The Perubar Ba-Pb-Zn VHMS deposit, Central Peru

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The Perubar Ba-Pb-Zn VHMS deposit, Central Peru

POLLIAND, Marc, FONTBOTÉ, Lluís

Abstract

The mid-Cretaceous volcano-sedimentary sequences of the Casma Group in Central Peru hosts several Volcanic Hosted Massive Sulphide (VHMS) deposits. The main mine is on the Petubar Ba-ZnPb VHMS deposit, formed by the Graciela, Juanita, Cecilia Norte and Cecilia Sur massive sulphide (±barite) bodies. The stratigraphy of the Casma Group at Peru bar consists of four units, mainly composed 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 sulphide lenses are found on top of an impure limestone interval and in close spatial association with andesitic to rhyodacitic lavas, hyaloclastites and pyroclastic rocks. The Perubar deposit was strongly dislocated shortly after its deposition following an active faultblock subsidence event.

Massive sulphide lenses were separated from their main feeder zones and partly mobilized along the margin of deeply subsiding blocks. Once put back in its original position the Perubar VHMS deposit presents a typical [...]

POLLIAND, Marc, FONTBOTÉ, Lluís. The Perubar Ba-Pb-Zn VHMS deposit, Central Peru. In:

Sherlock R.L. and Logan M.A.V.

VMS Deposits of Latin America

. Geological Association of Canada, Mineral Deposits Division, 2000. p. p. 439-446.

Available at:

http://archive-ouverte.unige.ch/unige:78475

Disclaimer: layout of this document may differ from the published version.

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MARC POLLIAND AND LLU(S FONTBOTE

Section des Sciences de la Terre, Rue des Marafchers 13, CH-1211 Geneva 4, Switzerland

ABSTRACT

The mid-Cretaceous volcano-sedimentary sequences of the Casma Group in Central Peru hosts sev- eral Volcanic Hosted Massive Sulphide (VHMS) deposits. The main mine is on the Petubar Ba-Zn- Pb VHMS deposit, formed by the Graciela, Juanita, Cecilia Norte and Cecilia Sur massive sulphide (±barite) bodies. The stratigraphy of the Casma Group at Peru bar consists of four units, mainly com- posed of submarine volcanic rocks and pyroclastic deposits ranging from basaltic-andesitic to rhyo- dacitic composition, intercalated with volcaniclastic sandstones, tuffaceous mudstones, siltstones, and impure limestones. The massive sulphide lenses are found on top of an impure limestone interval and in close spatial association with andesitic to rhyodacitic lavas, hyaloclastites and pyroclastic rocks.

The Perubar deposit was strongly dislocated shortly after its deposition following an active fault- block subsidence event. Massive sulphide lenses were separated from their main feeder zones and partly mobilized along the margin of deeply subsiding blocks. 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) sulphides (±barite), (iv) banded barite and Zn, (Fe>Pb) sulphides, and (v) banded barite- pyrite. Typical footwall hydrothermal alteration zones have been recognized and range from a strong- ly chloritized-sericitized core to a peripheral extensive quartz-sericite alteration zone. The geological evolution recorded in the Casma rocks of the Cocachacra district suggest the existence of a local sub- marine caldera system at Perubar. The deposit probably formed at shallow water depth (<500 m?), as indicated by the presence of limestones in the basin at the time of mineralization.

INTRODUCTION

The mid-Cretaceous volcano-sedimentary sequences of the Casma Group in Central Peru (Fig.

1) hosts several Volcanic Hosted Massive Sulphide (VHMS) deposits (Vidal, 1987). The main mine is on the Perubar Ba-Zn-Pb VHMS deposit (Polliand et al., 1999, Vidal, 1987), mainly formed by the Graciela, Juanita, Cecilia Nmih and Cecilia South massive sul- phide (± barite) bodies (Fig. 2). The mine, which is owned by Perubar S.A. (Glencore), is situated in the Cocachacra mining district (11 °55'S 76°34'W), 50 km east ofLima (Fig. 2). From 1978 to 1999, 5.6 mil- lion tonnes of massive sulphide ore have been extract- ed from the four main orebodies, with an average grade of9.9% Zn and 1.4% Pb. Previously, about 10 million tonnes of barite ore were mined.

In this communication, we present partial results of an ongoing project at the University of Geneva,

439

and mainly focus on the tectonic, volcanic and sedi- mentary 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.

