Metalliferous sediments deposited on and near spreading ridges show contrasting geochemical signatures depending on whether deposition occurred in a restricted basin filled with anoxic dense brines, or in an open ocean characterized by oxidized and well-circulated seawater. Metalliferous sediments of the Atlantis II Deep, which are precipitated from ca. 60 °C dense brines, display a wide range of mineralogical and chemical facies. The most abundant facies are enriched in Fe-oxyhydroxides or hydrous Fe-silicates or metal sulphides; Mn-oxides, carbonates and anhydrite are locally important. Terrigenous and biogenic components are minor. Base and precious metals are notably enriched in the sulphide-rich facies. By contrast, chemical precipitates in open-ocean settings consist of Fe-Mn-oxyhydroxides that were deposited from dilute hydrothermal plumes, commonly admixed with biogenic carbonate-rich ooze. On a carbonate-free basis, open-ocean metalliferous sediments show much less enrichment in trace metals relative to sediments in the Atlantis II Deep but have higher contents of rare-earth elements (REEs).

The shale-normalized REE patterns of metalliferous sediments in the Atlantis II Deep show positive Eu anomalies, but lack Ce anomalies. Due to the low REE content of the precipitates, even minor aluminosilicate detritus (10%) can affect REE patterns. The ‘baseline’ of the REE pattern is determined mainly by the aluminosilicate component of the metalliferous sediment, with the Eu anomaly resulting from the hydrothermal component. Nd isotopic variations can be explained largely by the mixing of Nd provided by aluminosilicate detritus with lesser dissolved Nd derived from a basaltic-hydrothermal source. These two sources can also account for the Pb isotopic variations reported in previous studies. The majority of Sr is derived from evaporites that flank the brine deep, with a smaller contribution from underlying basalts.

In open-ocean settings, plume particulates that formed above high-temperature vents acquire seawater-type REE patterns and Nd isotope ratios soon after discharge; these features are maintained as the plume drifts hundreds to a few thousand kilometres from the ridge axis. Departures from seawater isotopic and REE signatures can occur if detrital material (basaltic or terrigenous) is present in the metalliferous sediment. The Sr-isotope ratios of open-ocean sediments are, like Nd, dominated by seawater Sr. For Pb, a basaltic component can be isotopically identified in metalliferous sediments up to 1000 km from the axis.

Where low-temperature fluids discharge through biogenic sediments on ridge flanks, as at the Galapagos hydrothermal mounds, Fe-rich smectites (nontronite) are formed, together with minor Mn-oxides. The nontronites have very low contents of REEs, with seawater-type patterns, although Ce anomalies are less negative than those of deep Pacific seawater. Nontronites formed on active ridges or intra-plate seamounts display a range of REE patterns resulting from mixtures of hydrothermal fluid and normal seawater.

In open-ocean basins, the REE patterns and Nd-Sr-isotope ratios of chemical precipitates generally reflect those of ambient deep seawater. In closed (or restricted) anoxic basins subject to hydrothermal input, the precipitates can have REE patterns and Nd-Sr-Pb isotope ratios that differ considerably from those of ambient seawater. These features bear on the interpretation of geochemical data from ancient exhalative deposits including iron formations.

沉积在扩张脊上和附近的含金属沉积物显示出对比鲜明的地球化学特征，这取决于沉积是发生在充满缺氧浓盐水的受限盆地中，还是发生在以氧化和循环良好的海水为特征的开阔海洋中。亚特兰蒂斯 II 深海的含金属沉积物，从约 60 °C 的浓盐水，显示出广泛的矿物和化学相。最丰富的相富含铁羟基氧化物或含水铁硅酸盐或金属硫化物；锰氧化物、碳酸盐和硬石膏在当地很重要。陆源和生物成分很少。贱金属和贵金属在富含硫化物的相中特别富集。相比之下，公海环境中的化学沉淀物由从稀释的热液羽流中沉积的 Fe-Mn-羟基氧化物组成，通常与富含生物碳酸盐的软泥混合。在不含碳酸盐的基础上，与亚特兰蒂斯 II 深海的沉积物相比，公海含金属沉积物的痕量金属含量要低得多，但稀土元素 (REE) 的含量更高。

亚特兰蒂斯II深部含金属沉积物的页岩归一化REE模式显示Eu正异常，但缺乏Ce异常。由于沉淀物的 REE 含量低，即使是少量的铝硅酸盐碎屑 (10%) 也会影响 REE 模式。REE 模式的“基线”主要由含金属沉积物的铝硅酸盐成分决定，而 Eu 异常则由热液成分决定。Nd 同位素的变化在很大程度上可以通过由铝硅酸盐碎屑提供的 Nd 与来自玄武岩热液来源的较少溶解的 Nd 的混合来解释。这两个来源也可以解释先前研究中报告的 Pb 同位素变化。大部分 Sr 来自卤水深处侧翼的蒸发岩，下伏玄武岩的贡献较小。

在开阔的海洋环境中，在高温喷口上方形成的羽状颗粒物在排放后不久就获得了海水型稀土元素模式和 Nd 同位素比率；随着羽流从山脊轴线漂移数百到数千公里，这些特征得以保持。如果含金属沉积物中存在碎屑物质（玄武岩或陆源），则可能会偏离海水同位素和 REE 特征。开放海洋沉积物的 Sr 同位素比与 Nd 一样，以海水 Sr 为主。对于 Pb，可以在距轴 1000 公里的含金属沉积物中鉴定玄武岩成分。

在低温流体通过山脊侧翼的生物沉积物排出的地方，如加拉帕戈斯热液丘，形成了富含铁的蒙脱石（绿脱石）和少量的锰氧化物。尽管 Ce 异常不如深太平洋海水的负值小，但非特列岩的 REE 含量非常低，具有海水型模式。在活动海脊或板内海山上形成的非特隆岩显示出一系列由热液和普通海水混合产生的稀土元素模式。

在公海盆地中，化学沉淀物的 REE 模式和 Nd-Sr 同位素比通常反映周围深海水的那些。在受热液输入影响的封闭（或受限）缺氧盆地中，沉淀物的 REE 模式和 Nd-Sr-Pb 同位素比与周围海水的有很大不同。这些特征与对包括铁地层在内的古代喷出沉积物的地球化学数据的解释有关。