Nature and origin of the protolith succession to the Paleoproterozoic Serra do Navio manganese deposit, Amapa Province, Brazil
Date
2012Author
Chisonga, B.C.
Huizenga, J.M.
Gutzmer, J.
Beukes, N.J.
Metadata
Show full item recordAbstract
Until its closure in 1997, the Serra do Navio deposit, located in Amapá Province, Brazil, was one of the most
important sources of high-grade manganese ore to the North American market. The high-grade manganese
oxide ores were derived by lateritic weathering from metasedimentary manganese protoliths of the Serra do
Navio Formation. The local geological context and nature of this protolith succession are not well understood,
due to poor surface outcrop conditions, and intense deformation. However, based on similar age, regional
tectonic setting and lithology the Paleoproterozoic volcanosedimentary succession that includes the Serra do
Navio Formation is widely believed to be similar in origin and laterally equivalent to the Birimian Supergroup
in West Africa. For the present investigation several diamond drill cores intersecting the protolith succession
were studied. Detailed petrographic and whole rock geochemical studies permit distinction of two
fundamental lithological groups comprising of a total of five lithotypes. Biotite schist and graphitic schist
lithotypes are interpreted as former metapelites. A greywacke or pyroclastic protolith cannot be excluded for
the biotite schist, whereas the graphitic schist certainly originated as a sulfide-rich carbonaceous mudstone.
Rhodochrosite marble, Mn-calcite marble and Mn-silicate rock are grouped together as manganiferous
carbonate rocks. Manganese lutite constitutes the most probable protolith for rhodochrosite marble, whereas
Mn-calcite marble was derived from Mn-rich marl and Mn-silicate rock from variable mixtures of Mn-rich
marl and chert.
The sedimentary succession at the Serra do Navio deposit is similar to that encountered at many other black
shale and chert-hosted Mn carbonate deposits. A metallogenetic model is proposed, predicting deposition of
manganese and closely associated chert in intra-arc basins, in environments that were bypassed by distal
siliciclastic (carbonaceous mud) and proximal pyroclastic/siliciclastic detritus. Positive Ce anomalies and
δ13CVPDB values of −4.3 to −9.4 per mill suggest that manganiferous carbonates derived during suboxic
diagenesis from sedimentary Mn4+ oxyhydroxide precipitates. Metamorphic alteration of manganese
carbonate–chert assemblages resulted in the formation of Mn-silicates, most importantly rhodonite and
tephroite; porphyroblastic spessartine formed where Mn-carbonate reacted with aluminous clay minerals.
Microthermometric studies of fluid inclusions in spessartine porphyroblasts suggests that peak metamorphic
conditions reached the upper greenschist facies (1–2 kbars and 400–500 °C). Retrograde metamorphism is
marked by partial re-carbonation, expressed by the formation of small volumes of rhodochrosite, and Mncalcite
that are closely associated with quartz, chlorite and minor amounts of sulfides related to postmetamorphic
veinlets. It is this metamorphosed succession that sourced the high-grade manganese oxide
ores during prolonged lateritic weathering
URI
http://hdl.handle.net/10394/17538http://www.sciencedirect.com/science/article/pii/S016913681100062X
doi:10.1016/j.oregeorev.2011.06.006