Geochronology and Nd isotope geochemistry of the Gameleira Cu–Au deposit, Serra dos Carajás, Brazil: 1.8–1.7 Ga hydrothermal alteration and mineralization

Abstract

The Gameleira deposit is one of several important Cu–Au deposits associated with the late Archean (ca. 2.7 Ga) volcanic rocks of the Itacaiúnas supergroup in the Carajás mineral province, southeastern Pará. It comprises mainly biotite- and sulphide-rich veins and quartz–grunerite–biotite–gold hydrothermal veins that cut andesitic rocks. It is interpreted as representative of the Fe oxide Cu–Au class of deposit. Sm–Nd isotopic data indicate an age of 2719±80 Ma (MSWD=3.0) and ε Nd( T) of −1.4 for the host meta-andesites. Metavolcanic rocks and cogenetic gabbros give an age of 2757±81 Ma (1 σ) with ε Nd( T) of −0.8. This is considered the best estimate for the crystallization age of the Gameleira volcanic and subvolcanic rocks. Negative ε Nd( T) and Archean T DM model ages (mostly between 2.8 and 3.1 Ga) suggest some contamination with older crustal material. The andesitic/gabbroic rocks are cut by two generations of granite dykes. The older has striking petrographic and geochemical similarities to the ca. 1.87 Ga alkali-rich Pojuca granite, which is exposed a few kilometers to the northwest of the deposit. The younger is a leucogranite with a U–Pb SHRIMP age of 1583+9/−7 Ma. Neodymium isotopic analyses of the two generations of granites indicate a strong crustal affinity and possible derivation from reworking of the Archean crust. The quartz–grunerite–gold hydrothermal vein yields a Sm–Nd isochron (MSWD=.83) age of 1839±15 Ma (1 σ) with ε Nd( T) of −9.2. Pervasive potassic alteration, represented by the widespread formation of biotite in the country rocks, is dated by Ar–Ar at 1734±8 Ma, and a similar age of 1700±31 Ma (1 σ) is indicated by the Sm–Nd isochron for the biotite–sulphide veins. Similar to that for the quartz–grunerite vein, the ε Nd( T) value for the sulphide-rich veins is strongly negative (−8.2), thereby suggesting that the original fluids percolated through, leached, or were derived from igneous rocks with an Archaean Nd isotopic signature. The geochronological data suggest that the Gameleira Cu–Au mineralization is related to a Paleoproterozoic (ca. 1.83 Ga) episode of hydrothermal activity and is not Archaean. The younger ages of ca. 1.70–1.73 might be interpreted as products of the lower blocking temperatures of biotite in relation to the Ar–Ar and Sm–Nd isotopic systems. Combined with previous geochemistry and stable isotope data, the Nd isotopic data suggest that the mineralizing fluids were derived from, or strongly interacted with, a Paleoproterozoic crustal granite, possibly similar in age and composition to the Pojuca granite.