Geology and Mineral Resources of Paraguay A Reconnaissance

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.