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Bendezú, R., and Fontboté, L., 2002, Late timing for high sulfidation cordilleran base metal lode and replacement deposits in porphyry-related districts: the case of Colquijirca, central Peru. SGA News, nr. 13, p. 1, 9-13.

 

Late timing for high sulfidation cordilleran base metal lode and replacement deposits in porphyry-related districts:
the case of Colquijirca, central Peru

R. Bendezú ^1,2 and L. Fontboté ¹

¹ Section des Sciences de la Terre, Université de Genève, Switzerland
² Sociedad Minera El Brocal S. A., Lima Peru

Introduction

"Cordilleran base metal lode" deposits (Einaudi, 1977 and 1982) or "cordilleran veins" (Sawkins, 1972) depicted as parts of the upper construct of porphyry copper systems, have the following main characteristics (e.g., Einaudi, 1982): (i) high sulfidation state of the mineral assemblage with associated advanced argillic to sericitic alteration, (ii) strong zoning from Cu cores to external Zn-Pb(Ag) ores, (iii) massive texture and sulfide content of up to more than 50% in volume, considerably higher than in other porphyry-related ore type, and (iv) basically a common suite of enriched metals, that is, Cu-Zn-Pb±(Ag-Au).
It is documented from different parts of the world that numerous sulfide-rich veins, some of them that can be classified as "cordilleran base metal lodes", are superimposed on the upper parts of porphyry copper deposits and cut earlier veins of the potassic and phyllic alteration assemblages (e.g., Meyer and Hemley, 1967; Brimhall, 1979; Fréraut et al., 1997; Brathwaite et al., 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. This is the case of the polymetallic belt of central and northern Peru, specifically in the Colquijirca, Cerro de Pasco, and Hualgayoc districts where important parts of the base metal mineralization are spatially related to Au high sulfidation epithermal ores (Fontboté and Bendezú, 1999; Bendezú and Fontboté, 2002; Baumgartner et al., 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 My) high sulfidation epithermal precious metal mineralization.

Geometric and temporal relationship of ore types at Colquijirca

In the center of the Colquijirca district, a 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 (Pocobamba Formation) to the north, and marine, Triassic-Jurassic, nearly pure limestones and dolostones (Pucará Group) 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 et al., submitted) 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 (12.9-12.4±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 et al., 1984).

Fig. 1. Geological map of the Cerro de Pasco, Colquijirca, and Atacocha districts showing the distribution of the different porphyry-related mineralization types. Geology compiled from Angeles (1999), Johnson et al., (1955), Brocal S.A. staff and own data. [Click on the figure to enlarge it]

The precious metal high sulfidation epithermal mineralization includes pervasive alteration of almost the whole diatreme-dome complex to assemblages of quartz - alunite - dickite -kaolinite ± (pyrophyllite - zunyite - illite) in mineralized areas and to kaolinite - illite ± (smectite) - sericite - chlorite - 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 (e. g., Summitville, in Gray et al., 1994), deep portions of non-oxidized ores contain characteristically <5 vol% of finely disseminated sulfide minerals, mainly pyrite-enargite and very minor sphalerite. The ⁴⁰Ar/³⁹Ar ages (11.6 to 11.3±0.1 Ma) on alunites from different parts 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 and 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 and 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 (see below).

Fig. 2. North-south longitudinal section of the Colquijirca district with main ore and alteration associations. ⁴⁰Ar/³⁹Ar determinations from Bendezú et al. (submitted). [Click on the figure to enlarge it]

Hosted by carbonate rocks of the Tertiary Pocobamba Formation, 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 pyrite-galena-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. [Click on the figure to enlarge it]

Discussion

On the basis of the current knowledge on the lifespan of a single intrusion related hydrothermal system (< 40000 years; e.g., Marsh et al., 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 My, Bendezú et al., 2002) is representative of a relatively long-lived hydrothermal activity which probably was sustained by multiple pulses of shallow intrusions (Cathles et al., 1997), as indicated by the existence of several hundreds meter-wide stocks, specially on the immediate edge of the diatreme vent.
In the Maricunga belt, Chile, based on detailed mapping and ⁴⁰Ar/³⁹Ar geochronology, Muntean and 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 to 0.6 My) 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.

Fig. 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. [Click on the figure to enlarge it]

Conclusions

The relative sequence of events and the absolute ages available in the Colquijirca district 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 (e.g., Hualgayoc, Cerro de Pasco) represent high-level equivalents of the superimposition of deeper mineralization events noted by Brimhall (1979) and Einaudi (1982, 1994) in the upper part of porphyry copper and/or skarn deposits.
The results presented here have important implications for exploration, 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 (e.g., San Gregorio) and for lateral distances as far as more than 4 km from the intrusive center.

Acknowledgements

A previous version of the manuscript was reviewed by R. Goldfarb and E. Marsh (Denver). The present investigation is being carried out with the support of the Sociedad Minera El Brocal S.A. and the Swiss National Science Foundation (FN 2000-062000.00).

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