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Show INTER-MOUNTAI- Going further on in the degrees east. same direction with the dip and crossing the basin toward the steeper mountain side, the dip increases rapidly and somewhat regularly for over a mile; then there is a sudden steepening, so that it passes within a short distance from an average of 25 or 30 degrees to about 60 degrees. From the line of this the dip is very unisteepening form up to the top of the mountain ridge which separates Mercur basin from Pole canyon, averaging about 60 The relative depresdegrees. sion of the Mercur basin, as compared with the higher hills which stand to the north and to the south, along the line of strike, causes the outcrop of the strata, in passing through the basin, to deviate from a straight line in the form of a distinct bulge to the northeast. This appearance is not indicative of any great irregularity in the strata, but is due simply to the recession of the rocks along the dip with the greater i erosion. AGE OF TIIE STRATA. Fossils were collected from the vari- ous strata and submitted to Mr. Chas. Schurbert of the geological survey, who determined the age to be the carboniferous. It seems that the boundary between the lower and upper carboniferous can not be well defined, but the gold ledge of Mercur is thought to be in great blue limestone of the lower carboniferous. ERUPTIVE ROCKS. In regard to eruptive rocks, Mr. In the Mercur district Spun says: there are two distinct varieties of closely related eruptive rocks, which form sheets or small dikes in the great blue limestone. Both these rocks belong to the class of quartz porphyries, although they are very dissimilar in appearance. One of these varieties is found in greatest thickness and freshest condition in the vicinity of Eagle Hill, on the divide which separates the Mercur Basin from Sunshine and the southern end of the range. This rock in its freshest condition is nearly pure white, with a grayish, brownish, or sometimes pinkish tinge; it is compact and fine grained, and breaks with a conchoidal fracture and a rough texture. On weathering, the rock loses cohesion and becomes a compact very chalky mass, so soft as to be easily impressed with the finger nail, or finally a loose pow'der. The Eagle Hill porphyry seems to be split up chiefly into two principal sheets, which are well exposed on the sides of Eagle Hill. Both are in a general way parallel to the bedding of the limestone, so that they are true sheets. In the Mercur Basin the uppermost of the two principal sheets is shown east of Eagle Hill on the ridge south of Mercur. This sheet is probably between 250 and 300 feet thick. and does not pass through the basin. The second sheet, which is estimated to be fine-grain- ed strati-graphical- ly N MINING REVIEW. and to split into several small sheets. In the lowest part of Mercur Basin at the head of Lewiston canyon they are much decomposed; they are also intimately connected with the ore deposition. There are here three small sheets, averaging ten or fifteen feet in thickness. The same general sheet of porphyry has been traced to the northern side of Lion Hill, at a point overlooking Ophir canyon . The porphyry has its greatest development around Eagle Hill and it seems probable that somewhere in this vicinity exists the channel through w'hich it came up from below. On the southern side of Eagle Hill the upper sheet occupies a considerable area; the lower sheet seems to be split up into two thin sheets one of which runs around and joins the upper body, while the other persists in the bottom of Sunshine gulch, past the Glencoe and the Sunshine mines, till it disappears in the foothills south of Sunshine. Nearly all these outcrops are on the northeast limb of the anticline, where the rocks dip to the northeast The other variety of porphyry, called by miners and explorers birdseye forms part of two porphyry conspicuous eminences which have been called Porphyry Hill and Porphyry Knob. This rock does not in any way resemble the porphyry just described. under which the gold ores are found has been more ore less altered to a dark gray or blackish shaly looking material, called by the miners black shale; that the larger part of the ore consists of the great blue limestone underlying this black shale, altered to various depths by replacement of silica, the altered porphyry or black shale being also more or less impregnated with gold and in places affording pay ore. Portions of the sulphide ores knowrn as black shale also are thought to be an alteration product of the limestone. MINERALIZATION. I deduce the following as the main points of the report: Numerous nearly vertical fissures exist in the limestones, reaching up to and above