The quartz vein-type gold deposits are widely hosted by the Neoproterozoic (Xiajiang Group) epimeta- morphic clastic rock series in southeastern Guizhou Province, China. The Zhewang gold deposit studied in this paper occurs in the second lithologieal member of the Pinglue Formation of the Xiajiang Group. Trace element geochemis- try of host rocks, quartz veins and arsenopyrite has revealed that ore-forming fluid was enriched in sulphophile ele- ments such as Au, Ag, As, Sb, Pb and Zn, and simultaneously concentrated some magmaphile elements such as W and Mo, which probably provides some evidence for multi-stage mineralization or overprinting of magmatic hydro- therm. Quartz veins and arsenopyrite were characterized by depletion in HFSE and enrichment in LREE. Hf/Sm, Nb/La and Th/La imply that the ore-forming fluid was probably a NaC1-H20 solution system enriched in more C1 than F; Th/U values reflect the strong reducibility of the ore-forming fluid, coincident with the sulfide assemblages. The values of Co/Ni reflect that magmatic fluids may have partly participated in the ore-forming process and Y/Ho values have proved that the ore-forming fluid was associated with metamorphism and exotic hydrotherm which has reformed former quartz veins during late mineralization. The concentrations of REE, Eu anomalies and Ce anomalies of this deposit display that ore-forming elements mainly were derived from host rocks and possibly from a mixed deep source, and the ore-forming fluid was mixed by dominant metamorphic fluid and minor other sources. The physical-chemical conditions of ore-forming fluid changed from the initial stage to the late stage. The metamorphic fluid is responsible for the mineralization. Therefore, the Zhewang gold deposit is classified as a quartz vein-type gold deposit which may have been reformed by magmatic fluids during the late stage. 相似文献
The U-Pb and Sm-Nd dating of deep crustal rocks from the Bergen Arcs system helps resolve enigmatic aspects of the tectonic evolution of the Caledonian Orogen in western Norway and yields insights into the arrested stages of eclogite development within the granulites of the area. The U-Pb dating of zircon from one of the eclogite facies shear zones yields an upper intercept age of 945 ± 5 Ma [all errors two standard deviations (2σ)], which is similar to other zircon ages from the granulite facies protolith. The age is interpreted to represent the time of late Proterozoic (Sveconorwegian) granulite metamorphism. The U-Pb ages of sphene and epidote show that the eclogites formed early in the evolution of the Caledonian Orogen (pre-Scandian phase) at about 460 Ma. An eclogite facies quartz vein yields a Sm-Nd whole rock-garnet isochron of 440 ± 12 Ma that may reflect the onset of cooling immediately after peak eclogite facies conditions, although the Sm-Nd systematics reveal some isotopic disequilibrium within the sample. In tandem with previous 40Ar/39Ar age determinations from, an adjacent eclogite of 450 Ma for hornblende and 430 Ma for muscovite, these data indicate that < 30 Ma elapsed between formation of the eclogites and the initial stages of cooling and exhumation to at least mid-crustal levels. This corresponds to minimum cooling rates of 14 °C/m.y. The timing relations suggest that the formation and exhumation of these eclogites from the overlying Caledonian Nappe wedge in western Norway are related to an early phase of crustal subduction during or somewhat before the major phase of continent-continent collision.
The short period of time between the formation of the eclogites and the initial stages of exhumation and rapid cooling is consistent with the only partial and localized transformation of the granulite to eclogite. Isolated occurrences of eclogite within the granulite, the formation of eclogite along metasomatic fronts and the formation of hydrous eclogite facies minerals within the “dry” granulite all point to the importance of fluids in the transformation and re-equilibration of the granulite to eclogite. Together, field and isotopic data demonstrate that both the localized and limited access of fluids and the rapid cycling of continental crust through the deepest portions of the orogen to upper crustal levels resulted in the preservation of the arrested stages of eclogite formation and survival of the granulites metastably through eclogite facies conditions. 相似文献