The geology, petrology, palynology and geochemistry of Permian coal basins in Tanzania

International Journal of Coal Geology 55 (2003) 157 – 186 www.elsevier.com/locate/ijcoalgeo

The geology, petrology, palynology and geochemistry of Permian coal basins in Tanzania: 2. Songwe-Kiwira Coalfield P. Semkiwa a, W. Kalkreuth b,*, J. Utting c, F. Mpanju d, H. Hagemann e a

Geological Survey of Tanzania (Madini), Ministry of Water, Energy and Minerals, P.O. Box 903, Dodoma, Tanzania b Instituto de Geocieˆncias, Universidade Federal do Rio Grande do Sul, Av. Bento Goncßalves, 9500, 91501-970 Porto Alegre, RS, Brazil c Geological Survey of Canada (Calgary), 3303-33 St. NW, Calgary, Alberta, Canada T2L 2A7 d Tanzania Petroleum Development Corporation, P.O. Box 2774, Dar es Salaam, Tanzania e Lehrstuhl fu¨r Geologie, Geochemie und Lagersta¨tten des Erdo¨ls und der Kohle, RWTH Aachen, Lochnerstr. 4-20, 52064 Aachen, Germany Received 12 December 2002; accepted 4 June 2003

Abstract This study provides coal quality, petrological, palynological and geochemical (Rock Eval) data on Permian coal seams and associated shales and mudstones of the Karoo Supergroup of the Songwe-Kiwira Coalfield, Tanzania. The coal seams, which have a cumulative thickness of 6.80 m, occur in the shale – coal – sandstone facies of the Mchuchuma Formation of Artinskian to Kungurian(?) age. Coal quality data (calorific values, volatile matter contents) and vitrinite reflectances indicate high volatile C bituminous to high volatile A bituminous coals, having relatively high ash yields (22 – 49 wt.%) and highly variable sulphur contents (0.17 – 9.2 wt.%). They could be used to fuel small-scale power generation units thereby providing electricity to nearby towns and villages. Also, the coals could be used as a substitute for wood, which is becoming increasingly scarce. In rural Tanzania, charcoal is still the main energy source for cooking, and wood is used extensively in brick kilns and for making roofing tiles. Petrological analysis indicated that the coals are dominated by dull to banded dull lithotypes, with seams at the base of the Mchuchuma Formation enriched in inertinite macerals (up to 83 vol.%), whereas up-section vitrinite contents increase. Palynological analyses indicated that the assemblage in the lower Mchuchuma Formation (Scheuringipollenites assemblage) is dominated by trilete spores, whereas in the remainder of the section, non-taeniate disaccates dominate (Scheuringipollenites – Protohaploxypinus assemblage). Facies critical macerals suggest for most seams a marsh/wet forest swamp depositional setting, which is consistent with the palynological data. Rock Eval analyses indicate type II/III kerogen, with Tmax (jC) values ranging from 426 to 440, corresponding to the early stage of hydrocarbon generation. Thermal Alteration Indices (2 to 2+) and vitrinite reflectance levels (0.60 – 0.83 Ro (%) support the Rock Eval maturity assessment, and despite the predominance of terrestrial-derived organic matter, there is evidence of oil generation and expulsion in the form of cavity and fracture filling exsudatinite. D 2003 Elsevier B.V. All rights reserved. Keywords: Tanzania; Songwe-Kiwira Coalfield; Coal quality; Petrology; Palynology; Kerogen types; Maturity

* Corresponding author. Tel.: +55-51-33166355; fax: +55-51-33167302. E-mail address: [email protected] (W. Kalkreuth). 0166-5162/03/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0166-5162(03)00108-3

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1. Introduction The topic of this report is the Songwe-Kiwira Coalfield, which is situated near the northern tip of Lake Nyasa (Fig. 1). Coal reserves (proven and indicated) are in the order of 616  106 t and are currently being exploited by medium-size underground coal mines (Kiwira and Ilima Coal Mines). A comprehensive report on the Permian Karoo Coal Basins in Tanzania, which included previous research on geological setting, coal resources and coal quality, palynology and biostratigraphy, was presented by Semkiwa et al. (1998); this contained new data on the petrology, palynology and geochemistry of Permian coal-bearing strata in the Rukwa Basin (Namwele-Mkomolo, Muze and Galula coalfields) (Fig. 1). The only previous study in the Songwe-Kiwira Coalfield related to coal seam characterization using coal petrological methods is that of Semkiwa (1992). In that study maceral group composition, vitrinite reflectance (coal rank) and fluorescence parameters for the maceral sporinite were reported for four samples collected from the underground workings. Predominant components were vitrinite (up to 69 vol.%), whereas inertinite and liptinite contents were significantly lower (both had maximum values of 28 vol.%). Vitrinite reflectances indicated high volatile A bituminous coals (0.73 –0.85% Rmax). Spectral fluorescence parameters (lambda max, Q and chromaticity values) determined for sporinite were in agreement with coal rank as determined by vitrinite reflectance. The aim of the present study was to investigate the petrology, palynology, geochemistry, coal quality and paleodepositional environment, the thermal maturity and source rock potential of the Permian coal seams and associated organic-rich sediments in the SongweKiwira basin.

2. Geological setting The Songwe-Kiwira Coalfield is situated some 50 km west of the northern tip of Lake Nyasa (Fig. 1) and consists of grabens and half grabens with structures following similar trends to the present day rift valley and the older Precambrian fault zones. The depocenter

contains about 900 m of Karoo sediments, which are exposed along a north – northwest trending outcrop with an average width of 5 km (Fig. 2). There are two main blocks of Karoo rocks forming the north – south trending Ivogo/Kabulo Ridges in the west and the Ilima Hill in the east. Dividing these two blocks is the post-Karoo Kiwira fault. The Ivogo Ridge is separated from the Kabulo Ridge by the deeply incised Mwalesi Gorge. Sections of the Lower Karoo deposits are exposed along the Mwalesi Gorge and the Lema River near Ilima Hill (Fig. 2). Karoo rocks extend southwards from Songwe-Kiwira to northern Malawi where they include coal measures of the Nkana coalfield. The detailed geology of Songwe-Kiwira Coalfield was first described by Harkin (1955). The Karoo beds have an average dip of 25– 30j to the east and are unconformably overlain by fluviatile current-bedded sandstones of Cretaceous age. The basal Karoo sediments (Fig. 1) consist of the glaciogenic Idusi Formation (K1 of McKinlay, 1965) as described in the Ruhuhu Basin (Diekmann, 1993). The Idusi Formation at Songwe-Kiwira is restricted to the basal diamictite (15 – 20 m thick) of the Lisimba Member. The diamictite is composed of sub-angular and frequently faceted clasts with components of Ubendian rocks. The clasts have an average diameter of 4– 8 cm with occasional cobbles up to 40 cm near the base. This sequence is overlain by sandstones and laminated (varved) siltstone with dropstones (Fig. 3). The coal-bearing strata of the Songwe-Kiwira Coalfield are correlated with the shale –coal –sandstone facies of the Mchuchuma Formation of the Ruhuhu Basin (Semkiwa, 1992). At Songwe-Kiwira, the Mchuchuma Formation varies in thickness from nil to 200 m. The strata consist of alternating shale, mudstone, siltstone, sandstone and coal (Figs. 2 and 3). At Mwalesi Gorge, the sequence overlies the Idusi Formation with an angular unconformity of 9j on a partially eroded succession of the Lisimba Member of the Idusi Formation, whereas at Ilima, it overlies directly weathered basement rocks. The coal-bearing Mchuchuma Formation is divided into an upper and lower shale –coal facies, separated by about 40 m of a thick Intermediate Sandstone Member (Fig. 3). The thickest and best developed coal seams in the Mwalesi Gorge area occur in the lower shale –coal facies and unconformably overly the

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Fig. 1. Distribution of Karoo basins in Tanzania and locations of coalfields. Inserted table shows the age relationships of the coal-bearing formations.

