Geology and Mineral Resources of Paraguay A Reconnaissance

88 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYseveral kilometers. It has been explored by one pit 6meters long, 4.5 meters wide, and 3.6 meters deep.This excavation exposes veinlets of pyrolusite 2 centimetersor more in thickness; locally there are masses 6to 10 centimeters thick, but even the best specimenscontain considerable amounts of silica. Results ofchemical analysis of one specimen, doubtless carefullyselected, were seen, which showed the material to contain85.65 percent MnO2 (equivalent to 54.90 percentMn), 6.94 percent BaSO4 and 0.23 percent SnO2 . Thepresence of tin in this material is surprising. There isno doubt that some manganese ore of commercial gradeexists at Emboscada. However, the available evidencemakes it seem very unlikely that any large quantitiesof ore will be found there.YAGUARON DEPOSITThe deposit, which is not believed to be of commercialsize or grade, is in a prominent but relatively smallhill just southwest of the town of Yaguaron (fig. 54).The steep slopes of the hill expose a total thickness ofabout 50 meters of brown, thick-bedded and crossbedded,very fine grained arkose. This rock, shown onplate 1 as part of the Triassic Misiones sandstone issomewhat soft in places but elsewhere it is cemented bysilica to a hard quartzite.The manganiferous material forms an irregular pipelikezone a few meters in diameter that extends almostvertically to near the top of the hill. Within this zonemany joints are filled with narrow irregular veinlets ofblack oxides of manganese. The sandstone itself is alsoimpregnated with small spherical bodies of similaroxides. There is no visible major fracture or fault; themanganese mineralization seems to have concentratedin the zone of joints."Mining" consists of the breaking off of pieces of outcroppingrock and collection of loose fragmental mate­rial from the surface. The material so gathered iscobbed to pieces 4 to 10 centimeters in diameter andsorted by hand on the basis of color. None of the sortedmaterial appeared to contain more than 10 percentMnO2, and it is very doubtful that any quantity ofbetter grade material could be produced.MERCURYDe Mersay (1860) reported that mercury ore wasfound in 1779 at San Miguel. He stated further thatsamples that were sent to Spain for analysis gave unsatisfactoryresults and that no further development orexploration was attempted.It is problematical whether the reported material consistedof the red sulfide mineral, cinnabar, or was someFIGUKE 54. Coarse-grained, coarse-bedded, manganiferous arkose, believed to be part of the Misiones sandstone just west of Yaguaron. The beds dip 1° W.;the eastward slope of the hill is controlled by crossbedding. Just left of the central palm tree, joints and interstices in the sandstone are impregnatedwith manganese. Mandioca, a staple starch crop of Paraguay, in the foreground.