Geological, sedimentological and geochemical studies of the Boleo copper-cobalt-zinc deposit, Santa Rosalia, Baja California, Mexico

Abstract

The Santa Rosalia basin is a NW-SE elongated and fault limited basin, which evolved from Upper Miocene to Pleistocene time. The transtensional Upper Miocene Santa Rosalia basin located in central Baja California, consists of almost 500 m of nonmarine to marine sedimentary rocks, with tuffaceous beds interbedded in its lower part. The sedimentary rocks consist of the Boleo Formation, which is 200 to 300 m thick and is dominated in its lower part by marine transgressive sediments and evaporite bodies followed by 170 to 300 m of clastic coarsening upward fan-delta marine-nonmarine sequence. The clastic sequence presents at least three well organized upward coarsing cycles (90-100 m thick each one). Each cycle represents a prograding fan-delta formed probably as a consequence of large and repeated vertical movements of the basin floor with respect to source areas, during the early stage of the opening of Gulf of California. Coeval with the deposition of the clastic sequence of the Boleo Formation there was extensive ash volcanism which originated from a volcanic center north of the basin. Copper-cobalt-zinc stratiform ore bodies (mantos) are hosted within the tuffaceous fine facies in the bottom of each sedimentary cycle. At least five mantos were recognized named as 4, 3, 3(1), 2 and 1. Ore minerals and their textures, the geometry of the mantos and elemental zonation, suggest a diagenetic origin for the different ore bodies. The lowest manto, 4, in the district seems to have a geological correspondence with the Lucifer manganese deposit north of the district associated with hot spring mineralization. Manto 3, 3(1), 2 and others all of which occur at a higher stratigraphic level than manto 4, are associated with mineralized fluids sourced from NW-SE faults. Fluids moving through the conduits crossed an oxidation-reduction boundary with the consequent precipitation of sulfide under reducing conditions. Sulfur and carbon-oxygen isotopes studies support anoxic conditions and a bacteriological origin for the sulfur and a mixture of seawater and freshwater during the formation of the mantos. Cu/Co, Cu/Zn and Co/Zn ratios and Cu-Co and Zn absolute values in the mantos support a horizontal and vertical zoning produced by low temperature up-ward moving solutions. The spatial and temporal correlation of the Boleo deposit with the opening of the Gulf of California indicate that the rifting setting might be the source of the metals and the same time might have induced the migration of the ore fluids.