GEOLOGICAL AND SEDIMENTOLOGICAL SETTING The mid-Cretaceous Casma Group was deposited during the development of the Peruvian Huarmey- Cafiete extensional marginal basin (Benavides, 1999;

Atherton and Webb, 1989) during Aptian to Middle Albian times. According to these authors, the basin is 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 man- tle. From its central pmi to the east, the Casma basin shows a marked polarity: a deep, mainly basaltic

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POLLIAND & FONTBOTE

N

GEOLOGICAL MAp OF CENTRAL PERU

QUATERNARY

~

0

Alluvions 'X' VMS deposit CENOZOIC VOLCANIC &

VOLCANOSEDIMENTARYROCKS

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Calipuy Group COASTAL BATHOLITH (CRETACEOUS TO NEOGENE)

Late Andean plutonic rocks (intrusive in the Calipuy Group)

Q Tonalite and granodiorite E!:l Adamellite

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MESOZOIC VOLCANIC &

VOLCANOSEDIMENTARYROCKS

Casma Group

(Aptian to middle Albian)

Figure. 1. Geological map of central Peru, modified from Megard (1978).

central facies grading progressively to a shallower, more acidic, and generally more pyroclastic eastern facies (Atherton and Webb, 1989). The rocks of the western pmt of the basin are not exposed. 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 (Fig. 3 and 4). The Footwall Unit (1) consists of a thick sequence (> 2krn) of interbedded marine vol- canogenic mudstones, siltstones and sandstones inter- calated with submarine tuffs and basaltic to andesitic lava horizons with calc-alkaline and tholeiitic affini- ties. 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 (Fig. 3). The overlying 100 to 150m

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thick Prospective Unit (2) is characterised by the onset ofbimodal volcanism and consists of submarine volcanic rocks and pyroclastic deposits ranging from basaltic-andesitic to rhyodacitic composition interca- lated with volcaniclastic sandstones, tuffaceous mud- stones, siltstones, and impure limestones. The mas- sive sulphide lenses are found in this unit, at about 80 to 100 m above the last encountered sill of the Footwall Unit, ju t on top of an impure limestone horizon and in clo e spatial a sociation with and.esitic to rhyodaciti lavas byalocla tite and pyroclastic rock (Fig. 3 and 4). The 50 to 100m thick overlying Hangingwall Unit (3) con i ts of simj far a· emblage · than fi und in the underlying Prospective Unit' bL.lt contains more abundant felsic volcanic rocks and 1 characterized by exten ive mass flow slump , and polymictic breccias. The Upper Unit (4) consists of m re than 200 m of relatively hallow marine vol- canic and pyrocla tic rocks po sibly representing

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c-:~Quall:runn lleposlt

~(nllu\•lon, colluvion)

~mr.~l'ossiblv slumpNI llili.llmn~ lvesulrlde

I t + t t

\ . prrsent tin ' rolull ( till ncivc)

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loeulion of normnl- 1111 volcnuotcctonic

yns~dlmonlar

roult

r;-, Post-batholithic diorltic

~dike, sill and apophysis 1+1 Coastal batholith (tonalite, L±.J monzogranite and granodiorite) rJil. Pre-batholithic dioritic L2J dike, sill and apophysis

Casma Group

.,....----1 (comprising the Footwu/1 Unit, Pro~peclive U11iJ.

/fanp,ingwa/1 Unit and Uppt!r Unit on the local Cocachacra dhtrict nomenclarure)

~

lava, hyaloclastite, breccia, pyroclastite, volcanoscdimcntary rocks, sill

Impure limestone

Figure. 2. Peru bar deposit geological map, Cocachacra mining district (largely modified from Perubar staff maps).

debris flow deposits.

From Upper Cretaceous to Eocene, the Huarmey- Cafiete marginal basin underwent compressive tecton- ic episodes and intrusion of the Peruvian Coastal Batholith, resulting in a progressive uplift and folding of the basin and development of contact metamo- prhism aureoles in the vicinity of intrusive contacts.

The Casma rocks in the Cocachacra mining dis- trict are located in a roof pendant (Fig. 1) intruded by two granodioritic plutons of the Peruvian Coastal Batholith and underwent contact metamorphism up to amphibole (volcanics), biotite-sillimanite (volcani- clastic mudstones and sandstones) and pyroxene-gar- net-calcosilicate (calcareous sediments) hornfels facies. Pyrite-pyrrhotite-magnetite assemblages formed in iron-rich massive sulphide zones. Whole-

rock geochemistry of the volcanic host rocks did not significantly change since the VHMS hydrothermal event, as also confirmed by sulphur isotope data showing that closed system conditions prevailed dur- ing contact metamorphism (Polliand et al., 1999).

