Ore mineralogy of the Rodu-Frasin Au-Ag deposit, Metaliferi Mountains, Romania

The Rodu-Frasin Neogene volcanic structure and associated Au-Ag mineralization are located in the north-eastern part of the Metaliferi Mountains, being part of the Baia de Aries - Rosia - Bucium metallogenetic district of the "Golden Quadrilateral".

The Rodu-Frasin region's geology consists mainly of Frasin dacite dyke and dome, polymictic volcanic and phreatomagmatic breccias and related volcano-sedimentary deposits, Badenian volcanic and sedimentary rocks, and Cretaceous sedimentary rocks.

Hydrothermal alteration in the region is pervasive and widespread throughout the volcanic structure and surrounding Cretaceous formation. Five main types of hydrothermal alterations have been described: potassic, propylitic, phyllic, silicic and carbonate. Argillic alteration is present only locally.

In the area of Rodu-Frasin deposit, the ore occurs in a structurally complex environment, influenced by faults and fractures oriented in two or more directions. Au-Ag-base metals mineralization is genetically associated with hydrothermal breccias and phreatomagmatic fractures.

Ore minerals consist of sulfides, gold, carbonates, adularia and quartz. They have been prevalently emplaced as veins, breccia bodies and disseminations in open fractures and breccias in the Rodu diatreme, and as stockworks, veins and disseminations in relationship to the Frasin dome structure.

The mineralized veins contain carbonates, quartz, pyrite, sphalerite, galena, chalcopyrite, tetrahedrite and gold. Magnetite and hematite, probably formed under mesothermal conditions, have been identified only as metasomatic substitutions of possible deep-breaking Cretaceous limestone clasts.

The deposition of the ore seemed to have a pulsating nature with the evolution taking place, possibly, in three stages to which the following mineral assemblages were described: 1. magnetite, hematite - pyrite, marcasite - quartz and pyrite - quartz ± base metal sulfides, in the first stage (mesothermal?); 2. arsenopyrite, Au - base metal sulfides - quartz - adularia, “chinga”, pyrite, Au - quartz - adularia and base metal sulfides - calcite, aragonite, dolomite, ankerite, ± rhodochrosite ± kutnahorite - quartz - adularia, in the second stage (epithermal low sulfidation) and 3. quartz - pyrite - marcasite - carbonates dominant rhodochrosite - Au and alabandite - rhodochrosite - quartz in the third stage (epithermal low sulfidation).

Gold is present in various proportions, either as small grains or as sub-microscopic occurrences and has been petrographically identified as electrum. The individual grains in native state have been observed as thin sheets on pyrite, sphalerite, rhodochrosite, calcite and quartz or as short wires and sheets in geodes. Local gold concentrations are common at the intersection of the locally-called “chairs” with “crosses” veins.

Acknowledgments

This work was supported by two Romanian Ministry of Research and Innovation grants, CCCDI – UEFISCDI, project number PN-III-P4-ID-PCCF-2016-4-0014 and PN-III-P1-1.2-PCCDI-2017-0346/29, within PNCDI III.