Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (7) Citing Articles via Google Scholar Google Scholar Articles by Chiaradia, M. Articles by Fontboté, L. Search for Related Content GeoRef GeoRef Citation

Radiogenic Lead Signatures in Au-Rich Volcanic-Hosted Massive Sulfide Ores and Associated Volcanic Rocks of the Early Tertiary Macuchi Island Arc(Western Cordillera of Ecuador)

Massimo Chiaradia and Lluís Fontboté

Section des Sciences de la Terre, Département de Minéralogie, Rue des Maraîchers 13, CH-1211 Geneva 4, Switzerland

Corresponding author: email, Massimo.Chiaradia{at}terre.unige.ch

An early Tertiary ensimatic island arc, whose remnants are known as the Macuchi unit, is exposed in the occidental portion of the Western Cordillera of Ecuador. The Macuchi unit consists of two volcaniclastic and volcanic sequences: the Basal Macuchi, of Paleocene age, which is the lower part of the arc pile, and the Main Macuchi, of Eocene age, which is stratigraphically higher. The Main Macuchi hosts the Macuchi, La Plata, and El Patiño Au-rich volcanic-hosted massive sulfide (VHMS) deposits. Despite higher contents of SiO₂ and lower contents of MgO than rocks of the Basal Macuchi, the least evolved rocks of the Main Macuchi have lower or similar concentrations of incompatible elements and higher Sc/Y, V/Ti, and Ti/Zr ratios. This suggests derivation of the Main Macuchi rocks from a depleted source compared to the Basal Macuchi sequence. The VHMS orebodies are characterized by a mineral assemblage including chalcopyrite, pyrite, low Fe sphalerite, bornite, covellite, digenite, tennantite, and by quartz-sericite and quartz-chlorite alteration. Lead isotope compositions of the volcanic rocks of the two sequences(²⁰⁶Pb/²⁰⁴ Pb = 18.65-19.10, ²⁰⁷Pb/²⁰⁴Pb = 15.53-15.67, and ²⁰⁸Pb/²⁰⁴Pb = 38.20-39.00) define mixing trends between a MORB-type reservoir and an upper crustal source. Volcanic rocks of both sequences display positive correlations of ²⁰⁷Pb/²⁰⁴Pb with MgO, which are compatible with fractional crystallization and assimilation of oceanic crust by a parent magma enriched at its source by radiogenic lead of pelagic sediments. The isotopic compositions of the orebodies overlap largely the compositional field of the Main Macuchi rocks, suggesting derivation of the majority of ore lead from leaching of the Main Macuchi sequence, even though small contributions from the Basal Macuchi cannot be ruled out. Lead isotope compositions are internally homogeneous within each of the three orebodies but vary beyond analytical uncertainties among the deposits (²⁰⁶Pb/²⁰⁴Pb = 18.835 ± 0.009, ²⁰⁷Pb/²⁰⁴Pb = 15.613 ± 0.005, ²⁰⁸Pb/²⁰⁴Pb = 38.535 ± 0.018 for Macuchi; ²⁰⁶Pb/²⁰⁴Pb = 18.859 ± 0.007, ²⁰⁷Pb\²⁰⁴Pb = 15.638 ± 0.005, ²⁰⁸Pb/²⁰⁴Pb = 38.643 ± 0.028 for El Patio; ²⁰⁶Pb/²⁰⁴Pb =19.023 ± 0.008, ²⁰⁷Pb/²⁰⁴Pb = 15.660 ±0.003, ²⁰⁸Pb/²⁰⁴ Pb = 38.786 ± 0.015 for La Plata). The isotopic difference between Macuchi (²⁰⁷Pb/²⁰⁴Pb = 15.613 ± 0.005) and El Patiño (²⁰⁷Pb/²⁰⁴Pb=15.638 ± 0.005), separated by a few hundreds of meters, suggests fluid homogenization only at a local scale, a possible result of high-level emplacement of magmatic chambers, and consequent reduced size of the convective cells. Consistent covariations between metal ratios and isotopic compositions of the orebodies suggest also that leaching by hydrothermal fluids of different portions of the Main (and Basal?) Macuchi resulted in a distinct metal geochemistry of the orebodies. The highest ²⁰⁷Pb/²⁰⁴Pb value measured in the Eocene Main Macuchi volcanic rocks and associated ores is among the most radiogenic recorded in ensimatic island-arc systems and would require assimilation of unrealistic amounts of pelagic sediments (up to 36 wt %). In agreement with whole-rock geochemistry, it is more likely that this signature results from mixing of radiogenic lead equivalent to assimilation of commonly accepted pelagic sediment amounts (i.e., <10 wt %) with a residual mantle, depleted in incompatible elements such as low radiogenic, MORB-type lead. Melts extracted from a residual source are candidates to be enriched in Au and other chalcophile elements. The strati-graphic association and isotopic affinity of the Au-rich VHMS ore minerals with the rocks of the Main Macuchi sequence suggests, therefore, a possible petrogenetic control on the formation of the VHMS deposits of the Macuchi arc.

This article has been cited by other articles:

				M. Chiaradia, D. Tripodi, L. Fontbote, and B. Reza Geologic Setting, Mineralogy, and Geochemistry of the Early Tertiary Au-Rich Volcanic-Hosted Massive Sulfide Deposit of La Plata, Western Cordillera, Ecuador Economic Geology, January 1, 2008; 103(1): 161 - 183. [Abstract] [Full Text] [PDF]

				M. Chiaradia and A. Paladines Metal Sources in Mineral Deposits and Crustal Rocks of Ecuador (1{degrees} N-4{degrees} S): A Lead Isotope Synthesis Economic Geology, September 1, 2004; 99(6): 1085 - 1106. [Abstract] [Full Text] [PDF]