STRATIGRAPHIC, PETROGRAPHIC, AND GEOCHEMICAL STUDY OF INTERBEDDED GYPSUM AND CU-ORE DEPOSITS IN THE BOLEO FORMATION IN BAJA CALIFORNIA SUR, MEXICO

Economically important copper and cobalt ore deposits are found in sedimentary layers of the Santa Rosalia Basin along the central Baja Peninsula of Mexico. Wilson and Rocha (1955) defined the stratigraphy of the Santa Rosalia Basin with a basal limestone overlain by basal gypsum followed by five major sedimentary cycles that host copper mineralization in ore beds or sheets called "mantos." Most of the gypsum in the Boleo Formation is uplifted and exposed along the coast of the Gulf of California and on San Marcos Island. In 2022, exploration borehole surveys were conducted to determine the amount of reserves and were used to make Core DDB10. The stratigraphic data of Core DDB10 establishes that the gypsum is deposited at the same time as the copper mineralization and provides a unique opportunity to investigate the relationship between the brine that deposited the gypsum and the ore fluid. Sulfur isotope analysis will be used to distinguish the salinity of the water and determine the amount of seawater versus the freshwater runoff that reached the basin. Elemental and petrographic analysis will determine composition and texture changes within the gypsum. Textural changes in the gypsum should constrain the temperature of the ore fluid.