Monument No. 1 channel (Channel No. 36), Mystery Valley, Mineral Mining District, Navajo County, Arizona, USAi

Regional Level Types
Monument No. 1 channel (Channel No. 36)	Channel
Mystery Valley	Basin
Mineral Mining District	Mining District
Navajo County	County
Arizona	State
USA	Country

In 1954 the Monument No. 1 channel was the scene of intensive mining activity, primarily as the result of a discovery of a new ore body in channel fill formerly thought to be barren. Before that, in the period 1942-50, the major producer was the Monument No. 1 mine, operated by the Vanadium Corporation of America. This mine was in basal strata at the east end of a large channel remnant. The ore deposit pinched out and in 1950 mining was discontinued and the adits were caved as a safety measure. From then until 1953 the area lay unclaimed, although some work was done in another small mine about a quarter of a mile distant. This mine, known as the Monument No. 1 Annex, is in a weakly mineralized mass of the Shinarump member of the Chinle formation about 150 feet long and 50 feet wide. During 1952, several Navajo Indians reprospected the area of the Monument No. 1 mine. Production records do not exist and it is assumed that no ore was produced. In 1953, a new mine, the Mitten No. 2, owned by the Foutz Mining Co. (now the Industrial Uranium Co.) was opened in the flank of the western part of the channel remnant. The mine was in weakly mineralized ground and produced less than 100 tons of ore. In early 1954; however, a new ore body was discovered in this part of the channel fill as a result of a drilling program by the U.S. Atomic Energy Commission, and as of January 1955, it was this ore body that was being mined through the new Monmnent No. 1-Mitten No. 2 mine workings ( Witkind, 1961).

The center of the Monument No. 1 area is at latitude 36°57'24" N. and longitude 110°14' W. The area is on a prominent ridge west of the Kayenta-Mexican Hat road. The mines are reached by an ungraded trail that leads Northwest from the Kayenta-Mexican Hat road and ascends to the mine portals by a series of switchbacks.

The ridge is capped by remnants of the Shinarump member of the Chinle formation that stand about 30 feet above the general ground surface formed on the Moenkopi formation (fig. 22). Scattered across this Moenkopi surface are deposits of unconsolidated eolian sand as much as 10 feet thick.

Most of the Shinarump member has been eroded from the area near the Monument No. 1 mine and it is only to the west, near Oljeto Creek, that the Shinarump member is preserved. There, however, it is concealed beneath a dune sand and alluvial cover that may be as much as 80 feet thick, although it likely averages 20 feet. The Shinarump member forms the gently dipping east flank of the asymmetrical Oljeto syncline and the dip averages 3° to
the southwest.

Remnants of the Shinarump member represent part of a former widespread sheet of conglomeratic sandstone. Dissection, however, has been so extensive that the uppermost beds of Shinarump member as well as part of the subjacent strata of Moenkopi age have been largely removed. The result is that strata of the Monument No. 1 channel now appear as two ridges whose alignment is to the north west. The two ridges, however, do not everywhere reflect the true width of the channel, for locally, part of the channel strata have been eroded. When these remnants are viewed in cross section it is apparent that in places only the east flank of the channel fill is preserved. The channel is estimated, by extrapolation from the preserved channel remnants, to have been about 280 feet wide and to have been cut about 50 feet into the Moenkopi formation. The channel curves to the northwest.

The two channel remnants, of the Shinarump member form topographic highs. The channel curves to the northwest. It is uncertain if the Monument No. 1 Annex is in a part of the same channel. The southern channel remnants, which contain the mine workings, trend about N10W, and the northern remnant trends about N55W. Sedimentary rocks in the Monument No. 1 channel have the same appearance as strata of the Shinarump member found elsewhere in the Monument Valley area, Arizona. The basal conglomeratic sandstone grades vertically into a massive sandstone in the uppermost beds. Conglomerate lenses are scattered throughout the channel fill. They retain their identity only for short distances, grading laterally into massive sandstone. In a few places, small scours, filled with conglomeratic sandstone, have been cut into the massive sandstone that forms the uppermost channel fill.

Three lithologic units can be differentiated in the basal channel fill: (1) trash-pocket conglomerate, (2) calcite-cemented sandstone, and (3) silica-cemented sandstone. The trash-pocket conglomerate consists of well-rounded pebbles of quartz, chert, and quartzite, as well as angular claystone fragments and fossil plant matter, all in a matrix of coarse-grained sandstone. The calcite-cemented sandstone is a light-tan hard massive crossbedded unit well cemented by calcium carbonate. These calcite-cemented sandstone lenses are composed of well-rounded fine to coarse grains of quartz, chert, and quartzite in an exceedingly tight carbonate cement. The quartz grains lack authigenic overgrowths in marked contrast to those grains in the silica-cemented sandstone. The third lithologic type, the silica-cemented sandstone, is composed of rounded to angular coarse grains of quartz, chert, and quartzite, all enclosed in a loosely knit matrix of chalcedonic cement. A characteristic feature is authigenic overgrowths on the quartz grains.

