A sulphur isotopic study of the Navia gold belt (Spain)

Abstract

The Navia gold belt is located in the West Asturian-Leonese Zone of the Iberian Variscan Orogen. The host rocks of the mineralization are quartzites, sandstones and black shales of Cambro-Ordovician age. The gold belt extends along 35 km and has five major veins: Penedela, Encarnita, Fornaza, Carmina and S. Jose. The ores belong to at least four associations having contrasting mineralogies and textures. The δ 34S values for individual mineral phases reflect the polyphase metallogenic history. The older association (Stage 1) is Fe-Mn-rich and is made up of spessartine, grunerite-dannemorite and quartz, with magnetite, pyrrhotite and chalcopyrite as metallic phases. The mineralization of Stage 1 is followed by the As-rich Stage 2 with quartz, arsenopyrite and pyrite. The δ 34S values for pyrite range from 14.9 to 19.9 per mil ( n = 16), and for arsenopyrite from 13.2 to 17.3 per mil ( n = 7). The observed isotopic homogeneity likely implies isotopic equilibrium at the scale of the gold vein. Stage 3 contains a coarse-grained base metal sulphide-rich association. The δ 4S values for sphalerite range from 16.4 to 20.6 per mil ( n= 16), and for galena from 17.0 to 18.7 per mil ( n = 11). δ 34S sp > δ 34S gl suggests that the sulphur isotopic fractionation of the ore-forming system had reached equilibrium. The youngest crosscutting mineral association (Stage 4) consists of Pb-Sb sulphosalts, bornite, electrum and quartz. The δ 34S values for sulphosalts range from 9.7 to 15.8 per mil, showing the lightest results of the Navia sulphides. The relatively tight clustering of δ 34S values of the Au-related sulphides, and the results of fluid inclusions and paragenetic studies, can be interpreted to indicate that the hydrothermal fluids of the last three stages were dominated by H 2S. In the H 2S predominant field, sulphide minerals precipitating from solutions would exhibit δ 34S values similar to the δ 34S ΣS value of the ore fluid. The heavy δ 34S ΣS of the Navia fluids is consistent with leaching of sulphur from the host rocks. The main sulphur source could be diagenetic pyrite from the siliciclastic rocks of the Cabos and Luarca Formations, which exhibit δ 34S values from 8.3 to 21.2 per mil. An additional sulphur-source in Stage 3 would be the leaching of disseminated sphalerite and galena present in Cambrian carbonates.