Geology, Pb Isotope Geochemistry and Ore Genesis of the Liziyuan Gold Deposit, West Qinling Orogen, Central China
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The Liziyuan gold deposit, situated on the south side of the Shangdan suture zone, West Qinling Orogen, occurs in metamorphic volcanic rocks (greenschist facies) of the early Paleozoic Liziyuan Group and in Indosinian Tianzishan monzogranite. Orebodies in the Liziyuan gold field are controlled by the ductile‐brittle shear zone, and by thrusting nappe faults related to the Indosinian orogeny. In detail, this paper analyzed the geological characteristics of the Liziyuan gold field, and the Pb isotopes of the Lziyuan host rocks, granitoids (Tianzishan monzogranite and Jiancaowan syenite porphyry), sulfides, and auriferous quartz veins by multiple‐collector inductively coupled plasma mass spectrometry (MC–ICP–MS). In addition, previous data on the sulfur, hydrogen, and oxygen isotopes were employed to discuss the possible sources of the ore‐forming fluids and materials, and to further understand the tectonic setting of the Liziyuan gold deposit. The sulfides and their host rocks (Lziyuan Group), Tianzishan monzogranite and Jiancaowan syenite porphyry, and auriferous quartz veins have similar Pb isotopic compositions. Zartman's plumbotectonic model diagram shows that most of the data for the deposit fall near the orogenic Pb evolutionary curve or within the area between the orogenic and mantle Pb evolutionary curves. In the Δβ‐Δγ diagram, which genetically classifies the lead isotopes, most of the data fall within the range of the subduction‐zone lead mixed with upper crust and mantle. This indicates that a complex source of the ore lead formed in the orogenic environment. The δ 34 S values of the sulfides range from 3.90 to 8.50‰ (average 6.80‰), with a pronounced mode at 5.00‰–8.00‰. These values are consistent with that of orogenic gold deposits worldwide, indicating that the sulfur sourced mainly from reduced metamorphic fluids. The isotopic hydrogen and oxygen compositions support a predominantly metamorphic origin of the ore‐forming fluids, with possible mixing of minor magmatic fluids, but the late stage was dominated by meteoric water. The characteristics of the Liziyuan gold deposit formed in the Indosinian orogenic environment of the Qinling Orogen are consistent with those of orogenic gold deposits found worldwide.Keywords:
Orogeny
Greenschist
Isotope Geochemistry
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The Meguma group of lithic greywacke, feldspathic quartzite, slate siltstone, and argillite is Early Ordovician or older in age and has undergone both regional and contact metamorphism. Both types of metamorphism have resulted in recrystallization and locally in orientation of newly formed minerals. Metasomatism and retrogressive metamorphism are subordinate and only locally important. Regionally metamorphosed rocks are divided into greenschist and almandine–amphibolite facies, although some assemblages cannot be assigned with certainty. Locally, biotite and garnet isograds are mappable within the greenschist zone.Relationships between regional metamorphism and structural elements (folding) show that deformation preceded regional metamorphism. Intrusion of granitic rocks has produced a zone of contact metamorphism (hornblende–hornfels facies) that is superimposed upon regional greenschist facies rocks, which shows that granite emplacement occurred after the regional grade was reached. Gold–quartz veins are confined to areas lying in the greenschist zone of regional metamorphism, which suggests that the almandine–amphibolite zone is not favorable.
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Greenschist
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Abstract In the Mona Complex of Llŷn, a narrow, steep belt of fine grained schists known as the Penmynydd Zone of Metamorphism lies between a deformed melange (Gwna Group) and a unit of gneisses and plutonic rocks (Sarn Complex). Rocks within and adjacent to this Zone are characterized by mylonitic textures. The highest grade Penmynydd rocks on Llŷn reached greenschist and, locally, glaucophanitic greenschist fades conditions. The schist belt is interpreted as a major shear zone formed as the Sarn Complex moved up against the lower grade Gwna Group. A previous interpretation of the schists (Shackleton 1956) as being the result of a prograde metamorphism from sub‐greenschist facies Gwna Group to anatectic granitic and gneissic rocks of the Sarn Complex is rejected.
