Field observation of fossil manganese nodules and crusts in the the West Sicilian Jurassic limestone beds

Fossil ferromanganese crusts, nodules and pavements are contained in stratigraphically condensed pelagic limestones from the Middle Jurassic of western Sicily; and a similar association of iron-manganese deposits with condensed pelagic sequences is notable throughout the Tethyan region. The west Sicilian concretions contain goethite, haematite and todokorite as fine-grained phases; their internal structure consists of iron-manganese segregations in a calcite matrix. Electron-microprobe study indicates that the amount of iron and manganese varies considerably in nodules from different localities, notwithstanding the disparity in carbonate content; and the same is true of the minor elements. The most consistently established inter-element relationships are Ni, Ba with Mn, and Ti with Fe - but these associations are by no means invariably developed. For western Sicily, submarine volcanism seems to have been the most immediate source of supply for the formation of the concretions; and this may well have been the case elsewhere in the Alpine- Mediterranean region; however, the Jurassic was a time of widespread sedimentary iron-ore formation in epicontinental northern Europe, and river drainage must have also supplied some manganese to the Tethyan Ocean. These mineral concretions are interpreted as being formed on ancient limestone (non-volcanic) seamounts. The presence of algal stromatolites in many of the condensed sequences suggests formation within the photic zone; and this relatively shallow-water origin may have affected the minor-element composition of the concretions, particularly with reference to copper and vanadium.西西里西部中侏罗统地层中，凝缩远洋石灰岩（stratigraphically condensed pelagic limestones）内赋存着化石状铁锰结壳（ferromanganese crusts）、锰结核（nodules）与铁锰质铺盖层（pavements）；特提斯（Tethyan）构造域内，铁锰矿床与凝缩远洋沉积序列的类似组合广泛产出，已成为显著的地质特征。西西里西部的结核含有针铁矿（goethite）、赤铁矿（haematite）与钡镁锰矿（todokorite）等细粒相矿物；其内部结构表现为方解石基质中发育铁锰分凝体。电子探针（electron-microprobe）分析结果显示，尽管不同产地结核的碳酸盐含量存在差异，但各结核的铁、锰元素含量均存在显著变化；微量元素的分布特征亦是如此。已明确的最稳定的元素间关联为：镍（Ni）、钡（Ba）与锰（Mn）呈相关关系，钛（Ti）与铁（Fe）呈相关关系——但此类元素组合并非在所有样品中均发育。对于西西里西部而言，海底火山作用（submarine volcanism）似乎是结核形成最直接的物质来源；阿尔卑斯-地中海（Alpine-Mediterranean）区域的其他地区大概率也是如此。但在侏罗纪时期，北欧陆表海（epicontinental）环境中广泛发育沉积型铁矿床，河流径流（river drainage）也必然为特提斯洋输送了部分锰质。此类矿物结核被认为形成于古老的石灰岩质（非火山型）海山之上。多数凝缩沉积序列中产出的藻类叠层石（algal stromatolites）表明，结核形成于透光带（photic zone）环境；这种相对浅水环境的成因，可能对结核的微量元素组成产生了影响，尤其是铜（Cu）与钒（V）的含量特征。