petrography and mineralogy

Petrography and Mineralogy

CHAPTER 3

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 Mn-

silicates, 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.

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3.1 Biotite schist

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.

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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.

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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 xx x xxx xxx Sillimanite

DH114-O xx x x xxx xxx Graphite

DH114-P xx xxx xxx x x Pyrite

DH116-N xxx x xx xxx x K-feldspar

DH116-O xx x xx xxx x Titanite

DH116-Q xx x xxx xx x x

DH116-R xx xxx xxx x Pyrite

DH116-S xxx x xxx x

DH116-T x xx x xx x Graphite

DH140-I xxx xx xx Pyrite

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.

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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).

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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 Graphite Schist

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 well-

preserved 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.


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.

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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 xx x xx xxx x x Rhd. Pyroph.

DH116-K x xx xxx x xx x K-fsp, Pyrite

DH116-L xx x xx x xx x Mn-calcite

DH116-P x x xx xx x x *Rhd., Pyrite

*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.

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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).

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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.

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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 Mn-carbonate schist


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.


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

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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 Spessartine Rhodochrosite Biotite Rhodonite Tephroite Amphibole Others DH114-A x x xxx xxx x Mn-calcite

DH114-I x xx xxx x xxx Cobaltite

DH114-M x xxx xx x x xx x Mn-calcite

DH116-A xx xx x xx x Mn-calcite

DH116-B xx x xx x xxx Mn-calcite

DH116-C xx xxx xx x x

DH116-D xx xx xx xxx Pyrite

DH116-F xx xxx xxx Chlorite

DH116-G xx x xxx xxx Mn-calcite

DH140-A xx x xx xx xx

DH140-B xx x x xxx xx Mn-calcite

DH140-C x xx xxx xx xx

DH140-D x xx xx xxx x x

DH140-E x xx xx xx xx Zircon

DH140-F x x xx xxx xx Cobaltite

DH140-G xx xx xx xx xx x Mn-calcite

DH140-H xx x xxx xxx Chlorite

DH140-J x x xx x xxx xxx x

xxx: Major (> 10%)

xx: Minor (1 – 10%)

x: Trace (< 1%)

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Fig. 3.5: Hand specimen photographs depicting several types of Mn-carbonate schist. A: Granular Mn-

carbonate 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.

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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)

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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 fine-

grained 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


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 Mn-calcite Marble


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 Mn-

silicates.

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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 Pyroxene

DH114-D xx x x xx xx x xx Graphite

DH114-E x x xx x Rhodonite

DH114-K x x x xxx x xx Rhodonite

DH116-H x x xx x x x Rhodonite

DH116-J x xx x xxx xx Sulphides

DH140-O x x x x xx x Rhodonite

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.

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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.

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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).

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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 Rhodochrosite Marble


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 x xx xxx x xx xx x Titanite

DH114-F x xx xxx xx x Mn-cal.

DH114-G xx xxx xx x x

DH114-H x x x x xx xx Amphibole

DH114-L xx xxx xx x Mn-calcite

DH114-M x xxx xx x x Amphibole

DH116-E xxx xxx xxx xx Graphite

DH116-I xxx x xxx x xx xx Pyrite, Apatite

DH116-M xxx xxx xx Mn-cal.

DH140-L xx xx xxx xx x Sulphides

DH140-M xxx xx x x xx Amphibole DH140-N xx xxx x x Mn-cal.

xxx: Major (> 10%) xx: Minor (1 – 10%) x: Trace (< 1%)

39


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


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.

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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).

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3.6 Mineral Paragenesis and Discussion

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 Mn-

calcite 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 1st 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

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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 + CO2 + H2O - (1)

3MnCO3 + SiO2 + Al2Si2O5 (OH)4 = Mn3Al2 Si3O12 + 3CO2 + 2H2O

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 K-

feldspar 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.

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Fig. 3.11: Mineral paragenesis chart for the Serra do Navio deposit. Abbreviations: (Rhd-i, ii, iii): 1st, 2nd

and 3rd generation rhodochrosite respectively, (Mnc-i, ii, iii): 1st, 2nd 3rd generation Mn-calcite respectively,

(Qtz-i, ii, iii): 1st, 2nd and 3rd generation quartz; (S-i, ii, iii): 1st, 2nd and 3rd 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.

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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)

MnCO3 + SiO2 = Mn SiO3 + CO2

Rhodochrosite + Quartz = Tephroite + Carbon dioxide (rhodochrosite < quartz) - (3)

2MnCO3 + SiO2 = Mn2 SiO4 + 2CO2

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 3rd younger

generation of sulfides is represented by pyrite precipitated into some mm-thick post-

tectonic/hydrothermal veinlets. These veinlets, which also contain quartz, Mn-carbonates

and Mn-silicates, reflect retrograde growth, fluid influx and brittle deformation.

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