Porphyry to Epithermal Transition in the Agua Rica Polymetallic Deposit, Catamarca, Argentina: An intergrated petrologic analysis of ore and alteration parageneses

MARTA FRANCHINI; AGNES IMPICCINI; DAVID LENTZ; FRANCISCO JAVIER RÍOS; SOL O´LEARY ; JOSEFINA PONS; ABEL ISIDORO SCHALAMUK

ORE GEOLOGY REVIEWS

ELSEVIER SCIENCE BV

Año: 2011 vol. 41 p. 49 - 74

0169-1368

Agua Rica (27°26′S?66°16′O) is a world class Cu?Au?Mo deposit located in Catamarca, Argentina. In the E?W 6969400 section examined, the Seca Norte and the Trampeadero porphyries that have intruded the metasedimentary rock are cut by interfingered igneous and hydrothermal heterolithic and monolithic breccias, and sandy dikes. Relic biotite and K-feldspar of the early potassic alteration (370° to N550 °C) with Cu (Mo?Au) mineralization are locally preserved and encapsulated in a widespread, white mica+quartz+rutile or anatase halo (phyllic alteration) with pyrite+covellite that suggests fluids with temperatures ≤360 °C and high f(S2). The Trampeadero porphyry and the surroundingmetasedimentary rockwith phyllic alteration havemolybdenite in stringers and B-type quartz veinlets and the highest Mo grades (N1000 ppm). Multistage advanced argillic alteration overprinted the earlier stages. Early andalusite±pyrite±quartz is preserved in the roots of the argillic halo rimmed by an alumina?silica material and white micas. This alteration assemblage is considered to have been formed at temperatures ≥375 °C from condensed magmatic vapor. At higher levels, pyrophyllite replaces muscovite and illite in clasts of hydrothermal breccias in the center and east sector of the study section, suggesting temperatures of 280 to 360 °C. Clasts of vuggy silica in the uppermost levels of the central breccia, indicates that at lower temperatures (b250 °C), fluids reached very lowpH(pHb2). In this early stage of the advanced argillic alteration, hydrothermal fluids seem to have not precipitated sulfides or sulfosalts. Hydrothermal brecciation was concurrent with fluid exsolution (↑Δ V), which precipitated intermediatetemperature advanced argillic alunite (svanbergite+woodhouseite)±diaspore±zunyite as breccia cement along with abundant covellite+pyrite+enargite±native sulfur±kuramite at intermediate depths and in lateral transitional zones to unbrecciated rocks. This mineral assemblage indicates temperatures near 300 °C,oxidized and silica-undersaturated hydrothermal fluids with high sulfur fugacity to prevent gold precipitation. Multiple generations of pyrite, emplectite, colusite, Pb- and Bi-bearing sulfosalts, and native sulfur with Au and Ag, accompanied by alunite introduction in the upper level breccias, probably occurred at lower temperatures, but still high sulfur and oxygen activity. An independent Zn and Pb (as galena) mineralization stage locally coincideswith Au?Ag and sulfosalts, and advanced at depth, controlled by fractures and overprintingmuch of the previous mineralization. A later paragenesis of veinlets of alunite+woodhouseite+svanvergite+pyrite± enargite that cut the phyllic halo suggests temperatures ~250 °C and without woodhouseite+svanvergite, temperatures ~200 °C. Kaolinite occurs in the phyllic halo as a late mineral in clots and in veinlets thus, in this zone, the fluid had cooled enough for its formation.