From Oligocene to Pliocene, the Perubar deposit underwent successive Andean orogenic pulses, gener- ating dextral normal-slip faults mainly activated along corridors corresponding to pre-existing NNE- SSW trending crustal-penetrating discontinuities. It resulted mainly to dextral translations along the NNE- SSW trending Corte Ladrones and Split faults (Fig. 2).

ORE SETTING

The massive sulphide orebodies at Perubar were strongly dislocated shortly after their deposition

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POLLIAND & FONTBOTE

uu

HwU

[ZJ

Andesitic dyke

c:.:J

Coastal Batholith

I

L

I

Upper Unit (>200 m thick)

D

Hangingwall Unit (50 to I 00 m thick) - Prospective Unit (100 to 150m thick)

D

Footwall Unit (>2 km thick)

-- ~

v v

EJ D

'

Massive sulfide ±barite Massive pyrite-magnetite

Stockwork ~Slump

: • Synsedimentary breccia Lava (undiferentiated,

basaltic to rhyodacitic)

Limestone (or altered

+

metamorphic equivalent, i.e. calcosilicate hornfels) Dioritic sill

Figuer. 3. Stratigraphic column of the Casma Group outcroping in the Cocachacra mining district area.

section 1 NE

{

i

Figure. 4. Cross sections of the Perubar deposit. Note the deeply synsedimentary subsided blocks. For cross section location, see Figure. 2.

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A

Stockwork/replacive feeder zone

Mined orebody contour (put back in its supposed

original position)

Active volcanotectonic normal synsedimentary fault during ore deposition

Future location of volcanotectonic normal

synsedimentary faults

Figure. 5. Aptian-middle Albian schematic reconstitution of the Peru bar deposit in its supposed original setting. Compare the outlines of the mined ore bodies with those shown in Fig. 2.

following an active fault-block subsidence event.

Massive sulphide lenses were separated from their main feeder zones and partly mobilized along the margin of subsiding blocks, producing massive sul- phide slumping and brecciation. Figure 2 shows the present-day setting of the different massive sulphide and stockwork bodies of the Perubar deposit and Figure 5 an Aptian-middle Albian reconstitution of the deposit in its supposed original setting. Once put

back in its original pos1t1on (Fig. 5), the Perubar VHMS deposit presents the following zonation: (i) a central pyrite-pyrrhotite-sphalerite ( + rare chalcopy- rite) feeder zone represented by the Webs and Rimae stockwork orebodies (Fig. 2); (ii) a transition zone, grading from stockwork-ore to more replacive-ore, mainly following impure limestone horizons and cor- responding to the sphalerite and calcite-rich Rimac-D orebody (Fig. 2); (iii) massive sulphide (± barite)

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POLLIAND & FONTBOTE

lenses (i.e. Leonila-Graciela, Juanita, Cecilia Norte and Cecilia Sur, Fig. 2). In addition, the massive sul- phide lenses comprise a typical proximal-to-distal internal zonation, grading from: (i) massive pyrite- pytThotite-magnetite; (ii) massive Zn, (Fe>Pb) sul- phides(± barite); (iii) banded barite and Zn, (Fe>Pb) sulphides; (iv) banded barite-pyrite. Copper is almost absent in the system.

HYDROTHERMAL ALTERATION

Typical footwall hydrothermal alteration zones (or their metamorphic equivalents) have been recognized (Polliand et al., 2000). 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 wide- spread 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.

GEOLOGICAL EVOLUTION

In the Cocachacra district at the end of the rela- tively deep marine sedimentation of the Footwall Unit f01med during a main subsiding pha e carbonate sed- imentation started abruptly indicating a very rapid uplift to a relatively shallow-water environment. Thi dynamic uplift was rapidly followed by the on et

or

bimodal volcanism together with an increment f the volcanic activity. The Perubar VHMS depo it fonned during this period of coeval bimodal volcanism and carbonate sedimentation, while the tectonic regime returned to (incipient) sub iding condition .

Almost contemporaneou ly ( lightly befor or after) to the deposition of the ore a dioritic 'ill-like stock intruded the footwall equence. Gib on et al.

(1999) interprete large, sill-like, multiphase ubvol- canic 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 Peru bar 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

Table 1. Typical whole-rock geochemistry for footwall volcanic rocks. Sam~les 1, 3 and 5 are strongly altered samples.

Samples 2, 4 and 6 are weakly altered samples (but ±regional Na-metasomatlsm).

