Mineralogy and geochemistry of tellurides and pyrite from the Laowan Au−Ag−Te deposit, Qinling−Dabie orogenic belt, central China: Implications for ore genesis

Abstract

The Laowan Au − Ag − Te deposit (containing greater than 100 t Au), located at the center of the Qinling − Dabie orogenic belt in central China, represents a breakthrough in gold prospecting in the Tongbai district. A tunnel geological survey and microscope-based mineral identification, combined with EMPA/EDS mapping and LA-ICP-MS analysis, were used in this study to determine the mineral types present. The sequence of formation and symbiosis were determined using the Au-hosting mineral geochemical element typomorphism to provide a basis for Au enrichment and the sources of material and ore − forming fluids. The results indicate that: (1) Tellurides were widespread in the main metallogenic auriferous pyrites, consisting of petzite, calaverite, tellurobismuthite, tetradymite, volynskite, and hessite. The vertical zonation of different telluride combinations indicates that mineral distribution shows the vertical zoning characteristics of upper Ag and lower Bi. (2) Petrographic analyses indicate that 5 generations of pyrite formed during the ore-forming stage (Py1, Py2, Py3, Py4, and Py5). LA-ICP-MS analytical data showed that Py3 and Py4 were rich in Au − Ag − Bi − Te, with a correlation between gold and tellurium. Auriferous pyrites were rich in both As and Te, and the enrichment of Au was unrelated to As. Te is an important scavenger of gold, the effect of As is not obvious. (3) From 250 °C to 200 °C, logfTe2 and logfS2 decreased from − 13.0 to − 8.3 and − 14.7 to − 8.6 to − 15.2 to − 11.2 and − 16.7 to − 10.5, respectively. fTe2, fS2 and temperature were the conditions controlling variations in the telluride combination. (4) The occurrence of tellurides, the correlation of Au − Ag − Bi − Te elements and enrichment of Te and Bi, combined with previous isotopic geochemical data, indicate that the ore-forming fluids and metals might be stemmed from the mantle-derived magmatic system, which could have provided materials, sufficient heat energy, volatiles, and components for the gold mineralisation.