约翰斯顿岛附近海域铁锰结壳矿物学和地球化学研究

MINERALOGY AND GEOCHEMISTRY OF FERROMANGANESE CRUSTS FROM JOHNSTON ISLAND EEZ

铁锰结壳存在两种不同的产状，一种是铁锰氧化物围绕“核心”生长形成的球状结壳，另一种是铁锰氧化物在基底岩石上生长形成的被状结壳。本文对Ｒ／ＶＫａｉｍｉｋａｉ－Ｏ－Ｋａｎａｌｏａ船１９９３年１１月在约翰斯顿岛附近海域取得的铁锰结壳进行了矿物学和地球化学分析。该区铁锰结壳主要组成矿物为δ－ＭｎＯ＿２，Ｍｎ／Ｆｅ比值为２．０２，富Ｃｏ（０．８８１％），贫Ｃｕ（０．１％），稀土总量（ΣＲＥＥ）１９１８．２７ｐｐｍ，具明显的Ｃｅ正异常（δＣｅ＝２．６），是典型的水成成因铁锰结壳。除Ｃｕ、Ｎｉ金属外，该区结壳中的稀土含量也已接近陆地稀土矿床的稀土品位。对球状结壳来说，表面光滑的上部壳层富δ－ＭｎＯ＿２，Ｍｎ、Ｃｏ、Ｎｉ、Ｃｕ、Ｐ含量较高，而表面粗糙的下部壳层Ｓｉ、Ａｌ含量相对较高。而且上部壳层的Ｃｅ正异常高于下部。对被状结壳而言，Ｃｏ、Ｎｉ等金属含量由基岩向结壳表层增加。作者认为造成上述变化的主要原因是基底岩石和海水间的反应。Ｒ型因子分析得出四个主要因子：三价稀土元素因子，Ｍｎ、Ｆｅ、Ｃｏ、Ｔｉ、Ｋ、Ｎａ、Ｖ、Ｍｏ、Ｃｄ、Ｃａ、Ｐ因子，Ｔｉ、Ｃｕ、Ｃｄ、Ｚｎ因子，和Ｓｉ、Ａｌ因子。分别代表了铁锰氧化物的水成成矿作用（因子?

A total of 1360kg of ferromanganese-crust-bearing samples were collected from 10sites in the Karin Seamount Range and Keli Ridge of Johnston Island exclusive economic zone during November,1993 cruise of University of Hawaii research vessel Kaimikai-O-Kanaloa, Sixty eight ferromanganese crust samples and subsamples were analyzed min-eralogically and chemically for 36 elements.The crusts are dominantly composed of δ-MnO_2.They are rich in Co(0.88%)and rare earth elements(ΣREE=1918ppm)andpoor in Cu(0.1%).They show a significant Ce anomaly(δ Ce=2.6)and have lowerMn/Fe ratio(2.02).This is characteristic of marine hydrogenous Mn-Fe oxides.Therare earth element content of Johnston Island ferromanganese crusts is very close to thegrades of land-based rare earth element deposits in China,There are two growth stylesof crust.The first is as a spherical crust around loose debris,The second is as a sheet-like crust on the outcropping rock of the seamount.There are chemical and mineralogicaldifferences between these two growth styles.The spherical crusts exhibit a smooth sur-face structure in the upper Mn Fe layers.This upper area is mineralogically richer in δ-MnO_2 and chemically richer in Mn,Co,Ni,Ca and P than the lower part of the crustwhich is richer in Si and Al.The lower part has a rough surface structure.The sheet-likecrust growth style exhibits a Co content which increases from the substrate to the crustsurface.The compositional variation is mainly controlled by reaction between substrateand seawater.R-mode factor analysis was performed on the samples. Eight percent of variance can be described by four factors. Factor 1,the trivalent rare earth element fac-tor,with 36% of the variance assumed to represent hydrogenous mineralization of Mn-Fe oxides,particularly for rare earths. Factor 2 with 23% of the variance has strong pos-itive loading associated with Mn,Fe,Co,Ti and a strong negative loading associated with Ca and P.It is also a hydrogentic factor and indicates an inverse relationship between ferromanganese oxides and calcium phosphate growth.Factor 3 with 13%of the variance has positive loadings on Ti,Cu,Cd and Zn.It may be a submarine weathering factor or indicate a hydrothermal input.Factor 4 with 7% of variance has strong positive loading for Si and A1.It is an aluminosilicate detrital mixing factor.