Geology of the Explorador Ag-Au vein system and Selene mining district, Apurimac, Perú

The emerging Selene mining district in southern Perú is in a Miocene volcanic
center and inferred caldera complex. A set of two elliptical ring structures approximately
5–7 km and 13 km in diameter, developed in pyroclastic rocks and interpreted
as a caldera setting, encircle a central stratovolcano and most mineralized systems of
the district including the Explorador Ag-Au vein system. Basal rhyodacitic pyroclastic
flows underlie the entire region, and are superimposed by a conic pile of andesitic lava
flows and epiclastic material of a stratovolcanic edifice. Several late-stage flow-dome
complexes and domes of rhyodacitic to rhyolitic composition intruded the stratovolcano
and ring structures, and host clusters of low- to intermediate-sulfidation veins
and a central high-sulfidation epithermal system with pervasive advanced argillic
alteration.
The Explorador Ag-Au quartz-adularia vein system, which is currently mined at
a rate of 750 TPD (tons-per-day), extends some 2400 m along an average N55°E
strike, and consists of the main vein plus a number of splays and parallel structures
within a 200 m wide corridor that probably formed in a dextral strike-slip fault zone.
Mineralization is recognized over a vertical interval of up to 350 m. The well-defined
paragenetic sequence is characterized by early, banded quartz-sulfide with adularia
and illite bands. Subsequent stages consist of lattice-textured quartz after calcite and
massive milky quartz. Ore mineralogy primarily associated with the early, banded
quartz-sulfide stage is characterized by ruby silver, argentite, and minor tetrahedrite
group sulfosalts (tennantite-tetrahedrite-freibergite) together with base metal sulfides.
Grade distributions display a steeply-dipping ore shoot geometry open at depth.
The pattern of Ag/Au ratios is independent from grade distribution and shows a systematic
subhorizontal zoning with ratios increasing downward.
A geologic model is proposed for the district that distinguishes four magmatic
stages and two to three main mineralizing stages, based on field observations and
geochronological data. New 40Ar/39Ar ages demonstrate that deposition of volcanic
rocks and emplacement of flow-dome complexes all occurred in early to middle
Miocene times. The first magmatic stage deposited rhyodacitic pyroclastic rocks on
regional scale until 16.2 ± 0.2 Ma. A second stage entailed construction of an
andesitic stratovolcanic edifice that terminated about 15.1 ± 0.1 Ma. Subsequent
emplacement of rhyodacitic flow-dome complexes and domes was closely followed
by on-set of hydrothermal activity. Alunite veinlets of the central high-sulfidation
system gave a weighted mean 40Ar/39Ar plateau age of 14.62 ± 0.05 that overlaps within error margin with biotite ages of the Explorador (14.6 ± 0.1 Ma) and Parcachata
(14.5 ± 0.1 Ma) flow-dome complexes. Mineralization of distal intermediatesulfidation
Ag-Au veins such as the Explorador vein is interpreted to have occurred
later at around 14.2 Ma. Afinal magmatic stage emplaced rhyolitic domes associated
with a separate mineralizing stage.
The close temporal and spatial relationship of late-stage magmatic phases with
mineralization suggests a genetic link. A striking feature of the Selene district is the concurrence of a wide spectrum of mineralization and alteration styles interpreted as
a zoned continuum of a single hydrothermal system with lateral and vertical fluid
evolution away from a common source.