Geological and geochemical controls for epithermal Au-Ag-Te (Pb-Zn) mineralisation at Coranda-Hondol and the Brad-Sacaramb basin mineral district of western Romania

The 'Golden Quadrilateral' of W. Romania is a rich mineralised magmatic province hosting major porphyry-Cu and epithermal Au-Ag deposits. The mineralisation is associated with extensive magmatism emplaced along a series of NW-SE trending pull-part basins (e.g. Brad-Sacaramb, Rosia Montana). These basins developed during the Miocene, owing to the opposite sense rotation of the ALCAPA and Tisza-Dacia micro-continents, which facilitated extension-related melting of the subduction modified sub-continental lithospheric mantle. The largest of these basins, the Brad-Sacaramb basin contains extensive calc-alkaline, amphibole-rich, porphyritic andesite and dacite volcanics and sub-volcanic intrusions. LA-ICP-MS U-Pb analysis of zircons indicate that these magmatic rocks were emplaced during the Mid-Miocene 9.7 - 13.1 Ma. The rocks display a temporal geochemical evolution from early "normal" island arc magmatism (13.0 - 11.3 Ma) to adakite-like, characterised by high Sr/Y and La/Yb ratios, emplaced after 11.4 Ma. This shift in the geochemistry resulted from the cessation of plagioclase fractional crystalisation in the adakite-like magmas, coupled with the onset of amphibole crystalisation in the lower crustal MASH zone, which depleted the melts in MREEE-HREE. This change in the crystalising assemblage may have related to an increase in the dissolved water content of the melts. High zircon Ce/CE[sup]* (Ce[sup]4+/Ce[sup]3+) and less negative Eu/Eu[sup]* indicate that the high water content of these melts was accompanies by highly oxidising conditions. These conditions may have been important in preventing early sulfide saturation, a process considered to be unfavourable for the formation of economic Au and Cu deposits. K-Ar analysis from hydrothermal illites indicate that lo low to intermediate sulfidation epithermal and porphyry-Cu mineralisation in the Brad-Sacaramb basin occurred between 9.7 and 12.3 Ma. The epithermal deposits typcially developed <0.5 Myrs after the cessation of magmatism in the immediate vicinity. One of the most prospective deposits in the basin is the Coranda-Hondol deposit (~4.8 Moz at 1.4 g/t Au). Coranda-Hondol is a telluride-rice, intermediate sulfidication Au-Ag deposit, hosted by a series of andesitic stocks (emplaced ~12.6 Ma) and siliciclastic sedimentary formation. The deposit displays a variety of mineralisation styles with pervasive pyrite dissemenations and base-metal sulfide and sulfosalt-telluride -rich veinlets. Microthermonetry of fluid inclusions indicate that the mineralisation was facilitated by a dilute (<10 wt% NaCl) low temperature (~250[degrees]C) fluid. Stable isotopes ([delta][sup]18O[sub]fluid: 6.1 - 9.4 [parts per thousand], [delta]D[sub]fluid: -39.8 - -74.3) support a magmatic source for the fluid which mixed with meteoric waters during the waning stages of the hydrothermal system ([delta][sup]18O[sub]fluid: -0.3 - 1.6). Boiling of the ore fluid during brecciation events triggered the precious metal mineralisation. The Au is predominately hosted within the crystal lattice of textual complex and isotopicalle light ([delta][sup]34S: -19/4 - -6.5 [parts per thousand]) colloform pyrite grains (up to ~450 ppm) developed within these boiling zones.