大厂锡-多金属矿床喷气沉积成因的证据—矿床地质、地球化学特征

EVIDENCE FOR EXHALATIVE ORIGIN OF THE DACHANG TIN-POLYMETALLIC SULFIDE DEPOSITS--THEIR GEOLOGICAL AND GEOCHEMICAL CHARACTERISTICS

本文通过对大厂锡-多金属矿床地质-地球化学特征的研究,阐明了本矿床按矿化类型可分为层状矿化及脉状矿化两大类。层状矿化是同生沉积的,严格产于喷气沉积岩层内。脉状矿化是矿床形成时及其后不同阶段变形-变质作用的产物,故又可进一步分为补给脉、梯状脉等4种类型,在成因上即有联系又有差别。对锡石的研究表明,本矿床不同矿化类型中锡石的矿物学及微量元素地球化学与其产状有着相应的一致性,进一步提供了有关矿床成因的信息。特别是作者对沙利文矿山喷气沉积型锡石进行了微量元素分析,其结果与大厂层状矿化者十分相似,证明了二者具有相同的形成机制。

Cassiterite occurs in marketedly diverse forms in the Dachang tin-polymetallic sulfide field. There are at least two different types of mineralization: concordant stratiform continuous beds and discordant veins. The former is strictly confined within the siliceous host strata, suggesting syngenetic sedimentary characteristics of the bedded mineralization, and the main orebodies themselves consist of these alternating cassiterite-rich thin layers and silici- ous rocks; the latter includes ladder veins within single beds, fissure veins or veinlets and large main veins. The veinlets transect a number of beds, but never penetrate the host strata; the large veins, however, usually cut all stratigraphic units from D_3~1 to D_3~3. Detailed investigation indicates that the primary stratiform main orebodies were originally sea-floor chemical precipitates from discharged hydrothermal fluids, and later underwent a complex, prolonged and continuous spectrum of deformation-metamorphism. This process commenced with diagenetic ompaction and lithification, and culminated in regional metamorphism, deformation and granitic lutonism.Successive deformation-metamorphism events not only reshaped the stratiform conformable orebodies, but also formed apparently different types of veins dy remobilization of ore-forming components from the primary Sn-enriched strata, and greatly changed the structure, texture and mineralogical composition of the primary ores. Such phenomena as the secondary enlargement of syngenetic K-feldspar, the aggregated recrystallization of exhalative sulfides and the boudinage in primary bedded ores might have been formed during the diagenetic compaction stage or Indosinian-orogenies.A special study of cassiterite shows that mineralogical characteristics and trace element geochemistry of this mineral from different types of mineralization are apparently affiliated with their modes of occurrence. Ti, Fe, Sc, V and W contents of cassiterite occurring in bedded mineralization are much lower than the contents of these elements in cassiterite from the large main veins related to granite, but things are just the opposite for Zn, Mn, Ba and S contents. Geochemical behavior of W in cassiterite from the various types of mineralization, in particular, demonstrates that cassiterite with high content of W was generated from the hydrothermal fluids affiliated with granite. Therefore, W and Mn+Ba appear to be in distinctly negative correlation. This undoubtedly indicates that Mn-and Ba-high cassiterite occurring in stratiform orebdies is related to submarine hydrothermal exhalative system. For the sake of comparison, we analysed the composition of pure seperated cassiterite from the Sullivan Mine of Canada which is generally considered to be of typical exhalative origin. Cassiterite from the Sullivan Mine is strikingly similar to that from the Dachang Mine in mineralogical and trace element geochemical characteristics, suggesting that these two ore districts share the identical formation conditions. The trace element geochemical data of cassiterite from sedimentary exhalative deposit presented in this paper are of great significance to the origin of some other tin deposits in the world.