Two billion years of evolution enclosed in hydrothermal rutile: Recycling of the São Francisco Craton Crust and constraints on gold remobilisation processes

Abstract

The Quadrilátero Ferrífero in the São Francisco Craton, Brazil is known for its world-class gold deposits hosted by the Rio das Velhas greenstone belt (RdV). As in other greenstone belts, gold is also concentrated in late Archean/early Paleoproterozoic supracrustal units; in this case within the Moeda Formation. This unit has been compared to the Witwatersrand basin where there has been a long-term debate over the nature of the gold and whether it is detrital (placer), hydrothermal or hybrid (modified placer). Presently, links between gold in the Moeda Formation and RdV are tenuous. To enlighten this area of ambiguity, a new approach using chemical, isotopic and mineral inclusion data from detrital rutile found in the gold-bearing horizons of the Moeda Formation is presented. Grain textures and mineral inclusions have contributed to establishing a primary hydrothermal origin for the rutile, related to As-Fe-Cu-Sb-Pb-W rich mineralising fluids. Fe, Nb and V indicate that the likely source of the gold-related rutiles is the Archean banded iron formations within the RdV, where most of the lode-gold occurs. Average Nb/Ta values of these rutiles are lower than average continental crust ratios which suggests fractionation driven by fluid precipitation and/or competing mineral phases. U-Pb data yield primary crystallisation ages older than deposition ages (> 2.58 Ga), supporting the detrital nature of these rutiles. Rutiles record

Detrital rutile tracks the first appearance of subduction zone low T/P paired metamorphism in the Palaeoproterozoic

“…and 3 from the Eriboll Fm.) and rutile grains were handpicked after common mineral separation procedures (see Pereira et al, 2019). Sample locations and U-Pb data for these rutile grains are presented and discussed in Pereira et al (2020).…”

“…To retrieve peak-T estimates from the Zr-in-rutile geothermometer, rutile grains were screened to assess the process by which they were formed. High field strength elements (HFSE), Sn and W can be used to distinguish some of these different rutile growth processes (Agangi et al, 2019;Pereira et al, 2019). Some key features of magmatic-related rutile include very high W, Sn, Sb and Nb concentrations (e.g.…”

“…Meinhold et al, 2010;Agangi et al, 2019) or very high Nb/Ta ratios. Rutile is also known to form in ore mineralisation settings yielding very high Sb/V, Sn, Sb and W. Authigenic rutile and/or rutile related to low-T fluids yields low Nb/Ta ratios, as Ta becomes more compatible with rutile in the presence of aqueous fluids, combined with low Zr/Hf ratios (Pereira et al, 2019). Because published analyses of detrital rutile usually lack full trace element composition reporting, only a small fraction of the available U-Pb detrital rutile data could be included in this work.…”

Detrital rutile ages can deduce the tectonic setting of sedimentary basins

“…Detrital rutile and zircon U-Pb ages are presented and discussed using only the ± 5% discordant data, which increases the confidence in the ages used and their significance. This is particularly relevant for Palaeoproterozoic, Archaean and Hadean mineral ages, where variable degrees of Pb-loss or age mixing can produce concordant isotopic ratios but meaningless geological ages (Pereira et al, 2019).…”

Hydrothermal monazite trumps rutile: Applying U-Pb geochronology to evaluate complex mineralization ages of the Katbasu Au-Cu deposit, Western Tianshan, Northwest China

“…Monazite and rutile are common accessory minerals in different types of gold deposits, and their U-Pb isotopes are often used to determine the ore formation ages of gold mineralization (Brown et al 2002;Sarma et al 2008;Pereira et al 2019).…”