Tectonic controls and fluid evolution of auriferous quartz veins in the La Codosera area, SW Spain

Dee, Stephen James (1991) Tectonic controls and fluid evolution of auriferous quartz veins in the La Codosera area, SW Spain. University of Southampton, Doctoral Thesis.

Abstract

During the Hercynian orogeny in SW Spain, at the Southern Margin of the Central Iberian Zone, veins were formed during three different stages of the local transpressive deformation sequence: during early ductile deformation; through granite intrusion; to late brittle faulting. Fluid inclusions in seven vein types have been investigated and show a change in composition over time. Peak metamorphism coincided with early ductile folding and associated thrusting which formed the Central and Southern Ridges. Fluid inclusion data, combined with mineralogical constraints and vitrenite reflectance geothermometry, constrain this metamorphism to 410-425^oC and 3.1-3.4 kbar. A low salinity aqueous fluid (c.6.4wt% equiv NaCl) of high density was in equilibrium with the peak metamorphic assemblage of quartz-albite-muscovite-chlorite. After emplacement of the Tres Arroyos Li Pegmatite, in the roof zone of the Alburquerque Granite, a high temperature (> 400^oC) magmatic fluid of low salinity (c.6.6 wt% equiv NaCl) mixed with a lower temperature fluid derived from the contact metamorphic aureole at pressures in excess of 1.5 kbar. At an early stage the fluid unmixed into an aqueous phase and a carbonic phase with significant quantities of N_{2 +} CH₄, generated by fluid interaction with pelites. The fluid was locally buffered by a quartz-muscovite-albite assemblage within the pegmatites. Late kinematic pyrite-arsenopyrite-gold bearing quartz veins within the Central Ridge and Los Algarbes areas were formed on obliquely reactivated thrust faults and as structures related to high-angle right-lateral faults. This late right-lateral antithetic bookshelf faulting occurred along a releasing bend in the Badjoz-Cordoba shear zone. These structures provided favourable sites for fluid infiltration. Gold occurs within pyrites and arsenopyrites either as a lattice component or as finely disseminated particles. During the late brittle faulting, relatively reduced carbonic fluids (H₂O-CO₂-N₂-CH₄ with H₂S as the dominant dissolved sulphur species) unmixed at c310^o and 1.5 kbar and CO₂-N₂-CH₄ were partitioned into the vapour phase. A late retrograde boiling event occurred at 250^oC and > 750 bars after gold mineralisation. High contents of N₂ (up to 8.7 mole%) are associated with the mineralised veins produced by fluid-rock interaction either locally or at the fluid source. The lack of extensive wallrock alteration and thermodynamic calculations suggest that the fluids were partially buffered by a quartz-muscovite-albite±k-feldspar assemblage in the wallrocks. Stable isotope data (δ18O, δD & δ^34S) are most consistent with a metamorphic source for the mineralising fluids deeper in the crust. Unmixing oxidised the fluid and preferentially partitioning H_2S into the vapour phase promoting the deposition of gold, by destabilisation of the reduced sulphur complexes.

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