Petrography and Mineralogy
Petrographic and mineralogical studies were conducted on a comprehensive suite of 50 samples taken from three diamond drill cores (see section 2.4.3) by means of optical microscopy (transmitted and reflected light), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). These three drill holes expose a sequence of five (5) distinct lithotypes, which are identified based on their mineralogical and textural characteristics.
These compare rather well with lithotypes recognized by Scarpelli (1973) despite the fact that we decided to apply a different nomenclature.
Biotite schist is a fine to medium-grained foliated rock with average grain sizes of
300µm. It is commonly grayish and is the least Mn-rich of all the lithologies at Serra do
Navio, being dominantly composed of biotite, quartz, plagioclase and spessartine. The contact with graphite schist is usually gradational. Graphite schists are characteristically dark grey with some rare medium grey varieties. They are usually finely laminated metapelites with some stratabound sulfide stringers visible in hand specimen. The presence of conspicuous graphite, albeit in minor proportions, is diagnostic.
Three carbonate rich lithotypes are distinguished, based on Mn content and carbonate mineralogy. Mn-carbonate schist is a medium grey rock with medium to coarse interlocking grains up to 5 mm size. It is characterized by the presence of two Mnsilicates, tephroite and rhodonite. Mn-calcite marble is a massively textured light to medium grey rock, and contains both Mn-calcite and rhodochrosite. This lithology grades into rhodochrosite marble, which is a medium grayish rock dominated by recrystallized rhodochrosite with some spessartine garnet.
21

Petrography and Mineralogy
3.1.1 Mesoscopic Description
This lithotype is commonly light to medium gray in color, but some samples are dark grey due to an abundance of sulphide minerals. It is usually fine to medium-grained (100
- 600µm) and displays a weak to distinct foliation (Fig. 3.1). Sometimes, foliated laminae consisting predominantly of biotite lamellae alternate with beds composed of subhedral spessartine porphyroblasts. Samples of biotite schist are dense and compact with some disseminated sulphides associated with equigranular quartz. The rock is cross cut by mm
- cm thick quartz-rich veinlets and displays cm-scale micro folds (Fig. 3.1). Petrographic and mineralogical analyses reveal the presence of biotite, spessartine, plagioclase in addition to quartz and chlorite (Table 3.1).
3.1.1 Microscopic Description
Biotite schists are marked by a rather consistent mineralogical composition marked by the abundance of biotite, quartz, plagioclase and spessartine, associated with minor chlorite, amphibole and K-feldspar. Few detrital grains of rutile and zircon have been recognized. Very fine grained (~40µm) and subrounded rutile predates most of the minerals in this rock and seems to have been overgrown by biotite; Rutile is aligned parallel to the foliation. Spessartine is dominantly subhedral with some infrequent euhedral porphyroblasts that have rounded – corroded edges, and may reach grain sizes up to 2mm. Spessartine hosts poikilotopic inclusions of paragenetically early quartz as well as rare titanite and ilmenite, and some examples are concentrically zoned (Fig. 3.2a).
A main generation of quartz is frequently intergrown with plagioclase. Quartz is usually anhedral and granular while plagioclase is subhedral and sometimes platy. Locally these two minerals fill interstitial spaces between spessartine porphyroblasts (Fig. 3.2b) and fractures that cut across the spessartine porphyroblasts, an indication that the formation of plagioclase and closely associated quartz postdates porphyroblast growth. Plagioclase and quartz form distinct layers that alternate with biotite and chlorite to define a discrete foliation (Fig. 3.2c). Biotite is tabular and subhedral with average grain sizes of 300µm.
22

Petrography and Mineralogy
It is sometimes columnar and lathlike, commonly conspicuously restricted to the foliated parts of the rock (Fig. 3.2d).
Fig. 3.1: Hand specimen photographs of the biotite schist. A: Typical appearance with well developed bedding parallel foliation. Microscopic studies reveal that quartz and plagioclase alternate with biotite to define the lamination (DH114-N). B: Medium-grained pale gray biotite schist very rich in plagioclase
(DH116-P). C: Gentle open centimetric micro folds in plagioclase and quartz rich (light colored laminae) biotite schist (DH116-O). Note almost gneissic appearance of this sample.
23

