Gold-rich VHMS deposits of the Western Cordillera of Ecuador: mineralogy, lead isotope and metal geochemistry

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Book Chapter

Reference

Gold-rich VHMS deposits of the Western Cordillera of Ecuador:

mineralogy, lead isotope and metal geochemistry

CHIARADIA, Massimo, FONTBOTÉ, Lluís

Abstract

The Western Cordillera of Ecuador hosts three Au-rich Cu-Zn YHMS deposits within an EarlyTertiary island arc sequence, now accreted to the continent. The deposits are characterized by pyrite-chalcopyrite mineral parageneses including bornite, covellite, digenite, tennantite, and are affected bya quartz-sericite alteration. Lead isotope compositions are homogeneous within each deposit but differsignificantly among the three orebodies, even if two of them are only l.5 km apart. Lead isotopecompositions of the orebodies overlap those of the volcanic host rocks and are consistent with leadderived from mixing of pelagic sediments, mantle and oceanic crust. Geological setting mineralogy,and gold grades of the VHMS depos its of the Western Cordillera of Ecuador bear some similarities to those of recent and present-day massive sulphides of the western Pacific.those of recent and present-day massive sulphides of the western Pacific.

CHIARADIA, Massimo, FONTBOTÉ, Lluís. Gold-rich VHMS deposits of the Western Cordillera of Ecuador: mineralogy, lead isotope and metal geochemistry. In: Shelock, R. and Logan M.A.V.

VMS deposits of Latin America . Geological Association of Canada, 2000. p. 333-339

Available at:

http://archive-ouverte.unige.ch/unige:29441

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

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GOLD-RICH VHMS DEPOSITS OF THE WESTERN CORDILLERA OF ECUADOR: MINERALOGY, LEAD ISOTOPE AND METAL

GEOCHEMISTRY

MASSIMO CHIARADIA AND LLUiS FONTBOTE

Section des Sciences de Ia Terre, Rue des Maraichers 13, CH-1211 Geneva 4, Switzerland

ABSTRACT

The Western Cordillera of Ecuador hosts three Au-rich Cu-Zn YHMS deposits within an Early Tertiary island arc sequence, now accreted to the continent. The deposits are characterized by pyrite- chalcopyrite mineral parageneses including bornite, covellite, digenite, tennantite, and are affected by a quartz-sericite alteration. Lead isotope compositions are homogeneous within each deposit but differ significantly among the three orebodies, even if two of them are only l.5 km apart. Lead isotope compositions of the orebodies overlap those of the volcanic host rocks and are consistent with lead derived from mixing of pelagic sediments, mantle and oceanic crust. Geological setting mineralogy, and gold grades of the VHMS deposits of the Western Cordillera of Ecuador bear some similarities to those of recent and present-day massive sulphides of the western Pacific.

INTRODUCTION

Gold-rich Cu-Zn VHMS deposits occur in the Western Cordillera of Ecuador within an Early Tettiary island arc sequence known as Macuchi Unit (BGS and CODIGEM, 1999; Chiaradia and Fontbote, submitted a and b). In this contribution we discuss geochemical and lead isotope data of these deposits (Macuchi, La Plata, El Patifio) and associated volcanic rocks focussing on (i) lead sources in the VHMS ores and host rocks, (ii) pyrite-chalcopyrite mineral assemblages, including bornite, covellite, digenite, tennatite, enargite, and (ii) the elevated gold grades (4.8 g/t at La Plata and 7.6 g/t at Macuchi) of the VHMS deposits.

GEOLOGICAL SETTING

The VHMS deposits studied are hosted by the Paleocene-Eocene Macuchi Unit, an intraoceanic island arc accreted to the continent during the Early Tertiary (Fig. 1: Cosma et al., 1998; Reynaud et al., 1999). The volcanic and volcaniclastic lithologies of the Unit are the result of submarine eruptions ranging from basalt to basaltic andesite, accompanied by less- er dacite and rhyolite, with tholeiitic to calc-alkaline affinity (BGS and CODIGEM, 1999). A lower

sequence of Paleocene age (herein called Basal Macuchi), including primitive basaltic pillow lavas, and a more evolved upper sequence of Eocene age (herein called Main Macuchi), including basaltic andesite and andesitic pillow lavas, can be distin- guished within the Macuchi Unit (Fig. 2; BGS and CODIGEM, 1999; Chiaradia and Fontbote, submitted a and b). The VHMS ore deposits are hosted at different levels of the Main Macuchi sequence (Fig. 2).

