Geological Features And Mineralization Of Skarn Cu-Au Deposit In Galale,Tebit

Based on a large-scale geological mapping and previous studies,the geological features and ore-controlling factors of the Galale Cu-Au deposit were systematically analyzed in this paper.The writer discussed and analyzed the typical characteristics of Skarn.the geochemical characteristics of mineralization rock,genesis of the rocks,characters and sources of the ore fluids of the deposit.Finally the writer discussed the driving mechanism of mineralization and give a modal of the deposit.The main strata outcropped in this district is dolomite and dolomitic limestone of the Jiega Formation of Lower Cretaceous,which is underlie by interbedded sandstone,siltstone and rhyolitic-dacitic volcanic of the Langjiu Formation of Lower Cretaceous.There are at least three phases of deformation,the first period formed NW-trending folds and NE direction tensional cross faults,the second one lead to NE-trending folds,NE longitudinal compression faults and small-sized tensional cross faults in a NW direction,the third period formed nearly NS-trending normal faults.According to the time order,the intrusive rocks in the mining area include Porphyritic quartz diorite of late Yanshanian,diorite-porphyrite,quartz diorite,granodiorite and granitic porphyry.The typical magnesian skarn formed near the contact zones between quartz diorite,granodiorite and dolomite,dolomitic limestones of the Jiega Formation,and the skarn contained two periods,the skarn of earlier period is composed of olivine,humite,a small amount of spinel,garnet and diopside,the wet skarn of later period is composed of serpentine,phlogopite and a small amount of epidote,superimposed on the stem skarn or the outside,form many lithologic zones,such as Olivine+spinel+humite±garnet±phlogopite zone,phlogopite+serpentine±olivine±garnet±diopside zone,serpentine+phlogopite+epidote±diopside zone,serpentine±phlogopite of marble zone.The ore body is mainly hosted in the serpentine+phlogopite zone,which is mostly described in the form of stratiform,stratoid,massive and other irregular structures.Moreover,the main metallic minerals are magnetite,chalcopyrite,bornite,chalcocite,natural gold and so on,accompanied with silicification,chloritization and carbonation.The mineralization in this region is closely related to the emplacement of granodiorite and quartz diorite,controlled by NE,NW-trending folds and relative faults.The skarns are controlled by potential lithologic such as lamellar dolomite and sandy dolomite of middle Jiega group,and the ore body located in the skarn zones where have high permeability.The granodiorite and quartz diorite which has closely relationship with mineralization belong to medium-high K calc-alkaline granite.This two kinds rocks show similar chondrite-normalized REE patterns and primitive mantle normalized trace element spectrum,they are both enriched in LILEs?Rb,Ba,K?and depleted in HFSEs?Nb,Ta,Ce,Ti?,and having the character of post-collisional magmatite.The LA-ICP-MS U-Pb shows the weighted mean ages of granodiorite is 84.07±0.66Ma and the weighted mean ages of quartz diorite is 86.52±0.44Ma.The rocks were formed in the post-collisional area of continental collision.The ore-forming fluids of Galale deposit belongs to the NaCl-H₂O system.Ore-forming fluids was in high temperature?460?⁶00??and high salinity??:14.3%²4 wt%NaCl;?:39.5%⁶7.3 wt%NaCl?in the early mineralization stage.When it comes middle of the mineralization stage,the temperature dropped to 251?⁴14?and the salinity reduced to?:5.71%¹7.52 wt%NaCl;?:35.6%⁵0.9 wt%NaCl.At last the temperature and the salinity down to 72?²75?and 4.2%¹3.1wt%NaCl in the late stage of mineralization.The H,O isotope indicated that the the water of the Ore fluids mainly came from magmatic water,and mixed with some meteoric water in the mid-late stage of mineralization.The value of?¹⁸O is 5.6‰⁸.8‰and?D is-82‰^-108.9‰,which belongs to serpentine and epidote who were formed in the main stage of mineralization.The have the same feature with rebalanced mix-magma;The value of?¹⁸O and?D of quartz and calcite is?¹⁸O:-2.5‰⁰.3‰and?D: -70.9‰^-110.4‰,which means,some meteoric water mixed in to the Ore fluids.Combined with all the understanding above,the metallogenic model is summarized.The collision between Gangdese plate and South-Qiangtang plate caused the partial melting of mantle and crust.The mixed magma which has lots of metal element moves up along the early NW and NE fractures.Forming the granodiorite and quartz diorite which has closely relationship with mineralization.The mixed magma differentiated a large number of high temperature and high salinity fluids,and the ore-forming fluids reaction with dolomite and dolomitic limestone of the Jiega.With the ore-forming fluids mixed with meteoric water,the fluids'boiling,the pressure and temperature lastly goes down and the decline of pH,resulting in the precipitation of metal element.