Chemical, Mineralogical and Environmental Aspects of the ... Elemental ratios such as Co/Ni and Se/S have been used to distinguish several ore forming environments instead of abundances

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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


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),


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,


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


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


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.


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).


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.


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.


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.


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


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.


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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Chemical, Mineralogical and Environmental Aspects of the ... Elemental ratios such as Co/Ni and Se/S have been used to distinguish several ore forming environments instead of abundances