Geology and Mineral Resources of Paraguay A Reconnaissance

16 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYis known that the northern part of the mapped area,in the vicinity of Quiindy, is made up mostly of graniteand also that granite makes up most of the area betweenCaapucu and Quyquyo. Most of the remainder appearsto be porphyry. The contacts between the two kindsof rock are very irregular but are comparatively easyto trace on the ground and even on aerial photographs,because the porphyries almost invariably have astreaky appearance in photographs that the granites donot. The age relations of granite and porphyry arenot known.The surface of the granite ranges from nearly levelto gently rolling, with even less relief, in general, thanthe surface of the porphyry, which here and thereforms small steep-sided hills, that rise 50 to 100 ormore meters above the surrounding plains. The extensiveareas of swamp that characterize these rocks,particularly the granite, suggest that they are relativelyunfractured and unjointed, and therefore, unusuallyimpermeable.Similar lateritic soil profiles are formed on the graniteand porphyry; both are very different from profilesformed on the sedimentary rocks. There is no evidenceas to the age of the laterization but by analogywith similar laterites in other parts of the world, itmay well be an ancient laterite and represent a quitedifferent climatic environment than the present one.The surface material consists of a few centimetersof loesslike fine sandy loam that supports lush grasses,but very few trees. This material changes abruptly to1 to 2 meters of white to buff sandy clay that gradesdownward to fresh granitic rock. The clay containsrounded grains and small euhedral crystals of freshquartz. Most of the quartz grains and crystals arecoated with 1 to 3 millimeters of limonite; nodules oflimonite % to 2 centimeters in diameter are scatteredthroughout the clay. The limonite and quartz materialstend to concentrate at the top of the clay,beneath the loam, and are cemented in places to ahardpan that forms an excellent and widely usedmaterial for road surfacing. In places there are largeresidual boulders of fresh granitic or porphyritic rocksin the soil; locally the lateritic soil is missing and thereare fairly large smooth outcrops of unaltered rock.So far as known, the granite itself contains nomineral deposits, though it offers distinct promise asan attractive building stone. The porphyries, on theother hand, not only have potential use as building stonebut contain nearly all the iron and copper deposits,as well as the one known deposit of pyrophyllite, insouthern Paraguay (p. 90).Granite and related rocks. In general, the granite ismade up of interlocking grains of pink and white feldsparsin approximately equal amounts, 1 to 2 centimetersin diameter, with smaller, nearly sphericalgrains and euhedral crystals of clear-gray quartz.The only dark mineral is biotite, largely altered in mostplaces to a soft brown or green material, which formsvery small clusters interstitial to the quartz and feld­spars. In most exposures the near-surface rock ispartly weathered and crumbly, but in a few places, ason the road 8 kilometers southwest of Quyquyo", thegranite is brilliantly fresh and would form an attractivepolished building stone.A typical specimen of the granite (P-62), collecteda short distance east of Barrerito consists of glassyquartz, pink alkalic feldspar, and dark biotite, withporphyritic crystals of white feldspar a centimeterlong. The rock is apparently much the same as abiotite-gneiss studied by Goldschlag (1913a, p. 54)from Estancia Machuca-cue near the Rio Apa; andis also similar to granite from many other parts of theworld. Figure 8 shows partly chloritized biotite anda few opaque (black) ore grains. The feldspars aresodic plagioclase and orthoclase, both somewhat sericitized.There are a few euhedral deep-brown grainsof allanite(?) and also some sphene. A chemicalanalysis of this rock is given in table 1.Comparatively small dikes and irregular masses ofaplite and, to lesser extent, pegmatite are widely distributedin both the granitic and porphyritic rocks andin some of the older metamorphic ones. Some arestrongly sheared and fractured and contain considerableamounts of epidote on fracture faces. At kilometer156 on the Caapucu-Villa Florida highway, streaks ofcomparatively coarse-grained dull-green hornblenditewere noted in the aplite. Similar small hornblendicbodies are found in many localities.Good examples of typical aplites (specimen P 74, 75)are shown in figures 9 and 10; chemical analyses ofthese two rocks are reported in table 1. The rockshown in figure 9, taken from along the road betweenBarrerito and Quyquyo, is particularly interestingbecause it is porphyritic and closely resembles some ofthe quartz porplryries (see fig. 14) in external appear­ance. It is a red felsic rock, with large glassy quartzphenocrysts. Unlike the porphyries, however, it has adistinctly granular texture throughout.Porphyry. The predominant porphyritic rock is aquartz porphyry, close to rhyolite in composition;most of it is reddish brown but some is dark gray. It ischaracterized by the presence of irregular grains andeuhedral crystals of clear glassy quartz 3 millimetersto 1 centimeter in diameter, together with irregulargrains of dull-pink feldspar that are usually smaller inany given specimen than the quartz grains. Theseare set in a dense dull-brown to gray stony groundmass.The rock varies widely in the relative proportions of