Page 101 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

100 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
masses of porphyry, shrinkage and settling cracks developed, and along these fractures came, first, the mineralizing solutions of the district, and later, as a last stage in the igneous activity, the residual portions of the still-molten body of magma welled up to form the rhyolite dikes of the region, and the extensive rhyo- lite flows, tuffs and breccias.
Locally, the relationship between the folding of the stratified rocks, the fracturing of these beds, and the intrusion of the por- phyritic mass can be seen in the cross canyons, such as the mid- dle fork of Percha Oreek passing through Kingston and in the gulch to the north near the Lady Franklin mine.
ORE DEPOSITS
In the Kingston district the ore occurs in pipes along steeply inclined fracture planes and in beds and pockets in the Fussel- man and Montoya limestones below the Percha shale. The ore solutions moved upward along the margins of the igneous intru- sions and along related fractures in the limestone farther away from the contacts, and in a large measure they deposited the metals in the tops of the arches that were formed before the pe- riod of fracturing and intrusion. The principal deposits are located in a narrow belt on the west side of the main mass of monzonite porphyry. Here the limestone is rather sharply arched in such a manner that ore-bearing solutions traveling up- ward along the west side of the monzonite were diverted by im- pervious beds of Percha shale and rose along the underlying beds of eastward-dipping limestone, until they reached the crest of the arch, where the minerals were deposited in bedded or blanket form and in irregular pockets. On the east side of the monzon- ite, where the sedimentary beds dip to the east, no important de- posits of ore are known. The west limb of the arch just described is cut off by a northward-trending fault, which apparently dips steeply to the east at an angle that would cause it to intercept the monzonite mass at a moderate depth below the deepest present workings in the district. Some ore is known in this fault and in the adjacent beds under the shale, but it occurs in minor amounts compared with the ore that formed at the crest of the arch and that came up next to the monzonite porphyry intrusion. The ore occurs in bunches, spaced at somewhat regular intervals of about 50 feet, according to local report, along fractures that strike N. 40° W. and are intersected by another series of minor northeast fractures which usually carry no ore. The largest ore bodies oc- cur where fractures of these two series intersect. Narrow string- ers of sulfide ore usually fill the northwest fractures, and by means of these leads, prospecting from one ore body to the next is accomplished. The ore bodies are connected horizontally un- der the Percha shale along the northwest fractures, the relation- ship being that of beads spaced at wide intervals along a thread, and they also extend downwards with rapidly diminishing cross