Geology and Mineral Resources of Paraguay A Reconnaissance

IGNEOUS AND METAMOEPHIC ROCKS 33a diabase porphyrite from Cerro Howyi. This hill isnot shown by name on recent maps, but from Carnier'sdescription it appears to be a prominent conical hillabout half way between Yataity and Coronel Martinez,west of Villarrica. It contains phenocrysts of augite,olivine, and feldspar as much as 0.5 centimeters long ina glassy groundm.ass that contains smaller crystals offeldspar, brown hornblende, magnetite, and apatite.The chemical analysis of this rock is given in table 4.OLIVINE BASAIT NEAR ASUNCIONThick layers of columnar basalt characterized byprominent phenocrysts of olivine cap Cerro Tacumbu,on the south edge of Asuncion; Cerro Lambare, a fewkilometers south; and several other hills both northeastand southeast of the city. Most of these deposits arequarried extensively for building and paving stone.They appear to be remnants of a once-extensive lavaflow, but the fact that similar basalt has been found inseveral deep water wells in Asuncion indicates that thebasalt may be, at least in part, intrusive and sill-like inform, rather than wholly extrusive.Milch (1895) described the rock on Tacumbu as afeldspar-free olivine basalt or limburgite. The rock isdense dark brown to black, with many large phenocrystsof olivine and some of augite and magnetite.The olivine phenocrysts, some showing rapid growth inone direction, are colorless to light green and very fresh,with only a little iron hydroxide on some fracture planes.Some crystals are homogeneous olivine, but others containgreater or smaller amounts of magnetite and glassas inclusions. The augite phenocrysts are generallylarger than those of olivine. They are light in color,hence, only slightly pleochroic; zonal structure is clearlyrecognizable. The groundmass consists of augite crystalsvery irregularly distributed through glass. Theaugite forms stubby prisms as well as angular androunded grains; with magnetite it tends to rim thephenocrysts. Some of the glass is entirely free of inclusionsbut elsewhere it is filled with thin small greenor colorless prisms and needles. Some of these crystalsare believed b}T Milch to be feldspar, but they are sofew that the rock is designated as essentially feldsparfree.Harrington (1950) also briefly describes the basaltson Cerro Tacumbu and Cerro Lambare, in particularemphasizing the amygdular character and absence ofolivine (at least macroscopic) in the latter place.__,FIGURE 25. Basalt (specimen P-92) from Foz de Iguacu, Brazil. Shows basalt dissolved in chalcedony-chlorite vein material. X 20.