Structucture and age of the Peña Colorada Iron deposit (Colima): a possible fanerozoic equivalent of the IOCG-type deposits

Abstract

The Peña Colorada iron deposit is made up by three different mineralized bodies: (1) an upper massive magnetite body, up to 20 meters thick, sub-concordant with the regional stratification, that contains decimetric to metric fragments of a granatite rock completely replaced by K-feldspar; (2) a lower disseminated magnetite body, also subconcordant with the regional stratification, with a maximum thickness of 150 meters and made up by pyrite–magnetite–pyroxene rhythmic alternances with poiquilitic K-feldspar masses (episienite “sensu lato”); and (3) a mineralized polymictic breccia, with evidences of hydraulic fracturing, with a diatrema-like morphology, cutting the whole stratigraphical serie at Peña Colorada. The latter includes xenoliths of “low-Ti nelsonites” (in the sense of Fooses and Grauch), found exclusiverly at the lower breccia outcrops.

In this paper we report the first two K–Ar absolute ages obtained from the main magnetite bodies at Peña Colorada. A fresh K-feldspar from one episienite mass of the lower disseminated body gave an older age of 65.3±1.5 Ma, whereas a K-feldspar sample from the pseudomorfosed granatites gave a younger age of 57.3±2.1 Ma. These ages place the age of this deposit between the upper Cretaceous– Paleocene transit and the middle Paleocene. As the dated bodies represent the second and third genetic events (of five) in the origin of the Peña Colorada deposit, it is possible to assert that the mechanisms that gave origin to Peña Colorada acted, at least, during a 4.4 Ma period.

The disposition of the different mineralized bodies, their textural characteristis and the difference in ages among them suggest that the Peña Colorada iron deposit formed due to recurrent mineralizing events in a discrete cortical volume and within a relatively wide time lapse (>4 Ma). All these facts are against a skarn origin caused by the intrusion of a magmatic rock. Morevoer, we propose that the Peña Colorada deposit has strong affinities with the IOCG (Iron–Oxide–Copper–Gold deposits of Phanerozoic age, similarly to the Fe deposit of Cerro de Mercado in Durango.