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The Pena Colorada mineralized area contains several iron oxide-apatite (IOA) deposits around the arguably richest known iron resource in Mexico. The Lower Cretaceous volcano-sedimentary host rock sequence has been subjected to several episodes of hydrothermal alteration, each accompanied by a distinct episode of faulting and intrusion (calc-alkaline to tholeiitic). Faulting is partly associated with the reactivation of cryptic structural corridors in basement rocks. High-resolution 40Ar/39Ar and apatite fission track (AFT) dating of this deposit and the adjacent Arrayanes prospect reveal the following sequence of events that range from the latest Cretaceous to the earliest Eocene: (1) intrusion of a 67.6 ± 3.5 Ma magnetite-bearing diorite with associated skarn/skarnoid metamorphism that was coeval or predated N–S to NNW–SSE faulting; (2) approximately 63.26 Ma syenite-like potassic alteration with disseminated magnetite, predated by N–S to NNW–SSE faulting and postdated by WNW–ESE faulting; (3) intrusion of 62.0 ± 2.5 Ma diorite and 59.39 ± 0.21 Ma andesite dikes that predate the main mineralization event at 55.72 to 54.84 Ma of large semi-stratabound massive and disseminated bodies; (4) intrusion of a 53.3 ± 3.0 Ma magnetite-bearing gabbro and 53 ± 2 Ma pegmatoid magnetite + fluorapatite veins at the Arrayanes prospect, which overlap the WNW–ESE faulting; (5) 50.70 to 48.18 Ma polymictic magnetite breccia as the last stage of mineralization in the area, predated by E–W faulting; and (6) reactivation of WNW–ESE faults and later NE–SW faults. Therefore, the total age span of the Pena Colorada deposit ranges between approximately 19 and 23 million years. The closeness in age between intrusions and mineralization in the Arrayanes prospect, their confinement between the WNW–ESE and E–W structural domains, and mingling or mixing structures between gabbro and diorite with an associated magmatic magnetite enrichment support the idea of a close genetic link between such intrusives and IOA hydrothermal mineralization. All mineralization events at Pena Colorada are associated with pervasive potassic to propylitic alteration, whereas at Arrayanes they are associated with dominant sodic alteration instead. Alteration features are suggestive of relatively shallow and deep formation of these deposits, respectively . Event 3 thermally reset fluorapatite in fragments of pegmatoid magnetite + fluorapatite + diopside associations (dated at 59 ± 2 Ma, AFT) within the polymictic breccia, which were sampled from a deep orebody (still to be found) and that would be likely associated with event 1 or 2. Consequently, exploration endeavors at depth at Pena Colorada may be considered promising. In this study, we use numerous geological and geochemical proxies to constrain the likeliest genetic model for the Pena Colorada and neighboring deposits: (a) the nearness in time and space between hydrothermal mineralization and magnetite-rich, tholeiitic, relatively oxidized intrusive rocks; (b) the occurrence of key mineral associations (i.e. magnetite + fluorapatite ± diopside veins); (c) the exclusive occurrence of fluorapatite in lieu of other apatites; (d) the composition in key major cations (Ca, Fe, Na, Mn) in fluorapatite; (e) the correlations between Ni/Cr vs. Ti values, between Ti+V vs. Ni/(Cr+Mn) values, between Ti+V vs. Al+Mn values, and Mg contents in magnetite; (f) pyroxene thermometry; (g) log f(O2 ) values calculated from Mn contents in fluorapatite; and (h) normalized REE patterns, and ΣLREE and ΣHREE contents in fluorapatite. These proxies indicate that IOA deposits in the Pena Colorada area have a hydrothermal origin with a strong magmatic influence (magmatic-hydrothermal iron oxide, or MHIO, deposits) that formed under high oxygen fugacities and “moderate” temperatures, and with a high geochemical affinity with IOCG and Kiruna-type deposits or the general IOCG “clan” (for both hydrothermal minerals and associated hypabyssal rocks). Relatively high Ti contents in magnetite, and high Ce and low Eu contents in fluorapatite in these deposits (with respect to typical compositions in IOCG “clan” deposits) are geochemical features still in need of further explanation. The correlation between regional and local structural domains and the geochronologic study in this paper constrain the possible ages of such domains as follows: (1) the N–S to NNW–SSE domain can be bracketed between 67.6 and 63.26 Ma, (2) the WNW–ESE domain between 63.26 and 59.39 Ma, (3) the E–W domain between 54.84 and 50.70 Ma, (4) the WNW–ESE to NW–SE domain is younger than 48.18 Ma, and (5) the NE–SW domain is still active. The structural analysis also shows that the massive orebody at Pena Colorada is partially stratabound but its emplacement was also controlled by low-angle Laramide faults, and that hydrothermal fluids were preferentially driven through volcanosedimentary rocks. The latter characteristic is not only a matter of the stratigraphic distribution of relatively pervasive versus impervious rocks but also of the lateral distribution of such rocks due to N–S strikeslip faults. As additional results of this study, we determined that the conglomerates atop the host volcano-sedimentary sequence that were initially attributed to the Cerro de la Vieja Formation cannot be older than 67.6 Ma, and that the IOA deposits at Pena Colorada would be formed at depths of only a few hundred meters.

Geochronology of Mexican mineral deposits. VII: the Peña Colorada magmatic-hydrothermal iron oxide deposits (IOCG ‘clan’), Colima