Sm-Nd age of the fazenda brasileiro gabbro, Bahia, Brazil: example of robust behavior of the Sm-Nd isotopic system under extreme hydrothermal alteration

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 with REE data, 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.

Gold deposits in poly-deformed metasedimentary rocks: a case study of the C1-Santaluz gold deposit, Itapicuru Greenstone Belt, Northeast of Brazil

“…The Fazenda Brasileiro gold deposit is located in the meridional part of the RIGB, and has been the subject of several studies (Kishida 1979, Marimom et al 1986, Xavier 1987, Reinhardt 1988, Reinhardt & Davison 1989, Teixeira et al 1990, Mello et al 1996, Alves da Silva et al 1998, Vieira et al 1998, Silva et al 2001, Pimentel & Silva 2003. The C1-Santaluz deposit is located in the central portion of the belt (Fig.…”

Diamonds from the São Francisco and Amazon cratons, Brazil

“…In the Alto Paranaíba Province diamonds occur in kimberlites (e.g. Catalão, Três Ranchos, Canastra) and are widespread in 3and Frutal alluvial diamond (4) (adapted from Pimentel and Silva (2003) and Alkimin et al 1993). Fig.…”

Aplicação de isótopos radiogênicos na mineralização de ouro da mina Cuiabá, greenstone belt Rio das Velhas (MG)

“…Elementos terras raras são móveis sob condições de intensa alteração hidrotermal associada à vários tipos de mineralizações, como por exemplo o depósito de sulfeto maciço Kidd Creek(Campbell et al 1984), que apresenta fracionamento de ETR. Entretanto, existem exemplos de rochas com intensa alteração hidrotermal, em que o sistema Sm-Nd não foi afetado pelo hidrotermalismo, como por exemplo, no greenstone belt de Itapicuru -Depósito Fazenda Brasileiro(Pimentel & Silva 2003).Os dois conjuntos de rochas analisados, denominados basaltos andesíticos (Man) e toleiíticos (Mba) apresentam razões Sm/Nd distintas, com razão 0,22-0,23 para o Man e 0,26-0,28 para o Mba, observando-se que apesar de terem alteração hidrotermal, as razões Sm-Nd pouco variaram. Os resultados analíticos estão na Tabela 8.…”