Earth and Planetary Sciences ETDs

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The southern part of the Cerrillos mining district is underlain by a sequence of igneous rocks which have been altered and mineralized. The rocks are intrusive equivalents of the Oligocene (?) Espinaso Volcanics. Four stages of intrusion are recognized. A hornblende monzonite porphyry stock was emplaced during stage T1. Several small hornblende­augite monzonite stocks intruded stage T1 rocks during stagesT2 and T3. During stage T4, stocks of augite-biotite monzonite, hornblende-biotite monzonite porphyry, and andesineaugite-biotite monzonite porphyry were intruded into the older igneous rocks. The stage T1, T2, and T3 stocks are flanked by sills and laccoliths.

Three sets of near-vertical fractures are the major structural features. The trends strike northeasterly, north, and northwesterly.

Mining operations in the area probably date from A.D. 950, when Indians quarried Mount Chalchihuitl for turquois. Later, but prior to 1680, the Spanish extracted silver from the Mina del Tiro. More recently there was mining of both turquois and sulfides. The main period of activity was from the 1880's to about 1900. Since then only sporadic attempts at mining have been made. Since 1909 the district has produced about $420,000 worth of gold, silver, copper, lead, and zinc.

Vein deposits occur in several of the fractures trending northeasterly. Galena and sphalerite are the major ore minerals.

There is also one known disseminated mineral deposit in the area. It consists of chalcopyrite veinlets in fractured stage T1 monzonite porphyry. Malachite, azurite, and "copper pitch" replace chalcopyrite near the surface.

Alteration takes the form of propylitization, argillization, and supergene copper mineralization. Propylitization affects only rocks older than stage T4 and is therefore presumed to be the result of pervasive, but relatively weak, hydrothermal activity. Argillization has converted rocks to quartz, kaolinite, and other clays. However, the areas affected are widespread but individually not very large. Argillized areas have both hypogene and supergene minerali­zation associated with them, and their exact origin is in doubt.

Turquois is the most prominent supergene mineral. It occurs, at shallow depths, in nearly all argillized areas. Fractures of all three major structural trends are very evi­dent in turquois deposits, and the turquois usually forms thin, discontinuous veinlets in these and other cracks

Degree Name

Earth and Planetary Sciences

Level of Degree


Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Abraham Rosenzweig

Second Committee Member

J. Paul Fitzsimmons

Third Committee Member

Edgar F. Cruft



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Included in

Geology Commons