Geochemistry of subsurface Late Quaternary ironstones in Rajshahi and Bogra Districts,Bangladesh:implications for genetic and depositional conditions

Md.Sazzadur Rahman·Ismail Hossain·Pradip Kumar Biswas·Md.Abdur Rahim·A.S.M.Mehedi Hasan·Md.Ibrahim Adham

Abstract The present study dealswith the geochemistry of Late Quaternary ironstones in the subsurface in Rajshahi and Bogra districts,Bangladesh with the lithological study of theboreholessediments.Major lithofaciesof thestudied boreholes are clay,silty clay,sandy clay,f ine to coarse grained sand,gravels and sands with(fragmentary)ironstones.The ironstones contain major oxides,Fe2O3*(*total Fe)(avg.66.6 wt%),SiO2(avg.15.3 wt%),Al2O3(avg.4.0 wt%),MnO(avg.7.7 wt%),and CaO(avg.3.4 wt%).These geochemical data imply that the higher percentage of Fe2O3*along with Al2O3 and MnO indicate the ironstone as goethite and siderite,which is also validated by XRD data.A comparatively higher percentage of SiO2 indicates the presence of relative amounts of clastic quartz and manganese-rich silicate or clay in these rocks.These ironstones also have signif icant amounts of MnO(avg.7.7 wt%)suggesting their depositional environments under oxygenated condition.Chemical data of these ironstones suggest that the source rock suffered deep chemical weathering and iron was mostly carried in association with the clay fraction and organic matter.Iron concretion was mostly formed by bacterial build up in swamps and marshes,and was subsequently embedded in clayey mud.Within the coastal environments,the water table f luctuates and goethite and siderite with mud and quartz became dry and compacted to form ironstone.

Keywords Ironstone·Geochemistry·Goethite·Siderite·Bangladesh

1 Introduction

Iron,which is estimated to make up about 4.7%of the earth’scrust,is present in signif icant amounts in almost all sedimentary rocks.Morethan 400 Fe-mineralsarereported in different literatures;however,only iron ore minerals with morethan 15%of Fe arerelevant(H.G.Dill,personal communication).Aggregates of iron ore minerals,namely ironstone,are sedimentary rocks,either deposited directly as ferruginous sediment or created by chemical replacements that contains a substantial proportion of an iron compound.The iron minerals comprising ironstones can consist either of oxides(i.e.,goethite,limonite,hematite,and magnetite),carbonates(i.e.,siderite),silicate(i.e.,chamosite)or some combination of these minerals(James 1996).The studied subsurface ironstones are mostly light gray to brown in color,very hard and compact,and concoidal with spherical or elliptical nature.

These types of ironstones are occasionally found during the drilling of a shallow or deep water pump setup at a depth of about 45–84 m below the study areas(Fig.1).Dominant lithologies of these areas(from boreholes data)are clay,silty clay,sandy clay,f ine to coarse grained sand and sands with gravel and/or ironstones.Generally,the geochemical process for iron concentration is controlled by the dynamic tectonic evolution of earth(Mohanta 2007).The silicate mineral weathers to release Fe,which precipitates as goethite,chamosite,siderite and pyrite in the sedimentary geochemical environment depending upon the low temperature thermodynamics prevailing in the depositional site(Mohanta 2007).The presence of ironstones in the studied geological sectionsmay occur through different geochemical processes or genetic inheritances.

Fig.1 Location map showing borehole positions in Mohanpur,Rajshahi and Kahaloo,Bogra Districts,Bangladesh

