TILTING HISTORY OF THE SAN MANUEL-KALAMAZOO PORPHYRY SYSTEM, SOUTHEASTERN ARIZONA

The Laramide San Manuel-Kalamazoo porphyry system of Arizona has been pivotal in concepts of both extensional tectonics and alteration-miner alization zoning. This paper reexamines the tilting history in light o f new work in the region and reinterprets the geometry of the deposit. The porphyry mineralization occurs in and near an intrusion of Larami de San Manuel porphyry in Precambrian Oracle Granite. The area has an extremely complicated history of Tertiary crustal extension and fanglo merate deposition, but the blocks containing the two main fragments of the original orebody (separated by the San Manuel fault) were involve d in only the later parts of this history and are less tilted than oth er nearby blocks. Originally horizontal features of mid-Tertiary age a re tilted about 30 degrees, those of Laramide age about 35 degrees, an d those of pre-laramide age about 45 degrees to the northeast. Pa leom agnetism of the porphyry intrusion itself suggests tilting of about 33 degrees. The data thus suggest that postemplacement tilt of the Laram ide porphyry system was 30 degrees to 35 degrees and that virtually al l of it was mid-Tertiary in age. An earlier interpretation of greater tilt magnitude was apparently based in part on correlation with adjace nt areas showing greater tilt magnitude (as in the Tar Wash allochthon ) and in part on a choice of cross section that gives the impression o f a nearly recumbent attitude. However, Lowell's sections are oriented along the strike of both the orebodies and the productive porphyry in trusion, which are dipping tablo-cylindroidal bodies. Ore forms a holl ow envelope whose inner margin has an aspect ratio of about >4.3 (leng th)/>2.5 (downdip height)/1 (limb separation). The productive intrusio n, 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 t he original attitudes of intrusion and ore yields a common strike near N 75 degrees E and a steep dip to the south. This is a common attitud e for undisturbed productive porphyry dikes of Laramide age.