Characteristics of Chaucha Porphyry Copper Deposit, Ecuador

Abstract

The Chaucha porphyry copper deposit, located in the northern Andean Cordillera of Azuay Province, Ecuador, is related to the quartz dioritic Andean batholith intruded in the Paleozoic metamorphics and "Diabase and porphyrite formation" of Cretaceous age. The intrusive complex is constituted of quartzdiorite, granodiorite, tonalite and quartz porphyry. The granodiorite batholith has an oval form in plan elongated along the regional northeast trend with 11 km major and 4 km minor axis. The tonalite stock shows nearly circular outline with 2.7 km diameter, and appears to have intruded in the southern margin of granodiorite. Then quartz porphyry emplaced in the southern part of the tonalite stock.
Weak mineralization occurs anywhere within the tonalite, not in quartz porphyry. Relatively intense primary mineralization is localized in tonalite side on the contact with Paleozoic quartzite and has been enriched forming chalcocite blanket of 800 m in length, 400 m in width and thickness of several tense of meters.
Characteristic alteration of the district, if it could be called alteration, is the extensively distributed recrystallized biotite clot which might relate with quartz porphyry intrusion, however, it does not show any direct relation to the mineralization. Other alteration pattern of the area is controlled by the outline of tonalite stock. The propylitic alteration more or less is ubiquitous in tonalite stock. In some part along tonalite boundary relatively sericitized propylitic rock develops. Main orebody occurs in such comparatively intense sericitized area, though even in the orebody half of the wall rock is still showing incomplete propylitic alteration. Neither potash silicate core, nor usual zonal arrangement of wall rock alteration around orebody is found.
P. E. DAMON determined the age of Chaucha deposit as 9.77 m. y. Minor supergene enrichment may be caused by the relatively recent emplacement.