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Fig. 2. Geological map of the Songwe-Kiwira Coalfield showing locations of mine sites and outcrop locations (Mwalesi River—Section 1, Lema River—Section 2).

varvites of the Idusi Formation. The upper-shale coal facies contains thin coals with limited lateral distribution. The Intermediate Sandstone Member (Fig. 3) separating the two facies is composed of coarse, often pebbly arkosic sandstone with fining upwards sequen-

ces. The low clay content of the sandstone suggests a possible braided stream environment. The Mchuchuma Formation is overlain by the Scarp Sandstone Member (Fig. 3), which in the Ruhuhu Basin, southwest of Songwe-Kiwira has been

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Fig. 3. Correlation chart of sections and coal seams investigated in this study including position of sampled intervals. 7-TZ-224 sample number.

included in the Mbuyura Formation (Kaaya, 1992). The Scarp Sandstone consists of coarse, highly cemented, light coloured, micaceous sandstone with typical fining upwards sequences ranging in thickness from 0.5 to 5.0 m. The boundary between the Mchuchuma Formation and the overlying Mbuyura Formation is difficult to determine. It is tentatively placed at the last appearance of carbonaceous-rich sediments and the first occurrence of red to greenish coloured clay-rich sediments. The lower part of the Mbuyura Formation consists of red sandstone, massive sandstone units and clay-rich red beds. The latter reflects

an increasing degree of chemical weathering. The upper half is composed of clay/siltstone and sandstone dominated strata. Limestone beds, calcareous nodules and concretions are common which suggest a lacustrine environment of deposition. Karoo sediments of the Mhukuru Formation (Fig. 1) are not developed in the Songwe-Kiwira area suggesting a possible hiatus or erosion. Recently, Weiss (2001) described the microflora from this unit in the Ruhuhu Basin. The Mbuyura Formation is overlain by the Ruhuhu Formation with an angular unconformity. Similar to the type locality (Ruhuhu

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Basin), the formation consists of clayey limestones with ostracodes and marls. In the lower part of the formation, arkosic sandstone is common. The Ruhuhu Formation is overlain unconformably by the fluviatile current-bedded Red Sandstone Group of Cretaceous (?) age, which in the Songwe-Kiwira area is in the order of 600 m (McKinlay, 1965). In Malawi, the Red Sandstone Group contains Cretaceous Saurian remains. The northern part of the Songwe-Kiwira basin is covered by basalts of the Neogene Rungwe Volcanics.

Two 0.50-m seams are exposed at the base of a covered interval close to the top of the formation (Fig. 3A), and are correlated with #2 seam at Lema River (Fig. 3B). 3.2. Lema River (11 samples) At Lema River, a short section of the upper part of the Mchuchuma Formation is exposed (Fig. 3B) and provided samples for coal seams #1 (0.41 m) and #2 (upper bed: 0.15 m, lower bed: 0.30 m). 3.3. Kiwira Mine (38 samples)

3. Coal seam distribution and sampling Workable coal seams occur at the bottom and top of the lower shale –coal facies within the Mchuchuma Formation and these seams were sampled for analysis (Fig. 3) along with a few samples from thin and discontinuous seams developed in the upper part of the shale – coal facies. The coal seams have been named from top to bottom of the sequence as seam Number 1 to 9 (Chinese Coalfield Investigating Team, 1979). A total of 132 coal, carbonaceous shale and mudstone samples were collected from outcrop sections and the two underground mines (Kiwira and Ilima Coal Mines). The coal outcrops were frequently weathered at the surface, and care was taken to obtain fresh material. Coal seams were described megascopically at outcrop or mine sites following a somewhat modified Stopes Heerlen System (Bustin et al., 1989). The coal seams were sampled for petrographic analysis as full seam channel samples and/or as lithotype samples to study in-seam variations. A representative split of each sample was provided for geochemical (Rock Eval) and palynological analyses. Fig. 3 provides a correlation chart of outcrop and subsurface sections with identification of seams and sampled intervals. 3.1. Mwalesi River (53 samples) At the Mwalesi River section (Fig. 3A), the Mchuchuma Formation comprises an approximately 90-m-thick succession of interbedded coal, mudstone and carbonaceous shale. The coal seams identified in this section (from base to top) are seams #9 (0.50 m), #6 (0.30 m), #3b (2.00 m) and seam #3a (3.00 m).

A subsurface section at Kiwira Mine (Fig. 3C) provided samples of seam #3b (1.80 m), seam #5 (1.60 m), seam #6 (0.97 m) and seam #9 (0.05 m). The total number of samples collected at this location (coals, shales and mudstones) is 38. 3.4. Ilima Coal Mine (30 samples) Ilima Coal Mine provided samples for coal seams #3a (0.61 m) and #3b (2.03 m) (Fig. 3D).

4. Analytical procedures Samples for petrographic analysis were prepared using standard procedures (Bustin et al., 1989). Analytical methods included incident light microscopy to determine coal rank by measurement of vitrinite reflectance (Bustin et al., 1989). Coal maceral composition was determined using both white light and blue light excitation. The macerals were identified and counted (500 counts/sample, including minerals) according to the nomenclature defined by the International Committee for Coal and Organic Petrology (ICCP, 1963, 1971, 1998). Throughout the text and on figures, maceral groups are reported in vol.% on a mineral matter-free basis (m.m.f.). At the time of analysis, the new ICCP inertinite classification (ICCP, 2001) had not been established. Therefore, the inertinite maceral sclerotinite is reported here, although semi-quantitative re-examination of sclerotinite-rich samples (2 – 5 vol.%) suggests that the major part of the sclerotinite would be classified as funginite according to the new classification.

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The organic geochemistry of the coal, shales and mudstones and the evaluation of their hydrocarbon generation potential are based on Rock Eval pyrolysis techniques (Espitalie´ et al., 1977; Peters, 1986). Coal quality data (moisture, volatile matter, fixed carbon, ash yields, sulphur content, calorific value) were determined according to ASTM standard procedures (ASTM, 1991). Palynological samples were processed using standard preparation techniques (Wood et al., 1996). Palynomorphs in each sample were identified, and wherever possible, 250 specimens were counted to obtain quantitative data.

