The large Cerro de Pasco Cordilleran base metal deposit in central Peru
is located on the eastern margin of a middle Miocene diatreme-dome
complex and comprises two mineralization stages. The first stage
consists of a large pyrite-quartz body replacing Lower Mesozoic Pucara
carbonate rocks and, to a lesser extent, diatreme breccia. This body is
composed of pyrite with pyrrhotite inclusions, quartz, and black and red
chalcedony (containing hypogene hematite). At the contact with the
pyrite-quartz body, the diatreme breccia is altered to
pyrite-quartz-sericite-pyrite. This body was, in part, replaced by
pipelike pyrrhotite bodies zoned outward to carbonate-replacement Zn-Pb
ores hearing Fe-rich sphalerite (up to 24 mol % Fes).
The second mineralization stage is partly superimposed on the first and
consists of zoned east-west-trending Cu-Ag-(Au-Zn-Pb) enargite-pyrite
veins hosted in the diatreme breccia in the western part of the deposit
and well-zoned Zn-Pb-(Bi-Ag-Cu) carbonate-replacement orebodies; in both
cases, sphalerite is Fe poor and the inner parts of the orebodies show
typically advanced argillic alteration assemblages, including aluminum
phosphate Sulfate (APS) minerals. The zoned enargite-pyrite veins
display mineral zoning, from a core of enargite-pyrite +/- alunite with
traces of Au, through an intermediate zone of tennantite, chalcopyrite,
and Bi minerals to a poorly developed Outer zone hearing
sphalerite-galena +/- kaolinite. The carbonate-hosted replacement ores
are controlled along N 35 degrees E, N 90 degrees E, N 120 degrees E,
and N 170 degrees E faults. They form well-zoned upward-flaring pipelike
orebodies with a core of famatinite-pyrite and alunite, an intermediate
zone with tetrahedrite-pyrite, chalcopyrite, matildite, cuprobismutite,
emplectite, and other Bi minerals accompanied by APS minerals,
kaolinite, and dickite, and an outer zone composed of Fe-poor sphalerite
(in the range of 0.05-3.5 mol % Fes) and galena. The outermost zone
consists of hematite, magnetite, and Fe-Mn-Zn-Ca-Mg carbonates. Most of
the second-stage carbonate-replacement orebodies plunge between 25
degrees and 60 degrees to the west, suggesting that the hydrothermal
fluids ascended from deeper levels and that no lateral feeding from the
veins to the carbonate-replacement orebodies took place.
In the Venencocha and Santa Rosa areas, located 2.5 km northwest of the
Cerro de Pasco open pit and in the southern part of the deposit,
respectively, advanced argillic altered dacitic domes and oxidized veins
with advanced argillic alteration halos occur. The latter veins are
possibly the oxidized equivalent of the second-stage enargite-pyrite
veins located in the western part of the deposit.
The alteration assemblage quartz-muscovite-pyrite associated with the
pyrite-quartz body suggests that the first stage precipitated at
slightly, acidic fin. The sulfide mineral assemblages define an
evolutionary path close to the pyrite-pyrrhotite boundary and are
characteristic of low-sulfidation states; they suggest that the
oxidizing slightly acidic hydrothermal fluid was buffered by phyllite,
shale, and carbonate host rock. However, the presence in the
pyrite-quartz body of hematite within quartz suggests that, locally, the
fluids were less buffered by the host rock. The mineral assemblages of
the second mineralization stage are characteristic of high- to
intermediate-sulfidation states. High-sulfidation states and oxidizing
conditions were achieved and maintained in the cores of the second-stage
orebodies, even in those replacing carbonate rocks. The observation
that, in places, second-stage mineral assemblages are found in the inner
and outer zones is explained in terms of the hydrothermal fluid
advancing and waning.
Microthermometric data from fluid inclusions in quartz indicate that the
different ores of the first mineralization stage formed at similar
temperatures and moderate salinities (200 degrees-275 degrees C and
0.2-6.8 wt % NaCl equiv in the pyrite-quartz body; 192 degrees-250
degrees C and 1.1-4.3 wt % NaCl equiv in the pyrrhotite bodies; and 183
degrees-212 degrees C and 3.2-4.0 wt % NaCl equiv in the Zn-Pb ores).
These values are similar to those obtained for fluid inclusions in
quartz and sphalerite from the second-stage ores (187 degrees-293
degrees C and 0.2-5.2 wt % NaCl equiv in the enargite-pyrite veins: 178
degrees-265 degrees C and 0.2-7.5 wt % NaCl equiv in quartz of
carbonate-replacement orebodies; 168 degrees-999 degrees C and 3-11.8 wt
% NaCl equiv in sphalerite of carbonate-replacement orebodies; and 245
degrees-261 degrees C and 3.2-7.7 wt % NaCl equiv in quartz from
Venencocha). Oxygen and hydrogen isotope compositions oil kaolinite from
carbonate-replacement orebodies (delta(18)O = 5.3-11.5%o, delta D = -82
to -114%o) and on alunite from the Venencocha and Santa Rosa areas
(delta(18)O = 1.9-6.9%o, delta D = -56 to -73%o). Oxygen isotope
compositions of quartz from the first and second stages have 6180 values
from 9.1 to 1.7.8 per mil. Calculated fluids in equilibrium with
kaolinite have delta(18)O values of 2.0 to 8.2 and delta D values of -69
to -97 per mil; values in equilibrium with alunite are -1.4 to -6.4 and
-62 to -79 per mil. Sulfur isotope compositions of sulfides from both
stages have a narrow range of delta(34)S values, between -3.7 and +4.2
per mil; values for sulfates from the second stage are between 4.2 and
31.2 per mil. These results define two mixing trends for the ore-forming
fluids. The first trend reflects mixing between a moderately saline
(similar to 10 wt % NaCl equiv) magmatic end member that had degassed
(as indicated by the low delta D values) and meteoric water. The second
mixing indicates condensation of magmatic vapor with HCl and SO(2) into
meteoric water, which formed alunite.
The hydrothermal system at Cerro de Pasco was emplaced at a shallow
depth (similar to 500 m) in the epithermal and upper part of a porphyry
environment. The similar temperatures and salinities obtained for the
first stage and second stages, together with the stable isotope data,
indicate that both stages are linked and represent successive stages of
epithermal polymetallic mineralization in the upper part of a porphyry
system.