Table of Content

20 March 2002, Volume 8 Issue 1

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Article

Study on Metallogenic System and Its Significance to Mineral Resource and Environmental Protection

ZHAI Yu-sheng, DENG dun, PENG Run-min, WANG diang ping

2002, 8(1):  1-8. 

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Metaorogenic system refers to a natural system with ore-forming function and within certain geological time-space zone. It includes the geological factors controlling ore-formation and preservation the ore-forming processes and their products．The essentials of metallogenic system theory contain four major points：① Ore-formation is a complex dynamic process inc1nding its onset，continuation and finish；④ The products of ore-forming processes include ore deposit series and anomaly series They together compose the ore-forming network；③ Metallogenic system studies the whole process of formation，change and preservation of mineral deposits；④ The metallogenic systems are investigated under certain ore-forming geological settings,and clasified by tectono-dynmnic environments．The significance of metallogenic system study lies in：① to promote the development of metallogeny and benefit the mutual penetration between ore deposit geology and other disciplines of geosciences；② to build strategic view and integrated thought in ore prediction and prospecting to accelerate regional ore exploration and mineral resource potentia1 assessment；③ To provide scientific base for evaluation of environmental quality and environmental protection by studying noxious contents in metallogenic systems and their po1lution. Combining the systematic and historical points of view，the study on metallogenic system will develop further to serve the mineral resource supply and environmental protection

Experimental Study on Evolution of REE in Magmatic Fluid Phase in the F．Rich Granite System

LI Fu-chun1 2, ZHU Jin-chu2, Qi Liang3, RAO Bing2, PAN Gen-xing1

2002, 8(1):  9-15. 

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Experiments of fractional crystal1ization under high T-P conditions demonstrate that during crystallization of F-bearing alumina-oversaturated granitic magma,an evolutional trend of REE concentrations in fluid phabe coexisting with melt phase is observed： When temperature drops from 750℃ down to solidus(about 570℃)，the REE concentrations gradually decrease. This phenomenon is consistent with the geological fact that REE contents in the upper zones of rare-metal granites are lower than in the deeper zones ．REE mostly tend to disperse in some rockforming minerals(such as biotite)and some ore-forming minerals(such as fluorite and cassiterite)．The rest of REE partition into fluid phase．The evolution characteristics of REE in the F-rich alumina-oversaturated granite system are not favorable for forming hydrothermal deposits of REE.

Dynamical Analysis of Vertical Transport for Partially Molten Magma in Asthen0sphere

DONG Jun, PENG Sheng-lin, LAI Jian-qing, SHAO Yong-jun

2002, 8(1):  16-24. 

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The research on mantle mass creep is one of the most important subjects in the field of geology and ore-forming dynamics．It has received a considerable attention in the past century．A new tendency in this aspect is the development of mathematical modeling analysis for the movement of partially molten magma. For the magma ascent by asthenosphere rock flow, some simplified dynamical models are studied and discussed in this paper Firstly，the complex magma movement is simplified to one-dimensional problem of vertical movement so that the basic physical process might be understood．Secondly．Some assumptions are made to get a series of simplified formulae aecorring to the basic principles of fluid flow in a porous medium, for example，the asthenosphere rock is treated as a porous medium saturated with liquid(partially molten magma)and supposed to move vertically at a constant velocity Therefore，some characteristics related to the mechanism of transport of partially molten magma could be obtained from these formulae It is worthy to emphasize that a formula of critical velocitiy could be yielded It shows that the partially molten magma would be produced at some level planes while the rock velocities are smaller than the critical velocity；and it would be produced in some delayed region while the rock velocities are greater than the critical velocity．Finally，some simple numerical calculations have been done by applying the formulae to the partially molten magma of basalt．All these conclusions show the reasons why the energy and the metallogenetic materials could move upward from the asthenosphere to the earth’s surfac, which is consistent with the geologic background and observation，and the magma transport might be of great importance for the further research on mantle creep flow and the dynamics of ore-forming processes．

Mineral Chemistry and Geochemical Characteristics of Metagranitoids from the Ultrahigh Pressure Metamorphic Belt，East Dabie Mountains and Their Geological Significance

SUN Hai-ting1 WANG Ru-cheng1, XU Shi-jin1, FANG Zhong1, XU Shu-tong2, WU Wei-ping2

2002, 8(1):  25-39. 

