Anomaly threshold estimation and data normalization using EDA statistics: application to lithogeochemical exploration in Lower Cretaceous Zn–Pb carbonate-hosted deposits, Northern Spain

A lithogeochemical survey of Lower Cretaceous sediments (1293 samples) in the Basque–Cantabrian basin (northern Spain) was carried out in order to estimate the mean contents of major oxides and Ba, Co, Cu, Ni, Pb, Rb, S, Sr, V, Zn and Zr in different lithological subsets and to identify anomalies related with Zn–Pb carbonate-hosted mineralization. After a detailed evaluation of different anomaly recognition methods, using both real and synthetic data, the cut-off values used in the Exploratory Data Analysis (EDA) were selected as thresholds. A prior classification of the rocks (based on chemical, mineralogical and petrographical data) guarantees appropriate homogeneous (with respect to major oxides) rock-groups during the evaluation of the data. A data standardization method was developed and applied to the samples collected from the Carranza sector (n=330). Zinc and Pb values for each sample were normalized to the particular threshold and interquartile range of their corresponding rock-type. These normalized values were plotted in geochemical maps. The results reveal the presence of Zn (up to 0.6%) and Pb (max. 1380 ppm) anomalies concentrated in carbonate facies, mainly in limestones, dolostones and dolomitic limestones. Magnesium-rich carbonates host the most significant anomalies. In the Carranza area, the great majority of outliers occur around known Zn–Pb mineral occurrences and are concentrated at the edge of the carbonate platform. In some cases fault control of the anomalies is indicated.