It is a very remarkable phenomenon that the present subsurfaceironstoneshave complex genetic history.In this case,the Bengal Basin is an ideal museum in understanding weathering,erosional and tectonic history.Raymo and Ruddiman(1992)also established that chemical weathering in the Himalayas and the global climate have potential connection.The study areas are within the southern slope of the Bogra Shelf within the Paleoproterozoic Stable Platform f lank of the Bengal Basin.Main aims of the study are to outline the lithology of the boreholes,identify the rock types,and assess the genetic history of ironstones and depositional conditions.The geochemistry of the rocks with the overall lithologies of the boreholes helps to reveal the mineralogical composition of the rocks,and its depositional conditions,as well as the interacting factors including provenance,weathering and tectonism.The geochemical data also helps with understanding the ore,its utility and the recovery of valuable metals of the rock.There are few studies on denuded sedimentary rocks in the Bengal Basin from Himalaya(Najman et al.2008;Ferdousy 2011;Hossain et al.2014),asthere are not much study on geochemistry in these areas.Due to the lack of studies on ironstone and iron-formation deposition in these areas,this research may help geologists reveal the new geoenvironmental condition of these areas.

2 Geological setting

The Bengal Basin is an asymmetric basin.The sediments increasein thicknesstoward the southeast,with the thickest being about 22 km thick(Curray 1991).The basin was shaped by the dynamic interaction of three plates,namely the Indian,Eurasian and Burmese sub-plates(Fig.2).Uddin and Lundberg(2004)reported the tectonics of the basin,which is primarily related to the rifting due to the separation of the Indian Plate from Antarctica-Australia during the Gondwana break-up,and then its movement,initially northwestward and then northward.Most likely during the Late Eocene,the initial collision with the Burmese sub-plate resulted in the rising of an Eocene island arc,which created the Bengal Basin to the west and the Irrawady Basin in the east(Fig.2).After the separation of these basins during the Miocene,the Indian Plate subducted beneath the Burmese sub-plate with anti-clockwise rotation and ultimately the Bengal Basin commenced closing in thenortheast and gradually turned into aremnant basin(Uddin and Lundberg 2004).Bangladesh comprises a major part of the Bengal Basin.Several scientif ic articles have already been published regarding the regional geology of the Bengal Basin and its surrounding areas(Reimann 1993;Goodbred and Kuehl 2000;Hossain et al.2018 and others).Representative lithological succession of the study areas is recorded in Table 1.

Bangladesh is divided into two major tectonic units:the Palaeoproterozoic stable platform in the northwest and the Bengal Foredeep on the southeast,which are separated by the Hinge Zone.The northwest stable platform is divided into three major elements:Bogra shelf,Rangpur saddle and Dinajpur slope.Geographically,thestudy arealiesbetween a part of the Rangpur saddle and Bogra shelf.The study areas cover the entire area of Keshorhat Pourashava,Mohanpur,in the Rajshahi district(latitude 24°35′59.11′′and longitude 88°39′45°3′′E)and some important samples are also collected from Kahaloo Upazila in the Bogra district(latitude 24°50′02.2′′N and longitude 89°18′08.7′′E)(Fig.1).

Fig.2 Dynamic nature of the Bengal Basin with interaction of three plates(Indian,Eurasian and Burmese sub-plate).Figure also showing two prominent basins of the Irrawady Basin(east)and the Bengal Basin(west).Hinge zone indicates the edge of the Indian Craton(modif ied after Steckler et al.2016;Rangin 2017)

Table 1 Stratigraphic succession of Bogra slope and adjoining areas of shelf zone,Bengal Basin(slightly modif ied after Zaher and Rahman 1980;Islam 2001;Hossain et al.2007)

3 Materials and methods

Subsurface ironstone samples were collected from Keshorhat,Mohanpur at 61–67 m depth and Kahaloo at 46–55 m depth during drilling of testing wells and then preserved in the polyethylene bags.Later these samples were dried up primarily by natural sunlight.From there,seven(7)samples were analyzed using the X-ray Fluorescence Spectrometer(XRF)method at the Institute of Mining,Mineralogy and Metallurgy,BCSIR,Joypurhat following the procedures of Goto and Tatsumi(1994,1996),and following instrumental precisions and standards discussed in Hossain et al.(2014).Sample preparation techniques of the studied rocks for XRF analysis were also followed according to Hossain et al.(2014).In the analytical results,accepted highest uncertainties for XRF major and minor elements are～2%and trace elements are＜10%.The X-ray diffraction(XRD)analysis is also supplemented for petrographic/mineralogical studies.The XRD patterns were recorded using a GBC-Emma,Australia diffractometer(Cu-Kαradiation generated at 35.5 kV and 28 mA),equipped with a 1°divergence slit,a secondary monochromator,a Solid-State Detector and a sample changer(sample diameter 16 mm).The samples were investigated from 10°to 80°2θwith a step size of 0.02°2θand scan speed of 4.01°/min where measuring time of 4.987531E-03°per step.For specimen preparation,the top loading technique was used.

