THEORETICAL BASIS FOR METALLOGENIC STUDIES OF METALLIC ORE DEPOSITS

In this paper the author preliminarily discusses some problems on the basic theories in studies of regional metallogeny of metallic ore deposits in the light of his knowledge gained in a comprehensive study of regional geology and metallogeny from the late 1950’s up to now. 1. The features of the regional geochemical field are the prerequisite of material for the uneven regional distribution of metallic ore deposits. The primary and secondary halos of elements reflect the features of regional geochemical fields in various area.Secondary halos can serve as an indication in search for outcropping ores, while primary halos not only can serve as an indication for ore finding, but also can clearly show the kinds and contents of ore-forming elements in strata of various ages and rocks of various types in work areas; therefore, it lays a material basis for the uneven distribution of ore deposits on a regional scale. 2. The components of the metamorphic series in the folded basement in the continental crust constitute one of the important material sources for metallic ore mineralization. First, the folded basement rock series is "primary source beds". In ancient uplift areas, these ancient ore-bearing rock series unaerwent a long period of weathering and erosion. A part of ore-forming material was transported into rivers, lakes and seas and deposited, thus giving rise to sedimentary deposits in favourable geological environments and "enigenetic source beds" in unfavourable environments or under conditions of insuficient ore-forming material. Second, the basement rock series in depression areas was subject to partial melting at greater depths, thus causing the ore-forming material in the basement rock series to enter molten magmas.Therefore, the ore-forming feature of molten magmas is directly influenced by the ore-bearing feature of the basement rock series. This feature is called the "theory of influence by basement components" and "metallogenic specialization of basement components". As this kind of ore-forming magmas originates by subsequent melting,it is called"epigenic parent rock. Third, after the folded basement of continental crust formed, the metaltic ore-forming elements in the basement rock series, like water on the earth have been constantly evoluting and circulating during the development of the geologic history according to the "law of conservation of matter". Ibis regularity is called the"theory of evolution and circulation of metallic elements". Finally, the feature of the components of the basement rock series devermines the origin and features of the regional geochemical fields in vamous areas, which is called the "theory of origin and feature of the regienal geochemical field". 3. With regard to endogenic metallic ore-forming processes, the conditions of "internal and external causes" may transform into each other. When magmatic rocks are actually parent rocks, they are the "internal cause"; when magmatic rocks act as the heat source for ground water and strata （connate） water like a big heating furnace, they are the "external cause". Country rocks are the "external cause" only when they control the features of the genetic and industrial types of ore deposits, and they are the "internal cause" when they are source beds and provide material for ore formation. So there are no impassable gaps between the "internal and external causes" and between endogenic and exogenic deposits; On the contrary, they can transform into each other. 4. Post-magmatic residual hydrothermal fluids are present in very small amounts;larger amounts of hydrothermal fluids are derived from ground water and strata water heated （or baked） by heat sources of magmas as well as metamorphic water and subsurface hot brines. The four types of hydrothermal fluids can all form hydrothermal deposits, but of these the most important is the exogenic water with magmas as the"heating furnace". For this reason the concept of "post-magmatic hydrothermal deposits should be revised as follows: endogenic hydrothermal deposits refer to those formed with magmas as the heat source and exogenic water as the water source. 5. Many important large- and intermediate-sized deposits in China are not of a single genetic type, but result from association of orebodies of two or more genetic types. Ore deposits characterized by association of such poly-genetic orebodies are called "poly-genetic deposits", of which those formed in the same metallogenic epoch are called "synchronous poly-genetic deposits" and those formed by superimposition of orebodies of two or more metallogenic epochs are called "poly-epochal and poly-genetic deposits".