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To investigate factors affecting the evolution of nitrogen system in rocks during progressive metamorphism, unmetamorphosed to greenschist-facies pelitic samples were collected from both pelitic and (pelitic members of) psammitic rock sequences along two transects of the Taiwan mountain belt. Among all samples, greenschist-facies metapelites from the Tananao metamorphic basement of this mountain belt exhibit the most variable and depleted nitrogen content and δ15N values. The observations are difficult to interpret and can only be accounted for by complicated tectonic/metamorphic history of the basement complex. On the other hand, irrespective of the different metamorphic grades, samples from the metamorphosed cover strata, which have been subjected to only one stage of metamorphism, show comparable nitrogen and N-isotope composition. The only exception is that greenschist-facies metapelites from the pelitic members of psammitic rock sequence (i.e., the G(Ps) group samples) of the cover strata are enriched in the 15N isotope by at least 1‰. Taking Al content as a normalizing factor, averaged N/Al and LOI/Al show systematic decrease through progressive metamorphism. Given the non-homogeneous nature of the sedimentary protoliths, the comparable N-isotope composition among the unmetamorphosed to zeolite-facies (i.e., S-Z) group, prehnite-pumpellyite-facies (i.e., PP) group and greenschist-facies (i.e., G) group samples from pelitic rock sequences can be explained by rock devolatilization with limited nitrogen depletion (<10%). High δ15N composition of the G(Ps) group samples might be mainly due to more extensive nitrogen depletion (∼25%). It is suggested that during metamorphism, the pelitic rock sequence may approximate a closed system, while thick psammitic rock sequence may behave more like an open system, facilitating nitrogen depletion and N-isotope fractionation even for thin pelitic members within a thick psammitic sequence. Lithology, in addition to other factors, therefore plays a role in controlling the evolution of the nitrogen system during low-grade metamorphism. The present study implies that element transport by dehydration fluid in subduction zones may largely take place along fluid channel ways.
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Pelite
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Metamorphic minerals are widely developed in Laoling Group of southern Jilin Province.Based on a detailed analysis of the metamorphic minerals in rock samples,the authors hold that the main minerals in lower sub-group of Laoling Group can be divided into two generations:M1 and M2,which represent two metamorphic stages.Researches on metamorphic assemblage,distribution characteristics and division of metamorphic facies show that M1 is characterized by low-to-moderate temperature regional dynamic heat flow metamorphism which can be divided into two metamorphic facies:lower greenschist facies and high greenschist facies.By contrast,M2 is represented by local thermal metamorphism which can be divided into three metamorphic facies:lower greenschist facies,high greenschist facies and lower amphibolitic facies.Comprehensive studies show that metamorphism of the sub-group of Laoling Group in southern Jilin Province is characterized by reduction of influencing area and decrease of intensity from early to late.This tendency suggests hat the earth's crust became stable in Paleo Proterozoic.
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Several dolomitic marble lenses were found at the western margin of the metadiorite subzone in the central basic belt of the Brno Massif. Phase relations and mineral compositions indicate two metamorphic events in surrounding amphibolites. Thermometric estimations indicate metamorphic conditions of early metamorphism in epidote amphibolite facies conditions (for 6 kbar T = 650 - 690 °C) and later in greenschist facies conditions. Mineral assemblages in marbles: Tr + Cal ± Chl ± Qtz and Tr + Dol ± Chl indicate metamorphism only in greenschist facies. Tremolite formed throuhg metasomatic reactions of SiO2-rich hydrothermal fluids with dolomite in centre of marbles boudins. This process is related to tectonic deformation of pure dolomitic marbles during metamorphism in greenschist facies.
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Metasomatism
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continental collision
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