Geology and ore deposits of Baja California, Mexico

Abstract

The geologic record in Baja California probably begins with the Upper Jurassic. A series of slates, quartzites, cherts and metavolcanics strongly suggestive of the California Franciscan is exposed along the western coast. The Franciscan (?) rocks were deposited in a synclinal trough that was probably an extension of the Californian Franciscan trough. In the Lower Cretaceous this series was isoclinally folded and resulting land submerged to received a sea that also occupied a narrow geosynclinal trough, the axis of which lay east of the earlier one. Here were laid down shales, sand-stones, conglomerates and interbedded volcanics. Major diastrophism abruptly ended this period of sedimentation in the Mid-Cretaceous; Cretaceous and older beds were intensely folded along north-northwest axes; the Baja California composite batholith invaded the folded rocks, and intrusion was followed by formation of tungsten, iron, copper, manganese and gold deposits.This erogenic cycle closely resembles the Nevadan cycle in California except in one particular: the Sierra Nevada batholith carries a major gold belt along its western border, but only unimportant, scattered gold deposits accompany the Baja California batholith.In Upper Cretaceous and early Tertiary time the west flanks of the peninsula were recurrently submerged. In the Upper Miocene an elongated uplift occupied the site of the present gulf, and a thick series of volcanics poured out from fissures along its crest. Uplift continued in the Pliocene, but the whaleback was broken by a zone of great faults along its western crest, corresponding to the east coast of the present peninsula. The crestal portion of the uplift dropped as a half-graben, the free side swinging down like a trapdoor along the major faults, to form the present Gulf of California.Evolution of the Gulf with its western frame, the peninsula of Baja California, bears striking resemblance to that of the Great Basin and its western frame, the Sierra Nevada.Probably at some time during the Tertiary the silver-gold-base metal deposits of El Triunfo formed along a thrust fault. In the Pliocene, probably just before the major faulting along the eastern border of the peninsula, hypogene copper and manganese solutions rose along fissures which, with the later major faults, were probably localized by the steep eastern wall of the Baja California batholith. In places the fissures broke through the floor of the shallow sea and deposited copper and manganese syngenetically with tuff falling into the sea by explosive volcanism. The great Boleo copper deposits were formed in that manner, but important manganese ore bodies are epigenetic, deposited by hydrothermal solutions in preexisting rocks.