Clay Minerals Associated with the Kuroko Deposits in the Hokuroku District, with Special Reference to Relation between X-ray Patterns and Modes of Occurrence of Sericite Minerals

Abstract

Clay minerals showing a zonal distribution in and around several Kuroko deposits in the Hokuroku district, Akita Prefecture, are outlined together with some investigations on x-ray patterns of sericite minerals.
The ore deposits of the Shakanai, Hanaoka (Matsumine), Kosaka (Uchinotai and Uwamuki), and Furutobe mines, on which the present study has been made, belong to typical Kuroko deposits occurring in altered felsic volcanics of Miocene age, and consist successively of silicified rhyolite and pyroclastic rocks, gypsum ore zone, siliceous ore (or pyrite ore) zone, yellow ore zone, black ore zone, ferruginous chert in ascending order. Intensely argillized rock which is mostly derived from tuff is distributed within and surrounding the ore deposits.
The clay mineral occurring most abundantly in the alteration zone is sericite minerals, which are fine-grained and often form monomineralic clays. Sometimes they are associated closely with sulfide minerals in the ore deposits. X-ray diffraction analysis has revealed that they are interstratified minerals of sericite-montmorillonite as well as sericite. The interstratified minerals usually contain only small amounts of expandable layers and show subtle variations of basal reflections, but occasionally contain up to 40 per cent expandable layers and have long spacing (25Å or longer) basal reflections. The variations of basal reflections were investigated in relation to their modes of occurrence by measurements of peak widths at the half height of about 10Å and 5Å reflections (ω₁ and ω₂) and of the peak positions of 10-12Å reflection. The results indicate that those associated with the silicified rocks and the siliceous ore are sericite or interstratified sericite-montmorillonite containing very small amounts of expandable layers, and that most from the other portions of the deposits are interstratified sericite-montmorillonite with a trend of having more expand-able layers in the upper or outer portions of the deposits. The polytypes of sericite minerals are 2M₁, 1M, and 1Md, the last of which is found only in the interstratified minerals.
Mg-chlorite is secondly widespread, particularly being associated with the gypsum ore. The chlorites associated with the ferruginous chert and in the altered pyroclastic rocks distant from the ore deposits are usually ferromagnesian.
In addition to these minerals, following various clay minerals are found locally in the alteration zones: interstratified Mg-chlorite-saponite, sudoite, interstratified sudoite-mont-morillonite, interstratified sericite-sudoite, montmorillonite, nacrite, dickite, kaolinite, and pyrophyllite. The fact that the clay minerals distribute zonally with a close relation to the ore deposits may indicate their origin to be connected directly to the mineralization of metal ores, and may suggest primary formation of the interstratified sericite-montmorillonite.