Hawley and 2 co-authors of the U.S. Geological Survey mapped the Temple Mountain district, Emery County, Utah with their report issued in 1965. The area is located on the southeast flank of the San Rafael Swell, one of the major topographic and geologic features of the western part of the Colorado Plateau. . Some of the oldest mines in the plateau are in this district, which is one of the few places where plateau-type uranium ores are associated with collapse structures and faults and with highly altered rocks.
The rocks cropping out in the district are sedimentary and range from Permian to Jurassic in age. With the exception of two thin marine limestones, the
Kaibab Limestone and the Sinbad Limestone Member of the Moenkopi Formation, they are clastic rocks that were deposited on land or very near shorelines.
The limestones and coarser grained rocks of all clastic units are locally petroliferous.
The oldest rocks exposed are light-colored generally fine grained sandstones of the uppermost 80 feet of the Coconino Sandstone of Permian age. These
rocks are overlain by another Permian formation, the Kaibab Limestone, which averages about 25 feet in thickness but is locally thin or absent. The Moenkopi Formation, about 600 feet of Lower and Middle (?) Triassic rocks, disconformably overlies the Kaibab, and it, in turn, is unconformably overlain by the Chinle Formation of Late Triassic age. The Chinle Formation is divided into four members; the lower two, the Temple Mountain and Monitor Butte Members, are mapped as one unit that averages about 25 feet in thickness. The Temple Mountain and Monitor Butte Members are overlain disconformably by the Moss Back Member, 60-130 feet thick, which grades upward into the Church Rock Member, about 245 feet thick. The Temple Mountain, Monitor Butte, and Church Rock Members are mainly composed of fine-grained red and purple rocks, but the intervening Moss Back Member is mainly light-colored sandstone and conglomerate. The Moss Back is the main ore-bearing unit of the district and was deposited by a great generally northwest trending river system.
The cliff-forming Glen Canyon Group overlies the Chinle and forms the massive San Rafael Reef, which bounds the San Rafael Swell. The group totals
about 1,200 feet in thickness and consists of the Wingate Sandstone of Late Triassic age, the Kayenta Formation of Late Triassic (?) age, and the Navajo
Formation of Triassic (?) and Jurassic age. The youngest exposed rocks are siltstones and thin limy beds in the lower part of the Carmel Formation of
Middle and Late Jurassic age.
The rocks generally strike about N. 55° E. and dip southeast away from the axis of the northeast-trending San Rafael anticline. Eastward across the
district the amount of dip increases gradually from a minimum of about 2° to about 10° near the main outcrop belt of the Chinle Formation; there the dips
steepen southeastward in a monoclinal flexure whose axis is parallel to the regional strike. Farther east the dip flattens to 9° or less. The generally
homoclinal structure of the district is locally distorted by high-angle normal faults, northwest-trending crossfolds, and collapse structures.
The collapse structures (or collapses) are down-warped or downfaulted masses of altered rocks that are roughly oval shaped in plan. The largest of the five
collapses exposed in the district is at Temple Mountain. East-west it is about 3,000 feet long, and it comprises rocks of at least the Moenkopi, Chinle, and
Wingate Formations. The Temple Mountain collapse possibly is a composite structure formed from two or more main collapse elements arranged along one of the synclinal crossfolds of the district.
The collapses probably formed at least in part as the result of collapse or sag of younger rocks into voids formed by the solution of carbonate and possibly
siliceous rocks. They are believed to have formed at about the same time as the folding of the San Rafael anticline. Most of the uranium deposits of the district are unoxidized and are composed chiefly of a uraniferous carbonaceous mineraloid termed "asphaltite." The largest deposits are in the Moss Back Member of the Chinle Formation, but near the Temple Mountain and other collapse structures small uranium deposits occur in other rock units and in the collapse structures themselves. Most of the ore bodies can be classed as either rolls or tabular bodies. Most of the ore produced has come from deposits in the Temple Mountain mineral belt, a northeast-trending zone about 9,000 feet long. Ore bodies in the belt generally
are oriented northwest and are as much as 700 feet long and 15 feet thick.
The distribution of ore was probably controlled by several factors, among which are collapse structures, the monoclinal upwarp underlying the San Bafael
Reef, and paleochannels in the Moss Back Member. On a smaller scale the distribution of ore was controlled by mudstone splits, the shape of sandstone
lenses, and, locally, fractures. In comparison with equivalent barren rocks, the ores are greatly enriched in selenium, arsenic, and lead in addition to the principal ore elements uranium and vanadium. Iron and other rarer elements such as copper, chromium, and zinc are enriched to a lesser degree. In terms of weight percent, iron, vanadium, uranium, selenium, arsenic, chromium, and zinc are relatively abundant constituents of the ore.
The unoxidized ores contain, in addition to asphaltite, small amounts of pyrite, montroseite, sphalerite, uraninite, ferroselite, and, less commonly, other
metallic minerals. Pyrite is the most common sulfide; it has several modes of occurrence and is found in the Moss Back Member in ores and in both uraniferous and non-uraniferous coals. Uraninite occurs as relatively coarse irregular grains and possibly as very fine round blebs in the asphaltite. Some of the uranium in the asphaltite is either in submicroscopic uraninite or in an organic complex. The gangue consists chiefly of the detrital grains of the host sandstones but includes some dolomite, clay minerals, and chromium-bearing mica clays that formed as alteration products. Petroleum is widespread in barren, unaltered host sandstones; a residual interstitial asphalt is common in altered sandstones near ore-bearing zones.
Asphaltite and closely associated ore minerals were deposited relatively late in the paragenetic history of the rocks; they formed after the hardening of the
coals that are contained in the deposits. Some of the pyrite, sphalerite, and possibly other metallic minerals formed earlier, before the hardening of the coals. Although most of the asphaltite formed after the deposition of the closely associated metallic minerals, the association suggests that asphaltite and closely associated minerals formed about contemporaneously.
At Temple Mountain, ore deposits in the Moss Back Member are generally richer in vanadium and arsenic nearer the collapse structure than farther
from it. Selenium is most strongly enriched in the ore deposits of the Temple Mountain mineral belt at a median distance from the collapse. The distribution
of other elements tends to parallel that of either vanadium and arsenic or selenium but in general is more complex. In individual ore bodies, chromium,
selenium, vanadium, and a few other elements are distributed asymmetrically across roll or other sharp ore-body contacts.
Many of the rocks of the district are altered, and the effects of alteration are especially noticeable on Temple Mountain and near the other collapse structures. Solutions removed calcite and petroleum and formed clay minerals, dolomite, and siderite in the rocks near ore deposits or collapse structures. Both the color and composition of the mudstones in and near ore were changed during alteration.
The ore and associated alteration products, such as dolomite and chromium-bearing mica clays, may have formed about concurrently from carbon dioxide-bearing aqueous solutions that were introduced into the Moss Back Member and other strata through the collapse structures. The asphaltite was derived
mainly from petroleum that was displaced by the introduced solutions; it was deposited near stabilized interfaces that formed between the introduced solutions and the petroleum contained in the rocks. These interfaces are now marked by sharp ore contacts.
Some of the metals found in the ore deposits, such as uranium, selenium, and arsenic, probably had their source at a considerable distance from the ore
deposits; others, such as chromium and vanadium, may have been partly derived from nearby wallrocks during alteration. Some components of the introduced solutions, such as carbon dioxide, may have had an igneous origin.
For the 7 Plates generated by this report, see: https://pubs.er.usgs.gov/publication/b1192
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