Ultimas interpretaciones para yacimientos de alta sulfuración
cordilleranos de metales base y de reemplazamiento en distritos
relacionados a pórfidos:
el caso de Colquijirca, Perú central.
R. Bendezú
1
1,2
y L. Fontboté
1
Section des Sciences de la Terre, Université de Genève, Switzerland
2 Sociedad Minera El Brocal S. A., Lima Perú
Introducción
Los yacimientos "Cordilleranos de metales base" (Einaudi, 1977 y 1982) o "vetas
cordilleranas" (Sawkins, 1972) son representadas como partes de una construcción superior
de sistemas de pórfidos de cobre, tienen las siguientes características principales (ejemplo,
Einaudi, 1982): (i) un estado de alta sulfuración del ensamblaje mineral con alteración
argílica avanzada asociada a sericítica, (ii) fuerte zoneamiento desde núcleos de Cu a
minerales externos de Zn-Pb(Ag), (iii) textura masiva y contenido de sulfuros en mas del
50% en volumen, considerablemente mas alto que otro tipo de mineralización relacionado a
pórfidos, y (iv) básicamente un lugar común para metales enriquecidos, esto es, Cu-ZnPb±(Ag-Au).
Esto está documentado en diferentes partes del mundo que numerosas vetas ricas en
sulfuros, algunos de los cuales que pueden ser clasificados como "yacimientos cordilleranos
de metales base", están súper impuestas en las partes superiores de yacimientos de pórfidos
de cobre and cut earlier veins of the potassic and phyllic alteration assemblages (ejemplo,
Meyer y Hemley, 1967; Brimhall, 1979; Fréraut y otros, 1997; Brathwaite y otros, 2001).
Einaudi (1977, 1982, 1994) points out the late stage character of the fluids that form this
type of base metal-rich deposit within the moderately deep porphyry-skarn environment. At
shallower environments, these fluids can also generate different styles of high sulfidation
cordilleran base metal lodes and replacement bodies. Este es el caso de las franjas
polimetálicas del Perú central y norte, específicamente en los distritos Colquijirca, Cerro de
Pasco y Hualgayoc donde partes importantes de la mineralización de metales base estan
espacialmente relacionadas a los minerales epitermales de alta sulfuración de Au (Fontboté y
Bendezú, 1999; Bendezú y Fontboté, 2002; Baumgartner y otros, 2002).
One of the open fundamental questions is to determine when cordilleran base metal lodes
and replacement deposits are formed in the evolutionary history of porphyry-related
hydrothermal systems. Are these ores formed from late stage fluids, as those responsible for
sulfide-rich "D"-veins in porphyry copper deposits and, according to Muntean and Einaudi
(2001) and at least in the Maricunga belt, for coetaneous high sulfidation epithermal precious
metal mineralization? Or do they correspond to even later fluids? This contribution presents
geometric and 40Ar/39Ar geochronological evidences indicating that at Colquijirca high
sulfidation cordilleran base metal lodes and replacement deposits are post (~0.6 Ma) high
sulfidation epithermal precious metal mineralization.
Relación geométrica y temporal de los tipos de mineral en Colquijirca
En el centro del distrito de Colquijirca, area 4 km2, Miocene, mainly dacitic, diatreme-dome
complex intrudes a >300-m-thick sequence of folded carbonate rocks, continental Eocene
limestones, marls and detritic sediments (Formación Pocobamba) to the north, and marine,
Triassic-Jurassic, nearly pure limestones and dolostones (Grupo Pucará) to the south
(Figs. 1, 2). The entire sedimentary pile surrounding the diatreme neck, including red beds
of the Permo-Triassic basement, has subsided about 400-500 m (Fig. 2). A high precision
40Ar/39Ar survey (Bendezú y otros, presentado) reveals two pulses of high sulfidation
hydrothermal activity at 11.6±0.1 to 11.3±0.1 Ma (precious metals) and at 10.8 to 10.6
±(0.06-0.1) Ma (base metals). They post-date formation of the volcanic complex (12.912.4±0.1 Ma.) as determined from 40Ar/39Ar dating of biotite samples from less altered
adjacent dacitic domes. Previous K/Ar dating from other areas of the volcanic complex is
consistent with the new data (Vidal y otros, 1984).
Figura 1 Plano geológico de los distritos de Cerro de Pasco, Colquijirca, y Atacocha
mostrando la distribución de los diferentes tipos de mineralización relacionados a los
pórfidos. Geología recopilada de Angeles (1999), Johnson y otros, (1955), staff de Brocal
S.A. staff y datos propios.
