The alteration and mineralization in Zhijiazhuang skarn iron deposit, Hebei province

The Zhijiazhuang skarn iron deposit is in Laiyuan County, Hebei Province and is now being mined in open pit. Biotite granite, porphyritic granite, alaskite and diorite porphyry are discovered in the sampl but only biotite granite is closely related to the mineralization. The altered wall rock is dolomite interbeded with chert stripes of the Mid-Peroterozoic Gaoyuzhuang Formation. Along with the emplacement of biotite granite the dolomite was thermally metamorphosed to dolomic marble, forsterite marble and periclase marble. The intrusive rocks underwent albitization. Four stages of contact metasomatism are noted. In the first stage, biorite granite was altered to calcic skarn which consisted mainly of diopsidic pyroxene, andradite and trace schorlomite, seyrigite, while the marbles were replaced by diopside, forsterite and minor spinel which comprised the magnesian skarn. The Th of inclusions in andrdite is >540 degree C; diopside in calcic skarn 450-520 degree C; diopside in magnesian skarn 370-420 degree C. During retrograde alteration, the calcic skarn was replaced by epidote with minor tremolite phlogopite, chlorite and serpentine. In magnesian skarn, chondrodite replaced forsterite, fine grained diopside and minor tremolite replaced diopside, then they all were replaced by phlogopite, chlorite and serpentine. Magnetite began to deposit after the formation of phlogopite and before serpentinization mainly in exoskarn, followed by the deposition of ludwigite which was replaced later by szaibelyite. The decrepitation temperature f magnetite is 345 degree C; afterwards minor serpentine, calcite, chlorite and pyrrhotite, pyrite, chalcopyrite, galena, sphalerite were precipitated in the sulfide stage. Most of the sulfides were found in magnesian skarn adjacent to endoskarn. The Th of fluid inclusions in calcite of this stage is 250-300 degree C. In the final carbonate stage, minor calcite and aragonite filled in fissures, apophyllite grew in vugs or replaced early minerals, and brucite replaced periclase. The size of exoskarn is much larger than that of endoskarn. Roughly the skarns are zoned with respect to diopside/andradite ratio in calcic skarn, diopside/serpentine ration in magnesian skarn, the intensity of retrograde alteration and ore petrography. According to chemical composition of inclusions in diopside, the physiochemical conditions of the fluids are inferred: salinity 51.01% NaCleq proximal, 47.87% NaCleq distal; pressure 23 MPa; fO2 about 10-23 Pa; pH 7.58