4 Lithology and lithofacies of study areas

Lithological analyses were carried out from the boreholes Mohanpur(MR),Rajshahi and Kahaloo(KB),Bogra,which were drilled about 70 and 60 m deep,respectively.The representative lithological sections are presented in Fig.3a,b.These two boreholes were drilled by the Department of Public Health Engineering(DPHE)and the Institute of Water Modeling(IWM),and contain dominantly clay,silty clay,sandy clay,f ine to coarse grained sand,gravels and sands with ironstones.On the basis of color,grain size,stickiness and visible minerals of these lithologies,four distinct sedimentary facies are recognized in these boreholes.The individual lithofacies are described below:

Clay lithofacies:This facies shows light gray to yellowish brown clay with some organic matter.It isgenerally very sticky and soft.Sometimes very thin layers of silty or sandy clay lithofacies are also present,which are generally light gray to yellow color and few patches.It also contains a considerable amount of vegetal matter.Thicknesses of the clay lithofacies range from a few centimeters to 13 m(Fig.3).Clay lithofacies in the KB borehole show only a few centimeters of clay layers,whereas the MR borehole demarks two thick clay layers,ranging from trace to 13 m and 15 to 18 m in thickness(Fig.3).This is an indication of an abrupt decrease of hydrodynamic conditions or a low energy standing water environment,away from the active channels(Ponte′n and Plink-Bjo¨rklund 2007).Occasionally present silty or sandy clay lithofacies indicate cool and calm environmental conditions,usually coastal,thought sometimes they may deposit in f lood plain or lacustrine environments(Islam and Hossain 2006).

Sand lithofacies:These lithofacies have different sublithofacies,mainly(1)very f ine to f ine sand,(2)medium sand and(3)coarse sand.These are mainly light gray in color,angular to sub-round grained,and composed of quartz,mica,dark and greenish minerals.Usually very f ine to f ine sand beds range from a few centimeters to 7 m,medium sand beds range from a few centimeters to 10 m,and coarsesand bedsrangefrom afew centimetersto 14 m(Fig.3).Generally very f ine to medium sands indicate a deltaic or shore/beach environment with a low velocity of current.The coarse sand is deposited near the source area of the river,within or near the active channel,where the current energy essentially is found in a f luvial environment(Ponte′n and Plink-Bjo¨rklund 2007).

Sandy Gravel lithofacies:This lithofacies is variegated in color,and is commonly a mixture of sand to gravel size deposits.Thicknesses of these beds range from 2 to 5 m(Fig.3).Sandy gravel indicates a continental environment near the source or few distances from the source of a river and very high velocity isrequired for thedeposition of such lithology with a f luctuation of energy(Rahman et al.2004).

Sand with ironstone lithofacies:This lithofacies consists of a mixture of sand,gravel and gravely ironstone.These fragmental mixtures are mostly light gray to brown color,gravel or boulder size,very hard,dense and compact,spherical or elliptical nature with concoidal and sharp edges,and composed of iron-rich minerals with siliceous components.The samples of the studied ironstones were collected from boreholes MR and KB,with depths of 61–67 m and 46–55 m,respectively(Figs.3,4).

These rarely present ironstone samples have enormous geological signif icanceto thesestudy areasaswell asto the regional geology of Bengal Basin.Geochemical analyses of these samples can provide some answers to solve the geological mystery of the genesis of this fragmentary ironstone lithofacies.

