Alteration mineralogy, lithochemistry and stable isotope geochemistry of the Murgul (Artvin, NE Turkey) volcanic hosted massive sulfide deposit: Implications for the alteration age and ore forming fluids

The Murgul (Artvin, NE Turkey) massive sulfide deposit is hosted dominantly by Late Cretaceous calc-alkaline to transitional felsic volcanics. The footwall rocks are represented by dacitic flows and pyroclastics, whereas the hanging wall rocks consist of epiclastic rocks, chemical exhalative rocks, gypsum-bearing vitric tuff, purple vitric tuff and dacitic flows. Multi-element variation diagrams of the hanging wall and footwall rocks exhibit similar patterns with considerable enrichment in K, Rb and Ba and depletion in Nb, Sr, Ti and P. The chondritenormalized rare earth element (REEs) patterns of all the rocks are characterized by pronounced positive/negative Eu anomalies as a result of different degrees of hydrothermal alteration and the semi-protected effects of plagioclase fractionation. Mineralogical results suggest illite, illite/smectite + chlorite +/- kaolinite and chlorite in the footwall rocks and illite smectite kaolinite and chlorite illite in the hanging wall rocks. Overall, the alteration pattern is represented by silica, sericite, chlorite and chlorite-carbonate-epidote-sericite and quartz/albite zones. Increments of Ishikawa alteration indexes, resulting from gains in 1(20 and losses in Na2O and the chlorite-carbonate-pyrite index towards to the center of the stringer zone, indicate the inner parts of the alteration zones. Calculations of the changes in the chemical mass imply a general volume increase in the footwall rocks. Abnormal volume increases are explained by silica and iron enrichments and a total depletion of alkalis in silica zone. Relative K increments are linked to the sericitization of plagioclase and glass shards and the formation of illite/smectite in the sericite zone. In addition, Fe enrichment is always met by pyrite formation accompanied by quartz and chlorite. Illite is favored over chlorite, smectite and kaolinite in the central part of the ore body due to the increase in the (Al + K)/(Na + Ca) ratio. Although the REEs were enriched in the silicification zone, light REEs show depletion in the silicification zone and enrichment in the other zones in contrast to the heavy REEs' behavior. Hydrothermal alteration within the hanging wall rocks, apart from the gypsum-bearing vitric tuffs, is primarily controlled by chloritization with proportional Fe and Mg enrichments and sericitization. The 8180 and 8D values of clay minerals systematically change with increasing formation temperature from 6.6 to 8.7%. and 42 to 50%0 for illites, and 8.6 and 52%0 for chlorite, respectively. The 0-and H-stable isotopic data imply that hydrothermal-alteration processes occurred at 253-332 C for illites and 136 C for chlorite with a temperature decrease outward from the center of the deposit. The positive 834S values (20.3 to 20.4%.) for gypsum suggest contributions from seawater sulfate reduced by Fe-oxide/-hydroxide phases within altered volcanic units. Thus, the hydrothermal alteration possibly formed via a dissolution-precipitation mechanism that operated under acidic conditions. The K-Ar dating (73-62 Ma) of the illites indicates an illitization process from the Maastrichtian to Early Danian period.