Genesis and composition of borates in the metasomatically altered dolomitic, rhodochrosite, and calcareous marbles of Japan

The paper addresses the composition and genesis of endogenous borates from hypabyssal skarn deposits of Japan (Honshu Island) that were formed after dolomitic, rhodochrosite, and calcareous marbles in the contact aureoles of magmatic intrusions of diverse felsicity (from granites to diorite-monzonites). Metasomatic bodies formed at the prograde stage of the mineral formation are characterized by clearly expressed zoning of primitive type. Borates occur in the calciphyres at the Neichi, Kaso, and Rito mines, and are developed in calcitic marbles at the Fuka mine. Depending on initial composition of carbonate rocks, borates are represented by suanite, kotoite, jimboite, and takedaite in the outer zones of spinel-forsterite and galaxite-jacobsite-tephroite calciphyres or calcitic marbles, respectively. It was shown that early borates are subjected to hydration that is expressed in variable deficit of boron. At the next stages of hydrothermal mineral formation, they are replaced by pertsevite, wiserite, sibirskite, and other borates.     

Genesis and composition of endogenous borates in the skarns of the Eastern and Central Pyrenees

This paper addresses the genesis and composition of endogenous borates and other minerals from the magnesian-skarn aureoles at the contacts between dolomites and Hercynian granitoid intrusions of the Eastern and Central Pyrenees (Querigut and Costabonne peak massifs in France, and the Monchi deposit in Spain). It was shown that these occurrences and other magnesian skarns in the Sierra Morena Range, Spain, genetically belong to the periclase depth facies: zoned metasomatic aureoles of dolomites of primitive structure complicated by the development of periclase marble zones. The near-contact zones of the intrusions are represented by the granitoids of increasing basicity and alkalinity, which indicates the assimilation of host rocks by overheated granitic magmas. The postmagmatic stage was marked by the formation of magnesium and magnesium-iron borates of diverse composition in calciphyres and marbles, replacement of forsterite by humites in the calciphyres, and the development of silicates of decreasing Mg mole fraction after pyroxene skarns; the latter is accompanied by magnesium migration into the outer zones of the aureole. It was determined that the studied deposits of France contain boron minerals (kotoite, suanite, and pertsevite) previously unknown for this region in association with fluoborite and late szaibelyite. In skarns of Spain, the high-Fe borates are represented by monomineral aggregates of fine-prismatic parallel oriented crystals of vonsenite or its coarse-grained masses. It is conceivable that sulfide-bearing magnesian skarns and calciphyres of the studied deposits contain magnesium hydroxysulfides: tochilinite (after pyrrhotite) and valleriite (after chalcopyrite).     

Composition and genesis of endogenous borates from the Pitkáranta ore field,Karelia

The borate mineralization of the Pitkáranta skarn field of Karelia is localized in metasomatically altered Proterozoic dolomites. In the contact aureole of rapakivi granites, the zoning of magnesian skarns includes spinel-diopside or fassaite skarns with syngenetic magnetite and spinel-forsterite calciphyres surrounded by periclase marbles, which confirms their hypabyssal genesis. Stringer-stockwork bodies developing in the brecciation zone at the roof show a primitive zoning consisting of an inner diopside and an outer forsterite calciphyre zone grading into a dolomitic marble. All these zones inherited the Ca/Mg ratio of the primary carbonate rocks. Rhythmically banded textures observed in the skarns and calciphyres of the deposits studied suggest their formation under thermodynamically disequilibrium conditions typical of hypabyssal metasomatites. Magnesium and magnesium-iron borates in marbles and calciphyres and beryllium borates in greisens were formed during the postmagmatic stage. Data are reported on the chemical composition and genesis of suanite, kotoite, ludwigite, hulsite, pertsevite, fluoborite, szaibelyite, and humites from the Hopunvaara, Klara, Lupikko, and Herberz deposits. The deficit of boron in magnesian borates is related to their endogenous hydration. Data on hambergites and berborite are given according to E.I. Nefedov.     