La mineralización epitermal de alta sulfuración de metales preciosos incluyen alteración
pervasiva de almost the whole diatreme-dome complex to assemblages of cuarzo - alunita
- dickita -caolinita ± (pirofilita - zunyita - illita) en areas mineralizadas y a caolinita - illita ±
(esmectita) - sericita - clorita - calcite beyond them. Shallow Au-(Ag) "vuggy silica"
orebodies, as much as 100 m in vertical extent, exist mainly in the central portion of the
complex, mostly within the diatreme breccia and pyroclastic infill, following a morphology
apparently controlled by permeability (Fig. 2). As in other Au high sulfidation epithermal
deposits (ejemplo, Summitville, en Gray y otros, 1994), deep portions of non-oxidized ores
contain characteristically <5 vol% of finely disseminated sulfide minerals, mainly pyriteenargite and very minor sphalerite. Las edades 40Ar/39Ar (11.6 a 11.3±0.1 Ma) en alunitas
de partes diferentes of the diatreme-dome complex, including a strongly altered dome 2 km
south of the diatreme vent, indicate that the precious metal high sulfidation epithermal
mineralization constitutes the first recognizable hydrothermal pulse within the Colquijirca
district.
The economically more important cordilleran base metal lodes and replacement deposits
represent the other distinctive type of mineralization in the district. The ores consist of at
least 150 Mt of Cu-Zn-Pb-Ag±(Au-Bi) high sulfidation, carbonate-hosted, massive
replacement (Fontboté y Bendezú, 1999, 2001). Replacement temperatures range from
about 300°C, near the diatreme-dome complex, to 150°C, in external positions. The ores
contain typically between 20 and 40 vol.% sulfide minerals, mainly in the form of flat,
elongated mantos and irregular stacked bodies that extend from the external margins of the
diatreme vent into the carbonate rocks (Figs. 1 y 2). Exploration drilling indicates that the
bodies have their roots within the diatreme complex, in deep subvertical narrow veins
composed mainly of enargite-pyrite-quartz-alunite and minor pyrophyllite (Fig. 2). The veins
have been intercepted between 500 m and >750 m depth with a mineral association similar
to that of the core of the mantos (ver abajo).
Fig. 2. Sección longitudinal norte sur del distrito de Colquijirca con las principales
asociaciones minerales y de alteración. Las determinaciones 40Ar/39Ar según Bendezú y
otros (presentado).
Hospedados en las rocas carbonatadas de la Formación Pocobamba del Terciario, the
mantos extend continuously for almost 4 km in length to the north, where they virtually
attain the same shallow topographic level of the precious metal high sulfidation epithermal
ores. Along this extension, the mantos are zoned in all directions from a Cu core of high
sulfidation mineralization composed of enargite - pyrite - quartz - alunite ± (luzonite,
colusite, barite; Zone I). The surrounding economic ore zones include pyrite - chalcopyrite dickite - kaolinite - siderite - quartz± tennantite, Bi-Ag sulphosalts, bornite, alunite, barite,
quartz; Zone II); pyrite - sphalerite - galena - chalcopyrite - dickite - kaolinite - quartz ±
(siderite, hematite, magnetite, alunite; Zone III); and an outermost known zone of pyritegalena-sphalerite-siderite±(kaolinite, dolomite, Zn-bearing carbonates; Zone IV). A similar
zoning is also present to the south (Fontboté and Bendezú, 2001), where ore bodies
develop not only within the Pocobamba Formation, but mainly in Lower Jurassic carbonate
rocks of the Pucará Group, which constitutes the host rock of San Gregorio, one of the
biggest Zn-Pb deposits in Peru (> 70 Mt @ Zn+Pb~10 %). Replacement at San Gregorio
took place at very low temperature (</= 150°C), yielding fine-grained and not easily
recognizable ores.
The superimposition of the cordilleran base metal lodes and replacement deposits on the
Au-(Ag) high sulfidation epithermal mineralization is evidenced from crosscutting
relationships. In the northern flank of the diatreme vent, alteration selvages from the
massive enargite-quartz-alunite ores overprint weakly argillized/propylitized dacitic domes
and phreatomagmatic breccias related to the Au-(Ag) mineralization. To the east of the
diatreme vent, the argillic associations related to the Au(Ag) ores are cut by narrow
sphalerite-galena-pyrite veins. These geometrical relationships are totally consistent with
40Ar/39Ar absolute ages obtained on alunite associated with the enargite-pyrite and
sphalerite-galena zones of the "cordilleran base metal lodes", which have ages between
10.8 and 10.6 ±(0.06-0.1) Ma, i.e. about 0.6 My younger than the precious metal
mineralization.
