CHROMIUM 



115 



PODLFORM DEPOSITS 



The podiform chromite deposits are lenticular or 

 rudely tabular pods, but some defy description as to 

 form. They range in size from a few pounds to sev- 

 eral million tons. Most pod deposits yield less than 

 1,000 tons of ore each, but most of the production 

 comes from ore bodies that contain 100,000 tons or 

 more. Less than a dozen deposits that originally con- 

 tained 1,000,000 tons or more are known. Most pod 

 deposits are of the high-chromium type, but these 

 deposits also are the only source of high-aluminum 

 chromite. Podiform deposits occur in irregular peri- 

 dotite masses or peridotite-gabbro complexes of the 

 alpine type, which are restricted to mountain belts 

 such as the Appalachians and Urals and to belts that 

 surround most of the Pacific Ocean. Within many 

 large complexes, the chromite deposits are localized, 

 generally near contacts between peridotite and gab- 

 bro ; but where there is no gabbro, deposits appear to 

 be scattered at random. 



Podiform chromite deposits are scattered along the 

 Pacific Coast of the United States from the Kenai 

 Peninsula in southern Alaska to southern California 

 and along the Appalachian Mountains from northern 

 Vermont to Georgia. The principal concentrations of 

 deposits or districts are in the Klamath Mountains in 

 southwestern Oregon and northern California, in the 

 western slopes of the Sierra Nevada, in the Coast 

 Ranges of California, in eastern Oregon, and in the 

 Maryland-Pennsylvania (State Line) district (Thay- 

 er and Miller, 1962 ; Thayer, 1966 ; Thayer and Ramp, 

 1969) . The largest single high-grade ore body proba- 

 bly was at the Woods mine in Pennsylvania, which 

 yielded between 100,000 and 200,000 tons of lump 

 ore before 1900 (Pearre and Heyl, 1960) . Statistical 

 analysis of recorded production from 456 mines in 

 the United States between 1916 and 1953 has shown 

 that of 517,000 long tons of ore shipped the largest 

 20 mines provided about 72 percent, 42 mines yielded 

 15.7 percent, and 394 mines yielded only 12.3 percent 

 of the ore. These figures are believed to typify podi- 

 form deposits, and accordingly, they show that most 

 deposits of this type are economically insignificant. 



The high-chromium chromite ores of the Eastern 

 Hemisphere, except for the Great Dyke, are mostly 

 in podiform deposits that are concentrated in three 

 major tectonic belts: (1) the Ural Mountains, (2) 

 the so-called Tethyan mountain chains between the 

 Alps and Himalayas, and (3) around the western 

 margin of the Pacific Ocean. The largest single well- 

 described deposit is the Coto ore body in the Philip- 

 pine Islands (Stoll, 1958), which originally contained 

 about 15 million tons of high-aluminum chromite ore. 



The Tiebaghi mine in New Caledonia and the Gololan 

 ore body in Turkey each yielded more than a million 

 tons of high-chromium chromite. The southern Ural 

 Mountains appear to include the greatest known con- 

 centration of large podiform deposits in the world, 

 but no modern figures on size of deposits are 

 available. 



DISRUPTED STRATIFORM DEPOSITS 



Tectonic disruption and dismemberment of strati- 

 form deposits have led to misidentification of those 

 in some districts as podiform. Recognition of parts 

 of a once-continuous stratiform sheet can provide an 

 economic incentive for systematic exploration to find 

 missing segments, whereas a similar program in a 

 podiform chromite district could be financially ruin- 

 ous. No large podiform deposits are known in Pre- 

 cambrian rocks, and no true stratiform deposits are 

 known in rocks younger than about middle 

 Precambrian. 



Recent cooperative investigations by the U.S. Geo- 

 logical Survey and the Companhia de Pesquisa de 

 Recursos Minerals and Departamento Nacional da 

 Produgao Mineral, under the auspices of the U.S. 

 Administration for International Development, have 

 revealed that the chromite deposits near Campo For- 

 moso, Bahia, formerly believed to be isolated blocks 

 actually are segments of a highly faulted layered 

 complex 10 to 11 miles long. Massive layers that 

 range from about 1 to 6 feet in thickness are of high- 

 chromium (metallurgical) grade. Large amounts of 

 high-iron chromite have been recovered by washing 

 of disseminated ore that has been weathered ir- 

 regularly to maximum depths of about 150 feet. Be- 

 cause a thick layer of debris from quartzite cliffs 

 above the ore zone covers much of the area, the 

 mines originally were located where this cover was 

 thin. Determination by drilling that the deposits are 

 semicontinuous under the debris has led to plans for 

 production of 100,000-200,000 tons of ore a year 

 (Mining Jour., 1972). Recent prospecting indicates 

 that similar, although probably much smaller de- 

 posits, occur in the surrounding region. Podiform 

 deposits, mostly of high-aluminum chromite, occur 

 in small peridotite masses and in the Tocantins com- 

 plex (Berbert, 1973; White and others, 1971) in cen- 

 tral Goias along a north-south belt about 1,000 miles 

 long. The lode deposits are small, and concentrations 

 of residual chromite in soil have been the principal 

 sources of production. Because exploration for 

 chromite in Brazil is at an early stage, the estimates 

 in table 24 are regarded as very tentative. The Bahia 

 deposits are believed to have the largest minable 

 reserves, by far, in the Western Hemisphere. 



