Reddish Alteration Process And Its Contribution To Gold Mineralization In Sizhuang Gold Deposit,Northwestern Jiaodong Peninsula

More than 90%of the proven gold resources,which are closely related to reddish alteration zone along NE-NNE trend structure,are located in the northwest region in Jiaodong Provience.In addition,the robust constraints on reddening alteration process and its contribution to gold mineralization are rare.Sizhuang gold deposit is sited in the south Jiaojia fault,and nearly 70%proven resources are directly hosted in reddish zone.On the basis of careful observation on space-time distribution,alteration intensity and mineral assemblages of reddish zone and and careful measurement of structural deformations,we conducted microphysiography,feldspar EPMA and LA-ICP-MS element analyses,combined with H-O isotope,multiple geochronological analysis and thermodynamics simulation to probe the physicochemical processes of reddish alteration and its contribution to gold mineralization.Reddish alteration zone along Jiaojia fault and secondary structure shows large surface distribution and liner alteration.Mineraloology studies show that the weak redsish alteration controlled by secondary structure occur in the nucleus of the plagioclase,which is converted into the albite,mica and a small amount of potassium feldspar.At the same time,hematite in this stage.The weak alteration zone along Jiaojia fault switched to strong reddish zone and produced large amount of k-feldspar during further fluid-rock reaction.The mass balance calculation shows that Si O₂,K₂O and was moved Na₂O,Ca O,Al₂O₃,TFe O,Mg O were emigrated during the reddish alteration process.Compared with magmatic K-feldspar,hydrothermal K-feldspar shows higher K₂O(average 16.02 ppm)?TFe O(average 0.05 ppm)?Ga(average 15.16 ppm)?Zn(average3.06 ppm)?Cu(average 0.51 ppm)?Pb(average 53.86 ppm),and lower Na₂O(average 0.88ppm)?Al₂O₃(average 19.33 ppm)?Sr(average 702.15 ppm)?Ba(average 4836.02 ppm)?Cs(average 0.45 ppm),which indicates that the reddish fluid is active and the mineralization element is enriched during the alteration process.Also,?Eu(average 26.06 ppm)in reddish K-feldspar was lower than that in magmatic K-feldspar(average 126.08 ppm),significantly suggesting f O₂ of reddish fluid oxygen is slightly lower than the magma f O₂.The combination of the REE in feldspar ande pyrite,and H-O isotopes show that the reddish fluid derived from high temperature Linglong magmatic fluid,it also shows that the reddish fluid source is different from metallogenic fluid.Minerology study reveals the permeability of the reddish rock were improved by the formation of the fracture of the interlocking pore in plagioclase during alteration process and cracks produced by later tectonic stress.The process of hydrothermal K-feldspar replaceing plagioclase caused rock volume expansion and fracture,and reduces rock compressive strength,which provides favorable surrounding rock conditions for late fault activity,fluid migration and materials precipitation in the ore-forming period.Mineral assemblage characteristics and thermodynamic simulation results in the mineralization stage show that the p H value 4.2 during the process of K-feldspar shifted into sericite,which is significantly lower than the p H value of the ore-forming fluid under the same temperature and pressure condition,indicating that the p H value of the system in the process of hydrothermal K-feldspar etching into sericite may further lead to gold precipitation.