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Polliand, M. and Fontboté, L. (2000) The Perubar Ba-Pb-Zn VHMS deposit, Central Peru: a possible Andean Kuroko-type equivalent in a shallow water caldera. Volcanic environments and massive sulfide deposits International Conference, Hobart, Tasmania, 16-19 November 2000. Extended abstract, accepted.

The Perubar Ba-Pb-Zn VHMS deposit, Central Peru:
a possible Andean Kuroko-type equivalent in a shallow water caldera.

M. Polliand, & L. Fontboté

Department of Mineralogy, University of Geneva, Switzerland

1. INTRODUCTION

The mid-Cretaceous volcano-sedimentary sequences of the Casma Group in Central Peru hosts several Volcanic Hosted Massive Sulfide (VHMS) deposits (Vidal, 1987). One of the major Ba-Zn-Pb VHMS deposits is Perubar, formed by the Graciela, Juanita, Cecilia North and Cecilia South orebodies (Polliand et al., 1999, Vidal, 1987). The mine, which is owned by Perubar S.A. (Glencore), is situated in the Cocachacra mining district, 50 km east of Lima. From 1978 to 1999, 5.581 Mio tons of massive sulfide ore have been extracted from the four main orebodies, with an average grade of 9.9% Zn and 1.4% Pb.

The aim of this study is to try to constrain the tectonic, volcanic and sedimentary environment that prevailed during formation of the Perubar deposit as an example of Mesozoic VHMS deposits in an ensialic back-arc marginal basin in the Central Andes.

2. GEOLOGICAL AND SEDIMENTOLOGICAL SETTING

The mid-Cretaceous Casma Group was deposited during the development of the Peruvian Huarmey-Cañete extensional marginal basin (Benavides, 1999; Atherton and Webb, 1989) during Aptian to Middle Albian times. Always according to these authors, the crust splitted and the basin was floored by mantle material. This is in agreement with the 3.0 g/cm³ arch-like structure found beneath the basin and considered by Couch et al. (1981) to be due to fracturing and insertion of material from the mantle. From its central part to the east, the Casma basin shows a marked polarity: a deep, mainly basaltic central facies grading progressively to a shallower, more acidic, and generally more pyroclastic eastern facies (Atherton and Webb, 1989). The rocks of the western part of the basin are not exposed on land. The Casma rocks outcroping at Perubar are considered to be part of the eastern facies.

The stratigraphy of the VHMS hosting Casma Group at Perubar consists of four main units. The Footwall Unit (1) consists of a thick sequence (> 2km) of interbedded marine volcanogenic mudstones, siltstones and sandstones intercalated with submarine tuffs and basaltic to andesitic lava horizons with calc-alkaline and tholeiitic affinities. Towards the top of this unit, basaltic and andesitic lavas, hyaloclastite breccias and peperites become more predominant and indicate an increasing and more proximal volcanic activity. In addition, a dense network of gabbroic to dioritic sills intruded the Footwall Unit. The overlying 100 to 150 m thick Prospective Unit (2) is characterised by the onset of bimodal volcanism and consists of submarine volcanic rocks and pyroclastic deposits ranging from basaltic-andesitic to rhyodacitic composition intercalated with volcaniclastic sandstones, tuffaceous mudstones, siltstones, and impure limestones. The massive sulfide lenses are found in this unit, at about 80 to 100m above the last encountered sill of the Footwall Unit, just on top of an impure limestone horizon and in close association with andesitic to rhyodacitic lavas, hyaloclastites and pyroclastic rocks. The 50 to 100 m thick overlying Hangingwall Unit (3) consists of similar assemblages than found in the underlying Prospective Unit, but contains more abundant felsic volcanic compounds and is characterized by extensive mass flows, slumps, and polymictic breccias. The Upper Unit (4) consists of more than 200 m of relatively shallow marine volcanic and pyroclastic rocks possibly representing debris flow deposits.