Petrography and Mineralogy
Biotite laths appear microfolded and closely associated with spessartine (Fig. 3.2e).
Closely associated with biotite is fine-grained (<40µm) apatite that is concentrated in the biotite/chlorite-rich layers and that may be pre-metamorphic in origin. Olive green amphibole is noticeable along the foliation planes, usually in the biotite rich bands. It is medium-grained and anhedral (~350µm) but only occurs in trace amounts. In some rare cases graphite plates are intergrown with amphibole and biotite making it difficult to recognize their paragenetic order.
Table 3.1: Mineralogy of biotite schist from petrographic studies and X-ray powder diffraction.
Sample # Quartz Spessartine K-feldspar Biotite Plagioclase Chlorite Amphibole Others
DH114-N
DH114-O
DH114-P
DH116-N
DH116-O
DH116-Q
DH116-R
DH116-S xxx x xxx x
DH116-T
DH140-I
DH140-K xx xx x xx x x xxx: Major (> 10%) xx: Minor (1 – 10%) x: Trace (< 1%)
Pyroxene is only present in accessory amounts but is distinguishable by its anhedral and irregular columnar form. Some samples of biotite schist (e.g. DH114-N) are characterized by the occurrence of sillimanite. The needle-shaped sillimanite is always associated with biotite or spessartine, suggesting a cogenetic origin. Tiny platy crystals of chlorite are distinctly associated with biotite. They are seldom larger that ~100 µm but are prominently anhedral and rugged. Chlorite and biotite are noticeably lacking as poikilotopic inclusions in spessartine and could have therefore formed later. But chlorite partially replaces biotite, and indication of retrograde metamorphic processes. This indicates that spessartine was the first to form, followed by biotite and chlorite.
24

Petrography and Mineralogy
Fig. 3.2: Photomicrographs (A – E) and SEM-BSE images (F) illustrating the textural relationships in biotite schist. A: Growth zoning in spessartine poikiloblasts with inclusions in the inner core but missing from the rim (DH114-O); B: Quartz and plagioclase filling interstitial spaces and fractures between spessartine porphyroblasts (DH140-Q); C: Discrete bands of quartz/plagioclase alternating with biotite/chlorite layers to define the foliation (DH116-R); D: Biotite laths, often restricted to the well foliated parts of the rock (D114-N); E: Very large spessartine porphyroblast cogenetic with biotite (DH116-
T); F: Chalcopyrite occurring as inclusions in, and as adjacent grains around spessartine (DH140-O).
25

Petrography and Mineralogy
Of the sulphides, only chalcopyrite is included in spessartine (Fig. 3.2f). Other sulphides, including pyrite, molybdenite, gersdorffite and possibly a second generation of chalcopyrite, are disseminated throughout the matrix and occasionally in the foliation plane. Very late microscopic pyrite veinlets cross cut spessartine. It thus appears that there are multiple generations of sulphides (Section 3.6).
3.2.1 Mesoscopic Description
Graphite schist is a very dark grey finely laminated metapelite with sub-mm to mm thick bedding parallel foliation. Stringers of fine-grained sulphides prominently mark the wellpreserved sedimentary lamination (Fig. 3.3). The transitions between biotite schist and graphite schist are gradational, but sometimes sharp, with graphite becoming more abundant in the graphite schist units.
3.2.2 Microscopic Description
The mineralogy of this lithotype is variable. Biotite and graphite are associated with variable amounts of quartz, plagioclase and the Mn-rich silicates, i.e. spessartine and tephroite. It appears as if the entire mineral assemblage of this lithology is metamorphogenic (Table 3.2). Biotite is commonly aligned along the foliation plane
(Fig.3.4a). It is frequently columnar and subhedral with average grain sizes of ~200µm.
Biotite is usually intergrown with plagioclase and graphite. Graphite forms minute flakes
(~30 – 100µm) that are dark gray and lath-like with uniform anhedral – subhedral grain shapes (Fig.3.4). Although it is sometimes randomly oriented, it is commonly associated aligned along the foliation plane with biotite, a mineral with which it formed concomitantly. Quartz is of submillimetric grain sizes and anhedral, but very often concentrated in laminations along the foliation.
26

