| Pakistan is rich in coal resources,which amount to about 186 billion tons.In this study an attempt has been made to study the proximate parameters,trace elements and major element oxide concentrations,maceral and mineral compositions of Padhrar and Darra Adam Khel coalfield to get knowledge about the source,genesis,enrichment mechanism,sedimentary medium conditions,and enrichment factors of elements combined with the knowledge of coal sedimentary environment and mineralogy and also to evaluate the potentially valuable and toxic elements that are of great economic significance for the clean utilization of coal.The Paleocene Padhrar and Darra Adam Khel coalfields located in Central Salt Range Punjab Province and Khyber Pakhtunkhwa Province,Pakistan,respectively.Padhrar coal has not been studied in detail with respect to the mineralogical,concentration and distribution of trace elements and Darra Adam Khel coalfield was first discovered by Malkani in2012 but due to security reasons,it was impossible to conduct research work in this area.The total 37 samples were Padhrar and Darra Adam Khel coal,12 samples(A1,A2,A3,A4,A5,A6,B1,B2,B3,B4,B5,B6)were collected from Padhar and 25samples(M1S1SK,M1S2SK,M1S3SK,M1S4SK,M1S5SK,M2S1MB,M2S2MB,M2S3MB,M2S4MB,M2S5MB,M2S1NM,M2S2NM,M2S3NM,M2S4NM,M2S5NM,M4S1AC,M4S2AC,M4S3AC,M4S4AC,M4S5AC,M11S1SK,M11S2SK,M11S3SK,M11S4SK,M11S5SK)were collected from Darra Adam Khel coal.The Ro max%analysis,coal petrography,inductively coupled plasma mass spectrometry ICP-MS\-ICP-OES,X-ray diffraction,X-ray fluorescence spectrometry(XRF)and optical microscopy was performed to study the mineralogical characteristics,coal petrological characteristics,trace element characteristics and organic geochemical characteristics of the samples were studied.The following understandings have been obtained.1.Coal Chemistry and Vitrinite Reflectance:The maximum vitrinite reflectance(Romax%)value of A1,B1,M1SK,M2MB,M2NM,M4AC and M11SK were 0.48%,0.50%,0.44%,1.06%,1.03%,0.97%,0.43%,respectively.The Padhrar coal A1 and B1 were characterized as low-rank coal and medium-rank I coal respectively.The Darra Adam Khel coal M1SK and M11SK were characterized as low-rank coal and M2MB,M2NM,and M4AC were characterized as medium rank III coal.Based on the analysis in the above samples,the vitrinite reflectance(Ro max%)in Darra Adam Khel and Padhrar coal ranges from0.431.06,0.78 with an average and 0.480.50 with an average of 0.49 respectively.Darra Adam Khel coal is fine coal,which indicates strong structural compression,while Padhrar coal is lump coal,which indicates less structural compression.The northern margin of the Indian plate is occupied by an active fold and thrust belt of the Salt Range,formed in response to the collision between the Indian and Eurasian plates.The tectonic activities may be the main causes of high maximum vitrinite reflectance and fine Darra Adam Khel coal.The mechanism of coal seam metamorphism cannot be discussed in-depth,however,it should be noted that the metamorphism of coal is obviously affected by heat and pressure.After deducting the important factor of burial depth,the gradual variation characteristics of coal rank surface can be connected with the regional metamorphic history and source of local metamorphism,which has important indicative significance.The Padhrar coal shaft A and shaft B were characterized by low to medium moisture contents,low to medium ash yields,high volatile matter coal,high sulfur contents respectively.The Darra Adam Khel coal M1SK,M2MB,M2NM,M4AC,and M11SK were characterized as low moisture contents.The Darra Adam Khel coal M2MB,M2NM,M4AC,and M11SK are characterized by low ash yields,high sulfur,and high volatile matter coal respectively.In contrast,M1SK was characterized by medium-high sulfur content,medium ash yield,medium volatile matter coal on average respectively.The main sulfur in Padhrar and Darra Adam Khel coal is pyritic sulfur and the high sulfur content indicates seawater influence.The only high sulphur content in coal is not sufficient evidence to evaluate the deposition environment that's why Sr/Ba and other parameters were also discussed.2.Coal Mineralogy:The mineral phases identified by optical microscopy and X-ray diffraction(XRD)in Padhrar coal were pyrite,quartz,calcite,coesite,magnetite,hydronium jarosite,rodolicoite,and clay minerals.The minerals identified by optical microscopy and X-ray diffraction(XRD)in Darra Adam Khel coal M1SK,M2MB,M2NM,M4AC,and M11SK are quartz,pyrite,calcite,volborthite,magnetite,giuseppettite,eucryptite,ulvospinel,barroisite,franklinite,coesite,and clay minerals.The berry granular aggregate pyrite may be the product of early diagenesis,which is formed by the replacement of filling of bacterial reservoir residues