Geochronology and ore-forming fluids in the Jinchang skarn gold-copper deposit,Yinan County,western Shandong Province

The Jinchang skarn Au-Cu deposit is located in Yinan County,western Shandong Province,China.It is hosted by Neoproterozoic to Cambrian sedimentary clastic sequences intercalated with limestone and marble units,intruded by adamellite and granitic porphyries.In the skarn facies,andradite and diopside comprise the main prograde skarn minerals,whereas epidote,chlorite,tremolite,actinolite and calcite are the principal retrograde skarn minerals.Single grain and low background Rb-Sr isochrone dating on biotite from prograde and retrograde skarn gives 133 ± 6Ma and 128 ± 2Ma,respectively.Four types of fluid inclusions were distinguished:(1) daughter mineral-bearing inclusions;(2) vapor-rich inclusions;(3) CO2-H2O inclusions,and(4) salt-saturated inclusions.Fluid inclusions associated with garnet(prograde skarn stage) display high homogenization temperatures and high salinities(530 ～570℃;39.9% ～60.4% NaCl equiv.).Fluid inclusions associated with epidote(retrograde skarn stage) also record higher homogenization temperatures and salinities(451 ～ 580℃;39.3% ～ 60.7% NaCl equiv.).Fluid inclusions associated with quartzsulfide stage show two intervals of homogenization,320 ～ 450℃ and 108 ～ 291℃.Some fluid inclusions in garnet contain hematite or magnetite daughters,suggesting an oxidizing magmatic environment.Overall,the Jinchang deposit is interpreted as an oxidized Au-Cu skarn based on the mineralogy and fluid inclusion characteristics.