LOCATION AND SURFACE FEATURES
The Bristol Range, Highland, and Ely Range quadrangles make up the larger part of a. rectangular area 35 miles north and south by 24 miles east and west, which lies 19 miles west of the Nevada-Utah line and about 250 miles southwest of Salt Lake City. The district lies within the Great Basin, a semiarid region of alternating mountain ranges and intermontane plains floored largely by outwash from the mountains.
The plain, which slopes away from the ranges, stands between 4,700 and 6,000 feet above the sea. The Bristol and Highland Ranges, which are separated only by a low gap, form an almost continuous north-south range that rises about 2,500 feet above the highest part of the surrounding plain, to general altitudes of 8,000 to 9,000 feet, though the highest point, Highland Peak, reaches 9,395 feet. A lower range, the Ely Range, with a northwesterly trend, lies farther east and nearly in touch with the Bristol-Highland Range. The town of Pioche lies midway on the. eastern foot of the Ely Range.
ROOKS OF THE PIOOHB REGION
The rocks of the ranges are Paleozoic sediments, Tertiary (?) lavas and intrusive rocks, and Pliocene (?) tuffs.
The Paleozoic sediments have a total thickness of nearly 18,000 feet. Over 8,000 feet of the Cambrian has been measured without reaching its base. The lowest Cambrian formation is a quartzite, of which only the upper 1,500 feet is exposed, and this is followed by 1,200 feet of shale, 400 feet of limestone, aoid 150 feet of shale. Above this second shale the upper three-fourths of the Cambrian consists of limestone and dolomitic limestone. It is in the quartzite and in the limestone interbedded in and bounding the shales that the main ore bodies of the district have been found. Above the Cambrian comes 1,795 feet of Ordovician limestone, with some interbedded dolomite and with a 50-foot quartzite a, third of the way down from the top; 75 feet of Silurian dolomite; 3,000 feet of Middle Devonian dolomite with thin interbedded quartzite near the top, followed by 550 feet of Upper Devonian limestone; 3,775 feet of Mississippian limestone with a heavy quartzitic sandstone just below the middle; then, at the top of the series, 700 feet of Pennsylvania n limestone.
The Paleozoic formations are tilted, bent, and faulted, but sharp folding is extremely rare. Dips between 10° and 30° are common, but there are few greater than 40°. The rocks are cut by many large and innumerable small normal faults, which in many parts of the area are difficult to recognize because they cut thick formations of similar rocks. A large thrust fault occurs along the west side of the Bristol-Highland Range.
A thick series of lavas with interbedded tuffs lies along the flanks of the ranges and makes up a large part of the hilly northwest corner of the Bristol quadrangle. The lavas lie unconformably on the Paleozoic sediments and consist mainly of dacite, latite, and andesite, with some basalt and a little rhyolite. About 6,000 feet of lavas and tuffs were measured in Condor Canyon, near the south end of the Ely Range. The age of the lavas is not sharply fixed; they may be early Tertiary or even late Mesozoic. It is not unlikely that they are the result of volcanic action extending over a long period. The lavas are tilted and faulted, though their average dip is less than that of the Paleozoic sediments.
At Blind Mountain, on the west side of the Bristol Range, stocks of quartz monzonite and dikes of similar composition cut an infaulted block consisting of sediments, chiefly Devonian, and lavas. Both the sediments and the lavas are metamorphosed, and a belt of marked metamorphism extends south along the west base of the Bristol-Highland Range for several miles. The quartz porphyry dikes of the region, including those near Pioche, are believed to have come in at the same time.
The plain west of the Bristol-Highland Range is a. valley of interior drainage floored with outwash from the mountains, coarser near the range but finer and making a clay flat along the west side of the Bristol Range and Highland quadrangles. The valley east of the Bristol-Highland Range, however, has outside drainage by way of Meadow Valley to the Virgin and Colorado Rivers. The stream in upper Meadow Valley and its tributaries have cut through the surface wash from the mountains and laid bare several hundred feet of white, yellow, and red water-laid Pliocene tuffs that lie in nearly the position in which they were laid down. This erosion has developed a striking badland topography in the Pliocene beds.
HISTORICAL SUMMARY
The general succession of events may be summarized as follows:
1. Sedimentation during most of Paleozoic time from Cambrian to Pennsylvanian.
2. Uplift, slight warping, and erosion.
3. Volcanism of perhaps late Mesozoic or early Tertiary time, producing lavas and tuffs. This period of volcanism may have lasted a long time and spanned one or more of the epochs of faulting.
