The Study Of Early Cretaceous Skarn Mineralization In Longyang Ore Concentration Area, Baoshan Block, Sanjiang Region, SW China

The Longyang ore concentration area is located in the northern Baoshan block,Sanjiang region,SW China,in which the Early Cretaceous skarn Fe-Cu-Pb-Zn-Au deposits are widely developed.They display a horizontal zonation that Fe-Cu-Pb-Zn deposits in the center and Au deposits in the margin.This paper mainly focuses on the representative Hetaoping skarn Pb-Zn deposit,Jinchanghe skarn Fe-Cu-Pb-Zn deposit and Heiniuwa skarn Au deposit in this area,to trace the origin of ore-forming materials,construct the process of ore-forming fluid evolution,and reveal metal precipitation mechanism.The skarn Fe-Cu-Pb-Zn mineralization?represented by Hetaoping and Jinchanghe?in the center displays a vertical zonation that includes,from the bottom to top,ilvaite skarn associated with Fe mineralization,garnet skarn associated with Cu mineralization and pyroxene and actinolite skarn associated with Pb-Zn mineralization.Fluid inclusion study shows that the ore-forming fluids of Hetaoping and Jinchanghe are a dominantly NaCl-H₂O system,with minor CO₂ and N₂.The fluids in both deposits have similar evolution histories,they are characterized by high temperature?>500°C?and high salinity?>17 wt%NaCl equiv?in prograde stage,and evolved to moderate temperature and moderate salinity in retrograde stage,then to moderate-low temperature and moderate-low salinity in sulfide-ore stage,and finally to low temperature?¹00°C?and low salinity?<1 wt%Na Cl equiv?in post-ore stage.The temperatures and salinities generally display a positive correlation.The?⁶⁶Zn values of sphalerites from Jinchanghe are 0.07–0.19‰,significantly lower than that of the wall rock(?⁶⁶Zn=0.28-0.39‰),revealing that zinc is from magma.In addition,C-O-S isotopic studies also demonstrate that the ore-forming fluids are originated from magma,with a significant amount of meteoric water and wall rock carbon and sulfur mixed into the hydrothermal system during their evolution.The mixing of low-temperature and low-salinity meteoric water was considered to be the most important factor controlling the precipitation of metal,whereas fluid boiling and rock-water reaction were suggested to have played subordinate roles in ore deposition.The skarn Au mineralization in the margin is represented by Heiniuwa Au deposit.Based on field investigation and petrographic and mineralogical observations,three main mineralization stages have been identified in this deposit,which developed pyrite and arsenopyrite in stage I,and developed pyrrhotite,chalcopyrite,sphalerite and galena in stage II and barren quartz and calcite in stage ?.Fluid inclusion studies show that eutectic temperatures vary from–48.5°C to–36.7°C,indicating the ore-forming fluid of Heiniuwa is a typical H₂O–NaCl–CaCl₂ fluid system.From stage I to stage ?,the temperature and salinity of ore-forming fluid decrease continuously,showing a positive correlation with the value of X_{Na Cl}?NaCl/?NaCl+CaCl₂??.Combined with oxygen isotopic analysis,the temperature and salinity decrease from stage I to stage ? were led by the mixing of meteoric water.The Na/Ca ratios of ore-forming fluid also decrease from stage I to stage ? suggests that the interaction with carbonate wallrock is significant during fluid evolution,remobioling the Ca ion from carbonate wallrock.The?³⁴S values of sulfides from Heiniuwa are-5.7‰to4.6‰,indicating that the sulfur has a magmatic origin,with a little sulfur from wall rock.The LA-ICP-MS analysis of sulfides show that stage I pyrites are the main gold-bearing mineral.In summary,the Fe-Cu-Pb-Zn mineralization in center and Au mineralization in margin in Longyang ore concentration area both belong to the Early Cretaceous skarn system.The ore-forming fluid originated from exsolution of beneath concealed pluton,which is characterized by high temoerature and high oxygen fugacity,form skarn Fe-Cu-Pb-Zn mineralization in proximal zone.A part of ore-forming fluids migrating to distal zone,with temperature and oxygen fugacity decreasing,develop Au mineralization.