Reconstruction of Middle Tertiary Extension and Laramide Porphyry Copper Systems, East-Central Arizona

Abstract

In east-central Arizona, overlapping sets of Tertiary normal faults dismembered, variably extended, and exposed up to 15 km of the upper crust including portions of several Late Cretaceous to Paleocene (Laramide) igneous centers and their associated porphyry copper systems. These exposures enable both a rigorous evaluation of the nature of extension in the upper crust and systematic reconstruction of the 3-dimensional distribution of several major porphyry copper centers.Synthesis of existing geological data and new mapping provide the basis for reconstructions of district and regional scale cross sections through an area comprising about 4,000 km2 centered on the Dripping Spring Mountains of east-central Arizona. The study area is located within a highly extended portion of the Basin and Range province and encompasses the Globe-Miami, Superior, and Mineral Creek (Ray) mining districts and numerous other deposits and related occurrences.The field evidence and the reconstructions demonstrate that sequential sets of initially steeply dipping normal faults generated multiple half-grabens and associated sedimentary fill. Complex overlap of >10 sets of these half-grabens led to aggregate extension of about 100% across the study area, but the amount of extension locally varies from less than 20% to well over 400% depending on the amount of overlap and direction and amount of displacement on the various fault sets. These fault sets were not kinematically linked and do not merge into a master fault at depth, but are inferred to feather into a broader zone of mid-crustal flow, which may resemble the characteristics of nearby metamorphic core complexes.Reconstructions at regional (10's of kms) to copper deposit (a few kms) scales demonstrate that multiple deposits, prospects, and other hydrothermal features in the Globe-Miami, Superior and Ray districts are dissected portions of originally fewer, larger hydrothermal centers. These restorations delineate exposures ranging from near paleosurface to locally >10 kms paleodepth and enable comparisons of different systems and of well mineralized portions along with their roots, tops, and margins.