Circular 38 — Geology of the Luis Lopez Manganese District, Socorro County, New Mexico

By Alfred T. Miesch, 1956, 31 pages.

The Luis Lopez manganese district is located in the Chupadera Mountains, a north-south trending range which lies a few miles southwest of Socorro, in central New Mexico. The Chupadera Mountains are made up almost wholly of volcanic rocks, ranging in composition from basalt to rhyolite, but consisting chiefly of rhyolite, tuff, and welded tuff of rhyolitic composition.

The core of the range is massive rhyolite, over 1,000 feet thick, covering an area of about 10 square miles. The other volcanic units are restricted mainly to the northern and southern parts of the range. Though similar, the rocks cannot be correlated directly from north to south; accordingly, they are divided into two sequences. The northern sequence is made up chiefly of tuff, rhyolite, and volcanic breccia with interlayered rhyolite and andesite flows. The southern sequence is welded tuff, rhyolite, and andesite. All these units are older than the massive rhyolite, which is intruded by a plug and dikes of rhyolite in the north and by latite dikes in the south.

The Chupadera Mountains are surrounded by fanglomerates of presumed Santa Fe age. These sediments are semi-consolidated to consolidated sand, gravel, and some clay. They are intruded by basalt sills, dikes, and plugs and are capped by basalt flows at many localities.

Semi-quantitative spectrographic analyses of seven volcanic rocks and a complete chemical analysis of the massive rhyolite are presented. Within the Chupadera Mountains, faulting and tilting of the older volcanic rocks resulted from the intrusion of the massive rhyolite. Later faulting, associated with the formation of the Rio Grande trough, is marked on the eastern front of the mountains by a north-south trending en echelon fault system.

Small manganese deposits are present throughout the area, but the major manganese deposits discovered so far occur in breccia zones or steeply dipping fault zones in the massive rhyolite. Manganese oxide minerals, chiefly psilomelane, occur as banded botryoidal vein fillings or as stockworks of veinlets cementing breccia. The ores are considered to be of hydrothermal (epithermal) origin.