Gold Metallogenesis In Northeastern Hunan,Jiangnan Orogen

Northeastern Hunan,located in the central of Jiangnan orogenic belt,is an important part of Jiangnan orogenic gold metallogenic belt.The region has experienced a long evolutionary history and multistage tectonic activity,forming a series of gold deposits（spots）.Despite the favorable geological conditions of metallogenesis,a large amount of studies have been investigated on gold deposits in northeastern Hunan,and have obtained considerable achievements.However,the multi-period of metallogenesis leads to various understandings of the metallogenic dynamics,the origin of ore-forming fluids and materials on the gold deposits in the region,which has seriously restricted the study of metallogenesis.With the large-scale gold mining in the region,the shallow resources are nearly depleted,and there is an urgent need to expand the search object.The Xiaojiashan gold deposit is a newly discovered gold deposit in the Liling area of northeastern Hunan,and the investigation for this deposit is significantly lower than that of other gold deposits in northeastern Hunan.The deposit is located between the Changsha-Pingjiang Fault and the Liuyang-Hengdong Fault,and hosted in the low-grade metamorphic slate and greywacke of Huanghudong Formation of the Neoproterozoic Lengjiaxi Group.The orebodies are controlled by the NE-trending structure,and the gold-bearing quartzankerite veins/veinlets are formed in the inter-/intra-layer detachment faults and ductile to brittle shear zones,which is a typical tectonically controlled gold deposit in the region.The deposit is characterized by multi-stage gold mineralization.The significant differences in the morphology and mineral assemblages of each stage gold-bearing quartzankerite veins,and the geochemical information make the Xiaojiashan gold depsit a good window to study the same type of gold deposits in the region.The identification of the metallogenesis and genesis type and the establishment of metallogenic model for the Xiaojiashan gold deposit can not only provide a new scientific basis for the investigation of this type of gold deposit,but also have an important reference for the research of the surrounding deposits as well as the regional prospecting.In this study,taking the Xiaojiashan gold deposit in northeastern Hunan as the case,based on the detailed geological investigation of the deposit,we carried out research on genetic mineralogy,fluid inclusions,stable isotopes and radioisotope chronology to better constrain the gold mineralization and genetic type of the Xiaojiashan gold deposit and to establish the genetic model.The main results of this study are as follows:（1）Based on the vein cross-cutting relationships and mineral assemblages,the gold mineralization at Xiaojiashan can be divided into three stages,namely: I)quartz-ankerite-pyrite-gold,II)quartz-ankeritepolymetallic sulfide-gold;III)quartz-ankerite.The stage I and II are the primary stages of gold mineralization.（2）The EPMA major elements and LA-ICP-MS trace element analysis of the main gold-bearing minerals pyrite（fine-grained porous Py1 and coarse-grained Py2 from stage I,core-rim-textured Py3,core Py3 a and rim Py3 b,from stage II）indicate that the composition of oreforming fluids has a significant variation from stage I to stage II.The oreforming fluids in stage I are relatively enriched in Co,Ni,Se and Te,but depleted in As and Au.Comparatively,the ore-forming fluids in stage II have higher As,Au,and Zn,but lower Co,Ni,Se and Te.Combined with the sulfur isotopic compositions of Py1,Py2,Py3 a and Py3 b,the Au-and As-poor Py1 and Py2 are associated with the metamorphic fluids at depth,while the Au-As-rich Py3 a and Py3 b may have the participation of the deep magmatic fluids.The EPMA major elements and LA-ICP-MS trace element analysis of the arsenopyrite（Apy）,chlorite（Chl1,Chl2 and Chl3）and sphalerite（Sph）indicate that the upper temperature of 234 to 381℃ and 266 to 343℃ for the stage I and stage II,respectively.The mineralization environment is reducing（lgfO₂: -53.9 to ～-₂7.9,-52.2 to -34.1）and medium-acidic environments with the sulfur fugacity（lgfS2）of-14.8 to -8.3 and -11.4 to -8.5 for the stage I and stage II.The mineralization temperature,oxygen fugacity and p H have a decreasing from Py1 to Py2 and Py3 a to Py3 b,respectively.