Geology and Mineral Resources of Paraguay A Reconnaissance

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40 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYnorth of Acahay, is a fine-grained dark-gray rock, withminute quartz veinlets, reddish spots and a little pyritevisible with a hand lens. The weathered surface isbleached white to a depth of 1 centimeter. Close to thenorth edge of the town there are exposures of redbasaltic scoria also.Microscopically, the agglomerate is seen to be a siliceousrhyolite agglomerate. Anhedral quartz and sodicplagioclase phenocrysts are enclosed in a micro crystallinefelsic aggregate (fig. 30).The chemical analysis of this rock, specimen P-58b,table 1, resembles those of the Precambrian granite, porphyry,and aplite. The question naturally arises as towhether this is another instance of the commonly observedassociation of basalt and rhyolite or merely aspatial association of two unrelated rocks. If theformer is true, then the rhyolite would be contemporary,broadly speaking, with the Acahay basalt, and thereforefar younger than the Precambrian age attributedto the other granitic and rhyolitic rocks of Paraguaydiscussed in this report. Field studies should decidethis.PORPHYRY AND AGGLOMERATE FROM THE YPACARAI HIGHWAYQUARRYThe quarry that yielded most of the rock for thepaved highway between Asuncion and Eusebio Ayalais just east of the railroad and about midway betweenYpacarai and Pirayu. Though mapped as a singlebody of igneous rock on plate 1, there are actuallyfive small rounded knolls, each appearing to be geologicallycomplex. The knolls are rudely circular in planand range from 200 to 1,000 meters in diameter. Theyrise only 10 to 50 meters above the swampy plain of theYpacarai depression.Only the easternmost knoll of the cluster (the one onwhich the quarry was opened), was examined by Eckel,but, from nearby ground and aerial observation theother four appear to be similar in shape and geologicmakeup.The quarry, part of which is shown in figure 31, isnearly circular, with an irregular floor. It is about 100meters in diameter and 10 meters deep along its western,or highest, edge.The dome-shaped hill consists of black to brownshale, siltstone, and sandstone all intruded and brec-FIGUEE 30. Rhyolite agglomerate (specimen P-58b) from the Acahay caldera has a microprotoclastic texture with quartz, q, filling microscopic fractures.The dark patch, x, to the left is a xenolith. X 20.
FJGUEE 31. Panorama of highway quarry at Ypacaral. The rocks are largely rhyolite agglomerate, with muua ^ oreccia made up of altered sedimentary rocks in igneous matrix.
Page 1 and 2: Geology and MineralResources of Par
Page 3 and 4: UNITED STATES DEPARTMENT OF THE INT
Page 5 and 6: IVCONTENTSMineral resources._______
Page 8 and 9: GEOLOGY AND MINERAL RESOURCES OF PA
Page 10 and 11: INTRODUCTION(1947, 1952) are the on
Page 12 and 13: GEOGRAPHYFIGURE 1. Index map showin
Page 14 and 15: GEOGRAPHYbisects the Republic of Pa
Page 16 and 17: GEOGRAPHY 9thorny semidesert plants
Page 18 and 19: GEOLOGY 11chiefly logs, rough lumbe
Page 20 and 21: IGNEOUS AND METAMORPHIC ROCKS 13and
Page 22 and 23: 15FIGURE 7. Arkosic sandstone (spec
Page 24 and 25: IGNEOUS AND METAMORPHIC ROCKS 17FIG
Page 26 and 27: IGNEOUS AND METAMOKPHIC ROCKS 19acc
Page 30 and 31: IGNEOUS AND METAMOEPHIC ROCKS 23TAB
Page 32 and 33: 25FIGURE 18. Ehyolite (specimen P-3
Page 36 and 37: IGNEOUS AND METAMORPHIC ROCKS 29PET
Page 38 and 39: IGNEOUS AND METAMORPHIC ROCKS 31DIA
Page 40 and 41: IGNEOUS AND METAMOEPHIC ROCKS 33a d
Page 42 and 43: GEOLOGICAL SURVEY PROFESSIONAL PAPE
Page 44 and 45: 36 GEOLOGY AND MINERAL RESOURCES OF
Page 50 and 51: 42dated by two kinds of porphyry. O
Page 52 and 53: 44FJGTJEE 34. Porphyry (specimen P-
Page 68 and 69: 60lower third of the series consist
Page 74 and 75: 66possible Cretaceous age. Baker (1
Page 82 and 83: 174 GEOLOGY AND MINERAL RESOURCES O
Page 95 and 96: MINERAL RESOURCES 87a pivot, show s
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Geology and Mineral Resources of Paraguay A Reconnaissance

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