Deposits associated with ultramafic-mafic complexes in Mexico: the Loma Baya case

Several ultramafic-mafic complexes of assumed ophiolitic affinity that contain ore deposits occur mostly along the Pacific margin of Mexico and the boundaries of the Guerrero composite terrane, which have been described in association with various tectonomagmatic settings. These are (1) roots of volcanic arcs, (2) ophiolites, (3) oceanic basins, (4) continental fragments underlying a magmatic arc, and (5) subduction complexes or rift-transform magmatic suites. However, the petrogenesis of such ultramafic-mafic complexes and the associated ore deposits remains ill defined to date, as their comprehensive study virtually lacks an appropriate geochemical approach. Ultramafic-mafic complexes with known or estimated ages in Mexico are mostly of Paleozoic and Mesozoic ages, and very few of them formed once the crustal dynamics of the Mexican Pacific margin switched from generally extensional to compressional tectonic settings during the Late Cretaceous. A few deposits, aged Late Cretaceous to Paleocene, were interpreted to have formed in association with tectonomagmatic settings different from most deposits in the Guerrero composite terrane. To move forward in the understanding on how these mafic-ultramafic complexes and their associated ore deposits formed, the genesis of the chromitite deposits within the Loma Baya ultramafic-mafic complex (southernmost Guerrero composite terrane) is hereby endeavored. The combination of the geochemical study of both major and trace elements in chromite (including platinum-group elements; PGE), the mineralogy, PGE geochemistry and radiogenic isotopes (i.e., Re-Os) in platinum-group minerals (PGM), provides new assessment on the genesis of this ultramafic-mafic complex. From the obtained results it stems that the Loma Baya complex and the chromitite deposits associated with it are closely related with the formation of a back-arc basin in an active supra-subduction zone during the Lower Cretaceous, and the parental melts resulted from re-melting of an ultramafic protolith of Paleozoic (or older) age. The melts that originated in such back-arc basin were of tholeiitic affinity and produced Al-rich chromitites with a moderate enrichment in PGE, associated with rates of partial melting of a depleted mantle source of at least 20%. Such geological framework is in frank disagreement with previous work, thus highlighting the necessity of further research in other Mexican ultramafic-mafic complexes to better constrain their geodynamic setting of formation. The obtained Re-Os model ages in PGM of the Loma Baya chromitite deposits (ca. 300 and 130 Ma) suggest a possible correlation between these deposits and those in the Californias, in the sense that both areas record similar geological processes (mantle rock melting, re-melting, and chromitite emplacement) within similar tectonomagmatic environments and time frame for their occurrence. Such possibility further highlights the necessity to carry on similar studies in chromitite deposits elsewhere along the Pacific margin in Mexico and southwestern USA, in order to better reconstruct its tectonomagmatic evolution during the Mesozoic.