Petrogenetic implications of the mineral-chemical characteristics of scheelite and associated phases from Miniki Gol (Chitral), NW Pakistan

摘要

The stratabound tungsten mineralization in Chitral, northern Pakistan, lies within the Hindu Kush terrane and is located to the north-west of the Main Karakoram Thrust (MKT) that represents the suture zone between the Kohistan complex and Asian plate. The area has undergone amphibolite facies metamorphism followed by retrogression and at least two episodes of deformation. Scheelite occurs mainly in the calc-silicate quartzite and subordinate tourmalinite associated with metapelite of the Jurassic Arkari Formation at Miniki Gol. The scheelite host rocks are intruded by leucogranite, which is exposed ~400 m east of the studied tungsten mineralization. The leucogranite was emplaced after the culmination of the amphibolite facies metamorphism. The scheelite- bearing calc-silicate rock consists of clinozoisite, quartz, calcicamphibole, plagioclase, chlorite, biotite, calcite, sphene and garnet. This mineral assemblage correlates with a skarn-type environment. Scheelite is intergrown with clinozoisite in the calc-silicate rock and also associated with tourmaline and spessartine-rich garnet in the tourmalinite. The extensive tourmalinisation leading to the formation of abundant (i.e., up to 80 modal%) tourmaline in the scheelite-bearing tourmalinite appears to have been caused by a post-magmatic hydrothermal activity. The mentioned leucogranite in the area might have acted as a source of fluids for this borondominated metasomatic process. The abundant development of epidote, making up to 60% by volume of the calc-silicate rock, signifies its formation through a hydrothermal phenomenon. The chemical compositions of tourmaline, clinozoisite, sphene, garnet and scheelite from the calc-silicate rock also support their hydrothermal origin. Similarly, the compositional characteristics of both tourmaline and spessartine-rich garnet from the tourmalinite indicate the precipitation of these phases from evolved acidic magmaderived fluids. More importantly, the investigated scheelite contains significant amounts of Zr (up to 0.46 wt% ZrO_2) and Ta (up to 0.35 wt% Ta_2O_5), which are believed to replace W in scheelite from post-magmatic environments. As all the mentioned minerals are associated and some of them even intergrown with scheelite, it appears that the tungsten mineralization also is genetically linked to the solidification and emplacement of leucogranite in the area. Although itself generally unmineralized, the Miniki Gol leucogranites average W content (5.3 ppm) is higher than that of low-Ca granites.