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UNITED STATES MINERAL RESOURCES 



dike) of magnetite pyroxenite intruded the zone of 

 weakness at the contact of the biotite pyroxenite 

 with the country rocks. A younger mass of syenite 

 cut both the magnetite and the biotite pyroxenite, 

 and alkalic syenite dikes cut the biotitite core. 

 Fenitized Belt rocks and a small nepheline syenite 

 mass nearby suggest the presence of a carbonatitic 

 mass at depth. Biotite in the biotite pyroxenite, and 

 to a small extent in the magnetite pyroxenite, is 

 altered to hydrobiotite and vermiculite; the biotite 

 of the dense core is virtually unaltered. 



Boettcher (1966) concluded that the hydrobiotite 

 is the result of high-temperature alteration (pre- 

 sumably hydrothermal) , whereas the vermiculite is 

 the result of surficial alteration by circulating 

 ground water. The evidence for hydrothermal altera- 

 tion is not conclusive to me; a deiinite answer will 

 probably not be gained until hydrobiotite is found 

 below the level of ground-water circulation. A fairry 

 intensive search of the literature has not turned up 

 any such occurrences. The Mining Journal (1952) 

 reported that vermiculite had been proved by drill- 

 ing to a depth of 800 feet ; as the deposit is located 

 in mountainous terrain and underlies a ridge with 

 from 800 to 1,200 feet of relief on three sides, it is 

 very possible that the vermiculite from the bottom 

 of the 800-foot hole was still in the zone of circulat- 

 ing ground water, the zone of surficial alteration. 

 Many hundreds of thousands of tons of vermiculite 

 have been produced from this deposit since mining 

 began about 1925, and the deposit is far from 

 depleted. 



Vermiculite in the dunite-type (type 2) deposits 

 of the Blue Ridge province also is found only in the 

 zone of weathering. It occurs in veins and lenses 

 along the commonly serpentinized contact of dunite 

 and pyroxenite bodies with the metamorphic country 

 rocks and with crosscutting pegmatites, and along 

 fractures within the bodies. The largest deposits are 

 at the contacts, but the highest concentrations are 

 along the internal fractures. Where the country rock 

 also is mafic (hornblende gneiss or biotite schist), 

 the vermiculite may occur on both sides of the con- 

 tact. In these deposits, vermiculite typically occurs 

 in lenses that are as much as 4-5 feet thick and 

 50-60 feet long. A few lenses are 20 feet thick and 

 more than 100 feet long (Hunter, 1950) . 



There is considerable variety among the deposits 

 in the layered metamorphic rocks (type 3). Some 

 of the amphibolite and biotite schists may have been 

 sedimentary rocks, others intrusive or extrusive 

 rocks, but all deposits are characterized by granitic 

 or syenitic crosscutting pegmatites. The deposits in 

 the second most important group in the United 



States are of this type, in the Enoree and Enoree- 

 Waldrep districts of South Carolina, where many 

 deposits have contained 20,000 to 50,000 tons of ore, 

 and some as much as 100,000 tons of ore. In these 

 districts, weathering is so intense that rock types 

 are difficult to identify and the contact relations are 

 very obscure. Hunter (1950) suggested that the 

 vermiculite developed by deep weathering of biotite 

 from lenses of pyroxenite in contact with biotite 

 gneiss and schist. Only fragments of pyroxenite 

 have been found in the deposits; the pyroxenite 

 bodies may have been metamorphosed to amphibolite 

 schists, which have been mapped in the Enoree area 

 by McClure (1963). All the deposits are cut by 

 stringers of pegmatite, and small masses of "gran- 

 ite" have been reported. Hunter (1950) described 

 vermiculite lenses 450 feet long and 150 feet wide, 

 with a depth to unaltered biotite of about 80 feet; 

 these dimensions suggest pluglike bodies rather than 

 dikes or layered rocks, but most of the deposits have 

 length-width rations of 5:1 or greater — consistent 

 with favorable zones in layered rocks. 



The most numerous deposits, but also those with 

 the smallest amounts of vermiculite, are in the 

 layered metamorphic rock (type 3). Characteristic- 

 ally, the deposits are in biotite schists, amphibolite 

 schists, or mica-amphibolite schists, and are cut by 

 pegmatites. The deposits are only a few feet thick, 

 although they may be several hundred long, and the 

 depth of weathering is usually no more than a few 

 tens of feet. Ore content usually ranges from a few 

 hundred tons to several thousand tons. Hagner 

 (1944) described several deposits of this type in 

 Wyoming, and Clabaugh and Barnes (1959) did so 

 for Texas. Descriptions of deposits in other States 

 are given in publications li.°ted in "Selected Refer- 

 ences." 



In general, the geologic relations of all the types 

 of deposits suggest that pyroxenes (diopside and 

 augite) , amphiboles (hornblende and tremolite) , and 

 olivine in the ultramafic rocks were altered by solu- 

 tions and volatiles from intrusive syenites, carbona- 

 tites, and pegmatites (supplying mostly OH) ; and 

 biotite, phlogopite, serpentine, and chlorite were 

 formed. Supergene alteration by circulating ground 

 waters later removed the alkalis, redistributed mag- 

 nesium, and added H2O as interlayer water mole- 

 cules to form vermiculite. The position of 1:1 regu- 

 larly interstratified hydrobiotite in the sequence is 

 not certain (Boettcher, 1966), but I believe that it, 

 too, is a supergene alteration product. 



In order of significance, the vermiculite deposits 

 of the United States are in Montana (almost two- 

 thirds of the United States production), the Inner 



