Until its closure in 1997, the
Serra do Navio deposit, located in A
map谩 Province, Brazil, was one of the
most i
mportant sources of high-grade
manganese ore to the North A
merican
market. The high-grade
manganese oxide ores were derived by lateritic weathering fro
m metasedi
mentary
manganese protoliths of the Serra do Navio For
mation. The local geological context and nature of this protolith succession are not well understood, due to poor surface outcrop conditions, and intense defor
mation. However, based on si
milar age, regional tectonic setting and lithology the Paleoproterozoic volcanosedi
mentary succession that includes the Serra do Navio For
mation is widely believed to be si
milar in origin and laterally equivalent to the Biri
mian Supergroup in West Africa. For the present investigation several dia
mond drill cores intersecting the protolith succession were studied. Detailed petrographic and whole rock geoche
mical studies per
mit distinction of two funda
mental lithological groups co
mprising of a total of five lithotypes. Biotite schist and graphitic schist lithotypes are interpreted as for
mer
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 fro
m Mn-rich
marl and Mn-silicate rock fro
m 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 未13CVPDB values of 鈭?#xA0;4.3 to 鈭?#xA0;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 掳C). Retrograde metamorphism is marked by partial re-carbonation, expressed by the formation of small volumes of rhodochrosite, and Mn-calcite that are closely associated with quartz, chlorite and minor amounts of sulfides related to post-metamorphic veinlets. It is this metamorphosed succession that sourced the high-grade manganese oxide ores during prolonged lateritic weathering.