UNIVERSITY OF GENEVA - DEPT OF MINERALOGY

Chiaradia, M. & Fontboté, L. (2000) Radiogenic lead signatures in Au-rich VHMS ores and associated volcanic rocks of the Early Tertiary Macuchi island arc (Western Cordillera of Ecuador). Volcanic environments and massive sulfide deposits. International Conference, Hobart, Tasmania, 16-19 November 2000. Extended abstract, accepted.

Radiogenic lead signatures in Au-rich VHMS deposits
and associated magmatic rocks of the Early Tertiary Macuchi island arc
(Western Cordillera, Ecuador)

Massimo Chiaradia and Lluís Fontboté
Section des Sciences de la Terre, Rue des Maraîchers 13, CH-1211 Geneva 4, Switzerland

We report new geochemical and lead isotope data on three Au-rich Cu-Zn VHMS deposits (Macuchi, La Plata, El Patiño) and associated volcanic rocks from an island arc sequence, now accreted to the continent, in the Western Cordillera of Ecuador (see also Chiaradia and Fontboté, submitted). The present contribution focuses on (i) lead isotope compositions of the VHMS's and host rocks, which are among the most radiogenic reported for ensimatic island arcs, (ii) the elevated gold grades (4.8 g/t at La Plata and 7.6 g/t at Macuchi) of the VHMS deposits which are characterized by a mineral assemblage including bornite, covellite, digenite, tennatite, enargite, and (iii) the metallogenic implications of the stratigraphic association and isotopic affinity of the VHMS deposits investigated with volcanic rocks issued from a depleted mantle source.

The VHMS deposits studied are hosted by the Paleocene-Eocene Macuchi Unit, an intraoceanic island arc accreted to the continent during the Paleocene. The volcanic and volcaniclastic lithologies of the Unit result from submarine eruptions ranging from basalts to basaltic andesites, accompanied by lesser dacites and rhyolites, with tholeiitic to calc-alkaline affinity. 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 distinguished within the Macuchi Unit. The VHMS ore deposits are hosted at different levels of the Main Macuchi sequence, whereas no analogs are known in the Basal Macuchi.

Recent studies on the geotectonic evolution of the Western Cordillera and of the Coastal Plain of Ecuador (Jaillard et al., 1997; Reynaud et al., 1999) point to accretion to the continent of two or more Cretaceous oceanic plateaus with overlying intraoceanic island arcs during Paleocene, cessation of arc activity, subduction jumps to the west, and onset of new arcs on top or adjacent to the older ones.

Depletion in incompatible elements and higher V/Ti, Sc/Y ratios suggest derivation of the Main Macuchi rocks from a source more depleted than that of the Basal Macuchi and of N-MORB. This result has a justification in the geotectonic model presented previously, according to which multiple subduction-related extractions from the same mantle reservoir might have occurred, thus yielding superimposed arc sequences derived from a progressively depleted source.

Lead isotope compositions of the Basal and Main Macuchi volcanic rocks lie along linear trends in conventional plots (²⁰⁶Pb/²⁰⁴Pb=18.66-19.09, ²⁰⁷Pb/²⁰⁴Pb=15.53=15.67, ²⁰⁸Pb/²⁰⁴Pb=38.20-39.00). The Main Macuchi rocks are consistently more radiogenic than those of the Basal Macuchi sequence and display covariations of ²⁰⁷Pb/²⁰⁴Pb values with Cr/Zr, Cr/Nd ratios indicative of an assimilation-fractional crystallization (AFC) process. The shift towards low ²⁰⁷Pb/²⁰⁴Pb values, which accompanies fractional crystallization (i.e., decreasing Cr/Zr and Cr/Nd), indicates that l.s. oceanic crust was assimilated by a parent magma with initially high ²⁰⁷Pb/²⁰⁴Pb lead. Pelagic sediments, incorporated in the mantle magma source, are therefore the only ultimate possible source of radiogenic lead present in the Main Macuchi sequence. The highest ²⁰⁷Pb/²⁰⁴Pb value measured in the Eocene Main Macuchi volcanic rocks is among the most radiogenic recorded in ensimatic island arcs and would require assimilation of unrealistic amounts of pelagic sediments (up to 35 wt.%). In agreement with rock geochemistry, it is likely that this signature results from mixing of radiogenic lead equivalent to assimilation of commonly accepted pelagic sediment amounts (i.e., 1-10 wt.%) with a mantle depleted in its low radiogenic lead (and other lithophile elements) by former extraction event(s).

