Abstract

The Laramide San Manuel-Kalamazoo porphyry system of Arizona has been pivotal in concepts of both extensional tectonics and alteration-mineralization zoning. This paper reexamines the tilting history in light of new work in the region and reinterprets the geometry of the deposit. The porphyry mineralization occurs in and near an intrusion of Laramide San Manuel porphyry in Precambrian Oracle Granite. The area has an extremely complicated history of Tertiary crustal extension and fanglomerate deposition, but the blocks containing the two main fragments of the original orebody (separated by the San Manuel fault) were involved in only the later parts of this history and are less tilted than other nearby blocks. Originally horizontal features of mid-Tertiary age are tilted about 30 degrees , those of Laramide age about 35 degrees , and those of pre-Laramide age about 45 degrees to the northeast. Paleomagnetism of the porphyry intrusion itself suggests tilting of about 33 degrees . The data thus suggest that postemplacement tilt of the Laramide porphyry system was 30 degrees to 35 degrees and that virtually all of it was mid-Tertiary in age. An earlier interpretation of greater tilt magnitude was apparently based in part on correlation with adjacent areas showing greater tilt magnitude (as in the Tar Wash allochthon) and in part on a choice of cross section that gives the impression of a nearly recumbent attitude. However, Lowell's sections are oriented along the strike of both the orebodies and the productive porphyry intrusion, which are dipping tablo-cylindroidal bodies. Ore forms a hollow envelope whose inner margin has an aspect ratio of about >4.3 (length)/>2.5 (downdip height)/l (limb separation). The productive intrusion, around which ore is wrapped, has a similar aspect ratio in the same plane (strike N 58 degrees E, dip 47 degrees SE). Reconstruction of the original attitudes of intrusion and ore yields a common strike near N 75' E and a steep dip to the south. This is a common attitude for undisturbed productive porphyry dikes of Laramide age.

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