29 research outputs found
Volcanic facies architecture, hydrothermal alteration and subsea-floor replacement at the Neves Corvo deposit, Iberian Pyrite Belt
Contribution to research project ARCHYMEDESII-POCTI/CTA/45873/2002.Three felsic volcanic sequences constitute the host succession to the Neves Corvo
VHMS deposit. The lower volcanic sequence (late Famennian) consists of a rhyolitic
fiamme-rich facies association that comprises polymictic and overall graded quartzphyric
fiamme breccia units (up to 60 m thick). These units have pyroclastic origin
and constitute the substrate to the rhyolite facies association (intermediate volcanic
sequence). The rhyolite facies association (late Strunian) comprises intervals of
coherent quartz-feldspar-phyric rhyolite (up to 10 m thick) that are enclosed by much
thicker intervals (up to 250 m) of jigsaw-fit and clast-rotated monomictic rhyolite
breccia. Laterally these breccias grade to beds of monomictic rhyolite breccia that
alternate with crystal-rich sandstone. The units defined by the rhyolite facies
association are rhyolitic lavas. The massive sulfide orebodies (late Strunian) directly
overly the lavas or are interleaved with relatively thin (up to 50 m) intervals of mudstone. The upper volcanic sequence (early Visean) consists of a thin interval of
monomictic dacite breccia. The host succession to the Neves Corvo orebodies thus
comprises proximal to source vent deposits from submarine explosive and effusive
eruptions. However, the ore-forming process relates both in time and space with the
rhyolitic lavas, which are coeval with the mineralization.
Neves Corvo is well known for its high-grade Cu ores and unique cassiterite
mineralization. Ore-related hydrothermal activity overprints an early metasomatic
stage and relates with a multi-sourced hydrothermal system, responsible for early
stringer and massive cassiterite deposition and subsequent massive sulfide oregeneration.
In the Corvo orebody, the early deposition of massive cassiterite ores was
fed by an independent stockwork in a tectonically-bounded alignment. Textural and
petrographic analyses, geochemistry and oxygen-isotope data indicate brusque
flushing of the tin-bearing fluid into seawater after minimal fluid-rock interaction
during up flow.
Massive sulfide-related hydrothermal alteration is essentially stratabound and
controlled by permeability contrasts. Alteration zonation is classical, consisting of an
inner chlorite/donbassite-quartz-sulfides-(sericite) core that grades into sericitequartz-
sulfides-(chlorite) and paragonite-quartz-sulfides-(chlorite) peripheral
envelopes. The aluminous hydrothermal alteration mineralogy coupled with elemental
and stable isotope geochemistry indicates very low pH, unusually high maximum
interaction temperature and predominant low-sulfidation alteration/mineralization
conditions. Textural and mass-balance analyses show extensive silicate-sulfide
replacement in the coherent volcanic rocks of the footwall sequence, and disseminated
replacement mineralization in the volcaniclatic/sedimentary units
Late Strunian age : a key time frame for VMS deposit exploration in the Iberian Pyrite Belt
Estimate of geological environments favorable for the formation of massive sulphide deposits is an important goal to the exploration companies working in the
Iberian Pyrite Belt (IPB), the main European VMS base metals province, with giant deposits such as Neves Corvo, Aljustrel (Portugal), Rio Tinto and Tharsis
(Spain). Palynostratigraphic research programs using more than 40 exploration boreholes (>30 km length) allowed the dating of the sediments of the
Volcano-Sedimentary Complex (upper Devonian to upper Viséan), that host the massive sulphide deposits. Research is based on detailed palynomorphs study.
Careful work was focused on dark grey and black shale units that host stockwork and massive ore mineralizations. Felsic volcanic U/Pb age data was also used
to confirm the sediment age.Late Strunian (miospore biozone LN, 360.7 ± 0.7 Ma -362 Ma) sediments host the massive sulphide ore at Neves Corvo, Lousal,
Caveira and Montinho, mines located in the Portuguese IPB sector. In Spain similar data was obtained at Aznalcollar and Tharsis. The agedata show a
favourable geological period of ~2 Ma were paleogeographic conditions were extremely favorable to hydrothermal fluid circulation and VMS deposits formation.
Late Strunian age therefore becomes one key exploration guide in IPB
Geology of the Rosário-Neves Corvo antiform, Iberian Pyrite Belt, Portugal : new insights from physical volcanology, palynostratigraphy and isotope geochronology studies
The lithostratigraphic sequence in the Rosário–Neves Corvo antiform comprises the Phyllite–Quartzite Group, whose top is of Famennian age, the Volcanic
Sedimentary Complex, of Strunian to upper Visean age, and the Mértola Formation (the lower unit of the Baixo Alentejo Flysch Group) of upper Visean age.
The volcanic sedimentary complex comprises a lower sequence of Strunian (Late Famennian) age and an upper sequence of lower to upper Visean age.
Detailed mapping of the antiform towards NW of the Neves Corvo mine, supported by palynological dating, identified two new lithostratigraphic units: the
Barrancão member (upper Famennian) ascribed to the Phyllite–Quartzite Group and made up of laminated dark shales with siliceous lenses and nodules, and
the Ribeira de Cobres Formation of the Volcanic Sedimentary Complex, containing shales, siltstones and fine volcaniclastic rocks. Based on zircon U–Pb
isotope dating, five discrete felsic magmatic events were identified at approximately 354, 359, 365, 373 and 384 Ma. This suggests that the volcanic activity
in the area has extended for about 30 Ma, in a context of high regional heat flow as indicated by the geochemical signatures of the felsic volcanic rocks. The
characteristics of magmatism and the depositional environment indicated by the sedimentary record should therefore have been highly favourable for massive
sulphide formation. However, evidence of massive sulphide mineralization in the study area is still to be found. Moreover, reconstruction of the volcanic facies
architecture demonstrated that the volcanic units in the Rosário area are strongly dominated by coherent facies typical of the inner part of thick lavas/domes.
In fact, most of their external part, the more favourable location for possible massive sulphide mineralization, is missing. Palynological dating indicates a
significant hiatus, recognised between the lower and upper sequences of the volcanic sedimentary complex, which implies erosion of the top of the volcanic
centre, where VHMS deposits could possibly have formed. However, lateral areas of this volcanic centre, eventually preserved at depth, have good potential to
host massive sulphide mineralization