DESCRIPTIVE MODEL OF LOW-SULFIDE Au-QUARTZ VEINS

By Byron R. Berger

Model 36a

APPROXIMATE SYNONYMS Mesothermal quartz veins, Mother Lode veins.

DESCRIPTION Gold in massive persistent quartz veins mainly in regionally metamorphosed volcanic rocks and volcanic sediments.

GEOLOGICAL ENVIRONMENT

Rock Types Greenstone belts; oceanic metasediments: regionally metamorphosed volcanic rocks, graywacke, chert, shale, and quartzite. Alpine gabbro and serpentine. Late granitic batholiths.

Age Range Precambrian to Tertiary.

Depositional Environment Continental margin mobile belts, accreted margins. Veins age generally post-metamorphic and locally cut granitic rocks.

Tectonic Setting(s) Fault and joint systems produced by regional compression.

Associated Deposit Types Placer Au-PGE, kuroko massive sulfide, Homestake gold.

DEPOSIT DESCRIPTION

Mineralogy Quartz + native gold + pyrite + galena + sphalerite + chalcopyrite + arsenopyrite pyrrhotite. Locally tellurides scheelite bismuth tetrahedrite stibnite molybdenite fluorite. Productive quartz is grayish or bluish in many instances because of fine-grained sulfides. Carbonates of Ca, Mg, and Fe abundant.

Texture/Structure Saddle reefs, ribbon quartz, open-space filling textures commonly destroyed by vein deformation.

Alteration Quartz + siderite and (or) ankerite + albite in veins with halo of carbonate alteration. Chromian mica + dolomite and talc + siderite in areas of ultramafic rocks. Sericite and disseminated arsenopyrite + rutile in granitic rocks.

Ore Controls Veins are persistent along regional high-angle faults, joint sets. Best deposits overall in areas with greenstone. High-grade ore shoots locally at metasediment-serpentine contacts. Disseminated ore bodies where veins cut granitic rocks.

Weathering Abundant quartz chips in soil. Gold may be recovered from soil by panning.

Geochemical Signature Arsenic best pathfinder in general; Ag, Pb, Zn, Cu.

EXAMPLES

Grass Valley, USCA (Lindgren, 1896)

Mother Lode, USCA (Knopf, 1929)

Ballarat Goldfield,
Victoria, AUVT (Baragwanath, 1953)

Goldfields of Nova Scotia, CNNS (Malcolm, 1929)