Page 178 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 175
earlier stages, with rhyolite and rhyolite tuffs following. Latite and latite porphyry occur in the lower andesite as dikes and small sills and as flows of limited extent on the andesite surface. To the southeast of the district near the Lake Valley railroad station the andesite laps gently over the dipping sediments. A fault has caused a slight displacement here, and a small intrusion that looks like fine-grained monzonite in hand specimens is visible in the andesite flows nearby. Northwest of the faulted block of sediments, andesite flows are in contact with Fusselman lime- stone for over 3 miles along the escarpment. On the southwest side of the sedimentary fault block rhyolite is in contact with the sediments, and to the northwest the higher parts of the Rhyolite Range are uniformly capped with rhyolite flows and tuffs. At the base of the rhyolite flows is a glassy member 3 to 10 feet thick, which is an excellent horizon marker as far north as the Kingston district, A few dikes and' irregular masses of latite or monzonite were noted in this outlying area.
STRUCTURAL RELATIONS
As seen on the map, Plate X, and on the various drawings prepared by Clark 54 and adapted by Gordon, 55 the Lake Valley district consists essentially of a tilted fault block of Ordovician to Mississippian sediments projecting through and entirely sur- rounded by volcanic flows. The sedimentary fault block within which the known ore deposits of the Lake Valley district occur is elongated in a northeasterly direction, corresponding to the strike of the beds. The strike of these beds is in general N. 45° E., and the dip 15° to 20° SE. Along the northwest escarpment of this block, the El Paso (Ordovician) limestone is in contact with Tertiary andesite. The minimum throw of this fault cannot be less than 625 feet. On the dip slope the topmost beds of the block disappear at an angle of about 20° under andesite flows. Some faulting also has occurred along this border of the block, although it is minor in amount, as is shown in two shallow shafts that begin in andesite and enter the underlying sediments at elevations that would indicate a displacement of 50 feet or less. At the southwest end of the fault block the beds are sharply terminated by a fault trending N. 10° W., along which the rhyo- lite of Porphyry Hill has been dropped relatively until it is in contact with the sediments. This fault marks the southwest termination of the known ore in the district.
Faulting in the region commenced at the time of the earliest extrusions or even before, and it continued during the whole of Tertiary time, due largely to the weight of the lava flows and the movement of the intrusive magmas. Following the period of andesite extrusions came the period during which the monzonites and latites of the region were intruded. The field relationships
54Clark, Ellis, op. cit., pp. 138-167.
55Gordon, C. H., op. cit. (U. S. G. S. Prof. Paper 68), pp. 278-281.