Geochemical features of the endogenous hydration of magnesium borates

The hydration of kotoite, suanite, and szaibelyite with ultimate brucitization was considered by the example of skarn deposits of endogenous magnesium borates. This process involves isomorphic substitution of hydroxyl groups for equivalent amounts of boron radicals. The degree of kotoite hydration varies from 2 to 40%. The hydration products reach Mg₂[BO₃](OH) and Mg₃{[BO₃]_1.5(3OH)_0.5}₂, are isostructural with orthoborate, approach Mg₃[BO₃](OH)₃, and do not contain szaibelyite. Kotoite in association with humites is replaced by Si-bearing pertsevite with variable F content. In contrast, suanite is directly replaced by szaibelyite, with preservation of its relics or inheritance of crystal shapes. The composition of szaibelyite also changes owing to the partial substitution of hydroxyl for boric radical (in hydroxylszaibelyite) or an increase in H₂O content (in hydroszaibelyite), which does not rule out its brucitization. The hydration of borates is caused by a decrease in boron content in hydrothermal solutions with decreasing temperature. The borate assemblages studied are characterized by low F content, the increase of which leads to the appearance of F-bearing low-and moderate-Si pertsevites in humite-kotoite calciphyres. The investigation of hydration became possible owing to the direct determination of boron content in borates. The obtained data were compared with experimental studies on the hydrothermal synthesis of kotoite, suanite, szaibelyite, and fluoborates. The established geochemical tendencies in the hydration of boron minerals, which is accompanied by partial migration and loss of boron into the adjacent rocks, are important for estimating the quality of borate ores in magnesian-skarn deposits.     

Skarn-greisen deposits of the lost river and mount ear ore field,Seward Peninsula,Alaska, United States

The boron, tin, tungsten, beryllium, and fluorite deposits of the York Range, Seward Peninsula, represent the continuation of the Asian segment of the Pacific ore belt and are globally conjugate with the Verkhoyan-Chukotka ore province of Northeastern Russia. They are localized in the alteration aureoles of dolomites and limestones of the Paleozoic Port Clarence Formation at the contact with the Mesozoic leucocratic granites and genetically belong to the magnesian-skan ore formation. Ore-generating process developed in marbles, skarns, and greisens under hypabyssal conditions in several stages and was accompanied by sequential formation of polymineral assemblages. Early mineralization is represented by magnetite in prograde pyroxene skarns after dolomites. Postmagmatic ore stage is represented by formation of endogenous borates, including their tin-bearing Mg-Fe species, in the magnesian skarns, superposition of calcareousskarn assemblages containing calcium borates, borosilicates, and scheelite, formation of cassiterite and wolrframite in the greisenized granites, and precipitation of sulfides, chrysoberyl, and fluorite. The mineral composition of the rocks and ores was formed under the influence of F-bearing hydrothermal solutions, which caused the presence of fluorine in borates, rock-forming silicates, and the replacement of calcite by fluorite. Boron, tin, beryllium, and fluorine participate at all the stages of endogenous process, but the mineral modes of their occurrence are varied, which is confirmed by data on their chemical composition. The results of studying the skarns and ores of the Alaska deposits are of great applied and scientific significance, and can be used for study of skarn-greisen deposits localized at the contacts of carbonate rocks with granite intrusions of the Pacific ore belt and other world’s regions.     

辽宁古元古代地体中富电气石岩石的成因：蒸发岩硼源的证据

辽宁东部古元古界底部地层(南辽河群)中赋存着大型的硼酸盐矿床，含矿层位中广泛分布含电气石的变粒岩和电英岩。空间上这些含电气石的岩石与硼酸盐有着密切的联系，电气石可以作为区域硼矿找矿的标志。已有研究结果表明，该地区的硼酸盐矿床是变质蒸发岩成因。本研究对该区不同产状的电气石和硼酸盐的地质特征，全岩和矿物成分、硼同位素组成进行了分析。本区的电气石包括层状和脉状两大类，而电气石的富集与硼酸盐关系密切，电英岩往往分布在硼酸盐矿体的上盘。而矿体的下盘一般不产出富电气石的岩石。当长英质脉体穿过硼酸盐矿体时，脉体中往往会富集电气石。含电气石岩石的全岩地球化学分析表明，它们的REE及其他微量元素特征以及相关性关系与周围不含电气石的同类岩石十分相似，反映出一种成因上的联系。本区电气石主要属于镁电气石一铁电气石系列，靠近硼矿体的电气石比远离硼矿体的电气石更加富镁，有着更高的Mg／Fe比值。电气石和硼酸盐的硼同位素成分分析显示出二者在同位素组成上的相似性，前者比后者的δ^11B稍低，这可能是由于热液活动过程中同位素分馏的结果。电气石的硼同位素组成在空间上显示出变化规律：远离硼酸盐矿体的电气石的δ^11B值(-5．2‰- 3．6‰)比矿体附近的电气石低(平均 10．5‰)。以上空间和成分上的关系表明硼酸盐可能是形成电气石主要的硼来源，电气石是在热液过程中通过淋滤下伏含硼蒸发岩中的硼形成含硼热液，在与上覆沉积物交代过程中形成含电气石岩石。电气石的条带是热液顺层选择交代的结果。本区电气石与硼酸盐的关系表明，层状电气石可以通过含硼热液交代的方式形成。变质地体中的层状电气石岩石的出现可能与变质蒸发岩有关。这一认识对区域硼矿勘查工作和变质地体的沉积环境分析有借鉴意义。     