Petrography and Mineralogy
Table 3.2: Mineralogy of graphite schist as determined by petrography and X-ray powder diffraction.
Sample # Quartz Spessartine Plagioclase Biotite Graphite Tephroite Amphibole Others
DH114-J
DH116-K
DH116-L
DH116-P
*K-fsp: K-feldspar, Pyroph: Pyrophanite, Rhd: Rhodonite xxx: Major (> 10%) xx: Minor (1 – 10%) x: Trace (< 1%)
Quartz frequently shows a recrystallized mosaic texture; Occasionally, quartz is associated with plagioclase. K-feldspar occurs sporadically as small (~100µm) subangular grains displaying typical Karlsbad twinning. Spessartine is observed to have formed as porphyroblasts in a matrix of quartz/plagioclase as well as biotite. It exhibits euhedral to irregular grain outlines and has abundant poikilotopic inclusions of quartz and sulphides. Irregular aggregates of spessartine poikiloblasts (~2mm) are sometimes intergrown with pyrite (Fig. 3.4b). Rarely some euhedral spessartine is almost free of inclusions. In some samples spessartine is partially replaced by tephroite.
Tephroite is tabular, sometimes porphyroblastic, but is mostly medium grained (~400µm) and of anhedral shape (3.4c). Tephroite tends to be randomly distributed and is usually intergrown with rare grains of rhodonite. Rhodonite and tephroite appear to locally replace spessartine. Rhodonite is subordinate both in general abundance as well as size, to tephroite, but also tends to be porphyroblastic. The mutual cross cutting relationship between rhodonite and tephroite suggests a contemporaneous crystallization.
Amphibole is minor and occurs adjacent to spessartine garnets. Wherever present, amphibole is characterized by irregular dense aggregates intergrown with some rare grains of titanite. Of the sulphides, pyrite is dominant, and gives the rock a dark shiny color. It is intergrown with spessartine porphyroblasts, showing that the sulphide precipitated during spessartine growth.
27

Petrography and Mineralogy
Fig. 3.3: Hand specimens of the compact and dark colored graphite schist. A: Very fine-grained graphite schist with sulphide stringers along the foliation/bedding planes (DH114-K); B: Flaky graphite randomly oriented with biotite, quartz and sulphides (DH116-P).
28

Petrography and Mineralogy
Fig. 3.4. Photomicrographs illustrating important petrographic characteristics of graphite schist (A, D, E– photomicrographs; B, C, F – SEM-BSE images). A: Biotite and graphite flakes aligned along the foliation.
(DH114-E); B: Spessartine porphyroblast intergrown with pyrite showing concomitant growth. Note the apparent rotation in the spessartine defined by pyrite (DH114-J); C: Tephroite and spessartine replaced by cobaltite (DH114-J); D: Chalcopyrite and chalcocite enclose quartz (DH114-J); E: Pyrite restricted to the foliation plane showing that it formed contemporaneous with deformation. (DH116-P); F: Pyrophanite replacing ilmenite (DH114-J). In each case the tiny black mineral flakes are graphite.
29

Petrography and Mineralogy
Chalcocite occurs rarely associated with chalcopyrite (Fig.3.4d). Pyrite and chalcopyrite aggregates occur astride regular elongate pyrophanite grains (Fig.3.4e) with no evidence of replacement or exsolution. Cobaltite and niccolite are rare and fine grained (~60µm).
Cobaltite straddles a late Mn-calcite veinlet showing that it formed at the same time of immediately after veining. These Mn-calcite veinlets are fracture-hosted and usually of sub-mm thickness. They cross cut both tephroite and rhodonite. Growth zoning is observed between pyrophanite and ilmenite (Fig.3.4f)
3.3.1 Mesoscopic Description
Mn-carbonate schist is a characteristically medium to coarse-grained rock, with a grain size range of 0.8 – 5mm, and is massively textured with interlocking grains (Fig. 3.5). It is light to medium gray and has a pinkish tint, with coarse granular patches of rhodonite prominent and crosscutting veinlets (mm-thick) of rhodochrosite, Mn-calcite and quartz.
Abundant coarse-grained Mn-silicates, commonly tephroite and rhodonite (Table 3. 3) are diagnostic for this lithology. Mn-silicates predominate over the Mn-carbonate.
3.3.2 Microscopic Description
Mn-carbonate schist is marked by the predominance of tephroite, rhodonite and spessartine garnet. Mn-carbonates (rhodochrosite and Mn-calcite) are minor but paragenetically important constituents as are quartz, biotite and amphiboles. The occurrence of rhodonite and tephroite is characteristic for this lithology. Rhodonite is very coarse, sometimes averaging ~5mm; It is typically coarser-grained than spessartine and is abundant in almost all the samples (Table 3.3) It is tabular, subhedral – anhedral and granular, and very often intergrown with tephroite. Rhodonite replaces the carbonate minerals and quartz, and sometimes spessartine (Fig. 3.6d), but may locally alter to amphibole. Rhodonite has poikilotopic inclusions of sulphide minerals, including cobaltite, alabandite and pyrite. Cobaltite and alabandite are irregularly shaped while
30

