西秦岭大桥金矿硅质岩硅氧同位素特征及成因探讨

Si-O Isotopic Characteristics and Genesis of the Siliceous Rocks from the Daqiao Gold Deposit in West Qinling

西秦岭大桥金矿床位于岷县-宕昌和舟曲-成县大断裂之间,金矿体赋存于三叠系下部灰岩、泥质板岩岩性不整合界面处的硅质角砾岩中。硅质角砾岩呈似层状、板状和透镜状产出,倾向于断裂带分布两侧,矿石构造以角砾状、被膜状、皮壳状、细脉-网脉状、孔隙状构造为主。矿石矿物主要为黄铁矿、白铁矿、磁黄铁矿、毒砂、闪锌矿、方铅矿、黄铜矿,蚀变以硅化、绢云母、方解石为主,矿化元素组合为Au、Sb、Ag、As、Hg、W。利用氧同位素地质温度计估算的硅质岩形成温度为142～218℃,硅质岩δ^(30)Si值为-0.7‰～0.4‰,δ^(18) O为12.8‰～17.9‰,与热水-热泉华型金矿床地质特征及同位素特征一致。综合研究认为,大桥金矿硅质岩为热水-热泉形成,硅质角砾岩是在断裂带的周期性破裂-自封闭条件下,由流体超压引发的岩石破裂形成。

The Daqiao gold deposit is situated between the Minxian-Dangchang and Zhouqu- Chengxian regional faults in west Qinling. The ore-body is hosted within the siliceous breccia at the lithological unconformity interface between limestone and argillaceous slate in the lower part of Triassic strata. The breccia shows layered, tabular and lenticular shapes, occurring in the both side of the fault. The ore structures include primarily breccia, quilt-membrane, crusty, vein- stockwork and pore. The ore minerals are mainly pyrite, binarite, pyrrhotite, arsenopyrite, sphalerite, galena and chalcopyrite. The alterations of country rocks are composed of silicification, sericitization and calcilization. The element associations related to the mineralization are Au, Sb, Ag, As, Hg and W. By the oxygen isotope thermometer, the estimated formation tempera-tures of these siliceous rocks are around 142-218℃, their 330 Si and 318 0 values are - 0. 7‰-0. 4‰ and 12.8‰-17.9‰, respectively, which are similar to the ones of the hot spring gold deposits. Combined with other geological characteristics, it＇ s believed that the siliceous rocks are formed by rock fracturing due to fluid overpressure under the hot-spring environment nearby the periodic fault zone.