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
