宁芜型铁矿床交代蚀变过程中稀土元素地球化学和活化转移

GEOCHEMISTRY AND MOBILIZATION OF THE RARE EARTH ELEMENTS IN METASOMATIC ALTERATION OF THE NINGWU-TYPE IRON DEPOSITS

在简要论述宁芜型主要亚类铁矿的地质特征和蚀变分带的化学特征及交代蚀变成矿模式的基础上,着重讨论各蚀变带的岩石和矿物的稀士元素分布特征,探讨其在交代蚀变过程中时空分布及活化转移的规律和原因。

The metallogenetic model of the Ningwu-type iron deposits is established by the authors primarily on the basis of the tempo-spatial distribution and evolution of metasomatic alteration and related metallization. The three main stages (from the early to the late) and zones (from the lower to the upper) are summarized as follows: 1) formation of albitized zone as a result of sodiummetasomatism; 2) formation of iron ore bodies and meta-altered zone due to iron magnesium-calcium-metasomatism; 3) formation of leuco-altered zones including silicified, aluminized and argillized subzones in response to acidic or hydrogenmetasomatism.The formation and evolution of this metasomatic sequence and zonation are dependent on a number of factors such as temperature, aH+/aNa+, pH and separation of compounds or components (such as acid-alkali, solvent-solute, etc.) in the ore forming solutions. Regular changes in rock-forming elements, REE and ore-forming elements with vertical zoning of metasomatic alteration represent those in the composition of ore-forming solutions with time, space, and extent of their interaction with porphyrites and their country rocks.The REE, especially LREE, contents of the albitite are one order of magnitude lower than those of the porphyrites. All this goes to show that the sodium-metasomatic zone is consistently corresponding to the mobilized-extracted zone for REE, irnh, magnesium, phophorus and sodium in hydrothermal solutions, which is necessary for the formation of stable REE and Fe complexes.The REE contents of the iron-magnesium-calcium-metasomatic zone, especially of the apa-tite-and diopside-bearing rocks, are generally higher than those of the porphyrites. The REE contents of apatite are so high that it is believed to be the carrier of REE, especially LREE, and thus an economic mineral with respect to REE.The REE contents of the silicified subzone are one order of magnitude lower than those of the porphyrites and volcanic rocks. Plot of REE against SiO2 shows s strong negative correlation, so the silicified subzone is the REE-leached zone. High REE contents are recognized in the argillized rocks because the clay minerals serve as the carrier of REE.