Fig. 3. Extension of the diagram of Muntean and Einaudi (2001), which represents the
timing of A veinlets, D veinlets and precious metal high sulfidation epithermal veins in the
Maricunga belt, to the high sulfidation cordilleran base metal environment, using the data of
the Colquijirca district.
Discusión
On the basis of the current knowledge on the lifespan of a single intrusion related
hydrothermal system (< 40000 años; ejemplo, Marsh y otros, 1997), the total duration of
the hydrothermal activity recognized in each of the pulses in the Colquijirca district (in the
range of 0.2-0.35 Ma, Bendezú y otros, 2002) is representative of a relatively long-lived
hydrothermal activity which probably was sustained by multiple pulses of shallow intrusions
(Cathles y otros, 1997), as indicated by the existence of several hundreds meter-wide
stocks, specially on the immediate edge of the diatreme vent.
En la Franja Maricunga, Chile, basados en mapeos detallados y en geocronología
40Ar/39Ar, Muntean y Einaudi (2001) favor a model whereby high sulfidation epithermal ores
and porphyry-related late stage "D" veins are temporally and genetically linked and both
postdate by several hundreds of thousands of years the early A veining (Fig. 3). At
Colquijirca, the long time gap (0.5 a 0.6 Ma) between the precious metal epithermal
mineralization and the younger cordilleran high sulfidation base metal ores indicates that
the latter formed by even later hydrothermal fluids, and combining with the results from the
Maricunga belt, they would also postdate the hypothetical "D"-vein stage (Fig. 3).
The formation of cordilleran base metal lodes in the Colquijirca District seems to represent
a late phase of the complex evolution history of a magmatic cycle active for at least two
million years through multiple intrusions related to a single intrusive center.
Figura 4. Schematic cross section through the Colquijirca district showing the spatial and
temporal distribution of the different ore types. Presence of disseminated porphyry copper
mineralization is only hypothetical, the erosion level does not allow any insight in deeper
parts of the system.
Conclusiones
La secuencia relativa de los eventos y las edades absolutas disponibles en el distrito de
Colquijirca firmly establish, for the first time, a late timing for cordilleran base metal ores,
which are mainly epithermal and display high sulfidation mineral assemblages. They
postdate the early precious metal-bearing, high sulfidation epithermal mineralization. Based
upon the sequence of events at Colquijirca, it can be hypothesized that similar crosscutting
relations present in epithermal ores at other districts (ejemplo, Hualgayoc, Cerro de Pasco)
represent high-level equivalents of the superimposition of deeper mineralization events
notados por Brimhall (1979) y Einaudi (1982, 1994) in the upper part of porphyry copper
and/or skarn deposits.
Los resultados presentados aquí tienen importantes implicancias para la exploración,
porque because many classic districts known for their porphyry copper and/or Au-(Ag)
epithermal deposits may host concentrations of "cordilleran base metal lodes" at any
spatial position upward from the porphyry environment. These may occur at levels as
shallow as the epithermal environment, which in carbonate rocks may be characterized by
fine-grained and not easily recognizable Zn-Pb mineralization (ejemplo San Gregorio) and
for lateral distances as far as more than 4 km from the intrusive center.
Agradecimientos
Una versión previa del manuscrito fue revisada por R. Goldfarb y E. Marsh (Denver). La
presente investigación fue llevado a cabo con el soporte de la Sociedad Minera El Brocal S.A.
y la Swiss National Science Foundation (FN 2000-062000.00).
Referencias
Angeles, C., 1999, Los sedimentos cenozoicos de Cerro de Pasco: estrastigrafía,
sedimentación y tectónica. In Sociedad Geológica del Perú, Volúmen Jubilar N° 5, 103-118.
Baumgartner, R., Fontboté, L., and Jobin, Y., 2002, carbonate hosted zinc-lead highsulfidation mineralization at the Cerro de Pasco deposit, Peru. XI Congreso Peruano de
Geologia, Lima.
Bendezú, R. and Fontboté, L., 2002, Au(Ag) epithermal and "cordilleran base metal lodes
and replacement bodies" two different high sulfidation mineralization types. Examples from
the Colquijirca District, central Peru. XI Congreso Peruano de Geologia, Lima.