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The studied metagranitoids are located in the ultrahigh pressure metamorphic belt in the Dabie Mountains and belong to sub-alkaline granites. They are chemically rich in SIO2 and poor in CaO and AL2O3 The surrounding rocks are ultrahigh-pressure metamorphic gneisses with eclogite enclaves．The total REE contents of the most samples are(100～200)x10-6 ．All samples have obviously negative Eu anomalies ．Their protolith was originated from Neoproterozoic ganites．Combined with geochemical characters of the metagranitoids，they are thought to be relatd to paleo-orogeny．There are numbers of metamorphic overgrowth zircons in the granitoids， which are composed of mixing magmatic core and metabolic overgrowth rim. Combination of mineral chemistry and U-Pb ages of zircons indicates that the metagranitoids were or inated and metamorphosed from paleo-granites，but not Mesozoic remelting granites formed later than ultrahigh pressure metamorphism．There are two kinds of garnet in the rocks,one is poor in Mn，another is rich in Mn．By using geothermobarometry of garnet-botite and garnet-phengite mineral pairs，it can be proved that metagranitoids underwent metamorphism at pressure 0．6～0．8GPa and temperature 420～ 500℃. Some indirect evidence shows that the metagranitoids might have experienced a Mesozoic ultrahigh pressure metamorphism ．

An EPR Study on Mollusk Shells from Loess-Paleosol Sequences in the Loess Plateau，Central China

SHENG Xue-fen1, CHEN dun1, JI Jun-feng1, SUI Yun-xia2

2002, 8(1):  40-45. 

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Electron paramagnetic resonmnce(EPR)spectrum won induced to decipher the paleoclimatic change information in the mollusk shells from the Luoe huan and Xifeng sections in the Loess Plateau．centra1 China The EPR analysis results showed the presence of trace amount of Mn2+ in the mollusk shells carbohate and no obvious tribal discrepancy of the EPR spectrum in tensities among different species of modern mollusks. The EPR intensities of the mollusk shells from the loess are significantly different from those in the paleosols The EPR signals of Mn2+ in the mollusk shells from the loess, which were formed under cold oonditions, are stronger than those from the paleosols which were developed under warmer conditions．Therefore，the Mn2+ content in the mollusk shells from the loess is higher than that from the paleosols It is concluded that during the glacial period, mollusks lived in more reducing environment．0n the contrary， during the interglacial period．the environment won more oxidizing. Thus the content of Mn2+ in the mollusk shells reflects the redox oonditions of the environment. The EPR signals of Mn2+ in the mollusk shells from loess and paleosols provide a new approach to reveal important information of environment changes.

Paleoplate Tectonics and Regional Geology at Bayan Obo in Northern Inner Mongolia

ZHOU Jian-bo, ZHENG Yong-fei, YANG Xiao-yong, SHU Yong, WEI Chun-sheng, XIE Zhi

2002, 8(1):  46-61. 

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Regional geology and tectonic characteristics were investigated for Bayan Obo and surrounding areas in northern Inner Mongolia The results place constraints on paleoplate tectonic units, the tectono-sedimentary settings of Bayan Obo Group and the features of tectonic assemblages in this region. The Bayan Obo Group Call be divided into three sedimentary associations which represent the transition processes of sedimentary environments within rift basins from interior continent at Mesoproterozoic to continetal margin at Neoproterozoic，and to active continental margin at Early Paleozoic． There are important differences in geology and tectonics between southern and northern regions along Bayanjiaolake-Kuanggou Fault as a boundary．The rocks in the southern region uncomfomlably overlies the rocks of Seertengshan Formation in Wutai Group and thus the sedimentary rocks of continental margin are developed on the continental basement of North China．Some strata of carbonate and shale underwent intensive metasomatism of mantle-derived fluid at Early Paleozoic，forming a large scale REE mineralization. Regional metamorphism overprinted both metasomatic and unmetasomatic strata again at Late Paleozoic．The Bayan Obo Group in the northern region is characterized by development of ophiolite me1ange，imbricate thrust and tightly synclinal fold，corresponding to accretionary complex scrapped off during plate subduction A preliminary scheme is presented to outline the geodynamic processes of tectonic evolution associated with plate subduction in Bayan Obo Region.

