The La Florida lead-zinc deposit (Cantabria, Spain), which is hosted by Urgonian carbonates, is located on the border of the Basco-cantabric Lower Cretaceous basin. The stratiform orebody, which is composed of sphalerite, galena, and barite in iron-dolostones, occurs in an envelope of dolostone. Organic matter studies have been carried out on samples from this deposit to understand its genesis and later evolution. The results of this work reveal the immatury of the autochthonous organic matter despite a slight paleothermal anomaly centred on the deposit. Various alteration processes have affected the hydrocarbons associated with autochthonous organic matter and (or) allochthonous bitumens, namely geochromatographic fractionation, water-washing, and biodegradation. Together with previous geological work, the results of this study are consistent with epigenetic and (or) diagenetic emplacement of the mineralization. The proposed genetic scheme involves hydrothermal fluids that would have acquired at least some of their characteristics during their upward migration through the Mesozoic cover. These solutions probably brought in the metals, the sulphates, and the organics all necessary for in situ hydrogen sulphide production and ore genesis. Sulphate reduction probably proceeded mostly at the expense of allochthonous organics by bacteria introduced by meteoric water infiltrations.

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