The Paleozoic-Mesozoic Metallogenesis In The Tongbai Area,Central China

The Qinling Orogen was mainly generated by two epochs of accretionary-collisional orogenic processes during the Paleozoic and Mesozoic,which was ascribed to the closure of the Shangdan Ocean and Paleo-Tethys Ocean,respectively.This orogenic belt is well-endowed with numerous Au,Mo,Ag-Pb-Zn deposits that predominantly formed in the Late Triassic to Early Cretaceous,with rare Paleozoic varieties documented.In addition,the mineralizing expression and superposition among different episodes of orogeny are poorly constrained.To better understand the polymetallic metallogenesis in the Tongbai area in response to Qinling composite orogeny,this thesis anatomy of three deposits,including the Huoshenmiao Fe deposit,Yindongpo Au deposit and Poshan Ag deposit.We performed detailed field investigation,petrological and mineralogical observation,geochronology,and isotopic geochemistry studies;those,when combined with previous geological data,allow us to tightly constrain the time,genetic type and tectonic setting of polymetallic deposits in the Tongbai area.Furthermore,the thesis summarizes the metallogenic condition and regularity of polymetallic deposits,which may have implications for future mineral exploration in the Tongbai area.The Huoshenmiao iron deposit is localized in the northern flank of the Heqianzhuang anticline,which is proximate to the gabbroic diorite dikes in the Tongbai area.Iron mineralization occurs as lenticular and stratiform orebodies that developed along stratification and lithological contact zones and are hosted in chlorite-amphibole schist,migmatitic gneiss,and marble of the Erlangping Group.Hydrothermal alteration and mineralization in the Huoshenmiao deposit consist of(1)prograde skarn stage represented by anhydrous silicate minerals such as garnet and diopside,(2)retrograde skarn stage forming hydrous silicate minerals such as epidote,hornblende,actinolite,(3)magnetite stage characterized by massive or disseminated magnetite and minor hematite;(4)quartz-sulfide stage marked by quartz,pyrite,and lesser amount of chalcopyrite and chlorite,and(5)carbonate stage illustrated by calcite aggregates or veins.The garnet,diopside,epidote,and quartz contain appreciable daughter mineral-bearing,polyphase fluid inclusions,suggesting a hypersaline fluid was responsible for the Huoshenmiao deposit.Two samples of hydrothermal garnets from ores yield LA-ICP-MS U-Pb ages of437±9 Ma and 437±10 Ma,which agree well with zircon U-Pb ages of 438±4 Ma and436±3 Ma for gabbroic diorite and 430±4 for granite intruded close to the Huoshenmiao deposit.The gabbroic diorite has low concentrations of Si O₂ and K₂O,with high contents of Mg O and Fe₂O₃^T,and shows similarities with intrusions associated with iron skarn deposits globally.Gabbroic diorite and granite are characterized by enriched in large ion lithophile elements(Ba,K,Sr)and light rare earth elements,depleted in high field strength elements(Nb,Ta,P,Ti),have whole-rock(⁸⁷Sr/⁸⁶Sr)_i,?_Nd(t),and zircon?_Hf(t)values of 0.7039?0.7042,3.32?4.33,and 13.0?14.9,respectively.These geochemical and isotopic characteristics suggest that the Early Paleozoic intrusions and the Huoshenmiao iron skarn deposit were affiliated with arc magmatism and magmatic-hydrothermal activity triggered by the subduction of the Shangdan oceanic plate.Gold mineralization in the Yindongpo deposit occurs as stratiform,lenticular,and subordinate vein-type orebodies hosted in carbonaceous metavolcanic-sedimentary rocks of the Waitoushan Formation,Erlangping Group.Orebodies are structurally controlled by the thrust shear zones along two limbs and dilational areas of the hinge zone of the Heqianzhuang anticline.The mineralizing succession consists of an early metallogenic epoch comprising quartz-pyrite stage,quartz-carbonate-sulfide stage and quartz-calcite stage,and a late metallogenic epoch with generating minor fluorite-pyrite veins that infill fractures of previous orebodies and wall rocks.Two samples of rutile grains from the early metallogenic epoch yield LA-ICP-MS U-Pb ages of 395±9 Ma and 400±13 Ma,which are consistent with the time of greenschist-to amphibolite-facies metamorphism of the Erlangping Group.The carbonate minerals from various ores in the early metallogenic epoch have?¹³C_V-PDB=–4.9?–3.9‰,?¹⁸O_V-SMOW=7.9?9.3‰.Quartz from pre-,syn-and post-mineralization samples have?¹⁸O values of 13.2?14.1‰,4.9?13.9‰,and 4.8?10.7‰.Fluid inclusions in auriferous pyrite yield ³He/⁴He and⁴⁰Ar/³⁶Ar ratios of 0.51?2.27 Ra and 439?1201,respectively.The magmatic isotope signatures mirror that the mineralizing fluid in the Yindongpo deposit was most likely derived from metamorphic devolatilization of the volcanic rocks of the Erlangping Group.Sulfides from the early metallogenic epoch have?³⁴S_V-CDT=–2.13?3.28‰,with Pb isotope compositions(²⁰⁶Pb/²⁰⁴Pb=17.008?17.152,²⁰⁷Pb/²⁰⁴Pb=15.402?15.493,²⁰⁸Pb/²⁰⁴Pb=38.254?38.564)agreement with that of pyrite hosted in carbonaceous metamorphic rocks.In comparison,pyrite from the late metallogenic epoch has?³⁴S=–35.96?