379 Onyedika et al., Chemical, Mineralogical and…
Futo Journal Series (FUTOJNLS)
e-ISSN : 2476-8456 p-ISSN : 2467-8325
Volume-2, Issue-2, pp- 379 - 392
www.futojnls.org
Research Paper December 2016
Chemical, Mineralogical and Environmental Aspects of the Umuobom Pyrite Deposit, Anambra Basin, Nigeria.
1Onyedika, G.O., 2Achusim, U.A.C., 3Onyekuru, S.O. and 1Ogwuegbu,
M.O.C. 1Department of Chemistry, Federal University of Technology, P.M.B. 1526, Owerri,
Nigeria. 2Department of Science Laboratory Technology, Imo State Polytechnic, Umuagwo,
Owerri, Nigeria. 3Department of Geology, Federal University of Technology, P.M.B. 1526, Owerri,
Nigeria. Correspondence email: [email protected]
Abstract
The Umuobom pyrite ore in Orlu, Imo State, Nigeria, deposit was characterized based on its morphology, chemical microstructure and trace elements. The chemical content evaluation was achieved by dry crushing of the sample and acid digestion, then analyzed using the inductively coupled plasma – optical emission spectrophotometry. Other instruments utilized are the Scanning Electron Microscope-Electron Dispersive Spectrophotometer (SEM-EDS), and X- ray Diffractometer (XRD).The results obtained show that the ore deposit is predominantly pyrite (FeS2) with about 98 % dominance. Traces of other elements such as sodium, potassium, magnesium and calcium associated with the deposit occur in less than 0.01 %. Quantitatively, Si and Al were found to be 0.54 and 0.31 % respectively existing in foam like manner as aluminum silicate mineral. The x-ray mapping showed the spatial distribution of the elements within the ore matrix in a random manner with iron and sulphur in predominance. Also, the nature, mechanism of formation and incorporation of impurities into the pyrite ore was established. Hence, a veritable tools for the identification of the pyrite stratigraphic surfaces.
1.0 Introduction
Pyrite concretions occur as minor constituents of most rocks and are typically
abundant in rocks that are rich in organic matter such as black shale and coal
(Berner, 1970). Pyrites are formed in anoxic marine sediments by the reaction of
dissolved sulphide produced by microbial sulphate reduction with detrital iron-
bearing minerals. The availability of reactive iron minerals probably limits pyrite
380 Onyedika et al., Chemical, Mineralogical and…
formation in many of the most commonly studied sedimentary environments, notably
continental margin sediments and as a result, there is considerable interest in
defining the reactivity of iron minerals towards dissolved sulphide (Berner, 1984).
Ehinola et al,(2010) studied the paleo-environmental significance of pyritic nodules
from the Lokpanta Oil Shale intervals in the Petroleum System of Lower Benue
Trough, Nigeria. Four types of authigenic minerals were recognized from
petrography and x-ray diffraction (XRD) studies. They included pyrite, barite,
anhydrite and calcite. Further studies by Ofoegbu, (1984) revealed that the igneous
activity in the trough provided the source of heat required for the transportation of the
hydrothermal solution (sea water) that leached basaltic rocks in the area. These
leachates according to Ehinola et al. (2010) in elemental form led to the formation of
the pyritic nodules.
Recently, many materials characterization employ different modern techniques which
include Mossabauer spectroscopy (Hang et al, 1994). The high sensitivities of other
instruments such as scanning electron microscopy (SEM), energy dispersive
spectroscopy (EDS), optical microscopy and laser ablation inductively coupled
plasma mass spectroscopy (LAICP-MS) have resulted in the detection of many trace
elements down to the level of pg g-1. Interestingly, several mineral characterizations
have exploited these techniques to obtain reliable and classical results (Brander,
1999; Sengupta et al., 2008; Rizvi et al., 2010).
Elemental ratios such as Co/Ni and Se/S have been used to distinguish several ore
forming environments instead of abundances (El Shazy et al, 1957; Loftus-Hills and
Solomon, 1967). However, both criteria appear to have limitations in characterizing
syngenetic sedimentary pyrites. Co/Ni ratio of less than unity are believed to indicate
sedimentary origin (Loftus-Hills and Solomon, 1967).