Strain Features and Strain Capacity Under Microscope

WANG Shi-feng, LIU Rui-xun

2002, 8(1):  62-67. 

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The application of microstructures in deformation analysis is well known recently. However, the marks used in stain measurement are restricted in fossils， colites and some minerals．In this paper，the authors depicted some mierostructures for strain measurement，such as dominostructure, pudding structure， rumpled fold， rootless fold，etc. A1l these microstructures wil1 benefit the construction of balanced cross section and the measurement of the shorten amount of a constricted area. In each of the five examples，the first thing is to direct length of fore-deformation(L)along the biggest stress direction in field of vision，then calculate strains in defferent fields of vision and find out the biggest strain，and calculate the line capacity e= (L-L0)／L0) For example，Guishan Group in North Huaiyang tectonic belt of Dahie mountain is monoclinic strata．The micro-deformation here develops very well. The micro-fold structure is one of the deformation types with a strain capacity of 0.57. It means that the strata shortening is 57％.

Post-Orogenic Tectono-Magmatism and Its Implications for Evolution of Orogenic Belts

LI Xiao-yong, GUO Feng, WANG Yue-jun

2002, 8(1):  68-78. 

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The traditional plate tectonics and its derivative plate collision theory have been sucessfuly applied in explaining the plate motion and formation of orogenic belts However，they are now in the face of many difficulties in explaining many geological phenomena after the final convergence of plates，such as rapid uplifing of orogenic belts，exhumation of high to ultra-high pessure metamorphic rocks and extensive magmatism,and so on. Post-orogenic magmatism, as one of the most important manifestations to reflect the adjusment and transition of themal-stress structure in orogenic belts，its petrogenesis is crucial for understanding the entire evolution of orogenic belts．It generally shows lithological assosiations characterized by：(a)bimodal volcanism ；(b)high-K calc-alkaline to shoshonitic characters and(c)massive granitic plutons. Due to their melting sources having been modified by previously subducted continental／oceanic crust，the post-orogenic basaltic magmas generally exhibit similar geochemical characteristics to those of island arc volcanic rocks. For instance, strong LILE and LREE enrichment and significant HFSE depletion. Comparing with thoze of IAB，they usually possess higher K2O 十Na2O and((Fe203+FeO)／((MgO)，((K2O)／((Na2O)ratios as well as higher Sr contents and Zr／Y rarios. Additionally,they exhibit enriched Sr and Nd isotopie ratios in relation to those of MORB and OIB，which are determined by the degree of LILE and LREE enrichment as well as the characters of subducted crust. For the basaltic magmas，they are derived from decompressional melting of metasomatized continental lithospheric mantle formed during previous plate subduction in response to unrooting of the thickened lithosphere，whereas the felsie magmas can be generated either by means of differentiation of basaltic melts in crust-level magma chamber，or remelting of lower／middle crust as a result of basaltic underplating. The mechanism for extensive melting during post-orogenic stage is closely related to extensive horizontal movement along lithosphere-scale faults that triggers lithospherie extension and thinning．For most of the post-orogenic magmatism in eastern China，which is characterized by low MgO an d high Si02 contents and absence of OIB-like melts and had undergone strong magma differentiation before their emplacement，a diffusive rather than rapid extension impacted by upwelling mantle plumes is preferred.

The Application of Direct Current Prospecting to Underground Thermal Water Investigation：Exemplified by the Fashui Hot Spring，Zixi County，Jiangxi Province

GONG Yu-ling1 2, WANG Liang-shu1, TANG Hong-zhi2, GUO Ling-zhi1, LIU Qing-cheng2

2002, 8(1):  79-85. 

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Located in a tectonic uplift region in Zixi county，Jiangxi province the Fashui hot spring along the Erkou—Shixia fault belt is a typical intermountain basin type hot spring with a geothermal anormaly area 1ess than lkm．In order to investigate the distribution of underground thermal water of the hot spring，we used the direct current prospecting to determine the fracture belt bearing underground thermal water with the near-source field electric profiling．Furthermore，the inclination，depth and the water-bearing characteristics of the fracture zone were investigated by using electric sounding and combined profiling on key profile lines The positions of boreholes are projected for development and utilization of heat energy of the hot springs.

