Circular 34 — Geology of Disseminated Copper Deposit near Hillsboro, Sierra County, New Mexico

By Frederick J. Kuellmer, 1955, 46 pages. 

A funnel-shaped intrusive mass of quartz monzonite porphyry with an outcrop area of one-half square mile and a copper content of 0.35 weight percent occurs in the Animas Hills near Hillsboro, Sierra County, New Mexico. The intruded rocks are early Tertiary (?)dark-colored andesitic flows, tuffs, and agglomerates with small feldspar and hornblende phenocrysts which, together with the matrix, have been altered variably to aggregates of chlorite, sericite, calcite, and epidote. The quartz monzonite porphyry is light-colored and ranges in grain size from very fine to coarse, with large orthoclase cryptoperthite phenocrysts. Many monzonitic dikes, roughly radial to the main intrusive mass, occur throughout the Animas Hills. A biotite latite dike and basalt dikes are younger than the monzonitic rocks. Thin accumulations of alluvium cover parts of the area.

Jointing and fracturing throughout the area trend predominantly in directions ranging from N. 30° E. to N. 65° E. and from N. 40° W. to N. 65° W. Veins of sulfides and disseminated sulfides are fo~nd in both the porphyry and the intruded rocks. Sulfide minerals appear to be most abundant in quartzose and brecciated areas.

As the shape of the intrusive body is a dominant factor in.an evaluation of the amount of possible ore it may contain, or of the possibilities for ore in older rocks underlying the volcanics, the configuration of the intrusive quartz monzonite porphyry was investigated not oniy by field geologic methods but by five magnetometer profiles. These investigations suggest strongly that the intrusive rock decreases in diameter with depth.

Published descriptions of copper-porphyry-type intrusives show that inward dipping contacts, funnel shapes, or even floors are common. Intrusives of this type. owe their shape to the excess of magmatic pressures over the confining pressures of the host rocks, with approach toward the surface. lateral as well as upward expansion of such hypabyssal magmas appears to be best attributed to their gaseous phase.