Gold behavior during endogenic and supergene alterations of sulfides in magnesian skarns

The paper presents materials on the genesis of gold deposits of the magnesian-skarn association. It is demonstrated that sulfides are precipitated at these deposits late in the course of the mineral-forming process and often contain visible and fine gold. Post-sulfide mineral-forming processes resulted in the widespread development of hydroxisulfides: tochilinite and valleriite in high-Mg rocks and borate ores affected by serpentinization, brucitizatin, and szaibelyitization. The newly formed hydrosulfides inherit gold from the replaced sulfides. The endogenic or supergene decomposition of tochilinite and valleriite in endogenic and supergene environments stimulates the dissolution of the fine-grained gold and its remobilization, first, by hydrothermal solutions and, subsequently, by meteoric waters. The possibility is discussed of the later regeneration of gold as a consequence of electrochemical processes or at geochemical barriers. The deposition of “newly formed” gold in weathering crusts and placers is discussed, along with the significance of this process for assaying the potential of the weathering crusts and placers. It is emphasized that a significant role in this process is played by cryogenic processes, which can increase gold concentrations in naturally occurring solutions and facilitate its later regeneration. The data presented in this paper are compared with data on gold and PGE deposits of other genetic types, which are hosted in ultramafic rocks and carbonatites, i.e., rocks petrochemically similar to magnesian skarns. It is demonstrated that the occurrence of hydroxisulfides in the ores is a significant geochemical and technological problem during the exploration for sulfide ores and their mining and processing. The magnesian skarn ores of the deposits discussed in this publication were determined to be a significant source of both primary and placer gold and, perhaps, PGE also. The materials presented in the paper characterize the behavior of gold in the endogenic and supergene processes at magnesian skarn deposits.     

Types of Pyrophyllite Deposits in Foldbelts

Abstract. Pyrophyllite deposits can be divided into five types on the basis of geology and genesis. The first two types are associated with hydrothermally altered rocks in felsic and intermediate volcanogenic suites. They are characterized by their metasomatites and their subsequent mineralogic transformations under varying volcanic conditions. The third type includes deposits and occurrences of metamorphic-metasomatic genesis, which is caused by transformations of terrigenous-sedimentary interbeds in felsic volcanics under greenschist facies conditions. The fourth type is associated with low and mid-temperature stages of hydrothermal vein formation at the limits of volcanogenic and metamorphic strata. The fifth type comprises pyrophyllite occurrences in weathering crusts on metamorphic strata and metasomatite.
The formation conditions and distribution of raw pyrophyllite deposits were influenced by the geodynamic situations and geochemical conditions, such as character of tectonic dislocations, volcanism and chemical composition of hydrothermal solutions.     

中国镁质碳酸盐岩型温石棉矿床的成矿规律

镁质碳酸盐岩型温石棉矿床是成矿热液交代镁质碳酸盐岩生成的,一般均产于地台环境。按地台稳定性不同,成矿环境分为地台隆起区的基底建造类型和地台拗陷区的盖层建造类型。产于地台隆起区的矿床,以前震旦纪镁质碳酸盐岩为控矿层位。产于太古代及早、中元古代深变质结晶基底中的矿床,由区域变质及混合岩化过程中的变质热液交代碳酸盐岩成矿。产于晚元古代浅变质褶皱基底(扬子准地台)中的矿床,以岩浆期后热液或接触交代(湿矽卡岩)热液为成矿热液来源。产于地台拗陷区的矿床,均以震旦纪镁质碳酸盐岩为控矿层位。其成矿时期较晚,一般与拗陷区内岩浆活动同期,由岩浆期后热液或接触交代(湿矽卡岩)热液对镁质碳酸盐岩进行交代成矿。     

Metasomatic transformations of carbonate rocks observable in quarries of Riverside, California, United States

