GRADE AND TONNAGE MODEL OF PLACER Au-PGE

By Greta J. Orris and James D. Bliss

REFERENCE Orris and Bliss (1985).

COMMENTS Placers used for this model are predominantly Quaternary in age and alluvial in nature. Many of the placer deposits contain a mix of depositional environments and energy level--deposits along minor tributaries have been worked with deposits downstream on a higher order stream, bench (or terrace) gravels have been mined with more recent deposits on valley floor. Some of the placers included in this model were formed by complex glacial-fluvial processes. Deposits not included in this model are those primarily cataloged as desert placers, pre-Tertiary or Tertiary age placers, beach placers, eolian placers, residual placers, eluvial placers, and gravel-plain deposits. These types, however, may be minor components of those deposits selected to be included. In most cases, the grade and tonnage figures are for districts or for placer operations within one mile (1.6 km) of one another. For some placers, early production figures were missing due to poor records of early gold rush work. In most cases, reserve figures (if a reserve is known) are not available. Some tonnage figures were estimated from approximate size of workings. Some grades were based on very limited information and in some cases extrapolated from information on manpower figures, type of equipment used, and estimates of the total contained gold produced.

Cutoff grades are dependent on the mining methods used to exploit placers. Methods of placer mining included in this model are as diverse as the depositional environment. These methods include panning, sluicing, hydraulic mining, and dredging. Draglines were used to mine some placers. Cut-off grades are also dependent on the value of gold during the period, or periods, of operation.

Some placer deposits were excluded due to grade or tonnage figures not compatible with the majority of placers found in the model. Placers exploited through drift mining exhibit grades that are too large and tonnages that are too small to be included in this model. Similarly, the large regional placers formed at the junction of mountainous areas and an adjacent plain or valley were excluded because they can be mined with large-volume dredges which are economic at grades not viable under other conditions. Both grades and tonnages of these placers are incompatible with this model.

Placer sizes were initially recorded in terms of cubic meters and the grades recorded as grams per cubic meter. In order to conform to other deposit models herein, deposit volume and grades have been converted to metric tons and grams per metric ton using 2.0 metric tons per cubic meter--the average density of wet sand and gravel. Gold grade is correlated with tonnage (r = -0.35) and with silver grade (r = 0.66, n = 16). See figs. 196, 197.

Figure 196. Tonnages of placer Au-PGE deposits. Individual digits represent number of deposits. [Because of limitations on the reproduction of the original figure, these numbers cannot be distinguished in the figure.]

Figure 197. Precious-metal grades of placer Au-PGE deposits. A, Gold. B, Silver.

DEPOSITS

Name Country

Adelong Creek AUNS

Alma (Mills) Placer USCO

Araluen Valley AUNS

Bannack USMT

Big Badja River AUNS

Blue River USCO

Boulder River USMT

Bullrun Placer USOR

Buxton Creek CNBC

Camanche USCA

Cobweb Diggings AUNS

Copper Basin USAZ

Corduroy Creek USID

Crooked Creek USID

Cullengoral AUNS

Deep Gravel USOR

Dixie Placer USOR

El Dorado USMT

Elkhorn Creek USMT

Elliston USMT

Fall Creek USID

Foots Creek USOR

Forest Creek USOR

French Gulch USCO

George Prezel USID

Georgia Gulch USCO

Gold Run (Summit Co.) USCO

Gold Run (Boulder Co.) USCO

Golden Rule USID

Green River USUT

Horse Praire USMT

Humbug Creek USOR

Hundred Dollar Gulch USID

Iowa Gulch USCO

Jembaicumbene Creek AUNS

Jordan Creek USID

Lamb Creek USID

Llano de Oro USOR

Lowe Placer USCO

Lower Beaver Creek USCO

Lowland Creek USMT

Lynx Creek USAZ

Missouri Creek USCO

Mitchell Creek USMT

Nugget Creek (South Fork) USID

Ophir USMT

Pactolus USCO

Picuris USNM

Pioneer USMT

Prickly Pear Creek USMT

Rio Challana BLVA

Rio Chimate BLVA

Rio Tuichi (upper reach) BLVA

Rio Yolosano BLVA

Rio Yuyo BLVA

Sand Creek USID

Schissler Creek USID

Snowstorm area USCO

Sterling Creek USOR

Sumpter Bar USOR

Swan River USCO

T93-R77W Placer USCO

Vermilion River USMT

Wellington AUNS

Wombat Creek AUNS