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

摘要

Niuxinshan is a typical example of the numerousmesothermal gold deposits formed during Mesozoic tectono-magmatic reactivation of the Archean North China Craton ineastern Hebei province. Gold occurs in quartz-sulfide lodes inArchean amphibolites and also in greisen zones in the MesozoicNiuxinshan granite stock. Four mineralization stages can berecognized from early to late: (1) quartz-K-feldspar, (2) quartz-pyrite, (3) quartz-polysulfide, and (4) quartz-carbonate. Goldmineralization mainly occurs in stages 2 and 3. Fluid inclusions inquartz and fluorite from greisen zones in the Niuxinshan granite,and inclusions in vein quartz and sphalerite from stages 1 to 3 inthe amphibolites, have been studied by microthermometry. Threecompositional types of inclusions are recognized: type 1 (Tpl) areH2O-CO2-bearing inclusions and include primary (Tpl-P) andsecondary (Tpl-S) inclusions. These are found in quartz andfluorite from the greisen zones as well as in vein quartz andsphalerite from stages 1 to 3. The Tpl-P inclusions are consideredto represent the gold-bearing hydrothermal fluids. Type 2 (Tp2-S)are secondary H2O-CO2+solid phase inclusions in fluorite fromthe greisen zones. Type 3 (Tp3-S) are secondary aqueousinclusions with a solid phase which coexist with the Tp2-S influorite from the greisen zones. The Tp1-P inclusions showvariable VCO2 (commonly 0.3 to 0.6) and XCO2 values (mainly 0.1to 0.4). The salinities of inclusions cluster around 3 to 11 wt.%NaCl equivalent and their homogenization temperature to theliquid phase (Th(L)) fall dominantly in the range of 260 to 360℃.The compositional variations of inclusions in stage1 probably result from exsolution of magmatic fluidsat various stages; immiscibility or boiling of thefluids can be ruled out. The compositional variatinosof inclusions in the greisen zones and in vein stages2 and 3 are attributed to cooling, mixing (dilution),and necking-down of the fluids. The Tp1-S and Tp2-S inclusions show salinities of 3 to 6 wt.% NaClequivalent and XCO2 values of 0.04 to 0.17. Th(L)clusters at 240 to 260℃. The Tp3-S inclusions havesalinities of 3 to 6 wt.% NaCl equivalent and Th(L)of 170 to 240℃. Isochoric reconstructions, combinedwith oxygen and sulfur isotope geothermometry ofmineral pairs, give trapping P-T conditions for thegold-bearing fluids. The greisen zones formed at 310to 460℃ and 1.3 to 3.7 kbar; stage 1 veins at 300 to430℃ and 1.2 to 3.7 kbar; stage 2 veins at 290 to 380℃ and 1 to 3 kbar; stage 3 veins at 250 to 350℃ and1 to 3 kbar. H2P-CO2 fluids with low to moderatesalinities and moderate to high densities (0.66 to1.01 g/cm3) dominated at early mineralization stages,and evolved towards H2O-richer and CO2- and less salinefluids through time. The retrograde P-T evolutionprobably resulted from regional uplift and cooling ofgold-bearing hydrothermal fluids. The gold bisulfidecomplex was dominant in the fluids duringmineralization and gold deposition was mainly inducedby decreases of temperature and pressure, as well asdestabilization of the bisulfide complex during sulfidization of wall rocks.