with colloidal and fine-grained pyrite.The berry granular aggregates pyrite reflects an anoxic environment.In coal,it is distributed in the form of nodules,veins,and lumps,and some are filled in vitrinite.In this study,pyrite is mostly tuberculous,occasionally filled with cells.Nodular pyrite was formed in the early diagenetic stage,and cell-filled pyrite was formed in the late diagenetic stage.Colloidal and fine-grained pyrite is the product of syngenetic sedimentation of sediments in the early diagenetic stage,representing an anoxic environment.Spherulitic pyrite may be the result of filling holes with colloidal pyrite,which is embedded in organic matter pores under the action of surface tension,the colloid crystallizes into an approximate spherulite structure.The modes of occurrence indicate the syngenetic origin of the pyrite,which may be related to the activities of the algae and bacteria.The quartz generally exists in two forms,the quartz is commonly distributed in form of quartz vein and another form grey-black smooth‘porphyritic'distribution.The quartz was observed in fine intergranular form with a smooth surface and clear outline in Padhrar and Darra Adam Khel coal.Calcite is commonly found in Padhrar and Darra Adam Khel coal in the form of cell filling and vein filling.Cell filling calcite is mostly an authigenic mineral,and vein filling calcite is mostly epigenetic.Calcite may be formed due to the influence of groundwater activity or the invasion of low-temperature hydrothermal fluid rich in calcium.3.Coal Maceral:The macerals in Padharar coal shaft A and shaft B were mainly composed of vitrinite,inertinite,and liptinite.Vitrinite includes desmocollinite,thin banded telocollinite,with a small amount of telinite,vitrodetrinite,and often corpogelinite.The secondary inertinite includes lenticular or irregular shape semifusinite,oxyfusinite,and inertodetrinite,and a small amount of micrinite and macrinite are distributed in desmocollinite,occasional secretinite,fuginite,pyrofusinite.Liptinite in Padhrar coal includes sporinite,cutinite,and liptodetrinite.The macerals in Darra Adam Khel coal were vitrinite,inertinite and liptinite.The primary vitrinite macerals in Darra Adam Khel coals include vitrodetrinite,lenticular desmocollinite,and thin banded telocollinite.The inertinite macerals Darra Adam Khel coals include lenticular or irregular semifusinite,oxyfusinite and inertodetrinite,micrinite,and macrinite distributed in desmocollinite and secretinite.Liptodetrinite was a primary liptinite maceral in Darra Adam Khel coal has a small amount of sporinite and liptodetrinite macerals.The high vitrinite content in both Padhrar and Darra Adam Khel coals indicate wet and anoxic conditions of peat formation.The vitrodetrinite macerals in Darra Adam Khel coals also indicate that these coals deposited in lagoons.The inertinite is the secondary maceral in both Padhrar and Darra Adam Khel coal but the inertinite content in Darra Adam Khel coal are higher than Padhrar coal,show water table fluctuation.The vitrinite contents in Padhrar coal are high than Darra Adam Khel coal.The inertinite and liptinite content in Darra Adam Khel coal are higher than Padhrar coal.The vitrinite and inertinite ration(V/I)in Padhrar coal were greater than 4 which represents the strong humid conditions.The V/I in Darra Adam Khel coal was greater than 1 and less than 4 except M11SK that is greater than 0.25 and less than 1.The VI ratio in most samples is more than 1,which indicates that the coal seams are dominated by woody plants.The Darra Adam Khel coal represent high moist-shallow water conditions except M11SK which represents dry and weak water conditions.The seawater influence on Padhrar coal is higher than Darra Adam Khel coal.4.Coal Geochemistry:(1)Compared to Chinese coal average values,the Padhrar coals has a lower concentration of Si O2,Ti O2,Al2O3,Mg O,Ca O,Na2O,K2O,P2O5,and a higher concentration of Fe2O3.The concentration of Si O2,Ti O2,Al2O3,Mg O,Ca O,Na2O,P2O5 in Darra Adam Khel coal M1SK are higher,while the other major elements oxides in Darra Adam Khel coals are in a lower concentration.The trace elements in the coal samples were analyzed and compared to the world's averages of low-rank coal.According to the value of concentration coefficient CC(ratio of element concentration)in the study area(Padhrar and Darra Adamkhel coals)vs World coal,the trace elements with concentration coefficient 5<CC>10 are significantly enriched in Padhrar coals were Be,Zn,Ge,Mo,Ta,Co,Co,and W.The significantly enriched trace elements in Darra Adam Khel coals M1SK were Cr and Zr;Ag and Sn in M2MB;Sn in M2NM;Li in M4AC;Ag in M11SK.W and Co were significantly enriched in Darra Adam Khel coal that may be caused by the addition