辽东硼矿床矿石的稀土元素特征及其地质意义

辽东硼矿床赋存于辽东古元古代的镁质火山-沉积地层中.矿石与含矿岩系具密切的交代继承关系：（1）矿石与含矿镁质岩石具交代继承的矿物组成、结构构造;（2）矿石的稀土元素特征主要受含矿岩系岩石的控制,在混合岩化弱的含矿岩系中,矿石的稀土元素特征与含矿镁质岩石相似,而在混合岩化强的含矿岩系中的矿石与混合岩的稀土元素特征相近;（3）辽东的硼矿床的形成是在混合岩作用过程中形成的含硼成矿流体交代镁质岩石的结果.     

硼的地球化学性质及其在俯冲带的循环与成矿初探

硼是广泛应用于化工、农业、材料科学及核工业领域的重要元素。硼与氢的核聚变反应是未来具备运用潜力的清洁能源。硼作为典型的亲石元素,是高度不相容元素。硼元素容易富集于蚀变洋壳及蛇纹石化地幔橄榄岩中。而在板块俯冲过程中,由于硼具有强的流体活动性,会优先赋存于流体中。因此,当蛇纹石化的大洋岩石圈及覆于其上的沉积物在俯冲过程中发生脱水,这使得弧前地幔楔发生大规模的蛇纹石化。此时大量硼元素很可能随俯冲流体释放并封存于弧前地幔楔中。目前已发现的超大型硼矿床主要位于聚合型板块边缘,尤其土耳其拥有世界上最大的硼酸盐储量。我们推测这些矿床的形成基础条件之一可能与弧前高度蛇纹石化的地幔楔有关。尤其是在洋 陆俯冲环境,弧前蛇纹岩或蛇绿混杂岩首先通过俯冲侵蚀再循环到火山弧岩浆中,使得岩浆更富集硼。随后弧火山喷发大量富硼的火山岩、岩浆热液及水气。在岩浆冷却过程中,硼元素析出、沉淀于火山表面,并伴随风化、侵蚀过程汇聚至碰撞造山带的封闭湖盆之中。此外,干冷的气候条件下也进一步促进了硼的成矿。我国具有形成大型、超大型硼矿的地质条件,应加大研究及探勘力度,并适当购买硼作为战略储备。     

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。     

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.      

硼矿床含硼地层的二元结构模式

通过对与沉积作用有关的硼矿床结构的研究表明，含硼地层都具有二元结构，即下部是富含硼的凝灰岩、安山岩或酸性岩，是成矿的物质基础；其上是弱盐化－强盐化环境下形成的含硼矿层的蒸发岩沉积，蒸发岩环境是成矿的必要条件。二元结构的含硼地层的形成与裂谷构造、板块俯冲带和碰撞带的发生、发展、演化紧密相关。硼矿床含硼地层二元结构模式的提出，将硼矿床成矿机制进一步深化，具有理论意义和指导找矿的实际意义     

辽东古元古宙成矿带中的变质蒸发岩及其意义

辽宁东部－吉林南部的古元古宙变质岩系中存在一些成分特殊的岩石。这些岩石的出现与蒸发岩有关，其突出特征是富含K、Na、B、Mg等组分。蒸发岩是特殊环境下（干旱气候）的沉积产物，其物质组成以Na、K、Ca的盐类矿物为主，还可含硼酸盐等组分。识别蒸发岩，尤其是变质蒸发岩的标志可以归纳为以下几点：（1）岩相及岩性组合；（2）特殊的岩石产状和组构；（3）高盐度的流体包裹体溶液和（4）硼、磷等同位素数据。辽东古元古宙大型硼矿床中电气石岩与硼酸盐之间的密切 成因关系提供了认识SEDEX型金属矿床中广泛分布的电气石岩和成矿流体成因的钥匙。     

Occurrence of rare borate minerals: Veatchite-A,tunellite, teruggite and cahnite in the Emet borate deposits,Turkey

The Emet borate deposits were formed in two separate basins, possibly part of an interconnected lacustrine playa lake, in areas of volcanic activity, fed partly by thermal springs and partly by surface streams. The borates are interlayered with tuff, clay and marl. Limestone occurs above and below the borate lenses. Sediments in both basins are similar but borate minerals show different mineralogical and geochemical features in the two basins.The Emet borate deposits are the only deposits known to contain any of the minerals veatchite-A, tunellite, teruggite and cahnite. Principal minerals are colemanite, with minor ulexite, hydroboracite and meyerhofferite.Thermal springs associated with local volcanic activity are thought to be the source of the borates. The initial brines from which the borates crystallized are deduced to have been high in sulphite and sulphate, low in chloride, and hence it is assumed that the initial brines were fed at all times by abundant calcium and boron with minor amounts of arsenic, strontium and sulphur. Realgar, celestite and native sulphur are almost ubiquitous in borates and sediments, and appear to have formed at all stages during deposition and diagenesis.The early colemanite, meyerhofferite, ulexite and teruggite nodules were probably formed directly from brines penecontemporaneously within the unconsolidated sediments below the sediment/water interface, and continued to grow as the sediments were compacted. Later generations of colemanite occur in vugs, veins and as fibrous margins to colemanite nodules. Tunellite appears to have formed during diagenesis with enrichment of Sr in some places. Diagenetic alterations include the partial replacement of colemanite by veatchite-A, cahnite, hydroboracite and calcite.     

