Suclan, Catherine2025-08-062025-08-062025-08-06http://hdl.handle.net/10393/50731https://doi.org/10.20381/ruor-31299The Hishikari deposit is a high-grade low-sulphidation epithermal gold deposit in southwestern Kyushu, Japan. The mineralization age data by previous researchers suggest that the Hishikari Lower Andesite is contemporaneous with the mineralization. The relationship between mineralization and volcanism is unclear where this study aims to characterize the parental magmas of the Hishikari Lower Andesites. The Hishikari Lower Andesites collected ~11 km west of Hishikari mine consist of basalts (MgO ≤ 5.7 wt%) and basaltic andesite lavas (MgO ≤3.4wt%) containing Mg-rich clinopyroxene (Mg# ≤ 82) and normal zoned olivine (~Fo 82-66, Mn = 2090 - 4260 ppm, Ni = 150 - 690 ppm) with magnetite inclusions (0.66 wt% V). Samples collected from the Hishikari mine area are basaltic andesites, andesites and dacites. The andesites (1.8 - 3.7 wt% MgO) are characterized by containing clinopyroxene (Mg# ≤ 84) and orthopyroxene (Mg# ≤ 70). The basaltic andesites consist of Mg-rich basaltic andesite lavas (MgO ≤ 7.3 wt%) and pyroclastic rocks (MgO ≤ 3.3 wt%) containing Mg-rich clinopyroxene (Mg# ≤ 84) and olivine phenocrysts (Fo77, Ni ~ 500 ppm, Mn ≤ 3600 ppm) with Cr-bearing (~1.6wt% Cr, 0.46wt% V) magnetite inclusions. The presence of Mg-rich basaltic andesites suggests partial melting in the mantle for volcanism during the mineralization. Primitive mantle-normalized trace element patterns are similar in all rocks and show a subduction geochemical signature with depletions in Nb and Ti. The rocks collected ~11km west of Hishikari mine show similar ⁸⁷Sr/⁸⁶Sr values (0.704510 to 0.704754) and ɛNd values (-1.4 to +0.4) to the basaltic andesites in the Hishikari mine area (⁸⁷Sr/⁸⁶Sr = 0.704373 - 0.704500; ɛNd = -0.04 to +0.5) suggesting a similar mantle source. In the Hishikari mine area, the andesites show higher ⁸⁷Sr/⁸⁶Sr values (0.704701 to 0.706168) and lower ɛNd values (-5.3 to +0.3) than the basaltic andesites. The andesites fall along a mixing hyperbola between the basaltic andesites and the Shimanto sedimentary basement rocks suggesting that the parental magmas interacted with sediments from the Shimanto basement. In samples collected west of Hishikari, Fe²⁺/Mg ratios of the olivine equilibrium melt show that olivine crystallized in a more evolved composition than the bulk rock, suggesting early crystallization of Cpx likely due to elevated water contents in the parental magmas. Cpx exhibits weak Eu anomalies in the mafic samples (Eu/Eu* = 0.64 to 1.1) and no correlation with Sr, suggesting that the parental magmas were mildly oxidized to contain high Eu³⁺/Eu. A clear positive relationship between Ni and Fo components in olivine, the lack of sulphides observed in samples and low bulk rock S content (<0.01wt%) suggests low S contents in the parental magmas. Our samples are similar in lithology, mineralogy and bulk rock compositions to two samples from the Yamagano Kinzan andesite as well as Pliocene-Pleistocene volcanic rocks in large volcanic fields of Hisatsu and Hokusatsu in Kyushu. These geochemical similarities are possible evidence for a similar mantle source beneath south-central Kyushu.enHishikariandesitebasaltic andesiteolivinemagmavolcanismKyushuclinopyroxenemagnetiteHishikari Lower AndesiteThesis