Petrography and Mineralogy pyrite is euhedral. Tephroite is also quite abundant, though subordinate to rhodonite and unusually coarse-grained (~4mm). It has anhedral shapes and is characteristically fractured (Fig.3.6e) along its edges, with some granular interlocking grains displaying a mosaic texture. It is locally replaced by Mn-calcite (Fig. 3.6f). Rhodonite and tephroite mutually enclose each other and are intimately intergrown (Fig. 3.6g), a textural relationship that indicates that they are cogenetic. Biotite occurs as tabular laths adjacent to tephroite and rhodonite, an indication that they formed concomitantly (Fig. 3.6h).
Niccolite and gersdorffite are extremely fine grained (~30µm), usually anhedral and irregular but often occupying the interstices between grains of tephroite and rhodonite.
Table 3.4: Mineralogy of Mn-carbonate schist as determined by petrographic studies and X-ray powder diffraction
Sample # Quartz
DH114-A
DH114-I
DH114-M
DH116-A
DH116-B
DH116-C
DH116-D
Spessartine Rhodochrosite Biotite Rhodonite Tephroite xx x xx x xxx
xx xxx xx x x
DH116-F
DH116-G
DH140-A
DH140-B
DH140-C
DH140-D
DH140-E
DH140-F
xx xxx Chlorite xx x x xx xxx x xx xx xxx x x
DH140-G
DH140-H
DH140-J xx xx xx xx xx x xxx: Major (> 10%) xx: Minor (1 – 10%) x: Trace (< 1%)
31

Petrography and Mineralogy
Fig. 3.5: Hand specimen photographs depicting several types of Mn-carbonate schist. A: Granular Mncarbonate schist characterized by pinkish rhodonite and tephroite (DH140-A); B: Very coarse grained (~2 –
5mm) sample showing intergrown rhodochrosite, rhodonite and tephroite (DH140-E); C: Coarse grained
(>2mm) sample with a ~8mm rhodochrosite filled veinlet crosscutting it (DH140-C).
Spessartine is poikiloblastic and may reach sizes of 3mm. Most of the poikiloblasts are anhedral with rough and jagged edges that have in most cases undergone replacement by amphiboles. Spessartine is characterized by poikilotopic inclusions (Fig. 3.6a) that include paragenetically older quartz, carbonates and pyrite. Later pyrite tends to fill the spaces between spessartine grains (Fig.3.6i).
Mn-calcite grains are interlocking and sometimes fill interstitial spaces between spessartine porphyroblasts. Mn-calcite is anhedral and fine grained, frequently ~250µm.
32

Petrography and Mineralogy
Fig. 3.6: Photomicrographs illustrating different petrographic features of Mn-carbonate schist. A:
Poikilotopic spessartine with regular edges set in a Mn-calcite matrix (DH114-A); B: Mn-calcite replacing spessartine. Note the irregular remnants of spessartine porphyroblasts (DH116-D); C: Granular rhodochrosite, subordinate to Mn-calcite associated with tephroite and rhodonite (DH116-A); D:
Rhodonite and Mn-calcite replacing tephroite and regularly shaped porphyroblasts of spessartine (DH116-
B); E: Coarse (>4mm) anhedral tephroite typically fractured and replaced by Mn-calcite along the grain boundaries and fractures (DH140-A); F: Mn-calcite replacing tephroite (DH140-D)
33

Petrography and Mineralogy
Fig. 3.6 (contd.). G: Mutual crosscutting relationships between rhodonite an tephroite. Note the irregular remnants of Mn-calcite within the euhedral spessartine (DH114-A); H: Metamorphic assemblage of biotite, tephroite, rhodonite, Mn-calcite and sulphides (DH116-G); I: Late pyrite constrained on the boundaries of spessartine (DH116-B); J: Medium grained granular pyrophanite with pyrite inclusions. Note the finegrained blebs of yellow-brown alabandite on the edges of pyrophanite (DH116-G).
Mn-calcite has in places been overgrown and in others replaced, by rhodonite and tephroite. Mn-calcite may locally replace spessartine (Fig. 3.6b). Rhodochrosite is medium grained and has a granoblastic texture (Fig. 3.6c). This older generation of rhodochrosite tends to alter to tephroite, sometimes rhodonite and may replace spessartine.
Granular quartz is anhedral and of variable grain size, though averages of ~200µm are widespread. It is quite abundant, occurring as inequant clusters, sometimes as poikilotopic inclusions in spessartine. Amphibole is subhedral and forms short stubby
34

