庐枞火山岩盆地正长岩与水反应动力学实验研究

Kinetics experimental study of syenite and water interactions in Luzong volcanic basin

为了模拟庐枞火山盆地基底的水热反应和正长辉长岩-二长岩-正长岩与水相互作用,深入剖析深部流体携带金属来源的可能性,本文利用开放流动体系的叠层流动反应器实验研究25~435℃、23~35MPa条件下二长岩-正长岩与水相互作用的化学动力学过程。研究表明,造岩元素、成矿元素和痕迹元素,在低温至水临界态的升温过程中溶解产物在水中的浓度(或溶解速率)不断变化,它们的最大溶解速率出现在不同温度。主要元素Si、Al等最大溶解速率出现在300~400℃范围,即,在跨越水的临界温度时出现溶解速率的涨落。或者说,正长岩-H2O体系中元素的溶解速率主要是温度的函数,此外,压力也对大部分元素的溶解速率产生影响,但是,对于各元素溶解速率随温度而变化的基本规律影响不大。实验重要的发现:相当于中地壳的温度即300~400℃时反应很特殊。硅酸盐矿物的硅被强烈溶失和矿物岩石会垮塌。实验表明:相当于中地壳温度(300~400℃)发生的二长正长岩与水相互作用对庐枞火山盆地深部矿床形成产生重要影响。长期的含金属流体活动造成了金属矿石的形成。水/岩反应动力学过程的温度因素:不同金属元素的最容易溶解和沉淀温度的区别导致了金属-蚀变分带性。实验还揭示,纯水与二长正长岩的反应可以淋滤出大量有色金属元素,在溶液的浓度可以到几百×10^(-9),如Ni、Mn、Cu、Zn,它们从二长正长岩里被溶解释放速率也很高。以往,研究表明火山盆地铁矿的物质来源于围岩。这次实验研究显示:水与二长正长岩的反应同样影响着盆地金属矿的形成和盆地深部、上中地壳的流体性质。实验对于理解上中地壳发生的水岩相互作用也有普适性意义。

In order to understand the mechanism of water and alkali-gabbro-pyroxene monzonite-syenite interactions in Luzong volcanic basin,and to reveal deep origin of the metal-bearing fluids,it is necessary to carry out kinetics experiments of water-rock at high temperatures and pressures.The kinetics experiments of syenite in water are performed in the temperature range of 25~435℃ and at pressures of 23~35 MPa using flow through packed bed reactor.Experimental results show that dissolution rates of various elements change with temperature from 25 to 435℃ and at pressure of 23~35 MPa.The maximum dissolution rate of many elements（Si,Al,K,Na etc.） of syenite in water occurred in the temperature range of 350 to 400℃,i.e.within the critical state region of water.The maximum dissolution rates of various elements occur at different temperatures.Another important impact factor of the reaction between syenite and water is pressure.Dissolution rates of most elements of syenite in water decrease during increasing pressure from 23 to 35 MPa.And also,flow rates affect dissolution rate of syenite in water.These results provide evidence that there are lots of metal elements in reacted solutions and the concentrations of metal elements are up to hundreds ×10^-9,such as Ni,Mn,Cu,Zn.Dissolution rates of these elements is higher.Previous studies indicate that the metal origin of iron-deposit in the Middle-Lower Yangtze River Valley is derived from hydrothermal alteration of country rock.The experimental results indicate that the interactions of syenite and water affect the formation of deposits and change the composition of the aqueous fluids in volcanic basins and in the upper and middle crust.Long term activities of metal-bearing fluids cause the formations of metal deposits.Different depositional temperature of various elements is resulting in a complex metal-alteration zoning in volcanic basins.