302铀矿床绿泥石特征及其与铀成矿的关系

通过对302铀矿床的蚀变矿物绿泥石进行电子探针成分分析和矿物学研究,提出该矿床的绿泥石分属4种成因类型,具有4种产出形态;探讨了绿泥石的形成环境和温度,并以此对铀成矿环境进行推断。302铀矿床的成矿过程可表述为矿前期在相对较高温度的热液流体作用下,黑云母发生绿泥石化蚀变,随后热液继续交代长石,形成长石蚀变型、裂隙充填型绿泥石,进而在成矿期热液温度相对较低的条件下形成浸染型绿泥石。     

南岭中段黄沙矿区223铀矿床绿泥石特征与铀成矿关系

绿泥石化是华南热液铀矿床重要的蚀变类型之一。本文通过对南岭中段黄沙矿区223铀矿床绿泥石的岩相学和电子探针成分分析,区分出4种产出状态的绿泥石,识别了绿泥石的化学成分类型,计算了绿泥石的形成温度、n(Mg)/n(Fe+Mg)等相关指数,讨论了绿泥石形成机制环境及其与铀成矿的关系。研究表明223铀矿床绿泥石主要分为黑云母蚀变型、长石蚀变型、裂隙充填型和铀矿共生型4种产出类型,为富铁的蠕绿泥石,形成于还原环境,形成温度为200~282℃,属于中温热液蚀变;绿泥石的形成机制主要有溶蚀-沉淀结晶和溶蚀-迁移-沉淀结晶2种方式。绿泥石化改变了岩石物理-力学性质、原岩中铀的赋存状态,提供了成矿热液部分铀源和有利于铀富集成矿的地球化学环境。     

苗儿山中段向阳坪铀矿床绿泥石特征及其成岩成矿意义

向阳坪铀矿床是近年在苗儿山地区发现的重要矿床,绿泥石化是向阳坪铀矿床重要的蚀变类型和找矿标志。本文对向阳坪矿床发现的铀-绿泥石型矿石的蚀变矿物学特征进行了系统地观察,结合电子探针原位微区分析,查明了绿泥石相关矿物的共生组合关系,获得了其化学定量分析结果,划分了绿泥石的种类及其形成条件,在此基础上探讨了绿泥石性质及其对铀成矿的启示。结果显示,向阳坪铀矿床绿泥石可分为黑云母蚀变型、裂隙充填型、铀矿物相关型和黏土矿物吸附铁镁质转变型4种。铁硅协变图解表明向阳坪矿床主体为铁镁绿泥石,部分为蠕绿泥石,含少量的密绿泥石。据经验公式计算所得绿泥石形成温度变化范围为190~265℃,平均239℃,属中低温热液蚀变,其形成机制包括溶蚀-结晶和溶蚀-迁移-结晶2种。绿泥石化为铀成矿过程提供了所需的环境,促进了铀的活化、迁移并最终沉淀成矿。     

鄂尔多斯盆地纳岭沟铀矿床绿泥石特征及地质意义

纳岭沟铀矿床位于鄂尔多斯盆地北部,具有明显的后期热液作用的特征,矿体空间展布主要受控于绿色-灰色砂岩的过渡界面,与绿泥石化的蚀变砂岩关系密切。通过对纳岭沟铀矿床不同颜色砂岩中的绿泥石进行详细的岩相学研究和电子探针化学成分分析,依据绿泥石的成因与共生矿物的关系,识别出绿泥石主要的3种类型:填隙物型绿泥石,片状与黄铁矿共生型绿泥石以及黑云母蚀变型绿泥石;同时通过绿泥石的Fe-Si图解确定了纳岭沟铀矿床不同颜色砂岩中的绿泥石主要为铁镁绿泥石和密绿泥石。根据Al/(Fe+Mg+Al)-Mg/(Fe+Mg)的关系图解确定出不同颜色砂岩中的绿泥石具有铁镁质流体和泥质两种来源,通过绿泥石中主要阳离子与镁的关系图解和计算得出的绿泥石形成温度共同确定出绿泥石是多期次的中低温热液流体作用的产物。综合研究表明,纳岭沟铀矿床的绿泥石形成至少经历了温度稍高的还原性流体和温度稍低的氧化性流体等两个期次的流体作用,稍高温的还原性流体与成矿关系更为重要。与绿泥石形成有关的热液流体作用不仅带入了部分铀,还促进了铀的活化和运移。     

