Page 132 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 131
Basalt.—Quaternary basalt in the form of flows is found as residual patches at various places in the district. The largest of these is on the mesa just north of Hillsboro, where basalt about 25 feet thick rests on gravels and agglomerates of Tertiary age. Another nearby residual patch has about the same thickneE, The top of Black Peak consists of basalt 75 feet thick, and on the flanks of the hills at points 1 mile and 11/2 miles to the west are two other smaller patches. Black Peak is believed to be the source of the basalt flow in this region, as underground workings in the peak have encountered and passed completely around what appears to be the breccia filling in the throat of the old vent.
STRUCTURAL RELATIONS
The south-central part of the Hillsboro district, which con- tains the most valuable lode deposits, consists chiefly of extrusive andesites and latites and intrusive monzonite with related dikes. In a drill hole from the bottom level of the Rattlesnake mine, limestone underlying the extrusives was encountered at a depth from the surface of approximately 1,150 feet, and these extrusive rocks may be underlain by limestone in most of the district. At the southeastern corner of the district a faulted block of Fussel- man limestone strikes slightly west of north and dips to the east. The original position of the topmost bed of Fusselman limestone along the fault scarp was about 250 feet above the collar of the Rattlesnake shaft, so that there is a difference between the two elevations of the top of the limestone of not less than 1,400 feet, a fair measure of the total throw of the fault. West of this fault the Tertiary agglomerates abut against the limestone and against the andesite to the north, so that relations here are obscured. The monzonite mass near Hillsboro has evidently been intruded along the line of faulting at the point where the fault appears to take a sharp bend to the west. The flat remnant of basalt on the top of the hill showing the monzonite exposure and a similar flat remnant on the mesa north of Hillsboro indicate that all the faulting along this zone was not confined to a single fracture plane, and although no evidence of a fault was found in the Ter- tiary agglomerates west of the main fault scarp, it is quite clear that there has been a total vertical movement of approximately 250 feet between these two masses of basalt. South of the Rio Percha beyond the limits of the map of the district, the main fault scarp of the Fusselman limestone can be traced for over half a mile through Tertiary agglomerates and Palomas gravel which here make both walls of the break. The zone of faulting is from 12 to 20 feet in width, and it is marked by a slickensided outcrop of silicified, brecciated and iron-stained material that has resisted erosion and now stands as a bold outcrop above the surrounding gravels.
North of the central area proper, faulting is again evident along the contact of the limestone with the andesite. Little is