江西永平Cu-W矿床白钨矿地球化学特征及其对矿床成因的指示

Trace element geochemistry of scheelites from Yongping Cu-W deposit in Jiangxi:Implications for ore genesis

江西永平Cu-W矿床是钦杭成矿带东部一个大型Cu-W矿床。针对该矿床的成因,一直存在着海底喷流沉积型与矽卡岩型矿床的争论。文章针对该争论,通过对永平Cu-W矿床的白钨矿开展微量元素分析,研究了成矿流体性质、来源和矿床成因。永平Cu-W矿床发育3种类型白钨矿:退化蚀变阶段暗色均质白钨矿Ⅰ-1;亮色均质白钨矿Ⅰ-2;石英-硫化物阶段具有环带结构的白钨矿Ⅱ。白钨矿中Mo含量和Eu异常能够指示成矿流体氧化还原性。白钨矿Ⅰ-1富集Mo元素,并呈负Eu异常,指示氧化性;白钨矿Ⅰ-2和白钨矿Ⅱ中Mo含量减少,并且呈正Eu异常,指示成矿流体的氧逸度降低。永平Cu-W矿床所有白钨矿均呈明显的轻稀土元素富集模式,与典型矽卡岩型白钨矿稀土元素特征相一致,而明显不同于石英脉型矿床白钨矿中稀土元素或重稀土元素富集模式。白钨矿具有高Mo和低Sr元素,与岩浆-热液白钨矿特征一致,而明显不同于变质来源的白钨矿,指示成矿流体来源于岩浆。白钨矿的Y/Ho比值范围为19~43,与似斑状黑云母花岗岩(Y/Ho=25~30)相似,明显不同于石炭系叶家湾组(Y/Ho=34~75),指示成矿流体主要来源于岩浆。白钨矿地球化学特征指示永平Cu-W矿床为矽卡岩型矿床。

The Yongping deposit is a large-sized Cu-W deposit in the Qin-Hang metallogenic belt.The ore genesis of submarine exhalative sedimentary versus skarn types has been debated for a long time.The trace element concentrations of scheelite from the Yongping deposit were analyzed for constraining mineralization conditions and the genesis.There are three types of scheelite from the Yongping deposit:(1)homogeneous scheelite with dark shade in CL image from retrograde alteration stage(SchⅠ-1);(2)homogeneous scheelite with light shade in CL image from retrograde alteration stage(SchⅠ-2);and(3)scheelite with oscillatory zoning from the quartz-sulfide stage(SchⅡ).Mo and Eu substitution is of particular relevance,and shows redox sensitivity.The SchⅠ-1 is Mo enriched and shows negative Eu anomaly,indicating an oxidized fluid,whereas the SchⅠ-2 and SchⅡare Mo depleted and show positive Eu anomaly,indicating a reduced fluid.All scheelites from the Yongping deposit have relative LREE-enriched chrondrite-nomalized rare earth element patterns,which are consistent with those from the skarn deposits but are different from those in metamorphic deposits.The high values of Mo and low values of Sr in scheelite suggest a magmatic-hydrothermal origin instead of metamorphic origin.The similarity in Y/Ho ratios between scheelites and granite indicates that the ore-forming fluid was derived from the granite.In summary,these data strongly support that the Yongping deposit is a skarn-type deposit.