201和361铀矿床中绿泥石的特征及其形成环境研究

绿泥石化是201和361铀矿床最重要的蚀变类型之一。本文在岩石薄片观察的基础上,采用电子探针分析技术研究了绿泥石的共生组合与形貌特征,测定了87个代表性绿泥石的化学成分,并据此划分了绿泥石的化学类型,计算了绿泥石的形成温度、n(Al)/n(Al Mg Fe)值等相关指数,讨论了绿泥石的形成机制及其与铀成矿的关系。研究表明:①岩石中绿泥石主要呈脉状、黑云母假象或团块状等产出,具有蠕虫状、叶片状等形貌特征;②绿泥石的Fe/Si图解显示201和361铀矿床中绿泥石主要为铁镁绿泥石和蠕绿泥石,少数属密绿泥石;③根据Battaglia提出的经验方程式计算了201和361铀矿床绿泥石的形成温度变化于179~276℃之间,且主要介于230~260℃之间;④绿泥石主要形成于还原环境,其主要的形成机制是溶解-沉淀和溶解-迁移-沉淀。     

相山铀矿田云际矿床绿泥石特征及其地质意义

云际铀矿床是相山矿田典型的碱交代型矿床,其围岩和矿石中的绿泥石与铀矿化关系密切。笔者在详细的岩矿鉴定基础上,利用电子探针分析了绿泥石的化学成分特征,探讨绿泥石的形成环境及其地质意义。研究表明,该矿床的绿泥石可以分为黑云母假象型和填隙型两种产出形态,均属于相对富铁的铁镁绿泥石和密绿泥石,指示其形成于相对的还原环境中;绿泥石的形成温度为188~244℃,均值为213℃,属于中低温热液作用范围;从矿前期到成矿期,绿泥石表现出逐渐由Mg取代Fe的特征,暗示热液流体有向碱性方向的演化趋势;绿泥石主要有溶蚀-结晶和溶蚀-迁移-结晶两种形成机制。     

江西乐安居隆庵铀矿床绿泥石特征及地质意义

位于相山铀矿田西部的居隆庵铀矿床绿泥石化十分强烈,本文在对钻孔岩心样详细的野外和室内岩相学观测基础上,利用电子探针技术研究了绿泥石的产出状态及共生组合关系,并测定了其化学成分,探讨了该矿床绿泥石地球化学特征及其与铀矿成矿的关系。研究表明:①该矿床存在黑云母蚀变形成的绿泥石、长石蚀变形成的绿泥石、脉状绿泥石和与铀矿密切共生的绿泥石共4种类型绿泥石;②该矿床以蠕绿泥石和铁镁绿泥石为主,个别为鲕绿泥石,其形成温度介于190.5~269.9℃之间,平均为224.5℃,属于中低温条件;③该矿床绿泥石形成于还原环境,形成机制分为溶解-沉淀机制和溶解-迁移-沉淀机制两种;④绿泥石化过程改变了围岩的物理化学性质,改变了铀在岩石中的赋存状态并促使铀的预富集。     

赣南草桃背铀矿床中绿泥石地球化学特征及成矿指示意义

赣南草桃背铀矿床位于会昌地区河草坑铀矿田,是一个大型火山岩容矿的铀矿床,成因机制存在较大争议。矿床内发育有大面积的花岗岩和火山岩,矿体主要赋存在火山隐爆角砾岩和震碎花岗岩中。文章以该矿床内花岗岩、火山岩及矿石中广泛发育的蚀变矿物绿泥石为研究对象,采用电子探针及LA-ICP-MS原位微区分析技术对其矿物化学组成进行分析。分析结果表明,花岗岩和矿石中的绿泥石主要为富铁的鲕绿泥石和蠕绿泥石,火山岩中的绿泥石则为相对富镁的密绿泥石。各类绿泥石阳离子置换关系主要以Fe、Mg置换为主,同时存在一定的Tschermak（TK）和二八—三八面体（DT）替换机制。矿石中绿泥石具有相对较高含量的Ti、Li、Be、B、Zn、Ga、Ge、Sn、Cs、U、Rb和Ba等元素,并显示出较低的Th/U比值。矿石中绿泥石的结晶温度区间为201~269℃,平均值为242℃,属于中低温热液作用的产物。矿石中绿泥石的氧逸度（log f O₂）变化于-48.4~-41.2之间,平均值为-44.5,硫逸度（log f S₂）变化于-7.5~+2.8之间,平均值为-1.8,草桃背铀矿床的铀矿化主要形成于低氧逸度、高硫逸度的环境。     

