Lukachukai Mountains, Apache County, Arizona, USAi

Regional Level Types
Lukachukai Mountains	Mountain Range
Apache County	County
Arizona	State
USA	Country

REGIONAL STRUCTURE:

The Lukachukai Mountains are a north-bending structure 110 miles long and 50 miles wide. To the west the rocks dip gently toward Black Mesa basin which is separated from the Defiance uplift by the Sheep Creek, Chink, and Rock Mesa monoclines. To the east, strata dip steeply along the Defiance monocline into the San Juan Basin. The northeast limit of the Defiance uplift is marked by the Toadlena anticline, which, in conjunction with the Chuska syncline, on its southwest flank, trends northwest for nearly 45 miles oblique to the long axis of the Defiance uplift. The Lukachukai Mountains lie in the northwestern part of the Chuska syncline. A smaller structure, known as the Red Rock monocline, turns away from the northeast flank of the Toadlena anticline near Red Rock Trading Post and trends north for about 12 miles. The monocline dips to the east about five degrees or less. Few faults are present in the area surrounding the Lukachukai Mountains.

LOCAL STRUCTURES:

The Chuska syncline and Toadlena anticline are the dominant structural features of the Lukachukai Mountains area. From Mesa I, the axis of the Chuska syncline strikes about N. 45 W. for 4½ miles to the north side of Mesa VII, turns to N. 65 W. and continues for 3½ miles to the west edge of Mexican Cry Mesa. Beyond Mexican Cry, the axial line swings westward, then northward for several miles before the fold dies out. The axis plunges to the northwest but not at a constant rate. From Mesa I to the southeast side of Mesa VI, the plunge is about one half degree; from Mesa VI to the east side of Mexican Cry Mesa the plunge is only one-tenth degree; across Mexican Cry Mesa the plunge is again about one half degree. The Chuska syncline is sharply asymmetric. The steeply dipping limb of the syncline faces southwest opposing the regional dip off the Defiance uplift. The axis of the Toadlena anticline nearly parallels that of the syncline and plunges northwest at a variable rate. Although the Toadlena anticline may be the surface expression of a deep-seated fault, no faults as such are known in the area.

DESCRIPTIONS OF DEPOSITS:

HOST ROCKS: Ore bodies in the Lukachukai Mountains are in the Salt Wash Member of the Morrison Formation, but sub-ore grade deposits have been found in the overlying Recapture Member, and in the Chinle Formation. The stratigraphic position of host units within the Salt Wash ranges from 30 to 80 feet above the Salt Wash-Bluff contact, roughly in the two middle quarters of the Salt Wash. Neither ore nor protore is known in the lower 20 feet of the Salt Wash, but protore may occur at any other stratigraphic position within the member.

LITHOLOGY:

The host sandstone units, ranging from 10 to 40 feet in thickness, are white, gray, limonitic brown, or red, and contain mud galls, claystone splits, and mudstone pebble conglomerate lenses. The host sandstone changes from its normal color of pink or reddish-brown to gray or tan in the vicinity of ore bodies, which usually contain red, brown, and black stains. The sandstones are fine-grained, lenticular, and cross-stratified; carbon is locally abundant, particularly in sandstones deposited by east-and southeast-flowing streams. Claystone and/or siltstone units, which are laterally continuous across one or two mesas, nearly always underlie and frequently overlie the host units. The vertical interval of the host unit through which ore is distributed seldom extends through the total thickness of the host unit; instead, barren rock nearly always separates the ore
from the bottom and frequently from the top of the host unit.

The most common occurrences of uraniferous material are: 1) in cross- stratified sandstone containing red, brown, and black stains and cements which give the ore a characteristic mottled or banded appearance, 2) in limonite-stained, cross-stratified sandstone associated with halos and bands of limonite, 3) in and around carbonaceous plant material, 4) in mudstone pebble conglomerates or associated with claystone splits and partings, and 5) as joint fillings. Sandstones containing some interstitial clay, or having irregular bedding seem to be preferred loci for the deposition of ore. Calcium carbonate concretions and bands, most of which are stained dark gray or reddish-black, are commonly associated with ore, but similar bands and concretions, though most are unstained, are common in barren rock.

MINERALOGY:

In any of the above partly or completely oxidized occurrences, tyuyamunite, the uranium vanadate, is by far the most common ore mineral. It may be irregularly disseminated, concentrated in lenses, or distributed in bands. It may fill the sand interstices, or only coat sand grains, or it may replace calcite and carbon. Other vanadium minerals which have been identified include corvusite, pascoite, hewettite, metarossite, vanadium clays, and possibly montroseite (S. R. Austin, personal communication ). In general, the vanadium to uranium ratio of the ores shipped from the Lukachukai Mountains has averaged 4:1. Uraninite has been identified as a replacement in carbonized wood and as a cement in some ore bodies that are not completely oxidized (Laverty and Gross, 1956).

