Geodiversity in the Natural Park “Porţile de Fier”: cave mineralogy and mineral deposits

The Iron Gates (“Porţile de Fier”) Natural Park is located in South-Western Romania and extends along the Danube Gorges and the affluent valleys. The Park is one of the biggest in Romania, having a surface of 115665.8 hectars and including 18 Natural Reserves. The geodiversity of the Iron Gates Natural Park is given by the distribution of a large variety of magmatic, metamorphic and sedimentary rocks, and particularly of limestones of Jurassic and Cretaceous age, affected by a large number of karst phenomena: caves, swallow-holes, gorges, dolina, lapies, uvalas.
   The most representative caves in the Park are those from Gura Ponicovei, Padina Matei and Gaura cu Muscă. All of them contain important deposits of fossil bat guano, with a large diversity of phosphate species, including apatite-(CaOH), taranakite, ardealite, brushite, monetite, francoanellite and leucophosphite. These mineral species generally occurs as crusts of yellow cream or reddish brown color deposed on the cave floor or on some speleothems, or, rarely, as earthy masses of white or white cream color. They were identified using a combination of X-ray powder diffraction, Fourier-transform infrared absorption and electron microscopy.
   In the upper basin of Mraconia Valley, a system of galleries oppened a tungsten-bearing skarn deposit, which develops at the very contact between crystalline limestones and a porphyric granodiorite of Mesozoic age. The skarn is mainly andraditic, but also contains plagioclase, potassic feldspar, feroactinolite, magnetite, epidote, apatite, vesuvianite and wollastonite. Four stages of mineralization overprint the primary skarn: (1) a high temperature stage conduced to the deposition of scheelite in the mass of skarn; a parallel deposition of quartz and molybdenite on cracks affecting the granodiorite mass is likely; (2) a second hydrothermal stage conduced to deposition of pyrite, chalcopyrite and calcite on the cracks and of impregnations of pyrite and chalcopyrite in the skarn mass; (3) a third hydrothermal stage conduced to the massive deposits of chalcopyrite, pyrite, sphalerite, galena, scarce pyrrhotite and tertahedrite, as veins and lenses in the skarn mass; (4) a low temperature hydrothermal stage yields the formation of bornite and covellite on chalcopyrite but also of hematite (specularite) on magnetite.