Abstract:

The Dashui gold deposit is located at the southern margin of West Qinling orogenic belt. Gold mine-ralization has close relationship with silicification, decarbonation and calcitization. Silicification is mainly chara-cterized by quartz or chalcedony replacing (dolomitic) limestone; residual debris of limestone or dolomitic limestone is commonly observed in quartz; natural gold exists in quartz grains or fractures, which indicate that the silicification, carbonation, and gold mineralization are synchronous. The veinlet-mesh-vein gold-bearing calcites of the ore-forming stage are mainly distributed at the edge of the silicification-decarbonation alteration zone or near the fractures, indicating the crystallization of calcite is coeval with the gold mineralization or a little later. The Sm-Nd isotopic system of eight gold-bearing calcite samples from this deposit was studied. The ¹⁴⁷Sm/¹⁴⁴Nd values and ¹⁴³Nd/¹⁴⁴Nd values of these calcites range from 0.069 489 to 2.243 873, and 0.511 673 to 0.514 369, respectively. All calcite samples fell on one line and yielded a precise Sm-Nd isochron age of (189.4±1.4) Ma with a MSWD=1.8. This age can represent the time of gold mineralization of the Dashui gold deposit. In addition, the mineralization age of the Dashui gold deposit is coincident with the intrusion of granodiorite magma, which is related to the Qinling Orogeny. What's more, rare earth elements geochemistry, Co/Ni ratios of pyrite and H-O isotope of calcite from previous studies also suggest that the source of ore-forming materials has close relationship with this granodiorite magmatism. Thus, the calcite Sm-Nd isochron age presented in this study provides chronological evidence to the argument that the gold mineralization of the Dashui gold deposit has a genetic relationship with the granodiorite magmatism.

Key words: West Qinling, Dashui gold deposit, calcite, Sm-Nd isochron