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

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.