AMPHIBOLE LEACHING AS A METAL SOURCE, LUCIFER CU-ZN-CO-MN DISTRICT, BCS, MEXICO

The leaching effects of hydrothermal fluids on hornblende crystals within a sequence of Comondú andesite, basaltic andesite, and trachyandesite flows below the Lucifer manganese mine 12 km WNW of Santa Rosalía, Baja California Sur, Mexico were evaluated by SEM/EDS elemental analysis. The fault concentration along the Arroyo Infierno appears to be the strongest controlling factor on hornblende leaching. Iron and manganese concentrations within magnesiohornblende phenocrysts vary along the traverse, decreasing from an average of 0.78 wt. % Mn and 24.05 wt. % Fe to an average of 0.03 wt. % Mn and 4.19 wt. % Fe from NE to SW towards a concentration of faults and veins below the Lucifer mineralization. In contrast, potassium and silica contents slightly increase from 0.62 wt. % K and 14.76 wt. % Si with degree of alteration to 0.89 wt. % K and 22.76 wt. % Si the SW. These trends reflect interaction between the Comondú volcanics and ore fluids that produced argillization of the glassy to aphanitic, feldspar-rich groundmass into smectites and other secondary phases.

Manganese deposition is thought to have occurred as the mineralizing fluids reached the surface or vented into the near-shore marine environment, precipitating the Mn ore via oxidation. The leaching and migration of Mn and Fe is recorded at the small scale of individual hornblende crystals as well as at a broader (kilometer) scale along the traverse. The network of faults and calcite-Mn oxide veins exposed in Arroyo Infierno are interpreted to be the plumbing system for the Lucifer Mn manto deposits.

The leaching and alteration trends observed at the Lucifer mine are consistent with ore-formation processes proposed for the Boleo mineralization, suggesting that the Cu, Zn, Co, and Mn in the Boleo mantos was also derived by leaching from the abundant Comondu volcanics underlying the Boleo mineralization.