Geology and Mineral Resources of Paraguay A Reconnaissance

30 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYFIGURE 22. Basalt (specimen P-92) from Foz de Iguacu, Brazil, showing a core of ilmenite, i, with a hull of pseudobrookite, ps. X 300.of pseudobrookite: all but two of them are in extrusivevolcanic rocks in cavities and on fissure walls.Other vugs are lined or filled: some with a darkgreenmicaceous mineral, others with a brown micaceousmineral accompanied by microscopic "dogtooth"calcite crystals. Both of these platy mineralswere studied by their X-ray diffraction pattern byJ. M. Axelrod. He found the pattern of" the darkmineral to resemble that of nontronite or saponiteFe2 (OH) 2SiO4O10-nH2O or 9MgO-Al2O3-10SiO2.16H2O(approximately) and that of the brown substanceto be mainly similar but also with a 7A spacing that may bechlorite, a kaolinite group mineral, or serpentine.A fine-grained phase of the basalt (specimen P-92b),which presumably chilled early and thus was imperviousto hydro thermal solutions, is shown in figure 24.The relations of the basalt to the replacing quartzchloriteaggregate are shown in specimen P-92, figure25. Basalt is shown at the bottom of the photograph,with a bordering phase of incompletely absorbedbasalt. Several partly assimilated basalt fragmentsare seen in the quartz-chlorite area. The basalt awayfrom, the contact appears to be normal olivine basalt,the brown olivine often being completely altered toiddingsite, although the pyroxene and calcic-plagioclaseis unaltered. The light-gray turbid areas of the illustrationconsist essentially of almost isotropic finegrainedchalcedonic quartz and light-green chlorite,with scattered pyrite and bright-red hematite. Theclear areas are quartz and radial chalcedony, containingabundant extremely minute long fibers, which may berutile or, possibly, pseudobrookite. A small group ofsuch, much larger than usual, are seen in the upperright center of the photomicrograph.CARMEN DEL PARANACarmen del Parana is 200 kilometers southwest ofFoz de Iguagu. It was not visited, but a representativespecimen (P-14) from, this locality was studied. Therock is quartz basalt, with amygdaloidal cavities, somelined with bright-green chlorite and filled with calcite,others with chlorite and quartz. The rock, believedby its owner to be copper ore, contains little if anycopper; the conspicuous green "copper staining" isceladonite or a related chloritic mineral. Microscopically,the spherical cavity shown in figure 26 consistsof chlorite with quartz. Irregularly shaped quartzparticles, usually with chlorite, are strewn about. Thedarker areas are green chlorite, the white are quartz.