西秦岭大水金矿床方解石Sm-Nd等时线年龄及其地质意义

大水金矿床位于西秦岭造山带南缘,矿床中金矿化与硅化、去碳酸盐化和方解石化密切相关。硅化主要表现为石英或玉髓交代(白云质)灰岩,在石英中常见灰岩或白云质灰岩的交代残余碎屑,在石英颗粒内或裂隙中见有自然金颗粒,说明硅化、去碳酸盐化和金矿化是同步的;成矿期的脉状-网脉状载金方解石主要分布于硅化-去碳酸盐化蚀变带的边缘或附近裂隙中,说明方解石的结晶与金矿化同步或稍晚。对该矿床成矿期的8件载金方解石样品进行了Sm-Nd同位素定年体系研究,方解石的147Sm/144Nd值变化于0.069 489~2.243 873,143Nd/144Nd值变化于0.511 673~0.514 369,获得Sm-Nd等时线年龄为(189.4±1.4)Ma,代表了大水金矿床的成矿年龄。大水金矿床的成矿时限与秦岭造山运动相关的碰撞型花岗闪长岩岩浆活动时限一致;此外,前人的稀土元素地球化学、黄铁矿Co/Ni比值及H-O同位素研究亦表明成矿物质来源与花岗闪长岩岩浆活动关系密切,因而本次获得的大水金矿床成矿年龄,在一定程度上为"大水金矿床的金成矿与花岗闪长岩岩浆活动存在成因联系"提供了年代学证据。

Calcite Sm-Nd Isochron Age of the Dashui Gold Deposit in West Qinling Orogenic Belt and Its Implications

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.