Mesozoic magmatic–hydrothermal iron oxide deposits (IOCG ‘clan’) in Mexico: A review

Abstract

Mesozoic magmatic-hydrothermal iron oxide (MHIO) deposits in Mexico have been identified either (1) in regions close to the paleo-Pacific convergent margin, in Baja California and southwestern-southern Mexico, and (2) in regions several hundreds of kilometers inland from that margin, in Chihuahua and Sonora. The formation of these deposits started during the transition from dominantly oceanic extensional tectonomagmatic environments (Aptian–Albian; see Camprubí, 2013) towards the continental thickening by means of subduction-related magmatic processes into the Cenozoic. Despite their need for further studies, in this paper we suggest that MHIO deposits in the type-1 regions, on the one hand, formed possibly in intra-arc continental settings, similarly to those that occur in the Coastal Andes Cordillera in South America. On the other hand, deposits in the type-2 regions are likely to have formed in back-arc continental settings, which are analogous to those in eastern Mexico during the Cenozoic (collectively included in the Eastern Mexico Alkaline Province). MHIO deposits in Mexico are typically associated with intrusive sequences that include gabbros, diorites and monzonites within plutonic ensembles that are largely granitic, where calc-alkaline- and tholeiitic-affinity intrusions display synchronicity at some degree and complex interactions (magma mixing or mingling). The oldest MHIO deposits known in Mexico are those in the northern half of the Baja California Peninsula (Early Cretaceous), in association with the Peninsular Batholits, and delineating a relatively ephemeral metallogenic epoch. Contrastingly, MHIO deposits in southwestern and southern Mexico started forming during the Late Cretaceous and continued forming during the Cenozoic, in association with the Silicic Large Igneous Province (SLIP) of the Western and Southern Sierras Madres and the related back-arc magmatic processes in the Eastern Mexico Alkaline Province. The age spans for the formation of individual MHIO deposits in southwestern and southern Mexico appear to be wider than those in the Baja California Peninsula, a feature that is consistent with the ages of the associated intrusions but may also explain the fact that MHIO deposits in southwestern-southern Mexico tend to be larger than those in Baja California. The case of the late Cretaceous to Eocene (ca. 71 to 48Ma) multi-stage Peña Colorada deposit in Colima, the largest iron deposit in Mexico, exemplifies a sort of telescoping process, in which (1) stages of mineralization that formed proximal to magmatic sources (skarns and skarnoids) were postdated by (2) distal to source stages (Kiruna-type stages and hydrothermal veins). It may be speculated that such evolution may occur in other MHIO deposits elsewhere that were associated with a steady magmatism or ‘magmatic supply’ unless rapid exhumation ‘quenched’, and ultimately stopped, this evolution. Such might be the case of MHIO deposits in the Baja California Peninsula, where intrusions migrated eastwards between 140 and 80Ma before being rapidly exhumed.The formation of MHIO deposits in regions close to the paleo-Pacific convergent margin started forming once the accretion of oceanic terranes was complete by the Early-Late Cretaceous boundary, and such formation is likely to have been terminated once the continental thickening reached a critical point during the Laramide orogenic pulse. As such pulse showed a north-to-south progression, it resulted in the termination of the formation of ‘coastal’ MHIO deposits earlier in the Baja California Peninsula than in southwestern and southern Mexico (Late Cretaceous and Eocene, respectively).