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Bulletin of the Geological Society of America Vol. 56, Pp BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 56, PP. 50S-SI4, 4 PUS., 3 FIGS. MAY 1945 SIMPSONITE AND THE NORTHERN BRAZILIAN PEGMATITE REGION BY FREDERICK H. POUGH CONTENTS Page Abstract 505 Introduction and acknowledgments 505 General description of the Alto do Giz 506 Mineralogy 508 Simpsonite 508 Microlite 511 Tantalite and its varieties 512 Bismutite and cyrtolite 513 Paragenesis 513 References cited 514 ILLUSTRATIONS Figure Page 1.—Typical tabular yellow-brown Alto do Giz simpsonite crystal 510 2.—Minute tabular, buff-colored simpsonite from Onca Mine 510 3.—Prismatic crystal fragment from Onca Mine 510 Plate Facing page 1.—Simpsonite Frontispiece 2.—Alto do Giz ! 506 3.—Kaolinite replacements 507 4.—Microlite and cyrtolite 510 ABSTRACT A study of the Alto do Giz, near Equador, Rio Grande do Norte, Brazil, reveals an interesting series of tantalum minerals including the rare aluminum tantalate, simpsonite. Crystals of this mineral are yellow brown, hexagonal in outline; c = .60978. It belongs to the hexagonal dipyramidal class Ceh. Hardness 7J, specific gravity is 6.81. It fluoresces blue to white in a cold quartz mercury vapor lamp with a filter. It is also found in white crystals at this mine and in buff-colored crystals at the Onca Mine, some miles away. It appears to be a replacement mineral of a short complex pegmatite sequence, crystallizing after microlite and tantalite. INTRODUCTION AND ACKNOWLEDGMENTS The extensive exploitation of the numerous pegmatite operations in northern Brazil is a relatively recent development, since the beginning of the war. In the years immediately preceding, when some nations foresaw a need for stockpiles of raw materials, there was a certain amount of beryl and tantalite production in the region, but the real impetus to the development of this desert area was supplied by the advent of the United States Purchasing Commission and their introduction of various 505 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/56/5/505/3425995/i0016-7606-56-5-505.pdf by guest on 28 September 2021 506 F. H. POUGH—SIMPSONITE AND BRAZILIAN PEGMATITE promotional activities. These included assistance in the form of money and contracts as well as by the supplying of mechanical equipment, technical assistance, and scien- tific supervision. The Brazilian Government, however, had already appreciated the potential value of the pegmatites of the Northeast, and in 1940 they established in Campina Grande, the rail head in Paraiba, an office and analytical laboratory to assist development by identifications and analyses. The writer was very fortunate in being able to visit the region twice, in 1943 and in 1944, and, while time did not permit extensive studies of many of the exposed pegmatites, he was able to pay considerable attention to a few, especially the Alto do Giz, chief source of the simpsonite. At the same time a fine survey was obtained of a large number of the operations giving a general picture of the pegmatites of the area. Scheelite occurrences and a fluorite mine were also seen. All are situated in the same area, for reasons which will appear later. The writer is indebted to many people in this study of the mineralogy of the Alto do Giz and the neighboring pegmatites. The occurrence of the brown hexagonal mineral, now identified as simpsonite, was first brought to his attention by Dr. Caio Guimaraes, who followed a preliminary announcement in 1942 (Guimaraes, 1942) with a published description of it under the name calogerasite (Guimaraes, 1944). Further specimens were seen in Rio de Janeiro at the petrographic laboratory of the Ministerio da Agricultura, Divisao do Geologia e Mineralogia with Dr. Othon H. Leonardos, and were generously presented to the writer for study by Dr. Evaristo P. Scorza, who is in charge of the laboratory. The subsequent trip to the Northeast and the visit to the locality was greatly facilitated by the extremely co-operative attitude of the Director, at that time, of the Divisao de Fomento da Producao Mineral, Dr. Avelino Ignacio de Oliveira, and his laboratory Director at Campina Grande, Paraiba, Dr. Onofre Pereira Chave. The owner and operator of the mine, Jose Marcelino de Oliveira, was very generous with specimens and information about the development of the deposit. Mention should also be made of the contributions in the form of information and comfortable living quarters wh:le in the field accorded by many members of the staff of the United States Purchasing Commission, headed in 1943 by Edward H. Page and in 1944 by James E. Moore at the field office in Campina Grande. The writer also wishes to express his appreciation to Professor Paul F. Kerr for