5. Results and discussion 5.1. Coal petrology Petrological coal seam characteristics are given in terms of (a) macroscopical appearance (lithotypes); (b) petrographic composition (maceral groups including Gelification Index (GI) and Tissue Preservation Index (TPI) based on maceral associations as defined by Diessel (1986) and mineral matter contents). 5.1.1. Lithotypes The coal seams are typically intercalated with numerous shale and mudstone partings and grade

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frequently into carbonaceous shale. The coal is composed predominantly of dull (D), banded dull (BD), to banded coal (BC) lithotypes. The shale and mudstone partings predominate in most seams and are the major contaminants responsible for the high ash yields. The macropetrographic characteristics of the coals from the Songwe-Kiwira coalfield are comparable to those observed in coal seams occurring in the shale – coal –sandstone facies of the Ruhuhu Basin (Semkiwa, 1992). 5.1.2. Petrographic composition 5.1.2.1. Gross petrographic composition. The overall petrographic composition of coal seams analyzed from the Songwe-Kiwira Coalfield is shown in Fig. 4. There is a large spread in the relative proportions of vitrinite to inertinite macerals, with essentially all samples from Kiwira Mine having more than 50 vol.% inertinite (Fig. 4). Higher vitrinite contents in excess of 50 vol.% occur in samples from the Mwalesi River section and in samples from Lema River and Ilima Mine (Fig. 4). Liptinite contents constitute up to 20 vol.%. It appears that gross petrographic composition of the coal seams is related to their stratigraphic position within the Mchuchuma Formation. Seams developed in the lower part of the formation (#5, #6 and #9, see also Fig. 3) have the highest inertinite contents (Fig.

Fig. 4. Ternary diagram showing maceral group distribution of full seam channel samples in the Songwe-Kiwira Coalfield.

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4). Seam #6 at Kiwira Mine and #9 seam at Mawalesi River have extraordinarily high inertinite values (83 vol.%) and, from a petrographic point of view, could well be correlative seams. Seams developed higher up in the stratigraphic column (#3A and #3B, see also Fig. 3) have intermediate inertinite values between 28 and 49 vol.% (Fig. 4). In the upper part of the section, the lower bed of #2 seam at Lema River and the lower bed of an unnamed seam at Mwalesi River have the highest vitrinite contents of close to 70 vol.%. The upper bed of seam #2 and the upper bed of the unnamed seam at Mwalesi have significant lower vitrinite values ( < 50 vol.%). The trend of decreasing vitrinite towards the top of the formation continuous with seam #1 at Lema River, which contains only 40 vol.% vitrinite but has 51 vol.% inertinite (Fig. 4). The petrographic similarities of #2 seam (upper and lower beds) at Lema River and the unnamed seam (upper and lower beds) at Mwalesi River suggest that these seams may be correlatives (Figs. 3 and 4). The lower beds of the seams are characterized by vitrinite contents of 71 and 66 vol.%, inertinite contents of 20 and 26 vol.% and liptinite contents of 9 and 8 vol.%, respectively. The upper beds contain 48 and 53 vol.% vitrinite, 41 and 42 vol.% inertinite and 11 and 8 vol.% liptinite. The seams also have petrographical similarities in individual maceral content such as collotelinite and fusinite abundance, and pyrite distribution. The upper beds of #2 seam at Lema River and Mwalesi River have pyrite content from 10 to 18 vol.%, whereas the lower beds have virtually no pyrite (nil to 0.2 vol.%). The high amounts of inertinite macerals recorded for many of the Songwe-Kiwira coal seams are observed frequently in Permian coals of Gondwana as has been reported elsewhere (Moodie, 1978; Correa da Silva, 1980; Navale and Saxena, 1989; Hunt, 1989; Semkiwa et al., 1998; Holz and Kalkreuth, in press). At Songwe-Kiwira, inertinite group macerals are dominated by inertodetrinite, fusinite and semifusinite. Others include macrinite, sclerotinite, micrinite and rare pyrolithinite (pyrolytic carbon). Vitrinite group macerals mainly consist of collodetrinite, collotelinite, pseudovitrinite, vitrodetrinite and rare telinite. Liptinite macerals are dominated by sporinite, resinite, cutinite and liptodetrinite.

5.1.3. In-seam petrographic variations 5.1.3.1. Mwalesi River. At the Mwalesi River, major coal development occurs in the lower shale – coal facies of the Mchuchuma Formation (Fig. 3). The coal zone has an approximate thickness of 13 m (Fig. 5), comprised of carbonaceous shale alternating with coal. The coal seams were identified as seam #3 (top), underlain by seam #3b (?) (middle and basal part) of the sequence. Seam #3a has a total thickness of 3.80 m, of which 3.03 m is coal. The seam is characterized by the dominance of thin bright layers at the base, banded coal in the central part and banded dull and dull lithotypes at the top. Thin fusain lenses are common (marked as F in Fig. 5). Maceral group contents are highly variable with respect to the position within the seam and lithotype association ranging for vitrinite from 36 to 72 vol.%, whereas liptinite and inertinite group macerals range from 7 to 18 and 19 to 49 vol.%, respectively. Mineral matter content ranges from 7 to 28 vol.% and is dominated by clay minerals and quartz. Pyrite content ranges from nil to 2.2 vol.% at the top of the seam. Seam #3b is developed in a 7.62-m sequence of alternating carbonaceous shale and coal, of which 1.08 m is coal. The seam is characterized by thin beds of coal at the top (Fig. 5), and a 0.68-m seam at the base of the succession. The thin layers of coal at the top are bright and banded bright, whereas at the base, the coal is dull to banded coal. Vitrinite group macerals range from 5 vol.% in a dull layer at the base to 72 vol.% in a bright lithotype in the upper part of the coal zone. Inertinite content ranges from 15 to 89 vol.% (Fig. 5), where the macerals inertodetrinite and fusinite are the most abundant components in samples enriched in inertinite. Liptinite content ranges from 3 to 13 vol.%, with the predominance of sporinite. Mineral matter content is below 25 vol.% in most samples (Fig. 5), except in a coal seam developed near the base of the coal zone, where mineral matter contents increase to 61 vol.% in the upper part of the seam. The mineral matter is, for the most part, represented by clay minerals and quartz (5– 44 vol.%), whereas carbonate occurs in significant amounts only in one sample (7-TZ-214, 16 vol.%). Pyrite content ranges from nil to 0.6 vol.%.

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Fig. 5. Coal seam characteristics for #3a and #3b seams, Mwalesi River section (for location see Fig. 2). Pattern used to describe lithology of partings, floor and roof rocks is the same as in Fig. 3. Abbreviations used for coal lithotypes are as follows: F = fibrous; D = dull; BD = banded dull; BC = banded coal; BB = banded bright; B = bright. GI vitrinite + macrinite/fusinite + semifusinite + inertodetrinite + micrinite; TPI telinite + collotelinite + corpogelinite (in situ) + fusinite + semifusinite/collodetrinite + vitrodetrinite + inertodetrinite + macrinite + micrinite.