of weathering material from granite around the sedimentary basin.The concentration of Co and W were very high in both Padhrar and Darra Adam Khel coal,may be related to the addition of weathering material of surrounding granite.The concentration of Mo and V in Padhrar coals shaft B and Darra Adam Khel coal M1SK were high respectively.The content of trace element Mo and V in seawater is higher than that in brackish water or freshwater,therefore the high concentration of Mo in Padhrar and V in Darra Adam Khel coal may be the cause of seawater influence.Vanadium is a relatively active metal element,which is easy to migrate and can be released by weathering of basalt and decomposition of dark minerals and transported to peat swamp.Therefore the content of V in Darra Adam Khel coals M1SK affected by seawater and mainly controlled by the supply of V-rich terrigenous detritus,which makes the V content generally higher.Due to the collision of the Indian and Eurasian plate,the Himalaya was formed and the impact of collision in these areas are high and may be the cause of enrichment of these elements.Compared to the upper continental crust,rare earth elements and yttrium(REY)concentration in Padhrar coal are higher than the world low-rank coal REY concentration(65.3?g/g)and about half of the REY concentration(135.9?g/g)in Chinese coal.The total REY concentration in Darra Adam Khel coal M1SK ranges from 160.42?g/g to 189.59?g/g with an average of 174.91?g/g,M2MB 24.36?g/g to 38.62?g/g with an average of 29.19?g/g,M2NM 38.69?g/g to 115.88?g/g with an average of 70.62?g/g,M4AC 36.08?g/g to 106.02?g/g with an average of 60.44?g/g,and M11SK 46.13?g/g to 121.31?g/g with an average of 75.64?g/g.The values of La,Ce,Pr,Nd,Sm,Gd,Dy,Y,Er and Yb in Padhrar and Darra Adam Khel coal are high and the values of Eu,Tb,Ho,Er,Tm and Lu in Padhrar and Darra Adam Khel coal are lower.Padhrar coals are classified as H-type and M-type enrichment with positive Ce,Eu,and Gd anomalies.The Darra Adam Khel coals M1SK,M2NM,and M4AC are classified as L-type and M-type enrichment with negative Ce,Eu and positive Gd anomalies.The Darra Adam Khel coal M2MB characterized by an H-type and M-type enrichment with Ce,Eu negative,and Gd positive anomalies.Simultaneously,M11SK is classified as L-type enrichment with Ce,Eu,and Gd negative anomalies.The high Sr/Ba and sulphur value in the studied coal samples reflecting the marine deposition environment.The content of Si O2+Al2O3 in coal is usually related to the terrigenous detrital influx,therefore,the contents of Si O2+Al2O3can reflect the coal accumulation environment.The(Fe2O3+Ca O+Mg O)/(Si O2+Al2O3)ratios are higher than 0.23.Hence,it can be concluded that the depositional environments of Padhrar and Darra Adam Khel coal were mainly influenced by seawater.The negative Ce anomalies in Darra Adamkhel coal also indicate a marine depositional environment.Ce N/Ce N*value is 0.91.0 in Padharar coal Shaft A,Shaft B,and Darra Admkhel M2MB show anoxic marine water.Ce N/Ce N*value is0.60.9 in M1Sk,M2NM,M4AC,and M11SK show suboxic marine water.(2)The 80?g/g Li value was reasonable to be taken as the minimum mining grade and 120?g/g as the economic grade or industrial grade.The Li concentrations in Darra Adam Khel coal M4AC are considered to be a mining grade.Germanium value in Padhrar coal is higher than those of the Chinese coals and World hard indicates that Ge in Padhrar coal has commercial value.The studies show that the large local coal reserves have enormous potential and need to be utilized to meet the Country's rising energy requirements.It must be indicated that the use of low-rank coal for power generation in Pakistan can be a huge benefit and can resolve the country's severe electricity problem.The concentration of As,Ni,Pb,Cu,Cd,Zn,and Cr elements were considered as potentially toxic elements in Padhrar and Darra Adam Khel coal that can cause great harm to the environment and human health.The harmful trace elements enriched in coal and coal series solid waste in the process of coal mining,washing,combustion,and transformation will cause serious pollution to the environment by acid rain or direct volatilization and ultimately endanger human health.The Padhrar and Darra Adam Khel coal are high-sulfur coal that will release harmful SO2 during combustion and can also cause acid rain so the high-sulfur Padhrar and Darra Adam Khel coals can cause huge damage to the environment and human health.The flotation,ultrasonication,acid washing,gravity separation,and other techniques should be considered for desulphurization ash reduction and removal of harmful trace elements before the utilization of Padhrar and Darra Adam Khel high-sulphur coal. |
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