The Punta del Cobre Formation, Punta del Cobre - Candelaria area, northernChile

The Punta del Cobre Formation hosts the iron oxide-rich Cu-Au(-Zn-Ag) depos its of the Punta del Cobre belt, Chile. Hydrothermal alteration associated with mineralization and/or contact metamorphism caused by the Coastal Batho lith exposed in the west resulted in new mineral assemblages and significan t changes in the geochemical composition of its rocks. This paper presents a redefinition of the Punta del Cobre Formation and discusses its geologic context. The Punta del Cobre Formation is subdivided, from bottom to top, i nto: (1) the 'Geraldo-Negro Member', composed mainly of massive andesitic v olcanic rocks (>300 m) and locally dacitic rocks (up to 200 m) that overlie the latter, and (2) the 'Algarrobos Member' (up to >800 m) comprising main ly volcaniclastic rocks, basalt andesitic and basaltic flows, and volcanic breccias. The Punta del Cobre Formation is overlain by the Chanarcillo Grou p, the lowest part of which is represented by alternating carbonate and vol caniclastic beds of the Abundancia Formation, which pass vertically and lat erally into limestones of the Nantoco Formation. The contact between the Pu nta del Cobre Formation and the overlying Abundancia Formation is transitio nal and defined by the first continuous bed of massive limestone or its met amorphosed equivalent. A pre-upper Valanginian age for the Punta del Cobre Formation is suggested by ammonites of Berriasian age found in the upper pa rt of the Algarrobos Member and Late Valanginian-Early Hauterivian fauna in the overlying Abundancia Formation. Despite the intense alteration, immobi le element compositions of the volcanic rocks of the Punta del Cobre Format ion enable the recognition of a calc-alkaline suite with a trend to more pr imitive, less differentiated magmas towards the top of the stratigraphic se quence, reflecting initial stages of basin development. The transgressive n ature of the sedimentary part of the Punta del Cobre Formation marks the on set of increased subsidence that eventually culminated in the Early Cretace ous marine backarc basin south of Copiapo (Chanarcillo Group). The present investigation shows that the Punta del Cobre Formation could be considered as the lower part of the Bandurrias Group, both having similar lithologies and geochemical affinities and occupying similar paleogeographic positions. (C) 2001 Elsevier Science Ltd. All rights reserved.