Page 126 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 125
ranted. The limestone-monzonite contact is vertical or nearly so, and it may continue without change in attitude to a considerable depth. On the other hand, if the apparently laccolithic mass is nothing more than an enlargement of the top of a sill, the con- tact may flatten out rapidly at any depth below the present work- ings to assume the normal dip observed for this sill at other points. If the contact is vertical or dips steeply to a considerable depth, valuable contact-metamorphic ores may have formed along the contact below the present workings, but if the contact flattens a short distance below these workings, the ore deposition may have been confined to the reentrant angle formed by the top sur- face of the sill and the steep east wall of the protruding lacco- lithic mass, and therefore may be of only small or moderate importance. Because of the absence of contact-metamorphic minerals in the Dictator tunnel in its first 360 feet, the writer be- lieves that the contact dips steeply for at least several hundred feet, and that this contact should be prospected. At a slightly increased depth below the horizon of the Dictator tunnel, if ore is found, it should be much less oxidized and more amenable to concentration than the ore near the surface.
ANIMAS HILLS
HILLSBORO (LAS ANIMAS) MINING DISTRICT
LOCATION AND GEOGRAPHY
The Hillsboro or Las Animas mining district is in the south- western part of Sierra County. It is crossed by the Rio Percha, a small river which flows in a winding easterly course through the southern foothills of the Animas Hills. Hillsboro, the county seat, is on the Rio Percha, just southwest of the district, at an elevation of 5,236 feet. It is connected by excellent highways with Kingston 9 miles to the west, Hot Springs 35 miles to the northeast, and Lake Valley 17 miles to the south. Rail shipments to and from the district are handled through Lake Valley. Plate VI is a detailed map of the lode mining portion of the district, covering an area of approximately 16 square miles, while Plate VII is a general map of the placer mining area, which includes 18 square miles of territory to the east of the lode mining ground.
The Animas Hills form part of an almost unbroken chain of hills that includes the Sierra Cuchillo in the northern part of the county and the Lake Valley Hills in the southern part. The Ani- mas Hills are fairly rugged and are more or less circular in out- line, surrounding a small erosional area known as Copper Flat, which drains to the eastward and is part of the Rio Percha drain- age area. The material eroded from Copper Flat has been laid down as an alluvial fan, which merges with the wide grada- tional plain and river terraces of the Rio Grande valley. (See Plate I.)