RESULTS AND DISCUSSION

Lead isotope geochemistry of the host sequence Lead isotope compositions of the Basal and Main Macuchi volcanic rocks (206pbf204Pb=18.66-19.09, 207Pbf204Pb= 15.53= 15.67, 208Pbf204Pb=38.20-39.00) define linear trends in conventional plots (Fig. 3), the Main Macuchi rocks being in general more radiogenic than the Basal Macuchi ones. The isotope trends indicate that lead in the Macuchi rocks derives from mixing of at least two different sources, which are iso- topically homogeneous on a scale of 200 km, i.e., the distance across which samples have been collected (Fig. 1). Accepting the ensimatic nature of the Macuchi arc, the 207Pb-rich source can only be repre- sented by pelagic sediments (Chiaradia and Fontbote,

Chiaradia, M. and Fontboté, L. (2000) Gold-rich VHMS deposits of the Western Cordillera of Ecuador: mineralogy, lead isotope and metal geochemistry.

In R. Sherlock and M.A.V. Logan, eds., VMS deposits of Latin America, Geological Association of Canada, Spec. Publ. nr. 2, 333-339.

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CIIIARADIA & FONTI30TE

..

₂ ₃ ₄ ₅ ₆ ₇

e

^sampled^localiti^es

Figure. I. Geological map of the Western Cordillera of Ecuador between 0° and 2°S showing the location of rock samples and VHMS deposits (modified from Cosma et at., 1998 and 11-om BGS and CODlGEM, 1999). Legend: I =pre-Cretaceous metamorphic basement rocks; 2=Cretaceous-Paleocene oceanic floor; 3=Cretaceous-Paleocene island arcs (including Paleocene Basal Macuchi);

4=Latest Cretaceous-Paleocene forearc; 5=Eocene island arc (Main Macuchi); 6=Tertimy-Quaternmy forearc basins; 7=Tertialy- Quaternary continental arc. The thick lines represent terrane boundaries and major faults. The geotectonic map in the inset shows the main teiTanes of Ecuador (modified from Litherland et at., 1994). Grey=oceanie terranes; white=continental terranes.

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GOLD-RICII VIIMS DEPOSITS OF TilE WESTERN CORDILLERA OF ECUADOR: MINERALOGY, LEAD ISOTOPE AND METAL GEOCIIE~11STRY

submitted b). MORB -crust or San Juan oceanic plateau crust (proposed by Cosma et al., 1998, as the poss ible basement of th e Macuchi arc) account for the

207Pb-

poor end-member (Fi g. 3). Vol canic ro cks of the Basa l and Main Macuchi sequences display also positive correlations of the 207Pbf204Pb ratio with MgO and other fractionation indexes (Ni, V

,

Ti/Zr, etc.). Thi s suggests their derivation from 207Pb- rich pa

rent

magmas that

have undergone fractional crys-

talli zation and assimi

lated low radiogenic

Pb of MORE-cru st or of th e San Juan oceanic plateau at shallow l eve

l

s. These co rre

l

ations exc lude assimilation of upper continental crust by the parent magm as reaffi rming pelagic

sediments

as the only source of the radiogeni c lead present in the Mac uchi Uni t (C hiaradia and Fontbote, submitted b).

GEOLOGY, MINERALOGY, METAL AND PB ISOTOPE GEOCHEMISTRY OF THE VHMS DEPOSITS

La Plata, situated about 60 km SW of Quito (Fig.

1), is hosted by the upper part of the Main Macuchi sequence ( Fig. 2). Lenses of mass ive and di sseminat- ed sulphides (several decimetres to a few metres th

ick

and up to 100 m in extension, Lehne, 1990), as well as veinlets occur within bleached and shea red Na- depleted andes ites that display silicifi cation and seric- itization . The ore

lenses

are zoned with a Cu-rich footwall and a Zn-ri ch hangingwall (Paladines, 1989;

Lehne, 1 990). A preliminary resource of 840,000 tonnes

@ 4.8 g/t Au, 54 g/t Ag,

4.1

% Cu, 0.7% Pb

and 4. 2

%

Zn has been reported (Cambior Annual Report,

J

998) .

Macuchi, situated 70 km SSW of La Plata (Fig. 1 ), is hosted by the lower part of the Main Macuchi sequence (Fig.

2).

The orebody is a lens of mass

i

ve sulphides, 1 25 m

l

ong and up to 1 8 m thi ck, in a sequence of basa ltic to basa ltic andesite volcanic and volcan

i

clasti c rocks with rarer fels

i

c tuffs, porphyri c andesites and dacites. The mineralization occurs as repl acement,

stockwork and

layered

sulphides.

