『Abstract
Until its closure in 1997, the Serra do Navio deposit, located
in Amapa(後のaの頭に´) 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℃). 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.
Keywords: Serra do Navio manganese deposit; Brazil; Lithogeochemistry;
Metamorphic petrology; Fluid inclusion microthermometry; Metallogenesis』
1. Introduction
2. Geological setting
3. Methodology
4. Petrography and mineralogy
4.1. Biotite schist
4.2. Graphitic schist
4.3. Mn-silicate rock
4.4. Mn-calcite marble
4.5. Rhodochrosite marble
4.6. Mineral paragenesis
5. Fluid inclusion studies
5.1. Fluid inclusion petrography and microthermometry
5.1.1. Type 1: H2O-NaCl inclusions (n =
59)
5.1.2. Type 2: Carbonic (CH4-rich) inclusions
(n = 28)
5.1.3. Type 3: Mixed aqueous-carbonic inclusions (n = 7)
5.2. P-T conditions of entrapment
6. Whole rock geochemistry
6.1. Meta-pelite group
6.2. Mn-carbonate group
6.3. Stable isotope geochemistry
7. Discussion and conclusion
7.1. Metamorphic overprint
7.2. Nature of the protolith succession
7.3. Metallogenetic model
Acknowledgments
References