Bendezú, R., and Fontboté, L., 2002, Spatial and temporal relations between "cordilleran
base metal lodes and replacement bodies" and precious metal high sulfidation epithermal
mineralization in the Colquijirca district, central Peru. Abstracts SEG Meeting Global
exploration 2002: Integrated methods for discovery, Denver April 14-16, 2002, 63-64.
Bendezú, R., Fontboté, L., and Cosca, M., Formation of cordilleran base metal lodes and
replacement deposits post precious metal high sulfidation epithermal mineralization in the
Colquijirca district, central Peru. Submitted to Mineralium Deposita.
Brathwaite, R.L., Simpson, M.P., Faure K, Skinner, D.N.B., 2001, Telescoped porphyry CuMo-Au mineralisation, advanced argillic alteration and quartz-sulphide-gold-anhydrite veins
in the Thames District, New Zeland. Mineralium Deposita., v. 36, 623-640.
Brimhall, G., H., Jr., 1979, Lithologic determination of mass transfer mechanisms of multiplestage porphyry copper mineralization at Butte, Montana: Vein formation by hypogene
leaching and enrichment of potassium silicate protore. Economic Geology, v. 74, 556-589.
Cathles, L.M., Erendi, A. J.H., and Barrie, T., 1997, How long can a hydrothermal system be
sustained by a single intrusive event? Economic Geology, 92, 766-771.
Einaudi, M. T., 1977, Environment of ore deposition at Cerro de Pasco, Peru. Economic
Geology., v. 72, 893-924.
Einaudi, M.T., 1982, Description of skarns associated with porphyry copper plutons,
southwestern North America. In Titley, S.R., ed., Advances in geology of the porphyry
copper deposits, south western North America: Tucson, Univ. Arizona Press, p. 139-184.
Einaudi, M.T., 1994, High sulfidation and low sulfidation porphyry copper/skarn systems:
Characteristics, continua, and causes. Society of Economic Geologists, International
exchange lecture, WEB. http:pangea.Stanford.edu/ODEX/marco-hilosulf.html
Fontboté, L and Bendezú, R., 1999, The carbonate hosted Zn-Pb San Gregorio deposit,
Colquijirca District, central Peru, as part of a high sulfidation epithermal system. In Stanley
et al., (eds.), Fifth Biennial SGA Meeting, Mineral Deposits: Processes to Processing, v. 1,
495-498.
Fontboté, L. and Bendezú, R., 2001, The carbonate-hosted San Gregorio and Colquijirca (ZnPb-Ag) deposits (central Peru) as products of an epithermal high sulfidation system. Proexplo
2001, Lima, Perú, Abril 2001, CD-ROM, doc. 18 p.
Fréraut, C. R., Ossandón, C. G., and Gustafson, L. B., 1997, Modelo geológico de
Chuquicamata. In Actas 8th Congreso Geológico Chileno, 3,1898-1902.
Gray, J. E., Coolbaugh, M. F., Plumlee, G. S., and Atkinson, W. W., 1994, Environnment
geology of the Summitville mine, Colorado. Economic Geology., v. 89, 2006-2014.
Johnson, R.F., Lewis, R.W., and Abele, G., 1955, Geology and ore deposits of the Atacocha
district, departamento de Pasco, Perú. U.S. Geological Survey Bulletin 975-E, 337-388.
Marsh T.M., Einaudi M.T., and McWilliams M., 1997, 40Ar/39Ar geochronolgy of Cu-Au and AuAg mineralization in the Potrerillos district, Chile. Economic Geology., 92: 784-806.
Meyer, C., and Hemley, J.J., 1967, Wall-rock alteration. In Barnes, H.L., ed., Geochemistry
of hydrothermal ore deposits, 1st ed., New York, Holt, Rinehart Winston, p. 166-235.
Muntean, J.L., and Einaudi, M.T., 2001, Porphyry-epithermal transition: Maricunga belt,
Northern Chile. Economic Geology., v. 96, 743-772.
Sawkins, F.J., 1972, Sulfide ore deposits in relation to plate tectonics. Journal of Geology, v.
80, 377-396.
Vidal, C., Mayta, O., Noble, D.C., and McKee, E. H., 1984, Sobre la evolución de las
soluciones hidrotermales dentro del centro volcánico Marcapunta en Colquijirca-Pasco.
Volumen Jubilar Sociedad Geológica del Perú, 10, 1-14.