西秦岭南亚带层控金硒矿床的赋矿地层年代与成矿时代

REIDENTIFICATION OF THE ORE-HOSTED STRATA AGE AND METALLOGENIC AGE OF THE STRATABOUND Au-Se DEPOSITS, THE WESTERN QINLING MOUNTAINS

西秦岭南亚带层控金硒矿床的赋矿地层为太阳顶群 ,为一套主要由炭质硅岩和炭质板岩组成的硅岩建造。对于太阳顶群的时代 ,至今仍众说纷纭 ,莫衷一是 ,大多数人认为属晚寒武世—奥陶纪 ,也有认为属晚震旦—早寒武世。作者通过对古杯类化石的鉴定以及Rb Sr、Sm Nd同位素等时线年龄测定后认为 ,赋矿的太阳顶群时代应属早寒武世 ,而非晚寒武世—奥陶纪 ,更不可能为晚震旦世。对金硒矿床中 2个成矿阶段石英样品进行40 Ar/ 3 9Ar快中子活化测定后获得马鞍型谱线特征。其中坪年龄 (71 .4 5± 0 .5 5 )～ (80 .0 5± 0 .6 5 )Ma、最小视年龄 (70 .2 4± 1 .93)～ (78.5 9± 2 .85 )Ma和等时线年龄 (6 9.4 5± 0 .38)～ (81 .6 1± 0 .5 5 )Ma均十分接近 ,说明所测 2件石英样品的40 Ar/ 3 9Ar年龄真实可靠 。

The stratabound gold and selenium deposits in the south subzone of the western Qinling Mountains include the Laerma (Edu) and the Qiongmo gold and selenium deposits and the Yaxiang gold and selenium ore occurrence. They constitute a new type of gold deposits discovered in China, which possess unique geological characteristics. They are hosted in Taiyangding Group, which is composed of a series of carbonaceous cherts and slates. However, the ore-hosted strata age has been confused because of lacking fossil and dependable data of isotopic age, which is identified by comparison between structure and lithology. The recently tendentious idea about the ore-hosted strata age is the Late Cambrian—Ordovician or the Late Sinian—the Early Cambrian. However, identification of the Archaeocyathus in chert in the lower Taiyangding Group shows that the fossil age ranges from the Early to Middle Cambrian. Combining with Rb-Sr and Sm-Nd isochron ages, the authors conclude that the ore-hosted strata age is the Early Cambrian. Two quartz samples collected from the Qiongmo Au-Se deposit in the western Qinling Mountains were measured by ^(40)Ar/^(39)Ar fast neutron activation techniques, and the spectra are characterized as being saddle-shaped. The sample measurements show a plateau age of (71.45±0.55)～(80.05±0.65) Ma, a minimum apparent age of (70.24±1.93)～(78.59±2.85) Ma and an isochron age of (69.45±0.38)～(81.61 ±0.55) Ma, and the three ages being close to each other, indicating that the ages of the quartz samples so far measured are reliable. The authors suggest that the time of formation of Au-Se ores corresponds to the Late Yanshan period.