Zircon SHRIMP U‐Pb Dating of the Tourmalinites from Boron‐bearing Series of Borate Deposits in Eastern Liaoning and its Geological Implications

This paper focused on the zircon sensitive high resolution ion micro-probeU-Pb geochronology of the tourmalinites from boron-bearing series of borate deposits in Eastern Liaoning. The zircons commonly have core-rim structures, most cores show oscillatory zoning in cathodoluminescence and plane polarized light images, suggesting a magmatic detrital origin. Ages of the magmatic detrital zircons from the hyalotourmalite samples (N13) and (N14) are 2175?±?5??Ma and 2171?±?9??Ma, respectively. Moreover, metamorphic zircon from the sample (N13) shows an age of 1906?±?4??Ma. Zircon core and rim from the hyalotourmalite sample (N02) record ages of 2171?±?6??Ma and 1889?±?62??Ma, which are explained as indicating the formation and metamorphic ages. Combined with the geological and geochemical studies, it can be concluded that the tourmalinites are formed during sedimentary exhalative mineralizations in the mid-Paleoproterozoic (~2170?Ma) and underwent the metamorphism in the late-Paleoproterozoic (~1900?Ma). The tourmalinites are the products of submarine acid volcanism in the extension rifting phase of the Liaoji Paleoproterozoic Rift, the rock-forming materials of which are derived from the mantle sources with recycling crustal contamination. The emergence of tourmalinites not only indicates the mid-Paleoproterozoic tectonic-magmatic processes, but also provides impetus, heat and material sources for the mineralization of borate deposits in Eastern Liaoning.     

中国钼矿床的时空分布及成矿背景分析

我国钼资源十分丰富,目前已发现钼矿床四百余个,它们具有成带分布的特点。本文在钼矿床地质特征基础上,系统总结了钼矿床和含钼矿床的成矿年代(依据辉钼矿Re-Os年龄),结果显示我国钼矿床空间上可分为东秦岭-大别、兴-蒙、长江中下游、华南、青藏和天山-北山六大钼成矿带;成矿时代上,钼成矿作用分为古元古代(1882~1804Ma)、早古生代(480~420Ma)、晚古生代(412~260Ma)、中生代印支期(251~209Ma)、中生代燕山期(194~77Ma)和新生代(65~13Ma)等六个阶段,主要集中于中生代和新生代。元古宙形成的钼矿床分布于东秦岭-大别钼成矿带,古生代钼矿床主要分布于天山-北山钼成矿带,中生代钼矿床在中国东部广泛分布,新生代钼矿床全都分布于青藏钼成矿带。我国古元古代钼矿床(1882~1804Ma)形成于古陆块之间俯冲碰撞背景下的岛弧环境(东秦岭-大别);早古生代钼矿床(480~420Ma)形成于不同构造单元由挤压向伸展转换的岛弧或陆缘弧环境(东秦岭-大别、兴-蒙和华南);晚古生代钼矿床(412~260Ma)形成于古亚洲洋壳俯冲的岛弧环境(兴-蒙);中生代印支期钼矿床(251~209Ma)形成于板块碰撞及后碰撞背景(东秦岭-大别、兴-蒙和天山-北山)或洋壳俯冲的背景(青藏);燕山期钼矿床形成于古太平洋板块俯冲转向及其后伸展体制下岩石圈减薄拆沉环境(东秦岭-大别、兴-蒙、长江中下游和华南),燕山晚期钼矿床(85~77Ma)形成于碰撞后的伸展背景(青藏);新生代(65~13Ma)钼矿床形成于印度板块与欧亚板块陆陆碰撞及其后的伸展背景(青藏)。我国钼成矿作用受到了环太平洋构造带(东秦岭-大别、兴-蒙、长江中下游和华南)、中亚造山带(天山-北山、兴-蒙)和特提斯构造带(青藏)三大构造体制的影响。     