The results of skarn-forming processes at contacts of the multiphase Southern California Batholith with carbonate rocks accessible to study in quarries in Riverside, California, involve prograde metasomatic transformations of marmorized dolomites and calcareous rocks in contact with granitic melts and contaminated magmas. The processes of contact assimilation are proved to have been controlled by the emplacement of granitic melts overheated relative to subliquidus melts (with the overheated melts prone to approach the composition of granodiorite, syenite, and gabbro) into skarnified marbles. The degree of magma overheating was evaluated based on G.F. Smith’s data on linear melting temperature variations for anhydrous intrusive rocks with various SiO₂ concentrations (<750°C for granites and >1100°C for contaminated rocks, ΔT 350°). This corresponds to the thermal regime of the development of mineralogically contrasting hypabyssal skarn aureoles: magnesian at contacts with granite magmas and calcic at contacts with melts of high basicity. The peripheral parts of the aureoles ubiquitously contain preserved zones of forsterite calciphyres and periclase marbles, whereas skarns at mafic intrusions consist of high-temperature silicates of decreasing Mg contents: monticellite, merwinite, melilite, and spurrite. Prograde and retrograde mineralforming processes in the metasomatic rocks and their facies affiliation are analyzed, and the chemical composition of the minerals are examined. The Riverside skarn aureoles are compared with other compositionally contrasting skarn aureoles that developed in contacts with granite magmas and melts of increasing basisity.     

Geochemical Behaviors of REE and Other Trace Elements during the Formation of Strata-bound Skarns and Related Deposits: A Case Study of the Dongguashan Cu (Au) Deposit, Anhui Province, China

REE and other trace elements in the altered marbles, massive skarns and ores, as well as garnet and quartz were determined in order to examine the behaviors of trace elements during hydrothermal alteration. It is demonstrated that the high-field-strength (HFS) elements Zr, Hf, Th and Nb were immobile while other trace elements were mobile during the formation of skarns and related deposits. REE and ore-forming elements such as Cu and Ag in hydrothermally-altered marbles and skarns were provided primarily by hydrothermal fluids. In the direction transverse of the strata, the more deeply the marbles were altered, the higher the total REE abundance and the larger the negative Eu anomalies would be. The chondrite-normalized REE patterns of skarns are similar to those of the marbles, but the former are distinguished by much higher REE contents and more remarkable negative Eu anomalies. Those patterns were apparently not inherited from the marble protolith, but were controlled by garnets, which were determine     

Heterovalent isomorphism in the magnesium-iron borates

Orthorhombic magnesium-iron ludwigite-vonsenite forms a continuous isomorphic series Mg₂Fe³⁺[BO₃]O₂-Fe ₂ ²⁺ Fe[BO₃]O₂; its composition at the magnesioskarn and other deposits varies from magnesian to ferriferous members. In addition, they demonstrate isovalent substitution of Mn for Mg (in pinakiolite, blatterite, and others) and practically complete substitution of Ni for Mg (in bonaccordite). Ferric iron in the borates is substituted by isovalent Al and Cr. The incorporation of Ti, Sn, Sb, and V via heterovalent substitution has been studied in less detail. Our research revealed new manifestations of Ti-and Sn-bearing borates. They are magnesioludwigite and azoproite with variable Ti content, as well as by Sn-bearing aluminian borates formed via the 2Fe³⁺ → (Ti⁴⁺ + Mg)⁶⁺ and/or (Sn⁴⁺ + Mg)⁶⁺ substitution. The incorporation of pentavalent elements according to the scheme 3Fe³⁺ → (Sb⁵⁺ + 2Mg)⁹⁺ or (V⁵⁺ + 2Mg)⁹⁺ is not excluded. The highest Ti borates were found in the marbles and calciphyres of the Tazheran deposit in the Baikal region and Nalednoe, Dokuchan, and Titovskoe deposits in Yakutia, where azoproites contain more than 50 and even higher 75 mol % of the Mg₂(TiMg)_0.5[BO₃]O₂ end member. Aluminum magnesioludwigites from Yakutia and Chukotka simultaneously contain tin and titanium. Mount Brooks, Alaska, contains tin-bearing azoproite or its tin-bearing varieties. New data are reported on Sb-and V-bearing orthoborates. Calciphyres of Alaska contain monoclinic magnesiohulsite (Mg,Fe)₂(SnMg) _0.5 ⁶⁺ [BO₃]O₂, which is replaced by schoenfliesite MgSn(OH)₆. The studied borate occurrences belong to hypabyssal magnesian skarns of the periclase and monticellite metasomatic PT facies at contacts of dolomites with granitoid intrusions of increasing alkalinity or leucocratic granites. Their formation was related to interaction between disequilibrium kotoite and early oxides and spinellides of various compositions, on the one hand, and, on the other hand, to the influx of Ti-and Sn-bearing hydrothermal solutions.     

