Geochemical Study of Ishikari Coals, Hokkaido.

SUMMARY

Various types of information about a coal seam's depositional environment and diagenesis can be obtained by investigating its mineralogical and major inorganic element composition, and forms of sulfur. The geology and coal petrology of the Ishikari coal field, one of the representative coal fields in Japan, have been well investigated. However, the problem of the depositional environments of the coal seams in the Ishikari Group remains partially solved. The Ashibetsu district, comprising the northeastern part of the Ishikari coal field, is an appropriate target to study the depositional environments of the coal seams in the Ishikari Group because of its location, the number of coal seams amenable to investigation, and the abundant geological information available regarding the coal quality and the environmental transitions of coal-bearing strata. The mineralogical composition, major inorganic elements, forms of sulfur, and sulfur isotopic composition of many coal seams in the coal-bearing formations of the Ashibetsu district have been studied. The results are discussed with respect to their depositional environments which formed the coals.

I. Mineralogical composition

(1) Silicate minerals

Silicate minerals identified from many coal seams are quartz, plagioclase, kaolinite, illite, illite/smectite mixed layer minerals, smectite, K-smectite, and chlorite.

Kaolinite is dominant in every coal seam. Illite appears near the roof, floor and intercalated sediments, but is very rare in relatively thick coal beds containing less than 10% ash, and is generally scarce in the coal seams. Trace amounts of chlorite exist near only a few floor sediments or in a few intercalated sediments. The great abundance of kaolinite, the moderate abundance of illite, and the scarcity of chlorite suggest that most of the coal seams were formed in a fresh water environment with almost no flow of water, even though a few areas were formed in a brackish water environment.

Quartz, plagioclase, illite/smectite mixed layer minerals, K-smectite, and chlorite are detrital minerals derived from tuffaceous or muddy sediments. Kaolinite occurred in low ash coals with high crystallinity and is thought to be authigenic. It seems to be most probable that smectite was diagenetically transformed from either an illite/smectite mixed layer mineral or K-smectite. These phenomena suggest that coal seams may provide suitable conditions for the formation of some clay minerals. Two kinds of illite/smectite mixed layer minerals are found in the coal seams. One is the same as that found in the intercalated tuff, and the other is the same as that found in the roof, floor, or intercalated shale. Illite/smectite mixed layer shale-type minerals have higher illite ratios than those of the tuffaceous type. From this observation it appears that illitization in the mud proceded through more complicated processes such as weathering and transportation rather than from the tuff when the mud deposited in the coal basin.

(2) Pyrite and iron sulfate minerals

Pyrite tends to occur near the roof or the floor of the coal seam and sulfide and sulfate minerals were not detected in the main parts of the coal seams save one, the Torashita-sanshaku seam. In that case, pyrite and gypsum occurred throughout the seam. Total sulfur content is also very low in the main parts of all the coal seams except the Torashita-sanshaku seam. However, total sulfur content is still low in the main part of the Torashita-sanshaku seam at a different location. This evidence suggests that most of the coal seams were formed in a fresh water environment, though a few areas were formed in a brackish water environment. This suggestion is consistent with the depositional environment inferred from the clay mineral composition.

Pyrite exists as very small, microscopically observable crystals even in the coals containing less than 0.5% total sulfur (S). Framboids are rarely present in low sulfur coals. The number of isolated crystals and of framboids increase as S content approaches 1%. Bigger framboids and clusters of framboids appear in coals containing more than 1% S. Pyrite spheres, formed from infilled framboids, and larger pyrite spheres, formed from infilled clusters of infilled framboids, are often observed in coals containing more than 2% S. All the pyrites of different shapes found in the coals of the Ashibetsu district were syngenetically formed as a result of the activity of sulfate reducing bacteria in different depositional environments.