5.1.3.2. Lema River. At Lema River, seams #1 and #2 of the upper shale –coal facies are exposed (Fig. 3). The coal zone has a thickness of 2.66 m (Fig. 6), of which 0.86 m is coal alternating with layers of carbonaceous mudstone and shale (Fig. 6). The coal is banded bright at the base (#2 seam, lower bed) and banded dull (#2 seam, upper bed). Seam #1 has a thin layer of vitrain (bright coal) at the seam base followed by banded dull and dull coal towards the top. In terms of maceral group distribution, there is a distinct trend from vitrinite-rich coal at the base (70.9 vol.%, #2 seam, lower bed) to vitrinite-poor coal at the top (40.3 vol.%, #1 seam), whereas inertinite shows the opposite trend (Fig. 6). Liptinite content remains the same regardless of stratigraphic position (9.1 –10.9 vol.%). Mineral matter content ranges from 18.4 to 43.6 vol.%, and occurs mainly in the form of

clay minerals and quartz. The upper bed of #2 seam is characterized by enrichment of pyrite (18 vol.%), where in the other coal beds, no pyrite was detected. 5.1.3.3. Kiwira Mine. At Kiwira Mine, the lower shale –coal facies of the Mchuchuma Formation include seams #3b, #5, #6 and #9 (Fig. 3). The proximity to the base of the Mchuchuma Formation is indicated by laminated varvites exposed approximately 5 m below the base of #9 seam. At this location, seams #3b and #5 were analyzed in terms of seam sub-sections, and the results are discussed below. Seam #3b is 1.86 m thick, of which 1.11 m is coal (Fig. 7). Partings are comprised of carbonaceous siltstone in the lower part of the seam and carbonaceous mudstone in the upper part of the seam. The lower part of the seam is mainly composed of banded

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Fig. 6. Coal seam characteristics for #1 and #2 seams, Lema River section (for location see Fig. 2). Pattern used to describe lithology of partings, floor and roof rocks is the same as in Fig. 3. Abbreviations used for coal lithotypes are as follows: F = fibrous; D = dull; BD = banded dull; BC = banded coal; BB = banded bright; B = bright. For explanation of GI and TPI, see legend Fig. 5.

coal (Fig. 7), whereas the upper part consists of banded coal, with banded dull and bright coal at the very top. Vitrinite macerals range from 17 to 98 vol.% and inertinite macerals range from 2 to 67 vol.%. The distribution of the maceral groups largely reflects the lithotypes in which they occur: higher contents of vitrinite and lower contents of inertinite in the brighter lithotypes, whereas the opposite trend occurs in the duller lithotypes (Fig. 7). Liptinite content ranges from < 1 to 17 vol.%, with sporinite being the most abundant maceral. Mineral matter content ranges from 6 to 42 vol.%, mainly consisting of clay minerals and quartz. Pyrite content is low (nil to 0.5 vol.%), except at the top of the seam, where a pyrite content of 4 vol.% (sample 8-TZ-134-93) was recorded. Seam #5 is 1.60 m thick (Fig. 8) of which 1.29 m is coal, the remainder is comprised of coaly mudstone,

carbonaceous sandstone and carbonaceous mudstone partings. The lower part of the seam mainly consists of banded coal and banded dull, whereas the upper seam is comprised of banded dull, banded coal and dull lithotypes. Results from maceral analyses indicate significant changes in petrographic composition from seam base to seam top. At seam base, vitrinite is the predominant component (Fig. 8), with up to 66 vol.%, and moderate inertinite (17 –22 vol.%) and liptinite contents (12 –19 vol.%). The remainder of the seam sub-sections (Fig. 8) is characterized by a drastic decline in vitrinite (1– 6 vol.%) and liptinite (1 –9 vol.%) contents. The organic matter in these samples is composed almost entirely of highly oxidized inertinite macerals (89 – 98 vol.%), with fusinite, inertodetrinite and semifusinite being most abundant. Mineral matter content ranges

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Fig. 7. Coal seam characteristics for #3b seam, Kiwira Mine (for location see Fig. 2). Pattern used to describe lithology of partings, floor and roof rocks is the same as in Fig. 3. Abbreviations used for coal lithotypes are as follows: F = fibrous; D = dull; BD = banded dull; BC = banded coal; BB = banded bright; B = bright. For explanation of GI and TPI, see legend Fig. 5.

from 26 to 65 vol.%, with highest values in the central part of the seam and at the top. The mineral matter is composed mainly of clay minerals and quartz; and carbonate is abundant in samples 8-TZ106, 112 and 113 (13 – 21 vol.%). Pyrite content ranges from nil to 0.8 vol.% except at seam base (1 vol.%, sample 8-TZ-104-93) and at seam top (2 vol.%, sample 8-TZ-115-93). 5.1.3.4. Ilima Mine. At Ilima Mine, seams #3a and #3b from the lower shale – coal – sandstone facies of the Mchuchuma Formation are mined (Fig. 3). Seam #3 is split into four beds of coal (Fig. 9) interlayered with carbonaceous siltstone, carbonaceous shale and silty mudstone. The total coal seam thickness is 1.25 m, of which 0.61 m is coal. The seam consists of

shaley coal at the base, followed upward by banded dull and dull coal and banded coal at the top. Vitrinite content ranges from 10 to 63 vol.% (Fig. 9), whereas inertinite content ranges from 28 to 61 vol.%. Liptinite content ranges from 9 to 29 vol.%. Mineral matter content is low to moderate (10 –16 vol.%), except in the dull coal, which is composed of 75 vol.% mineral matter, mainly in the form of clay minerals, quartz (56 vol.%) and carbonate (19 vol.%). Pyrite was recorded in the range from 0.2 to 2.2 vol.%, with the highest value occurring in sample 9-TZ-160 in the basal part of the seam. Seam #3b is 1.94 m thick (Fig. 10), of which 1.50 m is coal. The seam has four partings comprised of silty mudstone, carbonaceous mudstone and carbonaceous sandstone, and is overlain by carbonaceous shale and a

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Fig. 8. Coal seam characteristics for #5 seam, Kiwira Mine (for location see Fig. 2). Pattern used to describe lithology of partings, floor and roof rocks as in Fig. 3. Abbreviations used for coal lithotypes are as follows: F = fibrous; D = dull; BD = banded dull; BC = banded coal; BB = banded bright; B = bright. For explanation of GI and TPI, see legend Fig. 5.

thin rider seam (Fig. 10). The lower part of the seam shows a dulling-upward sequence (Fig. 10) from banded bright coal at the base to dull coal at the top. The upper part of the coal is brighter (banded coal to bright coal). In-seam variations in maceral groups and mineral matter content are largely controlled by lithotype distribution. This is particularly evident in the lower part of the seam, where vitrinite decreases from banded bright to dull coal, and inertinite and mineral matter increase. The overall range for vitrinite is 10 – 68 vol.%, for inertinite 22– 71 vol.% and for liptinite 6– 19 vol.%. Mineral matter content ranges from 5 to 31 vol.%, consisting almost entirely of clay minerals and quartz. Pyrite content is < 1 vol.% in all samples except in sample 9-TZ-151 at the top of the sequence, where pyrite content is 1.1 vol.%.

