Assessing the magmatic affinity and petrogenesis of granitoids at the giant Aktogai porphyry Cu deposit, Central Kazakhstan

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ID: 295991
2016
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Abstract
Most mineralized porphyries associated with large to giant oxidized porphyry Cu deposits show an affinity with high Sr/Y rocks, while barren or weakly mineralized granitoids show typical low Sr/Y features. The Aktogai giant porphyry Cu deposit occurs in the Koldar pluton and provides a good natural laboratory in which to investigate this relationship, while determining the petrogenesis of the pluton and its mineralization. Zircon U-Pb dating, mineral chemistry, whole rock geochemistry and Sr-Nd-Pb and zircon Hf-O isotopic analyses were carried out on the pre-ore granodiorite (the major component of the Koldar pluton) and on the mineralized granodiorite porphyry. Zircon U-Pb ages indicate that the pre-ore granodiorite and mineralized granodiorite porphyries were emplaced at 345 and 328 to 331 Ma, respectively. Distinctly higher apatite SO~3~ contents in the granodiorite porphyry relative to the granodiorite suggest an increase in *fO*~2~ during the petrogenesis of the mineralized porphyries (\>NNO+1). Although all rocks share similar geochemical characteristics (calc-alkaline, strong depletion in Nb, Ta and Ti, and enrichment in LREE and LILE), the pre-ore Koldar pluton has normal arc related magmatic features \[low Sr/Y and (La/Yb)~N~, high Y and Yb~N~\], while the granodiorite porphyries and diorite (trace component of Koldar pluton) exhibit high Sr/Y and (La/Yb)~N~, low Y and Yb~N~ features. All samples show similar Sr-Nd-Pb-Hf-O isotopic compositions \[(^87^Sr/^86^Sr)~i~ = 0.70369 to 0.70413, ε~Nd~ (t) = + 3.6 to + 5.6, (^206^Pb/^204^Pb)~i~ = 18.16 to 19.32, zircon ε~Hf~ (t) = + 11.8 to + 15.9, and δ^18^O = + 3.8 to + 5.9 ‰\], and very young whole rock T~2~DM (Nd) (640 -- 680 Ma) and zircon T~DM~^C^ (Hf) (320 -- 590 Ma) values, suggesting that they were probably derived from partial melting of juvenile lower crust. Geochemical patterns and partial melt modeling indicate that the high Sr/Y rocks were probably formed by partial melting of eclogitized, thickened lower crust, while the Koldar pluton formed by partial melting of normal thick lower crust. We propose that pre-ore low Sr/Y rocks were probably generated earlier via subduction of Junggar-Balkhash oceanic crust, and that the high Sr/Y rocks were formed later by partial melting of sulfide-enriched, thickened juvenile lower crust. High oxygen fugacity and the high melting temperature of the high Sr/Y rocks ensured that all sulfide was dissolved in the magma, which intruded the previously emplaced low Sr/Y pluton and resulted in significant mineralization.
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Authors Ming Cao, Guangming Li, Kezhang Qin, Noreen J. Evans, Eleonora Seitmuratova
Journal american journal of science
Year 2016
DOI
10.2475/07.2016.02
URL
https://doi.org/10.2475/07.2016.02
Keywords Keywords not found

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