熊耳山矿集区蒿坪沟Ag–Au多金属矿床绿泥石特征及其找矿意义

为理清蒿坪沟Ag-Au多金属矿床中多阶段矿化与热液蚀变之间的关系,文章选取与铅锌成矿阶段密切相关的绿泥石进行野外观察及电子探针分析。文章将蒿坪沟Ag-Au多金属矿床中的绿泥石分为3类：Ⅰ型分布在石英脉两侧的围岩中;Ⅱ型呈细粒、隐晶质填充于隐爆角砾岩基质;Ⅲ型与铅锌硫化物共生、或以蠕虫状广泛分布在石英颗粒间隙中。3种类型绿泥石均为斜绿泥石,并落在了铁镁绿泥石的范围内,指示其形成于偏还原的酸性环境中;在阳离子置换中,主要发生了Fe²⁺对Mg²⁺的置换,其余置换作用均不明显;3种绿泥石形成与镁铁质围岩关系密切。由校正后的绿泥石地质温度计估算出3种类型绿泥石的形成温度为196～239℃,属于中—低温热液蚀变范围。3类绿泥石与蒿坪沟Ag-Au多金属矿床银铅锌成矿阶段相匹配,对进一步找矿勘查具有重要意义。绿泥石化学特征表明岩浆热液参与了成矿流体的形成,绿泥石形成于熊耳山矿集区早白垩世大规模岩浆-成矿时期。     

山西铜矿峪铜矿床绿泥石特征及其地质意义

为探讨山西铜矿峪铜矿床绿泥石的成岩成矿意义,运用电子探针分析了矿床中斑岩型铜矿中的绿泥石。结果显示,矿区绿泥石可以分为与石英硫化物脉共(伴)生的绿泥石(Ⅰ型)、与方解石硫化物脉共(伴)生的绿泥石(Ⅱ型)、斑岩中的绿泥石(Ⅲ型)和围岩蚀变带中的绿泥石(Ⅳ型)4种类型。4种类型绿泥石主要为富铁种属的蠕绿泥石(铁绿泥石)和密绿泥石,指示其均形成于偏还原环境;在其结构的离子置换中均表现为Fe对Mg的置换,反映其形成都与铁镁质围岩有关,并都经历了多期次变质作用。由绿泥石地质温度计估算出4类绿泥石的形成温度为180~220℃,均属于中-低温热液蚀变范围。在铜成矿过程中,随着温度不断下降热液流体性质向酸性逐渐演化。     

与湘南白腊水锡矿床有关的骑田岭花岗岩的绿泥石化研究

湘南白腊水锡矿床位于骑田岭燕山期黑云母花岗岩中。绿泥石化是这一含矿花岗岩中常见的热液蚀变现象,对采自
钻孔的岩石样品研究发现,花岗岩的绿泥石化程度变化较大。本文选择三组绿泥石化蚀变程度显著不同的花岗岩样品进行
研究,探讨了与锡成矿密切相关的含矿花岗岩的绿泥石化作用。研究表明,不同程度绿泥石化花岗岩中的绿泥石,在产出
形式与化学成分上均表现出一系列明显不同的特征。依据绿泥石产出形式、化学成分、绿泥石化花岗岩地球化学等特征认
为,绿泥石对花岗岩造岩矿物的交代顺序为：角闪石、黑云母→斜长石、钾长石→石英;绿泥石形态特征的变化一方面与
绿泥石成分有关,另一方面与被其交代的造岩矿物晶体结构有关;花岗岩绿泥石化越强烈,全岩的烧失量和Zn含量就越
高,原岩的Cl丢失到蚀变流体中去的量也越多。随着花岗岩绿泥石化程度的加强,其绿泥石的主要成分含量会呈现出规律
性的变化。绿泥石的Si含量在绿泥石化过程中逐渐减少;长石遭受破坏溶解后释放出的Al在厘米尺度范围内发生迁移与再
分配,引起绿泥石Al含量的升高;Fe从热液中带入,蚀变花岗岩中绿泥石矿物含量的多少与水-岩反应体系中Fe的多少有
关;形成绿泥石所需的Mg主要来源于花岗岩暗色矿物内Mg的释放与再分配;Mn则主要由外部流体带入。     

Chloritization of the hydrothermally altered bedrock at the Igarapé Bahia gold deposit,Carajás,Brazil