4. Tilting and normal faulting.
5. Thrust faulting.
6. Quartz monzonite intrusions at Blind Mountain.
7. Normal block faulting of the Basin Range type.
8. Erosion of the faulted blocks to maturity and to essen-. tially the topography of to-day.
9. Outbursts of volcanic ash, probably in late Pliocene time, and the deposition of several hundred feet of water-laid tuffs in the valleys.
10. In Meadow Valley, valley cutting, which has produced a badland topography in the soft Pliocene tuffs and canyons where the streams cross the harder Paleozoic limestones.
ECONOMIC GEOLOGY
The Pioche district during four years in the early seventies was second only to the Comstock district in output of silver. The bonanza ore of those stirring times came from fissure veins in the Prospect Mountain quartzite, of Lower Cambrian age. In recent years the main interest has shifted to the bedded replacement deposits of silver-bearing lead-zinc sulphide ore occurring in the limestone members of the Pioche shale a type of ore body which was discovered accidentally during the prospecting of the fissure veins.
The ore deposits of the district comprise three groups (1) silver-bearing fissure veins in quartzite; (2) silver-bearing mineralized granite porphyry; (3) replacement deposits in limestone and dolomite. All of them appear to have been formed at about the same time, in the epoch of mineralization that occurred shortly after the intrusion of the granitic rocks and their allied dikes of granite porphyry and lamprophyre.
The entire present output of the district is coming from the replacement deposits in limestone and dolomite, but exploratory work is still in progress on the fissure veins and mineralized porphyry.
The replacement deposits include both replacement fissure veins and stratiform ("bedded") replacement deposits.
The replacement fissure veins dip steeply and cut across the bedding of the carbonate rocks in which they are inclosed. They are thoroughly oxidized, as deep at least as 1,100 feet, for on none of them have the mine workings penetrated to water level, and they are highly manganiferous and limonitic and low in silica. At-certain horizons stratiform replacement deposits extend out as lateral branches from the fissure veins. Deposits of this kind occur mainly in the Mendha limestone, Highland Peak limestone, and Lyndon limestone. The stratigraphic range is therefore at least 5,500 feet, and as some of the fissure veins extend down through the underlying Pioche shale the indicated range may exceed 6,500 feet. The most notable representatives of the replacement fissure veins are at the Bristol mine, where they yield silver-bearing copper-leadzinc ores. So far unique among the ore bodies of the district is the pipe of wad and pyrolusite ore at the Jackrabbit mine, the periphery of the pipe consisting of a girdle of extraordinarily coarse white calcite spar produced by the recrystallization of the surrounding limestones.
The stratiform replacement deposits that are attracting most attention occur at the intersections of steep fissures with the limestone members of the Pioche shale. In recent years the " bedded " ore of the Combined Metals mine has been of main interest. The ore is essentially a.n intimate intergrowth of pyrite, sphalerite, and galena. Although above water level, the ore is unoxidized. It extends on both sides of the mineralizing fissure as far as 100 feet. At the Prince mine ore beds were formed at seven successively higher horizons in the Pioche shale and the overlying Lyndon and Highland Peak limestones. The ore " beds " above water level in the Prince mine are thoroughly oxidized and consist of manganese-iron oxides low in silver, lead, and zinc. About 800,000 tons of ore carrying 2.5 to 3 ounces of silver to the ton, 3 per cent of lead, 35 per cent of iron, and 15 per cent of manganese has been shipped. The Prince mine was in 1915-1918 the premier producer of lead in Nevada. The ore " bed " that occurs here below water level consists of sphalerite, galena, and pyrite in a gangue of manganosiderite and minor quartz. This is the only locality in the district in which the primary source of the abundant oxidized manganese minerals (wad, pyrolusite, and braunite) has so far been found.
Some of the stratiform ore bodies the ore beds, as they are locally called were formed adjacent to exceedingly insignificant-looking fissures; and this dependence on inconspicuous mineralizing fissures is beyond doubt one of the most impressive features in the geology of the district. It opens the possibility that there may be many other bedded deposits which, like the Combined Metals ore bed, do not crop out. To find these ore bodies will be difficult, but their discovery will be aided primarily by applying skillfully a knowledge of the geologic column and by determining the faulting that has disturbed or changed the normal sequence of the strata.