Geology and Mineral Resources of Paraguay A Reconnaissance

22 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYFIGURE 15. Precambrian porphyry (specimen P-60), gray phase, associated with quartz porphyry of figure 14. Megascopically a gray stony rock with conspicuousgrayish-white feldspar phenocrysts, the section shows sodic plagioclase phenocrysts, /, and a few small chloritized and epidotized mafic mineralsin a felsic groundmass. Other specimens of similar appearance contain considerable quantities of quartz as phenocrysts. X 20.Chaco, 20-30 kilometers northwest of the river, and areonly 30 to 40 meters high. Quarried rock from thesehills that was seen by Eckel at Puerto Guarani resembledthe Precambrian quartz porphyry more closelythan the younger alkaline volcanic rocks and it is somapped on plate 1.FUERTE OL1MPOAccording to DuGraty (1865), grayish-brown denseporphyry, with grains of quartz and plates of oligoclase,makes up the small hill on which Fuerte Olimpo issituated on the upper Rio Paraguay. Several laterwriters have quoted him, but no further descriptionsare known to the author. The body is shown in figure 2as of younger Precambrian age. This assignment issupported only by the fact that most writers on Paraguaytend to characterize the Triassic or Tertiaryigneous rocks as something other than porphyry,whereas this is the term used by most of them in describingthe Precambrian quartz porphyries of the RioApa and Caapucu regions. The porphyry is reportedto contain barite deposits.ANALYSES OF PRECAMBRIAN IGNEOUS ROCKSTable 1 shows the results of chemical analysis of sixof the Precambrian rocks collected by Eckel from centraland southern Paraguay. To these are added threeof Goldschlag's anah'ses of rocks from the RJ'O Aparegion.An analysis of a specimen of rhyolite agglomerate(P-58b), collected by Eckel near the much youngercaldera at Acahay, is also included, though discussionof its geologic setting and meaning is reserved for thesection on alkalic rocks.Also, because of the close similarity of the chemicalanalysis and other features, the analysis of a rhyoliteporphyry from the highway quarry near Ypacarai isgiven here. However, a Precambrian age for this rockis doubtful; the matter is discussed further on pages42-43.Spectographic analyses of Eckel's analyzed rocks, aswell as of two pebbles of quartz porphyry collectedby him from the Tubarao tillite, of Pennsylvanian orPermian age, are shown in table 2.