小岩浆大流体成大矿与透岩浆流体成矿作用——以东秦岭-大别山成矿带钼矿床为例

"Small" magma and "big" fluid lead to form large scale deposit and transmagmatic fluid mineralization:Take for example of Mo deposits in eastern Qinling-Dabie mountain metallogenic belt

岩浆活动与内生金属成矿作用关系密切,备受国内外矿床学家的关注。表现为它们在时空上具有广泛的一致性,成矿与岩浆岩有关,成矿物质由同源岩浆分异演化而来。这便是著名的岩浆热液成矿理论,也称岩浆期后热液成矿理论,该理论把内生金属成矿系统看作是一个理想系统。东秦岭-大别山地区的钼矿-花岗岩关系研究表明,钼成矿与小岩体(小岩浆)关系密切,而大岩体/基与钼矿没有成生联系;地质事实表明,大型-超大型矿床往往广泛发育了大规模的热液(流体)蚀变(大流体),具有大的流体/岩浆比,其矿化蚀变范围是小岩体的几十甚至几百倍,表明成矿过程中必有外来流体的广泛参与。由于小岩体往往没有经过强烈的分异结晶作用,质量平衡计算表明,小岩浆体不可能产生足够数量的含矿流体和成矿物质;因此,成岩与成矿有本质的区别,成矿系统应是一个非线性的复杂性动力学系统。研究表明,东秦岭-大别山小斑岩体来源较深(下地壳),成矿流体来源于地幔,二者呈双层结构;岩浆实际上是沟通深部和浅部的通道,这种非岩浆分异的外来成矿流体我们称之为透岩浆流体。小岩体不是成矿的必备的条件,只有出现大流体时才能成大矿。东秦岭-大别山地区有200多个小岩体,但大型、超大型钼矿矿床仅有10余个,只有小岩浆(小岩体)大流体(强蚀变)成大矿,其余众多小岩体由于没有流体(蚀变)或流体少(弱蚀变)而不成矿或成小矿。由此可见,岩浆成矿系统实际上是一种流体(挥发分)过饱和系统或熔体-流体流及流体对熔体的强相互作用。当岩浆系统被加入大量源自地幔的高温高压含矿流体之后,系统将具有极大的活动能力,从而深部含矿流体沿裂隙快速上升到地壳浅部卸载成矿。为解释上述成矿特征,作者引入并厘定了透岩浆流体的概念。透岩浆流体被重新定义为透过岩浆活动并导致岩浆系统行为发生非线性变化的外来流体。据此,输入了含矿流体的岩浆可成矿,未输入含矿流体的岩浆不成矿。这种认识可以解释东秦岭-大别山地区大多数小岩体不成矿或只形成小矿的现象。

A close temporal and spatial relationship existed between magrnatic activity and the endogenous metal mineralization. In order to explain the relationship between mineralization and magmatic rocks, the （post-） magmatie hydrothermal ore-forming theory had been proposed. In other words, metallogenic materials are evolved from magmatie differentiation. In fact, this understanding considers the endogenous metal metallogenic system as an ideal system. The research on the relationship between Mo deposit and granite showed that the deposits were closely related to small intrusion （＂ small＂ magma ）, and were irrelevant to the batholith. A common characteristic of large or giant scale deposits is that the extensive development of hydrothermal alteration （ ＂big＂ fluid） and its scope is larger than small intrusion dozens or hundreds of times implying external fluid participated in the mineralization. There is no strong differentiation crystallization in the formation process of the small intrusion, so the small intrusion was unable to provide adequate ore- forming matter to form a giant deposit which can proved by the mass balance calculation. Therefore, rock-forming and mineralization have essential difference. The metallogenic system should be a nonlinear complexity dynamic system. Research shows that the magma of small intrusion is often derived from lower crust and ore-forming fluid is derived from the mantle showing double layer structure. Magma is actually a channel connecting the deep and shallow. The ore-forming fluid of the non-magma differentiation of what we call the transmagmatic fluid. The small intrusion is not an essential condition for mineralization. Only when the development of large-scale fluid can form large deposit. Although there are more than 200 small intrusions in eastern Qinling-Dabie Mountain, only 10 small intrusions associated with giant deposits. More typically, small intrusions did not develop mineralization because of no development fluid （hydrothermal alteration ） or less fluid （weak hydrothermal alteration ）. From the point of the current situation, magmatic metallogenic system is actually a fluid （volatile） supersaturated systems or strong interaction of melt-fluid flow and fluid in the melt. When a large number of deep high temperature and high pressure fluid is added into the magmatic metallogenic system, the system will have great activity ability which can effectively guarantee dissolved metal in deep in fluid to rapidly rise through the crack to shallow crust and mineralize. To explain the above metallogenie characteristics, the author introduce and define the concept of transmagmatic fluid. Transmagmatic fluid is redefined as external fluid which penetrate magma, the result is a nonliner change in the magma system. Therefore, the magma injected ore-forming fluid can form ore, the magma without ore-forming fluid cant form ore. This cognition can well explain why most small instrusions in eatern Qinling -Dabie mountain dont metallogenie or only form small ores.