The iron bearing minerals have diverse characteristics and the minerals themselves
are associated with impurities. The impurities are spatially distributed within the ore
bodies and have adequately been detected (Balasubramanian, 2000). Some iron
ores and their deposits have been characterized using different analytical techniques
and approaches to obtain high value results. Such techniques include the use of
diffuse reflection spectroscopy (da Costa et al., 2009), scanning electron microscopy
and optical image analysis (Donskoi et al., 2013; Hapugoda, et al., 2016),
381 Onyedika et al., Chemical, Mineralogical and…
mossbauer spectroscopy and x -ray diffraction ((MacDonald et al., 2010; Pachauriet
al., 2013; Khan, 2012; Rhan, 1997; Gomes, 2008;Takehara, et al., 2009). Delbem et
al., further studied the problems associated with iron ore analysis using reflected
light optical microscope and developed the digital image analysis denominated Opt-
lib to overcome the problem.
Today, Nigeria is looking inwards towards harnessing all her abundant solid mineral
deposits to improve her economy and create jobs. The characterization of some
Nigeria solid mineral deposits has been previously reported with less precise
instrument without proper documentation. ‘Many ore characterizations in Nigeria
were based on one or two classical instruments, which reported less accurate
information about the actual composition of the ore deposits’(Onyedika,
Onwukamike, Onyenehide & Ogwuegbu, 2015, P.307).The use of SEM-EDS, XRD
and ICP to characterize Nigerian minerals has been scantly reported (Onyedika et
al., 2011 & 2013).
Notably, rivers near the Umuobom pyrite deposit have been reported to have high
concentration of iron (Onyedika et al., 2012).Therefore, there is need to characterize
a nearby ore deposit within the Umuobom area to ascertain the source of the iron in
the river and its actual chemical and mineralogical composition, as well as
understanding the paleoenvironmental implications regarding hydrodynamics,
energy conditions and the prevailing oxic/anoxic regime in the source area. Hence,
this research will utilize modern analytical tools such as the ICP-OES, SEM-EDS, x-
ray mapping and XRD techniques to define the composition of the Umuobom pyrite
ore. This will also assist researchers understand the behavior of Umuobom iron ore
deposit during its utilization.
2.0 Description and geology of the study area
Umuobom and its environ lie between Ntueke and Umuakam at the northern fringes
of Ideato, Southeastern Nigeria. The study area is precisely delimited by Latitudes 5o
48’ and 5o 51’ N and Longitudes 7o 04’ and 7o 12’ (Fig. 1). The ore deposit being
characterized is situated along Amiyi – Arondinzogu road. The area is generally
characterized by a rugged topography with an average elevation of about 351m
above mean sea level (MSL). The relief is gentle to the North-West facing flank while
it is rugged and almost with steep escarpment on the South-East flank. Geologically,
382 Onyedika et al., Chemical, Mineralogical and…
this highland region appears to be a low asymmetrical ridge/cuesta of the Awka –
Orlu-Okigwe Uplands, which trend roughly North West to North East. The dominant
geological formations that underlie the area are the Eocene Ameki Formation and
Imo Shale. The former is sequence of unconsolidated or poorly consolidated sand
and shale, of about 305 m thick. It is underlain by the thick Imo Shale of Paleocene
age (Fig 2). The Imo Shale is comprised of shales, siltstones limestone which are
very fossiliferous. The Ameki Formation in the area is predominantly sandy with thin
claystone and siltstone bands, lenses and laminations. The sand is poorly-sorted,
may be cross-bedded and medium to coarse grained. These units, separated by
shale-siltstone-fine sand layers, may be as thick as 30 m in some places. The
unconsolidated sands are loose, friable and poorly cemented with thin shale layers
(Amangabara and Otumchere, 2016).
Fig 1: Location map of the study area (Insert map of Imo State)
5051’N 5051’N
5048’N 5048’N
704’E 7012’E
704’E 7012’E
Urualla
Ntueke
Umueshi
Umuagwo
Umuakam
UMUOBOM
Major Road
Other Roads
River
Towns
Study Location
LEGEND1km0
SCALE
N
383 Onyedika et al., Chemical, Mineralogical and…
Fig 2: Geologic map of the study area
3.0 Materials and Methods
3.1 Sample Collection
Large lump of the ore weighing 1 kg was collected from the Umuobom Ideato deposit
with the assistance of a staff of Nigeria Geological Survey Agency (NGSA), Federal
Secretariat complex, Owerri, Nigeria.
