Tesis Doctoral Ramiro Lopez (2006)

During the final stages of the Jurassic volcanism, and a as a result of the elevated geothermic gradient in
the upper crust, a regional hydrothermal activity took place all over the Deseado Massif, generating
epithermal mineralization in deepness, with some feeders reaching the surface.
In the studied area, several manifestations of the Jurassic hydrothermal activity were recognized:
epithermal veins and breccias with Au-Ag anomalies, hydrothermally altered rocks, calcareous
stromatolites and silica sinter; all of them representing different levels within the epithermal model.
The main mineralised structures in the area are: Eureka, La Mariana north and La Mariana south veins.
Eureka vein is 4.5 km long, with a N305° azimuth, commonly dipping 75° to the SW. There were
recognized seven pulses within the vein. Its main characteristics have regular variations, indicating a
three segments subdivision: northwest, centre and southeast.
Eureka northwest has a main vein outcrop ranging from 5 to 25 m in width, filled by quartz and
chalcedony with banded and breccia textures and a strong quantity of Fe-Mn oxides. In this sector the
vein is hosted in the andesitic lava flows and the subvolcanic andesitic body, both with intense propylitic
alteration.
Eureka centre is represented by three or four branches of 1 to 3 m in width each, with the same kind of
infill but with a strong participation of brecciated textures. In this case, the host rocks are ignimbrites with
a strong argillic and silica hydrothermal alterations.
Eureka southeast is the most regular of the three subdivisions. It has up to 2 m wide, and is composed by
a dense stockwork to breccia with clasts of quartz in a Fe-rich silica cement and with Fe oxides
associated. This structure is hosted in ash-fall pyroclastic facies.
La Mariana north vein is 200 m long, up to 2 m wide and with a N260° strike. It is filled by banded and
breccia quartz textures, occasionally porous with limonitic infilling. This vein is hosted in argillic and oxide
altered, grey to violet coloured, reworked volcanic facies.
La Mariana south vein is characterized by discontinuous outcrops along a 100 m long structure, and it
has up to 1 m in width. It has variable strike between N280° and N290°, and is vertical. It is composed by
a quartz stockwork, with some brecciated texture. It is very important the participation of Fe-Mn oxides. In
this case the host rock is the undifferentiated volcanic facies with a strong alteration (mainly oxidation).
Seven more sectors with evidences of hydrothermal mineralization were recognized: Vetilleo Norte, Cerro
Solo, Área Puesto Solo, Paragüitas, Brecha Centro Volcánico Cerro Tetis, Brecha Hidrotermal Central y
Brecha La Mariana. They are characterized be the presence of quartz or calcite veinlets, or silica
hydrothermal breccias, in general interpreted as upper levels of the epithermal system.
Also some surface manifestations of the hydrothermal activity are present in the area: carbonate
stromatolites and a silica sinter.
At same time this activity is present in the area as extended outcrops of hydrothermally altered rocks. The
main alterations are argillic, silicic and propylitic, occurring one or other depending principally on the
primary composition of the rocks.
The mineralogy, quartz structures and textures, hydrothermal alteration, metals geochemistry, fluid
inclusions, stables isotopes and lead isotopes were studied in order to understand and classify the
different epithermal evidences.
The mineralogy of the veins was studied by petrography, chalcography, electronic microscope and X-ray
analyses, characterizing two main systems in the area: one with silica (mainly quartz) and other with
Lopez, R., 2006. Tesis Doctoral. FCNyM-UNLP.