The Relationship of Laramide Stocks to Regional Structure in Central Arizona

Abstract

The area of this study is a northeast trending zone which is approximately ninety miles long, extending from Casa Grande to Globe, Arizona. A number of porphyry copper deposits and barren Laramide stocks occur within this zone. Geological mapping of the various Laramide granitic stocks shows that all of the stocks are composite, epizonal granitic stocks, which are all elongate in a N45°E to N80°E direction. Sulfide bearing veins also trend N45°E to N80°E. The configuration of the stocks defines the structural framework into which they were intruded. The consistent elongation of the granitic stocks shows that the stocks were intruded into a series of parallel, regional faults that trend approximately N60°E to N80°E. Isotopic dating of the stocks shows that they become progressively younger from west to east. Typical isotopic ages are 71 m.y. on the west portion of the zone, and 61 m.y. on the east portion. The porphyry copper deposits also show a trend of decreasing age from west to east. The granitic Laramide stocks are localized where the N45°E to N80°E regional faults intersect the Precambrian granite-Pinal Schist contact. In some cases the regional faults are parallel to the Precambrian granite-Pinal Schist contact, in which case the Laramide stocks are parallel to the contact. The Laramide quartz diorite stocks only occur along the "San Pedro hinge line" and farther to the east. Although the individual quartz diorite stocks are aligned in an east-northeast direction, the overall geographical trend of the quartz diorite is north-northwest. The quartz diorite stocks may also become progressively younger proceeding to the east. The defining of the regional faults or lineaments, based upon the orientation of the Laramide granitic stocks, permits a rather precise projection of these lineaments into areas of complete post-mineral rocks. This allows a large area of post-mineral rocks to be reduced to a rather small area, where exploration for porphyry copper deposits can be intensified.