Gypsum, szomolnokite, conquimbite, roemerite, melanterite, and hydronium jarosite were identified in some coal samples. These sulfate minerals are weathering products of pyrite. Gypsum appears in the early stages of weathering in calcite-rich coal. Iron sulfate minerals appear in calcite-poor coals from an early stage.

(3) Carbonate and phosphate minerals, and boemite

The carbonate minerals identified in the coal seams are calcite, ankerite, aragonite, siderite, and ferroan dolomite.

Calcite is the most abundant and ankerite is the second most abundant among the carbonate minerals studied. Both calcite and ankerite appear as cleat and fracture fillings in the coals. Aragonite is rare, and is thought to appear as a part of cleat and fracture fillings. The Ca, Mg,and Fe contents in the low ash and pyrite-free coals, rich in calcite and/or ankerite, are estimated to be nearly the same as those in plants or peats. This observation indicates that these minerals may have been formed in cleats and fractures by interaction of Ca, Mg, and Fe with CO₂ released from organic material during the stage of subbituminous coal, namely the second stage of coalification.

Siderite is not abundant and occurs as aggregates composed of fine grains or nodules of different sizes from less than 1 m to 10 m. Ferroan dolomite is rare and also occurs as aggregates of nodules of irregular shape, up to 100 m. Both siderite and ferroan dolomite are syngenetic minerals in the coals.

The phosphate minerals, apatite and goyazite, are found in the coal seams. Apatite is abundant. Detrital apatite appears in the mineral rich part of the high ash coal seams, and epigenetic apatite appears as fracture fillings in the low ash coal seams. Goyazite was detected in the samples that were relatively rich in apatite. Goyazite coexists with both detrital and epigenetic apatite. As P correlates well with Sr, these two phosphate minerals are thought to be closely related in origin.

Boemite is detected in most of the low ash coals. Organic Al may be the source of boemite.

II. Major inorganic elements

The results of the analysis of the major inorganic elements, Si, Al, K, Ti, Na, Mg, Ca, Fe,Sr, P, and Cl show that the contents of all these elements except Cl well reflect the abundance of the minerals containing each element in the coal seams.

The high correlation coefficient between ash content and Si, Al, K, Ti, and Na indicate that these five elements are associated with silicate minerals. Mg is mostly associated with carbonate minerals and partly with silicate minerals. Most of the Ca also exists as cabonate minerals, athough part of the Ca exists as apatite. The greater abundance of ankerite resulted in the relatively high correlation coefficient between Ca and Mg. Fe is divided into Fe in carbonate minerals and Fe in pyrite and iron sulfate minerals. Sr and a minor part of the Al are associated with goyazite, and P occurs as apatite and goyazite. Both organic and inorganic chlorine seems to exist though the content of Cl is low in the coal seams. Inorganic chlorine is thought to be incorporated into the coal seams by clay minerals because the correlation coefficient between Cl and both Na and ash is relatively high.

The systematic study on minerals and inorganic elements in coal is rare although the study of individual minerals or inorganic elements in coal has been carried out in response to the need for coal utilization. The results of this study are expected to be useful for coal utilization.

III. Forms of sulfur and their isotopic ratios

(1) Forms of sulfur

Ishikari coals are generally known as low sulfur coals. The five studied coal seams of theAshibetsu district also have low sulfur content, containing 0.22 ~ 1.65% total sulfur (0.58% onaverage), 0.0019 ~ 0.45% pyritic sulfur (0.004% on average), and 0.27 ~ 1.61% (d.a.f.) organic sulfur (0.60% (d.a.f.) on average). The vertical distribution of sulfur is such that pyritic and sulfate sulfurcontents are very low in the middle part and sharply increase near the roof of every coal seam and near the floor of one coal seam. Organic sulfur also has this same tendency, but it is not so clear as in the case of pyritic and sulfate sulfur. The average sulfur content in the middle part of the coal seams, except for the parts with increased sulfur content near the roofs or floor, are 0.48% total sulfur, 0.0055% pyritic sulfur, and 0.47% organic sulfur. The fact that most parts of every coal seam have low sulfur content and the largest portion of the sulfur is organic shows that the coal seams of the Ishikari coal field were formed in a fresh water environment. Moreover, the vertical variations of forms of sulfur mentioned above are clearly in harmony with the cyclothem, sandshalecoal seamshale, as seen in general at the depositional stage of coal seams in the Ishikari coal field. From these results, it is thought that peat started to be deposited at a time when the depositional environment was changing from brackish to fresh water, or after it had changed to fresh water, influenced by a marine regression, deposited in a fresh water environment, and terminated at the time when the environment was changing back to brackish water, influenced by a marine incursion. This interpretation explains how a coal seam largely deposited in a fresh water environment can have its roof and floor sediments deposited in a brackish water environment.

