Abstract

The Milpillas district, Sonora, México, is located in the Southwestern North America porphyry copper deposit province. The mineralizing events are associated with the late Cretaceous Cuitaca Granodiorite pluton and Laramide porphyry stocks. The host units are Mesozoic volcanic rocks correlated with the Mesa and Henrietta formations (Valentine, 1936). The Paleozoic sequence, include Bolsa, Abrigo, Escabrosa, and Martin formations, and the Proterozoic Cananea granite.

Field observations demonstrated that the Milpillas district was affected by similar regional tectonic stresses that affected other Southwestern North America porphyry deposits. Structural field mapping and collection of joint sets, veins, bedding; dikes, and faults were examined in the Milpillas and Mariquita projects; María mine; Alisos, Lucy, Toro; and the La Verde prospects. Detail structural analysis suggested the presence of local mechanical features produced by pluton-centered stress conditions in the Milpillas and Mariquita projects.

Six main principal paleo-stress directions were identified:

Stage #	Period	Paleo-stress	Observations
I	Jurassic	s1=NW65ºSE, slightly horizontal	Thrusting
II	Laramide	s1=NE70ºSW, slightly horizontal	Laramide magmatism
III	Mid Tertiary	s3=NE60ºSW, slightly horizontal	Uplift
IV	Late Tertiary	s1 = NW20ºSE, s2 vertical, s3 = NE70ºSW	Rotation of blocks
V	Pliocene	s3 = NE45ºSW	Basin and Range
VI	Pleistocene	s3 = NW75ºSE	Opening Gulf of California

Such stages were used to determine the most important structures favorable for the emplacement of porphyry copper deposits in the district. Analysis of landsat images and field observations indicate that the Cuitaca graben opened since Oligocene time. That event dismembered and concealed the copper deposits in the Milpillas district. The displacement directions of the ore bodies is controlled mainly by a northsouth ± 30º left lateral faulting, and a west north west ± 30º right lateral fault system.