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Show INTER-MOUNTAI- Geology and Hineralization of Hercur. WRITTEN BY lROF. MARCUS E. JONES. FORMATION. The mineral veins of Mercur lie in the lower part of the blue limestones of the lower carboniferous age. The lime- stones are in all over 4000 feet thick, but only the lower part is mineralized, that lying near the black shales carrying sponges and crinoids. These beds laid under the ocean till the close of the carboniferous age, probably hundreds of thousands of years, and were then upheaved above it, but not min- MINING REVIEW. N veins even in one place (the lower half of Lewiston canyon) at least 1000 feet. ent origin. It also covers the entire dome, lies along the bedding, is almost Fairfield canyon has not been cut down pure silica, and often stands out as a to or near the veins. Sunshine canquartz ledge, going by the name of yon has been cut down to the silver quartzite where it carries no values. vein from one end of the canyon to It is thickest and richest tow'ard Ophir the other along the strike. All the and thins out both in width and values canyons from Sunshine to Ophfr have toward Sunshine and the western side been cut down through all the veins. of the range. This vein wras worked The gulch or flat running down from once and a little rich ore taken out on the Eagle mine to the Golden Gate and the Mercur side, but it W'as abandoned thence up to the Sacramento has been long ago. It will go thirty worn down nearly to the upper gold ounces in silver w'here nearly it carries ore. vein. With the exception of the ground This vein is so wrell marked that it is lying, to the eastward of the Eagle used as the basis of the search for the and extending to the Golden Gate, the overlaying gold vein. dip of the veins is practically the same Below the silver vein are three as the slope of the hills on which the veins, within twro hundred feet of itgold on mines are located. the Sunshine side of the divide, but so During the tertiary age, after a still longer period, there were thrust through them, at least in part, two masses of eruptive rock at differVEINS. which into lifted them ent times, up The uppermost vein lies in broken was the limestone, great dome, whose apex which is often reduced to Pittsthe from Little dirt on the Mercur side of the camp, ridge running water This to tank. the but on the Sunshine side is often ceburg mine eralized. first rock is called birdseye porphyry by the miners and is nearly a- true granite, lacking the mica. It crops out above the Eagle mine. It, in my judgment, caused the mineralization of all the gold veins in the camp. There was probably a continuous slope from the Little Pittsburg mine to the Sunshine then, and the slope was greater on the east of the line than it was on the west, or the strata were more open, because we find greater minefalization on the east. The second eruptive rock appears to have come up later and formed the ridge which separates Mercur from Sunshine. This rock is a compact white porphyry, which seems to be destitute of mineral and seems to have shattered the strata above the Mercur mine, and to have tilted the rocks much more to the westward. The apex of the dome now is not much over S000 feet above the sea, but it must have been nearly double that when first formed. The mineral veins follow the slope of the dome along the bedding in all directions, and not far from the surface. The base of the dome sinks under the valleys in an irregular arc of a circle, beginning back of Cedar Fort, thence to Fairfield, thence to about half a mile south of Sunshine, thence sharply turning to the northward below Sunshine and extending to Ophir. The northern side of the dome is absent, because of the probably previous upheaval of the mineral-bearin- g belt at Ophir. The diameter of the dome is about six miles, though narrower east and west. On the east side of the canyon running down to Fairfield nearly all the blue limestones of the lower carboniferous age lie in place, forming a mountain some 10,000 feet high, but these strata are all above the mineral zones. In the same canyon the strata have been sharply folded, so as to lie on end, but in all other places they lie at an outward dip of 15 to 33 degrees. As you approach the crest of the dome there is little decrease in the dip up to 7000 feet altitude, thence it rapidly lessens to horizontal at the top, though there is only a small area that is level. Since the mineralization took place the action of the elements has worn deep canyons in the dome, radiating from the crest and cutting through the mented together by silica or composed of silicious sand. It is from four to ninety feet thick, seldom less than fifteen. It lies along the bedding; it shows no faults anywhere along the dip, but occasional ones along the strike, though none are great. By erosion it has in most cases two and sonie'times four apexes on the dip. It is sometimes split up into several smaller veins, with narrow lime strata between. There are in some mines other veins, a hundred feet or more apart, above this vein, but so far none of them carry values to warrant more than a mention of them here. The great gold vein continues throughout the entire dome, on or close to the surface, and assays everywhere, but paying values have been verified by me only from the upper end of the Marion to the Golden Gate and thence to the upper end of the Sacramento on the Mercur side of the camp, and from the Sunshine to the Malvern on that side of the camp. I expect values to be found from the Ophir divide (and maybe beyond it) to the Sacramento, a distance of tw'o and one-ha- lf miles one fourth mile wide, and long and from the top of the Sunshine divide to the Malvern, and possibly beyond it, a distance of at least three miles h mile wide. Paylong and ing values have been reported from many places at the western base of the dome from Sunshine to Ophir, but have not been verified by me so far, though should not be surprised if they were found, as will be show'n hereafter. This vein has been opened up by innumerable shafts, inclines and drifts for nearly seven miles on the strike. The largest work done is on the Golden Gate, where the vein is opened continuously for sixteen hundred feet on the strike and four hundred feet on the dip, showing a continuous ore body, wiiich is never less than ten feet thick and very often forty, and in one place ninety feet thick. In the Sunshine it will average over ten feet, while on the level it is forty feet thick. The ore averages about $9 to the ton on the Marion, $11 on the Sunshine, $12 in the Mercur, and still more in the Golden one-sixt- T 300-fo- ot Gate. Some fifty feet (more or less, according to circumstances) below' the gold vein lies a silver vein of wholly differ far none of them have been opened enough to determine the values. These veins are seldom less than four feet thick. On the Mercur side no work seems to have been done on these veins. From my view' of the mineral- ization there is no reason why there should not be many other veins in this formation not yet discovered. ORES. The ore of this camp is very uniform. On the Mercur side it is composed of pieces of lime rock from the size of dust to that of a mans head, incrusted with sulphide of gold and native gold, the latter about 25 per cent of the W'hole. There is no metal of mineralization at all in the pebbles or rock. In the crevices betw'een the pebbles there is more or less cinnabar, and arsenic in the form of orpment and realgar, but I have seen no silver, antimony, zinc, or lead, though there is a small quantity of iron. On the Sunshine side there is some native sulphur in addition, and the ore is quite silicious, carrying more or less gold within the silica as in ordinary ores. Throughout the camp all the ores practically are oxidized, the ore bodies being above the w'ater level, except in portions of the Golden Gate and the gulch running thence to the Eagle, where the formation seems to be so tight in some places that the ore is unoxidized, and therefore in the form of arsenical py-ri- te carrying gold. In the Golden Gate there is said to be an upper stratum of shale which is unoxidized, carrying large quantities of arsenic, while that below' is free from arsenic and oxidized, but to my mind the arsenical When the stratum is also oxidized. mines get to a sufficient depth to reach permanent water there wrill be a complete change in the ores, the values being contained exclusively in arsenical pyrite. All the gold veins seem to carry the same quality of ore. The silver vein has the silver in the form of small specks of silver sulphide, mostly, scattered through the quartz. THEORIES OF MINERALIZATION. The miners, never having seen ore in this shape before, have assumed that it was produced in some strange way, and have vied with each other in the grotesque theories of its origin, and, worst of all, have acted on those theories in the location of shafts, tunnels, etc. There is a theory that Mercur is a second Johannesburg, but that ore region has its ore in the form of an an- cient placer, where the pebbles have |