ABSTRACT
This study concerns the secondary mineral deposits found in an abandoned mining heritage of particular geological and cultural significance. The Aveleiras mine is an old water mine, belonging to St. Martin of Tibães Monastery, near Braga (NW Portugal), which through the last three centuries supplied the monastery until it was converted in a wolfram mine in 1940-62. By initiative of the Tibães Monastery, it will be partially rehabilitated into a geosite. Secondary mineral deposits were observed in this subsurface rock environment, including black deposits, mucolite-like stalactites and blue-green deposits, which cover the walls and ceiling of the Aveleiras mine. The formation of these speleothem features are potentially induced by bacterial communities. In order to evaluate their biogenicity, morphological, geochemical and mineralogical characterisation were conducted. Scanning electron microscopy examinations revealed the presence of microorganisms and microbial mats concealed within these mineral deposits. The typical morphology of biogenic manganese oxides, consisting of Mn-rich fibril aggregates embedded in extracellular poly-meric substances, were found in the black deposits. Elemental analyses performed by energy dispersive X-ray analysis showed enrichment in carbon and manganese, which evidences a relation of microorganisms to the formation of these secondary deposits. In addition, coccoid cells and rod-shaped bacteria were observed in all samples. The mucolite-like stalactites or snottites consisted of pale yellow slime material rich in S, K and Fe, suggestive of jarosite, as also corroborated by Fourier transform infrared spectroscopy. The blue-green colour deposits were formed by enrichment in Cu and As, and depletion in Si, Fe and Zn.
