SUPERIMPOSED MAGNETITE AND IRON OXIDE-CU-AU MINERALIZATION AT PRODUCTORA, CHILEAN IRON BELT

The Productura Prospect lies within the Chilean Iron Belt 15 km south-southwest of Vallenar. The prospect is located within a thick package of intermediate composition volcanic flows and ash-flow tuffs of the Jurassic-Lower Cretaceous Bandurrias Formation. The volcanic rocks are intruded by the dioritic Cachiyuyito intrusion (130 Ma from zircon U-Pb) and the granodioritic Ruta 5 composite batholith (border phase, 96 Ma from zircon U-Pb). Three hydrothermal systems are present in the Productura area: 1) Cachiyuyito magnetite-apatite system, 2) Productura iron oxide-Cu-Au system, and 3) Ruta 5 hematite-gold system. The Cachiyuyito system produced irregular replacement bodies of massive magnetite-(apatite) associated with sodic-calcic alteration commonly within 100m of the contact of the Cachiyuyito intrusion with the host volcanic rocks. The alteration resulted in replacement of host volcanic rocks by fine-grained albite-actinolite-magnetite and the formation of veins of actinolite, magnetite, and/or epidote. The Cachiyuyito stock has also undergone extensive sodic-calcic alteration. The Productura system partially overprints the Cachiyuyito system. Silicification and potassium-iron metasomatism dominate the Productura system and are concentrated along north-trending faults parallel to the adjacent Atacama Fault Zone. Potassium-iron metasomatism is subdivided into an early biotite-rich alteration assemblage with subsidiary potassium feldspar and a paragenetically later potassium feldspar-chlorite-epidote alteration assemblage; alteration mineralogy in both assemblages is lithologically controlled. The early biotite-rich assemblage is characterized by the presence of pyrite. The later alteration assemblage contains magnetite, hematite, and chalcopyrite. Potassium feldspar intergrown with chalcopyrite yields an Ar³⁹-Ar⁴⁰ age of 91 Ma, suggesting it is temporally related to the intrusion of the Ruta 5 batholith. Sulfur isotopic results for early pyrite (-5.4 to +1.2 ‰) and later chalcopyrite (-6.3 to -1.1 ‰) do not support a significant contribution of sulfur from Mesozoic evaporitic sulfate. A minor late hydrothermal event spatially related to the Ruta 5 intrusion produced propylitic alteration of the volcanic rocks associated with earthy hematite-quartz-gold veins.