Abstract:

The formation and evolution of plentiful alkalirich porphyries with deepxenoliths have an inevitable connection to the Cenozoic continental deformations, tectonisms, mantle magmas, anatectic fluids, and associated abundant polymetallic deposits. By the petrographical and petrochemical studies and the analyses using electron microprobe, scanning electron microscopy and energy spectroscopy, we confirmed that the connection was brought about by the deep geological processes and the action of orebearing mantle fluids. Microscopic traces and existing modalities of the processes and the action can be directly manifested as the following: （1）Narich glasses, which show as microlites and embryonic crystals under optical microscope, interpenetrated among deepxenoliths in a veined or disseminated manner. The colour of the glass changes with its composition and it is not uniform. The chemical composition of the glass is characterized by high Si, Al, Na and Fe, and low K, Ca, and Mg; the mineral composition is characterized by the assemblage of albite, amphibole, magnetite (specularite) and ilmenite. The microlite glass was formed from the upper mantle fluid before the xenoliths having been caught by the alkalirich magma.(2)Ferich glasses interpenetrated in a veined, agglomerated or disseminated manner among the host rocks and various xenoliths.(3)Black microcrystalline ironrich melt xenoliths as independent xenoliths existed in aegerine syenite porphyry. Both the above (2) and (3) entities are of submicroscopic crystalline texture under electron microscope, but almost no any demonstration of optical properties under transmitted or reflected light, only appeared as black opaque materials under optical microscope. Their chemical compositions are characterized as high Si, Al and Fe, and low Ca, Mg, Na and K. Their main mineral compositions are silicate and quartz, as well as moissanite, native iron with chromium, specularite with titanium, ilmenite, apatite etc. The unmixing texture, which is assumed to be caused by the unmixing of minerals between micrometal and nonmetal, indicates that the mantle fluid has the properties of magmatic melt and is unmixing with alkalirich magma. Alterations developed widely, such as hornblendication, silification and chloritization etc., and the corresponding retrograde reaction in order of pyroxeney→amphibole→biotite→chlorite resulted in the mantle fluid disseminating and replacing the host rocks and xenoliths Both of alkalirich magma and mantle fluid, which showed abovementioned three microscopic traces and modalities, coexisted and migrated together, but unmixed because of the differences of compositions and properties. On the basis of synthesis analyses and in combination with the theory of metallogenesis by transmagmatic fluid, we may draw the following conclusions: that the alkalirich porphyries were synchronically selfreplaced and alterated by the mantle fluid accompanying the process of alakalirich magma crystallizing while the fluid and magma was trapped well, and the orthomagmatic metallogenic system was formed in magma body or its depths, and the typical deposit of orthomagmatic metallogenic system is the Machangqing porphyry Modeposit; that if the perturbation of tectonization happened in the process of diagenesis and metallogenesis, the mantle fluid would enter into the contact zone between magma body and wall rock or strata next to the contact zone, and the replacing and alterating took place to form the contactmetasomatic metallogenic system, the typical ones of which are the Machangqing porphyry Cudeposit existing in skarnmarble zone, and the porphyry Audeposit existing mainly in stratum rocks; that if the Audeposit existed in porphyry body, it was commonly controlled by the fractures occurring after diagenesis; that if the deep fractures, as magma and fluid channels, were well developed, and environment was relatively open, the orebearing mantle fluid would flow far from the alkalirich magma along branch fractures, entered into different strata and rocks, and the replacing and alterating occurred along with the process of diagenesis to form the remote epithermal metallogenic system, the typical one of which is the Lanping Jinding superlarge PbZn deposit; in this process, the mantle fluid's properties were changed from magma→supercritical fluid→liquid by the changes of depth and environment, and the corresponding changes of physical and chemical conditions, and the moving fluid carried, activated and enriched orematerial to suitable places; it was the deep process and the action of fluid that accelerated crustmantle materials overlapping mineralization, facilitated the deep oreforming and benefited the formation of large and superlarge deposits.

Key words: alkalirich porphyry, ironrich melt xenoliths, Narich and Ferich glasses, unmixing action for mantle fluid and magma, fluid replacing and alternating mineralization, western Yunnan