Research On The Miaoliang Cryptoexplosive Breccia And Its Relation To Gold Mineralization In The Shanyang-zhashui Area,Shanxi Province

Miaoliang gold deposit is a cryptoexplosive breccia-type gold deposit discovered in Shanyang-Zhashui area of South Qinling in recent years.The formation of the cryptoexplosive breccia and the gold deposit is closely related to the intermediate-acid porphyry.No systematic study has been carried out since the discovery of Miaoliang gold deposit.On the basis of field geological researches,this paper studies the formation mechanism of the cryptoexplosive breccia and the genesis of the deposit by means of petrochemistry and fluid inclusions thermometry.The main conclusions are as follows:?1?There are two types of mineralization in this area:altered cryptoexplosive breccia-type and altered structural quartz vein-type.The main ore bodies is distributed in the cryptoexplosive breccia and controlled by its shape,and the ore bodies outside breccia is mainly controlled by structure.There are three metallogenic periods and five metallogenic stages in this deposit.The main gold bearing minerals are electrum,it occurs as intergranular gold and inclusion gold enclosed in pyrite or pyrrhotite.?2?There are three kinds of granitoids in the study area:granodiorite porphyry,biotite granite and quartz porphyry.Granodiorite porphyry belongs to high-K calc-alkaline series,they show metaluminous and display typical I-type granite characteristics,its initial magma was mainly derived from the partial melting of the lower crust;biotite granite exists in the cryptoexplosive breccia pipe in the form of breccia,belonging to shoshonite series and peraluminous granitoids,having the characteristics of typical I-type granite,and the origin of this granite is from crust-mantle mixed;quartz porphyry belongs to shoshonite series and weakly peraluminous granitoids,its initial magma was mainly derived from the crust.The trace elements are enriched in Rb,U,K,Nd and depleted in Nb,Ta Ti and P.The distribution curve of rare earth elements is generally inclined to the right,LREE is enriched and HREE is depleted,and there is no obvious Eu anomaly.?3?According to the contact relationships between pluton and its isotopic dating results,there are at least three stages of magmatic activity in the study area,with the ages of 172.3Ma for biotite granite,153.3Ma for quartz porphyry and 151.1¹48.9Ma for granodiorite porphyry.The forming mechanism of breccia and mineralization is as follows:quartz porphyry moves along the regional fault zone,cryptoexplosion occurs in the deep magma chamber,resulting in cryptoexplosive breccia formed and located in the subsurface.At the same time,the early biotite granite is brecciated.After that,granodiorite porphyry?vein?successively intrudes into the cryptoexplosive breccia,carrying a large number of ore-bearing hydrothermal fluids,resulting in multiple alteration and mineralization.The metallogenic age is after 148.9 Ma.?4?The fluid inclusions in the ore-bearing quartz vein are generally small.The type of inclusions is mainly gas-liquid two-phase?liquid-rich?inclusions,a small amount of gas-rich inclusions and three-phase inclusions containing CO₂.The coexistence of high-salinity and fluid-rich inclusions,gas-rich inclusions and three-phase inclusions suggests that fluid boiling happens.In the main metallogenic stage,the metallogenic temperature is mainly concentrated in 240²70?,and the salinity is5.86¹5.86%,belonging to the medium-low temperature,medium-low salinity and volatile-rich of H₂O-NaCl system.