Rock magnetic characterization of the mine tailings in Portman Bay (Murcia, Spain) and its contribution to the understanding of the bay infilling process

Abstract

The Portman Bay located in the SE Spain has suffered great changes since the 1950s associated with the mining activity. The direct discharge into the sea of mining waste has caused the silting of the bay in a few decades. In this study the magnetic properties of recent sands from a set of 52 samples from the bay and 16 samples from an opencast mine are magnetically analyzed. In particular, we have measured magnetic susceptibility, hysteresis loops, IRM-acquisition, SIRM-IRM and back-field-coercivity spectra, thermomagnetic curves and FORC diagram and took SEM micropictures. Data are correlated with classical magnetite content estimations derived from X-ray diffraction (XRD) in three separated grain sized (coarse, medium and fine grains) and heavy metal derived from X Ray Fluorescence (XRF) in samples from a parallel study at the same sampling sites and the same sampling campaign. The results indicate high concentrations of magnetite (up to 28%) compared to magnetically hard minerals that have not been quantified. Higher concentrations of magnetite are recorded in the central part of the beach, as indicated by Ms, SIRM and Kinit. Coercivity, however, indicates smaller values in the center of the Bay, suggesting a larger size of the magnetic particles. The S-ratio or high coercivity fraction is higher in older samples. Correlation of the magnetic properties with magnetite grain size indicates a positive correlation between Ms and medium and fine grain size fractions, determined by X-ray diffraction. In addition, magnetic properties show a direct relationship between magnetite content and the heavy metals Pb, Cu and As. Our results indicate that the deposition of magnetite was fast and properties have not been modified in depth. However, spatial location in the Bay is significant when we examine distribution of grain sizes. Also, results suggest that magnetite acts as captor of heavy metals.