Page 63 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

62 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
tiary and Quaternary detrital accumulations of the eastern foot- hills and the bolson plains.
STRUCTURAL RELATIONS
In broad outline, the main mass of the Black Range takes the form of an anticlinal dome of huge areal extent but with rela- tively slight upwarping and folding of the strata. The major axis of this dome is apparently about at the crest of the range and has approximately the same trend. Along the eastern bor- der of the dome, faulting took place, most of which appears to be of the normal type, and this limb was broken into many fault blocks. The general pattern of the faulting consists of a series of northerly trending breaks along which the flank of the dome is stepped down as one travels eastward, until finally the last of the blocks disappear beneath the bolson deposits and alluvial fans. Intersecting these main breaks are many transverse faults striking generally east. The high point of the dome appears to coincide with the high topographic point of the range near Hills- boro Peak northwest of Kingston, and along the east 'flank of the dome in this vicinity, faulting has exposed the geologic column to a depth of 600 feet below the top of the pre-Cambrian. To the north and south of Hillsboro Peak the beds dip away from the high point as they do to the east and west, but much more gently. On the western flank of the range less faulting is apparent, as most of this area is covered with rhyolite and rhyolite tuffs and breccias, but in general the flows dip gently to the west.
Figure 6 illustrates the cross section of the fault pattern in Palomas Canyon, where over 1,000 feet of Magdalena (Pennsyl- vanian) sediments may be seen in the steep walls of the canyon. Beneath this cross section is a heavy line, which indicates the original position of the beds along the line C-C' of Plate I, and shows that the faulting occurred in beds that had first been sub- jected to compressive stresses and had been slightly warped before the period of fracturing had occurred. In the deeper parts of the section near the eastern edge of the block, Lake Valley (Mississippian) limestone and Percha (Devonian) shale may be present in the creek beds. This same relationship of gentle folding prior to faulting can be seen in several other parts of the Black Range, though not always so clearly and conclusively as in Palomas Canyon.
ORE DEPOSITS
The ore deposits of the Black Range consist of (a) vein deposits in eruptive rocks and (b) replacement deposits in lime- stone. The vein deposits are principally in andesite, and the ores are gold-silver, gold-silver-copper, copper-silver, and lead- copper-silver ores. Some veins of this type extend into the lime- stone underneath or adjacent to mineralized blocks of andesite. In places along them contact-metamorphic minerals such as gar- net, epidote, calcite, hedenbergite, etc., replace the limestone of