Resource Potential and Exploration Benefit Prediction of Bamianhe Petroleum Play，Jiyang Depression

XIE Guo-jun1, JIN Zhi-jun1, CHEN Guang-xin2

2002, 8(1):  86-95. 

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The total amount of petroleum in an assessment unit can be estimated using Horvitz-Thompson estimator if the probability of finding a pool is proportional to its magnitude in the process of oil and gas discovery. But the estimator cannot be used straightly to obtain the total potential if the probability of finding a pool is not proportional to its magnitude. To solve the problem, a method that can be used to evaluate the individual pool size in the unit is presented using both Horvitz-Thompson estimator and the maximum probability theory with the asumption that the oil pool population obeys the Pareto distribution．In the meantime，the exploration benefit of the unit can be estimated using exploration simulation method on the basis of the predicted results by the above mentioned discovery process model. By using exploration simulation method，the petroleum potential of Bamianhe petroleum play in Jiyang Depression，Bohaiwan Basin，is forecasted and the future benefit of the play is predicted．In the process of prediction， the initial exploration stage with low efficiency and the normal exploration stage with the most amount of petroleum to be discovered are considered ．~eparately． The totaI arilount of the petroleum resource is forecasted to be 14 900 reserve units. The resource to be discovered amounts to 5 300 reserve units．most of which are hidden in many smaller pools. The exploration of Bamianhe play has entered into a mature stage．The future benefit will be less than 28 reserve units per exploration well and will decline slowly.

Study on Prediction of Muddy Fractural Reservoirs in Sikou Sag，Jiyang Depression

CI Xing-hua, JI Deng-wu, CHEN Han-Lin, YANG Shu-feng

2002, 8(1):  96-105. 

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In the southeastern part of Sikou sag in Jiyang depression．fractural reservoirs are found in muddy and shaly rocks of No．3 member in Shahejie Formation，and high oil production is obtained in some wells．The lithology of productive formation includes oil shale，oil mudstone，limy-mudstone and marlstone．Studies on core and thin-sections showed that there are 4 types of fractuxes：extensional fractures，shearing fractures，diagenetic shrinkage fractures and fractures due to abnormal pressure．The genetic mechanism and identification criteria of these fractures are different．Statistics sh owed that the extensional fraon re is the most important type among them. Regional tectonic stress，lithology, 1ithofacias ，thickness and depth of rocks are main factors influencing the extensional fracture distribution．Based on above analysis, the geological，mechanical and mathematical models were built up. Using 3-D finite element numerica1 simulation method，the authors made a regional stress and extensional fracture distribution prediction and pointed out 8 favored exploration target districts．

Influence of Fluid Pressure Change on the Deformation and Stress State of Surrounding Rockmass of Boreholes and Caverns

Ll Xiao-zhao1, ZHANG Di1, HUA An-zeng2

2002, 8(1):  106-112. 

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The distribution and change of fluid pressure have significant influence on the deformation and stress state of surrounding rockmass of the boreholes and caverns，which is related to many engineering phenomena. However，this is a dynamic coupling process involving several variables and responses, and has not been solved by analytical method yet. Also，study of the dynamic variation of fluid pressure can hardly he approached by physical mode1． In this article，we will，taking the discharging gas through borehole as an example，analyze the mechanism of some related engineering phenomena and appropriate countermeasures by applying numerical simulation and using the outcomes of experiments which were carried out on both macro and micro scales. There are two basic findings：1)During the process of gas discharge，pressure gradient of gas near the borehole wall decreased greatly，the Mohr’s stress cirde enlarged ，the stress state deteriorated and the plastic range expanded ．2)Outside the above area，both the maximum principle stress and the minimum principle stress increased，so did the effective stress and the influence range of stress redistribution. Based on the two findings described above，the following mechanisms for the deformation of surrounding rockmass in the process of gas discharge are proposed：in the area where the pressure gradient of gas decreased greatly，shear-and-tensile failure occurred．Just outside this area，the stress increased and its influence range enlarged, the deformation of surrounding rockmass accumulated accordingly. Furthermore，decrease of gas pressure against the grain surface could lead to the development of internal off-load crevice．These mechanisms has been illustrated by many related engineering phenomena．When gas pressure decreased， the conductivity coefficient of coal increased accordingly，and deformation of surrounding rockmass accumulated．Finally，we believe that gas discharge can release not only gas expansion energy，but also deformation energy of surrounding rockmass．Consequently, it is obviously helpful to reduce the possibility of outburst．

