Geology and Mineral Resources of Paraguay A Reconnaissance

IGNEOUS AND METAMORPHIC ROCKS 29PETEOGEAPHY OF THE BASALT AT FOZ DE IGUA£TJ, BEAZILThe basalt from near the town of Foz de Iguacuappears to be somewhat variable lithologically, asindicated by the several specimens collected by Eckel.Conditions during crystallization, that resulted invariations in color of the rock from gray to brown andthe action of hydrothermal solutions during the laterstages of crystallization of the magma are reflected invariations that resulted in formation of vugs andreplacement of rock by quartz-chlorite aggregates.Whether this variation is a special feature of the Foz deIguacu locality, revealed by the abundance of unweatheredexposures, or whether the Serra Geral lavas lesswell exposed elsewhere would show comparable diversity,is not known.A vuggy type of rather coarsely crystallized basalt(specimen P-92a) is shown in figure 21. There is anoteworthy amount of pseudobrookite as a result ofthe action of hydrothermal solutions acting on theilmenite of the basalt (specimen P-92, figs. 22 and 23).The pyroxene of this rock is almost completelyreplaced by opaque black oxide, perhaps ilmenite,FeO-TiO2 . However, immediately adjacent to the vugswithin the chlorite-chalcedonic quartz lining, thisblack oxide is mantled and in some places completelyreplaced by bright-golden-yellow prismatic pseudobrookiteFe2O3-TiO2. This mineral characteristicallyhas slight, hardly perceptible pleochroism and brilliantlow-order (0.030±) interference colors, is biaxial andoptically positive, and has negative elongation.The conversion of ilmenite to pseudobrookit e is thus:2FeO.TiO2+Fe2O3+O^2Fe2O3-TiO2 or 3FeO-TiO2+Fe3O4+20-^3Fe2O3 -TiO 2, and ilmenite is commonlyassociated with magnetite or hematite in basaltic rocks.Ramdohr (1950, p. 735-736) observesSo far, pseudobrookite is known almost without exception as agaseous alteration product in titanium-rich magmas, in opencavities, intergrown therein with hematite; and, very rarely,as a constituent mineral of foyaite-pegmatite. In the first case,it is formed indirectly or directly from titanomagnetite, throughoxidation at the temperature of hot fumaroles (e. g. AranyerBerg, Thomas Range, and Katzenbuckel), thus: 3FesO4+FeTiO3+20-»4Fe2O3-!-Fe2TiOs . Apparently, the hematite thusformed may often be removed.He also notes that pseudobrookite is easily producedby heating ilmenite and hematite in air.Fries, Schaller, and Glass (1942) list 15 occurrencesFIGURE 21. Brown moderately fine grained vuggy basalt (specimen P-92a) from Foz de Iguagu, Brazil. Well-formed calcic plagioclase phenocrysts,/, in amatrix of apparently similar but rather turbid feldspar. The black areas are pyroxene almost completely replaced by black iron oxide. Two large vugsare shown in the upper half of the illustration, with a lining, 1, of chalcedonic quartz and pale-green chlorite. Pseudobrookite is formed around thesevugs. X 15. Crossed nicols.