–33.90‰and shows moderate Pb isotope variations compared with the early metallogenic epoch.Both the sulfur and lead isotopes favor that the metals in the Yindongpo deposit were sourced from metavolcanic and metasedimentary rocks of the Erlangping Group.This view is partly supported by pyrite trace element compositions:the disseminated-stratiform pyrite from carbonaceous metamorphic rocks within the deposit contains minor amounts of Au,Ag,and Pb,whereas pyrite from amphibole-chlorite schist,chlorite schist in the Tongbai area is depleted in the mineralizing elements mentioned above,suggesting that the pyrite trace elements in the peripheral metamorphic rocks have been mobilized and exhausted.Thus,the Yindongpo is an orogenic gold deposit related to collisional orogeny between the northern and southern Qinling and associated regional metamorphism and deformation in the Early Devonian.The Poshan silver deposit is localized in the southern flank of the Heqianzhuang anticline and is hosted in carbonaceous metavolcanic-sedimentary rocks of the Waitoushan Formation.Orebodies present as stratiform and lenticular shapes,which are structurally controlled by interbedded fracture zones.Mineralizing succession in the Poshan deposit comprises an early and a late metallogenic epoch.The early metallogenic epoch is composed of polymetallic sulfide stage,sulfide and silver minerals stage,and quartz-calcite stage,with generating large number of pyrite,galena,sphalerite,chalcopyrite,and minor arsenopyrite,whereas the late metallogenic epoch forms numerous argentite and native silver that cemented on the surface of massive and vein-type ores.Two samples of apatite grains from the early metallogenic epoch yield LA-ICP-MS U-Pb ages of 127±9 Ma and 127±7 Ma,while hydrothermal calcite from the late metallogenic epoch ranges U-Pb ages from 8.55±0.26 Ma to 6.78±0.13 Ma.A variety of carbonate minerals from the early metallogenic epoch have?¹³C_V-PDB=–5.8?–2.2‰,?¹⁸O_V-SMOW=5.5?13.9‰,whereas calcite from the late metallogenic epoch has?¹⁸O_V-SMOW=11.3?20.6‰.Textural evidence and O isotope analysis indicate that the injection of meteoric water leads to a significant decrease of?¹⁸O values during quartz metamorphic recrystallization.Pyrite from the polymetallic sulfide stage is enriched in Ag,Cu,Zn,Pb,and Sb,whereas pyrite from the late metallogenic epoch contains many silver inclusions.Meanwhile,Pyrite intergrown with pyrrhotite hosted in leptynite has minor inclusions with Ag content up to 1.6 ppm.Sulfides from the early and late metallogenic epochs have?³⁴S_V-CDTvalues of–1.66?4.42‰and–41.70?–6.47‰,respectively.Lead isotopes of sulfides between two metallogenic epochs show negligible differences.The multiple isotopic evidence in combination with previous studies favors that enrichment of Ag-Pb-Zn and orebodies localization in the Poshan deposit are genetically related to regional metamorphism and deformation.Additional injection of meteoric water resulted in considerable variation of quartz O isotope and associated fluid in the early metallogenic epoch.The ore-forming fluid in the late metallogenic epoch is crustal/basin fluid.The metals were mainly originated from metavolcanic-sedimentary rocks of the Erlangping Group.The Liangwan pluton intrudes the Erlangping Group adjacent to the Poshan deposit and contains large number of dark enclaves.The intrusion and dark enclaves have zircon U-Pb ages of 127±1 Ma and are geochemically characterized by high-potassium calc-alkaline to shoshonitic series with high contents of Si O₂ and Al₂O₃.They have(⁸⁷Sr/⁸⁶Sr)_i=0.7070?0.7074,?_Nd(t)=-14.68?-13.62,with zircon?_Hf(t)=-16.9?-13.8.The geochemical signatures demonstrate that the Early Cretaceous intrusion was derived from partial melting of crustal components and is genetically related to the intracontinental orogeny and/or subduction of the Paleo-Pacific plate.The Late Mesozoic magmatic activity represented by the Liangwan pluton most likely has overprinted the Poshan deposit as recorded by apatite U-Pb ages.The Late Miocene crustal/basin fluid activity was responsible for re-enriching silver and forming high-grade ores in the Poshan deposit.Taken together,the polymetallic mineralization in the Tongbai area is the response to the Qinling composite orogeny.Pre-enrichment of Au,Ag and associated elements in the Erlangping Group and formation of the Huoshenmiao iron skarn deposit were ascribed to the subduction of the Shangdan oceanic plate and accretionary orogeny.Remobilization and enrichment of Au,Ag and relevant elements and formation of the Yindongpo and Poshan deposits were linked to the regional metamorphism and deformation due to the collisional orogeny in the Devonian.The overprinting/modification of the Poshan deposit and generation of the Laowan porphyry Mo deposit in the Tongbai area were attributed to the intracontinental orogeny and/or subduction of the Paleo-Pacific plate.The Cenozoic tectonic extrusion resulted from lateral growth of the Tibet Plateau induced pulsing crustal/basin fluid activities,which was accounted for the re-enrichment of silver in the Poshan deposit.Ore deposits in the Tongbai area are structurally controlled by folds and shear zones and hosted in the metavolcanic-sedimentary rocks,with their metallogenesis marked by multiple remobilization and overprint that as a product of Qinling composite orogeny.We propose that the carbonaceous metamorphic rocks of the Erlangping Group are favorable for gold and silver polymetallic deposits in the Tongbai area.The thesis highlights the significance of Paleozoic metallogeny associated with the subduction and closure of the Shangdan Ocean.