Geology and Mineral Resources of Paraguay A Reconnaissance

MINERAL RESOURCES 83than 1,000 meters (3,000 feet) of continental red bedswere found. The La Paz well, only 70 kilometerseast of the Santa Rosa well, showed that the Devonianshale had thinned from more than 1,200 meters toonly 120 meters in that distance, but that it wasunderlain by a thick series of Paleozoic beds, reportedlyvery different in appearance from, any of those in theSanta Rosa well. Conversely, the 3,149-meter Pirizalwell, 130 kilometers south of the La Paz well failed toreach the marine sediments, demonstrating an enormousthickness of continental red beds there. In theOrihuela well, 210 kilometers east of the Pirizal, nearly2,000 meters of red beds were penetrated and the drillpassed directly into an old quartzite, probably Cambrianand Ordovician in age, thus proving the absenceof Silurian and Devonian marine sediments in this area.Evidence indicates a deep basin in the marinesediments, filled with continental red beds of Mesozoicand Cenozoic age that centers somewhere near thesouthwestern border of Paraguay, not far from, thePirizal well. Whether this depression is due to foldingor to erosion and removal of the older marine sedimentsis unknown to the author; nor is the structure withinthe older beds known. It is clear, however, that theseolder beds rise to or above the surface eastward aswell as toward a large domelike structure that has anapex near the extrem.e northwest part of the republic.Despite the fact that only one well of the Union OilCompany the Picuiba found positive indications ofthe presence of oil and gas, the possibilities of eventualdiscovery of commercial petroleum or gas have not beenexhausted. Very thick carbonaceous marine shalebeds, possible sources for petroleum, are known in thearea. An extensive structural basin filled with alternatinglayers of shale and porous sandstone, which couldunder favorable circumstances serve admirably as rereservoirrocks for accumulations of petroleum, is alsoknown to exist in the area. Moreover, oil is producedin Bolivia in the anticlines on the western flank of thisbasin. The existence or location of minor folds, ancientshore lines, or other possible stratigraphic orstructural traps within the basin is of course quiteunknown, but it seems reasonable to suppose that suchfeatures may exist and may have been missed by theinadequate exploration that has taken place to date.Only further exploration can discover the unknownstructural features. Such exploration would necessarilybe both extensive and expensive; it would alsorequire larger amounts of equipment and technicalskills than were available in Paraguay when this reportwas written. The discovery of petroleum resourceswould mean so much to the national economy, however,that serious consideration should be given to developingthe ways and means of making further search for them.GEM STONESThere is a possibility, though only a remote one, thatsmall quantities of precious or semiprecious gem stonesmay someday be found in Paraguay. The reportedberyl in some of the pegmatites in the Rio Apa regionnaturally indicates the possibility that, as in Brazil,some may be in the form of emerald or of aquamarine,though admittedly no reports of such occurrences havecome to the author's attention.Similarly, comparison of the general geologic relationsin eastern Paraguay with some of the diamond-bearingareas farther north in Brazil, suggests that the basalconglomerate beds of the Lower Silurian Caacupe series,which locally mark the contact between these beds andthe Precambrian granitic rocks, might yield streamworndiamonds to thorough search. Such search, ifattempted, should be done in the full realization thatthe chances of finding diamonds are extremely slim,particularly since there is little or no knowledge as tothe sources of the Silurian conglomerate.Many of the beautiful amethysts and agates thatare produced in Brazil and Uruguay are derived fromcavity fillings in the Serra Geral basalts that are apparentlyidentical with those that cover a large part ofsoutheastern Paraguay. Some beautifully coloredagates and a few pale but otherwise gem-qualityamethysts have indeed been collected from the Para­guayan rocks from time to time. Active search amongthe basalt outcrops themselves or among the pebblesalong the Alto Parana and other streams that drain thebasalt area, would doubtless serve to discover additionalmaterial, though it is impossible to say in what quantityor quality.Some of the younger volcanic rocks, such as those atCerro Tacumbii in Asuncion, contain crystals of cleargreen olivine, or "evening emerald" as much as 1 centi­meter and more in diameter. It is probable that most,if not all, such crystals are so cracked or otherwiseflawed as to be worthless for gem material, but it wouldseem worthwhile to check this conclusion by furtherexamination of the rocks and of soils derived from them.GLASS SANDThe local glass industry depends almost entirely onalluvial deposits of fine sand that accumulate as barsin some of the streams. These form an inexhaustiblesupply of fairly good material, though none of thoseseen are clean enough to yield high quality glass. Thebest apparent source of cleaner material known to theauthor is the white saccharoidal sandstone member ofthe Caacupe series. This sandstone, which could bevery easily crushed, appears to be made up almostentirely of quartz grains in most places except for sma1 !