Statistical features of vein systems in the Hishikari epithermal gold deposit, Japan

Abstract

The Hishikari epithermal gold deposit consists of parallel vertical veins hosted by the Quaternary volcanic rocks (QVR) and Cretaceous sedimentary rocks (CSR) at the shallow and deep levels, respectively. In order to reveal the lithological influence on the evolution of vein systems in the Hishikari field, vein density, width, spacing and position were statistically examined using cores sampled from three boreholes. The statistical features of the veins identified were summarized as follows. (i) The QVR have lower mean vein densities than the CSR. (ii) The former has smaller deformation degrees (ratios of total vein width per length) than the latter. (iii) Cumulative frequencies of vein density obey negative-exponential distributions in both the types of rocks. (iv) Cumulative frequencies of vein width obey power-law distributions with exponents of about -1 in both the types of rocks, but consist of two segments in the CSR. (v) Cumulative frequencies of vein spacing obey lognormal distributions with coefficients of variation between 1.8 and 3.5 in both the types of rocks. And (vi) fractal dimensions of vein position are between 0.45 and 0.52 at measure lengths between 0.1 and 10 m without distinctive difference between the two types of rocks. The feature (i) indicates higher ductility of the QVR a gainst fracturing than the CSR. The feature (ii) implies that large deformation was accumulated in rocks themselves (e.g., alteration zones) in addition to the veins examined in the QVR compared to only the veins in the CSR. The feature (iv) suggests that the vein systems in the CSR reached a stage at which the several selected large veins grew as main conduit, that potentially provided pathways for ore-forming fluids to ascend from deep levels. The coefficients of variation of vein spacing and fractal dimensions of vein position (features v and vi) indicate clustered distribution of the veins in the field. This study might be helpful for a better understanding of lithological influence on the evolution of vein systems not only in the Hishikari field but also in other fields.