DESCRIPTIVE MODEL OF CREEDE EPITHERMAL VEINS

MODEL 25b

By Dan L. Mosier, Takeo Sato, Norman J Page, Donald A. Singer,

and Byron R. Berger

APPROXIMATE SYNONYM Epithermal gold (quartz-adularia) alkali-chloride-type, polymetallic veins (see fig. 106).

Figure 106. Cartoon cross section of typical Creede-type epithermal vein deposit.

fig 106

DESCRIPTION Galena, sphalerite, chalcopyrite, sulfosalts, + tellurides + gold in quartz-carbonate veins hosted by felsic to intermediate volcanics. Older miogeosynclinal evaporites or rocks with trapped seawater are associated with these deposits.

GENERAL REFERENCES Buchanan (1980), 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 29-4 m.y.).

Depositional Environment Bimodal and calc-alkaline volcanism. Deposits related to sources of saline fluids in prevolcanic basement such as evaporites or rocks with entrapped seawater.

Tectonic Setting(s) Through-going fractures systems; major normal faults, fractures related to doming, ring fracture zones, joints associated with calderas. Underlying or nearby older rocks of continental shelf with evaporite basins, or island arcs that are rapidly uplifted.

Associated Deposit Types Placer gold, epithermal quartz alunite Au, polymetallic replacement.

DEPOSIT DESCRIPTION

Mineralogy Galena + sphalerite + chalcopyrite + copper sulfosalts + silver sulfosalts gold tellurides bornite arsenopyrite. Gangue minerals are quartz + chlorite calcite + pyrite + rhodochrosite + barite fluorite siderite ankerite sericite adularia kaolinite. Specularite and alunite may be present.

Texture/Structure Banded veins, open space filling, lamellar quartz, stockworks, colloform textures.

Alteration Top to bottom: quartz kaolinite + montmorillonite zeolites barite calcite; quartz + illite; quartz + adularia illite; quartz + chlorite; presence of adularia is variable.

Ore Controls Through-going or anastomosing fracture systems. High-grade shoots where vein changes strike or dip and at intersections of veins. Hanging-wall fractures are particularly favorable.

Weathering Bleached country rock, goethite, jarosite, alunite--supergene processes often important factor in increasing grade of deposit.

Geochemical Signature Higher in system Au + As + Sb + Hg; Au + Ag + Pb + Zn + Cu; Ag + Pb + Zn, Cu + Pb + Zn. Base metals generally higher grade in deposits with silver. W + Bi may be present.

EXAMPLES

Creede, USCO (Steven and Eaton, 1975; (Barton and others, 1977)

Pachuca, MXCO (Geyne and others, 1963)

Toyoha, JAPN (Yajima and Ohta, 1979)