From Upper Cretaceous to Eocene, the Huarmey-Cañete marginal basin underwent compressive tectonic episodes and intrusion of the Peruvian Coastal Batholith, resulting in a progressive uplift and folding of the basin and development of contact metamoprhism aureoles in the vicinity of intrusive contacts. The Casma rocks in the Cocachacra mining district are located in a roof pendant intruded by two granodioritic plutons and underwent contact metamorphism up to amphibole (volcanics), biotite-sillimanite (volcaniclastic mudstones and sandstones) and pyroxene-garnet-calcosilicate (calcareous sediments) hornfels facies. Pyrite-pyrrhotite-magnetite assemblages formed in iron-rich massive sulfide zones. Whole-rock geochemistry of the volcanic host rocks did not significantly change since the VHMS hydrothermal event, as also confirmed by sulfur isotope data showing that closed system conditions prevailed during contact metamorphism (Polliand et al., 1999).

3. ORE SETTING

The massive sulfide orebodies at Perubar were dislocated shortly after their deposition following a very active fault-block subsidence event. Massive sulfide lenses were separated from their main feeder zones and partly mobilized along the margin of subsiding blocks, producing massive sulfide slumping and brecciation. Once put back in its original position, the Perubar VHMS deposit presents a typical proximal-to-distal zonation, from: (i) pyrite-pyrrhotite-sphalerite (+ rare chalcopyrite) stockwork, (ii) massive pyrite-pyrrhotite-magnetite, (iii) massive Zn, (Fe>Pb) sulfides (±barite), (iv) banded barite and Zn, (Fe>Pb) sulfides, and (v) banded barite-pyrite. Cu is quasi-absent in the system.

4. HYDROTHERMAL ALTERATION

Typical footwall hydrothermal alteration zones (or their metamorphic equivalents) have been recognized during the present study. They range from a strongly chloritized-sericitized core (stockwork zone) to a peripheral extensive quartz-sericite alteration zone, characterized by strong silicification, strong Na-depletion, and Ba and K-enrichment (Table 1). At the scale of the district, footwall rocks present widespread silicification and pyrite disseminations.

In addition, regional scale Na-metasomatism is recognized in the least altered mafic volcanic rocks (e.g., samples 2 and 4 in Table 1) and represents the background alteration at Perubar, attributed to early and/or late hydrothermal processes perhaps related to regional-scale burial metamorphism.

5. GEOLOGICAL EVOLUTION

In the Cocachacra district, at the end of the relatively deep marine sedimentation of the Footwall Unit, formed during a main subsiding phase, carbonate sedimentation started abruptly, indicating a very rapid uplift up to a relatively shallow-water level. This dynamic uplift was rapidely followed by the onset of bimodal volcanism together with an increment of the volcanic activity. The Perubar VHMS deposit formed during this period of coeval bimodal volcanism and carbonate sedimentation, while the tectonic regime returned to (incipient) subsiding conditions.

Almost contemporaneously (slightly before or after) to the deposition of the ore, a dioritic sill-like stock intruded the footwall sequence. Gibson et al. (1999) interprete large, sill-like, multiphase subvolcanic intrusions which are formed in several VHMS environments as the intrusive equivalent of deeper magma chambers that fed the volcanic succession.

Thus, the dioritic subvolcanic stock observed at Perubar may suggest the presence of a relatively high crustal magma chamber at the time of mineralization, which could have generated the local heat flow necessary to activate a seawater convection cell into the footwall volcanosedimentary pile.

Shortly after the VHMS mineralizing event, a very abrupt and chaotic subsidence regime started, spliting the seafloor into deeply subsiding small sub-basins delimited by volcano-tectonic faults spaced commomly at <<500 m. It resulted in the dislocation (downward translation, slumping and brecciation) of the Perubar VHMS deposit and emplacement of the chaotic Hangingwall Unit. Together with this ongoing strong fault-block subsidence, volcanic activity became more and more predominant. Subsequently, a major explosive volcanism event probably was responsible for the deposition of the Upper Unit volcanic/pyroclastic debris flow-like sequence.

