The Hishikari deposit, which is located in the northern part of Kagoshima Prefecture, Kyushu island, Japan, is an epithermal Au-bearing quartz vein-type deposit about 1 Ma old. The purpose of this study was to clarify the relation between concentrations of Au and trace elements such as rare earth elements (REEs) in Au-bearing quartz veins based on mineralization sequence, mineral assemblage and analytical data obtained by INAA and LA-ICP-MS and to determine the framework of the hydrothermal system forming the Au-bearing quartz veins.
The investigation was carried out using samples of extremely Au-enriched ores, 2, 130 and 3, 700 g/t Au, from Ryosen. No. 1 and Ryosen No. 5 veins in the Hishikari Lower Andesitic rocks, respectively, and typical Au ores of 200 g/t Au from Hosen No. 5 vein in alternation of shale and sandstone of the Shimanto Supergroup. The Au contents in the ores of Ryosen No. 1 and Ryosen No. 5 veins as determined by INAA decreased with the progress of mineralization, from 30, 300 ppm to 1.59 ppm and from 11, 500 ppm to 56.5 ppm, respectively. On the other hand, the Au content in the ores of Hosen No. 5 vein have increased from 5.7 ppm in the early stage to 57.1 ppm in the later stage.
The REE concentrations of quartz in the Au-bearing quartz veins were measured by LA-ICP-MS. The REE concentrations in quartz in the early stage of mineralization of Ryosen No. 5 vein were high and decreased gradually with the progress of mineralization. On the other hand, the REE concentrations in Hosen No. 5 vein were low in the early mineralization stage, which is characterized by a low Au concentration, and high in the later mineralization stage, which is characterized by a high Au concentration. A positive correlation was found between Au content and REE contents of these Au-bearing quartz veins. REE in these quartz veins would have been included in impurities of quartz crystals and/or clay minerals among grain boundaries of quartz crystals.
The estimated REE concentrations of the Hishikari hydrothermal solutions forming the Au-bearing quartz veins are 0.85 ppb for La, 2.18 ppb for Ce, 3.44 ppb for Nd, 0.76 ppb for Sm, 0.03 ppb for Eu, 0.24 ppb for Tb, 0.12 ppb for Yb and 0.05 ppb for Lu. These estimated concentrations of REE are different from those of REE in magmatic water and similar to those of REE in hot spring water, ground water and river water, which are of meteoric origin, in the area in which granitic rocks are distributed. REE in Au-bearing quartz veins of the Hishikari deposit were thought to have been extracted by water/rock interaction between hydrothermal solution and country. Quartz immediately after opening of veins has high Au and REE concentrations, and the concentrations of these elements gradually decrease with the progress of precipitation of quartz in the veins. These facts suggest that the water/rock ratio changed from low to high and that hydrothermal solutions related to later Au mineralization were interacted with country rocks depleted in these elements, that were depleted as a result of interaction between country rocks and hydrothermal solutions related to earlier Au mineralization.
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