Determination of base metals and gold in Pb-Sb sulfosalt mineral boulangerite using laser-induced breakdown spectroscopy and inductively coupled plasma optical emission spectrometry

Abstract

Gold (Au) is an economically important metal. Its determination is a challenging and crucial task due to its presence in low concentration and heterogeneous distribution in gold-bearing minerals. The suitability and performance of laser-induced breakdown spectroscopy (LIBS) to identify base and precious metals, especially gold, in a boulangerite sample was evaluated. X-ray diffractometer phase analysis was carried out to determine constituent phases, revealing the presence of boulangerite, tridymite and galena. Analysis of the optical emission spectra obtained on the boulangerite surface using a Q-switched Nd:YAG laser at a fundamental harmonic (1064 nm) in the 200–720 nm spectral range revealed the presence of Pb, Sb, S, Au, Cu, Ca, Fe, Mn, Zn, Si, Mg, Al, Na, Ni and Li. Most of the emission lines for the major elements constituting boulangerite (Pb, Sb and S) were observed in the shorter wavelength region. The resonance line of Au appearing at 242.79 nm and corresponding to the 5d106p2P3/2 → 5d106s2S1/2 transition facilitated its identification. The plasma temperature was calculated by the Boltzmann plot method using neutral Au lines which resulted as 11 295 ± 1100 K. A more accurate method known as the Saha–Boltzmann plot method was also employed to strengthen the plasma temperature calculation with good agreement. The electron number density was estimated using the Stark-broadened line profile of the neutral silica line at 288.15 nm and was found to be 7.63 × 1017 cm−3. The laser-generated plasma on the boulangerite surface was optically thin and fulfilled the local thermodynamic equilibrium condition. The LIBS compositional analysis results were cross-validated using scanning electron microscopy coupled with energy dispersive x-ray spectroscopy, inductively coupled plasma optical emission spectroscopy and Fourier-transform infrared spectroscopy, revealing an analogous composition. The results obtained using LIBS and other techniques displayed their ability as powerful and complementary tools for the compositional analysis of base and precious metals, especially gold, in gold-bearing minerals.