Abstract
Turquoise,a hydrous phosphate of copper and aluminum,is highly valued for its unique colour and historical significance.The similarity in colour,quality,and pattern be-tween turquoises from different localities may lead to confusion in determining their origins.This is particularly evident in the case of turquoises from Meiduk in Iran and Tongling in China(Fig.1).In Iran,turquoise deposits are typically associated with magmatic zones,specifically in the Meiduk mine,located 85 kilometers northwest of the Sar Cheshmeh porphyry copper de-posit in Kerman Province.The deposit is hosted by Eocene volcanic rocks of andesitic-basalt-ic composition with porphyry-type mineralization associated with two Miocene calc-alkaline intrusive phases.Five distinct zones of the hypogene alteration include potassic,potassic-phyllic,phyllic,and propylitic zones,which are rich in magnetite.Mineralization processes include stockwork,dissemination,veinlets,and veins rich in garnet,chalcopyrite,magne-tite,and anhydrite.Turquoise is found in transitional,leached,and supergene zones,pri-marily as fracture and seam fillers.In contrast,Chinese turquoises are more often found in sedimentary rocks,with significant exceptions in places like the Tongling mine in Anhui Province,which are hosted within magmatic rocks.The turquoise from Tongling is found in the Tongling area within the Middle-lower Yangtze Metallogenic Belt,an area characterized by complex tectonics and intense magmatic and metallogenic activities.The turquoise depos-its are associated with iron-copper polymetallic mineralization within Early Cretaceous vol-canic rocks.The turquoise from Meiduk exhibited a specific gravity range of 2.22 g/cm3 to 2.71 g/cm3,SWUV fluorescence from none to medium,and LWUV fluorescence from faint to strong,indicating diverse mineralogical compositions.SEM examination of turquoise from Meiduk reveals a variety of mineral morphologies.The turquoise displays closely packed ar-rays of elongated needle-like crystals,measuring 2 μm to 9 μm in length and 0.1 μm to 0.4 μm in thickness,indicating a dynamic growth environment.The density and alignment of these crystals suggest the space-constrained setting,likely influenced by rapid deposition from mineral-laden fluids,with crystals interlocked due to simultaneous nucleation events.Some crystals appear similarly elongated but are more dispersed,with greater separation be-tween individual crystals.In other areas,the elongated turquoise crystals are tightly interg-rown,creating a dense textural appearance,pointing to a stage where growth space became limited,resulting in an interlocking matrix.Additionally,some crystals radiate outward from a central point,forming a spherical pattern reaching about 18 pm.The turquoise from Tongling show a specific gravity range of 2.26 g/cm3 to 2.60 g/cm3,with consistent medium SWUV fluorescence and strong LWUV fluorescence.SEM examination reveals needle struc-tures,plate-like structures,and spheroidal aggregates composed of needle and plate-like mi-crocrystals.These spherical aggregates,some with diameters around 26 pm,exhibit the concentric growth structure covered by turquoise microcrystals,with crystal lengths of 8 μm.The surface needle-like microcrystals vary in size,approximately 3 μm in length,2 μm in width,and 0.2 to 0.6 pm in thickness.Optical microscopy,Raman and FTIR spectroscopy reported the presence of quartz,gypsum,iron oxides such as jarosite and goethite,biotite,sericite,pyrite,galena,bornite,graphene oxide,malachite,and azurite as major associated minerals of Meiduk's.In con-trast,the Tongling mine features minerals such as quartz,anatase,barite,sodium feldspar,illite,and malachite.Through EPMA and LA-ICP-MS results,the turquoises of Tongling exhibit similar av-erage iron content(1.28％and 1.26％respectively),but significant differences in copper content.The turquoises of Meiduk have the average copper content of 6.97％,whereas Ton-gling samples show the higher content of 11.38％.Na,K,and Ca concentrations are also higher in Meiduk samples,suggesting interaction with alkali-rich fluids and potassic altera-tion.Trace elements such as Ti,Cr,Zn,Se,and Mo serve as tracing agents for Meiduk samples,while Be and W are associated with Tongling samples.Regarding rare earth ele-ments(REEs),Meiduk samples show diverse 8Ce(0.14-4.62)and 8Eu(0.65-15.78)values,indicating a wide range of oxidation states and europium anomalies.The significant variability in LREE/HREE ratios(0.39-31.74)and ΣREE concentrations(0.25-240.72 ppm)suggests heterogeneous REE fractionation patterns.In contrast,Tongling samples display δCe(0.070-2.51)and δEu(0.238-4.87)ranges,with more consistent LREE/HREE ratios(0.128-10.2)and ΣREE values(0.069-4.08 ppm),indicating stable REE fractionation dynamics.This comparative study of turquoises from the Meiduk mine in Iran and the Tongling mine in China reveals significant similarities and differences in their geochemical and minera-logical characteristics.Both deposits are hosted within magmatic rocks and produce turquoise as a byproduct in open-pit copper mining operations,exhibiting comparable colors,patterns,and morphologies.However,distinct differences are noted in their mineral compositions and trace elements,with turquoise from Meiduk associated with a complex hydrothermal system rich in various sulfide and oxide minerals and higher copper,Na,K,and Ca concentrations,indicating alkali-rich fluid interactions.In contrast,the turquoise from Tongling,influenced by both volcanic and sedimentary processes,shows a higher copper content,presence of bar-ite,and different trace elements such as Be and W.The REE patterns also highlight the con-trasting geological histories and environmental conditions at each site,with Meiduk samples showing a broader range of oxidation states and europium anomalies compared to the more stable REE fractionation dynamics in Tongling samples.These differences underscore the importance of detailed geochemical and mineralogical analyses for accurate provenance deter-mination in gemmological and archaeological contexts.
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