The Phosphate Mineral Assemblages from La Viquita Pegmatite, San Luis, Argentina

La Viquita is a rare-element pegmatite of LCT signature, REL-Li subclass, spodumene subtype, that shows Fe > Mn mineral paragenesis instead of Mn > Fe, which is more common in the rare-element pegmatite population of the San Luis ranges. The phosphate mineral association of this pegmatite can be subdivided into (1) primary, with dendritic triphylite [(Fe/(Fe + Mn) = 0.72] and montebrasite–amblygonite as main phases; (2) metasomatic, with subsolidus replacement of triphylite by ferrisicklerite and heterosite; and (3) hydrothermal, with secondary growth of alluaudite at the expense of heterosite and wardite from montebrasite caused by Na-metasomatism. A Ca-rich influx under oxidizing conditions produced childrenite–eosphorite–ernstite, jahnsite-(CaMnFe), and kingsmountite. Apatite-group minerals are present throughout the processes. Very late-stage solutions formed millimetric crystals of hydroxylherderite associated with hydroxylapatite in cavities in K-feldspar.

Low-Temperature-Low-Pressure Mineral Paragenesis of the Lower-Middle Jurassic Volcanics in the Eastern Pontides, NE Turkey

<p>The Lower-Middle Jurassic volcanic rocks in the eastern Pontides were formed in a subduction zone under the extensional tectonic regime. These volcanic rocks were experienced seawater alteration during forming. They also were exposed to the burial metamorphism under the Cretaceous and Eocene aged formations. In addition, the Cretaceous and Eocene granitoids cut these volcanic rocks in some places and metamorphosed them. In this study, the mineralogical changes of the volcanic rocks they have experienced since their formation were examined.</p><p>Plagioclase (An<sub>>42</sub>) + augite/diopside (En<sub>38-52</sub>Wo<sub>25-46 </sub>Fs<sub>7-25</sub>) + Fe-Ti oxides (Fe<sup>+3</sup>/(Fe<sup>+3</sup>+Fe<sup>+2</sup>) > 0.80) ± magnesiohornblende (Mg/(Mg+Fe<sup>+2</sup>) > 0.92) are the main rock-forming minerals in these volcanic rocks. Mineralogical traces of seawater alteration are mostly masked by subsequent geological events. However, Na-enrichment of the plagioclases, increased <sup>87</sup>/<sup>86</sup>Sr<sub>(i)</sub> isotope ratios (0.70462 to 0.70611) and some clay minerals, laumontit,  analsime minerals, which are observed in the XRD peaks of some samples, refer to the alteration of the seawater. The pumpelyite (Fe<sup>+2</sup>/Fe<sup>+2</sup>+Mg = 0.60-0.90), chlorite (Fe<sub>total</sub>/Fe<sup>+2</sup> + Mg = 0.15-0.95), sphene, calcite, dolomite and secondary quartz minerals were formed during burial metamorphism. The Fe-Ti oxides reached the chemical re-equilibrium under the new P-T conditions (magnetite Fe<sup>+3</sup>/(Fe<sup>+3</sup>+Fe<sup>+2</sup>) = 0.40-0.62; ilmenite Fe<sup>+3</sup>/(Fe<sup>+3</sup>+Fe<sup>+2</sup>) = 0.01-0.20). Epidote (Fe<sup>+2</sup>/(Fe<sup>+2</sup>+Mg) = 0.75-0.95) accompany the mineral paragenesis in some areas affected by Upper Cretaceous and Eocene granitoids.</p><p>Temperature estimations using the chlorite geothermometer and the phase relationships on the P-T diagrams show that the volcanics were heated up above 200°C in the buried areas where the granitoids were not effective. The temperatures were above 250°C in the areas where the magmatic rocks were effective. Taking into consideration the thickness of the formations that overlie the Jurassic volcanics, it can be suggested that the pressure affecting the Jurassic volcanics reached up to 1.5 kilobars.</p><p>Acknowledgement</p><p>This work was financially supported by Scientific and Research Projects Unit of Karadeniz Technical University with grant # 8920.</p>

Evolution of mineral formation processes in Lower Vend terrigenous rocks of Nepsko-Botuobinskaya anteclise

Principal common factors of epigenetic occurrence in Lower Vend rocks of Nepsko-Botuobinskaya anteclise are shown in the article. Evolution of mineral formation processes is stated in the change of mineral paragenesis, which were formed on stages of regional background (stadial) lithogenesis of foundering — diagenesis, early and late katagenesis; and also as a result of local superimposed lithogeneous types complex combination – cataclastic, hydrothermal, metasomatic and dynamothermal activation. Local superimposed processes could acompony the period of Permo-Triassic trap magmatism. High degree transformation of Lower Vend terrigenous rocks of Nepsko-Botuobinskaya anteclise determinates its structure, composition and physical properties specificity, which should be taken into account in process of geological exploration of different mineral deposits.