The Igarapé Bahia gold deposit has developed from weathering of a near-vertical hydrothermal Cu (Au) mineralization zone. The unweathered bedrock composed of chlorite schists is mainly metamorphosed basalts, pyroclastic and clastic sedimentary rocks and iron formation. Contents and Fe/(Fe + Mg) ratios of chlorites increase from distal country rock towards the mineralization zone, which can be attributed to different water/rock ratios and locations in a hydrothermal system. In the hydrothermal system high salinity fluids convected through basin-floor rocks, stripping metals from the recharge zones with precipitation in discharge zones. The chlorite with lower Fe/(Fe + Mg) ratios indicates alteration by relatively unreacted Mg-rich fluids, occurring within recharge zones. By contrast, the chlorite with higher Fe/(Fe + Mg) ratios in the mineralization zone formed from solutions rich in Fe, Mn, Au, Cu, H₂S and SiO₂ within a discharge zone. The iron formation could also be formed within the discharge zone or on the basin floor from the Fe-rich fluids. The distal country rock with less chlorite content is a hydrothermal product at low water/rock ratios whereas the proximal country rock and the host rock with more chlorite content formed at high water/rock ratio conditions. The Al(IV) contents of chlorites indicate that the formation temperatures of these rocks range from 204 to 266 °C, with temperatures slightly increasing from distal country rock towards the mineralization zone.     

Infrared spectral reflectance characterization of the hydrothermal alteration at the Tuwu Cu–Au deposit, Xinjiang, China

Short-wave infrared (SWIR) reflectance spectroscopy was used to characterize hydrothermal minerals and map alteration zones in the Tuwu Cu–Au deposit, Xinjiang, China. The Palaeozoic hydrothermal system at Tuwu is structurally controlled, developed in andesitic volcanic rocks and minor porphyries. Hydrothermal alteration is characterized by horizontally zoned development of quartz, sericite, chlorite, epidote, montmorillonite and kaolin about individual porphyry dykes and breccia zones, as is shown by changes outward from a core of quartz veining and silicification, through an inner zone of sericite + chlorite to a marginal zone of chlorite + epidote. The alteration system comprises several such zoning patterns. Silicification and sericitization are spatially associated with Cu–Au mineralization. Zoning is also shown by compositional variations such that Fe-rich chlorite and Al-rich sericite occur preferentially toward the core and the most intensely altered parts, whereas Mg-rich chlorite and relatively Al-poor sericite are present on the margin and the relatively weakly altered parts of the hydrothermal alteration system. The compositions of chlorite and sericite, therefore, can be potentially used as vectors to Cu–Au mineralization. Montmorillonite and kaolinite, of probable weathering origin, are located near the surface, forming an argillic blanket overlying Cu–Au mineralization. Sporadic montmorillonite is also present at depth in the hydrothermal alteration system, formed by descending groundwater. Presence of a well-developed kaolinite-bearing zone on the surface is an indication of possible underlying Cu–Au mineralization in this region. Epidote occurs widely in regional volcanic rocks, as well as in variably altered rocks on the margin of the hydrothermal mineralization system at Tuwu. The widespread occurrence of epidote in volcanic country rocks probably reflects a regional hydrothermal alteration event prior to the localized, porphyry intrusion-related hydrothermal process that led to the Cu–Au mineralization at Tuwu.     

西藏阿里西部八面体假像透辉石、透闪石和绿泥石的化学标型特征

阿里西部超镁铁岩带八面体假像透辉石具有富钙贫铝的化学特点和降温降压的晶胞参数。八面体假像绿泥石具有富硅贫铝和贫铁的特点,它们都与地壳浅成热水产物的透辉石和绿泥石的化学特征完全不同。而它们可能是超镁铁岩由地幔上升侵位的早期所形成的热水交代产物。本文讨论了由尖晶橄榄石向八面体假像硅酸盐矿物转变的交代反应。     

Phyllosilicate minerals in the hydrothermal mafic–ultramafic-hosted massive-sulfide deposit of Ivanovka (southern Urals): comparison with modern ocean seafloor analogues