2 3 4 5 6

Subalkaline Subalkaline

basalt basalt Andesite Andesite Rhyodacite Rhyodacite

Si02 % 50.56 51.19 55.7 58.09 67.12 69.42

Ti02 % 0.88 1.16 0.82 1.1 0.55 0.23

A1203 % 17.7 18.86 16.5 17.32 10.5 14.9

Fe203 % 4.22 10.79 4.31 6.77 4.32 2.25

MnO % 0.16 0.19 0.19 0.2 0.09 0.05

MgO % 2.38 3.33 3.18 2.66 0.94 0.82

CaO % 8.36 7.1 4.01 5.6 2.73 2.23

Nap % 0.74 5.05 0.99 6.66 0.91 4.84

K20 % 7.22 1.06 8.84 0.52 4.27 3.18

P20s % 0.15 0.27 0.21 0.2 0.15 0.09

LOI % 2.35 0.4 3.11 0.66 1.14 0.71

Cr203 % 0.02 0 0.01 0.01 0.02 0.01

Ba ppm 33200 966 25100 521 47600 2433----

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the footwall volcano-sedimentary pile.

Shortly after the VHMS mineralizing event, an abrupt and chaotic subsidence regime stmted, spliting the seafloor into deeply subsiding small sub-basins delimited by volcano-tectonic faults spaced com- momly at less than 500 m. It resulted in the downward translation, slumping and brecciation of the Perubar VHMS deposit and emplacement of the chaotic Hangingwall Unit (Fig. 4). Together with this ongoing strong fault-block subsidence, volcanic activity became more and more predominant. Subsequently, a major eruptive event probably was responsible for the deposition of the Upper Unit volcanic/pyroclastic debris flow-like sequence.

CONCLUSION

Within the regional context of the Peruvian Huarmey-Cafiete extensional marginal basin, the complex sedimentary and volcanic evolution record- ed 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 sub- marine 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 pis- ton-like subsidence, as mentioned by Kokelaar (1992). The chaotic fault-block subsidence recorded in the Hangingwall Unit could conespond to such a piecemeal caldera collapse. The VHMS mineraliza- tion at Perubar most likely was emplaced in an intra- caldera incipient depression, prior to the main caldera collapse. This situation differs slightly from the post- caldera collapse emplacement generally observed in Kuroko deposits. The nanow collapsing sub-basins at Perubar probably developed in fault-splays above steep, crustal-penetrating discontinuities that focused plumbing of magmas, basement fracturing and hydrothennal fluid discharges at these sites

Therefore, considering the Perubar deposit mas- sive sulphide mineralogy, footwall alteration charac- teristics, volcanic environment and tectonic setting, the denomination ofKuroko-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 Copper 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 salin- ity higher than that of nmmal seawater, the hydrostat- ic 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.

ACKNOWLEDGEMENTS

This work is part of an ongoing PhD thesis project at the University of Geneva supported by the Swiss National Science Foundation grant FN 2000- 54150.98 and is a contribution to the Europe Science Foundation GEODE programme. We gratefully acknowledge W. Mueller and L. Oldham who insti- gated this project and transmitted us their knowledge of the deposit, the geologists and staff of the Peru bar mine for their help during the field work periods, especially R. Egoavil and C. Zumaran, and the geolo- gists from Glencore Peru, M. Steinmann and S.

Bureau, for logistical suppmt during the project.

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 Bricefio-Guarupe, L.

1981. Structures of the continental margin in Peru and Chile, in Kulm et a!., 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.

Megard, F. 1978. Etude geologique des Andes du Perou cen-

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POLLIAND & FONTBOTE

tral, Memoires ORSTOM, Paris, v. 86, 310 p.

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

Polliand, M., Fontbote, L., Bureau, S., Steinmann, M. and Egoavil, R. 2000. Ore setting and hydrothermal alteration at the Peru bar VMS deposit, X Congreso Peruano de Geologia, Resumenes, Sociedad Geologica del Peru Eds., Lima, p. 233.

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Polliand. M. Fontbotc L. and pangenberg, J. 1999. Trn . back sulfur isotope reequilibration due to contact met·l Clng

'111 r

phism: A case study from the Perubar VM clepo it, cntr·- Peru. Mineral Deposits: Proce to Processing, tanley 1 ~I (eels), Balkema, Rotterdam, v. 2, p. 967-970. a· Vidal, C. E., 1987, Kuroko-Type Deposits in the Midd[

Cretaceous Marginal Basin of Central Peru, Econ. Geot., :.

82, p. 1409-1430.

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