西藏多不杂斑岩铜(金)矿床磁铁矿元素地球化学特征及其对成矿过程的指示

Elemental Geochemical Characteristics of Magnetite and Its Indication of Mineralization Process in the Duobuza Porphyry Copper(Gold)Deposit,Tibet

多不杂铜(金)矿床是西藏多龙矿集区重要的斑岩型铜矿床之一。详细的岩心编录和岩相学研究显示,多不杂铜(金)矿床发育4类磁铁矿:磁铁矿-1(Mt_(1))反射色呈灰白色,它形粒状,部分颗粒包含在黑云母内部;磁铁矿-2(Mt_(2))反射色呈粉棕色,半自形-它形粒状,边缘被赤铁矿交代,颗粒内部见少量黄铜矿;磁铁矿-3(Mt_(3))反射色呈粉棕色,自形-半自形,粒度小,表面平整,主要产于角岩化蚀变内;磁铁矿-4(Mt_(4))反射色呈深灰色,颗粒间隙被黄铁矿、黄铜矿交代。Mt_(1)、Mt_(2)属岩浆磁铁矿或岩浆-热液磁铁矿的过渡类型;Mt_(3)、Mt_(4)属岩浆-热液磁铁矿的过渡类型。Mt_(1)、Mt_(2)、Mt_(4)磁铁矿形成温度大致在300~500℃,Mt_(3)形成温度明显低于其他三类磁铁矿,大致在200~500℃。4类磁铁矿具有明显的地球化学差异,其中Mt_(1)具高Ti、V,低Mn、低氧逸度的特征;Mt_(2)具有低V,高Mn、高氧逸度的特征;Mt_(3)具有低Ti,低Mn、Al,且Mn、Al含量与Ti呈正相关的特征;Mt_(4)具低V、高Mn、Si、Al,高氧逸度的特征。其中Mt_(1)形成时间最早,而Mt_(3)形成时间最晚。高Mn和低V的磁铁矿是多不杂矿床铜矿化的指示标志。

The Duobuza copper(gold)deposit is an important porphyry copper deposit located in the Duolong ore district of Tibet.Detailed core logging and petrographic studies show that the Duobuza copper(gold)deposit develops four types of magnetite.Magnetite-1(Mt_(1))with grayish-white reflective color,anhedral grain,and some are trapped in the biotite.Magnetite-2(Mt_(2))with pinkish-brown reflective color,subhedral to anhedral grain,with edges that are replaced by hematite,and containing a small amount of chalcopyrite.Magnetite-3(Mt_(3))is pinkish-brown,euhedral to anhedral grain,small in size with smooth surface,and mainly occurs in hornfelization alteration.Magnetite-4(Mt_(4))is dark gray,and its fissures and intergranular spaces are replaced by pyrite and chalcopyrite.Mt_(1) and Mt_(2) belong to the magmatic magnetite or magmatic-hydrothermal magnetite transitional types,while Mt_(3) and Mt_(4) belong to the magmatic-hydrothermal magnetite transitional types.Mt_(1),Mt_(2),and Mt_(4) magnetite formed at temperatures ranging from roughly 300℃to 500℃,while Mt_(3) formed at temperatures ranging from roughly 200℃to 500℃,which is significantly lower than those of the other three types of magnetite.The four types of magnetite have marked geochemical differences.Mt_(1) is characterized by high Ti and V,low Mn,and low oxygen fugacity;Mt_(2) is characterized by low V,high Mn,and high oxygen fugacity;Mt_(3) is characterized by low temperature,low Mn,Al,and positive correlation between Mn and Al content and Ti;Mt_(4) is characterized by low V,high Mn,Si,Al,and high oxygen fugacity.Mt_(1) is formed first,while Mt_(3) is formed last.The magnetite with high Mn and low V is an indicator of copper mineralization in the Duobuza deposit.