[S-CG53] 岩石・鉱物・資源

[SCG53-04] アメリカ・カルフォルニア州リオティント・ボロンオペレーション（鉱⼭）におけるホウ素鉱物の鉱物学的・地球化学的研究

*奥田 和峰¹、石田 章純¹、掛川 武¹ (1.東北大学 大学院理学研究科地学専攻)

キーワード：ホウ酸塩、プラヤ、クラマー鉱床、中新世

The origin of life heavily depends on the stability of ribose, a critical component of RNA. Recent studies suggest that borate minerals, such as colemanite and ulexite, play a vital role in stabilizing ribose under early Earth conditions, yet no direct evidence of borate-rich environments from the Hadean era has been discovered. This study investigates the Kramer borate deposit in California, one of the biggest and most well-known borate deposit, aiming to elucidate the mechanisms of boron accumulation and its implications for prebiotic chemistry. Geological survey at the Boron Operation revealed that boron ores were disseminated with pre-existing shale. The examined samples contain ulexite, kernite, and borax. All boron minerals were products through the early accumulation of boron in subsurface sediments followed by later enrichment and purification. It is found that examined ulexite contains pyrite and pyrrhotite, and borax contains stibnite and realgar. Local enrichment of REE is also found in ulexite with Fe-oxides. It is interpreted that Sb, As, and REE were brought by deep hydrothermal fluids in the boron accumulating sites. Sulfur isotope compositions of pyrite and pyrrhotite in ulexite range from -21 to +6.6 per mil. Stibnite and realgar in borax show sulfur isotope compositions between -23 to -40 per mil. Those sulfur isotope compositions indicate two distinct sources of sulfur for sulfide mineralization: one is deep fluidal H2S and the other one is biogenic pyrite (or H2S) in the shale. Such sulfur was introduced into the boron mineralization sites by multiple hydrothermal processes. These findings indicate that the Kramer deposit formed as boron and sodiumrich hydrothermal fluids interacted with calcium-rich brines in a closed basin environment. The zonation of sulfide minerals reflects evolving hydrothermal fluid chemistry. This study provides insights into the formation of borate deposits and their role in creating environments suitable for the origin of life.