ORE GEOMETRY:

The uranium deposits consist of one or more individual masses of ore surrounded or separated by protore. The term ore body, as here applied, refers to the composite extent of both ore and the surrounding protore. The individual masses of ore are here called ore shoots, and such shoots may range up to 350 feet in length. In exceptionally large and rich deposits, the aggregate length of the ore shoots may exceed 1,000 feet.

Nearly all ore bodies are elongate at least three times the width, and most of the ore shoots within the ore bodies are elongate at least twice the width. The overall elongation of every ore body is parallel to paleostream depositional trends measured in and near the ore bodies. More specifically, although the ore body may extend across several separate sand lenses presumed to be deposits in paleostream channels most of the ore shoots lie within and are elongate parallel to sand lenses. All of the ore bodies are lenticular in cross section. Thickness of the ore bodies ranges from one foot to 22 feet.

ORE DISTRIBUTION:

All the larger ore bodies and nearly all of the smaller ones are in a belt which trends slightly cast of north, oblique to the axis of the Chuska syncline. Within this belt, ore bodies are found in clusters, and the larger clusters are located either in reentrants at the heads of canyons or near the back end of the mesas. This distribution is probably the result of several factors. Deposits on the narrow, fingerlike mesas are most subject to oxidation and probably have been leached.

Drilling depths to the host unit are much greater toward the core of the mountains, and much of this area has been inadequately tested. Perhaps the apparent clustering of ore bodies near the rims is merely a result of greater drilling in these areas where drilling depths are shallow.

All ore bodies are on the southwest limb of the Chuska syncline with the exception of several large deposits on Mesa I and a small deposit on the northern tip of Mesa V which are located on the northeastern limb of the syncline. Within the ore belt only a very small amount of Salt Wash is preserved on the northeastern limb of the syncline. Thus the two occurrences strongly suggest that ore bodies at one time were present on the northeast limb but have since been removed by erosion. No direct relation is apparent between the fold and the location of ore bodies, ore clusters, or the ore belt.

As previously noted, all ore bodies are elongate parallel to paleostream depositional trends, but this is not true of all ore shoots. In many times, projections of ore which deviate from the paleostream depositional trend arc elongate parallel to prominent joint sets in the mines. Similarly, ore grade and thickness contours, the overall pattern of which closely follow sedimentary trends, show lobes and projections which parallel the prominent joint set. The largest ore shoot in the Hall Mine on Thirsty Mesa is roughly L-shaped, one branch being parallel to the sedimentary trend, the other parallel to the dominant joints. Thus, joint patterns bear a close relationship to the distribution of ore shoots within an ore body, but whether this relationship is a primary feature or is the result of secondary redistribution is unknown. No faults are present in the vicinity of the mines which were mapped.

ORE GUIDES:

In all of the mines mapped, the host unit in the vicinity of the ore bodies is predominantly gray, white, or limonitic-brown. At or near the edges of the ore bodies, these colors either abruptly abut or grade into the red color of the surrounding country rock. Data concerning whether the color change is a result of the passage of ore solutions is contradictory; however, because of the gray color in down dip oil tests in the San Juan Basin the author believes that at least some, if not most, of the red coloration is a result of oxidation of originally gray sandstone, and that not all favorably colored areas resulted from the passage of ore solutions which altered originally red rocks to gray. The ore bodies in all of the mines mapped are elongate parallel to paleostream depositional trends, and although the ore body may extend over several small channels, most of the ore shoots are elongate parallel to and lie within sand-filled channels on the order of 25 to 150 feet wide. The lateral extent of most ore shoots is controlled by the extent of the small channel in which the ore shoots lie, but some ore shoots and extensions are controlled by joints. The upper limits of ore shoots and ore bodies is often controlled by an overlying claystone, but control of the lower limits is not. Paleostrcam sedimentary channels, festoons, lineation, and rib-and-furrow trends measured in the mines did not always agree with channel trends outlined on mudstone:sandstone ratio maps of the same area. Over a limited area of one or two mesas, ore is confined to one or possibly two mappable, lenticular units which thicken and thin perceptibly. The ore bodies occur in units showing most rapid variation in thickness. Ore often occurs in muddy sandstones in preference to cleaner sandstones above or below. Carbon is widely distributed and locally abundant. Some ore is closely associated with carbon trash and logs, but as is common in oxidized deposits, the biggest part of the ore is not closely associated with carbon. In the Lukachukai Mountains carbon in the form of logs and branches is most abundant in sandstones deposited by east and southeast-flowing streams.