REGIONALLY EXTENSIVE BRINE-DOMINATED HYDROTHERMAL ALTERATION AND IOCG SYSTEMS, NORTHERN CHILE

Terrestrial saline hydrothermal systems create widespread yet rarely studied metasomatism in the upper continental crust. New mapping, petrography and geochemistry demonstrate that intense hydrothermal alteration affected well over half of the exposed upper crust along an 80 km transect across the late Mesozoic coastal batholith and related volcanic rocks of northern Chile. Alteration reflects two broad types of hydrothermal systems: (1) widespread circulation of non-magmatic brines driven by compositionally diverse intrusions that led to Na-Ca-K and acid metasomatism, leached metals in many areas, and precipitated Fe ± other metals elsewhere as S-poor Fe-oxide(±Cu±Au) [= IOCG] mineralization, and (2) localized introduction of magmatic waters from the felsic intrusions that caused K-silicate ± acid metasomatism and Cu(±Mo±Au) breccia and porphyry style occurrences.

Non-magmatic brines (evidenced from Sr, S, O, C isotopes, mass balance, petrology) created intense structurally (fault systems, intrusion margins) and stratigraphically controlled alteration of rocks to various combinations of sodic plagioclase, scapolite, and Fe-poor Mg(-Ca) silicates in zones that extend upwards of 25 km along strike, 5 km laterally and several km vertically. Related K-silicate, acid and skarn alteration and IOCG mineralization are common but more restricted, typically at high structural levels. Mass-balance indicates far more metals are removed in Na(Ca) alteration than deposited in IOCGs. Conversely, smaller (a few km max dimension) quartz-K-silicate and acid alteration assemblages with Cu-bearing but Fe-poor mineralization are restricted to the more quartz-rich, hornblende-bearing plutons, consistent with multiple lines of evidence for a magmatic-hydrothermal origin. Na-Fe-rich systems affected volumes >>10 times those modified by the clearly magmatic-hydrothermal systems. In aggregate >3/4 of the exposed crust exhibits well developed alteration.

These results are compatible with observations elsewhere in the American Cordillera and globally. Combined with energy and mass balance considerations they indicate that terrestrial brine-dominated hydrothermal systems have had a major impact on the upper crust in arcs and rifts. Supported by NSF EAR08-38157 and USGS MRERP 08HQGR0060.