2. METHODOLOGY 2.1 Sampling and sample preparation The type of sample selected for the geochemical mapping of the study zone was residual soil, which followed the recommendations of the Geochemical Baseline Program (Salminen, et al. 1998). During the field campaign from spring 2016 to spring 2017 up to 6.197 soil samples were collected, at an average sampling density of two samples per km2 (a grid of 500 × 1000 m) in N-S profiles, and an average weight of 3 kg. They were taken from 2 to 20 cm depth when possible due to the poor development of soil in the Pyrite Belt. Samples were collected as composite samples from five pits within 100 m2 (Martín-Méndez, et al. 2023). Soil samples were firstly sieved to < 3 mm during the field campaign. For quality control purposes, a field duplicate was taken at every 30 sample site, with an offset. All samples were shipped to a central sample preparation facility at the Geological Survey of Spain (IGME) and were processed by the protocol described by Mackovych et al. (2022). 2.2 Chemical analyses and quality control Chemical analyses of 64 elements were carried out at Actlabs Laboratories Ltd. (Ontario, Canada), after a tetra-acid digestion, by a combination of inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Al, Ca, K, Mg, Mn, Mo, Na, P, S, Ti and V), inductively coupled plasma mass spectrometry (ICP-MS) (Ag, Ba, Be, Bi, Cd, Co, Cr, Cs, Cu, Ga, Ge, Hf, Hg, In, Li, Nb, Ni, Pb, Rb, Re, Se, Sn, Sr, Ta, Te, Th, Tl, U, Y, Zn, Zr, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Tb, Ho, Er, Tm, Yb and Lu) and instrumental neutron activation analysis (INAA) (As, Au, Br, Fe, Hf, Ir, Na, Sb, Sc, W, La, Ce, Nd, Sm, Eu, Tb, Yb and Lu). Chemical element concentrations were determined using a tight internal and external quality control procedure. 2.3 Statistical analysis A comprehensive database integrating all analytical results, field‑campaign observations, and relevant supporting attributes was compiled for subsequent statistical analysis, following the methodological framework established by Reimann et al. (2008). Element concentrations falling below the instrumental detection limit (IDL) were assigned surrogate values equivalent to one‑half of their respective detection limits to enable robust statistical treatment. Univariate statistical analyses were conducted to characterize the geochemical distribution of individual elements. The variability of elemental concentration values, their distribution patterns, and the presence of multiple populations or outliers were assessed through the computation of several statistical parameters, which are summarized in Table 1. In addition, deviations from normal or log-normal behaviour were evaluated for each element following the approach outlined by Reimann (2005). 181
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