西藏甲玛铜多金属矿夕卡岩特征及成因意义

Characteristics and the genesis implication of skarn in the Jiama copper-polymetallic deposit,Tibet,China

甲玛铜多金属矿为西藏冈底斯成矿带中段的超大型矿床。通过研究矿床夕卡岩岩石学特征、岩石化学特征、稀土元素和微量元素地球化学特征,显示夕卡岩主要受早白垩世林布宗组角岩、板岩和晚侏罗世多底沟组大理岩之间的层间构造带所控制,主要呈层状、似层状产出,夕卡岩由上至下具有夕卡岩角岩-石榴子石夕卡岩-硅灰石夕卡岩-夕卡岩化大理岩的分带特征。夕卡岩主要为钙夕卡岩,具有弱铈异常、弱铕负异常和轻稀土富集等特征,同时富集大离子亲石元素(Rb、K、U、Th),并亏损Ta、Nb和Ti等高场强元素。夕卡岩地球化学特征受大理岩、花岗岩地球化学特征的制约和影响,指示夕卡岩与大理岩、花岗岩成因关系密切。角岩作为热液屏蔽层是夕卡岩连续分布的有利条件,矿区推覆-滑覆构造作用形成的角岩-大理岩层间扩容空间是夕卡岩铜多金属矿体大规模分布的关键因素。在成因上不支持"海底喷流沉积型矿床"观点,应该是与岩浆热液作用有关的接触交代型矿床。

The Jiama copper-polymetallic ore deposit is a super large ore deposit in the central Gangdese.The petrologic,petrochemical,rare earth and trace element characteristics of the skarn are studied in this paper.The skarn is distributed over the interformational detachment zone between Late Jurassic Duodigou Formation and Lower Cretaceous Linbuzong Formation as bedded and stratoid forms.It is distributed from top to bottom as a zoning of skarnization hornfels,garnet skarn,wollastonite skarn and skarnization marble.It is the calcareous skarn type,characterized by minor negative Eu anomalies,minor negative Ce anomalies and LREE enrichment,as well as the enrichment of large ion lithophile elements（LILE） such as Rb,K,U and Th and depletion of high field strength elements（HPSE） such as Ta,Nb and Ti.The geochemical characteristics of the skarn are restricted by that of marbles and affected by that of granites.This indicates that the skarn has something to do with marbles and granites.As a shielding layer from hydrothermal solution permeation,the hanging hornfels is favourable for forming the consecutive skarn.The lithologic interface between hornfels and marbles,a expanding space formed by the nappe and detachment movement,is a key factor for the large-scale distribution of the large skarn-type copper-polymetallic ore bodies.It is concluded that the Jiama deposit is not a submarine exhalation-sedimentation type but a magmatic hydrothermal contact metasomatic type ore deposit.