XIII Congreso Peruano de Geología.

Resúmenes Extendidos
Sociedad Geológica del Perú

ALTERATION PATTERNS AROUND THE CU-ZN

ANTAMINA SKARN DEPOSIT, ANCASH, PERU

Abraham Escalante1, Greg Dipple1, Richard Tosdal1, Manuel Pacheco2 & Eric Lipten2

Mineral Deposit Research Unit, University of British Columbia1; Compañia Minera Antamina S.A.2
aescalan@eos.ubc.ca

INTRODUCTION

The Antamina copper-zinc skarn deposit is located 270 km northeast of Lima in the eastern flank of the western
Peruvian Andes, at 9°32’S and 77°03’W, and at 4300 m altitude. This deposit provides an excellent site to
document the vertical extent of mineralization and fluid flow escape features because of its sheer size and the
topographic relief in the area. Fluid flow structures were examined adjacent to and above the skarn system to
characterize the nature of the fluid escape, and to identify the widest and most pervasive alteration halos.
Knowledge of distal alteration around a skarn deposit like Antamina may help to the identification of similar
concealed deposits.

DEPOSIT GEOLOGY

The Antamina skarn deposit (Fig. 1) is hosted in limestone and marls that form the transition between the Upper
Cretaceous Jumasha and Celendin Formations (Love et al., 2004; Petersen, 1965).
The deposit consists of several high-grade copper and zinc skarn ore bodies developed on a series of Miocene
high-level quartz-monzonite porphyries and dikes dated at 9.8 Ma (McKee et al., 1979) and 10.32 ± 0.09 Ma
(Love et al., 2004). This deposit is recognized in a NE-SW elliptical area of 4.5 x 2 km that coincides with a
subtle deflection zone of the NW trending, Incaic thrust and fold belt (Benavides, 1999; Love et al., 2004).
Thrust faults at Antamina are thought to have been reactivated as extensional structures in the Late Miocene at
the time of intrusion and skarn formation (Redwood, 1999).

Ore and hydrothermal alteration zoning are controlled by sedimentary layering, dike emplacement, and pre-
existing fold and thrust geometry (Figs. 2 & 3B). Four visible alteration zones are recognized around the central
quartz-monzonite porphyry: a pink to brown skarn altered intrusion (endoskarn), a brown to green garnet skarn
(exoskarn), a white and gray marble zone, a brown hornfels and marble zone, and a external zone of gray and
light green hornfels. Outside the hornfels zone fossiliferous gray limestone predominates.

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Sulfide mineralization is concentrated in massive sulfide bodies and crosscutting veinlets especially within
garnet-bearing rocks, and it comprises pyrite, chalcopyrite, and sphalerite, with lesser amounts of magnetite,
pyrrhotite, bornite, molybdenite, and copper-bismuth sulfosalts. Galena, bournonite, realgar and proustite appear
in traces amounts.
Distal alteration at elevations above and to the north of the Antamina pit is concentrated along quartz feldspar
porphyry dikes (QFP) that connect the Antamina stock to the poorly mineralized Condorcocha skarn (Fig. 2).
Meter-scale hornfels alteration aureoles enclose the dikes. These dikes are accompanied by multiple generations
of centimeter-scale calcite ± base metal sulfide veinlets and quartz + fluorite ± calcite ± sulfide veinlets. These
veinlets occur parallel and crosscutting the QFP dikes suggesting that the dikes acted as conduits for escaping
fluids.

FLUID FLOW PATTERNS AROUND ANTAMINA

Fluid conduits at Antamina are massive skarn, white marble and hornfels layers, quartz-feldspar porphyry dikes,
and pre-existing thrust faults. A systematic record of the macroscopic visible alteration features, trace element
geochemistry, oxygen and carbon isotopes and the fluorescence signature of the calcite veinlets around these
conduits were evaluated in order to determine the magnitude and extent of the water rock interaction and link it
to the mineralizing source.

VISIBLE ALTERATION

HOST ROCKS
Massive to laminated limestone to marly limestone is the predominant rock type between Antamina and
Condorcocha skarn deposits. Hornfels forms the outer alteration zone to both skarn systems and also occurs
adjacent to QFP dikes. Marble appears closer to the Antamina skarn whereas it is less abundant at Condorcocha.
Four different hornfels and marble units are recognized at Antamina based on color: brown, gray, green and
white. Color variation is related to the presence or absence of calc-silicate + sulfides and graphite. Gray hornfels
and gray marble units reflect thermal metamorphism of gray limestone of variable marl content. Brown hornfels
and brown marble derive their color from biotite. Some brown hornfels and marble are stratigraphically
controlled layers and may represent thermal metamorphism. Green hornfels is a local variation of gray and
brown hornfels associated with abundant calc-silicate + calcite + sulfide veins and lenses.
White hornfels and white marble reflect bleaching of gray hornfels and marble. This color change corresponds to
a loss of organic matter. Three scales of bleaching are recognized: decameter-scale pervasive massive bleaching
(Figs. 3A & 3C), meter-scale bedding-controlled bleaching (Fig. 3B) and centimeter-scale bleaching halos to
veins (Fig. 3D). Bedding controlled bleaching extends to approximately 200 meters and locally up to 500 meters
from the pluton and ore zone, representing also fluid escape paths. Centimeter-scale diffusional bleaching
represents limited leakage from planar hairline pyrite ± chalcopyrite ± sphalerite veins.

