The Pierina high-sulphidation epithermal Au-Ag deposit is located in the Cordillera Negra of north-central Perú, ~ 5km north of the coeval intermediate-sulphidation Ag–base metal Santo Toribio deposit, and in rocks of the Calipuy Supergroup. The deposit contains ~ 8 M oz Au, hosted in non-refractory iron oxides and, with heap-leach extraction, is one of the lowest-cost Au producers in the world. Stage Ia (15 Ma) and Ib (14.4 Ma) advanced-argillic alteration formed from mixed magmatic and meteoric waters. The main, 14.4 Ma event produced vuggy quartz alteration, focussed in a 16.9 Ma dacitic pumice-lithic tuff, and surrounded by quartz-alunite, quartz-dickite, and illite-montmorillonite alteration zones, the product of increased meteoric water contributions towards the periphery of the deposit. Stage II sulphide-barite mineralization introduced gold and silver, hosted submicroscopically in the disseminated high-sulphidation pyrite-enargite assemblage. Precipitation occurred from a low-to-medium - salinity magmatic fluid that mixed with meteoric waters at the site of ore deposition. A 14.1 Ma 40Ar/39Ar age for supergene alunite records the rapid incursion of meteoric waters into the deposit. The resulting oxidation of sulphides to schwertmannite, goethite, and hematite was facilitated by microbes, recorded by the stable-isotopic compositions of supergene barite+acanthite. Schwertmannite is enriched in Au and Ag and is the main precious-metal host. Regional pedimentation is inferred to have initiated retrograde boiling of the source magma chamber at 15 Ma, with renewed magma incursion triggering major alteration at 14.4 Ma. Both events involved the exsolution of an SO2-rich vapour and a more saline aqueous fluid. The latter migrated to Santo Toribio generating phyllic alteration and intermediate-sulphidation mineralization, whereas the SO2-rich vapours caused pyrite-bearing feldspar-destructive alteration along fluid pathways during ascent to Pierina. The subsequent single-phase, H2S-rich mineralizing fluid exsolved from the retracting magma at higher pressures. Vapour contraction during ascent along altered, unreactive pathways minimized the loss of S- and H+, optimizing the transport of Au as AuHS0. Watertable displacement resulting from pediment incision promoted the flow of groundwater into the epithermal environment where mixing with magmatic fluids precipitated gold. Supergene oxidation optimized conditions for microbial activity, a critical factor in the generation of economic mineralization. / Thesis (Ph.D, Geological Sciences & Geological Engineering) -- Queen's University, 2009-04-01 16:02:47.525

This thesis study is concerned with the genetic investigation of two epithermal gold deposits (Madendag and Kartaldag) in &Ccedil / anakkale, NW Turkey. The methodology comprises field and integrated laboratory studies including mineralogic-petrographic, geochemical, isotopic, and fluid inclusion analysis. Kartaldag deposit, hosted by dacite porphyry, is a typical vein deposit associated with four main alteration types: i) propylitic, ii) quartz-kaolin, iii) quartz-alunitepyrophyllite, iv) silicification, the latter being characterized by two distinct quartz generations as early (vuggy) and late (banded, colloform). Primary sulfide minerals are pyrite, covellite and sphalerite. Oxygen and sulfur isotope analyses, performed on quartz (&delta / 18O: 7.93- 8.95 &permil / ) and pyrite (&delta / 34S: -4.8 &permil / ) separates, suggest a magmatic source for the fluid. Microthermometric analysis performed on quartz yield a temperature range of 250-285 &ordm / C, and 0-1.7 wt % NaCl eqv. salinity. Madendag deposit, hosted by micaschists, is also vein type associated with two main alteration types: illite and kaolin dominated argillization and silicification, characterized by two distinct quartz phases as early and late. Oxygen isotope analyses on quartz (&delta / 18O: 9.55-18.19 &permil / ) indicate contribution from a metamorphic source. Microthermometric analysis on quartz yield a temperature range of 235-255 &ordm / C and 0.0-0.7 wt % NaCl eqv. salinity. The presence of alunite, pyrophyllite and kaolinite, vuggy quartz and covellite suggest a high-sulfidation epithermal system for Kartaldag. On the other hand, Madendag is identified as a low- sulfidation type owing to the presence of neutral pH clays and typical low temperature textures (e.g. colloform, comb, banded quartz).