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22 páginas.-- 6 figuras.-- 115 referencias.-- Additional Supporting Information may be found in the online version of this article: http://dx.doi.org/10.1111/sed.12431, Subsurface granitic environments are scarce and poorly investigated. A multi‐disciplinary approach was used to characterize the abundant moonmilk deposits and associated microbial communities coating the granite walls of the 16th Century Paranhos spring water tunnel in Porto city (north‐west Portugal). It is possible that this study is the first record of moonmilk in an urban subsurface granitic environment. The morphology and texture, mineralogical composition, stable isotope composition and microbial diversity of moonmilk deposits have been studied to infer the processes of moonmilk formation. These whitish secondary mineral deposits are composed of very fine needle‐fibre calcite crystals with different morphologies and density. Calcified filaments of fungal hyphae or bacteria were distinguished by field emission scanning electron microscopy. Stable isotope analysis revealed a meteoric origin of the needle‐fibre calcite, with an important contribution of atmospheric CO2, soil respiration and urn:x-wiley:00370746:media:sed12431:sed12431-math-0001 from weathering of Ca‐bearing minerals. The DNA‐based analyses revealed the presence of micro‐organisms related to urban contamination, including Actinobacteria, mainly represented by Pseudonocardia hispaniensis, Thaumarchaeota and Ascomycota, dominated by Cladosporium. This microbial composition is consistent with groundwater pollution and contamination sources of the overlying urban area, including garages, petrol stations and wastewater pipeline leakage, showing that the Paranhos tunnel is greatly perturbed by anthropogenic activities. Whether the identified micro‐organisms are involved in the formation of the needle‐fibre calcite or not is difficult to demonstrate, but this study evidenced both abiotic and biogenic genesis for the calcite moonmilk in this subsurface granitic environment., The authors acknowledge the Spanish Ministry of Economy, Industry and Competitiveness (MINEICO, project CGL2011-2569) and the Portuguese Foundation for Science and Technology, FCT (UID/GEO/04035/2013, UID/ECI/04028/2013, PEst-OE/CTE/UI0098/2011) and the LABCARGA| ISEP re-equipment program (IPP-ISEP|PAD’2007/ 08) for financial support. AZM thanks MINEICO for the ‘Juan de la Cierva – Incorporación’ postdoctoral contract (IJCI-2014-20443). PMMS thanks the Spanish Ministry of Science and Innovation for his contract associated with the ‘Research Programme in Technologies for the Assessment and Conservation of Cultural Heritage’ (TCP CSD2007-0005