Petrography and Mineralogy prisms with some conspicuous needle-shaped streaked crystals. It is fine to medium grained (~150µm) frequently occurring between the coarser grained rhodonite and tephroite. In some samples it overgrows and replaces spessartine, with which it is closely associated. Greenish pyroxene is minor, but tends to occur together with amphibole.
Several sulphide phases in this rock are associated with amphibole even though many are included in spessartine. Equant grains of pyroxene are randomly oriented and tend to accumulate with irregular but sporadic aggregates of titanite and graphite.
Infrequent zircons are minute in size, seldom larger 30µm, and are usually subhedral and subangular. They appear to be the only constituents that may possibly be of detrital origin. Pyrophanite may grow up to sizes of ~300µm and is associated with smaller pyrite aggregates (Fig.3.6j). Veinlets of rhodochrosite, quartz and Mn-calcite cross cut spessartine as well as tephroite and rhodonite. It is not uncommon for late but rare pyrite mm-thick veinlets to cross cut spessartine and rhodonite.
3.4.1 Mesoscopic Description
Samples of Mn-calcite marble are characteristically medium gray in color with a pinkish tint. They are often medium – coarse grained, some grains often up to 2mm. This rock has a massive texture with some granular interlocking grains and usually devoid of compositional/mineral or textural banding. It is cross cut by a network of cm-thick (Fig.
3.7) fractures that are filled by rhodochrosite, quartz and Mn-calcite. XRD analyses reveal a metamorphic mineral assemblage characterized by Mn-calcite, spessartine and tephroite (Table 3.4).
3.4.2 Microscopic Description
In this lithology Mn-rich carbonates (rhodochrosite and Mn-calcite) constitute consistently a prominent part of the mineral assemblage, and are more abundant than Mnsilicates.
35

Petrography and Mineralogy
Table 3.4: Mineralogy of Mn-calcite marble from petrographic studies and X-ray powder diffraction.
Sample # Quartz Spessartine Rhodochrosite Biotite Mn-calcite Tephroite Amphibole Others
DH114-C xx xx x xx xx xx x
DH114-D
DH114-E
DH114-K xx x x xx xx x xx x x xx Rhodonite x x x
DH116-H
DH116-J x xx x xxx Sulphides
DH140-O x x x x xx x xxx: Major (> 10%) xx: Minor (1 – 10%) x: Trace ( < %)
Mn-calcite forms nearly equidimensional grains that are usually cream colored and translucent in thin section. It has an average grain size of about 600µm and euhedral – subhedral grains with near triple junctions (Fig. 3.8b). Mn-calcite locally alters to spessartine (Fig. 3.8c) and forms remnant inclusions in tephroite. Quartz has a fine to medium grain size (200 - 400µm). It is often anhedral and displays irregular grain shapes.
The textures of quartz range from sub – to mosaic and the individual grains have occasionally sutured contacts; several grains are weakly strained and show mild undulatory extinction.
Spessartine is usually subhedral with sharp jagged edges. It is poikilotopic and contains inclusions of quartz, graphite, Mn-calcite and sulphides (Fig. 3.8d). It is interesting to note that pyrophanite exclusively occurs along the boundaries of spessartine grains.
Sulphides are disseminated throughout the carbonate matrix. Just like in other lithologies, tephroite and occasionally rhodonite are mutually intergrown and randomly oriented in clusters. Tephroite is randomly oriented; occasionally reaching centimetric sizes (~1cm) and has a poikilotopic texture (Fig. 3.8e). Rhodonite sometimes occurs as the inclusions in tephroite. While usually not as coarse as tephroite (being ~1mm) it nonetheless clusters in irregular blebs adjacent to tephroite.
36