his co-operation in the preparation of the report. A number of publications describing the occurrence of the pegmatites have been written in Brazilian reports and journals (De Almeida, Johnston, Scorza, and Leon- ardos, 1943), but little has appeared in English (De Almeida et al., 1944; Pough, 1945). Other papers on the region cc itain descriptions of the scheelite and fluorite deposits (Johnston, 1944; Johnstur and de Vasconcelos, 1944; Scorza, 1943), which occur in the roof pendants scattered irregularly through the region. GENERAL DESCR PTION OP THE ALTO DO GIZ As described by De Almeida et til., the region is a high plateau, about 7° S. Lat. with an average elevation of aboul 1600 to 1800 feet. The climate is dry; much of the region i' ' lost desert, but bee luse of the elevation and aridity it is not disagree- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/56/5/505/3425995/i0016-7606-56-5-505.pdf by guest on 28 September 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/56/5/505/3425995/i0016-7606-56-5-505.pdf by guest on 28 September 2021 BULL. GEOL. SOC. AM., VOL. 56 POUQH, PL. 3 FIGURE 1. LATHLIKE LAYERS IN A FORMER CRYSTAL WHICH WAS PROBABLY FELDSPAR The original crystal measured about 3 feet (Alto do Giz mine). FIGURE 2. REPLACED CRYSTALS OF SPODUMENE DISTRIBUTED THROUGH QUARTZ Boquerao mine. KAOLINITE REPLACEMENTS Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/56/5/505/3425995/i0016-7606-56-5-505.pdf by guest on 28 September 2021 GENERAL DESCRIPTION OF THE ALTO DO GIZ 507 able. A strong wind every evening makes the nights comfortable. Ordinary dirt roads are passable most of the time, and this is one of the best parts of Brazil in that respect. Travel is almost entirely by automobile; most of the mines can be easily reached, and roads to some of the better mines have been improved by the Com- mission so that compressors could be taken in. The simpsonite-bearing Alto do Giz (PL 2, fig. 2) is a typical pegmatite of the region but shows a few features not noted elsewhere. It is most notable for its varied mineralogy in the tantalum group of minerals. It is about 2 km. south of the main road from Equador to Parelhas, in Rio Grande do Norte, and is reached from a spot 8 km. north of the town. It belongs to and is operated by Jose Marcelino de Oliveira, of Equador, and has been an active tantalite producer since 1937. From this history, it is obvious that it is one of the richer mines of the region, for tantalite and beryl prices were far lower before the war need became so urgent. The dike occupies the summit of a ridge, parallel to and not far southeast of another ridge, nearer the road, on which is to be found the large dike of Alto Mamoes. In line with the latter deposit there are two openings on pegmatites directly opposite Alto do Giz. The pegmatite of Alto do Giz is a large one, outcropping over a distance of 1000 feet and showing a width of 40 to 50 feet. Surface development has made great cuts in the pegmatite at various places along the strike, and at the south end the most extensive develop- ment has taken place, leaving pits 40 feet deep. It lies above the granite batholith source and cuts a micaceous quartzite with pronounced schistosity roughly paralleling the N.40°E. strike. It rises approximately vertically through the layers of quartzite which dip about 40° W. In cross section the pegmatite is typical of the region, showing a heterogeneous band of mixed quartz, mica, and kaolinized feldspar along the contact, gradually grading out into the extremely coarse texture of a typical pegmatite dike. Here and there in this coarse mass are knots of fine-grained white mica, but in general only quartz and large pseudomorphs of feldspar crystals are to be seen. The core of barren quartz, left as a wall with cuts on either side, is about 12 feet thick. The economic minerals occur in the intermediate band of coarse material and have a slight tendency to be concentrated near the margin of the quartz core. None were seen in place, as is the characteristic experience in sporadic visits to any of these workings, but the site of earlier finds pointed out indicates this localization. Any tantalite found in the Northeastern pegmatites is promptly removed, so the occasional visitor will rarely see it in place. Production figures were obtained from the owner, who reported that by fall of 1943 the mine had yielded over 10 tons of tantalite and over 100 tons of beryl, with a little bismuth; the exact figure is not known. Extensive dumps were formed down the slope of the hill during the earlier operations when tantalite was less valuable, and these dumps, and the placers beneath them, are now being reworked by primitive jig and panning methods to recover the ores.
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