Page 76 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 75
son deposits of Palomas gravel by the later stream action of Cuchillo Creek and its tributaries.
INTRUSIVE ROCKS
Intrusive rocks are not definitely known to be present in the Chloride region. As explained in the section on the Sierra Cuchillo (see page 116), it is thought that the sill of monzonite, which is a prominent feature of that range, continues at least part way under the sediments and flows of the Chloride district, and that it was a part of the Cuchillo mass prior to the faulting that elevated the Sierra Cuchillo to its present elevation. The type of ore deposits in the area tends to bear out this opinion. It is not common to find large or continuous ore bodies that have emanated from a sill, and dikes are rare as offshoots from such intrusions. On the other hand, where some of the veins cut across limestone underlying the andesite, as at the Midnight mine, the limestone walls of the vein show evidence of contact-metamorphic action with the development of such minerals as garnet and epi- dote. These features suggest that the veins emanated from a mass of hot igneous rock at no great depth.
STRUCTURAL RELATIONS
Faulting has sliced the eastern flank of the Black Range arch or dome into a series of long, narrow blocks, which have in general been lowered with respect to one another from west to east. Cross faulting has divided these narrow strips into blocks of small and unequal dimensions, which have been elevated, low- ered and tilted in irregular fashion. The eastward-flowing streams of the area are along those cross breaks for the most part, and they have cut steep gorges; which open out here and there into parks, where the cross break intersects one of the ma- jor north-south faults. Near the heads of the streams the flow rocks are thick, and the top of the range has no visible outcrops of sedimentary rocks. Where the faulting has been pronounced, however, erosion has also been deep, and here the various faulted blocks show a base of limestone or Permian "Red Beds," with or without a residual capping of andesite or rhyolite. In places still farther east, blocks have sunk to such an extent that the andesite or rhyolite lies at the level of the valley floor, adjacent to the overlapping Palomas gravels.
ORE DEPOSITS
The chief ore deposits of this area are fissure veins in the
andesite. They are filled-fissure veins, with or without replace- ment of the wallrock. In the rhyolite is found some ore that is believed to be of later age than that in the andesite. Where the veins in andesite pass into limestone in depth, contact-metamor- phic minerals are developed for a short distance into the lime- stone walls, and the ore is usually higher in copper and lower in gold and silver than the average for the district. There are two vein systems, one which trends north-south and the other