the porphyry sheet. These fissures seem to form in zones, running northeasterly and apparently causing the main trend of the ore sheets to follow' that direction. These fissures, connecting with some body of uncooled igneous rock at unknown depth, form channels through which vapors charged with minerals ascended and, reaching the dense sheet of porphyry, more or less altered at the time, their mineral contents were deposited along the contact, chiefly downward through the limestone to varying depths. Very little gold wras deposited on the upper side of this porphyry sheet, but low grade ore has been found wrhere the altered porphyry wras sufficiently porous to allow the passage of vanors through it. The author decides that the porphyry sheet is too thin to admit of the theory of leaching by mineralized solutions, and deposition downward by them into the limestones. While the richer bodies of ore seem to exist along the zones of vertical Assuring, it is not likely that these fissures will themselves show any very profitable ore bodies. The ores were originally deposited in the form of sulphides as seen in the unaltered parts of the sulphide ores of the Golden Gate mine, the sulphides of mercury, arsenic and iron being found all along the gold ledge. There seems to be no particularly close relation between the gold and the various sulphides, as none of the specimens of pure cinnabar, realgar and pyrite assayed gave very high returns, and it is thought that much of the gold was originally deposited as a telluride, in the sulphide ores, In its freshest condition its general color is gray. The porphyritic crystals are well developed, and make up a large part of the bulk of the rock. They consist of light gray feldspars; also regularly disseminated biotite and occasional quartz crystals set in a greenish gray ground mass. On the edge of the Mercur Basin the birdseye porphyry consists of a single sheet, conformable with the stratification and writh the sheets of Eagle Hill porphyry. The horizon of the two porphyries is in a general way about the same, although near Porphyry Hill the sheet of the Eagle Hill variety is several hundred feet lower down than the birdseye porphyry. The latter occupies the summit of Porphyry Knob, where it has a columnar structure due to weathering. The porphyry outcrop, followed southeast, diminishes very OXIDIZED ORES. gradually in thickness for some distance, and then terminates northwestThe author says: It seems most can be the ward. The porphyry gold exists in the easily probable that ores oxidized in the free state, but in a to of runs the east Lion traced. It divided condition, and probvery finely Hill, and is exposed high up on the ably in a form somewhat different from northern wall of Ophir canyon. The that of ordinary metallic gold. The obtained by precipitation from Birdseye porphyry in the Mercur dis- gold solution is a fine powder, black, brown, trict, therefore, is only the edge of the purple or red in color, without metalmain mass. lic luster. He states that it has not two concludes the so that Mr. Spurr far been distinguished by the of varieties of porphyry are somewhat microscope, and that the process to reduced has oxidation it probably connected in origin, but does not decide an amorphous state, in which it is not as to their relative age. capable of amalgamation. VEIN MATERIAL OF THE GOLD LEDGE. The report states that the main sheet of the Eagle Hill porphyry splits just southwest of the Mercur Hill into three thin sheets, of which the low'est is the thickest and the uppermost the thinnest; and that the lowest is characterized by the occurrence of silver ores, wrhile the gold ores and Sunrise Hill. The greatest thick- are confined to the middle of the three, ness exposed here is upwurd and the uppermost has not been minprobably of 300 feet, but it seems to thin with eralized to any important extent. The great rapidity to the north and east, report decides that this porphyry sheet about 700 feet below the first, is found in its greatest thickness and with its rocks in the freshest condition on the spur between Eagle Hill 5 GENERAL DEDUCTIONS. The ore wTill not be found distributed all over the contact, but along sheeted zones, characterized by straight open fractures. The oxidized ores will not extend to any great depth, but will soon reach sulphide and telluride ores, but it is presumed that the ore bodies will increase in size and richness sufficiently to be profitably smelted. In prospecting it will be necessary to first identify, in the field, the gold ledge porphyry sheet, and one should follow the intersection of this sheet with the principal zones of northeast fracturing, and prospect dowmvard along these intersections and their vicinity. |