Geology and Mineral Resources of Paraguay A Reconnaissance

14 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYthrough mica schist, gneiss, and ultrabasic rocks.Harrington (1950) says that in the Sierra de Itayuru, alittle north of San Juan Bautista, this same series ofgneiss and schist contains intercalations of marble.At kilometer 176,' about 3 kilometers north of SanMiguel, there are several bands, 30 to 50 meters apartand each 2 to 3 meters wide, that are largely altered totalc. The original rock, as exposed in the road ditchesand in a shallow prospect pit (see fig. 58), was apparentlyan ultrabasic rock. The talc, which containsa few specks of chromite locally, ranges from whitethrough dull green to gray and from very smooth togritty texture. It contains a few narrow and irregularveinlets of white quartz and a cross-fiber mineral thathas the appearance of asbestos but which is shown byX-ray-diffraction methods to be fibrous talc.In addition to the talc, the belt of metamorphic rocksis known to contain deposits of magnetite and hematite(p. 85), some of them rich near San Miguel.On a hill 1 kilometer east of the main road, andabout 5 kilometers north of San Miguel there is a comparativelylarge body of white quartz in country rock1 As used in this chapter, kilometer numbers refer to markers along the main highways(Rutas). They measure distances from Asuncion.that has a rhyolitic appearance. The quartz outcropsover a roughly circular area about 50 meters in diameter.It is milk white and glassy, with a few faceted crystals,2 to 3 centimeters in diameter, in vugs. One smallbleb of coarse-grained galena was found on the dumpof a shallow prospect pit; a few specks of free gold arereported by former prospectors. This deposit ofquartz, obviously of hydrothermal origin, containing alittle galena and possibly some gold, lends some basisto reports that the early Jesuits found gold and mercuryore near San Miguel.In a strong shear zone near Villa Florida is whatappears to be a sheared granite (specimen P-70).(Throughout this chapter, field numbers assigned tospecimens collected by Eckel are retained so as toavoid confusion and repetition.) It is a thinly bandedrock, with layers of dark-gray glassy quartz andgreenish-brown epidote with feldspar. Microscopically,it shows sheared aggregates of highly alteredmicrocline, diopsidic pyroxene, and zoisite (garnet?)interlayered with quartz. The photomicrograph (fig. 6)shows the elongate lenses of feldspar in the coarselycrystallized quartz. The diopsidic pyroxene is associatedwith these feldspar pods. A more siliceous phaseFIGURE 6. Sheared Precambrian granite (specimen P-70), from shear zone near Villa Florida, g, quartz; /, feldspar (microcline) with mafic minerals. X 15.Crossed nicols.