Geology and Mineral Resources of Paraguay A Reconnaissance

IGNEOUS AND METAMORPHIC ROCKS 21FIGURE 14. Typical Precambrian quartz-porphyry (specimen P-67) reddish-brown phase, from the road between Quiindy and Caapucu. Phenocrysts areeuhedral quartz, q, and reddish sericitized sodic plagioclase, /, each as much as 2 millimeters in diameter, with a little chloritized biotite, b. The fine-grainedgroundmass, g, contains some ore grains, o, and shows striking flow structure. X 15.Petrographic descriptions of the granites, aplites, andgranite porphyries that are available indicate that theserocks are similar to those in southern Paraguay.The "biotite-gneiss" studied by Goldschlag (1913a,p. 54) from Estancia Machuca-cue is similar in mineralogyand chemical composition to the granite fromsouthern Paraguay, for example, specimen P-62 fromnear Barrerito. This suggests that much of the gneissdescribed by Carnier (IQllc), Goldschlag (1913a), andBoettner (1947), from the Rio Apa region is closelyrelated genetically and petrologically to the rocks ofsouthern Paraguay that are here called "granites."Goldschlag's "biotite-gneiss" is said to extend over alarge area from the Rio Pitonoaga to the Rio Apa. AtZanja Moroti it strikes N. 55°-60° E. and is cut bymany small veins of "quartzite," aplite, and pegmatite.It does not extend beyond the Rio Termentino. Goldschlag'sspecimens from Estancia Machuca-cue werefound by him to consist of microcline, albite-oligoclase,and quartz, with biotite, green hornblende, sphene,magnetite, and epidote.AMAMBAY REGIONEckel saw in Asuncion a rather large collection ofspecimens of metamorphic and igneous rocks that werereliably reported by their owner to have come from the"Amambay region, near Pedro Juan Caballero." Thereare no published geologic maps that show Precambrianrocks in the region cited and the locality descriptionavailable is so vague that no attempt has been made toportray them on plate 1.The specimens seen include crenulated mica schist,gneiss, and granite and quartz porphyry similar to therocks exposed near Caapucu in southern Paraguay.Coarsely bladed specular hematite and sheets of muscovitemica, streaked with brown and black materialand as much as 7 by 10 centimeters in size, were seenin the same collection.TRES HERMAN OS HILLS NEAR PUERTO GTJARANfCarnier (1913) mentions the Tres Hermanos hills,near Puerto Guarani, as probably related in origin androcks to the Pao de Acucar. They are in the Gran