Abstract
The Fazenda Brasileiro gold mineralization is hosted by a gabbroic sill, intrusive into metavolcanicmetasedimentary rocks of the Rio Itapicuru Greenstone Belt, São Francisco Craton. The 2.05 Ga old mineralization is associated with intense shearing and hydrothermal alteration, and the host gabbro is altered to a series of rocks rich in sericite, chlorite, actinolite, carbonate and quartz. Twelve whole-rock samples of the gold mineralization, representing varied degrees of alteration, from rocks with preserved igneous textures to the ore (quartz-carbonate-sulfide-chlorite), were studied by the Sm-Nd method. All analytical points resulted in an isochron (MSWD = 1.9) indicating the age of 2142 +/- 47 Ma (1s) and Epsilon Nd (T) of +1.2. Chlorite-sericite-carbonate rich hydrothermal rocks indicate the age of 2148 +/- 57 Ma and Epsilon Nd (T) of +1.1. The positive Epsilon Nd (T) suggest limited or no contamination with older continental crust, compatible with an oceanic setting for the tholeiites. Combined withREEdata, the Sm-Nd isotopic results reveal that the hydrothermal alteration, although intense, was unable to alter significantly the Sm/Nd ratios of the original igneous rocks and did not cause important scatter of the analytical points, providing a rare example of robust behavior of the isotopic system, even under intense hydrothermal alteration.
Highlights
During the last two decades, the Sm-Nd isotopic systematics has been applied to investigate the most varied aspects of crustal evolution, such as the petrogenesis of granite batholiths, continental crust formation and growth, sediment provenance, origin ofThe systematics is based on the alpha decay of 147Sm to 143Nd (λ 147Sm of 6.54 × 10-12/year)
In this study we report Sm-Nd isotopic data for whole-rock samples of extremely and variably hydrothermally altered and metamorphosed mafic complex, transformed mostly into micaschists, which hosts the Paleoproterozoic Fazenda Brasileiro gold mineralization, Itapicuru greenstone belt, Bahia
Samples analyzed have 147Sm/ 144Nd ratios varying between ca. 0.14 and 0.22 and their REE concentrations, roughly agrees with the observed by Silva et al (2001a, b): metagabbro samples have low LREE contents (Sm between 1.0 and 1.6 ppm and Nd between 3.2 to 4.6 ppm), while CLX and SUF hydrothermal rocks have higher concentrations of LREE
Summary
During the last two decades, the Sm-Nd isotopic systematics has been applied to investigate the most varied aspects of crustal evolution, such as the petrogenesis of granite batholiths , continental crust formation and growth, sediment provenance, origin ofThe systematics is based on the alpha decay of 147Sm to 143Nd (λ 147Sm of 6.54 × 10-12/year). In this study we report Sm-Nd isotopic data for whole-rock samples of extremely and variably hydrothermally altered and metamorphosed mafic complex, transformed mostly into micaschists, which hosts the Paleoproterozoic Fazenda Brasileiro gold mineralization, Itapicuru greenstone belt, Bahia.
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