Geology and Mineral Resources of Paraguay A Reconnaissance

MINERAL RESOURCES 89of the bright red iron hydroxide material that characterizesthe weathered rocks in some parts of the country.MICAMica is reported to occur in many pegmatitic bodiesin the northeastern part of the country, between Concepcionand the Rio Apa. Several have been exploredto some extent and have been examined by competentmining engineers, but no detailed descriptions or reportshave become available. However, many selectedspecimens of mica were seen in sheets as much as 5centimeters in diameter and there were many reports ofsheets 2 to 3 times as large as this. All of those seenwere so full of structural flaws or of dark inclusions offoreign ferruginous material that they would have littleor no value as sheet mica.Even though available evidence is scanty, it seemsreasonable to assume that mica is widely, and perhapsabundantly, distributed in the Precambrian rocks in thenorthern part of the country. It seems probable thatmost of the material will be suitable only for scrap mica,which brings very low prices on the world market. Itis possible, however, that good grades of sheet mica,which would bring much higher prices if competentlyextracted and graded, may exist in some places. Evenif promising deposits should be found, questions of economics,of transportation, and of living conditions inthis remote and largely unsettled region might play alarger part in the workability of the deposits than wouldthe intrinsic value of the mica itself.The most persistent and reliable reports of mica occurrencesrefer to the small area of Precambrian rocks10 kilometers north of Concepcion, and to the followingplaces near the Rio Apa: between "Reyes Cue" andArroyo Seco, near Arroyo Caracol, at Puente Sinho,and close to Bella Vista. All but the last of these aresketched and described by Boettner (1947); he saysthat the pegmatite dikes at Arroyo Caracol containlarge, badly flawed books of muscovite, associated withwhite and pink feldspar, quartz, beryl, and graphicgranite.In addition to the above localities, mica is also reportedfrom Estancia Hermosa, from Potrero Bocaya,from Estancia Saty y San Bias all localities in the RioApa region and from San Estanislao. This last reportis the only evidence known that suggests the presenceof Precambrian rocks in the vicinity of SanEstanislao.PIGMENTSMany materials that could be used as pigments in themanufacture of paints are known in various parts ofParaguay. The only source known to have beenworked in recent times is a deposit of ocher about 1,500meters southwest of Tobati. It is in the white sandstonedescribed (p. 55) as the white saccharoidal memberof the Caacupe series. Along several of the strongvertical, northeastward-trending fracture planes thatcharacterize the sandstone there are irregular areas inwhich the sandstone grains are cemented with ocherinstead of the more usual calcareous or siliceous cement.This ocher, which makes up approximately 2 to 7 percentof the rock, appears to be a finely divided clay thatis impregnated and colored by secondary limonite.The largest deposit, and the one that was being workedin 1952, is about 30 meters long, 2 to 3 meters wide and3 to 5 meters high. Part of it is shown in figure 47.The rock is soft and easily worked. Blocks are priedout with hand tools, hammered to small pieces, andthen disintegrated by soaking in water (fig. 55). Theheavier sand sinks to the bottom and is removed withshovels; the ocher is decanted off. The decantationprocess is repeated in 3 to 5 operations in small woodentroughs, and the resultant ocher, when dried in the sun,FIGURE 55. Quarry and paint factory near Tobati. The sandstone at the rightcontains yellow ocherous clay interstitial to the sand grains The yellow ocher isrecovered by crushing the sandstone and washing it in shallow trays beneath theshed.