Origin and diagenetic evolution of Ca–Mg–Fe carbonates in some coalfields of Japan

Abstract

Carbonate concretions, lenses and bands in the Pleistocene, Palaeogene and Upper Triassic coalfields of Japan consist of various carbonate minerals with varied chemical compositions. Authigenic carbonates in freshwater sediments are siderite > calcite > ankerite > dolomite B ferroan magnesite; in brackish water to marine sediments in the coal measures, calcite > dolomite > ankerite > siderite + ferroan magnesite; and in the overlying marine deposits, calcite > dolomite %-siderite.Most carbonates were formed progressively during burial within a range of depths between the sediment-water interface and approximately 3 km. The mineral species and the chemical composition of the carbonates are controlled primarily by the initial sedimentary facies of the host sediments and secondarily by the diagenetic evolution of pore water during burial. Based on the regular sequence and burial depth of precipitation of authigenic carbonates in a specific sedimentary facies, three diagenetic stages of carbonates are proposed. Carbonates formed during Stage I (< 500 m) strongly reflect the initial sedimentary facies, e.g. low Ca-Mg siderite in freshwater sediments which are initially rich in iron derived from lateritic soil on the nearby landmass, and Mg calcite and dolomite in brackish-marine sediments whose pore waters abound in Ca2+ and Mg*+ originating in seawater and calcareous shells. Carbonates formed during Stage I1 (500-2000 m) include high Ca-Mg siderite, ankerite, Fe dolomite and Fe-Mg calcite in freshwater sediments. The assemblage of Stage I t carbonates in brackish-marine sediments in the coal measures is similar to that in freshwater sediments. This suggests similar diagenetic environments owing to an effective migration and mixing of pore water due to the compaction of host sediments. Carbonates formed during Stage 111 (> 2000 m) are Fe calcite and extremely high Ca-Mg siderite; the latter is exclusively in marine mudstones. The supply of Ca is partly from the alteration of silicates in the sediments at elevated burial temperatures. After uplift, calcite with low Mg content precipitates from percolating groundwater and fills extensional cracks.bearing region is a small lininic basin in which a peaty-lignite bed 10-15 m thick is developed. . ' o P J Fig. 1. Location map with palaeogeography of the Palaeogene age (partly modified from Shimoyama Rr Iijima, 1976).

Berthierine and Chamosite in Coal Measures of Japan

“…The Utatsu berthierine, for example, is derived from sideritic rock in carbonaceous, kaolinitic shale of freshwater coalswamp deposits. This seems to reflect the fact that the siderite in these deposits is a low-Ca, low-Mg variety with nearly an end-member composition (Matsumoto, 1978;Matsumoto and Iijima, 1981). On the other hand, intermediate-to high-Mg berthierine is found in both modern and ancient, brackish water and shallow-marine sediments.…”

“…The abundance of Fe 2+ in berthierine in the coal measures indicates that the reaction progressed under reducing conditions and in the presence of abundant plant remains. Most sideritic rocks in freshwater coal swamp deposits form at a burial depth less than 300 m during early diagenesis (Matsumoto, 1978;Matsumoto and Iijima, 1981). The siderite to berthierine transformation should, therefore, occur at greater depths.…”

“…Siderite is the dominant carbonate in berthierine rocks of coal swamp deposits in the Paleogene and Upper Triassic coal measures and makes up 45-85 wt. % of the rocks (Matsumoto, 1978;Matsumoto and Iijima, 1981). It occurs as cryptocrys- , [~s of berthierine rock in coal measures of Japan.…”

Sandstone diagenesis related to varying burial depth and temperature in Greymouth Coalfield, South Island, New Zealand

“…7) is interpreted to reflect secondary growth of the coarser variety, probably involving partial recrystallisation of the spherulitic form. Matsumoto & Iijima (1981) suggested that there is a genetic relationship between kaolinite and siderite in coaly deposits, but did not fully explain how formation of minerals requiring apparently two different sets of chemical conditions occurred. We suggest that minor variation of pH alone, under reducing conditions, can explain the co-existence of these minerals.…”

“…(1) at least a slightly alkaline pH (Garrels & Christ 1965;Todd & Monroe 1968;Pearson 1979), although it may be stable down topH 5.0 (Isphording & Lodding 1973); (2)negative Eh conditions (Todd & Monroe 1968;Isphording & Lodding 1973); (3) low dissolved sulphide, high dissolved carbonate (pCO 2 ), and high Fe 2+ activity (Garrels & Christ 1965;Curtis & Spears 1968;Isphording & Lodding 1973;Matsumoto & Iijima 1981;Tucker 1991). A nonmarine environment is the most common setting in which these conditions exist.…”