Abstract

The intrusives at Santa Rita and near-by Hanover differ widely in the extent of hydrothermal alteration. The Santa Rita intrusive is in many places highly kaolinized, sericitized, and silicified. The Hanover intrusive is to a large degree unaltered.

The study has centered around the hydrothermal alteration of the intrusive and to a lesser degree, alteration in dikes and sills in the vicinity. On the basis of field and laboratory criteria, four stages of alteration in the Santa Rita intrusive have been selected: (1) Areas of negligible alteration; (2) Areas of significant visible changes in the feldspars and ferro-magnesian minerals; (3) Areas of argillic alteration; and (4) Areas of sericitization and silicification. Comparison of ore distribution with alteration indicates that the highest concentration of copper is in general related to Stage 3. Secondary enrichment of chalcocite has contributed to the formation of ore in the argillic area, but some chalcocite may have been originally introduced with the argillic alteration of the Santa Rita intrusive. Comparable copper deposition has not been observed in the Hanover intrusive.

A considerable amount of oxidation occurs in some portions of the deposit, and areas near the surface have been extensively leached, but the change in alteration minerals previously formed appears minor.

Studies at Santa Rita indicate the utility of applying zones of alteration in field mapping, to outline areas of greater and lesser promise from the standpoint of mineralization. Data of considerable aid to the general projection of a drilling program are also developed.

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