Dave Maher says we can find new porphyry copper systems in southeast Arizona by exploring for the roots of Laramide igneous systems (island arcs) and working our way up the geologic section. Dave presented the results of his recent PhD dissertation (UA, Dec 2007) at the Arizona Geological Society monthly dinner meeting last night.
And from his meeting abstract:
In east-central Arizona, overlapping sets of Tertiary normal faults [have] 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.... The field evidence and the reconstructions demonstrate ... extension of about 100% across the study area, but the amount of extension locally varies from less than 20% to well over 400%....
Now, the fact that many porphyry copper deposits in Arizona (and elsewhere) have been affected by mid-Tertiary extreme extension is not really news. What is news here, is that he has suggested a new way to explore for porphyry copper deposits - exploring by going in the opposite direction than the direction taken in the past. In the past, deposits have been found by looking for and very successfully finding the tops of deposits (hence, the low price of copper for such a long time interval until fairly recently). He suggests that explorationists wanting to find more copper - a good commodity to look for with the price up so high - should look for the roots of systems and work their way back up-section to probably fault-offset, mineralized portions of porphyry systems.