[H-RE13] 資源地質学

[HRE13-P06] Contemporaneous occurrence of spheroidal dolomites and framboidal pyrites in the Shimokawa deposit, Hokkaido, Japan

*中山 健¹、村山 雅史² (1.高知大学海洋コア総合研究センター、2.高知大学農林海洋科学部)

キーワード：下川鉱床、球顆ドロマイト、ヒマワリ状フランボイダル黄鉄鉱、タルク、硫酸還元

Primary dolomites are commonly associated with SEDEX deposits hosted in clastic rocks, but only known for volcanogenic massive sulfide deposits such as bimodal felsic, bimodal mafic and siliciclastic mafic sub-type deposits⁽¹⁾. It has slightly been reported in the submarine hydrothermal deposits and active hydrothermal fields such as the Middle Valley⁽²⁾, Kagoshima Bay⁽³⁾ and the Okinawa Trough ⁽⁴⁾. It became clear for the first time that spheroidal dolomites (SDs) containing framoidal pyrites (FPs) made up the carbonate matrix of pyrite ores in the Shimokawa deposit that was originated from the submarine hydrothermal deposit. This kind of pyrite ores containing SDs and FPs also occur in layered sulfides intercalated in argillites. The SDs with uniform diameter of 0.5 mm (±) are consisted of fibro radial dolomites and calcites infill the interstices of dolomites. Clear maltese crosses (black cross extinction) were observed under crossed polars. Fibrous dolomite crystals have a large number of inclusions and pores arranged parallel to the direction of fibers, and at the same time, pores that are arranged concentrically in the direction perpendicular to fibers. A ring nearly 10µm width consisting of fine talc crystals develops inside the SD. FPs embedded within the dolomite cements range in diameter from 2.4 to 20.3µm and relatively larger than that of syngenetic FPs (2.9~10.1µm) in unaltered argillites away from the sulfide deposits. FPs are overgrown by petal-like pyrite crystals with a length of ~31.5μm just like a sunflower and make aggregates. However no obvious spatial relationship between FP and SD is observed. TOC of argillites indicates 0.4-0.8wt%, whereas that of baked argillites adjacent to diabase sills records almost 0wt% due to thermal degradation of organic matter. Consequently CO₂, CH₄ and NH₃ caused by intrusion of sill are thought to have ascended via the interstices of the clastic particles and fissures, and seeped to the seafloor. Theδ¹³C _PDB of organic carbon in argillites shows -25.7‰ (n=53), whereas the δ ¹³ C _PDB of matrix dolomites and calcites indicates -5.5 ‰ (n=15) and -5.2‰ (n=4) respectively. That reflects the influence of pore seawater in unconsolidated sediments, which contain nearly 0‰δ¹³C _PDB. Since dolomite is highly ordered crystal, it is necessary to overcome the kinetic barrier which prevents the dolomite nucleation⁽⁵⁾. As indicated by the reaction as CH₄ + SO₄^2- → (HCO₃)^- + HS^- + H₂O, decrease of SO₄^2-, increase in alkalinity and Mg²⁺/Ca²⁺ caused by methane oxidation and bacterial sulfate reduction possibly promote the nucleation of dolomites and contemporaneous crystallization of FP. Although there have been reports that microbes were involved in the formation of spheroidal textures ⁽⁶⁾, but there is no positive evidence of microbial involvement in this study, and it is thought that heterogeneous nucleation due to increased supersaturation and rapid crystal growth, such as spherulites formed by polymers and quenched basaltic melts. We postulate that sulfide ores containing FPs and SDs in the Shimokawa deposit were precipitated under reduced environment just beneath the seafloor prior to the high temperature mineralization rich in chalcopyrites, pyrrhotites and sphalerites.

References

(1)Franklin et al.(2005) Economic Geology 100^th Anniversary Volume,523-560., (2)Fouquet et al.(1998) ODP Initial Report 169,153-203., (3)Yamanaka et al.(2013) JVGR, 258, 74-84., (4)Miyoshi et al.(2014) Springer Open,567-586., (5)James and Jones (2015) AGU, 446p., (6)Gunatilaka(1989) JSP, 57, 922-927.