5 Petrography and mineralogy of ironstones

Generally studied ironstones are very f ine-grained,noticeably composed of matrix of quartz grainswith clayey iron-bearing minerals,dominantly iron oxides and iron carbonates(e.g.,goethite and siderite).The microscopic study of KB-1 shows burf lower type matrices of light and yellow,orange or green color minerals(Fig.5a,b),which are mostly siderite,goethite with manganese-rich silicate bementite according to XRD data(supplementary data).Ref lected light photographs also show some developed metallic iron crystals in the rock(Fig.5c).On the other hand,the microscopic study of MR-7 authenticates the caviar type matrices of spotted light and yellow or green color minerals(Fig.5d,e),which has almost consistent mineralogical composition with the KB-1 sample(supplementary data).Ref lected light photographs show very few developed metallic iron crystals in this ironstone(Fig.5f).

Fig.3 Lithologs of a Kahaloo,Bogra and b Mohanpur,Rajshahi districts showing lithological types with sample locations

6 Geochemistry of ironstones

Compositions of major and trace elements of the subsurface Late Quaternary ironstones from the Rajshahi and Bogra districts,Bangladesh are presented in Table 2.The chemical compositions of the rocks(ironstones)exhibit relatively wide variations,especially Fe2O3*,which is comparatively very high with a wide range(61.71–73.87 wt%).The rocks also show wide variations of SiO2from 6.46(sample MR-6)to 24.46 wt%(sample MR-7),with an average of 15.27 wt%,Al2O3from 2.11(sample MR-6)to 4.54 wt%(sample MR-7)with an average of 4.00 wt%,MnOfrom 3.82(sample MR-7)to 11.68 wt%(sample MR-6)with an average of 7.66 wt%and CaO from 1.89(sample KB-1)to 4.91 wt%(sample KB-2)with an average of 3.43 wt%.Other oxides are less than 1 wt%(Table 2).In this study,the rocks have very high Fe2O3*,which indicates that the rock samples are ironstones.Generally the iron-rich sedimentary rocks,which contain 15 percent or more Fe of primary origin(depositional or diagenetic),are commonly referred to as ironstone or ironformation(James 1996;Al-Bassam and Tamar-Agha 1998).It is a very signif icant phenomenon that iron has extraordinary capability to form different minerals in response to different depositional and diagenetic environments.In general,the classif ication of the iron-rich rocks is based in large part on the mineralogy of the iron,the character,composition,and occurrence of the iron mineralsfound in sedimentary rocks(James1996).Usually mineral assemblages of ironstones vary widely,with hematite and goethite being the main components of red deposits(Young 1989)and siderite being the most important constituents of sedimentary ironstones(James 1996).Most ironstonescontain three different typesof iron minerals;oxides and oxyhydroxides,carbonates and silicates.Based on petrography and mineralogy,the studied rocks seem to have dominantly iron-carbonates,iron-oxides and considerable amount of iron-silicate (e.g.,bementite).In these ironstones,especially siderite or goethite have higher contents of Fe2O3*(60–75 wt%)and Al2O3(3–6 wt%)(James 1996;Al-Bassam and Tamar-Agha1998).In these regards,comparing these studieswith the geochemical data of seven(7)samples of the study areas,the constituent minerals of these rocks are mostly siderite and goethite,with few iron-manganese silicates.Although goethite usually contains about 5–10 wt%and siderite shows＜2 wt%SiO2(James 1996),the studied geochemical data show about 6.46–24.46 wt% (avg.15.72 wt%)SiO2,which is higher than the normal goethite and siderite(James 1996),which means the studied ironstones have considerable amounts of bementite-types minerals or clastic quartz(SiO2).These ironstones also have exceptionally higher MnO(3.8–11.68 wt%)with an average value of 7.66 wt%,which signify that these rocks abide considerable amount of manganese minerals.Accordingly,to concentrate Mn into signif icant sedimentary deposits,manganese needsto beoxidized to Mn(III)or Mn(IV)(Calvert and Pedersen 1996).These ions form oxyhydroxide minerals that are deposited in sediments(Calvert and Pedersen 1996;Armstrong 2008).Consequently,the presence of signif icant manganese content in the sedimentary record should imitate the history of manganese oxidation(Johnson et al.2016).These are the common behaviour of major oxides in ironstones:usually Fe2O3*and CaO show strong linear negative trends and Al2O3displays positive trends with increasing SiO2(Fig.6).Besides,MnOand Al2O3have strong positiveand negative correlation,respectively,with increasing Fe2O3*,while CaO shows an insignif icant positive trend(Fig.6).These trends demarcate signif icantly increasing iron and manganese minerals,as well as a few calcium minerals during hydraulic fractionation(Hossain et al.2014).It is very remarkable that all ironstones show relatively high Cr contents(375–974 ppm).It is noted that dominantly present siderite and goethite with few clay mineralsmay serveashost mineralsfor Cr.The Cr/Niratio in the ironstones is high and variable.The relatively higher Cr/Ni ratio in the studied ironstones is due to the association of Cr with clay(Tobia et al.2014).Rubidium and strontium contents are comparatively medium or slightly depleted with UCC and PAAS(Hossain et al.2014,Table 2).Most of the samples have a considerable amount of S(206–430 ppm),which indicate higher sulphur oxidation that increased iron-sulfur minerals(J?rgensen and Nelson 2004)in ironstones.High Cl(143–485 ppm)suggests more marine input in sediments.Generally,higher Cl contentsprove that the studied rockshave undergone evaporation at agreater ratethan the marine incursion(Worden 1996).