Petrography and Mineralogy
Fig. 3.7: Selected samples of Mn-calcite marble. A: Dark gray medium grained (<1mm) sample with a massive texture (DH116-A); B: Coarse-grained medium gray sample with prominent rhodochrosite vein
(DH114-C); C: A mm-thick veinlet of rhodochrosite crosscuts Mn-calcite (DH116-J).
Biotite crystallizes in submillimetric laths that are randomly oriented and very rarely define a weak foliation. Isolated biotite laths are embedded in recrystallized Mn-calcite and have random orientation. None of the biotite is included in spessartine or tephroite; biotite is cogenetic with or immediately formed after spessartine. Amphibole and pyroxene usually occur astride each other; they are frequently intergrown.
37

Petrography and Mineralogy
Fig. 3.8: Photomicrographs and SEM-BSE images depicting textural and mineralogical relationships in
Mn-calcite marble. A: Quartz in sub mm-thick veinlets that crosscut a Mn-calcite rich assemblage (DH114-
C); B: Tephroite porphyroblasts (~2mm) engulf and replace Mn-calcite (DH114-D); C: Mn-calcite replaces spessartine porphyroblasts. Note that the porphyroblast has older Mn-calcite inclusions and has been almost completely consumed by a younger generation of Mn-calcite (DH114-C); D: Subhedral spessartine crosscut by quartz and hosting poikilotopic inclusions of quartz, Mn-calcite and graphite (DH116-A); E:
Coarse tephroite intergrown with granular Mn-calcite. (DH116-J); F: Mn-calcite replacing tephroite along the grain boundaries (DH116-J).
38

Petrography and Mineralogy
Pyroxene tends to be anhedral and randomly oriented with grain sizes of ~200 - 300µm.
Amphibole on the other hand has an irregular at times almost prismatic habit, seldom reaching a grain size of 200µm. Younger quartz is present in a late stockwork of cm-thick fracture hosted veinlets (Fig. 3.8a). These veins crosscut amphiboles and Mn-calcite, and are thus paragenetically very late.
3.5.1 Mesoscopic Description
Rhodochrosite marble typically has a medium dark gray color and pinkish patches, with a sugary texture and almost equigranular grains that are on average ~500µm in size. It is compact and has a relatively low density with no discernible fabric. The mineral assemblage is dominated by rhodochrosite with quartz, minor tephroite and rhodonite
(Table 3.5), some of the latter visible in hand specimen (Fig. 3.9). A dense stockwork of late mesoscopic rhodochrosite and rhodonite veinlets that crosscut spessartine porphyroblasts is present in some samples. There are distinct generations of veinlets with the thicker quartz veinlets being younger as it crosscuts the older and thinner rhodochrosite veinlets (Fig.3.9a).
Table 3.6: Mineralogy of rhodochrosite marble as determined by petrography and X-ray diffraction.
Sample # Quartz Spessartine Rhodochrosite Biotite Rhodonite Tephroite Chlorite Others
DH114-B
DH114-F
DH114-G
DH114-H
DH114-L
DH114-M
DH116-E
DH116-I
DH116-M
DH140-L
DH140-M
DH140-N xx xxx xx x x x xxx xx x x xxx xxx xxx xx Graphite xx xx xxx xx x xxx: Major (> 10%) xx: Minor (1 – 10%) x: Trace (< 1%)
39

Petrography and Mineralogy
Fig. 3.9: Hand specimen photographs of selected rhodochrosite marble samples. A: Massively textured grey rhodochrosite marble with randomly oriented veinlets of rhodochrosite and quartz. Quartz is usually thicker (~3mm) and younger as it usually crosscuts rhodochrosite veinlets (DH116-M); B: Massive and granular-textured rhodochrosite marble dominated by rhodochrosite, Mn-calcite and quartz (DH114-B).
3.5.2 Microscopic Description
In this lithology, rhodochrosite is ubiquitous and usually subhedral with grain sizes of
300 - 600µm. It is occasionally flattened but commonly displays a granoblastic texture with occasional triple junctions (Fig. 3.10a). In some samples rhodochrosite is associated
40