3.2 Characterization Method
The phase composition was determined by x-ray diffraction (XRD) using the scintag
XDZ 2000 powder x-ray diffractometry with a graphite monochromator and Cu Ka
radiation. The morphology and microstructure were evaluated using the Hitachi
scanning electron microscope coupled with energy dispersive spectrophotometer.
The method is similar to that adopted by Zhiwei et al., (2014).
4.0 Results and Discussion
4.1 Phase Composition
Figure 3 is the XRD pattern of the ore obtained from the Umuobom deposit. From
the peaks shown in the results, pyrite (FeS2) was the only prominent peaks (Fig. 3).
Other minerals usually associated with crude ores were not detected by the XRD.
5051’N 5051’N
5048’N 5048’N
704’E 7012’E
704’E 7012’E
Urualla
Ntueke
Umueshi
Umuagwo
Umuakam
UMUOBOM
Ameki Formation
Imo Formation
LEGEND1km0
SCALE
N
384 Onyedika et al., Chemical, Mineralogical and…
The sharp diffraction peaks (p) observed from the XRD pattern shows good
crystallinity of the solid pyrite where (p) represents pyrite.
Fig. 3: XRD pattern of Umuobom pyrite ore
Table 1 shows the chemical composition of the Umuobom pyritic ore using the
inductively coupled plasma–optical emission microscope. It is observed that 45.75 %
of the total solid concentration is iron (Fe), 0.54 % silicon (Si), 0.31 % Aluminium
(Al), Magnesium (Mg), Calcium (Ca), Sodium(Na), Potassium (K), and Titanium (Ti)
were less than 0.01 % each, respectively. The calculated sulfur concentration was
found to be 52.57 % in the solid ore. Pyrite (FeS2) content in the ore was observed to
be 98.32 %. The small amount of silicon is associated with the quartz crystal phase
and possibly an amorphous phase of alumino-silicate, not easily detected by x –ray
diffraction (Fig. 3).
Table 1: Elemental composition of Umuobom pyritic ore.
Element Si Al Fe S Mg Ca Na K Ti Mo
Conc in
solid, %
0.54 0.31 45.75 52.57 ˂
0.01
˂
0.01
˂
0.01
˂0.01 ˂
0.01
˂0.01
Figure 4 is the SEM images of the Umuobom pyrite ore showing the crystal shape
and the growth strips within the matrix.
385 Onyedika et al., Chemical, Mineralogical and…
Figure 4: SEM images of the surface phases of the Umuobom pyrite ore and growth
strips
Figures 5 is the EDS analysis of the crystal images shown in fig. 4. It shows the EDS
analysis of the crystal image of the pyrite ore. The result showed high peak of only
iron (Fe). Figure 5 further shows sharp peaks for molbedum (Mo) that was not shown
in the x- ray diffraction graph of figure 3. This indicates that Mo is overlapped with
the peak of sulfur (S).
386 Onyedika et al., Chemical, Mineralogical and…
Figure 5: SEM-EDS analysis of the crystal image of the pyrite
Figure 6 is the x-ray map of the Umuobom pyrite ore surface showing spatial
distribution of the various elements present within the ore body matrix. The
distribution patterns of the elements were observed to be random. The different
colours separation shows different elements as they are different within the ore
matrix. The distribution also shows iron (Fe), and sulfur (S) in larger and correlative
manner. The image mapping for iron (Fe) and sulphur (S) were found to be dominant
compare to that of aluminium (Al) and and silicon (Si) and other impurities. This
further shows the predominance of FeS2 in the ore as the major constituent of the
deposit.
Figure 6: X-ray mapping of pyrite cluster
Aluminum (Al) and silicon have a coexisting relation, and are distributed in foam like
phases. The foam like phase is the aluminum silicate mineral which was not
detected by XRD (Fig.3), which shows that its composition is not very significant
(less than 0.01) as shown in table 1.
387 Onyedika et al., Chemical, Mineralogical and…
Although pyrite overgrowth is the predominant cement that forms between and within
pyrite framboids, such spaces may also be filled with quartz. Although it is typically a
minor constituent, in places quartz can actually form a matrix that surrounds pyrite
crystals and hold them in place.
The SEM-EDS point analysis on the aluminum silicate mineral found within the pyrite
ore is shown in figure 7. The EDS point analysis shows peaks for Fe, Si and Al with
Fe as the predominant peak.
