Proterozoic c-type eclogites hosting unusual TiFe±Cr±Cu mineralization in northeastern Brazil

Abstract

Abstract Mafic to ultramafic rocks occur at Bodoco, about 640 km west of Recife, cofolded with Early Proterozoic gneisses and migmatites, close to the tectonic contact with Middle to Late Proterozoic metasedimentary rocks of the Salgueiro-Cachoeirinha Group. The jadeite content of clinopyroxenes, the Fe/Mg partitioning between clinopyroxene and garnet, the Si activity in phengite, and the Na content of amphiboles in retrograde symplectites in gabbroic members, suggest a progressive metamorphic evolution up to C-type eclogites at 13 kbar/525° C, followed by a retrograde path of medium P/T ratio, to the greenschist facies. The final event transformed parts of these mafic rocks into banded amphibole gneisses and migmatites. The metagabbroic members contain up to 12% rutile. Metapyroxenitic members bear two types of massive ores: (a) cumulatic CrTi spinel bands and; (b) network ilmenomagnetite. The CrTi spinel is unusual in that it has a composition between ulvospinel, magnetite, and chromite and has a zonation of CrMg enrichment from core to rim which may be explained by peripheral Ti loss during high-pressure metamorphism. Several Ti occurrences in the Floresta area, 400 km west of Recife, including the 20 Mt Riacho da Posse deposit show striking similarities with the Bodoco occurrences: (1) Massive ores with high Ti/Fe ratios related to ultramafic rocks (olivine cumulates); (2) gabbroic intercalations composed of only garnet and amphibole, with the same mineral compositions recorded in Bodoco, lacking plagioclase but rich in rutile; and (3) migmatitic-gneissic wall rocks in the vicinity of the contact with younger metasedimentary rocks. A similar tectono-stratigraphic setting, a few km from the contact with the Late Proterozoic Macururo Group, and high pressure metamorphic host rocks (mafic granulites), has been described in the literature for the Serrote da Lage deposit of massive titanomagnetite with a 60 Mt interstitial copper mineralization. The only other eclogite occurrence recorded in Brazil, at Pouso Alegre, state of Minas Gerais, has an identical geological setting. Chemistry of the metagabbroic rocks of Bodoco and Floresta indicate an oceanic, tholeiitic-picritic character while the rare earth element patterns of the ultramafic rocks suggest a “modified ocean-floor peridotite” nature. Positive Co/Ni and very high Cu/Pd ratios in the sulphides also suggest an oceanic origin. All these features, with exception of the high Ti/Fe ratio, support an ophiolitic character of the mafic rocks. A detailed geochronological study may help to determine whether the mafic-ultramafic rocks represent relics of a true oceanic crust of the same age as the Salgueiro-Cachoeirinha Group, obducted onto older continental crust on an active Late Proterozoic continental margin, or if the proximity to the contact between basement and metasedimentary rocks is coincidental.