5.2. Coal quality and utilization potential Table 1 shows coal quality parameters determined for the major seams of the Songwe-Kiwira Coalfield. Ash yields are elevated in all seams, ranging from 22.37 to 49.30 wt.%. Sulphur contents are highly variable, ranging from 0.17 wt.% at the Kiwira Mine (#5 seam) to 9.20 wt.% in the #6 seam at the Mwalesi section. Calorific values of the seams range from 15.2 to 25.7 MJ/kg (dry) (Table 1). To determine ASTM coal rank, the calorific values were transformed from MJ/ kg into BTU/lb and calculated on a dry, mineral matter-free basis in accordance with Parr’s formula (Annual Book of ASTM Standards, 1991). In a similar manner, fixed carbon values were calculated

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Fig. 9. Coal seam characteristics for #3a seam, Ilima Mine (for location see Fig. 2). Pattern used to describe lithology of partings, floor and roof rocks is the same as in Fig. 3. Abbreviations used for coal lithotypes are as follows: F = fibrous; D = dull; BD = banded dull; BC = banded coal; BB = banded bright; B = bright. For explanation of GI and TPI, see legend Fig. 5.

to an ash-free basis as required by the ASTM standard. The data suggest (Table 1) that coals in the upper part of the coal-bearing sequence are high volatile bituminous C to C/B (Mwalesi River, #2 seam, upper and lower beds) and grade into high volatile bituminous B and A coals in the stratigraphically older seams at Mwalesi, Kiwira and Ilima Mines. Coal rank variations based on vitrinite reflectances (Table 1) show a similar pattern, although the reflectance levels determined for the lower seams would suggest a somewhat higher rank than that derived from calorific values. The discrepancies are most likely related to the known problems in the precision determining calorific values and fixed carbon in mineral matter-rich and/or vitrinite-poor

coals. The ASTM standard was designed for vitrinite-rich coals with low mineral matter content, whereas the sample material from Songwe-Kiwira is mineral matter-rich, and some samples are very poor in vitrinite as well. The Songwe-Kiwira coals are thermal coals based on relatively low rank (Rrandom 0.62 –0.83%, high volatile bituminous C/B-A) and high ash yields. They could be used to fuel small-scale power generation units thereby providing electricity to nearby towns and villages. Also, the coals could be used as a substitute for wood, which is becoming increasingly scarce. In rural Tanzania, charcoal is still the main energy source for cooking, and wood is used extensively in brick kilns and for making roofing tiles.

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Fig. 10. Coal seam characteristics for #3b seam, Ilima Mine (for location see Fig. 2). Pattern used to describe lithology of partings, floor and roof rocks as in Fig. 3. Abbreviations used for coal lithotypes are as follows: F = fibrous; D = dull; BD = banded dull; BC = banded coal; BB = banded bright; B = bright. For explanation of GI and TPI, see legend Fig. 5.

5.3. Palynology 5.3.1. Mwalesi River Seven samples were collected from the Mchuchuma Formation, and one from the Mbuyura Formation (Fig. 3). Assemblages from coal seams #6 and #9, in the lowest part of the Mchuchuma Formation, are dominated by trilete spores (e.g. Apiculatisporis levis, Brevitriletes cornutus, Horriditriletes filiformis, H. ramosus, H. tereteangulatus and Punctatisporites gretensis), whereas in the remainder of the section, non-taeniate disaccates generally predominate e.g. Scheuringipollenites maximus and S. ornatus (Figs. 11 and 12). Although the precise affinities of many of the spore taxa are uncertain, most of the triletes are generally considered to be derived from pteridophytes (ferns), whereas spores of arboreous and herbaceous

lycoposids, and tree ferns, were less common. The affinities of the gymnosperm (seed plant) pollen taxa are less well known, and non-taeniate and taeniate disaccate pollen are thought to have been derived from a variety of gymnosperm orders (Meyen, 1987). Taeniate disaccates are considered to be well adapted to arid climates. Thus, the abundance of presumed trilete pteridophyte spores in the lowest part of the Mchuchuma Formation suggests that ferns made a significant contribution to the flora, whereas seed plants were more important in the remainder of the section. Qualitatively and quantitatively, there are no obvious differences in the pollen and spore composition of coal seams #2, #3a and #3b. An exception is sample 7-TZ-222 from seam #2, which has an anomalously high proportion of trilete and monolete spores.

Locality Sample no.

Seam (m) #

Moist. V.M. F.C. ASH S C.V. C.V. F.C. ASTM a.r. (dry, (dry, (dry, (wt.%) (MJ/kg) (BTU/lb) (wt.%) rank wt.%) wt.%) wt.%) d.b. d.a.f. d.a.f.

Rank from Rr (%)

Rr (%)

S

N

VITR LIPT INER (vol.%, m.m.f.)

Mwalesi River

2,u 2,l 3A 3A 6 9 1 2,u 3B 5 6 3A 3B

2.46 3.38 2.20 2.39 2.61 2.77 2.41 1.91 1.58 1.81 1.96 1.35 1.39

HVB HVB HVB HVB HVB HVB HVB HVB HVB HVB HVB HVB HVB

0.71 0.71 0.65 0.70 0.82 0.80 0.67 0.62 0.70 0.72 0.83 0.69 0.83

0.04 0.05 0.05 0.05 0.08 0.09 0.05 0.04 0.07 0.04 0.08 0.05 0.07

50 50 50 50 50 50 50 50 50 50 50 50 50

53 66 36 44 31 3 40 48 56 17 8 50 62

Lema River Kiwira Mine Ilima Mine

7-TZ-223-93 7-TZ-221-93 7-TZ-181-93 7-TZ-184-93 7-TZ-95-93 7-TZ-97-93 10-TZ-175-93 10-TZ-171-93 8-TZ-135-93 8-TZ-117-93 8-TZ-102-93 9-TZ-154/162 9-TZ-152A-93

0.50 0.50 0.50 0.60 0.30 0.50 0.41 0.30 1.80 1.60 0.70a 0.60 1.94

27.32 24.15 31.21 28.78 21.49 16.12 28.24 29.14 23.35 14.66 21.27 21.41 29.44

44.32 48.19 43.03 45.32 34.81 42.47 45.33 43.21 36.25 37.61 52.29 29.30 40.19

28.36 27.65 25.77 25.90 43.70 41.42 26.43 26.70 40.40 47.73 26.44 49.30 22.37

3.62 0.89 1.34 0.98 9.20 0.79 0.50 4.53 0.46 0.17 1.28 0.87 0.54

20.5 21.5 22.9 22.5 15.3 17.3 22.8 23.7 18.7 15.2 22.7 15,653 25,748

12,872 13,014 13,687 13,483 13,127 13,518 13,773 14,518 14,301 13,479 13,709 14,448 14,617

67.43 72.51 61.86 62.47 74.06 81.34 65.80 63.69 66.34 80.74 75.74 64.92 54.08

HVB HVB HVB HVB HVB HVB HVB HVB HVB HVB HVB HVB HVB

C C/B B B B B B A A B B A A

B B C/B B A A C/B C B B A B A

5 8 17 7 11 14 9 11 11 6 9 23 9

42 26 47 49 58 83 51 41 33 77 83 27 29

Moist., a.r. = Moisture content, as received; V.M. = volatile matter; F.C. = fixed carbon; S = sulphur; C.V. = calorific value; Rr = arithmetic mean of random vitrinite reflectances; S = standard deviation; N = number of measurements; VITR = vitrinite; LIPT = liptinite; INER = inertinite; d.b. = dry basis; d.a.f. = dry ash-free basis; m.m.f. = mineral matter-free. a Total coal thickness of 6 seam is 0.97 m, analysed sample represents upper bed.

