Τίτλος:
The geology and mineralogy of the Stypsi porphyry Cu-Mo-Au-Re prospect,
Lesvos Island, Aegean Sea, Greece
Περίληψη:
The Stypsi Cu-Mo-Au-Re prospect, Lesvos island, is a shallow
porphyry-epithermal system hosted within a middle Miocene microgranite
porphyry, which was emplaced along NNE-, NW- and NE-trending structures,
within trachyandesites to trachydacites and felsic pyroclastics of the
Stypsi caldera. The mineralization comprises three stages: Stage I is
characterized by magnetite-actinolite +/- quartz in transitional and
banded quartz veins mostly developed within the microgranite and the
surrounding lavas related to calcic-potassic and propylitic alteration
of the host rocks. In the banded veins, quartz is botryoidal, suggesting
crystallization from a gel. Abundant vapor-rich inclusions in the bands
produce a dark grey to black color. Within the veins, magnetite,
chalcopyrite, bornite and native gold were followed in time by pyrite,
hematite, sphalerite and galena. Stage I mineralization was synchronous
and also postdates quartz formation in the veins, since it crosscuts
and/or fills vugs in the center of the veins. Sulfides are associated
with various combinations of K-feldspar, actinolite, epidote, chlorite,
and calcite gangue minerals. Pyrite-molybdenite-chalcopyrite (Stage II)
and late intermediate sulfidation epithermal veins (Stage III) overprint
earlier mineralization and alteration, and are associated with
sericite-calcite and sericite-kaolinite alteration of the porphyry
system and spatially associated lavas, respectively. Molybdenite is
widespread in the first two mineralization stages, with Re content
ranging from about 0.3 wt% in Stage I to 1.96 wt% in Stage H. A barren
silicic and advanced argillic (alunite-kaolinite) lithocap, is exposed
on top of the porphyry-style mineralization and is crosscut by
non-mineralized high-sulfidation epithermal chalcedony-barite veins.
Bulk ore analyses of surface samples from the Stypsi prospect yielded
values of up to 276 ppm Mo, 978 ppm Cu, up to 0.5 g/t Au, up to 3 g/t
Ag, and up to 70 ppb Pd and Re.
Fluid inclusion data indicate that the Stage I transitional and banded
quartz veins were deposited at 420 degrees C-530 degrees C and at
pressures up to 450 bars, from boiling hydrothermal fluids. The fluid in
the veins consists of a brine (40-61 wt% NaCl equiv) that coexists with
a lower salinity (6-14 wt% NaCl equiv) liquid phase and a low-density
vapor-rich fluid. Fluid inclusions in quartz of the Stage III
intermediate-sulfidation epithermal veins are characterized by
relatively low homogenization temperatures (231 degrees C-288 degrees C)
and salinities (up to 1.9 wt% NaCI equiv), which was the result of
subsequent dilution of the moderately saline fluids by circulating
meteoric water. This study verifies earlier works suggesting that
Au-enriched felsic magmas are able to crystallize Re-rich molybdenite,
and that Re may also be redistributed and enriched in later stages
during the deposition of porphyry-style mineralization. The Stypsi
prospect ressembles in many respects (e.g., Au grades, Cu/Mo ratios, the
Re content of molybdenite and the presence of ore-grade calcic-potassic
alteration), other porphyry Cu-MoRe-Au systems hosted in calc-alkaline
rocks in northeastern Aegean, Greece.
Συγγραφείς:
Voudouris, Panagiotis
Mavrogonatos, Constantinos
Melfos,
Vasilios
Spry, Paul G.
Magganas, Andreas
Alfieris, Dimitrios
and Soukis, Konstantinos
Tarantola, Alexandre
Periferakis,
Argyrios
Kolodziejczyk, Joanna
Scheffer, Christophe and
Repstock, Alexander
Zeug, Manuela