Fig.4 Photographs showing ironstones of Kahaloo,Bogra(a,b:siderite-goethite-enriched ironstone,streak slightly light yellow or gray in color,present light spots indicating mixtures of quartz or mud),and Mohanpur,Rajshahi(d,e:goethite-siderite-enriched ironstone,streak brown to yellow brown or orange in color,present few light spots also indicating mixtures of quartz or mud)

Fig.5 Petrographic microphotographsshowing ironstonesof Kahaloo,Bogra(a planepolarized photograph showing burf lower typematricesof light and yellow,orange or green color minerals,b cross nicol photograph also showing similar textural pattern with red/orange or deep green birefringences,c ref lected light showing some developed metallic crystals of iron minerals)and Mohanpur,Rajshahi(d plane polarized photograph showing caviar type matrices of spotted light and yellow,or green color minerals,e cross nicol photograph also showing similar textural pattern with light green birefringences,f ref lected light showing very few developed metallic crystals of iron minerals),Bangladesh

Table 2 Major and trace element compositions of the studied ironstones of Rajshahi and Bogra Districts,Bangladesh with the corresponding uncertainties

Fig.6 Harker major variations(in wt%)diagramsfor the investigated ironstoneswith error(uncertainty)bars(symbol:diamond:samples from Kahaloo,Bogra,and square:samples from Mohanpur,Rajshahi)

7 Discussions and conclusions

Major lithofacies of the studied boreholes are clay,f ine to coarse grained sands,sandy gravel,and sand with ironstones.Clay to f ine sand lithofacies suggest their depositional conditions as mainly low energy conditions,gently sloping intertidal areas or shallow marine neritic environment.Medium sands are deposited in deltaic or shore/beach environments,whereas coarse sand suggests somewhere near the source area of the river within or near the active channel,where the current energy isessentially high and found in a f luvial environment.Sandy gravel indicates the enormous f luctuation of energy within a continental environment.Sand with ironstone lithofacies may be paleoclimatic deposits,which are common in several semiarid regions of the world(Lal et al.2010).These types of ironstones also developed under temperate humid climatic conditions,typical of the post-glacial period(Dill and Techmer 2009).The comparatively light yellow color ironstones in Bogra(Fig.4a,b)may be siderite-enriched,which may be developed mostly syndiagenetically under reducing conditions.However,relatively higher reddish/brown ironstones in Rajshahi(Fig.4c,d)area indicates goethite-enriched,which may be developed syndiagenetically under oxidizing conditions(Dill and Techmer 2009).

