IANIGLA   20881
INSTITUTO ARGENTINO DE NIVOLOGIA, GLACIOLOGIA Y CIENCIAS AMBIENTALES
Unidad Ejecutora - UE
artículos
Título:
The Don Sixto mining project: a low sulfidation Au-Ag deposit of permian-triasic age, Mendoza, Argentina. Mineralogy, fluid inclusions, stable isotopes and 40AR/39AR age results
Autor/es:
MUGAS LOBOS, ANA CECILIA; MÁRQUEZ ZAVALÍA, MARÍA FLORENCIA; GALLISKI, MIGUEL ÁNGEL
Revista:
BOLLETTINO DI GEOFISICA TEORICA ED APPLICATA
Editorial:
ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA
Referencias:
Lugar: Trieste; Año: 2013 vol. 54 p. 291 - 293
ISSN:
0006-6729
Resumen:
The
Don Sixto deposit, formerly known as El Pantanito and La Cabeza, is located in
the southeastern Mendoza Province, in the San Rafael massif, and is one of the
few epithermal gold mineralizations related to the Permian-Triassic magmatism
in central western Argentina. Don Sixto is a low sulfidation Au-Ag deposit
The
local geology includes the Late Carboniferous Agua Escondida Formation
consisting of alternating beds of sandstone and shale. These rocks are
uncomformably overlain by the volcanic-pyroclastic units of the Choique Mahuida
Formation (Early Permian-Late Triassic) with rhyolite, rhyolitic ignimbrite, and
pyroclastic lenticular deposits. The rhyolitic subvertical dikes of the El
Portillo Group (Late Permian- Early Triassic) crosscut the previous sequences,
mainly with a NW-SE and N-S strike . Minor irregular bodies corresponding to
Don Sixto and Silicea breccias are locally significant. The hydrothermal
alteration is widespread and affects the ignimbrites and rhyolites that are
strongly to moderately silicified and moderately to strongly sericitized;
argillic and propillitic alterations are weakly to moderately developed.
The
mineralization occurs as Au-quartz veins and Au-dissemination in the
volcanic-pyroclastic units. Detailed studies allowed the identification of
seven stages of mineralization. The first sixth stages are quartz veins and the
last stage is represented by fluorite; most of these stages have banded, colloform,
and comb infilling textures. The presence of quartz veins with bladed calcite
replacement texture (bladed quartz) and quartz veins with adularia crystals are
indicative of boiling processes in the hydrothermal system.
The
microthermometric studies were performed in quartz and fluorite samples, in
petrographically defined fluid inclusion assemblages. Small (<10 μm),
primary, mainly irregular, biphasic (liquid-vapor) fluid inclusions were
analyzed to obtain melting (Tm) and homogeneization (Th) temperatures. For the
quartz veins, the salinity and homogenization temperatures have their maximum
value in the stage 4, related to a boiling process, with values up to 4.96%
NaCl eq. and <286.9ºC respectively. In fluorite, the salinity and
homogenization temperatures have lower average values with 1.05% NaCl eq. and 173.1ºC
respectively.
As
a complement, stable isotope studies for oxygen and sulfur were performed in
quartz and pyrite samples from the vein system. For the oxygen, the δ18O values
for the different quartz stages are in the range of 1.11 to 4.41?, relative to
VSMOW and the calculated δ value for the fluid is in the range of δ18OH2O=-6.92
to - 3.08?; these final results indicate a meteoric source for the oxygen in
the hydrothermal fluid. For the sulfur, the obtained δ34S values were 2.37 and
1.77?, relative to VCDT. As an approximation, the calculated δ value in the
fluid is δ34SH2S = 1.09?, which indicates a possible magmatic or even a mixed
source.
The
ore mineral association in Don Sixto includes major amounts of pyrite,
arsenopyrite, chalcopyrite and sphalerite, with lesser pyrrhotite, galena,
marcasite, magnetite, bornite, boulangerite and polybasite. The precious metal
bearing minerals are represented by gold of variable fineness, silver, acanthite,
uytenbogaardtite, stromeyerite and a selenium-enriched mineral association with
acanthite, polybasite and naumannite, together with the tellurium-enriched
stutzite and cervelleite. The main precious metal mineralization occurred in
the stage 4, associated to the boiling process. Chalcocite, digenite and covellite,
together with supergenic hematite, goethite and anglesite were recognized.
The
age of the alteration-mineralization process was obtained from adularia
crystals by the 40Ar/39Ar multiple step heat method, giving a result of
252.7±1.3 Ma; this age directly links the mineralization at Don Sixto to their
hosting Permian-Triassic magmatic rocks