Geology and Mineral Resources of Paraguay A Reconnaissance

32 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYFIGURE 24. Basalt (specimen P-92b) from Foz de Iguacu, Brazil. A fine-grained dark-gray dense rock, similar to that in figure 21, but not vuggy andmuch finer grained. The pyroxene (olivine?) appears to be orthorhombic (parallel extinction), and occurs as sparse porphyroblasts, p. The groundmassconsists of calcic plagioclase with abundant minute yellowish prisms of monoclinic pyroxene (strongly inclined extinction), and opaque blackore grains. The pyroxene (olivine?) is largely replaced by brown biotitic material. X 20.much as 1 centimeter in diameter, in a relatively finegrained groundmass. Alteration, due to weathering orother processes, has proceeded deeply along all jointsseen, so that the fresh rock is confined to nearly perfectspheres, 5 to 30 or more centimeters in diameter, surroundedby brown fibrous iron oxides and silicates andby red and brown clay minerals.As shown in the illustration (fig. 27) the rock is adiabase showing the characteristic texture; mineralogicallyit consists essentially of calcic plagioclase andaugite. No olivine or quartz was observed in thethin section.The effects of layering in a large diabase body mayproduce striking differences in composition; thus theabsence of olivine or quartz in the specimen studied byno means precludes their presence in lower or higherhorizons, respectively.OTHER OCCURRENCES OF DIABASEGoldschlag (1913a, p. 28) describes a coarse-graineddiabase from Arroyo Bolas-cue, a small stream 12 kilometerssouthwest of Caaguazu, on the road to Villarrica.It is a green to black rock that weathers to brown andconsists principally of macroscopic crystals of andesinelabradoritefeldspar and augite, with minor amounts ofmagnetite, apatite, and serpentine. No analysis isgiven.Olivine diabase caps the sandstone beds that make upCerro Domingo and Cerro Alberto, two of the peaks ofSierra Ybyturuzu, a part of the Cordillera de Caaguazuthat is also known as Sierra Villarrica, east of Villarrica;there is also an anrygdaloidal diabase on Cerro Domingo.Goldschlag (1913a), whose analyses are givenin table 3, describes the olivine diabase at the latterlocality as made up of visible crystals of albite-twinnedandesine-labradorite, with augite and olivine; accessoryminerals are apatite, zircon, and magnetite. Theamygdaloidal rock, whose cavities are filled with quartz,chalcedony, and zeolite (chabazite), contains crystalsof oligoclase-andesine feldspar, augite, and olivine in aglassy groundmass.With the Cerro Domingo diabase Goldschlag describesan amygdaloidal phase with chabazite. Thediabase overlies sandstone, which is found as xenolithsin the diabase.Carnier (19 lie) and Goldschlag (1913a) also describe