Bol©o is a 525 Mt sediment-hosted, stratiform Cu-Co-Zn district located along the eastern side of the Baja California Peninsula. Mineralization consists of low-grade (0.71% Cu, 0.06% Co, and 0.71% Zn) sulfides and oxides disseminated through a series of laterally extensive conformable horizons of laminated and brecciated claystone. Formation of such extensive mineralization implies a source(s) that is either highly enriched in metals or is volumetrically significant, or both. Possible metal sources for the Bol©o district and mechanisms for their extraction include: i) low to high temperature leaching of arc volcanic and basement plutonic rocks; and, ii) leaching of red-bed conglomerate by low temperature fluids.
Mass balance calculations were carried out in order to evaluate leaching of arc to early rift volcanic rocks, plutonic basement rocks and intraformational conglomerate. The calculated maximum metal yield from the conglomerate relative to the total metal budget of the entire Bol©o district is 14.3% for Cu, 81.3% for Co and 32.6% for Zn. Mass balance determinations indicate that up to 8.6% of the Cu, 4.1% of the Co and 16.6% of the Zn in the total metal budget could be produced from leaching arc andesite. Lead isotope compositions indicate leaching of the basement plutonic complex, but this source would have yielded only 23.2% of the Cu, 9.3% of the Co and 20.8% of the Zn in the Bol©o district metal budget. Additional metal enrichment of the hydrothermal fluid is postulated to occur by the admixture of magmatic volatiles. A magmatic component is inferred from: i) the inability of leaching to account for the total metal budget; ii) the spatial and temporal proximity of rift-related high-K andesite magmatism that is approximately coeval with the timing of sedimentation of ore mineralization; and, have the appropriately high 'O2 conditions of the high-K andesite melts required to transport large quantities of metals.
For more details about this thesis contact Dr. Andrew Conly