Petrography and Mineralogy with quartz. The granoblastic paragenetically early rhodochrosite occurs as poikilotopic inclusions in spessartine. Rhodochrosite and quartz are locally replaced by tephroite.
Veinlets of a late generation of rhodochrosite cross cut porphyroblastic spessartine garnet. On the other hand, quartz is subhedral and granular, frequently occupying the interstices between rhodochrosite grains. It has a uniform fine-grained size (<200µm) and is also included in spessartine porphyroblasts. Closely associated with rhodochrosite and quartz are subhedral grains of fine-grained apatite.
Rhodonite is medium-grained and anhedral. It is intimately associated with spessartine and there is no evidence of replacement of spessartine by rhodonite and vice versa.
Biotite is flaky and sometimes forms sheaf-like aggregates that are closely associated with spessartine. It occurs as platy crystals and is erratically distributed throughout the rock. Biotite alters to retrograde chlorite. Chlorite forms tabular and subhedral clusters of relatively finer grain size than biotite. A characteristic feature is the presence of chlorite that replaces spessartine (Fig. 3.10d), a feature also attributed to retrograde replacement.
Chlorite later lines the selvages of sub-mm thick tephroite veinlets.
Tephroite is anhedral and sometimes occurs as fractured aggregates replacing rhodonite and rhodochrosite. Tephroite and cobaltite are locally intergrown (Fig. 3.10e). In rare cases, Mn-calcite is found to replace tephroite. Sub-mm thick tephroite veinlets are, however, common and are usually associated with Mn-calcite and chlorite.
Tephroite evidently predates this generation of chlorite (Fig. 3.10f), even though they postdate Mn-calcite. Mn-calcite occurs in two generations. The first is as poikilotopic inclusions in spessartine, while the second is as very late micro-thick veinlets that are infilled by tephroite.
41

Petrography and Mineralogy
Fig. 3.10: Photomicrographs depicting textural and mineralogical relationships in rhodochrosite marble (A
– C, thin sections; D – F SEM-BSE). A: Granoblastic rhodochrosite with minor spessartine (DH114-L); B:
Poikilotopic spessartine with irregular jagged boundaries encloses carbonate and quartz (DH140-L); C:
Pyrophanite and sulphide minerals “pushed” away by the growth of spessartine (DH116-I); D: Chlorite replacing spessartine showing evidence of retrograde reactions (DH140-L); E: Large euhedral crystal of cobaltite with inclusions of tephroite (DH114-B); F: Submillimetre veinlets of chlorite and Mn-calcite crosscutting an aggregate of tephroite, which has rhodochrosite inclusions (DH140-N).
42

Petrography and Mineralogy
Petrographic and mineralogic studies reveal a consistent mineral paragenetic succession in the metasedimentary lithologies of the Serra do Navio deposit. It is possible to define a comprehensive pattern in the development of the mineral assemblage based on their textural characteristics and occurrence by focusing on both the Mn-rich and Mn-poor mineral assemblages. This trend in mineral paragenesis is reflected by pre-metamorphism
(sedimentation and diagenesis), peak metamorphism and retrograde metamorphism (Fig.
3.11) involving both retrograde reactions and hydrothermal fluid flow. The paragenetic chart shows that there are several generations of carbonate minerals, quartz and sulphides in the lithologies studied. For clarity, only the quantitatively and genetically relevant mineral phases are included in this paragenetic chart.
The textural characteristics and mode of occurrence show that rhodochrosite and quartz
(+/- Mn-calcite), although recrystallized during metamorphism, are paragenetically early and may have constituted the sedimentary precursor. Both rhodochrosite and/or Mncalcite are intergrown with quartz, usually with quartz being the subordinate constituent.
Either they are cogenetic or the carbonates precipitated earlier. Mn-calcite and quartz occur as poikilotopic inclusions in spessartine. As poikilotopic inclusions, quartz dominates over rhodochrosite.
Randomly distributed sulphide minerals in the carbonate/quartz matrix as well as poikilotopic inclusions in spessartine porphyroblasts represent a 1 st
sulphide generation.
Spessartine is often poikilotopic and is assumed to reflect peak metamorphic mineral formation. Whereas spessartine is notably minor in biotite schist, it is the most abundant and conspicuous garnet at the Serra do Navio deposit. Several authors have studies the precursor minerals responsible for spessartine formation in general (Dasgupta, 1990) and in Mn-carbonate rich environments in particular (Nyame, 2001 and Roy, 1981). Roy
(1981) postulates that spessartine in metamorphosed occurrences may have formed from
Mn-oxides and/or Mn-carbonates admixed with siliceous and argillaceous sediments. At
43

