Mineral chemistry of sphalerite from volcanogenic massive sulphide deposits: genetical implications

Though sphalerite is a common mineral in hydrothermal ore mineralization, there are still several open questions in terms of its mineralogical characterization. Many elements may appear in its structure such as Fe, Mn, Cu, Cd, Co, Hg, Ga, Ge, Ag, In, Tl, Se, etc. and some of them may correlate with the formation conditions (e.g., the amount of Fe strongly depends on formation T). For this reason, it is expected that composition of sphalerite my fingerprint the ore deposit type, although, until now only a few attempts were made to investigate this topic. As even technological/critical metals could be enriched in sphalerite (e.g. Co, Ga, Ge, Mn, In, etc.), this topic should be in the spotlight of modern research.

As a contribution to this field, we selected to study sphalerite from volcanogenic massive sulphide (VMS) deposits. Our investigation areas were chosen from the Neothethyan realm: Xylagani from the Hellenides, Greece; Gjegjan from the Dinarides, Albania and Lasail from the Semail Ophiolite, Oman. While Xylagani and Lasail has been identified as Cyprus-type VMS, the classification of Gjegjan ore deposit is uncertain. Our study supports its VMS related origin (more precise classification would need more detailed studies), but its host basalt formed most likely during the Triassic, advanced rifting stage, rather than during the oceanic stage of the Neotethys.

We performed petrographic microscopic, SEM-EDS and EPMA studies on sphalerite-bearing samples, aiming to characterise their texture, their mineral paragenesis, their mineral precipitation series and the mineral chemistry of sphalerite. It has been found that sphalerite mostly precipitated at the early stage of mineralisation, during the upbuild of the hydrothermal system, though at some places it also formed as a late product, during the waning of the system. Early sphalerite often suffered “chalcopyrite disease”, caused most likely by later Cu-rich fluid impulses. All studied sphalerite precipitated from a 238 °C to app. 60 °C warm fluid, characterised by an intermediate sulphidation state (logfS₂). We identified chemical anomalies in sphalerite, like Co in Lasail (up to 1230 ppm in massive suphide lens) and Ga in Gjegjan (up to 840 ppm in distal part of the mineralization). Fe (1,039–3,973 wt.%), Cd (950–3220 ppm), Mn (b.d.l.–2680 ppm) and Pb (b.d.l.–1260 ppm) was also presented in all samples and good correlations (R² > 0,64) were found between Fe–Zn, Cu–Zn, Fe/Cd–Zn and Fe/S–Zn.  Extreme Co enrichment of sphalerite could have been caused by a sudden change in the fugacity of S₂ and/or O₂ in the massive sulphide lens, while high Ga content favours low temperature conditions and may be accompanied by Ge anomaly as well. The identified Co and Ga anomalies not only may represent a new source of extractable raw materials for the investigated ore deposits, but understanding the reasons for enrichment of these critical raw materials can help to make mining more economical when exploring other similar ore deposits.