DESCRIPTIVE MODEL OF SADO EPITHERMAL VEINS

MODEL 25d

By Dan L. Mosier, Byron R. Berger, and Donald A. Singer

DESCRIPTION Gold, chalcopyrite, sulfosalts, and argentite in vuggy veins hosted by felsic to intermediate volcanic rocks that overlie older volcanic sequences or igneous intrusions (see fig. 106).

GENERAL REFERENCE Boyle (1979).

GEOLOGICAL ENVIRONMENT

Rock Types Host rocks are andesite, dacite, quartz latite, rhyodacite, rhyolite, and associated sedimentary rocks. Mineralization related to calc-alkaline or bimodal volcanism.

Textures Porphyritic.

Age Range Mainly Tertiary (most are 38-5 m.y.).

Depositional Environment Calc-alkaline and bimodal volcanism and associated intrusive activity over basement rocks composed of thick, older volcanic sequences or igneous intrusives (batholiths). Volcanic-related geothermal systems lack access to saline fluids from basement sources.

Tectonic Setting(s) Through-going fracture systems, major normal faults, fractures related to doming, ring fracture zones, joints.

Associated Deposit Types Placer gold and quartz-alunite Au.

DEPOSIT DESCRIPTION

Mineralogy Gold argentite + electrum + chalcopyrite. Sulfosalts and tellurides are moderate, galena and sphalerite are sparse. Gangue minerals are quartz pyrite adularia calcite. Chalcedony, adularia, kaolinite, rhodochrosite, chlorite, sericite, and barite are moderate to sparse.

Texture/Structure Banded veins, open space filling, lamellar quartz, stockwork, breccia pipes.

Alteration Silicification zoned by quartz + kaolinite + montmorillonite alunite; may have pervasive propylitic alteration of chlorite + calcite.

Ore Controls Through-going fracture systems; major normal faults, fractures related to doming, ring fractures, joints.

Weathering Bleached country rock, limonite, hematite, goethite, jarosite, alunite; argillization with kaolinite.

Geochemical Signature Au + Ag; Au + Ag + Cu.

EXAMPLES

Takeno, JAPN (Soeda and Watanabe, 1981)