DESCRIPTIVE MODEL OF PLACER PGE-Au

MODEL 39b

By Warren E. Yeend and Norman J Page

DESCRIPTION Platinum-group alloys and elemental gold in grains and (rarely) nuggets in gravel, sand, silt, and clay, and their consolidated equivalents, in alluvial, beach, eolian, and (rarely) glacial deposits derived from ultramafic sources.

GENERAL REFERENCES Boyle (1979), Wells (1973), Lindgren (1911), Mertie (1969).

GEOLOGICAL ENVIRONMENT

Rock Types Alluvial gravel and conglomerate and heavy minerals indicative of ultramafic sources and low-grade metamorphic terrane. Sand and sandstone of secondary importance.

Textures Coarse to fine clastic.

Age Range Tertiary to Holocene. Older deposits may have been formed but their preservation is unlikely.

Depositional Environment Marine (near shore), rivers and streams (medium to low gradient), desert (eolian) sand dunes, in-situ weathering.

Tectonic Setting(s) Paleozoic to Mesozoic accreted terranes, Tertiary conglomerates along major fault zones; low terrace deposits; high-level terrace gravels.

Associated Deposit Types Alaskan PGE deposits.

DEPOSIT DESCRIPTION

Mineralogy Platinum-iron alloys (isoferroplatinum with rarer ferroanplatinum, tetraferroplatinum, and tulameenite), platinum-iridium, gold, osmium-iridium alloys; magnetite, chromite, or ilmenite.

Texture/Structure Flattened, rounded edges, flaky, flour-sized alloys and gold; very rarely equidimensional nuggets.

Ore Controls Highest Au values at base of gravel deposits or on argillaceous to clayey beds within gravel sequence; metal alloys concentrated in "traps" such as natural riffles in floor of river or stream, fractured bedrock, slate, schist, phyllite, dikes, bedding planes, and in structures trending transverse to direction of water flow. For PGE, predominantly zoned "Alaskan" type ultramafic complexes and minor ophiolites as source rocks; streams or rivers usually head in regions of ultramafic rocks.

Geochemical Signature Anomalously high amounts of Ag, As, Hg, Sb, Cu, Fe, S, Cr.

EXAMPLES

Urals, USSR (Duparc and Tikonovitch, 1920; Mertie, 1969)

Goodnews Bay District, USAK (Mertie, 1969)

Choco, CLBA (Mertie, 1969)

Tulameen District, CNBC (O'Neill and Gunning, 1934)