Abstract
The Troia–Pedra Branca complex is the most extensive exposure of mafic–ultramafic rocks in the Borborema Province, northeastern Brazil. These rocks have been known for a long time, particularly because of their platinum group element (PGE) mineralization associated with chromitites. The Troia–Pedra Branca complex consists of a succession of serpentinites (after dunite), metachromitites, metaperidotites, hornblendites and metagabbros. PGE–bearing metachromitites are hosted by the serpentinite–peridotite unit, occurring as dispersed blocks, well preserved from weathering. However, host metadunites are poorly preserved, and are only accessed by drill core samples. Scanning Electron Microscopy (SEM) analysis in thin sections of selected metachromitite samples revealed that most of the platinum group minerals (PGM) occur in the chlorite–serpentine matrix, generally in contact with chromite grains. The main PGM are sperrylite (PtAs2), cooperite (PtS), irarsite ((Ir,Pt,Rh)AsS) and hollingworthite ((Rh,Pd,Pt,Ru)AsS). Within chromite grains, very few PGM were found, and sulfide inclusions are mainly chalcopyrite (CuFeS2), pentlandite ((Fe,Ni)9S8) and bornite (Cu5FeS4). Whole–rock geochemical data reveal that metagabbros are LILE–enriched and show subduction–related signature similar to that of Alaskan–type intrusions. Mineral chemistry of chromite and olivine is also compatible with arc–related Alaskan–type complexes. The U–Pb SHRIMP zircon age for a metachromitite sample yielded an upper intercept age of 2036 ± 27 Ma, which we interpret as the crystallization age. However, dispersion in the data implies that zircons lost variable amounts of radiogenic Pb at around 749 ± 54 Ma (lower intercept), which may be related to Neoproterozoic metamorphism. The age obtained for the Troia–Pedra Branca metachromitites (2036 Ma) is younger than the 2190–2130 Ma arc–related plutons of the area, and it is closely related in age to the 2.10–2.04 Ga syn– to late–collisional plutonism and high–temperature metamorphism. Therefore, this Alaskan–type mafic–ultramafic magmatism may be related to the post–collisional setting of the 2.2–2.0 Ga Eburnean/Transamazonian orogeny.
Highlights
IntroductionMafic–ultramafic rocks, including chromitites, and associated platinum group element (PGE) mineralization may form as part of oceanic crust (ophiolite), along arc–related magmatism (Alaskan–type complex) or in continental (non–orogenic) layered intrusions (e.g., Bushveld complex) (e.g., Naldrett and Cabri 1976, Naldrett 2004, Ripley et al 2005, Barnes and Lightfoot 2005, Naldrett et al 2012, Barnes and Ripley 2016).Alaskan– or Uralian–type mafic–ultramafic intrusions (Taylor 1967, Irvine 1974, Himmelberg and Loney 1995, Batanova et al 2005) are often reported in the literature as hosting PGE mineralization (e.g., Johan et al 1989, Garuti et al 2002, 2003, Ishiwatari and Ichiyama 2004, Ripley et al 2005, Kutyrev et al 2021)
The whole–rock geochemical data show that the metagabbros of the Troia–Pedra Branca complex have clear affinity to calc–alkaline basaltic series (Fig. 11), which is compatible with their high content of magmatic hornblende, and to minor biotite in the most evolved gabbro–diorite facies
The Troia–Pedra Branca complex consists of a succession of serpentinites, metachromitites, metaclinopyroxenite, hornblendites, metagabbros and minor metagabbro–diorites
Summary
Mafic–ultramafic rocks, including chromitites, and associated platinum group element (PGE) mineralization may form as part of oceanic crust (ophiolite), along arc–related magmatism (Alaskan–type complex) or in continental (non–orogenic) layered intrusions (e.g., Bushveld complex) (e.g., Naldrett and Cabri 1976, Naldrett 2004, Ripley et al 2005, Barnes and Lightfoot 2005, Naldrett et al 2012, Barnes and Ripley 2016).Alaskan– or Uralian–type mafic–ultramafic intrusions (Taylor 1967, Irvine 1974, Himmelberg and Loney 1995, Batanova et al 2005) are often reported in the literature as hosting PGE mineralization (e.g., Johan et al 1989, Garuti et al 2002, 2003, Ishiwatari and Ichiyama 2004, Ripley et al 2005, Kutyrev et al 2021). The Troia–Pedra Branca complex consists of the most extensive occurrence of mafic–ultramafic rocks in this province, and their chromitite occurrences have been known for more than thirty years, because of their PGE mineralization (e.g., Gomes et al 1981, Simões 1993, Angeli et al 1993, Angeli 2005, Angeli et al 2009, Barrueto and Hunt 2010). A better understanding of the petrogenesis of this mafic–ultramafic magmatism is still needed, to constrain the genetic model of its PGE mineralization In this manuscript, we provide field and petrographic observations, whole–rock and mineral geochemistry, and U–Pb (SHRIMP) zircon data for these mafic–ultramafic rocks. These new data provide information on the magma chemistry/source, characteristics of the PGE mineralization, and the tectonic setting of the Troia–Pedra Branca mafic–ultramafic complex, aiming to broaden the knowledge about PGE mineralization and its hosted rocks
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