辽东硼矿的成矿机制及成矿模式

为了解辽东硼矿的成矿机制及建立成矿模式, 分析了含硼岩系、镁质容矿岩石、区域变质作用及混合岩化作用、构造等四大控矿因素, 发现含硼岩系具富硼特征, 容矿岩石具富镁特点并可成为硼质的沉淀剂, 含硼岩系中硼在区域变质和混合岩化过程中得到进一步活化形成含硼热液, 含硼热液在有利的构造空间交代镁质岩石即可形成镁硼酸盐型硼矿.含硼岩系、镁质容矿岩石是硼矿形成的物质基础, 区域变质及混合岩化作用、构造活动是硼矿形成的必要条件.辽东硼矿成矿模式为原始火山-沉积初始富集和部分熔融含硼热液交代镁质岩石.      

Banded iron formations of the calciphyre-metabasite-gneiss association in the East European Craton

The paper presents characteristics of the least studied iron formations of the East European Craton (Archean banded iron formations of the calciphyre-metabasite-gneiss association), a typical member of granulite complexes of the Ukrainian Shield, Belarussian-Baltic region, and Voronezh crystalline massif. They are mainly composed of diverse metasedimentary rocks: aluminous gneisses; silicate-magnetite, magnetite, and barren quartzites; eulysites; calciphyres; and marbles associated with metavolcanic rocks. Data on chemical compositions of the metasedimentary rocks are summarized for the first time and their possible primary mineral composition has been reconstructed using the MINLITH software. It is shown that they could be formed from a lithogenetic series of sediments linked by gradual transitions and geochemical commonness of sediments: from fine-grained terrigenous insufficiently mature sediments to chemogenic sediments depleted in terrigenous material (ferruginous-siliceous, ferruginous-siliceous-carbonate, siliceous-carbonate, and carbonate sediments). The inferred primary mineral assemblage indicates sedimentation in the central parts of large paleobasins in a reducing environment characterized by deficit of oxygen and excess of carbon dioxide. Lithological specifics of the banded iron formations in different regions presumably reflect different distances of sedimentation zones from submarine hydrothermal discharge sites and sources of terrigenous material. The banded iron formations at the present-day erosion section of basement represent metamorphosed fragments of the lateral-facies zoning of rocks of the Archean sedimentary basins (or a single basin) of the East European Craton. Unlike other Early Precambrian banded iron formations of the East European Craton, rocks of the calciphyre-metabasite-gneiss association are marked by a high Mn content.     

我国海相火山铁铜矿床的成因类型及其某些成矿特征的讨论

建国以来,我国在火山矿床的地质勘探和研究工作方面都积累了丰富的资料,并且有不少关于矿床成因和类型划分方面的叙述。作者近几年来有机会接触到一些这方面的工作。这里试将海底火山活动的重要产物之一——海相火山铁铜矿床作一个比较系统的成因类型的划分。限于水平,不当之处请批评指正。     

Titanian clinohumite and geikielite in marbles from the Bergell contact aureole

The assemblage titanian clinohumite+forsterite +spinel+calcite is widespread in marbles from the eastern Bergell contact aureole (Switzerland/Italy). The Bergell titanian clinohumites vary considerably in composition (TiO₂: 0.19 to 2.05 wt%, F: 2.2 to 3.4 wt%). Electron microprobe analyses show that the titanian clinohumites contain less than detectable amounts of Co, Cu, Ni, Zn, Al, Cr and Cl. No trace of ferric iron could be detected by Moessbauer spectroscopy. Moreover, the Moessbauer spectra indicate that Fe²⁺ occurs only in one of the five octahedral positions in the crystal structure of the studied titanian clinohumite. Under the conditions of the contact metamorphism (600–650° C, 3 kb total pressure) the compositional variation along the exchange vector TiO₂M_–1 (OH,F)_–2 takes place at constantx _OH which is fixed by the pore fluid. Titanian clinohumite sometimes contains geikielite inclusions which strongly fractionate Fe and Mn relative to titanian clinohumite. The geikielites from the Bergell marbles are poor in Cr₂O₃, Fe₂O₃ and MnO, and thus different from those found in carbonatites, kimberlites and serpentinized ultramafic rocks.     

