Metallogeny of the Baiyangping Lead-Zinc Polymetallic Ore Concentration Area, Northern Lanping Basin of Yunnan Province, China

The Lanping Basin in the Nujiang-Lancangjiang-Jinshajiang （the Sanjiang） area of northeastern margin of the Tibetan Plateau is an important part of eastern Tethyan metallogenic domain. This basin hosts a number of large unique sediment-hosted Pb-Zn polymetallic deposits or ore districts, such as the Baiyangping ore concentration area which is one of the representative ore district. The Baiyangping ore concentration area can be divided into the east and west ore belts, which were formed in a folded tectogene of the India-Asia continental coUisional setting and was controlled by a large reverse fault. Field observations reveal that the Mesozoic and Cenozoic sedimentary strata were outcropped in the mining area, and that the orebodies are obviously controlled by faults and hosted in sandstone and carbonate rocks. However, the oreforming elements in the east ore belt are mainly Pb-Zn -Sr-Ag, while Pb-Zn-Ag-Cu-Co elements are dominant in the west ore belt. Comparative analysis of the C-O-Sr-S-Pb isotopic compositions suggest that both ore belts had a homogeneous carbon source, and the carbon in hydrothermal calcite is derived from the dissolution of carbonate rock strata; the ore- forming fluids were originated from formation water and precipitate water, which belonged to basin brine fluid system; sulfur was from organic thermal chemical sulfate reduction and biological sulfate reduction; the metal mineralization material was from sedimentary strata and basement, but the difference of the material source of the basement and the strata and the superimposed mineralization of the west ore belt resulted in the difference of metallogenic elements between the eastern and western metallogenic belts. The Pb-Zn mineralization age of both ore belts was contemporary and formed in the same metaliogenetic event. Both thrust formed at the same time and occurred at the Early Oligocene, which is consistent with the age constrained by field geological relationship.

Chronological Constraints on Late Paleozoic Collision in the Southwest Tianshan Orogenic Belt, China: Evidence from the Baleigong Granites

The Baleigong granites, located in the western part of the southwestern Tianshan Orogen(Kokshanyan region, China), records late Paleozoic magmatism during the late stages of convergence between the Tarim Block and the Central Tianshan Arc Terrane. We performed a detailed geochronological and geochemical study of the Baleigong granites to better constrain the nature of collisional processes in the Southwest Tianshan Orogen. The LA-ICP-MS U-Pb zircon isotopic analyses indicate that magmatism commenced in the early Permian(~282 Ma). The granite samples, which are characterized by high contents of SiO2(67.68-69.77 wt%) and Al2O3(13.93-14.76 wt%), are alkali-rich and Mg-poor, corresponding to the high-K calc-alkaline series. The aluminum saturation index(A/CNK) ranges from 0.93 to 1.02, indicating a metaluminous to slightly peraluminous composition. Trace element geochemistry shows depletions in Nb, Ta, and Ti, a moderately negative Eu anomaly(δEu=0.40-0.56), enrichment in LREE, and depletion in HREE((La/Yb)N=7.46-11.78). These geochemical signatures are characteristic of an I-type granite generated from partial melting of a magmatic arc. The I-type nature of the Baleigong granites is also supported by the main mafic minerals being Fe-rich calcic hornblende and biotite. We suggest that the high-K, calc-alkaline I-type granitic magmatism was generated by partial melting of the continental crust, possibly triggered by underplating by basaltic magma. These conditions were likely achieved in a collisional tectonic setting, thus supporting the suggestion that closure of the South Tianshan Ocean was completed prior to the Permian and was followed(in the late Paleozoic) by collision between the Tarim Block and the Central Tianshan Arc Terrane.

