Mineral assemblages in the tin (-indium) Santa Fe mining district, Bolivia

Abstract

The Santa Fe Mining District (SFD) is located in the Bolivian portion of the Central Andean Belt, which is an important tin province that hosts abundant world-class deposits, some of them with high indium content. In this work, we studied the mineralizations of Japo, Santa Fe and Morococala, in the SFD, in order to understand the mineralizing processes with focus in mineral chemistry and mineralogical assemblages. The SFD consists of a turbiditic Ordovician metasedimentary basement overlaid by a continuous sedimentary Silurian sequence represented by the Cacañiri, Llallagua and Uncia Fm. This sequence was folded and thrusted by an intense tectonic activity and is unconformably covered by the volcanic complex of the Morococala Fm., constituted by calc-alkaline lavas and tuffs of Miocene age (Grant et al., 1979). The ore mineralization is associated to felsic magmatism, represented by several generations of dykes and the San Pablo stock, of Oligocene-Miocene age (Grant et al., 1979). Ore mineralization occurs as veins and disseminations and consists of cassiterite, sulphides and sulfosalts. A wide shear zone and two fracture systems are developed: (1) N°40, dipping 60°W, with Sn-Zn, and (2) N°40, dipping 75°E, with Zn-Pb-Ag. Mineralogy was characterized by X-ray diffraction, scanning electron microscopy and electron probe micro-analyzer (EPMA). Mineral assemblages in the district are rather uniform but the relative contents among ore minerals vary from one deposit to another. Two main stages of mineralization are distinguished: (1) An early Sn mineralization represented by cassiterite; and (2) a late Sn and Zn-Pb-Ag mineralization, represented by sphalerite, galena, stannite group and sulfosalts. In Japo and Morococala cassiterite and sulphides as stannite, stannoidite, sakuraiite and kësterite predominate. EPMA analyses indicate that in these mines indium content ranges between 0.12 and 0.25 wt.% in cassiterite, reaches up to 0.9 wt.% in stannite, and attains the highest concentration in sakuraiite, with up to 2 wt.%. On the other hand, in the Santa Fe mine galena, sphalerite and Ag-sulfosalts predominate. Galena shows an average silver content of 0.30 wt.% and sphalerite has indium contents up to 0.24 wt.%. Several sulphides and sulfosalts with Se, Bi and Ag contents as ourayite, gustavite, matildite, jamesonite, jaskólskiite and bismuthinite also were found. Mineral deposition occurred in a multistage event: (1) Magmatic stage and injection of hydrothermal fluids rich in Sn; (2) metasomatism producing the alteration of metasedimentary sequences, synchronous to late magmatic stage; (3) late hydrothermal stage with sulphide deposition; and (4) supergene alteration rich in phosphates (mainly plumbogummite and crandallite). Grant, J., Halls, C., Sheppard, S., Avila, W., Snelling, N. (1979): K-Ar ages of igneous rocks and mineralization in part of the Bolivian Tin Belt. Econ. Geol., 74, 838-851.