Figure 7: SEM-EDS analysis of point analysis of foam like aluminum silicate mineral
within the pyrite (FeS2) matrix.
Further probe on the foam like image revealed the presence of trace aluminum
silicate within the pyrite crystals matrix. Figure 8 is the x-ray map of the foam phase
of aluminum silicate.
388 Onyedika et al., Chemical, Mineralogical and…
Figure 8: X-ray map of the foam phase of aluminum silicate.
The x-ray map in figure 8 shows the random distribution pattern and reveals false
colors of Al, Si, S and Fe. The colors of Al, Si and sulfur were distinct compare to Fe
and this point reveal that the foam like characteristics observed is the actual
aluminum silicate mineral concentration.
Figure 9 is the EDS peaks of the foam phase of aluminum silicate. The result in
figure 9 shows high peaks of aluminum (Al) and silicon (Si), moderate and small
peaks for sulfur (S) and iron (Fe), respectively. This confirms that the foam phase
crystal is actually the crystal formation of aluminum silicate within the pyrite ore body.
389 Onyedika et al., Chemical, Mineralogical and…
Figure 9: The EDS peaks of the foam phase of aluminum silicate.
4.2 Paleoenvironmental Implication
The significance of pyrite concretions lie in the fact that they constitute a sand-sized
population of sedimentary particles that forms far offshore in shale successions, and
that they have implications regarding hydrodynamics, energy conditions and
oxygenation of the water column.
The concentration of these concretions into lag deposits of several centimeters thick
implies reworking and winnowing of a considerable thickness of previously deposited
carbonaceous muds and substantial current velocities to accomplish their erosion
(Schieber, 1998).
In modern pyrite-generating environments, early diagenetic pyrite is usually of very
small grain size (20µm or less, whereas larger pyrite grains are thought to reflect
additional pyrite production during deeper burial of sediment (Canfield et al., 1992).
In that context the comparative large pyrite concretions found in the study area
suggests significant burial depth or abundance of available reactive iron in the
system. In the later, the supply of iron could be related to a deposition of
terrigeneous grains with unusual thick iron oxide, or the possibility of iron enrichment
390 Onyedika et al., Chemical, Mineralogical and…
from accumulation of large quantities of planktonic organisms (Van Leeuwe et al.,
1997).
Under fully anoxic conditions also, where excess hydrogen sulphide exist, pyrite
tends to be disseminated evenly as small grains, because all the iron released from
terrigenous grains are immediately precipitated. Under oxic bottom waters, the
sediment is often anoxic, but typically non-sulphidic in the surface layer and allows
localized pyrite accumulation (Berner, 1981; Brett and Allison, 1998). Because iron
can migrate through the sediments under the ensuing combination of anoxic and
non-sulfidic conditions, localized pyrite accumulation in anaerobic
microenvironments of decaying organic matter as recognized in the study area is
possible.
5.0 Conclusion
The combination of the spectrum of x-ray diffraction, SEM –EDS micrograph and
inductive coupled plasma - optical emission spectrophotometry, one observes the
presence of one well-defined mineral peak and crystalline pyritic phase. Trace
amount of alumino-silicate mineral was encapsulated in a foam phase manner within
the ore matrix. Molybdenum (Mo) was also found to be associated with the FeS
mineral in an insignificant amount. The Umuobom pyrite deposit is chemically and
mineralogically high grade pyritic ore. This is important information to prospective
miners and researchers.
Pyrite concretionary nodules are a type of iron placer that requires selective erosion
and winnowing of host sediment as recognized from the study area. Thus, they can
serve as important tools in sequence stratigraphic study as they can serve as useful
markers for identification of key stratigraphic surfaces.
Pyrite formation remains an active area of research. Significant progress has been
made in understanding the mechanism of its formation and incorporation of
impurities into pyrite ores. Despite the progress, some aspects of pyrite formation
are yet to be resolved. Further analysis such as Rare Earth Elements Analysis
(REEs) and Sulphur Isotope Geochemistry should be carried out to determine the
possible source provenance of the pyrite.
391 Onyedika et al., Chemical, Mineralogical and…
Acknowledgement
The authors are sincerely grateful to the Nigeria Geological Survey Agency for
providing the geologist that assisted us in the sample collection. Thanks also to the
Institute of Material Processing, Michigan Technological University for providing the
analytical tools.
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