6. CONCLUSION

Within the regional context of the Peruvian Huarmey-Cañete extensional marginal basin, the complex sedimentary and volcanic evolution recorded in the Casma rocks of the Cocachacra district, together with evidences of rapid uplift followed by abrupt fault-block subsidence suggest the existence of a local submarine caldera system at Perubar. According to Ohmoto (1996), the formation of submarine calderas played a key role in the genesis of VHMS deposits in the Japanese Hokuroku district. Under extensional basin settings, it is more likely to develop piecemeal caldera collapse rather than piston-like subsidence, as mentioned by Kokelaar (1992). The chaotic fault-block subsidence recorded in the Hangingwall Unit could correspond to such a piecemeal caldera collapse. The VHMS mineralization at Perubar most likely was emplaced in an intracaldera incipient depression, prior to the main caldera collapse. This situation differs slightly from the post-caldera collapse emplacement generally observed in Kuroko deposits. The narrow collapsing sub-basins at Perubar probably developed in fault-splays above steep, crustal-penetrating discontinuities that focused plumbing of magmas, basement fracturing and hydrothermal fluid discharges at these sites.

Therefore, considering the Perubar deposit massive sulfide mineralogy, footwall alteration characteristics, volcanic environment and tectonic setting, the denomination of Kuroko-type deposit seems to be appropriate for this Andean VHMS mineralization. However, it most likely formed at shallow seawater depth (<500 m?), as indicated by the presence of limestones in the basin at the time of mineralization. Moreover, the general low Cu content at Perubar suggests that the temperature of the system was lower than for most Kuroko deposits, as the hydrothermal fluid did not effectively transport this element. Considering a fluid temperature <300°C with a salinity higher than that of normal seawater, the hydrostatic condition required for the formation of a Zn-rich and Cu-poor VHMS deposit like Perubar might have been attained at a seawater depth <500 m.

This is a contribution to the GEODE program funded by the Europe Science Foundation.

7. REFERENCES

Atherton, M. P. and Webb, S. 1989. Volcanic facies, structure, and geochemistry of the marginal basin rocks of central Peru; Journal of South American Earth Sciences, v. 2, no 3, p. 241-261.

Benavides, V. 1999. Orogenic evolution of the Peruvian Andes: The Andean Cycle. Geology and ore deposits of the Central Andes, SEG Spec. Pub., v. 7, p. 61-107.

Couch, R., Whitsett, R., Huehn, B. and Briceño-Guarupe, L. 1981. Structures of the continental margin in Peru and Chile, in Kulm et al., eds., Nazca plate: Crustal formation and Andean convergence: Geological Society of America Memoir 154, p. 703-726.

Gibons, H. L., Morton, R. L. and Hudak, G. J. 1999. Submarine volcanic processes, deposits, and environments favorable for the location of volcanic-associated massive sulfide deposits, Reviews in Economic Geology, v. 8, p. 13-51.

Kokelaar, P. 1992. Ordovician volcanic and sedimentary record of rifting and volcanotectonism: Snowdon, Wales, United Kingdom, Geological Society of America Bulletin, v. 104, p 1433-1455.

Ohmoto, H. 1996. Formation of volcanogenic massive sulfide deposits: The Kuroko perspective. Ore Geology Reviews, 10: 135-177.

Polliand, M., Fontboté L. and Spangenberg, J. 1999. Tracing back sulfur isotope reequilibration due to contact metamorphism: A case study from the Perubar VMS deposit, Central Peru. Mineral Deposits: Process to Processing, Stanley et al. (eds), Balkema, Rotterdam, v. 2, p. 967-970.

Vidal, C. E., 1987, Kuroko-Type Deposits in the Middle Cretaceous Marginal Basin of Central Peru, Econ. Geol., v. 82, p. 1409-1430.