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Table 1 Coal quality parameters for Songwe-Kiwira coal seams

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Fig. 11. Qualitative pollen and spore vertical distribution chart for outcrop section at Mwalesi George. For location of section and stratigraphic position of samples, see Figs. 2 and 3.

The trends outlined above are comparable to those seen in the Mchuchuma Formation of the Namwele block of the Rukwa Basin (Semkiwa et al., 1998), where there is a similar abundance of

pteridophyte spores in the lower part of the section, but gymnosperm pollen is more abundant in the remainder. The lower assemblage was correlated with the Scheuringipollenites assemblage of Weiss

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Fig. 12. Quantitative pollen and spore vertical distribution chart for outcrop section at Mwalesi George. For location of section and stratigraphic position of samples, see Figs. 2 and 3.

(in Semkiwa, 1992), and the remainder to the Scheuringipollenites –Protohaploxypinus assemblage (Fig. 11). The single sample collected from the Mbuyura Formation was unproductive.

5.3.2. Lema River The upper part of the Mchuchuma Formation, exposed in the channel of the Lema River, was sampled (Fig. 3B). The pollen and spore assemblages (Figs. 13 and 14) are dominated by non-taeniate

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Fig. 13. Qualitative pollen and spore vertical distribution chart for outcrop section at Lema River. For location of section and stratigraphic position of samples, see Figs. 2 and 3.

disaccates e.g. Scheuringipollenites maximus and S. ornatus, and resemble that from the middle and upper part of the Mchuchuma Formation which is assigned to the Scheuringipollenites – Protohaploxypinus assemblage of Weiss (in Semkiwa, 1992). There is no obvious difference between the palynoflora of coal seams #1 and #2 (Figs. 13 and 14), but 1 m below seam #2, a single sample (10-TZ-166A) contains an anomalously high percentage (40%) of

the praecolpate pollen grain Marsupipollenites striatus. This taxon may have a medullosan affinity (Balme, 1970). 5.3.3. Kiwira Coal Mine The varvites from the upper part of the Lisimba Member, Idusi Formation, and the lower part of the Mchuchuma Formation were sampled (Fig. 3C).

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Fig. 14. Quantitative pollen and spore vertical distribution chart for outcrop section at Lema River. For location of section and stratigraphic position of samples, see Figs. 2and 3.

The assemblage in the Lisimba Member generally consists of poorly preserved pollen and spores. These are characterized by a few species of trilete spores, which are sometimes common, e.g. Zinjisporites eccensis and Punctatisporites gretensis, and common monosaccate pollen, e.g. Cannanoropollis obscurus and Plicatipollenites indicus. This assemblage is assigned to the Cannanoropollis/Plicatipollenites assemblage of Weiss (in Semkiwa, 1992).

Samples from the lower part of the Mchuchuma Formation contain an assemblage similar to that of the Mwalesi section described above. Coal seams #9 and #6 in both localities, with their abundance of trilete spores, are very similar qualitatively and quantitatively (Figs. 15 and 16) and are assigned to the Scheuringipollenites assemblage of Weiss (in Semkiwa, 1992). Samples from #5 differ in that they generally contain a very low proportion of trilete spores, the main

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Fig. 15. Qualitative pollen and spore vertical distribution chart at Kiwira Coal Mine. Sample locations are shown in respect to stratigraphic distance from Idusi/Mchuchuma contact. For location of Kiwira Coal Mine and stratigraphic position of samples, see Figs. 2 and 3.

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Fig. 16. Quantitative pollen and spore vertical distribution chart at Kiwira Coal Mine. Sample locations are shown in respect to stratigraphic distance from Idusi/Mchuchuma contact. For location of Kiwira Coal Mine and stratigraphic position of samples, see Figs. 2 and 3.

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exception being sample 8-TZ-106, where triletes are abundant. This seam is tentatively assigned to the Scheuringipollenites – Protohaploxypinus assemblage of Weiss (in Semkiwa, 1992). Assemblages from coal seam #3b are similar to those of seam #3b of the Mwalesi section and are assigned to the same Scheuringipollenites – Protohaploxypinus assemblage. 5.3.4. Ilima Coal Mine Samples of the Mchuchuma Formation were collected from coal seams 3a and 3b (Fig. 3D). The assemblage contains abundant non-taeniate disaccate pollen (Scheuringipollenites maximus and S. ovatus) and rare to common trilete spores (Apiculatisporis levis, Brevitriletes cornutus, Cirratriradites africanensis, Horriditriletes ramosus, H. tereteangulatus, Leiotriletes directus, Lophotriletes novicus, Microbaculispora micronodosa, Punctatisporites gretensis and Retusotrilets diversiformis). The assemblage (Figs. 17 and 18) is qualitatively and quantitatively similar to that of coal seams #3a and #3b of the Mwalesi River section (Figs. 11 and 12) and is assigned to the Scheuringipollenites –Protohaploxypinus assemblage of Weiss (in Semkiwa, 1992). 5.3.5. Probable ages of palynomorph assemblages The age of the Idusi and Mchuchuma formations was discussed in detail by Semkiwa et al. (1998) and is not repeated here. Based on palynology, the two formations have been correlated with the Dwyka and Ecca Groups of the main Karoo Basin (type locality) in South Africa. The upper part of the Idusi Formation was assigned to the Cannanoropollis/Plicatipollenites assemblage of the Upper Dwyka (Weiss and Wopfner, 1997). The lower part of the Mchuchuma Formation was assigned to the Scheuringipollenites assemblage of the Middle Ecca, and the upper part of the Mchuchuma Formation to the Scheuringipollenites –Protohaploxypinus assemblage of the Middle Ecca (Weiss, in Semkiwa, 1992). Whereas correlation of the nonmarine Permian of Gondwana with the marine stratotypes of Russia is still very uncertain, correlations have been made by previous workers incorporating data from Australia, e.g. Foster and Waterhouse (1988). These suggest that the upper Idusi Formation is Late Asselian to Early Sakmarian and the Mchuchuma Formation Artinskian to Kungurian (?) in age (Semkiwa et al., 1998; Weiss, 2001).

5.4. Facies analysis and depositional environment Diessel (1986) used coal petrographic analysis to assess depositional environments during peat accumulation. The study was aimed at examining the influence of larger scale depositional processes on the petrography of coals from Permian (Gondwana) sequences of eastern Australia. The results indicated that petrographic indices derived from maceral analysis (Gelification Index, Tissue Preservation Index) were similar for coal seams that had been formed in a similar depositional environment. The mire types identified in that study were marsh, fen, wet forested swamp, dry forest swamp, whereas depositional environments ranged from lower delta plain, upper delta plain, back barrier to piedmont plain (Diessel, 1986). The principal parameters used in the facies analyses are (a) the Tissue Preservation Index (TPI), which compares the proportions of coal macerals in which plant tissue structure is preserved with macerals in which the original botanical cell-structure is destroyed by humification or gelification, or by mechanical breakdown. It also reflects, in part, the input of woody vegetation; (b) the Gelification Index (GI), which contrasts the partially and completely gelified macerals with those that are ungelified. It largely reflects availability of moisture during the formation of peat and also during early diagenesis. Since its introduction in 1986, followed by subsequent modifications by Calder et al. (1991) and Lamberson et al. (1991), the TPI/GI concept has been used in numerous studies to define paleodepositional environments during peat accumulation (e.g. Paul et al., 1989; Kalkreuth et al., 1991; Calder et al., 1991; Kalkreuth et al., 2000). Recently, however, this concept has been questioned on the basis that modern peats accumulating in different environments may have a very similar petrographic composition (Wu¨st et al., 2001; Moore and Shearer, in press) and will likely result in coals of similar petrographic characteristics. Despite the possible shortcomings of the maceralderived TPI/GI concept when based on coal petrology alone, the authors believe that it is a useful tool to determine the depositional history of coals on the basis of their petrographic properties, especially when other facies parameters such as palynological

