冀西北宣龙地区菱铁矿的成因

本文以中晚元古代宣龙式铁岩中的菱铁矿为例,讨论碎屑岩中菱铁矿的成因。 宣龙式铁岩产于中晚元古代串岭沟组。成层状,层位稳定,分布较广,西起怀安龙泉寺,东至承德小平台一线均有出露,其中尤以宣龙地区发育最好。 宣龙地区的菱铁矿矿层薄,数量少,往往与赤铁矿构成混合矿体。该区菱铁矿分布规律性明显。横向上,由西往东逐渐增多;纵向上,由下至上由分散状变成层状,与赤铁矿间互产出。

THE GENESIS OF SIDERITE IN THE XUANLONG AREA OF NORTHWEST HEBEI

The Xuanlong type ironstone occurs between the Changzhougou Group and Chuan-lingou of Late Proterozoic.The distribution of the siderite is characterized by increasing laterally from the west to the east and changing from dispersal in lower part into interbedding with hematite in upper part.The main rock types of siderite may be divided into oolitic,microlitic,side-rite-hematitic and ehamosite-sideritic ironstone and,siderite sandstone etc.The replacement of hematite by siderite is obvious.The first and second generations of siderite occurred in early diagenetic stage and the vein siderite of third generation in later dia-genetic stage.The geochemistry,mineralization and mechanism of the formation of siderite are discussed herein.The Eh value calculated is in the specially low range(-0.25-0.35V)proposed by C.D.Curtis.The low Eh value and the narrow activity range of sulfide required for the formation of siderite can not,be satisfied in natural environments and siderite appears to be unstable in any water mass of sedimentary basins.The side-rite has not directly been precipitated as FeCO₃ in a sedimentary basin.During the process of diagenesis,organic matters take effect as a reductic agent that reduces ferric to ferrous iron under a certain Eh and pH condition,thus turning hematite into siderite.From the discussion above,the authors come to the conclusion that the paleogeo-graphical controll on siderite formation is indirect,but the content and structure of the organic matters as well as the flow of the pore fluid in rocks can be regarded as direct controlling factors.