Age of mineralization by basinal fluids at the El Soldado manto-type copper deposit, Chile: 40Ar/39Ar geochronology of K-feldspar

Manto-type copper deposits of the southern Central Andes are strata-bound, epigenetic mineral concentrations hosted by bimodal volcanic and volcaniclastic rock suites. They are economically important, but since they are not obviously related to intrusions, their genesis has been controversial, to some extent, because it has proven difficult to date the ore-forming event and distinguish it from the age of low-grade regional metamorphism. El Soldado, the largest manto-type copper deposit in central Chile, is hosted by marine volcanic and subvolcanic intrusive rocks of the Lower Cretaceous extensional arc. To discriminate between regional metamorphism and mineralization, we dated 11 samples of K-feldspar by the stepwise-heating 40Ar/39Ar method. Hydrothermally precipitated K-feldspar associated with copper sulfides precipitated at temperatures of >300 °C and range in age from 100.5±1.5 to 106.1±1.1 Ma (mean 103±1.6 Ma; n=8). Syn-mineralization K-feldspar gave an age of 102.5±2.0, which is interpreted to be the age of mineralization. K-feldspar unrelated to ore yields older ages between 109.4±1.1 and 112±2 Ma (mean=110.3±1.4 Ma; n=3) is variably reset by the younger ca. 103 Ma mineralization episode. The above ages are interpreted to represent the age of alkali redistribution and crystallization due to burial metamorphism of the host sequence. Fission track dating of apatite in the host rocks yields an age of ca. 90 Ma, indicating fast cooling of the system to less than ca. 100 °C after mineralization. Time–temperature modeling of fission track length data are compatible with initial fast exhumation, followed by slow exhumation and cooling to surface temperatures. High levels of atmospheric Ar were detected in all K-feldspar samples and are tentatively attributed to connate meteoric fluids incorporated into the basinal metamorphic mineralizing fluids.