The main alteration assemblage is quartz-seri cite, prevail- ing over qua rtz-chlorite. The Macuchi mine was ex pl oited from 1 94 1 to 1950 with an average ore grade of 5% Cu, 7.6 g/t Au and 8.5

g/t Ag (Stoll, 1962).

El Patino, a sm all er pyriti c orebody situated about 1.5 km NE of Macuchi, i

s found

at a hi gher strati graph-

ic level than Macuchi (Fig.

2).

The host basa ltic andes i tes display a

strong quattz-sericite alterati

on in proximity of the ore, s imilar to that observed at La Plata.

At La Plata and Macuchi the ore mi nerals display similar parage netic sequ ences and define

layered tex-

tures. The main ore minerals are early pyrite and c hal- co pyrite, the latter partly re pl ac ing py rite.

Chalcopyrite is in turn substituted by a bornite-pyrite associatio

n. At Macuchi bornite is replaced by covel-

lite-di genite (the latter hosting rare enarg i te) and by idaite. Low-F e sphalerite is coeval to or postdates bornite in both depos its. In general the paragenetic evo

lution of

sulphid es ind icates a progressive increase in sulfidati on con di tions with time. Native gold form

s

blebs of few tens of microns inside early chalcopyrite (Macuchi: Figure

4)

or is associated with bornite ( M acuchi and La Plata). At La Plata

gold

has bee n also found in association wi th late

ga

lena.

EOCENE

PALEOCENE

UPPER CRETACEOUS

Main Macuchi

Basal Macuchi

1/ ., ...

?

II 1/ 'I

~~ ~1/,, ::=~~~//\\

/J """'\\11 """'II \\ ~ /J ""\\II """'II

II fi 11 \\ \\ ~ II fi 11 \\ 1 Pinon/San Juan

7 r ~ ~ r ~

11-::::-,,~ 1! ""11-::::-\\~ l!

""'

^~

""'

" """'~ I ^c==J ^z

c::J

⁷

I 14

Figure. 2. Stratigraphic column of the occidental portion of the Western Cordillera of Ecuador at I ⁰S (modified from BGS and CODIGEM, 1999). Legend: I =oceanic crust of the Pinon Formation or San Juan Complex; 2=undifferentiated volcanic and volcaniclastic rocks of the lower part of the Macuchi Unit (Basal Macuchi); 3=basaltic pillow lavas;

4=1imestone; 5=undifferentiated volcanic and volcaniclastic rocks of the upper part of the Macuchi Unit (Main Macuchi) 6=basaltic andesite andesitic pillow lavas; ?=sandstones.

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,.Q

~ ~ 0 M

-

^,.Q~ t--0 M

,.Q

~ ~ 0 M

-

^,.Q

QO ~ 0 M

15.70

15.65

15.60

15.55

15.50

15.45

39.0

38.6

38.2

37.8

37.4 17.8

M

18

CIIIARADIA & FONTBOTU

Pacific sediments and Mn-nodules

18.2 18.4 18.6 18.8

206Pbf204Pb

uc

'

I I I I I I I I

-"'

uc

19 19.2 19.4 19.6

CJ

whole rock Main Macuchi whole rock Basal Macuchi

~ La Plata VHMS

e

Macuchi VHMS x El Pati-o VHMS

Figure 3. 207pbf204pb vs. 206pbf204pb and 208pbf204pb vs. 206pbf204pb plots showing the isotopic compositions of volcanic rocks and ore minerals of the Main Macuchi arc sequence. The source of data on Macuchi rocks and ores is Chiaradia and Fontbote (submitted a and b). The San Juan Complex isotopic compositions are from Mamberti et al. (1999). Sources ofEast Pacific MORB, Pacific pelagic sediments and Mn-nodules are various. Also shown for reference are the evolution curves of the upper crust (UC), orogen (OR) and mantle (M) (Zartman and Doe, 1981 ).

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GOLD-RICH VI·IMS DEPOSITS OF THE WESTERN CORDILLERA OF ECUADOR: MINERALOGY, LEAD ISOTOPE AND METAL GEOCHEMISTRY

. .

^•"'

.... -- ^...

'

-- ^···--

_I_'

-··-

' '

• • • • • • 0 • - . - , . . . . . - · • . . . ... • • • . . . . . • • -

. .. .. - ·r .. -

--· ... ep·. ·-· . --- ..

I

_. _.

.

^:

. Au ~

'

_ ^... ^. --· . ^·- ^.

^__^I

^:

^'

,. I

-

·-·:

1C4J

I"-·· •. --···

-

'

Figure 4. Microphotographs of ore minerals from the VHMS deposits of La Plata Abbreviations: Au=gold; cp=chalcopyrite;

dg=digenite; py=pyrite.

