Multistage quartz precipitation and mineralization in the stockwork veins of Altar porphyry Cu-(Au-Mo) deposit, Argentina, revealed by cathodoluminescence, LA- ICPMS, and fluid inclusions

LAURA MAYDAGÁN; MARTA FRANCHINI; DAVE LENTZ; AGNES IMPICCINI; FRANCISCO JAVIER RÍOS; ROGER RAY

ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

SOC ECONOMIC GEOLOGISTS, INC

Lugar: Litletton; Año: 2012

0361-0128

Abstract     The Altar porphyry Cu-(Au-Mo) deposit (802 mT, 0.42% Cu, 0.059 g/t Au) with     associated epithermal Au-(Ag-Cu) siliceous ledges is located in the Cordillera Principal     of San Juan (Argentina), within a new cluster of Miocene to early Pliocene porphyry     copper prospects. The mineralization at Altar is hosted in a complex stockwork with     different generations of veins and as disseminations, formed from several magmatic-     hydrothermal pulses and resultant repeated fracturing events. The veins were grouped     into EB (type1), A (types 2a and 2b chalcopyrite ± pyrite ± Au), B (type 3 quartz ±      molybdenite), D (types 5 and 6 of tennantite (chalcopyrite, bornite) ± quartz ± pyrite ±     gold ± tetrahedrite) and enargite + pyrite), and E (types 7 and 8 of quartz ± sphalerite ±     galena ± pyrite) types.      Cathodoluminescence (CL) of 2a, 3 and 5b veins revealed various cycles of quartz     deposition and textures that indicate processes of reopening, retrograde dissolution, and     corrosion. In 2a (A type) veins, Ql quartz hosts intermediate-density fluid inclusions     that are interpreted to represent the first fluid exsolved from the magma at lithostatic     pressures of  > 450 to 650 bars and temperatures of  > 550 °C. In 2a and 3 veins the first     generation of bright luminescence quartz (Ql) has high values of Ti and low Al     contents. The second generation of moderate luminescence quartz with euhedral growth     zoning (Qmz) is characterized by lower Ti and higher Al contents and may have     precipitated from a more acidic and cooler fluid. Episodically, fluid may boiled to form     a saline fluid and a low vapor density fluid registered in the fluid inclusions     assemblages hosted in the Qmz quartz. These episodes of fluid immiscibility (boiling)     could produce the observed retrograde dissolution and re-precipitation of quartz. The     youngest and less abundant reddish quartz (Qr) precipitated in the spaces left by the     retrograde dissolution of previous quartz. Qr quartz has the lowest Ti and high-variable     Al concentrations.      In type 5 veins (D type) the three generations of quartz have low Ti contents     suggesting that the formation of these veins was accompanied by a notable drop in the     fluid temperature. In 2a, 3, and 5b Altar veins, the mineralization accompanied the last     quartz generation (Qr). CL images together with the analysis of fluid inclusions     assemblages and trace elements in quartz, indicate that a drop in temperature over ~450     to 350°C, fluctuations in the pH, and (or) pressure of the hydrothermal fluids occurred     during the crystallization of the late quartz generation associated with the Cu-Au-, Mo-,      and Cu-Au-bearing sulfides and sulfosalts in the Altar deposit.