Sulfur isotope and metal variations in sulfides in the BIF-hosted orogenic Cuiabá gold deposit, Brazil: Implications for the hydrothermal fluid evolution

Abstract

Textural, chemical and multiple sulfur isotope analyses of sulfides from the Archean world-class Cuiabá orogenic Au deposit, Quadrilátero Ferrífero district, Brazil, were conducted to track distinct trace element signatures from syngenetic to epigenetic-hydrothermal pyrite types in different host units, and to deduce the nature and source of the mineralizing fluids. Gold mineralization in Cuiabá is hosted in greenschist facies volcanic and sedimentary host rocks belonging to the Archean Rio das Velhas Supergroup, especially in association with quartz veins and sulfide minerals. The BIF-hosted Fonte Grande Sul orebody is controlled by quartz veins and disseminated gold-bearing pyrite, pyrrhotite and arsenopyrite in the wall rocks. Five pyrite types are classified based on textural relations as spongy, syngenetic (Py1), porous early-(Py2), smooth main-(Py3) and smooth isolated and overgrown late-stage (Py4 and Py5) types variably present in metamorphosed carbonaceous pelites, BIF and andesite. Besides pyrite, pyrrhotite and arsenopyrite are also defined according to textural and paragenetic differences, and they display significant trace element contents. Trace element abundance maps and LA-ICP-MS analyses display trace element incorporations in Py1 (present only in carbonaceous pelite), Py2 and Py3 (in all host rocks), and Py4 and Py5 (only in BIF). Py1 yields high As, Co, Ni, Pb and Ag concentrations, whereas Py2 (formed by agglomeration of Py1) maintains high trace element concentrations with slightly less of those elements. Trace element incorporation in Py3 is similar to Py1 in carbonaceous pelite. Early-stage Py2 and main-stage Py3 in BIF and andesite have increased Co and Ni, but have less Au and As. Late-stage Py4 is characterized by lower trace element concentrations, whereas Py5 is further enriched in As, Bi, Co, Ni and Pb. The hydrothermal alteration is divided into early-, main- and late-stage, which correlate to the sulfide hydrothermal evolution. The study shows that carbonaceous pelite is pre-enriched in Co, Ni and Pb, whereas certain elements like Ag, Au, Bi and As are only hydrothermally concentrated either during the early, main or late stage in BIF and andesite. This geochemical pattern supports a syngenetic versus hydrothermal origin of distinct elements, including some trace elements now in the ore zones. Multiple sulfur isotope data suggest that mineralizing fluids at the Cuiabá gold deposit reflect a complex mixing of sulfur evolved from three possible sources: seawater, mantle, and reduced elemental sulfur. Spongy, syngenetic Py1 yields Δ33S values ranging from −2.28 to −0.25‰, separated into two ranges, the first from −2.28 to −1.97‰, and the second from −0.96 to −0.25‰. The first range suggests Py1 in the carbonaceous pelite was deposited in a seawater environment. These pyrites probably mixed with later mantle sulfur carrying a near-zero Δ33S signature, which is indicated by the second Δ33S range, or could have mixed with fluids sourced from sedimentary rocks at depth carrying a positive Δ33S signature. The second scenario is more common in sedimentary and diagenetic pyrites. An unequivocal contrast is shown by early-, main- and late-stage pyrite types (Py2, Py3, Py4 and Py5) in carbonaceous pelite, BIF and andesite, which present a continuous process of crustal assimilation values towards increasingly positive δ34S and Δ33S values. This confirms evidence for a sedimentary-derived sulfur source as also indicated by fluid inclusion data of the andesite-hosted orebody at the Cuiabá deposit.