Geology and Mineral Resources of Paraguay A Reconnaissance

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46 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYFIGURE 36. Shonkinite (specimen P-54a) from Mbocayaty. Note the large rounded hydronephelite, h, crystals at left. At the center right are "graphic"remnants of nepheline or hydronepheline, g, that have been partly replaced by sanidine. Light-green monoelinic pyroxene, p, showing good cleavage,and dark-brown biotite, b, are abundant; olivine, o, in the lower right edge, is enclosed in biotite. Pale-bluish apatite and black iron oxide are accessoryminerals. X 15.strongly pleochroic brown biotite grains, sphene, and apatiteare also present.P 54c, shonkinite. Specimen P 54c is similar to specimenP-54a, described above. Numerous olivine crystals are seenin figure 38 (just upper left of center, just left of middle loweredge, and elsewhere). Large pyroxene crystals, showing goodcleavage, are also abundant. In the middle of the left half ofthe field is a large crystal of hydronephelite. The white areasare sanidinic feldspar.P-54d and P-S4e, shonkinite. Specimens P-54d and P-54eare similar. Both are rather coarse-grained medium-dark rockswith prominent tabular feldspars. They are variants of specimensP-54a and P-54c and differ essentially in having no olivine,and very little titaniferous amphibole. Titaniferous amphiboleshows normal absorption in deep brown parallel to c, unlike thekataphorite amphibole found in the other rocks from thislocality. Specimen P-54d has been analyzed (table 5).Figure 39 (specimen P-54d) shows the abundant tabular(laths in cross section) feldspar, and titaniferous augite. Undercrossed nicols, many of the clear feldspars show turbid cores,indicating perhaps replacement of an earlier generation offeldspar.Specimen P-54e contains some analcite, with clear feldsparapparently replacing nepheline. Many of the feldspar crystalshave turbid inner zones.P-64f and P-S4g, shonkinite. Specimens P-54f and P-54gare similar, and differ from the others somewhat. No definitelath-shaped feldspars are seen; instead there are abundant roundedwhitish areas, as much as 0.5 centimeter across, which arereplacements of nepheline. Figure 40 shows specimen P-54f, inwhich two such replacements (with more or less hexagonal outline)are seen in the upper half of the field. The smaller and clearerone is isotropic, and may be analcite; the other appears to behydrouephelite. In the center of the field, to the right of alarge augite, is more of the hydronephelite aggregate enclosingclear feldspar. There is an abundance of mafic minerals,including olivine, in this rock. Hornblende, though, is absent.Specimen P 54g, shown in figure 41, contains abundantwell-formed crystals of colorless fresh olivine, augite colorlessinside but usually with a thick outer green zone (aegirineaugite)fresh red-brown biotite, and some kataphorite amphibole. This mineral shows a strong and unusual pleochroism:X, pale yellow, Y, wine-red brown, Z, yellow; a small opticangle; strong dispersion, v>p; and the amphibole type ofcleavage, against which an extinction of about 15° can bemeasured.This amphibole resembles a kataphoritic amphibole namedmagnophorite by R. T. Prider (1939) from a leucite-bearing
IGNEOUS AND METAMOEPHIC ROCKS 47, ',* , * - *^ S . Jf , ? ^..1^-filFIGTJEE 37. Nepheline syenite (specimen P-54b) from Mbocayaty. Coarsely crystallized orthoclase, /, with hydronephelite, ft, after nepheline, which isslightly darker, and sphene, s. The darker small areas are green pyroxene, a, and black ore grains, o, of magnetite-ilmenite. Much of the hydronepheliteretains the characteristic stubby hexagonal prism shape of the original nepheline; however, it is uniaxial positive with appreciably higher birefringencethan nepheline. X 15.TABLE 5.Chemical analyses of alkalic rocks[Analyses of A, B, and E by Trumbull with densities by L. N. Tarrant; analysis ofC, by Goldschlag; D, by Lindner]SiO 2 - - -Al2OsFeO.. . ......MgO ... ...Cao...-_ ______Na2O -. ... _.K20 .__....H.O- -H20+.. .... _.TiOs .....-.-.CO.. .........PsOs. -SOs.S....MnO ...BaO. ___ ____Less O . ..Norm...NOTE.AP-54d53.2916.104.502.982.323.783.459.42.201.231.11.02.71.50.03.13.1999.96.0299.942.68II-5-1-2BP-54g50.4111. 173.505.059.237.732.317.10.12.761.61.00.41.12.03.14.2099 89!0299.872.87III-6-1-2C48.5511. 975.734.414. 7710.064.864.30} 2.623.04Tr.100. 31III-6-1-3See explanation of specimen designations below.52.2020. 673.261.38.484.436.614.90f .36\ 3.56.141.54.12.04D. 09(Mn2O 3).0999. 872.463I-6-2-3EP-58a51. 2715. 728.611.533.725.983.714.15.871.821.64.01.43.17.1899.812.731I-5-2-3SpecimenNo.A (P-54d)_._T! CP-'Hfr}D ____ .E (P-58a) ........doRockLocalityDo.2.3 km west-southwest of Centurion.Sapucai, Ibytymi district.rock from Western Australia. Prider gives an analysis. Asecond paper by Wade and Prider (1941) gives a further description,with illustration of a peculiar "herringbone" structurewhich is also present in the Mbocayaty mineral. Figure 42shows this amphibole, with "herringbone" structure.The amphibole also resembles an amphibole from Wyoming,described by Cross (1897), which Cross states is almost uniaxial,and with a color scheme X, a pale yellow, b, red similar tohypersthene, Z, a bright yellow.ANALYSES OF AIKAIIC ROCKSFive chemical analyses of the alkalic rocks of Paraguayare available, three new ones (of nepheline syenite
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Geology and MineralResources of Par
Page 3 and 4: UNITED STATES DEPARTMENT OF THE INT
Page 5 and 6: IVCONTENTSMineral resources._______
Page 8 and 9: GEOLOGY AND MINERAL RESOURCES OF PA
Page 10 and 11: INTRODUCTION(1947, 1952) are the on
Page 12 and 13: GEOGRAPHYFIGURE 1. Index map showin
Page 14 and 15: GEOGRAPHYbisects the Republic of Pa
Page 16 and 17: GEOGRAPHY 9thorny semidesert plants
Page 18 and 19: GEOLOGY 11chiefly logs, rough lumbe
Page 20 and 21: IGNEOUS AND METAMORPHIC ROCKS 13and
Page 22 and 23: 15FIGURE 7. Arkosic sandstone (spec
Page 24 and 25: IGNEOUS AND METAMORPHIC ROCKS 17FIG
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Page 30 and 31: IGNEOUS AND METAMOEPHIC ROCKS 23TAB
Page 32 and 33: 25FIGURE 18. Ehyolite (specimen P-3
Page 36 and 37: IGNEOUS AND METAMORPHIC ROCKS 29PET
Page 38 and 39: IGNEOUS AND METAMORPHIC ROCKS 31DIA
Page 40 and 41: IGNEOUS AND METAMOEPHIC ROCKS 33a d
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Page 50 and 51: 42dated by two kinds of porphyry. O
Page 52 and 53: 44FJGTJEE 34. Porphyry (specimen P-
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Page 74 and 75: 66possible Cretaceous age. Baker (1
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Geology and Mineral Resources of Paraguay A Reconnaissance

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