Fernando Barriga

Discovery of a black smoker vent field and vent fauna at the Arctic Mid-Ocean Ridge

The Arctic Mid-Ocean Ridge (AMOR) represents one of the most slow-spreading ridge systems on Earth. Previous attempts to locate hydrothermal vent fields and unravel the nature of venting, as well as the provenance of vent fauna at this northern and insular termination of the global ridge system, have been unsuccessful. Here, we report the first discovery of a black smoker vent field at the AMOR. The field is located on the crest of an axial volcanic ridge (AVR) and is associated with an unusually large hydrothermal deposit, which documents that extensive venting and long-lived hydrothermal systems exist at ultraslow-spreading ridges, despite their strongly reduced volcanic activity. The vent field hosts a distinct vent fauna that differs from the fauna to the south along the Mid-Atlantic Ridge. The novel vent fauna seems to have developed by local specialization and by migration of fauna from cold seeps and the Pacific.

Geodiversity of Hydrothermal Processes Along the Mid‐Atlantic Ridge and Ultramafic‐Hosted Mineralization: a New Type Of Oceanic Cu‐Zn‐Co‐Au Volcanogenic Massive Sulfide Deposit

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Metallogenesis in the Iberian Pyrite Belt

One of the truly outstanding characteristics of the South Portuguese Zone (SPZ) is the abundance and dimensions of the massive sulphide deposits contained herein. The part of the SPZ where these ores exist or are likely to be found is well known in the literature as the Iberian Pyrite Belt (IPB). Also abundant, although modest in size and in economic significance, are manganese deposits, which usually are found not far from the massive sulphide deposits.

Giant pyritic base-metal deposits: The example of Feitais (Aljustrel, Portugal)

Abstract Detailed study of hanging-wall rocks and alteration above the giant Feitais orebody of Aljustrel (Iberian Pyrite Belt, Portugal) shows that cherts that constitute the immediate hanging wall to the sulfide deposit result from hydrothermal alteration of laterally equivalent Fe- and/or Mn-rich jaspers. Textures, geochemistry and lack of fossil radiolarians indicate that these jaspers are hydrothermal precipitates. Hydrothermal alteration of the jaspers took place concomitantly with sulfide precipitation, through reaction with hot ( 2+ -phyllosilicates. On the basis of the above, and considering also the results of previous studies on hydrothermal activity in the Aljustrel volcanic rocks and sulfide ores, we propose that sulfide precipitation may have taken place under a thin veneer of siliceous sediments. Present-day massive sulfide formation in the Guaymas Basin and in the Salton Sea, under hundreds of meters of sediments, may be approximate analogues in the sense that the presence of a caprock reduces dispersion of “black smoke” and causes more efficient precipitation.

Development of rodingite in basaltic rocks in serpentinites, East Liguria, Italy

Hydrogrossular replacement of plagioclase in basaltic rocks enclosed in serpentinite, and relationships between hydrogrossular, pumpellyite, vesuvianite are described. Rogingitic rocks are dominated by mixed layer chlorite-smectite, chlorite, pumpellyite, hydrogrossular and vesuvianite. In these rocks pumpellyite attains maximal Mg contents, and chemical analysis shows extreme removal of Na2O, K2O, TiO2 and SiO2 from the basalts, and increase in the Fe3+/Fe2+ ratio. Leaching during serpentinization by extremely alkaline solutions dominated by Ca-Mg(OH) may explain removal of components, but the oxidation may suggest that at an earlier stage dykes may have acted as input aquifers.

Extreme 18O-enriched volcanics and 18O-evolved marine water, Aljustrel, Iberian Pyrite Belt: transition from high to low Rayleigh number convective regimes

Abstract Quartz-eye keratophyric tuffs at Aljustrel, S. Portugal, Iberian Pyrite Belt possess unusually high, uniform whole rock δ-values up to 18.1%. with a mean of 16.7 ± 0.7%.. Because the quartz eye cores have δ18O 12.0 to 13.5%. the felsic tuffs may have had an original whole rock oxygen isotope composition of 10 to 11%. The deduced enrichments of up to +8%. in keratophyric derivatives is attributed to isotope exchange with abundant marine water under temperatures which diminished to ≤ 100°C, during thermally driven convective cooling. The isotopic uniformity requires elevated permeability of ∼ 10−8 cm2, such that the second critical Rayleigh number was exceeded, and drifting, non steady state convection cells dominated, with relatively smooth thermal structures. In mineralised counterparts of the tuffs beneath the Feitais-Estacao Zn-Pb-Cu massive sulphide orebodies, whole rock δ-values are ~12%. and quartz (13.3–15.4%.)-chlorite (3.2–6.3%.) fractionations correspond to temperatures of 220–270°C, and a calculated fluid δ18O of 1.4 to 5.7%. This local isotopic overprinting in vent domains of the regionally high 18O was induced by a stable, second stage convective regime imposed by drastic reduction of permeability accompanying capping of the geothermal discharge by hydrothermal cherts, and/or progressive spilitisation of the tuffs. At lower water/rock coupled with higher temperatures, the recirculating marine water underwent variable 18O enrichment up to 5.7%. by exchange with the high 18O tuffs. An upwards increase in δ18O quartz and δ-quartz reflects progressive cooling of the geothermal discharge from 15.4%. 270°C in the stockwork, through 18.3%. 220–240°C in the orebody, to 20.1%. 110–130°C in overlying hydrothermal cherts, probably induced by entrainment of ambient marine bottom water in sub-seafloor aquifers.

