Optimizing Mineral Resources with Automated Mineralogy Techniques: The Case of Colquiri in the Central Andean Tin Belt

Colquiri is one of several deposits from the Central Andean tin belt, where sphalerite and cassiterite are mined. Although this is a high-grade Zn-Sn deposit, processing results in a low overall yield, with significant amounts of zinc and tin being discarded as tailings. In this study, mineralogical research was conducted to identify the causes of the low yield, so that the flow diagram could be modified to improve recovery. Particle size was measured, and chemical and mineralogical analyses were performed using optical and electron microscopy and X-ray diffraction. The mineral chemistry of the ores was determined using electron probe microanalysis (EPMA), and mineral liberation analyses were performed to complete the characterization. Mineralization occurred in four stages: (1) formation of silicates and oxides; (2) main precipitation of sulfides, including pyrrhotite, sphalerite, and stannite; (3) precipitation of fluorite and the replacement of pyrrhotite by pyrite, which was then replaced by siderite; and (4) weathering of previously formed minerals. The run-of-mine material contains approximately 12 wt.% ZnO and 1.5 wt.% SnO2. The Zn concentrate contains up to 43.90 wt.% ZnO, and the Sn concentrate contains 52 wt.% SnO2. The final tailings still retain more than 3–4.5 wt.% ZnO and 1.2 wt.% SnO2. The average grain size of sphalerite is 200 µm, while that of cassiterite and stannite is 45 µm. The liberated fraction of sphalerite is 51.43%, and binary particles of sphalerite plus stannite account for 60 wt.%. Cassiterite is liberated at 54.68 wt.%. To increase the recovery of sphalerite (with stannite) and cassiterite, as well as the grade of the concentrates, it is necessary to reduce the particle size of the processed ores to less than 100 µm.