中国石炭纪铝土矿的地质特征与成因

世界石炭纪铝土矿主要产于中国,其形成条件因当时处于湿热的热带地区,构造长期稳定,准平原化排水条件好,强烈的化学及生物风化后产物经水流短程搬运,成胶体及碎屑不同程度兼有的混合沉积;往往经多次的震荡运动或地震活动成内碎屑浊流沉积;还遭受成岩、后生、表生及后期风化作用等的改造,而富集成矿。其中硬水铝石及地开石一般不形成于氧化的风化壳,主要是还原的成岩后生作用转变的,固相转化的氧同位素组成仍可具有“红土性质”或“风化壳性质”的保存性。

GEOLOGICAL FEATURES AND GENESIS OF CARBONIFEROUS BAUXITE IN CHINA

China is one of the most important Carboniferous bauxitebearing countries in the world.One hundred and more large and mediumsized Carboniferous bauxite deposits with the reserve of one billion tones have been found. About 75% of the total deposits occured mainly on the Middle Carboniferous karst basement of North China plateform, subordinately on the lower Carboniferous karst surface in G-uizhou. Only a few of them belong to geosynclinal karst bauxite (merely found in small-sized deposits in Mount Wushibei of Xingjiang ) . Nearly all of the bauxite are diasporite karst ones,with a few small sized bauxite deposits accompanied by boehmite. As a rule, diasporite, as well as dickite accompanied sometimes therein, are not formed under normal temperature and pressure, but derived from other minerals or substances in deuter-diagenesis period. The formation of the deposits is believed to take place near the ancient equator (deduced from theory of plate movement), resulted from severe chemical weathering under moist and hot climate, and from "consecutive formation" suah as short-distance transportation, deposition, diagenesis,deuterogene, hypergene and post-weathering. Very few deposits have undergone contact metamorphism. Differences manifest themselves in various parts and deposits. The material comes mainly from alumi-nasilicate rock and the weathering resideum of limestone. Colloform and debris are, in varying degrees, both the modes of transportation. Abundant good-quality b-auxites are formed mainly in lakes, lagoons and gulf, ore is closely related to t-he desilication desulphate and deferrization during the post-weathering and leaching. But those formed in marshes and rivers are inferior and small in scale. Due to the slight motions of shaking or frequent seismic activities ( including sub-kastification in pay bed after sedimentation in the crust of pene-plateform after deposition of bauxite, the above-mentioned main factors accompanied with the effect of flood and windstorm made the bauxite bed of deposition again or m-ore times eroded by the waves of waterflow and migrated faintly or subsided, even broken down. During this time, "interior fragment" was produced. And then the turbidity current deposit and windstorm deposit of bauxite was formed, (note: Tie author is the first man who put forward the newtheory "the bauxite of windstorm deposit" , but it is only an initial idea and needs a further study ) The bauxite of turbidity current deposit mainly occurs in the deep lake district. There the sorting and the thickness of bed is uneven ( several centermeters to meters ) . The sequence of grain is thick on bottom and fine upper, showwing a decrease in energy. The bauxite of windstorm deposit occurs mainly in the shallow lake district. There the bauxite is stored well. The thickness of flag bed is uniform ( from several centimeters to tens of centimeters ), with a sequence of fine grain on bottom and thick upper, howing a increase in energy. The author believes that both diosporite and dickite which sometimes contains constitutional water in Carboniferous bauxite in China is frequently formed by solidus reaction and solid transportation during diagenesis and deuterogene. The composition of O isotope in it is unchanged basically. It means a "preservation"; conversely, that formed by solution, the primary compositions of O isotope are exchanged ( see Fig.1) . Detailly, the diosporite is formed by alumina-gel in aocient crust of weathering( suspended transportation ) and colloidal solution at the initial stage of diagenesis of reductive environment ( including transformed by gibbsite). The phenomena of constitutional water ( means colloidal water from ancient crust of weathering), vegetating recrystallizing and sequence belong to the solid reaction.The exchange in diagenesis and deutero-water of 0 isotope is uot caused, so the O isotope possess preservation of "weathering crust" 5 and"laterite property" 7. It is derived from kaolinite deposited from ancient weathering crust and gel since the deuterogene present solid