Hydrothermal and geochemical evolution of the ore-forming system at Bolcana – Apuseni Mountains (Romania)

Abstract

The Bolcana Au-porphyry deposit is located in the southern Apuseni Mountains in western Romania. For- mation of the porphyry complex occurred in several distinct pulses within Neogene time under a prevailing post-subduction extensional tectonic regime. The porphyry system is well-preserved and -exposed with rem- nants of vuggy silica and quartz-alunite alteration proving preservation of the system’s roof at today’s ero- sional level and continuous drill holes of over 2 km length reaching into the basement. In order to resolve the procession of the deposit’s hydrothermal evolution petrographical studies were combined with in-situ meas- urements. Upon extensive vein petrography including CL-imaging, nine distinct vein-type groups were es- tablished, namely A-, banded, A-B- and B-type quartz, C- and D-type sulfide and M1- to M3-type magnetite veins. It turned out that the bulk of quartz vein-hosted non-quartz minerals were introduced before or after quartz precipitation, with sulfides clearly postdating all quartz veins. Vein and lithological sequence revealed that first A veins were formed during emplacement of the early medium- to coarse-grained porphyries, fol- lowed by focused large scale surface-venting magmatic-hydrothermal brecciation with concomitant repeated A to B and banded vein formation. Subsequent emplacement of amphibole-rich coarse- and fine-grained porphyries denote the termination of porphyry emplacement. Thereupon, a complete cycle of hydrothermal quartz veins, composed of sequent A, A-B and B veins, partially repeated and intermitted by M-type veins, superimposed the prior emplaced porphyries. M3-type veins postdate B veins but predate C- and D-type sulfide veins, which represent the endpoint of porphyry associated hydrothermal evolution. Formation tem- peratures of hydrothermal quartz veins were obtained by application of the titanium in quartz thermometer (TitaniQ). Titanium contents of successively formed quartz veins were acquired by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS). Ti-in-quartz temperatures are elevated by up to 180°C compared to temperatures obtained for secondary poly-phase fluid inclusion assemblages from the same veins, however the transition is gradual (TQtz = 700-500°C; Tsec. poly-phase FI’s = 530-410°C). A- and B-type quartz veins were formed at temperatures from 700-600°C and 550-500°C, respectively. Banded veins were formed at temperatures from 630 to 520°C. These temperatures allow reconstructing the fluid source from secondary ion mass spectrometry (SIMS) in-situ analysis of oxygen isotopes in the quartz (δ18OQtz, ranging from 9.54 ± 0.26 to 12.12 ± 0.35‰). Calculated fluid isotopic signature (δ18OFluid) range from 6.7 ± 0.8 to 10.9 ± 1.0‰, i.e. are within the magmatic field, suggesting porphyry vein formation to be derived exclusively by fluids of magmatic origin. On average δ18OQtz is distinct for all vein types (δ18OA-type Qtz = 10‰; δ18OB-type Qtz = 9.8‰; δ18Obanded-type Qtz = 11‰), while A and banded veins were formed from a fluid with equal oxygen isotopic composition (δ18OFluid (A and banded) = 8.9‰), late B veins formed from a slightly lighter one (δ18OFluid (B) = 7.6‰). Low-density vapor-only inclusions are the most wide-spread fluid inclusions. They coexist with high-salinity high-density poly-phase inclusions in A to B veins and with salt melt inclusions in banded veins. This all suggests prevailing high temperatures during porphyry quartz vein formation and pressure fluctua- tions to be the main driver of vein formation. The Cu-poor, Au- and magnetite-rich character of Bolcana can be explained by significant sulfur loss during formation of deep extending magmatic-hydrothermal breccias. The significant amount of porphyry-Cu-mineralization clearly postdates emplacement of individual porphyry phases and all hydrothermal quartz vein generations. As a result, Bolcana potentially experienced an early Au- and a late Cu-mineralization.