IMPLICATIONS OF NEW U-PB DATES FROM PORPHYRY COPPER-RELATED PLUTONS IN THE SUPERIOR-GLOBE-RAY-CHRISTMAS AREA, ARIZONA

Mid-Tertiary normal faulting affected the Laramide arc and associated porphyry copper deposits in southern Arizona, dismembering and tilting many of the systems and producing an exceptionally large range in depth of exposure of the magmatic-hydrothermal systems. The study area reconstructs to a 50-km long, 10-km deep slice along the arc, with copper deposits related to porphyry plutons of granodioritic to granitic compositions. To provide the necessary temporal constraints and to help correlate units between fault blocks, we have been conducting new geologic mapping and using the laser-ablation ICP-MS U-Pb method to date igneous zircons. These zircons exhibit highly variable degrees and sources of inheritance.

Plutons and porphyry deposits are widely distributed in this zone, with a large cluster of deposits near the Schultze pluton. Plutons range in age from 75-61 Ma, with dioritic rocks at the older end of the range. The main period of mineralization in the Schultze cluster, including the Globe-Miami, Pinto Valley, and Resolution deposits, is dated at ~64-63 Ma, although magmatism ranges from 69-61 Ma. Going to the south in the Tortilla and Dripping Springs Mountains, dioritic bodies are dated at ~75-74 Ma, whereas the porphyry systems extend to younger ages, from ~74 Ma at the Kelvin-Riverside to ~69 Ma at Ray and ~65 Ma at Christmas. Other intrusions modestly expand the age range within any one center.

The new U-Pb results are typically 5 to 10 my older than previous K-Ar dates. The mineralization events occur episodically over a span of 10 to 15 my; thus, it is difficult to call upon far-field triggers for mineralization (e.g., opening of the North Atlantic Ocean). Where geologic constraints exist, mineralizing plutons were emplaced following Laramide contractional deformation (reverse faults, folds) at any given locality. In spite of this relationship and the trend toward more felsic compositions in younger rocks, however, a systematic relationship to igneous composition is lacking, and there is no simple geographic progression in ages along strike of the arc. Thus, a convergence of a series of processes probably is required to explain the origin of porphyry copper deposits in Arizona, rather than a predominant, single tectonic or magmatic process.