Geochemical Assessment of Mineral Resource Potential in a Hg-Sb-Pb-Zn Mining Area: The Almadén and Guadalmez Synclines (South-Central Spain)

Abstract

The geochemical data from surface soils are often neglected or questioned when prospecting for ore deposits within active mining districts due to the “background noise” produced by anthropogenic pollution derived from mining activity. Large datasets on a national and international scale offer interesting possibilities to discover prospective zones. In the present work, data from the Geochemical Atlas of Castilla–La Mancha were treated in an area with an intense history of mining Hg, Pb, Zn, and Sb: the Almadén and Guadalmez synclines and the Alcudia Valley. The sampling grid was densified to adapt it to the scale of the main geological formations, and a cluster analysis was carried out to establish the relationships between the variables and a factor analysis to distinguish between geogenic and metallogenic factors. The results showed very high concentrations of some elements of prospective interest in local background areas: Hg (51 mg kg−1), Pb (1190 mg kg−1), and Sb (45 mg kg−1), with high variation coefficients. Cluster analysis unveiled a relationship between most of the local ore-forming metals (As, Sb, Hg, Pb, and Ag) with a clear correlation between Hg and organic matter, suggesting a notable contribution of Hg in soil. The factor analysis highlighted five factors, three geogenic and two ore-forming elements. Despite Hg being the main candidate to form a separate factor, its aerial deposition and a large number of outliers in the Almadén syncline contributed to integrating the element into a geogenic factor. Instead, factors F4 (Pb and Zn) and F5 (As and Sb) delimited the prospective areas in both synclines far from the known and the exploited mines. Some of these areas coincided with discovered mineralized zones, specifically in the area SE of a derelict Sb mine, confirming the usefulness of these datasets and statistical tools in areas with recent mining activity.

Biogeochemical prospecting of metallic critical raw materials: soil to plant transfer in SW Ciudad Real Province, Spain

“…Figure 2 shows the distribution of the concentrations of some elements of mining interest according to the geological domain to which they belong: the Alcudia anticline and the Almadén and Guadalmez synclines. As described by Barquero et al ( 2022 ), there are clear differences in terms of the presence of metallic mineralizations between these domains, with Hg mineralizations predominating in the Almadén syncline, Sb in the Guadalmez syncline, and Pb–Zn in the three domains, although with a greater presence in the Alcudia anticline and with high concentrations of Cu in the Guadalmez syncline (Higueras et al 2016 ). It can be observed that all the elements presented have similar averages in the three geological domains, although significant differences are evident in the Hg outliers in the Almadén syncline (Fig.…”

“…It is necessary to highlight the absence of notable differences between domains for Sb (Fig. 2 d), since it is an element which has been mined almost exclusively in the Guadalmez syncline, in addition to being an element with low mobility and very low bioavailability (Barquero et al 2022 ). The most reasonable interpretation is that Sb levels in Quercus leaves correspond to local background levels for this element in these lithological units, slightly higher than the local background levels in the Colline Metallifere (Monaci et al 2022 ).…”

“…The leaves were collected using pruning shears, at > 2 m high above ground, and were stored in paper envelopes and transported to the laboratory. At the same site, a composite sample of 2 kg of soil was collected from the A horizon (topsoil, 0–15 cm) using an Ejkelkamp soil sampler (see Barquero et al 2022 for more soil sampling details).…”

“…A 5-g aliquot was then blended with 0.15 g of agglutinant (dissolution of Elvacite 2046 PANalytical and ACETONA PURISS (CH 3 (CO)CH 3 ), UN 1090), inserted in aluminum vessels (RETSCH PP25) and compressed using a hydraulic hand press (SPECAC 250 kN), to obtain a pressed pill for use in the analytical procedure. Soil samples were air-dried at room temperature for 15 days and were then disaggregated, homogenized, and sieved to discharge the > 2-mm fraction; two aliquots were then obtained, one to determine the physicochemical parameters of the soil and the second was milled to < 100 µm in an automatic agate mortar (Barquero et al 2022 ).…”

“…The area is also characterized by the notable presence of metallic ore deposits, none of which are currently active: the Almadén syncline hosts the Almadén Mercury Mining District (AMMD), which has been the most important producer of this element worldwide (Saupé et al 1990; Hernández et al 1999 ); the Guadalmez syncline hosts some minor Pb–Zn and Sb vein-type deposits (Gumiel & Arribas 1987 ); and the Valle de Alcudia anticline hosts a large number of medium-importance Pb–Zn-Ag vein deposits, described by Palero et al ( 2003 ). The Sb deposits in the Guadalmez syncline are presently the target of detailed studies due to a renewed interest in this element by the European Union, who consider it to be a “critical raw material” (UE 2020 ), and has promoted a number of studies concerning this element in the southern Central Iberian area (Álvarez-Ayuso et al 2022 ; Barquero et al 2022 ; Esbrí et al 2023 ) in the context of the AUREOLE (tArgeting eU cRitical mEtals (Sb, W) and predictibility of Sb-As-Hg envirOnmentaL issuEs) project, funded by the European ERA-MIN3 program.…”