Wall rock alterations at the Gai Zn-Cu massive sulfide deposit (South Urals, Russia): Formation conditions and evolution

Metasomatically altered wall rocks at the Gai massive sulfide Zn-Cu deposit (D-2) were formed by multi stage mineral deposition. The earliest of these alterations are chlorite-sericite-quartz rocks forming an SN-trending zone , the upper part of which includes bodies of massive sulfide ores. These me tasomatic rocks were formed in the Eifelial under low temperature (<230-250 degrees C) acidic conditions. Later they experienced regional metamorphism of the prehnite-pumpellyite facies under temperatures of 300-350 degrees C or lower. The mica from the chlorite-sericite-quartz metasomatic rocks is fine-grained and appears to be a nonhydrated illite (the total number of in terlayer cations is 0.78-0.88 per cell after electron microprobe analyses) exclusively of 2M(1) polytype modification. This is evidence (taking into a ccount some new experimental data) that the initial micaceous material was composed of hydromicas. The Na/(Na+K) ratio for the enriched micaceous frac tion is rather high (0.15-0.30) as a result of Na-mica admixture (the admix ture was detected by electronographic technics). The Na-mica admixture rela tes to rather high Na activity in the fluid due to a high content of mafic minerals in the country rocks. The degree of veinlet and metasomatic silifi cation along with the Mg/Fe ratio in metasomatic chlorite increase in the d irection of the ore bodies, as do the temperature and solution acidity. The most acidic alteration occurred around the solution conduits, where serici te-pyrophyllite-quartz metasomatic rocks are common. Large zones of low-tem perature illite and Na-containing mica schists (paragonite + illite + K-Na micas of 2M(1) polytype according to electronographic and X-ray data) were formed within zones of intensive shearing around ore bodies before metamorp hism. After metamorphism the regressive process of the 2M(1) hydration of t he micas occurred along rare fracture zones. The data obtained confirm the near-surface conditions of the massive sulfide deposition.