Mineralogical characterization of gold alluvial sands preconcentrates in the mining districts of Huepetuhe and Delta 1, Madre de Dios Department, Peru

Artisanal and small-scale gold miners (ASGM) in the Madre de Dios Department, Peru, use mercury for gold processing, making it the primary source of environmental contamination. However, the lack of geochemical and mineralogical characterization of alluvial preconcentrates limits the search for alternative metallurgical processes. To address this, seven samples of alluvial preconcentrates (CQI-04, CQI-05, DAI-01, DAI-02, DAI-03, DAI-29, DAI-Hg-06) from two mining concessions in the Huepetuhe and Delta 1 mining districts were analyzed to determine their geochemical and mineralogical composition. The study included X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS), as well as X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), and sieve analysis with fire assay for gold quantification. The samples of the Puerto Belén concession, located in the Huepetuhe district, contain more than 80 % coarse gold particles (>250 µm), are rich in iron oxides (ilmenite, magnetite, hematite), quartz, and contain minor amounts of andalusite, zircon, and unidentified phases containing light rare earth elements (LREE). The samples of the Raul 1 concession, in the Delta 1 district, have 50 % medium-sized gold particles (106–250 µm), 28 % fine gold (<106 µm), and 23 % coarse gold (>250 µm), with dominant quartz content, followed by hematite, ilmenite, zircon, rutile, muscovite, and LREE-bearing phases. In both concessions, gold exhibits high purity (99 % Au, 1 % Ag). Optical and electron microscopy analyses reveal sub-rounded to elongated gold particles within ferritic and siliceous matrices. Understanding the geochemical and mineralogical characteristics of alluvial preconcentrates is essential for proposing mercury-free metallurgical alternatives that enhance gold recovery and enable the extraction of other economically valuable minerals such as iron, titanium, zircon, and rare earth elements.