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Fig. 17. Qualitative pollen and spore vertical distribution chart for seam #3a at Ilima Coal Mine. For location of Kiwira Coal Mine and stratigraphic position of samples, see Figs. 2 and 3.

and sedimentological characteristics of the seams and enclosing strata are considered. See also early studies on facies analysis of soft brown coal from the Rhein and Lausitz areas, Germany, where coal petrology was integrated with palynology including cuticular analysis and macropetrographic analysis to reconstruct the Tertiary mires (Teichmu¨ller, 1958; Neuy-Stolz, 1958; Benda, 1960; Schneider, 1966; Burgh, 1973; Brelie and Wolf, 1981a,b; Sontag and Schneider, 1982).

In the present study, the TPI/GI concept and evidence from palynological analyses have been applied in the Songwe-Kiwira Coalfield for the interpretation of the depositional environments and paleoflora during peat accumulation. The diagram shown in Fig. 19 is that of Lamberson et al. (1991), which uses relative amounts of vitrinite versus inertinite macerals and structured versus degraded macerals to define forest and marsh conditions during peat accumulation (Fig. 19A and B). Marshes

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Fig. 18. Quantitative pollen and spore vertical distribution chart for seam #3a at Ilima Coal Mine. For location of Kiwira Coal Mine and stratigraphic position of samples, see Figs. 2 and 3.

dominated by herbaceous plants are characterized by low TPI values, and depending on hydrogeological conditions, may give rise to low GI values (open marsh, where the peat is partially oxidized by occasional drop in the water table), or high GI values (clastic marsh, where the peat is buried rapidly followed by microbial attack). Forest swamps with dominantly arboreal plants are characterized by the predominance of structured over degraded compo-

nents (Fig. 19A), and the height of the water table will define the predominance of either vitrinite (high water table) or inertinite (low water table). According to this concept, coals developed in the lower part of the Mchuchuma Formation coal zone with TPI values < 1 (Fig. 19B) suggest formation in a transitional zone from open marsh to dry forest swamps. The spore assemblages with their high proportion of trilete fern spores, and less common trilete

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Fig. 19. Coal facies diagram for Songwe-Kiwira coals based on Diessel (1986) and Lamberson et al. (1991). (A) suggested possible degradation pathways of a forest swamp resulting in coals characterized by different proportions of vitrinite and inertinite maceral groups and within these groups having various proportions of components with preserved cell-structures and detrital components. (B) suggested depositional environments for Songwe-Kiwira coals. Sample identification the same as in Fig. 4. VIT = vitrinite, INERT = inertinite, SEMIFUS = semifusinite, FUS = fusinite, IDET = inertodetrinite, STRUC = structured, DEG = degraded.

lycopsid spores and monolete tree fern spores, also may indicate a relatively dry environment at the site of plant growth. The low GI values reflect the high inertinite contents of the seams, whereas the low

TPI values are a reflection of the high abundance of detrital macerals. Analysis of the top 0.70 m of seam #6 at Kiwira Mine (total seam thickness 0.97 m) showed significantly higher fusinite and semifusinite

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contents, resulting in a significantly higher TPI value (Fig. 19B), suggesting a deposition in a more terrestrial and drier environment. Seams #3B, #3A and #2 all have significantly higher TPI and GI values, suggesting deposition in a

wet forest-type swamp. This may explain why the microfloral assemblage is dominated by non-taeniate disaccate pollen, and why taeniate disaccate pollen and fern spores are less common. The lower bed of seams #2 at Mwalesi and Lema Rivers is separated from the

Fig. 20. Hydrogen indices, oxygen indices and Tmax (jC) values derived from Rock Eval analyses for (A) coals and (B) shale and mudstone samples in the Songwe-Kiwira Coalfield.

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other seams by significantly higher TPI values (Fig. 19B) as a result of the predominance of well-preserved botanical cell-structures in collotelinite. This compares with a relatively high proportion of trilete spores near the middle of the seams, but low values near the lower and upper parts. The uppermost seam #1 (Lema River) has a slight predominance of inertinite (mainly fusinite and semifusinite) over vitrinite suggesting a return to a somewhat drier environment (at or near the peat accumulation site) as indicated by a lower gelification index (Fig. 19B). However, the palynological data suggest no significant difference between seam #1 and seams #3B and #3A. 5.5. Rock Eval analysis In this study, evaluation of coal (48 samples) and organic-rich shales and mudstones (61 samples) in regard to hydrocarbon generative potential and thermal maturity is based on results from Rock Eval pyrolysis (Espitalie´ et al., 1977), vitrinite reflectances, spore colour alteration and microscopic observations on bitumen generation. Cross-plots of hydrogen index (HI) and oxygen index (OI) for Songwe-Kiwira basin samples are illustrated in Fig. 20A (coals) and Fig. 20B (shales and mudstones). Hydrogen index values range from 7 to 386 mg hydrocarbons per g TOC and Oxygen Index values range from 2 to 88 mg CO2 per g TOC. These values, not taking into account the few outliers (Fig. 20A and B), are indicative of kerogen type II/III. This observation is supported by the organic matter seen in palynological preparations, where exinous, and woody and coaly materials are predominant. No specimens of the alga Botryococcus were seen. The slightly higher oxygen indices shown by shales and mudstone are possibly related to mineral matrix effects (Katz, 1983). The values for Tmax (jC), a geochemical parameter describing the level of thermal maturity, are in the range of 426 – 440 jC (Fig. 20A and B), not considering a few very low and high Tmax values. Based on the Tmax values and the vitrinite reflectance of 0.62– 0.83% (Random), it is suggested that the strata are in the range of moderate thermal maturity corresponding to an early stage of hydrocarbon generation for type II/III organic matter that will produce both oil and gas.

