Intrusion Sequence and Hydrothermal Fluid Evolution of Encuentro Porphyry Cu-Mo-Au deposit, Northern Chile Public Deposited


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  • This dissertation is divided into two major chapters. The first chapter covers the geologyof the Encuentro deposit with emphasis in the intrusion sequence, vein mineral assemblages and distribution, and age of mineralization. The data used for this section includes field mapping, petrography, elemental analysis of biotite, and isotopic geochronology. The second chapter focuses on the pressure-temperature evolution of the magmatic-hydrothermal system at Encuentro and reconstruction of the evolution path for the mineralizing fluids. The basis of this chapter is the application of different geothermometers in both porphyry dikes and different veins present in the deposit.The Encuentro porphyry Cu-Au-Mo deposit is part of the middle Eocene to early Oligocene porphyry copper belt of northern Chile. New core logging observations, together with geologically constrained U-Pb and Re-Os geochronology documents the intrusion sequence and the timing of Cu-Au and Cu-Mo mineralization. A sequence of five porphyry dikes (Eep-1 to Eep-5) of dacitic composition is synchronous with and genetically related to mineralization. The Encuentro porphyry Cu-bearing dikes span from 42.3 ± 0.7 Ma to 41.2 ± 0.6 Ma, based on U-Pb zircon ages via LA-ICP-MS. The earliest intrusion (Eep-1) contains the highest Cu and Au grades; the metal content decreases systematically from oldest to youngest porphyry intrusions. Each porphyry intrusion developed a similar sequence of veins: (1) biotite, (2) early dark micaceous halos (EDM), and (3) a series of A-type quartz-K-feldspar-anhydrite veins (A1 to A5). In each sequence, biotite veins and EDM halos are cut by A-quartz veins. Cu-bearing sulfides are present in all these veins; however, A-type quartz veins contribute most of the Cu-Au and are the most voluminous vein type in the deposit (up to 56 vol.%, averaging 5 vol.% in rocks with >0.3 wt.% Cu). A-type veins are most abundant within and surrounding the upper parts of the porphyry dikes, and displays a concentric distribution in which the volume % of A-type veins is well-correlated with the Cu and Au ore grade.Other veins that are also stable with K-silicate alteration, include the vein sequence (4) B- type quartz-chalcopyrite±molybdenite, (5) quartz-anhydrite-molybdenite ± Cu-sulfides (QAM), and (6) magnetite±chalcopyrite veins. These veins postdate all porphyry intrusions, and offset all earlier biotite veins, EDM halos, and A-quartz veins. QAM veins and to a lesser extent B-type veins contribute the bulk of molybdenum mineralization in the deposit. Four Re-Os ages of molybdenite in different veins types range from 41.3 ± 0.2 Ma to 40.9 ± 0.2 Ma. These dates are consistent with crosscutting vein relationships and also with the 41.2 Ma age of the youngest porphyry (Eep-5) that B-type veins and QAM veins cut. Hence, most of the Mo mineralization likely occurred as a protracted single event immediately following the emplacement of porphyry phase Eep-5.Late veins include the sequence (7) chalcopyrite-pyrite±sericite veins, (8) chlorite- sericite-chalcopyrite±pyrite veins, and (9) D-type sericite-pyrite±chalcopyrite veins, and are all part of the sericitic alteration event. They show a zonal arrangement, where chalcopyrite-pyrite veins are dominant in the core of the deposit and are outwardly surrounded by chlorite-sericite veins, which are surrounded by D-type sericite-pyrite veins occupying the most distal parts of the system. The spatial distribution and similar mineral assemblages suggest that these three types of veins were likely originated from the same fluid. Changes in mineral ratios and development of wider quartz-sericite selvages towards distal areas were caused by a decrease in pH, as a consequence of cooling and acid dissociation. An Ar-Ar plateau age of muscovite in a D-type vein of 41.26 ± 0.15 Ma, suggests that Mo deposition and sericitic alteration took place in a short period of time. A late hydrothermal breccia with a tourmaline matrix, which is associated with the youngest (10) tourmaline-pyrite±chalcopyrite veins, was emplaced in the eastern part of the deposit. The crosscutting relationships and an Ar-Ar plateau age of tourmaline of 40.62±0.43 Ma indicate that these veins are the youngest manifestation of magmatic-hydrothermal activity at Encuentro.The thermal evolution of the magmatic-hydrothermal system was estimated by the application of different geothermometers such as: Ti-in-zircon, Ti-in-quartz, and Ti-in-biotite thermometers in Encuentro porphyry dikes; and by combining fluid inclusion microthermometry, Ti-in-quartz, and SEM-cathodoluminisecence in the different vein types present at Encuentro. These results suggest that Encuentro porphyry dikes crystallized at temperatures between 650°- 770°C and emanated from a concealed magma chamber located at ~6 km depth and lithostatic pressure of 1.8 kb. At least five cycles of porphyry magma injection are recognized. These dikes are associated with exsolved magmatic-hydrothermal fluids that mostly cooled and depressurizedbelow the brine-vapor solvus, and formed biotite veinlets, EDM halos, and A-type veins at temperatures between 550° and 700°C and pressures of 0.5-0.7 kb equivalent to depths of 1.8 to 2.5 km under lithostatic pressure. These are the P-T conditions under which most of the Cu-Au mineralization occurred at Encuentro. Subsequent batches of fluid up-flow from the concealed magma chamber continued, followed similar evolutionary paths in the two-phase brine-vapor field, and precipitated veins at similar but slightly cooler temperatures. B-type veins formed at similar conditions to A veins, between 540°-670°C and 0.4-0.7 kb; whereas quartz-anhydrite- molybdenite (QAM) veins formed at 540°-620°C and pressures of ~0.4 kb, both under lithostatic pressures. Later fluid input, likely from a deeper magma reservoir, reached the ore zone with higher acidity and lower temperatures than earlier batches, and therefore formed veins stable with sericitic alteration under hydrostatic pressure conditions. Indirectly estimated temperatures for D- veins are about 450°C, with pressures of 0.25-0.35 kb at conditions intermediate between lithostatic and hydrostatic pressures. Finally, the last magmatic-hydrothermal fluid released remained as a single-phase fluid without brine-vapor separation, and triggered the formation of tourmaline breccias and veins along the eastern flank of Encuentro at temperatures between 350° and 450°C and hydrostatic pressures of 0.15-0.3 kb. This data suggest that Encuentro porphyry deposit formed at relatively high-temperature and shallow depths (~2 km).
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