We have studied textural relationships and compositions of phyllosilicate minerals in the mafic–ultramafic-hosted massive-sulfide deposit of Ivanovka (Main Uralian Fault Zone, southern Urals). The main hydrothermal phyllosilicate minerals are Mg-rich chlorite, variably ferroan talc, (Mg, Si)-rich and (Ca, Na, K)-poor saponite (stevensite), and serpentine. These minerals occur both as alteration products after mafic volcanics and ultramafic protoliths and, except serpentine, as hydrothermal vein and seafloor mound-like precipitates associated with variable amounts of (Ca, Mg, Fe)-carbonates, quartz and Fe and Cu (Co, Ni) sulfides. Brecciated mafic lithologies underwent pervasive chloritization, while interlayered gabbro sills underwent partial alteration to chlorite + illite ± actinolite ± saponite ± talc-bearing assemblages and later localized deeper alteration to chlorite ± saponite. Ultramafic and mixed ultramafic–mafic breccias were altered to talc-rich rocks with variable amounts of chlorite, carbonate and quartz. Chloritization, locally accompanied by formation of disseminated sulfides, required a high contribution of Mg-rich seawater to the hydrothermal fluid, which could be achieved in a highly permeable, breccia-dominated seafloor. More evolved hydrothermal fluids produced addition of silica, carbonates and further sulfides, and led to local development of saponite after chlorite and widespread replacement of serpentine by talc. The Ivanovka deposit shows many similarities with active and fossil hydrothermal sites on some modern oceanic spreading centers characterized by highly permeable upflow zones. However, given the arc signature of the ore host rocks, the most probable setting for the observed alteration–mineralization patterns is in an early-arc or forearc seafloor–subseafloor environment, characterized by the presence of abundant mafic–ultramafic breccias of tectonic and/or sedimentary origin.Editorial responsibility: J. Hoefs     

川南普格玄武岩中沥青质杏仁体的矿物学与成因

川南普格玄武岩顶部发育了一系列杏仁体,沥青、绿泥石、石英、自然铜等矿物以各种产状出现在杏仁体中。系统的矿物学研究表明：杏仁体中的绿泥石为辉绿泥石,形成于中—高温富有机质成矿热液环境;沥青属于石油沥青,有机质来源于下伏地层的中二叠统阳新组（P2 y）生物碎屑灰岩;绿泥石、沥青、石英及自然铜等矿物是玄武岩成岩后的构造应力与晚期基性火山活动共同作用形成的含有机质成矿热液演化的结果,其形成过程为：玄武质岩浆末期（基性岩浆作用末期）→含铜的火山热液(火山热液作用）→构造应力→含有机质的成矿热液→绿泥石→第一世代沥青→乳白色石英→烟灰色石英、第二世代沥青、自然铜→辉铜矿→葡萄石。玄武岩中含沥青的杏仁体、晶洞以及构造破碎带可以作为该地区的铜矿化的重要找矿标志。     

滇黔桂地区卡林型金矿热液矿物地球化学

滇黔桂卡林型金矿是产于热水沉积岩及碎屑岩系的热液矿床，载金矿物为黄铁矿、毒砂等热液矿物，而石英、碳酸盐、萤石及粘土等矿物则是主要的热液蚀变矿物。矿化产于蒙脱石、高岭石粘土到伊利石、绿泥石粘土矿物的转变带，成矿深度在2000m以上，成矿热液来源于粘土矿物脱水产生的异常高压流体或沿断裂带天水循环形成的成矿流体。这种流体形成的热液矿物稀土元素分配显示中稀土富集的分配模式，并明显富元素钇，这种流体中形成的黄铁矿中Co／Ni值显示沉积到热液成因的过渡类型，因此与砂岩铜矿中热液矿物的稀土分布模式有某些一致性。     

Hydrothermal Alteration of Limestone and Mineral Exploration of Zn-Pb Skarn Deposits in the Sako-nishi Area of the Kamioka Mine, Central Japan

