CONTRASTING HYDROTHERMAL SYSTEMS RELATED TO EPISODIC MESOZOIC ARC MAGMATISM IN COASTAL NORTHERN CHILE

New deposit- to region-scale mapping and related lab studies demonstrate that multiple hydrothermal systems affected large volumes in the upper parts of the Mesozoic coastal batholith and coeval volcanic rocks of northern Chile (26-28˚S). Field relationships and geochronology (~500 dates incl. 60 new U-Pb on zircon, titanite) show that episodic magmatism between 240 and 60 Ma (Girardi and Barton, this meeting) drove (1) non-magmatic brine-dominated hydrothermal systems (IOCG clan), and (2) magmatic-hydrothermal systems (porphyry Cu clan).

These two broad system types differ in their types and volumes of hydrothermal alteration and in their relationships to magmatism. (1) Non-magmatic brines (geochemical, petrologic evidence) generated zones Na-Ca alteration that extend up to 25 km along strike, 5 km laterally, and 5 km vertically. Deep Na-Ca metasomatism removed Fe and base metals; shallower exposures contain some Na-Ca, but also K-silicate, acid and skarn alteration and varied types of S-poor Fe-oxide(±P±Cu±Au) mineralization. These features are best developed with dioritic rocks but occur with all magma types. (2) Conversely, magmatic-hydrothermal fluids generated smaller (2-3 km max dim) K-silicate ± H⁺metasomatism, little or no Na-Ca or Fe alteration, but numerous Cu(±Mo) breccia pipe and porphyry-style occurrences. Consistent with predictions for magmatic-fluid sources, these are restricted to Qz-rich, Hbl-bearing plutons. In aggregate >3/4 of the exposed crust exhibits well developed, often superimposed alteration systems; of this total, Na/Fe alteration systems comprise >90% by volume.

Hydrothermal alteration formed intermittently over 180 m.y., is most extensive in the shallow crust, and reflects superposition of many short-lived (<1 m.y.) thermal events. Secular patterns reflect petrologic linkages, levels of crustal exposure, and the magmatic pulse of the arc. The older, mainly granodioritic (240-190 Ma) western part of the arc is deeply eroded (6-12 km) and contains mainly the roots of porphyry and IOCG type systems. Younger pulses are largely quartz dioritic (140-100 Ma) or granodioritic (70-60 Ma); they are less exhumed retaining more of their volcanic superstructure, they contain extensive hydrothermal alteration, and they preserve widespread IOCG and some porphyry systems.