Antigorite in deformed serpentinites from the Mid-Atlantic Ridge

Deformed, non-psxeudomorphic serpentinites from fault zones in the Rainbow and Menez Hom areas, in the Mid-Atlantic Ridge, contain antigorite associated with variable amounts of chrysotile, while pseudomorphic or non-pseudomorphic lizardite + chrysotile serpentinites are the rule in this and other oceanic environments. A detailed TEM study of these deformed serpentinites shows that antigorite (polysomes m = 12 to 16) replaces chrysotile through dissolution-recrystallization, rather than through solid-state transition. This dissolution-recrystallization process is probably favoured by intense shear stress, the effects of which are preserved in the textures of these rocks. Oxygen isotope temperature estimates for these serpentinites fall well below 300 °C, confirming that Mid-Atlantic Ridge antigorite does not result from high-temperature prograde metamorphism, as it often does in other geological environments. Antigorite-bearing serpentinites, therefore, may occur locally in low-temperature, high-deformation settings, characterized by intense tectonic activity and major shear zones, as frequently found along the slow-spreading Mid-Atlantic Ridge. Technical difficulties may have limited the access to and sample recovery from important deformation settings, such as shear zones and fault scarps, thus explaining the relative scarcity of antigorite-bearing deformed serpentinites recovered from oceanic environments.

Geology and VMS Deposits of the Iberian Pyrite Belt

This Guidebook contains information to support the three SEG field trips included in the SEG Neves Corvo Field Conference 1997 (Lisbon, May 11–14, 1997). Collectively, these field trips cover the whole Iberian Pyrite Belt and beyond. Given that the Conference is aimed primarily at participants unfamiliar with the geology of the Belt, it was considered appropriate to introduce the subject prior to the field guides. Many studies have presented the overall characteristics of the geology and mineral deposits of the IPB (e.g. Carvalho et al., 1976; Strauss et al., 1977; Carvalho, 1979; Routhier et al., 1980; Barriga, 1990). Very recently, Carvalho et al. (1997) have summarized rather thoroughly the present state of the art concerning the IPB geology and metallogenesis. The present introduction is drawn largely from this. The Iberian Pyrite Belt (IPB) corresponds to an area of Devonian-Carboniferous volcanic and sedimentary rocks containing massive polymetallic sulfide deposits. This area forms an arcuate belt, about 250 km long and up to 60 km wide, trending westwards from near Seville in Spain to west-northwest in South Portugal. Both the eastward and westward extents of the belt are covered by Tertiary sedimentary rocks (Figure 1). The IPB is arguably the largest and most important volcanogenic massive sulfide (VMS) metallogenic province in the world. Some of its mineral deposits have been known and mined since the Chalcolithic era such as the Rio Tinto deposit, renowned for its historical role in the study of ore deposits. Only after the discovery of the large and rich copper-tin ore body of Neves Corvo (Southern Portugal) in 1977 has the true importance and potential of the IPB become fully appreciated. The original, pre-erosional amount of sulfides concentrated in about 90 known deposits are estimated at more than 1. 7 billion tons. Of this amount, about 20 percent has been mined, and 10-15% lost to erosion. This impressive amount of metals, in concentrations that range from small lenses with thousands of tons to giant bodies with hundreds of million tonnes, in such a relatively small area, represents an outstanding global geochemical anomaly of S, Fe, Zn, Cu, Pb, Sn and several other metals. The Iberian Pyrite Belt is located in the Southwest of the Iberian Peninsula, and comprises a large part of the Setubal and Beja districts in Portugal, and Huelva and Seville provinces in Spain (Figure 2). The region is an eroded peneplain, plunging gently to the

New hydrothermal activity and alkalic volcanism in the backarc Coriolis Troughs, Vanuatu

The Vanuatu Australia Vents Expedition (VAVE) to the Coriolis Troughs in southern Vanuatu during September 2001 aboard the RV Franklin discovered a new hydrothermall vent field-herein informally named Nifonea-and recent alkallic volcanic activity. The Nifonea field in the central Vate Trough was located by coincident light transmission and CH4 anomalies in a hydrothermal plume of ∼60 km2 extent, best developed between 1600 and 1750 m depth at ∼150 m above the seafloor. Extensive hydrothermal fauna and yellow-brown crusts and mounds cover an area of ∼1 km2. Very fresh, glassy, variably vesicular, sparsely phyric and aphyric basalt, trachybasalt, and basaltic trachyandesite (with ∼5-6 wt% combined alkalies at ∼ 51%-53% SiO2 and enriched light rare earth elements, Nb, and Zr) samples were dredged from youthful curtain, tube, and sheet flows, plus iron oxyhydroxide deposits. The alkalic composition of lavas in this tectonic setting is unique and attributed to thin ocean crust being developed in an incipient rifting phase involving a relatively low percentage of source-mantle melting. The Coriolis Troughs are among Earths most youthful backarc basins and thus provide valuable insights to incipient rifting and hydrothermal processes.

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.