Abstract: Crystalline limestone of the Sako-nishi area in the Kamioka Zn-Pb mine, central Japan, is depleted in ¹⁸O and ¹³C toward the center of mineralization due to interaction with hydrothermal fluids with a dominant meteoric water component. The relationship between isotopic composition and mineral assemblage, texture, the chemical composition of the minerals, and the bulk chemical composition in the limestone was examined. A decrease in the δ¹⁸OSMOW value correlated with: (1) increase of fine-grained calcite which is enriched in Mn and exhibits a bright cathodoluminescence, (2) progressive hy-drothermal alteration of clinopyroxene in the original limestone into tremolite within the weakly-altered zone, and into chlorite and actinolite within the strongly-altered zone, (3) dominance of hydrothermal chlorite in altered limestone having δ¹⁸O values of less than 10%. This chlorite was enriched in Fe compared to mafic minerals in the unaltered limestone. The enrichment of Fe and Mn was more conspicuous in calcite and chlorite in skarn deposits. The occurrence and chemical composition of hydrothermal minerals in the limestone, skarn, and ore indicate that the ¹⁸O–depleted zones were formed in the later stage from fluids, which were responsible for mineralization and skarnization, and for Fe and Mn enrichment. The Al, Mn, and Fe contents, and the ratios of Mg/(Mg+Mn+Fe), Al/Mg, and Mn/Sr in the hydrochloric acid leachate of limestone varied with decreasing δ¹⁸O and δ¹³C values, reflecting increases in high-Mn calcite and high-Fe chlorite. These indexes were useful for the identification of hydrothermally altered limestone. Furthermore, the potential score weighted by each index was more effective and accurate means of detecting promising mineralization zones. An anomalous potential score due to the presence of hydrothermal minerals in the outcropping limestone occurred along the Atotsu–1GO fault. This structure indicates that the skarn deposits of the Sako-nishi area belong to Mozumi-type Zn–Pb skarn deposits, in which fissures and faults served as major passages for the hydrothermal fluid. High-Mn carbonate and high-Fe chlorite widely occur in base-metal vein deposits and Zn-Pb type skarn deposits. Leaching of altered rock with hydrochloric acid in addition to stable isotope composition and cathodoluminescence imaging is effective for geochemical exploration for hydrothermal deposits because it makes possible the detection of the elemental composition of hydrothermal minerals such as chlorite and carbonate and because of the rapidity and convenience of analysis.     

Deposition of talc — kerolite–smectite — smectite at seafloor hydrothermal vent fields: Evidence from mineralogical,geochemical and oxygen isotope studies

Talc, kerolite–smectite, smectite, chlorite–smectite and chlorite samples from sediments, chimneys and massive sulfides from six seafloor hydrothermal areas have been analyzed for mineralogy, chemistry and oxygen isotopes. Samples are from both peridotite- and basalt-hosted hydrothermal systems, and basaltic systems include sediment-free and sediment-covered sites. Mg-phyllosilicates at seafloor hydrothermal sites have previously been described as talc, stevensite or saponite. In contrast, new data show tri-octahedral Mg-phyllosilicates ranging from pure talc and Fe-rich talc, through kerolite-rich kerolite–smectite to smectite-rich kerolite–smectite and tri-octahedral smectite. The most common occurrence is mixed-layer kerolite–smectite, which shows an almost complete interstratification series with 5 to 85% smectitic layers. The smectite interstratified with kerolite is mostly tri-octahedral. The degree of crystal perfection of the clay sequence decreases generally from talc to kerolite–smectite with lower crystalline perfection as the proportion of smectite layers in kerolite–smectite increases.Our studies do not support any dependence of the precipitated minerals on the type/subtype of hydrothermal system. Oxygen isotope geothermometry demonstrates that talc and kerolite–smectite precipitated in chimneys, massive sulfide mounds, at the sediment surface and in open cracks in the sediment near seafloor are high-temperature (> 250 °C) phases that are most probably the result of focused fluid discharge. The other end-member of this tri-octahedral Mg-phyllosilicate sequence, smectite, is a moderate-temperature (200–250 °C) phase forming deep within the sediment (～ 0.8 m). Chlorite and chlorite–smectite, which constitute the alteration sediment matrix around the hydrothermal mounds, are lower-temperature (150–200 °C) phases produced by diffuse fluid discharge through the sediment around the hydrothermal conduits. In addition to temperature, other two controls on the precipitation of this sequence are the silica activity and Mg/Al ratio (i.e. the degree of mixing of seawater with hydrothermal fluid). Higher silica activity favors the formation of talc relative to tri-octahedral smectite. Vent structures and sedimentary cover preclude complete mixing of hydrothermal fluid and ambient seawater, resulting in lower Mg/Al ratios in the interior parts of the chimneys and deeper in the sediment which leads to the precipitation of phyllosilicates with lower Mg contents. Talc and kerolite–smectite have very low trace- and rare earth element contents. Some exhibit a negative or flat Eu anomaly, which suggests Eu depletion in the original hydrothermal fluid. Such Eu depletion could be caused by precipitation of anhydrite or barite (sinks for Eu²⁺) deeper in the system. REE abundances and distribution patterns indicate that chlorite and chlorite–smectite are hydrothermal alteration products of the background turbiditic sediment.