdom of Gilpin County, Colorado, with which we have taken the liberty to compare it.

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