ALTERATION FEATURES AT SILVER BELL, ARIZONA

The Silver Bell Range, some 50 miles west of Tucson, Arizona, provides a varying array of igneous and sedimentary rocks in an area of widespread copper mineralization. Two deposits have been explored, Oxide on the south end of the Range and El Tiro 3 miles northwest. Both are essentially disseminated deposits and exhibit a series of hydrothermal metamorphic stages superimposed upon a variety of geologic conditions.

Early mined copper ores were largely formed by contact-metamorphic action along the boundaries between dacite porphyry or quartz monzonite and Paleozoic limestones. The contact deposits in the Mammoth-Union area at Silver Bell and pockets at El Tiro reportedly produced 100,000,000 pounds of copper. Disseminated ores explored in this investigation differ from the contact ores in being largely associated with quartz monzonite and to a minor extent dacite porphyry or dacite.

Major deformation has resulted in faulting of Precambrian alaskite on the southwest against Paleozoic sediments on the northeast. Major post-Paleozoic igneous activity has involved a dacite flow, a dacite porphyry intrusive, and quartz monzonite in sequence. Minor intrusive activity is represented by a series of dikes invading previously formed rocks and frequently following fault lines.

Hydrothermal copper-depositing solutions which followed the quartz monzonite have produced a halo of alteration minerals in alaskite, quartz monzonite, and the two dacites. The effects are observable chiefly in the quartz monzonite, but with equal intensity in limited areas of the other igneous rocks. Convergence in metamorphism with ultimate quartz-sericite mass replacement may be observed in all four rocks.

The alteration minerals are kaolinite, halloysite, montmorillonite, illite (or hydromica), sericite, chlorite, alunite, jarosite, barite, opal, secondary quartz, and adularia. Field criteria confirmed by laboratory study have been used to divide the four igneous rock types into alteration stages. An alteration map of the entire mineralized area shows these stages, and significant portions of the map are reproduced in the paper.

Certain aspects of the alteration at Silver Bell, Arizona, agree with stages described by Sales and Meyer (1948) at Butte, Montana; Levering (1949a) in the East Tintic area, Utah; Peterson et al. (1946) at Castle Dome, Arizona; and Kerr et al. (1950) at Santa Rita, New Mexico. Where the early rock contains ferromagnesian minerals a chloride stage is prevalent. Plagioclase is more susceptible to argillic alteration than orthoclase although both succomb. Kaolinite and illite (or hydromica) are intermediate. Sericite and quartz predominate toward the end of the hydrothermal stage at or near convergence. With the possible exception of East Tintic there also appears to be a striking parallel of the argillic stage with metallic mineralization. Silver Bell and Santa Rita agree closely in this respect.

Metallic mineralization appears to coincide with areas of advanced argillic alteration. This is shown most clearly in the quartz monzonite, the chief ore-bearing rock. Alteration zones followed on the surface aid in outlining areas for drilling and in excluding large unmineralized areas from consideration.