(2) Sulfur isotopic ratios

The results of the measurement of sulfur isotopic ratios for the coal seams provide another line of evidence regarding the depositional environments. The ³⁴S values of organic sulfur range from 6.7 to 25.5‰ for the whole coal seams, from 6.7 to 14.3‰, 11.5‰ on average, for the low sulfur portions containing less than 0.5% total sulfur, and from 11.1 to 22.4‰ 16.0‰ on average, for the low sulfur portions containing bet ween 0.5 and 1.0% total sulfur. The ³⁴S value of organic sulfur is enriched as total sulfur content increases. The ³⁴S values of pyritic sulfur are from 4.9 to 26.9‰ for the whole coal seams, from 4.9 to 12.7 ‰, 8.8 ‰ on average, for the low sulfur portions containing less than 0.5% total sulfur, and from 7.5 to 21.9‰, 16.9‰ on average, for the low sulfur portions containing between 0.5 and 1.0% total sulfur. The ³⁴S value of pyritic sulfur is also enriched as sulfur content increases. The ³⁴S values of total sulfur of Japanese coals and of both organic and pyritic sulfur of the Miike coals have the same trends as these results. These trends are quite different from those for the Australian and U.S. coals, for which the ³⁴S of organic sulfur becomes depleted as sulfur content increases.

The middle parts of the Ishikari coal seams, that contain low sulfur, containing 0.5 ~ 1.0% total sulfur, are possibly thought to have been affected by sea spray, air-borne marine sulfate, carried by rain water through plant growth and peatification occurred in a limnic peat basin because the peat basin was open to the seashore. As a result, the ³⁴S values of the low sulfur coals are enriched compared tothose of the Australian and U.S. coals. However, the trends of the ³⁴S values to be heavier with increasing sulfur content, as seen in the high sulfur parts near the roofs of the Ishikari coal seams and also seen in the whole Japanese coals, has not been well explained.

It is suggested that most of the coal seams were formed in a fresh water environment with almost no water flow as mentioned in the chapter on mineralogical composition. If the peat had been affected by sea spray in a stagnant water environment, or sea water had entered the peat basin very slowly, bacteria would have had to reduce both heavier and lighter sulfate and accordingly the complete conversion of sulfate to sulfide would have occurred in the peat. This explanation can explain the trend of both organic and pyrititic sulfur to be enriched with increasing sulfur content. On the other hand, the ³⁴S value of organic sulfur of the Australian and U.S. coals tends to be depleted as organic sulfur increases, showing isotopic fractionation. In such a case, the peat would have been formed in a deltaic environment with slow water flow.

The coal seams of the Yubari district in the southern Ishikari coal field contain low sulfur. Especially the coals from the Southern-Oyubari district are very low in both ash and sulfur. The total sulfur contents and the ³⁴S values of total sufur or organic sulfur are 0.14 ~ 0.34% and 1.9 ~ 8.5 ‰, respectively, indicating that these coals were formed in a fresh water environment. This implies that the plant growth and the peatification would have occurred in a stagnant, lacustrine peat basin far behind the Ashibetsu district without any influence by sea water.

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