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The colour of the pollen and spores in all the sections studied is medium to dark orange, indicating a Thermal Alteration Index of TAI 2 to 2+. This, according to the scale proposed by Utting et al. (1989), is approximately equivalent to a vitrinite reflectance value of 0.6 – 0.8 Ro%, and compares well with the actual vitrinite reflectances determined (Table 1). Organic geochemical analyses (soxhlet extraction, followed by gas chromatography – mass spectroscopy) on selected Songwe-Kiwira coals and carbonaceous shales (Mpanju et al., 1998) indicated that the n-alkane distribution is dominated by the occurrence of higher molecular weight (C20 +) n-alkanes and a distinct odd carbon number preference over the C25 – C31 range typical for higher terrestrial plant-derived organic matter. The predominantly terrestrial source for the organic matter is also indicated by the distribution of biomarkers such as the abundance of hopanes (Mpanju et al., 1998) and the predominance of C29 sterane over C27 and C28 sterane. High pristane/ phytane ratios (4.20 –8.00) suggest partially oxidizing conditions during accumulation of the organic matter. Despite the predominantly terrestrial source of the organic matter, which is commonly not considered to have a great oil generation potential, there is petrographic evidences for early generation and expulsion/migration of oil-like (bitumen) substances from the moderately mature source rocks (coal, shales, mudstone) at Songwe-Kiwira. The evidence is provided by the occurrence of exsudatinite (remobilized bitumen) in microfractures, cell-lumina of fusinite and voids in sclerotinite macerals; and the presence of fluorescing vitrinite macerals caused by impregnation of liptinic material (bituminization). The sources for this bitumen are most likely liptinite-rich layers within the coals and carbonaceous shales and mudstones.

6. Conclusions 6.1. Geology The structure of Songwe-Kiwira Coalfield is that of a half-graben to graben, which follows the present day rift valley trends in Tanzania and older Precambrian fault zones. The coalfield contains up to 900 m of

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Karoo sediments, with coal seams developed in the shale – coal – sandstone facies of the Mchuchuma Formation of Artinskian to Kungurian(?) age. 6.2. Coal distribution At Songwe-Kiwira, nine coal seams are recognized, with the economically important seams occurring in the lower part of the shale – coal – sandstone facies of the Mchuchuma Formation (seams #3a and #3b at Kiwira and Ilima Mines, and #3a at Mwalesi River). At Mwalesi River, cumulative net coal thickness for seams #9 through #2 is 5.91 m, with individual seam thicknesses of up to 3.80 m (seam #3a). At Kiwira and Ilima Mines, the main seam (#3b) has a thickness ranging from 1.86 to 1.94 m.

(Random = 0.62 –0.83%) and Thermal Alteration Index (TAI 2 to 2+), is high volatile bituminous C to B for seams of top of the coal-bearing interval, grading into high volatile bituminous A coals at the base. Ash yields as determined by proximate analysis range from 22 to 49 wt.%. Sulphur contents are highly variable, ranging from 0.17 (seam #5) to 9.20 wt.% (seam #6). The relatively high ash yields, along with the intermediate rank level, restrict the utilization potential of the Songwe-Kiwira coals to thermal coals for medium-sized power generation plants. Also, there is a potential for domestic use as a substitute for woodderived charcoal for cooking, and as a substitute for wood used in large quantities in rural Tanzania for brick making. 6.5. Palynology

6.3. Coal petrology The coal seams at Songwe-Kiwira are predominantly composed of dull, banded dull and banded coal lithotypes frequently grading into carbonaceous shales and mudstones. Petrographic composition of the seams is highly variable. Gross petrographic composition (maceral group distribution) is related to stratigraphic position, with seams from the basal part of the shale –coal – sandstone facies (seams #5, 6 and 9) all being rich in inertinite, whereas seams higher up in the section are characterized by higher vitrinite content. Petrographic in-seam variations are largely controlled by lithotype association, with inertinite and liptinite macerals abundant in the dull and banded dull lithotypes and vitrinite dominating in banded bright and bright varieties. Mineral matter content is high in all seams (16 –44 vol.%), with clay minerals and quartz dominating. Pyrite content, based on microscopic determination on channel and seam sub-sections, is in general low to intermediate ( < 2.2 vol.%), except in seam #2, the upper bed at Lema and Mwalesi Rivers (15 – 18 vol.%), and seam #9 at Mwalesi River (21 vol.%). Pyrite content slightly increases in seam sub-sections at the top and at the base of the seams. 6.4. Coal quality and utilization potential Coal rank at Songwe-Kiwira, based on calorific values (12827 – 14617 BTU/lb), vitrinite reflectance

The assemblage from the Lisimba Member, Idusi Formation is similar to that recorded by Weiss (in Semkiwa, 1992) with its abundance of monosaccate pollen (Cannanoropollis/Plicatipollenites assemblage) and is tentatively assigned a Late Asselian to Early Sakmarian age. The assemblages observed in the Mchuchuma Coal Formation (Scheuringipollenites and Scheuringipollenites –Protohaploxypinus assemblages) are similar to those recorded previously from the Rukwa Basin for which an Artinskian to Kungurian(?) age was tentatively proposed. (Semkiwa et al., 1998). 6.6. Facies analysis and depositional environment The high ash yields determined in all seams indicate that frequent flooding events occurred during the lifetime of the precursor mires, by which substantial amounts of clay minerals and quartz were deposited interlayered with decaying organic matter. Occasionally, the conditions for carbonate formation were favorable, as is evident by the occurrence of carbonate concretions and carbonate precipitation in fractures. Considering the relative amounts of facies –critical maceral associations determined in the seams, the stratigraphically older seams (#5, #6, and #9) all have inertinite contents > 50 vol.% suggesting a formation in a open marsh (inertodetrinite>semifusinite + fusinite) transitional to a dry forest swamp (semifusinite + fusinite>inertodetrinite) depositional setting. This is supported by the palynological data, where abundance

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of fern spores points to more arid conditions. Seams higher up section (seams #3b, #3a, #2 and #1) have both higher gelification indices and tissue preservation indices, suggesting formation in a wetter environment (wet forest swamp). This may explain the relative scarcity of taeniate disaccate grains in the assemblage. For seam #1 at the top of the coal-bearing succession, a return to somewhat drier conditions during peat accumulation is indicated, but no significant change was observed in the palynological assemblage. 6.7. Hydrocarbon generation potential of coals, shales and mudstones Hydrogen and oxygen indices suggest kerogen type II/III in coals and organic-rich shales and mudstones in the Songwe-Kiwira coalfield. The level of thermal maturity attained by the sediments and coals corresponds to the early stage of hydrocarbon generation for type II/III kerogen and will produce both oil and gas. No specimens of the alga Botryococcus sp., typical of type I organic matter were found in the palynological preparations. Petrographic evidence for early generation and expulsion/migration of oil-like (bitumen) from the mature source rock (coal, shales, mudstone) at Songwe-Kiwira Basin is provided by the occurrence of exsudatinite (remobilized bitumen in microfractures, cell lumina of fusinite, voids in sclerotinite macerals) and presence of fluorescing vitrinite macerals.

Acknowledgements The authors wish to thank the German Volkswagen Stiftung for financial support of this study. The Geological Survey of Tanzania (Madini) and the Tanzania Petroleum Development (TPDC) are highly acknowledged for logistical support during fieldwork. Songwe-Kiwira and Ilima Coal Mines, Tanzania provided logistical support and access to the mine sites and their support is gratefully acknowledged. The Geological Survey of Canada (Calgary) funded the collection and processing of palynological samples, and by providing Rock Eval data. The authors also wish to acknowledge the Lehrstuhl fu¨r Geologie, Geochemie und Lagersta¨tten des Erdo¨ls und der Kohle, RWTH Aachen, Germany for administrating

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the project and to provide access to microscope facilities in order to carry out the petrographic analyses of the samples. The manuscript benefited from critical suggestions made by C. Eble and N. Wagner.

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