The Alteration and Mineralization of Zhijiazhuang Iron Deposit, Hebei Province
- CHANG Zhaoshan,FENG Zhongyan
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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 porphry are discovered in the camp but only biotite granite is closely related to the mineralization. The altered wall rock is dolomite interbeded with chert stripes of the Mid-Proterozoic Gaoyuzhuang Formation. Along with the emplacement of biotite granite, the dolomite was thermally metamorphized to dolomitic marble, forsterite marble and periclase marble. The intrusive rocks underwent albitization. Four stages of contact metasomatism are noted. In the first stage, biotite 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 andradite＞540℃；diopside in calcic skarn 450～520℃；diopside in magnesian skarn 370～420℃;During retrograde alteration, the calcic akarn was replaced by epidote with minor tremolite, phlogopite, chlorite and serpentine. In magnesisan 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 mainl y in exoskarn, followed by the deposition of ludwigite which was replaced later by szaibelyite. The decrepitation temperature of magnetite is 345℃; Afterwards, minor serpentine, calcite, chlorite and pyrrhotite, pyrite, chalcopyrite, galena, sphalerite were precipitated in the sulfide stage. Most of the sulfides are found in magnesian skarn adjacent to endoskarn. The Th of fluid inclusions in calcite of this stage is 250～300℃. In the final carbonate stage, minor calcite and aragonite filled in fissures, apophyllite grew in vugs or replaced early minerals, 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 ratio in magnesian skarn,the intensity of retro grade alteration and ore petrography. According to chemical composition of inclusions in diopside, the physiochemical conditions of the fluids are infered: salinity : 51.01% NaCleq proximal, 47.87% NaCleq distal; pressure: 23 MPa,fO2 about 1/1023 Pa, pH: 7.58.