Alteration Cap Rock Formation Model

A Case Study Hosokura Ore Deposits in Northeastern Japan

Abstract

Hosokura Ag-Pb-Zn ore deposits consisting of fissure-filling veins of epithermal type, occur in andesitic lava and pyroclastics of the Miocene Hosokura Formation in northeastern Japan. The hydrothermal alteration is observed along the veins. Based on the mineral assemblages, the alteration around the veins is classified roughly into two zones, the potassic zone (SD, SS) and the argillic zone (M), from the vein side outwards. Characteristic minerals are K-feldspar and quartz in the former, montmorillonite and mixed layer minerals in the latter. The alteration zones are developed in brecciated zones; one along the ore vein which was originally fault breccia, while the other along the top of andesitic lava which was flow-top breccia. Width of the alteration zones is generally larger in the upper horizons wherever the permeable zones exist. It is considered that the neutral deep hot waters rise along the fracture zones (veins and fault breccia zones) and flow laterally into near-surface aquifer (flow-top breccia zones). These permeable zones changed to impermeable zones by alteration. The cap rocks are divided into the primary type and the secondary one, called the “alteration cap rock”. Based on the studies of the hydrothermal alteration in the Hosokura deposits, the alteration cap rock is interpreted as impermeable zone of near-surface aquifer sealed by alteration minerals which are precipitated by mixing of upflow of deep hot waters, and cold groundwater.