Quartz, sericite, calcite, chlorite and barite are the gangue minerals.

Ore samples of La Plata, Macuchi and El Patino display gold enrichments over silver and zinc when normalized to primitive mantle concentrations (Chiaradia and Fontbote, submitted a). Ore samples of the three deposits display also overall correlations of Au with Cu, Ag, As, Sb, Ba, and of Ag with As, Cu, Ba (Fig. 5).

The Pb isotope compositions of the three VHMS deposits fall along the trend defined by the volcanic arc rocks in conventional isotope diagrams and large- ly overlap the field defined by the Main Macuchi rocks (Fig. 3). Therefore, the VHMS deposits contain the same lead mixture of the volcanic rocks hosting them, i.e., MORB-type mantle enriched by pelagic sediments and either MORB-crust or oceanic plateau basaltic crust. The isotopic compositions are internally homogeneous within each of the three orebodies but vary significantly among the deposits, apparently

in relation to their stratigraphic position (Figs. 2 and 3). The relatively low radiogenic signature of Macuchi, situated in the lowest part of the Main Macuchi sequence, suggests mixing ofPb of the Main and Basal Macuchi (Fig. 3) possibly due to hydrother- mal convective cells straddling the two sequences.

The same mechanism can explain the isotopic com- position of El Patino which, nevertheless, bears a larger component of Main Macuchi lead (Fig. 3) being situated at a higher stratigraphic level than Macuchi (Fig. 2). El Patino requires perhaps a limited contribution from an additional source to explain its slightly lower 206Pbf204Pb ratios compared to the Main Macuchi rocks (Fig. 3).

CONCLUSIONS

The occurrence at Macuchi and La Plata of pyrite- chalcopyrite mineral assemblages, including covellite, bornite, digenite, idaite, tennantite, enargite may be typical of the high sulfidation VHMS type. However,

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CIIIARADIA & FONTilOTE

106 103

•

⁰

105

• _·~

⁰ ^,.-...., 102 ⁰ ⁰

,.-.... ⁸^0.. ₁₀4 ^R ⁰

5

8 ^0..

• ^. Ro ^co

5

^::l

-<

/;)1)

• u _~

10 0

103

• _Ill •

102 o.l _I 10 100 1000 ^I0.1 I 10 100 1000

Au (ppm) Au (ppm)

104 103

103 0©

,.-...., 8 ^0.. ₁₀₂

•

0 ,.-.... ⁸^0..102

•

₀ ⁰

5 5

Vl ..0 CD

-<

₀ ^if) ¹⁰ ₀

10

• •

- -

I I

0.1 ^I 10 100 1000 0.1 I 10 100 1000

Au (ppm) Au (ppm)

1os

•

¹⁰³

104

•

₀ ,.-..., ⁰⁰

,.-.... 8 ¹⁰² ₀ _. ₀

8 0 0 ^0..

103 ^0.. ⁰

0.. 0 0 ^"-'

0.. _bl)

•

...

• ^-<

= •

¹⁰

N 102

• •

10 I

to-2 102

I0-2 t o- 1 I 10 to2 L0-1 _I 10

BaO (wt.%) BaO (wt.%)

0 Macuchi

e

La Plata • El Pati-o

Figure 5. Element correlation diagrams for selected ore samples and reserves (R) of Macuchi, La Plata and El Patino.

in contrast with the argillic or advanced argillic alteration commonly associated with high sulfidation VHMS deposits (Sillitoe et al., 1996), only sericitic alteration, indicative of slightly acidic conditions, has

been observed. Also, different from typical Au-rich high sulfidation VHMS's (e.g., Sillitoe et al., 1993), La Plata and Macuchi occur in an arc sequence domi- nated by basic rather than by felsic magmatic rocks.

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GOLD-RICH VHMS DEPOSITS OF THE WESTERN CORDILLERA OF ECUADOR: MINERALOGY, LEAD ISOTOPE AND METAL GEOCIIEMISTRV

Lead isotope compositions indicate that lead (and by inference the other metals) of the VHMS ores is derived from the host volcanic rocks of the Main Macuchi sequence. As the latter, therefore, the VHMS ores contain radiogenic lead from pelagic sediments mixed with low radiogenic lead of MORB-type mantle and oceanic crust.

Lead isotope compositions of the three ore deposits investigated are internally homogeneous, but vary significantly among deposits. The isotopic difference between Macuchi and El Patino, separated by 1.5 km, suggests a lack of homogenization at the hundreds of metre scale.

The gold enrichment over silver and zinc in the normalized spectra of ore samples as well as the sig- nificance of geochemical and isotopic compositions of the Main Macuchi rocks, host to the VHMS deposits, discussed in Chiaradia and Fontbote (submitted a), indicate a possible petrogenetic control on the high gold grades of these deposits.