（3）The fluid inclusions studies of quartz associated with gold mineralization suggest that the hydrothermal fluids in stage I,II and III are H₂O-CO₂-NaCl（±CH4±N2）、H₂O-NaCl（-CO₂）and H₂O-NaCl fluid systems,respectively,with the fluid inclusion homogeneous temperatures ranging from 186 to 266℃,219 to 275℃ and 143 to 204℃,and salinities ranging from 3.19 to 7.86 wt.% NaCl equiv.,6.62 to 12.62 wt.%NaCl equiv.and 3.23 to 6.59 wt.% NaCl equiv.The ore-forming fluids of stage I,II and III are under condition of medium-low temperature and（medium-）low salinity.The temperature and salinity of stage II are higher than those of stage I.The mineralization pressure is estimated by the isovolume method and the homogeneous temperature of CO₂-bearing inclusions,ranging from 110.1 to 150.7 MPa and 85.7 to 103.9 MPa,respectively,corresponding to mineralization depths of 4.16 to 5.69 km and 3.24 to 3.92 km,which are mesozonal-hypozonal environments.（4）The H-O isotopic composition indicate that the ore-forming fluids are mainly derived from metamorphic fluids,and a magmatic fluids is probablely involved with the stage II.The sulfur isotopic compositions（by in-situ and single mineral）of pyrite in stage I have wide-range and low δ34S values（-33.30～-16.23‰）,in which the δ34S values of -33.30 to -16.23‰ and ～-₂5.41 to -15.45‰ for Py1 and Py2,respectively.Theδ34S values in stage I are significantly lower than that of ore-hosting strata（Neoproterozoic Lengjiaxi Group,δ34S = -13.1～-10.4‰）,indicating the sulfur is probably sourced from a metamorphic sulfur released from the strata at depth.The δ34S values of stage II are similar for the magmaticrelated sulfur reservoir（δ34S = 0±3‰）,and markedly different from those of stage I,with the δ34S values of -3.44～-1.72‰ and -1.00～-0.99‰for Py3 a and Py3 b,respectively,indicating the possible incorporation of magmatic sulfur for the stage II.The Pb isotopic analysis suggests that the Pb isotopic compositions of stage I and stage II are similar and have the charactered by the upper crust.The Pb source may be that the oreforming fluids extracted Pb from the strata of different end elements（deeper than Lengjiaxia Group）during the migration of ore-forming fluids.Comprehensive analysis of H-O-S-Pb isotopic data of gold deposits in northeastern Hunan indicate that the ore-forming materials and fluid are mainly metamorphic fluid system,and magmatic fluid system may be involved.（5）Au element in the Xiaojiashan gold deposit occur as the form of native gold in the voids,fractures and margins of pyrite and arsenopyrite,and as the form of invisible gold within the crystal lattice of pyrite and arsenopyrite.Au elements mainly migrate in the form of complex Au（HS）～-₂ in ore-forming fluids of stage I and II.The sulfidation of wall rocks,the precipitation of ankerite and the ore-forming fluids immiscibility are responsible for precipitation of gold ores,of which fluids immiscibility is an important factor in the formation of native gold.（6）The intrusive age of diabase cut by gold-bearing quartz-ankerite veins of stage I at depth is 221.5±0.8 Ma,providing a maximum age constraint for gold mineralization.The rutile age from gold-bearing quartz-ankerite veins of stage II yield the LA-TQ-ICP-MS U-Pb age of216.9±1.1 Ma.The above results show that the age of gold mineralization for Xiaojiashan gold deposit are ～ 216 Ma,indicating that the gold mineralization at Xiaojiashan was formed in the Late Triassic.Combined with this study and the mineralization age of gold deposits in northeastern Hunan,it is concluded that the Late Triassic is a significant period of gold mineralization in northeast Hunan.（7）Based on this study and regional data,it is considered that the formation of Xiaojiashan gold deposit is closely connected with Indosinian orogeny of South China.By comparing typical orogenic and intrusive-related gold deposits worldwide,the genetic type of Xiaojiashan gold deposit is considered to be orogenic gold deposit.