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(SUBJECT TO REVISION)
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Geology of the Warren Mining District
BY T. S. BONILLAS, * s. B., J. B. TENNEY, E. M., AND LEON FEUCHÈRE, * INGÉNIEUR CIVIL
DES MINES, BISBEE, ARIZ.
The main object of this article is to present the results of observations by the Copper Queen Consolidated Mining Co.'s geological department as an addition to the already published reports on the Warren district. Since these observations relate mostly to the ore deposits, little will be found here on the general geology that has not been covered by F. L. Ransome's admirable work on the Bisbee Quadrangle. A summary of this very important part of the study of the camp will be given, however, mainly to bring out the new facts discovered during the advance in underground development and the latest detailed study of structures and petrography of the mining area.
The Mule Mountains, in which the Bisbee ore deposits occur, constitute a chain with a northwest-southeast axis rising abruptly on the sides from the Sulphur Springs Valley on the northeast, as shown in Plate 1, and the San Pedro Valley on the southwest. The range starts at about the International Boundary near Christianson’s ranch, with low hills covering a width of 1 or 2 miles, and extending for about 3 miles to the northwest. Here the Gold Hill overthrust fault has caused an abrupt rise in elevation and from this point on the range widens out and the hills become very much more rugged. The highest point is about 6 miles farther. The total length of the range from the International Boundary is about 23 miles and its maximum width, about opposite the town of Bisbee, is 10 miles. The range finally ends at Government Draw, which is a pass about 2 miles wide, connecting the San Pedro and Sulphur Springs Valleys. Beyond this the Tombstone hills commence.
The range is divided into two parts by Tombstone Canyon, a deep canyon running through the southwest side of the range. It is along this canyon that the Borderland Route road takes its course, and in which the town of Bisbee is situated.
Geologically, also, as seen in Plate 1, the Mule Mountains are roughly divided by Tombstone Canyon into two parts. To the southwest is the pre-Cretaceous tract and to the northeast the Cretaceous. The difference in the two tracts is very marked physiographically. The pre-Cretaceous is very much cut up and is formed of rocks of diverse compositions, consisting of schists, granites, shales, and limestones. These rocks, when subjected to erosion, form a rugged topography with deep canyons and steep cliffs. The Cretaceous tract, on the contrary, is uniform in composition, constituted, for the most part, by soft sandstones, conglomerates,
Geology and Ore Deposits of the Bisbee Quadrangle, Professional Paper of the U. S. Geological Survey, No. 21 (1904).
and shales, which are but little faulted. The topography carved from them is characterized by gently sloping hills and draws with almost no clifis or deep valleys in evidence.
III. INTRODUCTORY GEOLOGY The oldest rocks of the range are pre-Cambrian schists and an intruded granite, which are separated from the overlying Paleozoic beds by a profound unconformity.
GOLD MILL BLOCK
PLATE 1.–GEOLOGICAL MAP OF THE MULE MOUNTAINS.
The Paleozoic beds represent an era of apparently uninterrupted deposition of sediments, starting with 440 ft. of quartzites having a basal conglomerate, followed by Cambrian limestones, OrdovicianSilurian quartzite, Devonian and Carboniferous limestones. The total thickness of the Paleozoic beds is approximately 5,000 ft.
At the end of the Carboniferous era a violent uplift took place accompanied by extensive faulting and intrusion of granite porphyry. During this uplift, and intimately associated with the intrusion, mineralizing solutions arose from which originated the orebodies of the camp.
Following this came a long period of erosion, which ended with a rapid subsidence, during which time 4,500 to 5,000 ft. of Cretaceous sandstones, shales, and limestones were deposited.
At the end of Cretaceous time a gentle uplift took place, lifting the range again above sea level. This was accompanied by some intrusion of rhyolite and monzonite.
During Tertiary and Quaternary ages, this land area was subjected to erosion, resulting in the present topography, the detritus filling in both the valley of the San Pedro and of the White River, one on each side of
IV. ROCKS OF THE DISTRICT
A. SEDIMENTARY ROCKS
Reference to Plate 2, showing a generalized geologic section, will help make clear the following description.
Name.-The oldest rock of the district is the Pinal Schist. It is composed of a uniform series of thinly laminated siliceous mica schists of unknown thickness. From the similarity in texture and stratigraphic relation to the later sediments, it has been correlated by Ransome? with the underlying schistose complex of the Pinal Range, which he called the Pinal Schist.
Distribution and General Structure. The Pinal Schist, being the basal crystalline rock upon which rest all the younger formations of the district, is exposed everywhere that erosion has stripped them off, and is encountered underground on sinking through them.
The best surface exposures are northeast of Mule Gulch and to the southwest of Tombstone Canyon. Smaller exposures are also found about 1 mile to the northwest of the town of Don Luis. Underground it is exposed where the Dividend Fault is cut, both in Copper Queen and in Denn ground. It also is found as dragged-in fragments in the contactbreccia mass around Sacramento Hill.
Lithology.—The color varies from light to dark gray, with tinges of green on fresh surfaces, and rusty in weathered specimens. The cleavage is rather imperfect, having a shiny satin-like surface.
The microscope shows the rock to be composed mainly of quartz and sericite. Sometimes the sericite is replaced in part by penninite, chlorite,
2 Loc. cit., p. 24.