BINGHAM PORPHYRY CU MAGMATISM: A REVIEW, DISCUSSION, AND COMPARISON WITH OTHER SYSTEMS

Unraveling the magmatic history of the Bingham porphyry Cu-Au-Mo system has been greatly aided by examination of the apron of coeval volcanic rocks closely flanking the deposit. Complexities that are readily apparent in the volcanic rocks are often obscured in the intrusive porphyries that cooled more slowly and suffered extensive alteration. Some examples of features present in volcanic rocks but lacking in porphyries include: 1) evidence for magma mixing in all intermediate-composition volcanic rocks, 2) altered olivine, 3) trace amounts of sapphire in some block and ash flows, and 4) less altered biotite. Very shallowly emplaced, narrow dikes or intrusions show some original magmatic characteristics that are not preserved in other intrusive or volcanic rocks. Foremost among these characteristics are barite- and apatite-rich inclusions present in pristine olivine phenocrysts from the most primitive rock type in the district. Equally striking are latite dike margins which are rich in magmatic sulfides. Consequently, how close are we to correctly interpreting magmatic compositions from either extrusive or intrusive counterparts?

Coeval volcanic rocks seen at Bingham are often not present at other deposits. What clues of a more complex magmatic history are sometimes present in intrusive rocks at other porphyry systems? Which of these may be important to the ultimate ore-forming process? Audience participation will be solicited.