Fluid inclusion microthermometry and the P-T evolution of gold-bearing hydrothermal fluids in the Niuxinshan gold deposit, eastern Hebei province, NEChina

Niuxinshan is a typical example of the numerous mesothermal gold deposits f ormed during Mesozoic tectono-magmatic reactivation of the Archean North Ch ina Craton in eastern Hebei province. Gold occurs in quartz-sulfide lodes i n Archean amphibolites and also in greisen zones in the Mesozoic Niuxinshan granite stock. Four mineralization stages can be recognized from early to late: (1) quartz-K-feldspar. (2) quartz-pyrite, (3) quartz-polysulfide, and (4) quartz-carbonate. Gold mineralization mainly occurs in stages 2 and 3. Fluid inclusions in quartz and fluorite from greisen zones in the Niuxinsh an granite, and inclusions in vein quartz and sphalerite from stages 1 to 3 in the amphibolites, have been studied by microthermometry. Three composit ional types of inclusions are recognized: type 1 (Tp 1) are H2O-CO2-bearing inclusions and include primary (Tp1-P) and secondary (Tp1-S) inclusions. T hese are found in quartz and fluorite from the greisen zones as well as in vein quartz and sphalerite from stages 1 to 3. The TF l-P inclusions are co nsidered to represent the gold-bearing hydrothermal fluids. Type 2 (Tp2-S) are secondary H2O-CO2 + solid phase inclusions in fluorite from the greisen zones. Type 3 (Tp3-S) are secondary aqueous inclusions with a solid phase which coexist with the Tp2-S in fluorite from the greisen zones. The Tp1-P inclusions show variable V-CO2 (commonly 0.3 to 0.6) and X-CO2 values (main ly 0.1 to 0.4). The salinities of inclusions cluster around 3 to 11 wt.% Na Cl equivalent and their homogenization temperatures to the liquid phase (Th (L)) fall dominantly in the range of 260 to 360 degrees C. The compositiona l variations of inclusions in stage 1 probably result from exsolution of ma gmatic fluids at various stages; immiscibility or boiling of the fluids can be ruled out. The compositional variations of inclusions in the greisen zo nes and in vein stages 2 and 3 are attributed to cooling, mixing (dilution) , and necking-down of the fluids, The Tp1-S and Tp2-S inclusions show salin ities of 3 to 6 wt.% NaCl equivalent and X-CO2 values of 0.04 to 0.17. Th(L ) clusters at 240 to 260 degrees C. The Tp3-S inclusions have salinities of 3 to 6 wt.% NaCl equivalent and Th(L) of 170 to 240 degrees C. Isochoric r econstructions, combined with oxygen and sulfur isotope geothermometry of m ineral pairs, give trapping P-T conditions for the gold-bearing fluids. The greisen zones formed at 310 to 460 degrees C and 1.3 to 3.7 kbar; stage 1 veins at 300 to 430 degrees C and 1.2 to 3.7 kbar; stage 2 veins at 290 to 380 degrees C and 1 to 3 kbar; stage 3 veins at 250 to 350 degrees C and 1 to 3 kbar. H2O-CO2 fluids with low to moderate salinities and moderate to h igh densities (0.66 to 1.01 g/cm(3)) dominated at early mineralization stag es, and evolved towards H2O-richer and CO2- and less saline fluids through time. The retrograde P-T evolution probably resulted from regional uplift a nd cooling of gold-bearing hydrothermal fluids. The gold bisulfide complex was dominant in the fluids during mineralization and gold deposition was ma inly induced by decreases of temperature and pressure, as well as destabili zation of the bisulfide complex during sulfidization of wall rocks.