DESCRIPTIVE MODEL OF DISTAL DISSEMINATED Ag-Au

By Dennis P. Cox

BRIEF DESCRIPTION

SYNONYM: Sediment-hosted Ag-Au, disseminated Ag
DESCRIPTION: Disseminated Ag and Au mainly in sedimentary rocks distal to porphyry Cu, skarns, and polymetallic veins (Graybeal, 1981).

TYPICAL DEPOSITS: Taylor, Candelaria, Star Pointer, Cove deposits, White Pine district, Nevada; Tecoma Utah; Vekol, Tombstone, and Hardshell, Arizona.
DISTINGUISHING FEATURES: This model is similar to sediment-hosted Au but has significantly higher Ag grades than that model (see Ag grades in grade and tonnage models for both). It also is characterized by higher geochemical background values

COMMODITIES: Ag, Au

OTHER COMMODITIES: Locally, Sb

ASSOCIATED DEPOSIT TYPES: Porphyry Cu, Cu skarn, Pb-Zn skarn, Au skarn, polymetallic veins, polymetallic replacement and replacement Mn deposits.

REGIONAL GEOLOGIC ATTRIBUTES

TECTONOSTRATIGRAPHIC SETTING: Continental margins.
REGIONAL DEPOSITIONAL ENVIRONMENT: Shelf and basinal sedimentary rocks are folded and faulted and intruded by I-type granitic rocks.
AGE RANGE: Mesozoic-Tertiary in Western United States; may be any age.

LOCAL GEOLOGIC ATTRIBUTES

HOST ROCKS: Carbonate and clastic sedimentary rocks.

ASSOCIATED ROCKS: Felsic hypabyssal or subvolcanic intrusions.

ORE MINERALOGY: Native Au, native Ag, electrum, argentite, Ag sulfosalts, tetrahedrite, stibnite, galena, sphalerite, chalcopyrite, pyrite, marcasite, arsenopyrite; at Cove deposits, stannite and canfieldite.

GANGUE MINERALS: Quartz, rhodochrosite, Ag-rich manganocalcite.

STRUCTURE AND ZONING: Ore minerals sparsely disseminated or in stockwork of thin quartz-sulfide veins.

ORE CONTROLS: Deposits commonly occur in skarn and polymetallic vein and replacement districts outboard of all other types of mineralization. Fracture permeability is the most important ore control, Primary rock permeability may be important locally

STRUCTURAL SETTING: Shear zones, axial plane fractures in folded rocks

ORE DEPOSIT GEOMETRY: Irregular bodies, locally conformable to bedding

ALTERATION: Silicification (Taylor, Star Pointer, Cove) and decalcification (Star Pointer) of carbonate rocks; sericite-clay in clastic rocks (Candelaria).

EFFECT OF WEATHERING: Leaching and redeposition of Ag as cerargyrite forms bonanza deposits (White Pine district, Nevada; Vekol, Arizona).

GEOCHEMICAL SIGNATURES: Ag+Pb+Zn+Cu+Sb+As+Zn+Pb+Mn+Cu+Ag+Au+As+ Sb+Hg+Te; Mn introduced at Cove, Candelaria, and Star Pointer. Ag:Au ratios are highly variable: Candelaria 400:1; Taylor, 143:1; Tecoma, 60:1; Purisima Concepion, 51:1; Hilltop, <2:1.