Inclusions in the basal channel fill consists of silicified wood, ranging from small fragments to logs more than 2 feet in diameter and 10 feet in length, as well as large amounts of carbonaceous matter and clay pebbles. Also included are angular fragments of light-gray siltstone interpreted as having been derived from the Moenkopi formation. Measurements of the trend of 15 silicified logs in the channel fill indicate a range from N10W to N85W, although most of the logs are aligned co-linear with the N10W strike of the channel.

The upper surface of the channel fill is irregular and is marked by depressions as much as 20 feet in diameter and 4 feet in depth, and by rounded hummocks which rise about 20 feet above the general surface.

Near the southernmost exposure of channel strata, a concentration of silicified logs on the surface is surrounded by very dark limonite-colored rocks. This limonitic coloration is not confined to one locality, for elsewhere on the surface, smaller limonite-colored areas are exposed. Unweathered exposures of channel fill lack the limonite coloration.

The channel is underlain by an altered zone in the uppermost strata of the Moenkopi formation that is about 2 feet thick along the channel flanks and increases in thickness to almost 5 feet below the channel. In the uppermost part of the altered zone beneath the channel, minute quantities of secondary copper minerals, such as azurite, malachite, and chrysocolla, are along the bedding planes and fill small fractures. The possible significance of the altered zone has been investigated (Alice D. Weeks, written communication, 1952). Specimens of both the red unaltered and the gray altered Moenkopi formation were chemically analyzed. Mrs. Weeks reports:

At Monument No. 1 mine, both red and gray clay contain quartz, hydromica, chlorite, and kaolinite. Chemical determinations of total iron, ferric and ferrous iron, titanium dioxide, and vanadium pentoxide, made by R. G. Milkey, showed that in all suites of samples total iron and ferric iron are higher in the red than in the adjacent gray sample. Although the ferrous-ferric ratio is higher in all the gray samples than in the adjacent red clay, the ferrous iron does not vary significantly between the red and gray of each set. To alter the red clay to gray, only l percent more or less of ferric iron pigment would have to be leached from the red. Hematite is too small in quantity or too fine grained to show in X-ray patterns of natural red clays.

Mrs. Weeks sought but found no evidence for or against a relationship of the altered zone to ore-mineralizing solutions.

Strata in the Monument No. 1 Annex are similar to those filling the Monument No. 1 channel except that light-gray oval clay pebbles, about half an inch long, are in a matrix of coarse-grained sandstone. The clay pebbles are so aligned as to give an impression of rudimentary horizontal bedding. Yellow uranium minerals are disseminated in the interstices of the sandstone near the pebbles, but they are absent elsewhere. A banded appearance results: yellow mineralized bands about 1 inch wide alternate with white barren bands, also about 1 inch wide. Close examination near some of the clay pebbles indicates that in some, yellow uranium minerals impregnate the sandstone to the very edges of the pebbles. In others, a halo about a quarter of an inch thick devoid of uranium minerals surrounds them.

Trash pockets of fossil plant matter are common in the rocks of the Monument No. 1 Annex. These pockets appear in cross section as irregular thin strips of black coaly substance (vitrain ?). In plan view, these pockets show as impressions of reed-like plant material in the sandstone.

Two ore bodies have been discovered in the basal channel fill of the south channel remnant. They were separated from one another by barren strata. Little is known about the size, shape, and distribution of the ore body mined by the Vanadium Corporation of America at the former Monument No.1 mine. The second ore body, mined through the Monument No. 1-Mitten No. 2 mine portals, was near the north end of the south channel remnant. It was about 675 feet long and about 75 feet wide, although in places it was as wide as 120 feet. It ranged in thickness from 1 foot to as much as 18 feet, but averaged 7 feet. This ore body trended N30W, and was collinear with the channel trend. In both longitudinal and cross section the ore body appeared planoconvex or biconvex, with its base commonly conforming to the channel floor. In places, both the trash-pocket conglomerate and the silica-cemented sandstone contained ore and formed ore bodies. Barren calcite-cemented sandstone lenses commonly were intercalated in the ore body. The ore was brillant blue black, principally due to the widespread distribution of the vanadium mineral corvusite. Scattered irregularly through the ore body were specks of yellow, green, and blue, representing secondary uranium (tyuyamunite), copper-vanadium (volborthite), and copper (azurite, malachite) minerals.

Copper minerals were common in the southern part of the Monument No. 1 mine workings, but were not found in the Mitten No. 2 mine workings. Semi-quantitative spectrographic analyses of the basal channel fill, however, indicated that copper was widespread.