Genetic Interpretation of the Quaternary Bottom Gravels in Huainan and Huaibei Coal Basins

GUI He-rong1 2, SONG Xiao-mei2, PENG Zi-cheng1

2002, 8(1):  113-120. 

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The bottom gravel of Quaternary system in Huainan and Hualbei mine districts of Anhui Province has been studied for the upper limit of coal mining. There are many viewpoints about the genesis of the“bottom gravel”.In this paper，the authors studied the depositional characteristics，described the morphology and surface textures of the Quaternary bottom gravels in these districts. It is proved that the bottom gravels in these two districts are glacial till(glacial deposit)．Although these two mine districts are located about 200km away from each other, chemica1 simi1arity of limestone and dolostone gravels for these two districts indicates that these gravels share a common source．A “stagnation and deposition-falling stacking” model is proposed for genesis of these bottom gravels．

The K-feldspar Megacrysts in Granites：A Case study of Microcline Megacrysts in Fogang Granitic Complex，South China

XU Xi-sheng, ZHOU Xin-min, WANG De-zi

2002, 8(1):  121-128. 

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K-feldspar megacrysts are very common in granitic rocks. They could be either microcline or monoclinic orthoclase，and their origins are still debated Microcline megacrysts，2 to 12cm long，are easily found in Fogang granitic complex，South China．They are usually euhedral in shape，common in simple Carlsbad twinning or grid-twinning，but quite often in cross interpenetration twinning．They are orientated with long-axes more or less parallel to the flow direction of the magma．Tiny minerals such as plagioclase，biotite．quartz etc, are poikilitically enclosed in them. In addition，the microcline megacrysts sometimes are surrounded by a thin plagioclase film forming mantled feldspar，and sometimes intergrown with quartz forming“pearl margin”．More petrographic features of subsolidus exsolution and oscillatory zoning can also be easily found under microscope X-ray powder diffraction analysis indicates that the triclinicity of the microcline megacrystis 0 963，tlo+ t1m= 0．970，t2o= t2m = 0．015，and therefore it is highly Si-Al ordered microcline. The microclines in groundmass of Fogang granitic complex are commonly in simple Carlsbad twinning or grid-twinning and exsolution texture，but with much smaller size and interstitial irregular shape． All the above mentioned petrolographic features show that the microcline megacxysts in Fogang granitic complex were crystallized from silicate melt．In order to further document the origin of the microcline megacrysts，we have also studied the microgranltoid enclaves in Fogang granitic complex as same microcline megacrysts have been found in these enclaves．These enclaves of quatz diorite or granodiorite composition with round shap,。needle-like apatite and biotite，ocular quartz and poikilitic texture, are typical magmatic enclaves formed by magma mixing．Therefore，the microcline megacrysts are really xenocrysts from the host magma． The detailed magmatic crystallization history could be addressed by using Swansons(1977) experimental diagram. At ca.850℃ ，plagioclase，biotite and K-feldspar crystallized simultaneously，but the K-feldspar has fllower nucleation density and higher growth rate than other minerals．Then plagioclase and biotite could be enclosed by the faster-growing K-feldspar，or simply were overtaken by the latter．However, as the mineral inclusions are usually concentrated at the centers of the K-feldspar, the outer parts of the K-feldspar were formed at a later stage．At ca .800℃ ，quartz started its crystallization, and its nucleation density was close to that of K-feldspar，but the growth rate of K-feldspar was much faster than that of quartz，and their intergrowth formed the“pearl margin”of the K-feldspar. At ca.600℃, even-shaped and small sized grainswere crystallized forming groundmass． The fact that the K-feldspar megacrysts and groundmass are microcline rather than orthoclase is significant for understanding the cooling history of the granitic complex. A possible mechanism is that the triclinic microcline was transferred from the monoclinic orthoclase during the slow cooling history．