太平洋海山磷酸盐的产状、特征及成因意义

Modes of occurrence and characteristics of phosphorates on Pacific Guyots and their genetic significance

为了更好地认识大洋富钴结壳的成矿作用,作者对与其伴生的海山磷酸盐的产状、化学成分、种类及X射线衍射特征等进行了研究。研究发现,海山磷酸盐(磷块岩或磷酸盐化岩石)的产状可划分为以下6种类型:①作为结壳的下伏基岩;②作为砾状、球状(椭球状)结壳和海山结核的核心;③呈胶结物状胶结老结壳的碎块;④呈细脉穿插于板状结壳中;⑤呈断续的“夹层状”及⑥呈碎屑状、浸染状等产于结壳中。海山磷酸盐的平均化学成分w(CaO)/w(P2O5)(1.62)与碳氟磷灰石的w(CaO)/w(P2O5)(1.621)几乎一致,而与氟磷灰石的w(CaO)/w(P2O5)(1.318)相差甚远,表明海山磷酸盐为碳氟磷灰石,其形态主要有针状-柱状微晶碳氟磷灰石、粒状微晶碳氟磷灰石及显微隐晶质碳氟磷灰石3种类型,它们在分布丰度上存在差异。研究表明,碳氟磷灰石的结构CO2含量介于5.73%～6.05%之间,属高碳氟磷灰石,其晶胞参数α变化于0.932～0.933nm之间,平均值为0.933nm。各种交代残余结构(交代角砾状结构、交代生物结构、交代凝灰结构、交代生物假象结构、交代填间结构等)表明,太平洋水下海山上的磷酸盐主要属交代成因。因此,积极开展海山磷酸盐的地球化学研究,对加快勘探和开发结壳资源,有着十分重要的理论和实际意义。

Geological features of phosphorates associated with cobalt-rich crusts on northwestern Pacific submarine guyots were studied systematically. Six obvious modes of occurrence of phosphorates （phosphorites or phosphorized rocks） have been found：① underlying substrate of cobalt-rich crusts; ②core of irregular globular, conglomeratic, nodular crusts; ③ cement of old generation crusts; ④phosphorate veinlets in crusts; ... intermittent ＂intercalated beds＂ or ＂veinlets＂ in crusts; and ⑤debris disseminations in crusts. Studies show that the average ratio of ω （CaO） / ω（P2O5） （1.62） of phosphates in submarine guyots is the same as that of ω （CaO） /ω （P2O5 ） （ 1.621 ） of carbonate-fluorapatite, but is obviously different from that of ω （CaO） / ω （ P2O5 ） （ 1.318 ） of fluorine-carbonate, suggesting that the phosphates from northwestern Pacific are species of carbonatefluorapatite. Three types of microneedle-microcolumnar, microgranular and microcryptocrystalline carbonatefluorapatite with different abundances have been found. The content of structural CO2 in the phosphates varies from 5.73 % to 6,05 %, implying that they belong to a type of carbonate-fluorapatite with high CO2 content.The crystal constant of α has a range from 0.932 nm to 0.933 nm with an average of 0.933 nm. Various replacement and remnant structures show that the phosphorates in submarine guyots are mainly of replacement origin. Geological significance of phosphorates formation （phosphorites or phosphorized rocks） is preliminarily discussed.