铜陵石炭系层状金属硫化物矿床稀土地球化学

REE GEOCHEMISTRY OF CARBONIFEROUS STRATIFIED METALL-SULPHIDE DEPOSITS IN TONGLING DISTRICT

本文揭示铜陵石炭系层状金属硫化物矿床中沉积围岩、侵入岩、蚀变岩及矿石矿物的稀土元素特征。不同产状,不同成分和来源的成矿热液,在不同成矿阶段中,各矿物组合中矿石矿物的稀土元素性状各异。

Tongling district is an important metall-sulphide ore-forming area in lower and middle Yangtze River ore-forming belt. During Late-Jurassic, high-alkaline potassium-rich neutral and neutro-acidic intrusions of syntectic type mainly in form of stocks intruded in carbonates of Carboniferous-Triassic formations. The chief ore-occurring strata of metall-sulphide orebodies are dolomite horizona which overly on fragments. The mineral association of stratified orebodies bears clear vertical and horizontal zonation. ΣREE, LREE and HREE of intrusive rocks decrease with increasing of their basic degree, while the LREE/HREE varies inversely. Between the ΣREE and LREE/HREE there is shown an evident negative correlation. The REE of carbonatites of different sedimentary facies change regularly, that is ,when the sedimentary environment change from shallow to deep and again to shallow, the ΣREE vary from low to high and again to low. In carbonatites the ratio of ΣREE to LREE/HREE shows a positive correlation LREE/HREE = 0.024 ΣREE+1.850, correlation coefficient r= 0.9581 and LREE/HREE changes in a narrow range with the increasing of EREE, indicating the realtively stable distribution of LREE and HREE in carbonatites. When thermometamorphosed, limestones and dolomites behave different REE geochemistry characteristica. Dolomites are transformed into dolomitic marbles, and the REE in them are over 4 times more than that in dolomites, whill the REE oflimestones have no change. In the stratified orebodies, the EREE, LREE / HREE and Eu abnormal coefficients for magnetites and gangue mineral taxoits are close each to other, but the Ce negative anomaly of magnetites is inferior to that of taxoites, moreover, Er, Tm, Yb, Y and so on in magnetites are obviously low. The ΣREE magnetites in orebodies occurred in contact zones are relatively low with evident Eu positive anomaly and no or weak negative anomaly of Ce, which is concerned with the rich REE in the gangue minerals-garnets. The REE of pyrites occurred in different parts are of obviously different characteristic. In the cataclastic zone of C24- C25 stratigraphic boundary surface, the pyrites which were formed in the hydrothermal process of atmospheric circulation infiltration water have the lowest ΣREE, and their Ce , Sm and Gd are all beyond the limit of determination; the pyrites which were formed in sedimentation (diagenesis) of conglomerate of the basal bed of Huanglong formation have higher ΣREE, and the values of LREE / HREE and δ5Eu are close to that of the rocks of the same horizon; the pyrites occurred in stratified orebodies and orebodies in contact zones have evident Ce negative anomaly , in addetion to a higher content of Co and Ni and Co/ Ni> 1, however, the ΣREE differ evidently because of different association of the gangue minerals. Chalcopyrites have different ΣREE when in different occurrence and different mineral association. Their highest ΣREE are shown in altered intrusious, of contact zones, whill the lowest ΣREE in the orebodies. The chatacteristic values of REE of siderites, sphalerites, galenites, melnikovites, etc, which are formed in the hydrothermal process of atmospheric circulation infiltration water, vary over a large range, especially when siderites and melnikovites unequally contain calcium impurities, such as calcites and ddolomites. The above mintioned data bring it to light that the ore-forming thermal solutions of different occurrence, composition and source must form different mineral association and have different REE content and characteristics in different ore-forming stage depending on different composition of wall rocks where the ore-forming thermal solutions run through.