Mineralogy, gold grades, metal association and possible petrogenetic control on gold enrichment of the Macuchi island arc VHMS deposits bear similarities with those described in recent Au-rich VHMS deposits from the western Pacific (Hannington et al., 1999).

ACKNOWLEDGEMENTS

We wish to thank Dr. Agustin Paladines (Universidad Central del Ecuador, Quito, Ecuador) for introducing us to the geology of the Western Cordillera of Ecuador. This study is funded by the Swiss National Foundation (grant# 2000-054150.98 to LF) and is a contribution to GEODE, a programme funded by the European Science Foundation.

REFERENCES

Aye, F., Cesbron, G. and Picot, P. ( 1983) Contribution de Ia petrographic et dela mineralogic

a

Ia connaissance des indices

a

Au, Cu, Zn, Pb, Ba, de Macuchi (equatcur). Note N. 924, Departement Gites Mineraux, BRGM, Orleans, France, 16 p.

BGS and COD! GEM ( 1999) Mapa Geologico de Ia Cordillera

Occidental del Ecuador entre 0°-l

o s

and I ⁰-2° S. Mision Britanica, CODIGEM, Quito, Ecuador.

Chiaradia, M. and Fontbot6, L. (submitted a) Radiogenic lead signatures in Au-rich VHMS ores and associated volcanic rocks of the Early Tertiary Macuchi island arc (Western Cordillera of Ecuador). Submitted to Economic Geology.

Chiaradia, M. and Fontbote, L. (submitted b) Assimilation of oceanic crust lead by radiogenic magmas of the Early Tertiary Macuchi island arc (Western Cordillera of Ecuador).

Submitted to Journal of Geophysical Research.

Cosma, L., LaPierre, H., Jaillard, E., Laubacher, G., Bosch, D., Desmet, A., Mamberti, M. and Gabriele, P. (1998) Petrographic et geochimie des unites de Ia Cordillcre occi- dentale d'equateur (0°30'S): implications tcctoniques.

Bulletin de Ia Societe geologique de France, 169, 739-751.

1-Iannington, M.D., Poulsen, K.H., Thompson, J.F.H. and Sillitoe, R.H. (1999) Volcanogenic gold in the massive sulfide environment. ln Barrie, C.T. and Hannington, M.D.

(eels.) Volcanic-associated massive sulfide deposits: process- es and examples in modern and ancient settings, Reviews in Economic Geology, 8, 325-356.

Lehne, R. W. ( 1990) The Cu-Zn-Au-Ag deposit of La Plata, Toachi), Ecuador. Jn Fontbote, L., Amstutz, G.C., Cardozo, M., Cedillo, E. and Frutos, J. (eels.) Stratabound ore deposits in the Andes. Special Publication of the Society for Geology Applied to Mineral Deposits 8, Springer-Verlag, 389-393.

Litherland, M., Aspden, J.A. and Jemie! ita, R.A. ( 1994) The metamorphic belts of Ecuador. Overseas Memoir II, BGS, 147 p.

Mamberti, M., Bosch, D., LaPierre, H., Hernandez, J., Jaillard, E. and Polve, M. (1999) Petrology and geochemistry ofMg- rich basalts from Western Ecuador: remnants of the Late Cretaceous Caribbean plateau? In Extended Abstracts of the 4th International Symposium on Andean Geodynamics, LSAG99, 4-6 October 1999, Gottingen (Germany), lnstitut de Recherche pour le Devcloppement.

Paladincs, A. ( 1989) Zonificaci6n geotect6nica y metalogcnia del Ecuador. Manana Editores, Quito, Ecuador, 192 p.

Reynaud, C., Jaillard, E., LaPierre, H., Mamberti, M. and Mascle, G.H. (1999) Oceanic plateau and island arcs of southwestern Ecuador: their place in the geodynamic evolution of northwestern South America. Tectonophysics, 307, 235-254.

Sillitoe, R.l-1., Hannington, M.D. and Thompson, J.F.H. ( 1996) High-sulfidation deposits in the volcanogenic massive sulfide environment. Economic Geology, 91, 204-212.

Stoll, W.C. ( 1962) Notes on the mineral resources of Ecuador.

Economic Geology, 57, 799-808.

Zartman, R. E. and Doe, B. R. ( 1981) Plumbotectonics - The model. Tectonophysics, 75, 135-162.

Gold-rich VHMS deposits of the Western Cordillera of Ecuador: mineralogy, lead isotope and metal geochemistry

Book Chapter

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