辽宁海城大房身稀有金属伟晶岩包裹体研究及找矿意义

A STUDY ON THE DAFANGSHEN RARE METAL PEGMATITE INCLUSION,HAICHENG, LIAONING AND ITS PROSPECTING SIGNIFICANCE

大房身3号伟晶岩脉是含铀、铌、钽等稀有金属伟晶岩矿床,分异较好,可分为4个岩相带。经对各带石英中包裹体的研究,表明含矿带含挥发份H_2O和CO_2较高,流体包裹体丰度较大,富含纯CO_2包裹体及含液相CO_2包裹体。该伟晶岩与围岩接触界线清楚,其边缘相含有围岩的捕掳体。伟晶岩结晶分异好,矿物成分简单,以长石、石英为主,其核心带产有巨晶状黑云母和微斜长石。包裹体气相成分中不含游离氧(O_2)。这些特点说明该伟晶岩是由硅酸盐熔浆在埋深大、较封闭的环境下,缓慢结晶分异而成。

The Dafangshen rare metal pegmatite deposit is situated at one place 4.7 km SE away from Haicheng county, Liaoning province. Where outcrop is widely Precambrian succession and locally Palaeozoic formations. No.3 Dafangshen pegmatite vein intrudes into the Precambrian potassic feldspar gneiss, biotite-potassic feldspar gneiss, hornblende plagioclase gneiss and hornblende gneiss. The No.3 Dafangshen pegmatite vein is a columar rock body, its cross section is nearly oval in shape. Its length is 80m, width 20m. Its strike is SE125 ?,dip angle is nearly 90 ?. It intersects the schistosisty of the schist and gneiss and there is sharp bounder line between the vein and the country rocks. The pegmatite is clearly zoned and there are four zones from outside to inward, namely: I , median grained plagioclace-quartz pegmatite zone; IT, graphic plagioclace-quartz pegmatite zone; 111, massive plagioclace ore-bearing zone; IV. microcline-quarts core zone. Inclusions are very abundant in the various zones of the pegmatite. They are of different kinds, such as solid inclusion, fluid inclusion. The former includes crystalline inclusion, melting crystal inclusion, melt inclusion (glass inclusion). The later includes liquid phase inclusion, gas phase inclusion, CO2bearing liquid phase inclusion and pure CO2 inclusion. According to the temperature measuring of the melt inclusion, it is inferred that the average petrogenetic temperature of the various zones are: zone 1, 1025 C; zone II, 9981;zone ID, 983tT and zone FV, 951t That is to say, the temperature decreases from the outer zone to the inner zone. By using the petrogenetic temperature of the various zones of the pegmatite and the density of CO2, thus the petrogenetic pressure of the various zones is obtained. They are: zone I , 3400 x 10~5Pa; zone n , 3150 x 10~5Pa; zone ffl , 2800 x 10~5; and zone N, 3000 x 10~5Pa; The salinity of the liquid phase inclusion are: zone I , 2.13 wt%; zone n, 8.6wt%; zone ffl, 13.7wt%; zone IV, 19.4wt%. That is to say, the salinity increases gradually from the outer zone to the core zone. The components of the gas phase inclusions are rich in H2O and CO2, while the liquid phase ones are rich in Na+, K+, Ca++, but are poor in Mg++ The inclusion typomorphic characteristics of the ore-bearing zone in the pegmatite are: 1), the abundance of the inclusion is large; 2), they are rich both in pure CO2 inclusions and in CO2 bearing liquid phase inclusions; 3) the gas phase components of the inclusions are rich in H2O (727.86ppm) and CO2(107.76ppm), they all are mineralizer which are profitable for the enrichment and formation of the ores such as U, Nb, Ta...the rare metal elements. All of the characteristics mentioned above have some certain significance in prospecting. The boundary between pegmatite and country rocks are clear. There are xenolithes of the country rocks in the marginal phase. Besides, the pegmatite shows well the phenominon of crystal differentiation, this manisfests that the pegmatite occurred there might be a rare metal element bearing silicate melt, which was formed slowly through crystal differentiation under high temperature and high pressure and in a somewhat close environment, it was by on means formed through the process of metasomatism.