FLUID SOURCES FOR IOCG (CANDELARIA, PUNTA DEL COBRE) AND PORPHYRY CU-STYLE MINERALIZATION, COPIAPO BATHOLITH, CHILE: GEOLOGIC AND SR ISOTOPIC CONSTRAINTS

Understanding the genesis of Fe-oxide(-Cu-Au) = IOCG deposits and their analogy, if any, to porphyry Cu systems remains unsettled. Most workers advocate a required role for magma-sourced fluids, whereas some advocate a major role for external brines (Barton & Johnson, 1996, Geology).

The composite, Early Cretaceous Copiapo batholith and its aureole host many IOCG deposits including those of Candelaria and the Punta del Cobre district. Regionally, IOCG and porphyry-type systems occur widely with the broadly coeval Chilean Coastal Batholith. Building on earlier work, new geologic mapping (>125 km²) in the Copiapo area has focused on the time-space development of hydrothermal alteration and its relationship to the complex igneous history. Spatial distributions and crosscutting relationships demonstrate that each phase of the batholith generated its own hydrothermal system with voluminous sodic-calcic (NaCa) associations and variable, but widely recognized K-silicate suites. Both types systematically vary in intensity and mineralogy with the composition of generative intrusions and with host rocks. NaCa is most intense with dioritic rocks and zones inward from sources beyond intrusive margins. Consistent with petrologic predictions, intrusion-hosted K-silicate styles vary: one group relates to upflow (cooling) of NaCa fluids; the rarer, porphyry-like suite relates to cooling of magmatic fluids from the more felsic plutons.

Most Copiapo isotopic results fail to discriminate clearly among material sources; however, new Sr isotope data on altered rocks combined with published data on fresh rocks and nearby sediment-hosted deposits show that Sr in IOCG mineralization, NaCa alteration, and IOCG-related K-alteration is 30 to >90% non-igneous in origin thus requiring major influx of brines from outside the batholith. Although our results imply that Copiapo batholith IOCG ores were dominated by non-magmatic fluids, the systems are complex and the source and concentration mechanism of metals remain to be firmly established. As in other areas (e.g., Yerington, NV), detailed mapping combined with careful geochemistry is required to constrain the evolution and roles of magmatic and non-magmatic components in igneous-related hydrothermal systems. We thank Phelps Dodge for support and permission to publish.