Geology and Mineral Resources of Paraguay A Reconnaissance

80 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYclay zone may represent weathering along an unconformitybetween argillite and limestone, particularlysince the overlying beds differ in attitude from thosebelow. It is equally possible, however, that the discolorationand softening are due to weak hydrothermalalteration.A series of limestones, some dolomitic, overlie theargillite. The entire series is about 50 meters thick,but only the lower 15 meters are exposed in the quarryface. The series has been sampled at the surface andis reported to be comparable in composition to therocks exposed in the quarry. No drilling or samplingof the beds in depth has been attempted. The bedsstrike northeastward and dip southeast, or into the hill,at a rather regular angle of 15°. They are 30 centi­meters to 1.30 meters thick. There are several continuousbeds of dolomitic limestone that are characteristicallyflesh pink to white in color and streakedor mottled. They are called "bacon rock" locally.The remainder of the series is made up of alternationsof dark-blue-gray crystalline limestone and greenish-gray limestone. The blue-gray rock, which is cut byveinlets of white calcite, is reported to contain anaverage of 95 percent CaCO3 . The greenish-gray rock,which has an average content of 80 to 85 percentCaCO3 , is made up of white to dark-gray crystallinelimestone with many irregular slickensided streaksand nodules of dark-dull-green chloritic material.Much of this rock could be mistaken easily for ser­pentine marble. All the rocks in the series are partlyto entirely recrystallized and many of them wouldform excellent ornamental marble.A series of strong, nearly vertical joints that trendnortheastward, 1 to 3 meters apart, cut all the beds.Some appear to be minor faults, with the beds droppedfrom a few centimeters to as much as 2 meters on thesoutheast sides. Many of the joints have been openedby solution, especially near the surface. The resultantopenings, 4 to 75 centimeters wide, are filled with coarsegrainedgranular calcium carbonate (caliche) thatcements a breccia of rock fragments. The caliche,which also coats the surface in places, is too high inmagnesia and silica to be used for cement; it, and thedolomite in the quarry, must be removed by handor avoided by selective quarrying.The rocks are cut by many irregular veinlets of whitecalcite, with an opening here and there that is linedwith crystals of calcite, dolomite, or mixed carbonate.One small grain of chalcopyrite was seen in one of theseveinlets and one irregular mass of white quartz, about12 centimeters in diameter, has been found. Thesefacts indicate rather clearly that the rocks have beensubjected to mild hydrothermal alteration.On the assumptions that the present practice ofsorting out the dolomite by hand continues to besuccessful, and that the higher beds, which will bereached soon in quarry operations, are no more dolomiticin depth than they are at the surface, the Vallemioperation seems to be assured of adequate raw materialsfar into the future. Even should these assumptionsprove incorrect it is certain that sufficient limestoneof usable grade can be found somewhere within the beltof outcrop of the Cambrian and Ordovician rocks.CLAYClay is one of the most widespread and abundantmineral resources of Paraguay, and the brick and tileindustry is by far the most important mineral industryat the present time. In addition to the common clays,there are many deposits of clay of pottery grade;they form the basis of a moderate-sized industry thatcould be much expanded.There are a few relatively modern plants that employsome machine methods. Good examples are the VargasPefia plant near the town of Ypacarai, which produceshollow structural tile, sanitary porcelain, sewer pipe,and other ware and the large plant at Aregua, whichproduces a large variety of cheap pottery. A greatmajority of the scores of brick and tile plants throughoutthe heavily populated regions, however, are tinyfamily affairs, producing only a few thousand brick peryear. In these, the clay is dug by hand from shallowpits, and moved by wheelbarrow to a central pointwhere it is thoroughly trodden by oxen attached to along sweep. For some purposes, straw or grass isadded to the clay at this stage. The clay is thenfurther kneaded by hand in small quantities, and packedtightly into crude wooden or metal molds. After therough tops are cut off by means of a wire the bricksare carried directly to an open grass-thatched dryingshed; thin slabs for tiles are first bent over a woodenform to give them the proper shape. After severalweeks of drying in the air the products are burned ina small wood-fired kiln. The final products vary considerablyin quality, but most are at least fair to good,and have a pleasing buff to light red color. Consideringthe amount of hand labor that must be expended oneach individual brick it is not surprising that bricksare roughly twice as large as the standard size usedin the United States and in Europe.REFRACTORY CLAYGood refractory clay, suitable for firebrick and otherhigh temperature uses, seems to be relatively scarce,but this may be due more to lack of adequate searchand testing than to actual scarcity. Some of theknown clay deposits have at least moderately goodrefractory properties. For example, the local clay nearthe old Quyquy6 iron furnace is reported to have yielded