ORO GRANDE DISTRICT.
The Oro Grande Mining District is located immediately east of the Southern California Railroad, at the town of Oro Grande. It is commonly supposed to embrace all the mines for some miles around, though, in fact each group of mines or hills has been given a separate name, but as these so-called districts are mostly without organization, all the claims and mines will be considered under one head.
The geology of the district about Oro Grande is complex, the formations being uplifted, greatly faulted, and broken, besides the intrusion of dikes of felsitic rock, diorite, and quartz porphyry. I had not sufficient time at my disposal when at Oro Grande to study out the somewhat intricate geological problem, and will describe the region in general terms.
Commencing at the town of Oro Grande, which stands on the bank of the Mojave River, the country rises in a gentle slope toward the hill half a mile distant; gently rolling hills are reached, which in turn give place to more rugged masses, and finally to a rough mountainous area, the hillsides being almost precipitous. The lowlying country about the base of the hills is mostly made up of schistose micaceous rocks, a quartzose mica schist predominating. The first hills of any consequence are eruptive, mostly a light-greenish felsite and a coarse-grained porphyritic rock. Beyond are prominent hills of a dense, hard quartzite resting upon a crystalline limestone, the highest hills being made up of practically the same materials (quartzite and limestone), in part schistose with some mica schist, jasper, and many intrusive dikes of all the previously mentioned eruptives, prominent among them being diorite of dark-green color.
THE EMBODY MINE.
Within half a mile of the town, and on a lower spur or ridge that makes down from the hills, is located the Embody Mine, which, during the excitement at this locality in 1890, attracted considerable attention. The gold-bearing material is quartzose, micaceous rock of somewhat friable character. The deposit, as I may term it, has the appearance of being an impregnation without definite form.
Were it not for the fact that the shoot of gold rock makes across the strike of the schists, it would resemble some bedded deposits found at the Homestake, Black Hills, South Dakota, where micaceous schists have been silicified and hornblende schists metamorphosed to chloritic schists, the whole carrying gold across a broad zone 1,600 feet in width and 6,000 feet in length. The gold occurs in shoots or vein-like zones, without defining lines of any character. At the Embody Mine, too, little development has been done to make any positive prediction as to the future of the mine. The formation strikes northeast and southwest, and dips 70° southeast.
The country is somewhat broken up, but no considerable masses of shattered rock were observed. The croppings are quite heavily stained with iron oxides of brown and red shades, and this mineralization can be traced some distance. Two shafts, one nearly 100 feet deep, the other about 30 feet, have been sunk on the deposit, exposing rocks of uniform character, all carrying some gold. The width of the gold-bearing zone is undetermined, but it is thought to be from 6 to 20 feet.
As far as I learned, a “mill-run” had never been made on the rock from this mine. Mining operations had been stopped and the property involved in some sort of dispute. The value of the rock was given to me as $8 or $10 per ton.
THE CARBONATE MINE.
The principal mine of this district, and the one which gave the camp its fame, is the Carbonate Mine. It was discovered by a man named Collins, who was working in a lime quarry near by. Collins found croppings of ore—limonite and manganese—containing silver. He developed the property somewhat, but it finally passed into other hands, and is now owned by a Los Angeles company, which has opened the mine quite extensively.
The formation which incloses the vein has a general trend northeast and southwest, the dip at the main workings being not over 20°. Here an inclined shaft has been sunk to a depth of 225 feet on the vein. At the bottom the shaft has attained a vertical depth of about 100 feet. Two veins of ore, consisting principally of silicious and earthy iron oxide and black oxide of manganese with carbonate of lime, sometimes crystallized, extend from the surface to the bottom of the incline. These veins are very irregular in size, varying from a mere seam to upwards of 2 feet in places.
The value lies in the lead carbonate and silver which accompany the gangue minerals. The two veins are curiously formed at contact with massive blue limestone and mica schist. The schist is from 1 to 4 feet in width, the ore lying both above and below it, the whole being inclosed between hanging and foot walls of crystalline limestone. At various points in the workings is a light-colored, much decomposed rock, resembling felsite, which has the appearance of having been injected between the strata in a thin sheet. It is a notable fact that where this buff-colored, granular appearing rock occurs in contact with the vein an enrichment of the ore is noticeable, and its absence is marked by the low value of the ore or no ore at all. In this incline, at a depth of 40 feet, a short drift has been run in on ore of good grade. At 180 feet from the collar of the shaft the discovery was made that caused this mine, and, in fact, the entire camp, to become at once the scene of excitement.
At this place a small wedge of crystalline, granular quartz and calcite appeared, and with it flakes of free gold. Just below the point of this discovery the wedge widened to several inches, and the rock was a mass of glittering sheets and shot-like pieces of gold. Assays of the material gave fabulous returns. The ore was broken down on canvas, and every ounce of it sacked on the spot. This was followed down some distance, but gradually thinned out below the 200-foot level, where drifts were run, one 50 feet northeast, the other 40 feet southwest. From these drifts considerable rich quartz was obtained.
In the southwest drift the formation is badly displaced and broken and the vein is lost, a fault having thrown it, but whether up or down could not be determined, as the adjacent rock was so badly fractured. Ore was found at the face and along the sides of some of the cuttings, and some free gold was found on the 200-foot level in the southwest drift. All work on this part of the mine had been suspended some time prior to my visit, the mine having been enjoined.
ORIGIN OF THE VEINS.
This question is one which finds its answer, it would seem, in the fact that the sheet of mica schist included between the heavy masses of limestone represents what at one time was possibly a bed of sandy clay or mud, which with the metamorphosis of the region has become a crystalline schist. The planes separating this schist and the limestone above and that below it were evidently planes of weakness, and when the forces which uplifted and fractured the strata exerted themselves these rocks slipped and ground upon each other, causing considerable crushing along these planes; possibly open crevices resulted in some portions. Ore was subsequently deposited in these interstitial spaces, partly by substitution of ore for limestone, no doubt, and partly by precipitation in the crushed mass of lime and schist. The injection of a sheet of felsite into the same plane of weakness can easily be conceived, as such intrusive bodies always follow the lines of least resistance. The extreme richness of the rock, together with its somewhat unusual association of quartz and calcite, attracted no little attention to the property at the time of its discovery.
A FISSURE VEIN.
Southwestward from the deposit just described is a vertical shaft 125 feet deep. This shaft has followed down what seems to be a fissure in the limestone, in which lead carbonate, some galena, limonite quartz, calcite, and manganese oxide occur. This ore was worth at Socorro, New Mexico, $50 per ton for the gold, silver, and lead it contained, and it was shipped there in quite large quantities.
A large stope commences on the northeast side of the shaft at a depth of 30 feet from the top and extends down to the 114-foot level. Considerable ore was standing in sight in the mine at the time of my visit, but nothing was being done. The company have had to stop work, as the owners of the lime quarry claim to have this property included in their patent.
OTHER CLAIMS.
There are scores of other claims in this interesting district, but little development has been done on them. Here and there are encouraging prospects, where carbonate of lead and oxide of iron have been found; but on the most promising of these only 10-foot holes have been sunk. The mineral zones are not well defined, and the prospectors have not the capital necessary to systematically prospect the hills.