Jorge M. R. S. Relvas

The relationship between ore deposits and oblique tectonics: the SW Iberian Variscan Belt

Abstract The Ossa Morena and South Portuguese Zones of the Variscan Belt of Iberia are interpreted to represent continental fragments that collided during the Variscan orogeny. Oblique northward subduction of an oceanic realm beneath the Ossa Morena Zone and subsequent collision induced thrusting and left-lateral transcurrent motion of crustal blocks and formation of a variety of ore deposits in both terranes. Most of the mineralization is related to dilational openings within thrusts and shear zones, extensional faults and pull-apart basins. A discontinuous diachronous vertical section from exhalative to deep mesozonal hydrothermal systems of Variscan age can be inferred. Volcanic-hosted massive sulphides are formed in third order pull-apart basins, but deeper related extensional structures are the loci for epithermal Hg, fluorite and Pb-Zn vein systems, Cu-Ni magmatic mineralization and iron-rich calcic skarns. Dilational regions along major shear zones also host mesozonal gold-bearing quartz veins. The overall Variscan mineralization pattern is inferred to be representative of an oblique collisional, (transpressional) geodynamic regime.

Gold mineralization in recent and ancient volcanic-hosted massive sulfides: The PACMANUS field and the Neves Corvo deposit

Gold mineralization in the Neves Corvo copper stockworks occurs in two geochemical associations: (1) Au+Co+Bi(±Te) and (2) Au+Cu+Ag(±Hg). Type 1 gold occurs in the deeper parts of the feeder zones. Its associated mineralogical assemblage is consistent with low sulfur activity of the fluid and high temperature (360°–400°C) conditions. The second type of gold geochemical association reflects pH increase and/or temperature decrease as the fluid moves upwards in the stockwork and reaches the base of the massive sulfide Neves Corvo is accompanied by high sulfidation parageneses. These might have resulted either from the long-lived maturation of the ore-forming system, which would have led to extreme zone refining effects, and/or from the late input of an external fluid component (possibly magmatic?) in the Neves Corvo hydrothermal system. In the PACMANUS hydrothermal field, native gold also occurs in both high and low sulfidation mineral associations. Gold occurs at the seafloor level either associated with: (1) chalcopyrite+sphalerite +tennatite, or (2) chalcopyrite+bornite ± covellite. At depth, instead, gold occurs in sphalerite as small native gold inclusions (silver-poor). Following the process of copper enrichment by zone-refining, gold frequently concentrates in the borders of the sphalerite grains.

Felsic pyroclastic and effusive volcanic facies hosting the Neves Corvo massive sulfide deposit, Iberian Pyrite Belt, Portugal

The combination of biostra-tigraphic data with the facies architecture of the Neves Corvo massive sulfide deposit allows dating of the two major volcanic events identified in the mine stratigraphy. Both volcanic episodes are represented by submarine and proximal to source vent deposits, with intrabasinal origin and rhyolitic composition. Thick pyroclastic pumice-rich facies of the late Famennian and youngerlavas of the late Strunian (latest Famennian) constitute the host sequence of the massive sulfides. The hydrothermal mineralizing event occurred after the emplacement of the rhyolitic lavas.

Osmium isotope systematics in the Iberian Pyrite Belt

New Re and Os analyses of pyrites from Aljustrel, Lagoa Salgada and Neves-Corvo deposits yield concentrations that are similar to those previously reported for the Rio Tinto and Tharsis deposits, all at the Iberian Pyrite Belt. Osmium contents in IPB pyrites increase from the deeper stockwork zones towards the upper massive sulphides. Pyrite samples from IPB sulphide orebodies plot on a major 187Re/188Os-187Os/188O linear array, yielding an age of 359±26 Ma and an initial 187Os/188O ratio of ca. 0.57. Five samples from Neves-Corvo sulphide-rich stockwork ores yield an age of 358 ±29 Ma and an initial 187Os/188O ratio of 0.49 ±0.07, in accord with palynological and isotopic age constraints for ore formation at the IPB. Pyrites from Neves-Corvo copper-tin ores have Re/Os ratios and osmium concentrations that are similar to those in pyrites from sulphide-rich stockwork ores; however, their highly radiogenic initial 187Os/188O ratios (4.89–7.85, if preserving original isotope system-atics) preclude derivation from the same sources as the remaining IPB sulphides. The overall Re-Os geochemical data conform to previous Sr, Nd and Pb isotopic studies supporting multiple sources for IPB metallogenesis.

Silica gel microtextures in siliceous exhalites at the Soloviejo manganese deposit, Spain

Well-preserved primary textural features were recognized in cherts and jaspers that form one of the manganese ore-hosting units of the Soloviejo Mn deposit, in the Iberian Pyrite Belt. These include (i) spherulitic, (ii) banded and (iii) brecciated. The spherulites (0.04—0.3 mm) consits of a central core of minute grains of hematite or quartz, rimmed by chalcedony or microcrystalline quartz. Banded jaspers show no spherulitic textures. Banded textures are made up by alterations of micro- to meso-bands of silica and hematite. They often show soft-sediment deformation features, such as microslumps, pinch-and-swell, and load cast. Diagenetic brecciation overprints these pristine textures. Breccias consist of jasper fragments (mm to cm), affected by total or partial recrystalization, cemented by different proportions of microcrystalline quartz. Primary textures in siliceous hydrothermal precipitates reflect different stages of formation and diagenetic maturation of its silica gel precursor. Their interpretation helps in constraining the initial conditions assisting the hydrothermal activity at the Soloviejo deposit, and its early evolving stages.

Portugal, science and resources in the last frontier

Portugal holds jurisdiction, and exclusive economic rights, over a very large area of the seabed. One of the main national objectives, at the present time, is to increase the rather limited knowledge that exists on the resources that may be contained herein.