Petrography and Mineralogy
Serra do Navio and also Nsuta (Nyame, 2001) it is very clear that spessartine forms at the expense of Mn-carbonates, with quartz and aluminous clay minerals.
Studies by Nyame (1998, 2001) reveal a precursor assemblage similar to the one at Serra do Navio. The quartz and Mn-carbonates as well as some Al-bearing clay minerals at
Serra do Navio are regarded as precursors for spessartines that is present in excess. Just as at the Nsuta deposit (Nyame, 2001), the aluminous phase that acted as a source for spessartine is not preserved. Since biotite does not occur as inclusions in spessartine, it evidently did not act as the direct alumina source for spessartine formation. The conclusion, therefore, is that whereas Mn-carbonates and quartz were present in excess, the Al-bearing phase was consumed during spessartine formation. This may also explain why muscovite is absent in the lithologies. Nyame (2001) has suggested a possible reaction to explain spessartine formation (Eq. 1),
Rhodochrosite + Quartz + Kaolinite = Spessartine + CO
2
+ H
2
O - (1)
3MnCO
3
+ SiO
2
+ Al
2
Si
2
O
5
(OH)
4
= Mn
3
Al
2
Si
3
O
12
+ 3CO
2
+ 2H
2
O
Graphite flakes commonly occur astride biotite and adjacent to spessartine and only rarely as inclusions. In graphite-rich lithologies Mn-carbonates are usually rare or completely absent. Graphite may have therefore originated from the metamorphism of organic compounds, not carbonates as suggested by Scarpelli (1973). The second generation of sulphides is characterized by pyrite intergrown with spessartine (Fig. 3.4b), cobaltite intergrown with tephroite (Fig 3.10e) as well as pyrite interstratified with biotite and graphite. This generation of sulphides is syn – to early post-tectonic.
Plagioclase is likely Na-rich albite. It is only present in the Mn-poor lithologies. This is because the precursor sediment was clay-rich or was a greywacke or even pyroclastics.
During metamorphism these minerals were transformed into plagioclase. Since Kfeldspar is rare, it appears that either most of the K was consumed during formation of biotite or that the clays were generally Na-rich and poor in K.
44

Petrography and Mineralogy
Fig. 3.11: Mineral paragenesis chart for the Serra do Navio deposit. Abbreviations: (Rhd-i, ii, iii): 1 st
, 2 nd and 3 rd
generation rhodochrosite respectively, (Mnc-i, ii, iii): 1 st
, 2 nd
3 rd
generation Mn-calcite respectively,
(Qtz-i, ii, iii): 1 st
, 2 nd
and 3 rd
generation quartz; (S-i, ii, iii): 1 st
, 2 nd
and 3 rd
generation sulphide respectively.
* Includes both sedimentation and diagenesis. NB: Only the relevant mineral phases are included in the chart.
Biotite is a metamorphic mineral that is texturally associated with spessartine, amphibole and pyroxene. Biotite does not, however, replace spessartine, neither is it included in it, yet it lies astride or wrapped around especially in lithologies where a weak foliation is present. Biotite and spessartine are thus cogenetic; the former is not precursor to the latter. Like plagioclase, biotite formed during metamorphic transformation of clays, possibly at the expense of K-feldspar. In Mn-rich lithologies, where a clay precursor is a minor constituent of the protolith, feldspars are absent. This indicates that biotite formed at their expense or that different clay minerals were responsible for the formation of biotite and feldspars.
45

Petrography and Mineralogy
The assemblage of tephroite and rhodonite marks the next stage in the metamorphic evolution of the Mn-rich lithologies at the Serra do Navio. The two are always mutually intergrown and truly cogenetic. They are observed to replace spessartine, Mn-calcite and quartz and are regarded as either late syntectonic or early post tectonic. This is because tephroite and rhodonite sometimes grow across the foliation. Depending on whether quartz or rhodochrosite were dominant, either tephroite or rhodonite may have formed
(Eq. 2 and 3):
Rhodochrosite + Quartz = Rhodonite + Carbon dioxide (rhodochrosite > quartz) - (2)
MnCO
3
+ SiO
2
= Mn SiO
3
+ CO
2
Rhodochrosite + Quartz = Tephroite + Carbon dioxide (rhodochrosite < quartz) - (3)
2MnCO
3
+ SiO
2
= Mn
2
SiO
4
+ 2CO
2
The formation of chlorite represents a third stage of the metamorphic evolution of the succession. Chlorite replaces both biotite and spessartine and is thought to have formed during retrograde metamorphic alteration and associated fluid flow. A 3 rd
younger generation of sulfides is represented by pyrite precipitated into some mm-thick posttectonic/hydrothermal veinlets. These veinlets, which also contain quartz, Mn-carbonates and Mn-silicates, reflect retrograde growth, fluid influx and brittle deformation.
46