Abstract
Major tin-tungsten deposits in Portugal are related to intrusions of
the Younger Series (300-280 Ma) of Hercynian granitoids. Mineralized
granites are 'specialized' by a specific increase or decrease of major,
minor and trace element contents in comparison with non-mineralized
occurrences. Component analysis is used to make distinctions between
the various geochemical types of barren and
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mineralized granites
(Chapter 2). A one-component model is characterized by positive
loadings for Rb, Sn, Cs, Li, Nb, F, Ta, Zn, Wand P20S and negative
loadings for Zr, Ba, Ti02 and Sr. In this one-component model the
component scores increase from barren to highly specialized granites.
In a two-component model component 1 (F, Li, Cs, Rb, Zn, Sn, P20S,
Nb and W) expresses a hydrothermal influence upon the granites, whereas
component 2 (Ti02, Ba, Zr, Sr, -Ta, -Nb, -Rb and -Sn) possibly
represents the magmatic differentiation trend. In this model the
composition of the individual granites concentrate in one of several
fields according to their loadings by the element parameters.
Muscovite granites, albitized and greisenized granites deviate clearly
in composition from the more common granites.
The geochemical variability of granites, greisens and schists of the
Panasqueira tungsten-tin deposit is investigated in great detail
(Chapter 3). The trace element contents of the Panasqueira cupola are
compared with those of other granite occurrences in the surroundings.
Only ore-bearing granites show a geochemical differentiation concurrent
with mineralization. The major elements illustrate the general
chemical evolution from muscovite-albite granite to greisenized
granite; trace elements, however, show in addition distinct groups of
granitoids. The general trend in the granitoids of Panasqueira is an
increase in Rb, Li, F, Sn, Cs, W, Fe203' P20S, Zn, As, Nb, Ta and Cu
contents while Na20, CaO, MgO, Sr, Zr, Ba and Ti02 contents decrease.
Rare earths are strongly depleted in the greisen. This alteration in
composition is the result of metasomatic and hydrothermal processes; closed in an early stage but thereafter opened due to the development
of sub-horizontal joints.
The schist along the hydrothermal ore veins at Panasqueira is
affected as well by the mineralizing fluids in the veins; tourmaline
and sericite alteration zones contain elevated amounts of the orerelated
elements Rb, Li, Cs, Sn, Wand Na20 and K20. Schist samples
were also taken at the surface and used for litho-geochemical
exploration purposes. The same elements, when plotted in contour maps,
outline the outcropping ore zones very well and can be used as a tool
in geochemical exploration (Chapter 4).
A major part of this investigation is dedicated to the physical
conditions under which the mineralization of Panasqueira was formed
(Chapter 5). Homogenization temperatures of fluid inclusions in the
minerals of Panasqueira point to an interval between 325 and 2500 C for
the early stages; salinities are between 5 and 10 wt% NaC1. In the
final hydrothermal stage homogenization temperatures dropped to about
100oC; the salinity is also lower, down to 3 wt% or less. Due to
trapping of C02-rich fluids against an impermeable granite-schist
contact, fluid pressures point to an 'overpressure' of about 1 kbar
before opening of the joint system. Opening of this joint system
caused vigorous boiling of the fluids in the ore veins at a depth of
about 1800 m.
Gas chromatographic analyses show next to C02 the presence of CH4
and N2' The presence of CH4 suggests a reducing environment during ore
deposition. The origin of the CH4 and N2 is possibly found by
assimilation of schist containing organic matter.
Stable isotopes of carbon, oxygen and nitrogen indicate a
homogeneous isotopic source for the early fluids. High overpressures
in the earliest stage and about constant oxygen, carbon and nitrogen
isotopes strongly favour a 'magmatic' origin for the initial
mineralizing fluids. Oxygen and carbon isotopes of the fluids differ
strongly in the latest stage and point to a large influx of meteoric
water
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