Tin transport and cassiterite precipitation from hydrothermal fluids

Cassiterite(SnO_(2))is the main ore mineral of tin in magmatic-hydrothermal tin deposits,but tin transport and precipitation mechanisms from hydrothermal fluids remain poorly understood.We critically evalu-ated aqueous tin speciation in hydrothermal fluids from extensive experimental data and thermody-namic modeling.Sn(II)chloride complexes in hydrothermal fluids exist mainly as SnCl^(+),SnCl_(2)(aq),and SnCl_(3).The revised Helgeson-Kirkham-Flowers model parameters of these three tin species and two tin ions(Sn^(4+) and Sn^(2+))were derived from the correlation algorithms among these parameters,and the standard molar properties of cassiterite were optimized to be internally consistent with the available thermodynamic dataset.These thermodynamic parameters,together with the available equilibrium con-stant equation of Sn(IV)chloride complexes,could reproduce the available solubility data of cassiterite in acidic solutions at 400-700℃under oxygen fugacity(f_(o2))levels buffered by hematite-magnetite(HM)or nickel-nickel oxide(NNO).These comparisons allow modeling chemical systems of SnO_(2)-NaCl-HCI-H_(2)O(liquid phase)to examine tin transport and cassiterite precipitation mechanisms under tin-mineralizing conditions:300--500℃,50-150 MPa,2 molal NaCI,and fo。levels from QFM(quartz-fayalite-magnetite)to HM.Sn(I)chloride complexes are commonly interpreted to dominate in aqueous tin speciation under f_(o2)=NNO,but our modeling results indicate that considerable contents of Sn(IV)chloride complexes also exist in those reduced fluids with high HCI contents,consistent with recent in situ high-temperature experiments and molecular dynamic simulations.The Sn(I)/Sn(IV)ratios in fluids depends onfo,temperature,and HCl contents.A considerable amount of Sn(IV)possibly exist in an early mineralization stage even under f_(o2)=NNO;if so,redox reactions are unnecessary to precipitate cassiterite from these mineralizing fluids.We find that even if the f_(o2) levels are constant,simple cooling can alter mineralizing fluids to be more oxidized(e.g.,from QFM to HM)and cause cassiterite precipitation,indicating that oxidizing agents are not necessary as previously thought.This explains why cassiterite can precipitate in host rocks(e.g.,sandstone or quartzite)that do not provide oxidizing agents.A simple rise in f_(o2),levels and pH neutralization(e.g.,greisenization)also cause cassiterite precipitation.Cassiterite solubility in oxidized acidic hydrothermal fluids(NNO<f_(o2),<HM)is high enough to account for the tin contents of fluid inclusions from typical tin deposits,but the mineralization potential of oxdized fluids is inferior to reduced fluids(f_(o2),≤NNO)under the same conditions.

Numerical Modeling of Deformation at the Baiyun Gold Deposit,Northeastern China:Insights into the Structural Controls on Mineralization

The Qingchengzi ore field is an important gold-polymetallic center of the North China Cra-ton.It has been recognized that the gold deposits in Qingchengzi were controlled by structures like litho-logical interfaces and fractures along mechanically weak bedding and foliation planes,but it still remains poorly understood how the structures affected the localization of the gold deposits.Finite element based numerical modeling was used to reproduce the deformation process of the Baiyun gold deposit during the mineralization period.Paleoproterozoic schist and marble are widely exposed in Qingchengzi,and a large part of the Baiyun gold ores occurs along the interfaces between the schist and the marble.The modeling results suggest that the mechanical contrast between the schist and the marble may be a major reason why the stress was localized along their lithological interfaces under a compressional stress regime.Two parts of their lithological interfaces were identified to be easily stress-localized and first fractured:the interface between the schist and its underlying marble at shallower levels and the one between the schist and its overlying marble at deeper levels.Stress concentration in these two parts is independent on the dipping angle and direction of the interfaces.Therefore,mineralizing fluids may have been concentrated into these two parts.The first one is consistent with the present ore bodies of the Baiyun gold deposit,and the second one could be considered for deep prospecting.These findings also provide implications for the structural controls of lithological interfaces on the mineralization in other gold deposits of this region.