DESCRIPTIVE MODEL OF CARBONATE-HOSTED Au-Ag

MODEL 26a

By Byron R. Berger

APPROXIMATE SYNONYM Carlin-type or invisible gold.

DESCRIPTION Very fine grained gold and sulfides disseminated in carbonaceous calcareous rocks and associated jasperoids.

GENERAL REFERENCE Tooker (1985).

GEOLOGICAL ENVIRONMENT

Rock Types Host rocks: thin-bedded silty or argillaceous carbonaceous limestone or dolomite, commonly with carbonaceous shale. Intrusive rocks: felsic dikes.

Textures Dikes are generally porphyritic.

Age Range Mainly Tertiary, but can be any age.

Depositional Environment Best host rocks formed as carbonate turbidites in somewhat anoxic environments. Deposits formed where these are intruded by igneous rocks under nonmarine conditions.

Tectonic Setting(s) High-angle normal fault zones related to continental margin rifting.

Associated Deposit Types W-Mo skarn, porphyry Mo, placer Au, stibnite-barite veins.

DEPOSIT DESCRIPTION

Mineralogy Native gold (very fine grained) + pyrite + realgar + orpiment ± arsenopyrite ± cinnabar ± fluorite ± barite ± stibnite. Quartz, calcite, carbonaceous matter.

Texture/Structure Silica replacement of carbonate. Generally less than 1 percent fine-grained sulfides.

Alteration Unoxidized ore: jasperoid + quartz + illite + kaolinite + calcite. Abundant amorphous carbon locally appears to be introduced. Hypogene oxidized ore: kaolinite + montmorillonite + illite + jarosite + alunite. Ammonium clays may be present.

Ore Controls Selective replacement of carbonaceous carbonate rocks adjacent to and along high-angle faults, or regional thrust faults or bedding.

Weathering Light-red, gray, and (or) tan oxides, light-brown to reddish-brown iron-oxide-stained jasperoid.

Geochemical Signature Au + As + Hg + W ± Mo; As + Hg + Sb + Tl ± F (this stage superimposed on preceding); NH3 important in some deposits.

EXAMPLES

Carlin, USNV (Radtke and others, 1980)

Getchell, USNV (Joralemon, 1951)

Mercur, USUT (Gilluly, 1932)