Age and metal source of orogenic gold deposits in Southeast Guizhou Province,China: Constraints from Re-Os and He-Ar isotopic evidence

The orogenic gold deposits in Southeast Guizhou are an important component of the Xuefeng polymetallic ore belt and have significant exploration potential, but geochronology research on these gold deposits is scarce. Therefore, the ore genetic models are poorly constrained and remain unclear. In the present study, two important deposits(Pingqiu and Jinjing) are investigated, including combined Re-Os dating and the He-Ar isotope study of auriferous arsenopyrites. It is found that the arsenopyrites from the Pingqiu gold deposit yielded an isochron age of 400 ± 24 Ma,with an initial ~(187)Os/~(188)Os ratio of 1.24 ± 0.57(MSWD = 0.96). An identical isochron age of 400 ± 11 Ma with an initial ~(187)Os/~(188)Os ratio of 1.55 ± 0.14(MSWD = 0.34) was obtained from the Jinjing deposit. These ages correspond to the regional Caledonian orogeny and are interpreted to represent the age of the main stage ore. Both initial ~(187)Os ratios suggest that the Os was derived from crustal rocks. Combined with previous rare earth element(REE), trace elements, Nd-Sr-S-Pb isotope studies on scheelite, inclusion fluids with other residues of gangue quartz, and sulfides from other gold deposits in the region, it is suggested that the ore metals from Pingqiu and Jinjing were sourced from the Xiajiang Group. The He and Ar isotopes of arsenopyrites are characterized by ~3 He/~4 He ratios ranging from 5.3 × 10~(-4) Ra to 2.5 × 10~(-2) Ra(Ra = 1.4 × 10~(-6), the ~3 He/~4 He ratio of air), 40 Ar=/~4 He ratios from 0.64 × 10~(-2) to 15.39×10~(-2), and ~(40)Ar/~(36)Ar ratios from 633.2 to 6582.0. Those noble gas isotopic compositions of fluid inclusions also support a crustal source origin,evidenced by the Os isotope. Meanwhile, recent noble gas studies suggest that the amount of in situ radiogenic ~4 He generated should not be ignored, even when Th and U are present at levels of only a few ppm in host minerals.     

湘东北金井地区花岗岩成因及地球动力学暗示：岩石学、地球化学和Sr-Nd同位素制约

出露于湘东北地区的燕山期花岗质岩岩体,呈岩基或岩株状广泛侵位于中元古界冷家溪群浅变质碎屑沉积岩中, 部分岩体为白垩纪红层覆盖。文章详细研究了金井岩体的岩石学、地球化学及Sr-Nd同位素特征,并与其围岩冷家溪群进行了对比。该岩体具有较高的SiO2(71．44-74．31wt．％)、K2O(3．86-4．98wt．％)和Na2O(3．24-3．84wt．％),以及较低的 FeO Fe2O3 MgO TiO2(均小于3％),结合高含量的Al2O3(13．25-14．89wt．％)及高ASI值(普遍大于1．1),为典型的强过铝质(SP)高钾钙碱性花岗岩。金井花岗岩富硅、碱和高Sr、Ba丰度、高LILE／HSFE和LREE／HREE比值,低镁铁质及REE 含量,表明金井岩体为高度分异的花岗岩,其源区的残留物中黑云母含量高。REE配分图和原始地幔多元素标准化图谱上 Rb、Th、U、K和LREE相对富集,而Ba、Sr和高强场元素(Nb、Ta、P和Ti)相对亏损,Eu可变的负异常(δEu:0．53-0．78),表明存在角闪石、黑云母、堇青石、磷灰石和钛铁矿等分异演化的影响,类似于同碰撞型花岗岩地球化学特征。高Rb／Sr(3-6)、低 Sr／Ba(0．2-0．7)、高P2O5含量(0．09-0．15wt．％)、低Al2O3／TiO2比值(40-70<100)、Zr含量(100-150ppm)以及FeO／MgO 等特征表明该过铝质花岗岩浆可能是在高温(±800℃)高压(-8kbar)奈件下、经中下地壳变质沉积岩黑云母脱水熔融形成。 CaO／Na2O的差异(>0．4和<0．2两组)以及Nd同位素的不同(高Nd和低Nd两组),类似于围岩冷家溪群砂质和泥质沉积岩特征,以及高Sr[(87Sr／86Sr)i=0．72204-0．72540]、低Nd[εNd(145 Ma)=-10--12]等均表明花岗质岩浆主要起源于中元古代冷家溪群变质砂岩(高Ca)和变质泥岩(低Ca)的部分熔融或中下地壳物质的熔融,但有少量地幔物质的加入。结合印支期以来华南及湘东北地区地球动力学演化史,作者认为金井燕山期花岗岩形成于陆内碰撞造山晚期,是160-140Ma 陆内俯冲(A型俯冲)碰撞造山作用增温减压体制达到鼎盛阶段的产物,标志湘东北地区由此全面转入陆内伸展阶段。