Mineralogical and geochemical spatial analyses of the Hop waste rock dump at the Roşia Montana gold mine, Romania

In this work, the 2.5 ha Hop waste-rock dump from the Roşia Montană gold mine (Romania) was investigated using mineralogical, geochemical, and geostatistical analyses. The Roşia Montană hydrothermal ore deposit is hosted in andesites and dacites of Neogene age piercing the prevolcanic sedimentary basement as breccia pipes [1]. The volcanic and subvolcanic rocks show pervasive adularia alteration with a phyllic overprint, as well as local silicification and argillic alteration. They host polymetallic sulphides and Au-Ag-Te mineralisations; they are present in epithermal veins, mineralized phreatomagmatic breccias, and stockworks [2]. The ore deposit was mined both underground and in open pit for more than 2000 years. Intensive supergenic alteration due to AMD processes is testified by acid sulphate waters and by the precipitation of large amounts of secondary minerals. The studied waste dump shows heterogeneities even at the field scale mainly evidenced by differences in colour. 22 samples were collected following a square cells grid. The particle size distribution was obtained by dry sieving. The mineralogy, petrography and geochemistry were studied by means of optical microscopy (reflected- and transmitted-light microscopy), SEM-EDS, XRPD, XRF and ICP-AES analyses. Samples were poorly sorted and could be classified as sandy gravel sediments. They show a pH value ranging between 3 and 8, with such strong variability strictly related to sulphur and carbonate content. Lithology, determined on the 250 to 125 m grainsize, is heterogeneous, comprising the following lithotypes: variably metasomatized porphyritic dacites, andesites, sedimentary flyschoid rocks and polygenic breccias. According to Acid Mine Drainage potential these lithotypes contains the following mineral species: AMD-producing sulphides (mainly pyrite with minor to trace sphalerite, chalcopyrite and marcasite) varying from << 1% up to 5%; neutral quartz, alkaline feldspar and micas with variable amounts of cryptocrystalline fragments that represent the aphanitic groundmass of the volcanic rocks; AMD-reducing carbonates (mainly calcite and dolomite with minor siderite and rodochrosite) that are absent or minor constituents with the exception of few samples where they represent about 5% of the total constituents. Amphiboles, felspatoids, apatite, rutile and zircon are the main accessory minerals. Secondary minerals that are formed as a consequence of AMD processes vary from 5 to 15 %; they are mainly represented by Fe-oxyhydroxides and oxides that occur as pseudomorphic replacement after primary sulphides, or as voids and fractures filling. They are also present as oxidation halos within the groundmass and as tiny coatings that rim sulphide and silicate grains. Ephemeral sulphates recognized on field were not preserved in the samples. Metallic paragenesis is rich comprising both heavy metals derived from the mineralisation (e.g. Cu, Zn, Ag, Pb) and from host rock (e.g. Rb, Sr, Zr). Further investigation on heavy metal geochemistry and mineral chemistry is ongoing to define the relationships between metals distribution and AMD processes also in order to evaluate the pollution potential of the dump.