Role of flashing in the formation of high-grade, low-sulfidation epithermal deposits: a case study from the Omu Camp in Hokkaido, Japan, The

Abstract

The Miocene low-sulfidation epithermal Hokuryu and Omui deposits of the Omu camp in northeastern Hokkaido, Japan, are small past-producers of high-grade Au and Ag ores. The quartz textures and distribution of ore minerals within vein samples were studied to identify the processes that resulted in the bonanza-grade precious metal enrichment in these deposits. Correlative microscopy involving optical microscopy, cathodoluminescence microscopy, and scanning electron microscopy was employed. The research shows that vein quartz exhibits a wide range of textures that represent primary growth patterns. In addition, textures indicative of recrystallization of silica precursor phases and replacement of other vein minerals were recognized. In the high-grade vein samples, which are crustiform or brecciated in hand specimen, ore minerals almost exclusively occur within distinct dark gray to black quartz bands. These bands alternate with barren, white to light gray quartz suggesting that ore deposition was episodic. The bands hosting the ore are colloform and composed of mosaic quartz. High-magnification microscopy reveals the presence of densely packed relic microspheres providing evidence that the mosaic quartz formed through recrystallization of a non-crystalline silica precursor phase. The ore minerals occur interstitially to the densely packed microspheres indicating that ore deposition was contemporaneous to the agglomeration of the microspheres. These colloform bands with relic microsphere textures are interpreted to have formed through rapid silica and ore mineral deposition within the veins at high temperatures, presumably involving temporary flashing of the hydrothermal system. Limited fluid inclusion data suggests that silica deposition occurred at a temperature of over 245-250°C implying that flashing occurred to a depth of over 400 m below the paleosurface. The ore-hosting colloform bands composed of agglomerated microspheres are texturally distinct from barren, colloform bands containing fibrous chalcedonic quartz bands formed at lower temperatures. The findings of this study are consistent with models linking the high-grade precious metal enrichment in low-sulfidation epithermal veins to episodic flashing of the hydrothermal system and have significant implications to the design of exploration strategies for bonanza-grade low sulfidation epithermal vein deposits.