Petrography and Sulphur Isotope Studies of Pyrites in the Muteh Gold Deposit

Abstract

Problem statement: The Muteh mining district is located in 70 km northeast of Golpaygan city within the Sanandaj-Sirjan metamorphic belt. There are 2 gold mines, 7 gold occurrences and numerous mineral indices in the Muteh gold district. There are few researches on Muteh gold district, but a detailed model is not clear yet. The aim of this study is to determine mineralogy of gold-bearing rocks and the role of these rocks in concentration of gold and to improve our knowledge about Muteh model. Approach: Detailed fieldwork carried out at different scales at the Muteh district. About 50 outcrops samples examined petrographically. Fifteen samples containing veinlets of sulfides and quartz selected for H, O and S stable isotope analysis. Petrography characterized by optical microscopy, Scanning Electron Microscopy (SEM) and X-Ray powder Diffraction (XRD) analysis. Results: Geological studies indicated that the study area show a major poly-phase metamorphism. Since the latest metamorphism was weaker than the earlier ones, the older rocks were affected by more intense metamorphism. The metamorphic rocks mainly consist of schists, quartzite, marble, amphibolite and gneisses. These rocks show two foliations (S₁ and S₂). The S₂ foliation is the major phase in the metamorphic rocks. Pyrite is the most abundant and the important gold-bearing mineral at the study area. Based on evidences of deformation (S₂) and crystallization, three main types of pyrites can be distinguished in the Muteh deposit: (1) pre-tectonic or gold bearing pyrite (2) syn-tectonic or disseminated pyrites along the foliation of the host rocks. (3) pyrite aggregates in the host rocks or in the metamorphic segregation quartz veins crosscutting the foliation of the host rocks. The sulfur isotope studies were carried out on pyrites within quartz veinlets, biotite schist and meta-volcanic rocks at the Muteh deposit. Five available data are highly variable even from the same types of hosted rocks and their δ³⁴S are +2.2, 6.6, 9.1, 13.9 and 16.9%₀. Conclusion: There are three generations of pyrite in Muteh gold district. According to isotope data, it seems that source of sulfurs were not homogenous. The values showed more than one geological event for generation of pyrites in the study area. Compositions of sulfur isotopes indicated several different sources or processes for the sulfide fluids. The sulfur of these pyrites might have derived either directly from regional metamorphism that produced the metamorphic fluid or through dissolution and leaching of pre-existing sulfide-bearing minerals.