New type of epithermal manganese mineralization from the Banská Hodruša precious and base metal deposit at the Rozália mine, Hodruša- Hámre, Slovakia

Abstract

The newly discovered unusual epithermal manganese mineralization was found at the 18 th level of the Rozália mine in the Štiavnica stratovolcano and it is hosted by a NE-SW oriented fault dipping 55° to SE. Based on its structural setting and cross-cut relationship to a pre-caldera vein it belongs to a large system of post-caldera veins related to horst uplift in the central zone of the stratovolcano. The vein occurs in andesite and consists of two contrasting parts, including a part dominated by white coarse-grained calcite and Mn-rich part represented by abundant johannsenite, younger rhodonite and Mn-carbonates (rhodochrosite, Mn-calcite), in association with minor quartz, calcite, adularia, chlorite, and clinozoisite. Ore minerals are represented by sphalerite, galena, pyrite, chalcopyrite and rare inclusions of acanthite and can be found exclusively in the Mn-rich part of the vein. Johannsenite forms greenish-blue, greenish-brown or brown radial aggregates reaching up to 7 cm in size. Its average empirical formula is Ca 0.96 (Mn 0.97 Mg 0.07 Fe 0.03 ) 1.07 (Si 1.98 Al 0.02 ) 2.00 O 6 . Rhodonite shows a significant compositional variation of Ca and Mn contents, ranging from Ca 0.48 Mn 4.56 (Si 5 O 15 ) to Ca 1.58 Mn 3.41 (Si 5 O 15 ) as well as minor amounts of Fe (up to 0.32 apfu), Mg (up to 0.14 apfu) and Al (0.06 apfu). Microthermometry of fluid inclusions hosted by johannsenite, rhodonite, sphalerite, calcite and Mn-calcite determined that the mineralization precipitated from fluids of low salinity (1.4 to 5.3 wt. % NaCl eq.) and moderate homogenization temperatures (187-318 °C), with a trend of simultaneous decrease in salinity and temperature, interpreted as mixing of fluids. Boiling of fluids was recorded in early calcite at temperatures from 275 to 279 °C, corresponding to pressure from 58 to 62 bars and paleodepth from 704 to 752 m. These values are similar to data from other post-caldera veins in the vicinity of the mine, as well as from other johannsenite-bearing epithermal deposits in the world.