UNRAVELING THE ORIGIN OF HYDROTHERMAL ALTERATION ZONES IN AN UPPER-CRETACEOUS PLUTON IN THE PERUVIAN COASTAL BATHOLITH

The ~130 Ma Cerro Media Luna Plutonic Complex (CMLPC) located near the city of Pisco is part of the Peruvian Coastal Batholith. It was identified as a copper porphyry system by Agar (1981), although the CMLPC is located within the Andean IOCG belt (Sillitoe, 2003). The Cretaceous Quilmana volcaniclastic sequence hosts the pluton. Gradational contact relations between the host rock and pluton and the absence of dates for the host rock make the determination of age relationships between the two units difficult. Mineralization is hosted in breccias within the volcanic host, which connects both lithologies in terms of the hydrothermal system as well. To better understand the nature and source of the fluids and type of mineralization of this hydrothermal system, fieldwork, petrographic and microthermometric data analyzed in quartz combined with H-isotope values from hornblende, actinolite, chlorite, and biotite, and paragenetic relationships are presented.

Hydrothermal events are recorded in moderately to pervasively altered host rocks, observed both in the field and under the microscope. Rock samples exhibit potassic, chloritic, epidotic, calcic-sodic, chlorite-epidote, sericitic, and propylitic alteration. Identified ore minerals are magnetite, ilmenite, hematite, specularite, and goethite. Fluid inclusions are grouped into three distinct families (biphasic, triphasic, and polyphasic) with initial ice-melting temperatures indicative of SO₂ bearing-fluids, with salinities having 10-41 wt.% NaCl and homogenization temperature ranges of 156-412°C (for primary) and of 60-464°C (for secondary). Hornblende and actinolite δD_v-smow values of –41 to –48‰ imply an oceanic hydrothermal fluid source. We interpret the fluids entrapped in the quartz grains as coincident with those that altered hornblende and actinolite. Apart from that, samples also show heterogeneous entrapment that, with variation in salinity and temperature as well as the increase in δD values could be indicative of evolving fluids throughout the plutonic body.

The CMLPC deposit fits better with an IOCG model than the copper porphyry model. It has zones of high magnetic susceptibility and transitional zones between high and low susceptibility observed in the field, a calcic-sodic paragenetic assemblage, a low K, high Na, Ca, and Fe geochemical composition, and the presence of iron ore.