Paragenesis of the Colorada copper sulphides, Cananea, Mexico

Abstract

The Colorada ore body is located within and along the contacts of a quartz porphyry stock that has been intruded into a series of massive, fine-grained, volcanic rocks, called the Henrietta formation. The ore body lies some seven or eight hundred feet beneath the present erosion surface which apically truncates the irregular stock. Roughly, the ore body has the form of an inverted hollow cone or crucible with a gash or split on its northeast side. The early gangue minerals form an irregular, pegmatitic ring-dike, and the succeeding sulphide mineralization is localized within and along the pegmatite ring. The ore minerals are composed chiefly of copper sulphides, molybdenite, and sulpho-salts of copper, principally luzonite and tennantite. All the minerals are of hypogene origin with the exception of minor quantities of "sooty" chalcocite in the upper levels of the mine. Blue chalcocite and covellite replace an earlier chalcopyrite and bornite intergrowth, the bornite being much more susceptible to replacement than the chalcopyrite. The textures of the chalcocite and covellite are particularly described in this paper, and the opinion is reached that these textures were formed by hypogene replacement, although it is admitted that some unmixing may have taken place. Deposition of chalcocite and covellite was followed by a later sulphide replacement, represented chiefly by luzonite and tennantite. Alunite is rather abundant throughout the ore body, and its close association with luzonite and tennantite is thought to indicate that these sulpho-salts were carried and deposited by solutions of acid character. The presence of both blue and gray chalcocite separated widely in the sequence of minerals leaves little doubt as to the distinction of these varieties at Cananea. In general the paragenesis of the minerals of the Colorada ore body is rather unusual and presents several new features with respect to assemblages and sequence. In structure and origin, the ore body is unique and invites special and intensive study, which has been undertaken by V. D. Perry.

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