From the geochemical data of the ironstones,theoverall chemical composition of the rocks shows comparatively wide variations;Fe2O3*is quite high(avg.66.6 wt%),indicating the constituent minerals of these rocks are siderite and goethite.However,the average percentage of SiO2is comparatively higher(15.72 wt%)than the common siderite and goethite,indicating the presence of relative amounts of manganese-rich silicate bementite or clastic quartz(SiO2).It is noted that manganese-rich silicate formation is usually restricted to marine environments wherein supply of iron(Fe)ishigh,conditionsare suboxic,and,most important,sediment inf lux is very low(Roy 1968;Curtis et al.1975).

The high Cr contents usually found in ironstones developed from basic and intermediate igneous rocks(Aubert and Pinta 1977).Generally,Co and Ni occur together with Fe in parent rocks,as Fe is more easily oxidized to Fe3+and readily precipitates.Rubidium and strontium are depleted,which indicate these elements are due to complete leaching of their host minerals during lateritization(Al-Bassam and Tamar-Agha 1998).These ironstones also conf irm signif icant amounts of MnO(avg.7.66 wt%),suggesting their depositional environments under oxygenated condition.Moreover,the presence of signif icant manganese content in the sedimentary record should imitatethehistory of manganeseoxidation(Johnson et al.2016).However,there are a couple exceptions to this general relationship where manganese enrichment in the sediments signif ies manganese oxidation above the redox boundary(Krauskopf 1979;Bellanca et al.1996).When there are high levels of soluble Mn2+(generally low Eh conditions trigger the production of reduced,soluble forms),this divalent ion can substitute for Ca2+in authigenic carbonate phases.Due to the high-potential redox chemistry of Mn,the geologic record of manganese deposits should ref lect the ancient oxygen availability and the paleo-environmental chemistry(Bellanca et al.1996;Maynard 2010).

The genesis of iron-bearing sediments has been one of themost debated topics in sedimentary petrology(Pettijohn 1984).The deposits were derived from a variety of parent rocks i.e.,metamorphic rock,basic and intermediate igneous rocks and pre-existing sediments of the surrounding areas(Ferdousy 2011;Hossain et al.2014).The source rock suffered deep chemical weathering in the neighbourhood,and the products(siderite,goethite,clay and quartz)were transported by river or channel to the depositional site.Iron was mostly carried in association with the clay fraction and organic matter(Tobia et al.2014).Several clay genetic processes were involved in the ironstone formation.Iron minerals(Fe-carbonate,Fe-oxides,Mn-silicates)mostly originated from bacterial build up in(mangrove)-swamps and marshes and were subsequently embedded in clayey mud.Iron is present in a number of mineral species(siderite and goethite)in sediments and these minerals also display different reactivities(Poulton et al.2004).The primarily formed carbonate in muddy sediments is rich in Fe2+and Mn2+,and poor in Mg2+and Ca+,which is established rapidly in reducing conditions(Curtis et al.1975).The Late Quaternary coastal stratigraphy of Bangladesh wascharacterized by alternating strata of organic and clastic minerogenic origin.Thesedimentary sequence of different areas in the Bengal basin reveals a precise relative sea level change (transgression and regression)during the Holocene or the post-glacial period(Dill and Techmer 2009;Ferdousy 2011;Salama et al.2014).Goodbred and Kuehl(2000)also reported that different Holocene stratigraphic units have traces of peats,mangrove woods and marine fossils,and evidently correspond to the progradational facies.Within these environments,due to the f luctuation of the water table,sideritegoethite with clayey mud and quartz became as dry and compacted as ironstone.Later,these ironstones were subjected to weathering and transported to form the present ironstones.

AcknowledgementsThe authors woud like to thank the Director,Institute of Mining,Mineralogy and Metallurgy(IMMM),Joypurhat,Bangladesh for hiscordial support to uscarrying out our geochemical and mineralogical analyses using XRF and XRD in the IMMM laboratory.The authors are grateful to Prof.S.Kabir for his effective proofreading of the manuscript and special thanks to M.A.Rahman for his kind support.We are also grateful to F.H.Tobia and an anonymous reviewer for their constructive reviews and comments.