Borate Deposits in Liaoning and Jilin Provinces, NE China: A Guide to Identifying Paleoproterozoic Collision Settings

１．ＩｎｔｒｏｄｕｃｔｉｏｎＦｉｇ．１．　ＲｅｇｉｏｎａｌｇｅｏｌｏｇｙｏｆｔｈｅｅａｓｔｅｒｎＬｉａｏｎｉｎｇＳｏｕｔｈｅｒｎＪｉｌｉｎａｒｅａ（ＡｆｔｅｒＺｈａｎｇ,１９８４）　　ＴｈｅＰａｌｅｏｐｒｏｔｅｒｏｚｏｉｃｖｏｌｃａｎｉｃｓｅｄｉｍｅｎｔａｒｙｓｅｑｕｅｎｃｅ,ｉ．ｅ．,ｔｈｅＳｏｕｔｈＬｉａｏｈｅＧｒｏｕｐａｎｄＮｏｒｔｈＬｉａｏｈｅＧｒｏｕｐ,ｅｘｔｅｎｄｓｂｅｔｗｅｅｎｔｗｏＡｒｃｈａｅａｎｃｒａｔｏｎｓ（Ｆｉｇ．１）．Ａｂｕｎｄａｎｔｍｉｎｅｒａｌｒｅｓｏｕｒｃｅｓ…     

Composition and geodynamic nature of the protoliths of diamondiferous rocks from the Kumdy-Kol deposit of the Kokchetav metamorphic belt, northern Kazakhstan

The distribution of rare earth elements was analyzed in the Early Cambrian diamondiferous calcsilicate rocks and gneisses, calciphyres, and marbles of the Kumdy-Kol deposit. These data were compared with the lithogeochemical characteristics of the sedimentary assemblages of weakly metamorphosed Late Precambrian graphite-bearing sedimentary rocks of the Kokchetav metamorphic belt. The obtained results allowed us to suppose that the protoliths of the Kumdy-Kol rocks were compositionally similar to the Late Precambrian graphite-bearing terrigenous-carbonate and sand-shale sequences of the continental shelf of the Kokchetav microcontinent, some of which were transformed in a subduction zone into diamondiferous rocks.     

Phase equilibria involving humite minerals in impure dolomitic limestones

The natural occurrence of critical assemblages among the phases clinohumite, calcite, dolomite, tremolite, forsterite, diopside, chlorite, and spinel in metamorphosed impure limestones, together with experimental and thermodynamic data, permits the calculation of phase equilibria governing the stability of clinohumite in terms of the variables P, T, and composition of a CO₂-H₂O-HF fluid. Equilibrium constant expressions are given for 23 equilibria that describe the stable phase relations between the above phases. Pure OH-clinohumite is considered to be metastable at relatively low pressures. The occurrence of clinohumite in natural marbles is the result of nonideal fluorine substitution which increases the stability of clinohumite. The stability field for clinohumite +calcite, governed primarily by the equilibrium 4forsterite+dolomite+H₂O = clinohumite+calcite +CO₂, expands to more CO₂-rich fluid compositions with increasing fluorine contents and decreasing total pressure. The F/(F+OH) ratio of clinohumite coexisting with calcite, dolomite, and forsterite is a sensitive indicator of the composition of the mixedvolatile fluid phase. The thermodynamic model is in good agreement with observed phase relations and can be used to gain useful information concerning the P-T-X _fluid conditions responsible for the formation of clinohumite.     

Boron isotopic composition of marine and nonmarine evaporite borates

Here we present the results of the boron isotopic analysis of borates from some well-known marine and nonmarine evaporite deposits. Combining new analyses with analyses from the literature yields a mean δ¹¹B value for nine marine evaporite borate analyses of 25 ± 4 permil (¹¹B/¹⁰B = 4.15 ± 0.02) and a mean for twenty-five nonmarine evaporite borates of −7 ± 10 permil (¹¹B/¹⁰B = 4.02 ± 0.04). Previous studies have shown that the boron isotopic composition of seawater (δ¹¹B ~ 40 permil, ¹¹B/¹⁰B ~ 4.20) is about four percent heavier than the boron isotopic composition of most crustal rocks and minerals. We suggest that our results for marine and nonmarine evaporite borates reflect this difference. Thus it should be possible to use boron isotopic composition as a tracer for boron of marine evaporitic origin. Applications include borate-bearing evaporite deposits and, perhaps, borate and/or borosilicate-bearing metamorphic rocks.