Ag-rich Tetrahedrite in the El Zancudo Deposit, Colombia: Occurrence, Chemical Compositions and Genetic Temperatures

Chemical compositions of tetrahedrite-Ag-rich tetrahedrite-freibergite solid solutions (Ag-rich tetrahedrite(ss)) and homogenization temperatures of fluid inclusions in quartz and carbonates of seventeen samples from nine veins in the El Zancudo deposit, Antioquia, Colombia, were investigated to reveal the origin of silver in Ag-rich tetrahedrite(ss), to derive their crystallization temperatures and to examine the relationship between chemical compositions of Ag-rich tetrahedrite(ss) and their crystallization temperatures. The ores consist of arsenopyrite, pyrite, sphalerite, Ag-rich tetrahedrite(ss), galena, boulangerite, andorite, owyheeite, diaphorite, jamesonite, miargyrite, bournonite, chalcopyrite, and electrum. Ag-rich tetrahedrite(ss) forms about 10 volume % of the total ores and is one of the most common and widely distributed sulfosalts in this deposit. Ag-rich tetrahedrite(ss) is rich in Ag (1.13 to 31.02 wt%) and Sb (22.93 to 29.82 wt%), and poor in As (0.06 to 2.43 wt%), consistent with the reported incompatibilities of Ag and As in Ag-rich tetrahedrite(ss). The Zn/(Zn + Fe)-, Ag/(Ag + Cu)- and Sb/(Sb + As + Bi)-atomic ratios exhibit some variations among the veins. Ag-rich tetrahedrite(ss) with higher Ag/(Ag + Cu) ratios coexist with diaphorite, whereas those with lower ratios are not associated with this sulfosalt. Ag-rich tetrahedrite(ss) in the assemblages of Ag-rich tetrahedrite(ss) + sphalerite and of Ag-rich tetrahedrite(ss) + bournonite + galena shows no Zn <-> Fe and Cu <-> Ag variations between core and rim, respectively, negating the possibility of solid state reaction during cooling. Ag-rich tetrahedrite(ss) is thus regarded as primary phase. Homogenization temperatures of primary fluid inclusions in quartz and carbonates co-existing with Ag-rich tetrahedrite(ss) define the mineralization temperatures of 134 to 263 degrees C. Independent crystallization temperatures of Ag-rich tetrahedrite estimated based on Zn/(Zn + Fe) and Ag/(Ag + Cu) ratios of the Ag-rich tetrahedrite(ss) associated with silver minerals such as miargyrite, andorite and diaphorite using Sack's thermochemical database lie in a range between 170 and similar to 250 degrees C. Both results are thus in good agreement.