VEINS
Crosscutting relations indicate at three-stage chronology of veining relative to the timing of mineralization.
White ptygmatic, centimeter- scale, discontinuous and sometimes fibrous calcite veins predate mineralization.
Syn-mineralization veins comprise porphyry-type veinlets, calc-silicate + calcite ± sulfides and calcite +
sphalerite + galena ± chalcopyrite + realgar veins throughout the skarn-marble-hornfels zone; and quartz +
fluorite ± calcite ± base metal sulfide veins associated to the QFP dikes in the periphery of the skarn. Calcite ±
sulfide veins (mainly pyrite) adjacent to QFP dikes persist for more than 1km from the mine and represent the
distal equivalent to quartz-fluorite-sulfide veins. Brown to orange red centimeter- scale carbonate veinlets (Fig.
3E), centimeter-scale, dark brown, Mn-oxide rich calcite veins (Fig. 3F), and local dolomite stringers are the
most distal expressions at elevations above the skarn. Post mineral veins are represented by white to light gray
planar, millimeter scale calcite veinlets with local trace amounts of pyrite.

SUMMARY OF MAIN POINTS

Visible alteration features are conspicuous close to the Antamina skarn and are bleaching of marble and hornfels,
and sulfide, quartz + sulfide ± fluorite and calc-silicate + sulfide veins. These features extend up to 200 meters
vertically from the main skarn body. Gray and brown marble and hornfels extend up to 800 meters laterally and
300 meters vertically above the pluton and ore zone. Outside the hornfels zone, visible alteration halos (calcite +
sulfide veins) are close to the QFP dikes.

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Fig. 3. A. Massive and bedding controlled bleached Fig.3. B. Bedding controlled bleached marble and hornfels
marble adjacent to skarn mineralization at Antamina crosscut by a QFP dike at the Quarry Bench (Antamina)

Fig. 3. C. Close up of the massive bleaching zone Fig. 3. D. Diffusion bleaching halos around calcsilicate +
showing some relicts of stylolites (Antamina) sulfide veins that cross cut gray marble units (Antamina)

Fig. 3. E. Orange to brown carbonate veinlets close Fig. 3. F. Mn-oxide rich calcite veins at the Fortuna area
to a Ag-base metal vein at the Fortuna area

TRACE ELEMENT GEOCHEMISTRY

Rock sampling along transects perpendicular to the major fluid escape paths indicate that trace element
abundances vary substantially within outcrops and consistently with proximity to these conduits. Some of the
threshold values (ppm) used for alteration halos definition were: Ag 0.1, As 10, Bi 0.1, Cu 10, Hg 0.01, Mn 800,
Mo 1, Pb 25, Sb 1, Tl 1 and Zn 100. Trace element halos, in some cases greater than 100m in thickness, surround
all major fluid conduits. High temperature indicator elements (Cu, Bi and Mo) form large geochemical halos
close to the skarn deposit, whereas moderate to low temperature indicator elements (Mn, Hg and Tl) define halos
at higher levels. Halos of 50m width in average for Ag, As, Pb and Sb appear at both depth and shallow levels.
These halos match the dominant sulfide minerals (galena, sphalerite) found in the veins associated with the QFP
dikes.

ULTRAVIOLET FLUORESCENCE

The basis for the use of this tracer is that ultraviolet fluorescence (UVF) of calcite veins with anomalous
manganese contents yields orange red fluorescent colors to the UV light (White, 1975). We observed that
orange-red ultraviolet fluorescence in calcite is restricted to zones of visible hydrothermal alteration and to syn-
mineralization veins. A UVF halo extends approximately 100m into marble adjacent to skarn and appears to

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track fluid circulation. In general, two zones of fluid flow and fracture permeability are identified with the UV
light on veins: the bleached marble halo adjacent to exoskarn and fracture permeability system centered on QFP
dikes above and distal to the skarn.

OXYGEN AND CARBON ISOTOPES

The isotopic composition of the unaltered limestone around Antamina ranges between 21 and 25 δ18O per mil
VSMOW, and 1 to 3 per mil δ13C VPDB. In general, there is a decrease in the δ18O and δ13C values of carbonate
units around the dikes and veins outside Antamina. These depletions mark major fluid escape paths at Antamina
and are consistent with a magmatic fluid source. It is worth noting that δ18O depletion halos are small and lie
within the zone of visible alteration (bleached and brown marble and hornfels) at depth. This may indicate the
effectiveness of marble as a seal to the mineralizing fluids. However, oxygen isotope depletion halos can be
traced in limestone up to 1000 meters horizontal and 500 meters vertically away from the Taco Pit. Oxygen
isotope zonation in limestone extends beyond the visible alteration zone at shallow levels indicating that fluid
escape through limestone was mainly fracture-controlled.

SUMMARY AND CONCLUSIONS

Distal alteration patterns around the Antamina

skarn deposit indicate that fracture
permeability dominates in all major fluid
conduits from deepest to shallowest levels.
The dominant fluid escape structures are
sedimentary layering and early-stage quartz
feldspar porphyry dikes (QFP). Visible
alteration is defined by the calc-silicate,
marble and hornfels halos, as well as by
centimeter-scale quartz, calcite, calc-silicate
and sulfide veins. Cryptic geochemical halos
are defined by trace element abundance,
ultraviolet fluorescence and oxygen isotope
composition (Figure 4).

Fig. 4. Visible and Cryptic distal alteration halos

around Antamina

Visible and cryptic alteration halos can be used as a model to explore skarn deposits in other areas with
geological features similar to Antamina.

ACKNOWLEDGEMENTS
This research was supported by Anglo-American Exploration (Peru), BHP Billiton, Compania de minas Buenaventura,
Compañia Minera Antamina, Noranda, Phelps Dodge Peru, Teck Cominco Peru and the Natural Sciences and
Engineering Research Council of Canada.

REFERENCES
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