The ore minerals display layered textures. Early pyrite is replaced by chalcopyrite. Chalcopyrite is substituted by the bornite-pyrite association. Bornite is replaced by covellite-digenite and by idaite. Digenite may host rare enargite. Low-Fe sphalerite is coeval to or postdates bornite. In general the ore mineral assemblages indicate a progressive increase in sulfidation conditions with time. Native gold forms blebs of few tens of microns inside early chalcopyrite or is associated with bornite. La Plata and Macuchi have high gold grades (4.8 and 7.6 g/t respectively) and, together with El Patiño, display gold enrichments over Ag and Zn when normalized to primitive mantle concentrations. Overall correlations of Au with Cu, Ag, As, Ba, and of Ag with As, Cu, Ba suggest the involvement of similar ore fluids at the three deposits.

The Pb isotope compositions of the three VHMS deposits overlap the trend defined by the volcanic arc rocks in both conventional isotope diagrams and fall within the range defined by the Main Macuchi rocks. Therefore, the VHMS deposits contain the same mixture of leads that the volcanic rocks hosting them, i.e., MORB-type mantle enriched by pelagic sediments and l.s. oceanic crust. The isotopic compositions are internally consistent within each of the three orebodies but vary significantly among the deposits.

La Plata and Macuchi have high gold grades (4.8 and 7.6 g/t respectively) and, together with El Patiño, display gold enrichments over Ag and Zn when normalized to primitive mantle concentrations.

The occurrence of mineral assemblages, including covellite, bornite, digenite, idaite, enargite, tennantite, at Macuchi and La Plata may be typical of the high sulfidation VHMS environment, but, in contrast with the argillic or advanced argillic alteration commonly associated with high sulfidation VHMS deposits, is accompanied by the development of sericitic alteration, indicative of only slightly acidic conditions. In addition, different from typical Au-rich high sulfidation VHMS's (e.g., Sillitoe et al., 1993), La Plata and Macuchi occur in an arc sequence dominated by basic rather than by felsic magmatic rocks.

Lead isotope compositions of the three ore deposits investigated are internally consistent, but vary significantly among deposits. The isotopic difference between Macuchi and El Patiño, separated by few hundreds of meters, suggests a lack of homogenization at the large scale possibly due to high level intrusions generating small hydrothermal convective cells.

A high pelagic sediment Pb component (up to 95%) has been identified in the VHMS ore minerals and host rocks, and results in highly radiogenic Pb compositions (the 15.67 ²⁰⁷Pb/²⁰⁴Pb ratio measured in the Main Macuchi basalts and in La Plata sulfides is among the highest known for ensimatic island arcs). A likely explanation, corroborated by host rock geochemical data, for the high proportion of pelagic sediment lead, is that commonly accepted amounts of pelagic sediments (i.e., 1-10 wt.%) were assimilated into primary melts derived from a Pb-depleted mantle source.

The stratigraphic and isotopic association of the VHMS orebodies of the Macuchi arc with volcanic rocks that are derived from a depleted source, i.e., candidates to be enriched in Au and other chalcophile metals (Hamlyn et al., 1985), indicate a possible petrogenetic control on the high gold grades of these deposits.

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

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

Chiaradia M. and Fontboté, L., submitted, 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.

Hamlyn P. R., Keays R. R., Cameron W. E., Crawford A. J., and Waldron H. M., 1985, Precious metals in magnesian low-Ti lavas: implications for metallogenesis and sulfur saturation in primary magmas. Geochimica et Cosmochimica Acta, v. 49, p. 1797-1811.

Hannington M. D. and Scott S. D., 1989, Sulfidation equilibria as guides to gold mineralization in volcanogenic massive sulfides: evidence from sulfide mineralogy and the composition of sphalerite. Economic Geology, v. 84, p. 1978-1995.

Jaillard E., Benítez S., and Mascle G. H., 1997, Les déformations de la zone d'avant-arc sud-équatorienne en relation avec l'évolution géodynamique. Bulletin de la Société géologique de France, v. 168, p. 403-412.

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, v. 307, p. 235-254.

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Radiogenic lead signatures in Au-rich VHMS deposits and associated magmatic rocks of the Early Tertiary Macuchi island arc (Western Cordillera, Ecuador)

Radiogenic lead signatures in Au-rich VHMS deposits
and associated magmatic rocks of the Early Tertiary Macuchi island arc
(Western Cordillera, Ecuador)