Reopening and retrograde dissolution in the early quartz veins of the Altar porphyry Cu-(Au-Mo) deposit, Argentina

LAURA MAYDAGÁN; FRANCHINI MARTA; FRANCISCO JAVIER RÍOS; DAVE LENTZ

Upsala

Congreso; 12th SGA Biennial Meeting; 2013

SGU, Geological Survey of Sweden

Cathodoluminescence (CL) images and fluid inclusion analysis in the early quartz veins of the Altar (31º 29? S, 70º 28? W) porphyry Cu-(Mo-Au) (Argentina) complex that are have yielded preliminary results on the characteristics of the hydrothermal fluids of the Altar system. Early type 2 veins show a complex growth history that revealed the tuff and the Middle-late Miocene upper existence of three generations of quartz: bright luminescence quartz (Ql), moderate luminescence quartz           and dikes and magmatic and hydrothermal with euhedral growth zoning (Qmz), and reddish and low luminescence quartz (Qr); and textures that indicate processes of reopening, dissolution, and corrosion.  The analysis of fluid inclusion assemblages hosted in the of veinlets and first generation of quartz (Ql) suggest the existence of a formed from several hydrothermal         first aqueous fluid exsolved from the magma at lithostatic pressures of >450 to 650 bars, temperatures of 425- 540°C and > 550 °C and low salinities of 3-7 wt % NaCl equiv that carried CO.2Primary and pseudo-secondary fluid inclusion B (type 3 quartz ± assemblages hosted in Qmz and Qr quartz generations     suggest two episodes of unmixing (boiling) of the D (chalcopyrite, hydrothermal fluids.   Coexisting saline and vapor-rich fluid inclusions hosted in and E (types 7 and 8 of quartz ± the Qmz quartz that homogenized at stemperatures (420°to 450°C) suggests that immiscibility of the magmatic?derived fluid occurred at hydrostatic pressures of ~300 bars.  Another episode of fluid unmixing occurred between 350° and 370 °C and ~150 bars. This last episode of fluid unmixing correlated with the dissolution of the Qmz quartz, followed by the precipitation of Qr quartz. The youngest chalcopyrite ± pyrite ± Au (capillary thin type 2b veinlets) that, together with disseminated mineralization in the potassic alteration, constitute the first pulse of Cu-Au- bearing fluids.