Geology and Mineral Resources of Paraguay A Reconnaissance

IGNEOUS AND METAMORPHIC ROCKS 17FIGURE 8. Coarse-grained granite (specimen P-62) from east of Barrerito on the road to Quyquyo; partly chloritized biotite, 6, with feldspar, / (orthoclase andoligoclase), partly sericitized, and quartz, g, and a few ore grains, o. X 20.groundmass and phenocrysts, so that some exposurestend to resemble granite or aplite.Good examples of the freshest rock, that wouldserve as beautiful polished building stone, can be seenon the Caapucu road at kilometer 134.On the road from Quiindy, north of Caapucu, porphyriticrocks are well exposed (figs. 11, 12). In placeswhere the clay formed by weathering processes hasbeen removed, the rock surfaces are studded with resistantquartz phenocrysts, as shown in figures 12 and 13.The microstructure of this rock (specimen P-67) isshown in figure 14. The rock is reddish brown, becauseof the abundance of reddish plagioclase phenocrysts.Where the reddish phenocrysts are not abundant, theporphyry is gray; quartz phenocrysts are few and whitefeldspars are present.The gray phase of the porphyry appears to be lessabundant than the reddish brown. Figure 15 shows atypical specimen of the gray phase taken from anexposure close to the rock shown in figure 14. Thethin section shows no quartz phenocrysts at all, butother specimens of outwardly similar rock contain afew euhedral crystals of quartz. The texture of thefelsic groundmass is obviously quite different from thatof the rock shown in figure 14. A chemical analysis ofthis rock (specimen P-60) is given in table 1.About 12 kilometers northwest of Caapucu, and somedistance west of ttie main highway, there is a largedeposit of pyrophyllite. The country rocks in thisvicinity are normal brown quartz porphyry, but thepyrophyllite (see p. 90) appears to be an alterationproduct, of a volcanic tuff that is probably interlayeredwith the porphyry.Associated with the quartz porphyries just described,notably just north of Caapucu, are many other varietiesof porphyry that, in the field, appear to range fromglassy rhyolite to dense gray quartz-free monzonite.Microscopic examination shows, however, that theyare probably textural variants of the quartz porphyriesalready described.One of these rocks (specimen P-31) forms the wallrock of the Del Puerto pyritic hematite deposit northof Caapucu. It is a brick-red fine-grained rock, withglassy quartz, red feldspar, and dark mafic phenocrysts.Pyrite is also disseminated through the rock, visiblewith a hand lens. Its microscopic character is shownin figure 16. At the lower left is a large quartz crystal,embayed by potassic feldspar, which, with quartz,