辽东-吉南地区硼矿床地质特征及成矿远景

产于辽东-吉南地区裂谷盆地中的古元古界辽河群里尔峪组为一套含硼岩系。其中划分的上、中、下三个含硼岩系分别赋存硼镁石-遂安石型、含磁铁矿-硼镁石型和硼镁铁矿型三类硼矿床。硼矿的发育与地层、围岩物质成分、围岩厚度及电气变粒岩等均有一定关系。通过分析研究,划分出营口后仙峪外围、风城和宽甸3个成矿远景区。并圈定出后仙峪硼矿外围、凤城县东汤镇西山、宽甸牛皮闸—大西岔、集安太平沟—小西沟、宽甸县老阳沟等4个找矿靶区。     

辽东-吉南硼矿的控矿因素及成矿作用研究

文章从区域成矿理论出发,分析了含硼岩系、镁质容矿岩石、区域变质及混合岩化热液交代作用、构造控制和改造等4大控矿因素在成矿过程中的作用。研究表明,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、构造改造活动是硼矿形成的必要条件;辽东-吉南硼矿的成矿作用为原始火山-沉积初始富集和部分熔融含硼热液交代镁质岩石的成矿作用过程。     

中国北方中新生代盆地砂岩型铀超常富集的驱动力

中国北方中新生代陆相沉积盆地存在大规模的砂岩型铀超常富集的现象,富集的驱动力究竟来自哪里?众所周知,砂岩型铀具有水成性、动态性、易聚散性等特征,非常活跃,这些特征决定了任何一次构造隆升对其超常富集和早期矿床的重置都起关键性作用,尤其是新生代构造的作用更加不容忽视。本文分析了新生代欧亚板块与印度板块、太平洋板块的时空构造特征及其与沿线盆地铀富集的时空关系,结果表明:①欧亚板块与印度板块在古新世开始碰撞,中新世经历快速隆升,形成现今巨型青藏高原和蒙古高原,改造影响了沿线中新生代盆地,形成一系列有利于砂岩型铀富集的大陆斜坡带,沿着北西西构造线昆仑山脉、天山山脉、祁连山脉等所围限的准噶尔、伊犁、塔里木、吐哈、柴达木、银额等盆地内侏罗纪、白垩纪、新近纪沉积地层中发育大规模铀的超常富集现象。②太平洋板块在渐新世-中新世低角度北西西向俯冲于欧亚板块之下,造成大兴安岭-太行山等系列山脉的快速隆升和沿线盆地内部褶皱逆冲断裂构造的形成,在海拉尔、二连、鄂尔多斯、松辽等盆地内侏罗纪、白垩纪、新近纪沉积地层中发育铀富集现象。③北方系列盆地砂岩型铀储层的时代无论是中生代还是新生代,铀成矿时间绝大部分集中在渐新世...     

大陆岩石圈伸展与斑岩铜矿成矿作用

华南地区自古生代以来一直属于陆内构造演化环境。华南陆内伸展型斑岩铜矿主要形成于早侏罗世、晚侏罗世和早白垩世3个时期,其中晚侏罗世成矿期与华南中生代大规模钨锡成矿作用的形成基本属于一个时期。这些斑岩型矿床的时空分布与同时期的俯冲带在时间上和空间上具有明显不协调的关系,且与俯冲有关的、后俯冲伸展背景以及陆陆碰撞有关的斑岩铜矿的线性分布特点明显不同,尤其是早白垩世斑岩铜矿的分布明显呈面状分布,与华南中生代地壳明显减薄的区域基本一致。虽然这3个时期的斑岩型铜矿在地球化学上显示出弧岩浆岩的特点,但是地质事实证明在这3个时期,华南岩石圈发生了明显的伸展作用,尽管每个时期华南不同地区岩石圈伸展的程度可能不同。因此,我们把华南这种类型的斑岩铜矿归称之为"陆内伸展型"斑岩铜矿。陆内伸展型斑岩铜矿的成矿机制可能是岩石圈伸展背景下软流圈上涌导致陆下岩石圈地幔或者下地壳被改造有关。     

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.     

辽吉地区含硼岩系中斜长角闪岩地球化学

辽吉地区古元古界含硼岩系经过了中高级区域变质和混合岩化作用,主要由黑云母变粒岩、浅粒岩、电气石变粒岩、斜长角闪岩及富镁质含矿岩石组成,局部形成层状混合花岗岩。斜长角闪岩在含硼岩系内各个岩组中都有分布,岩石化学分析显示斜长角闪岩具有富钠、富镁、高铁、高钛特征,MgO含量成多众数值分布,辽东地区斜长角闪岩MgO含量高于集安地区,并且各类斜长角闪岩中硼含量都较高。辽东地区B、Cr、Ni明显高于集安地区,显示为海相地幔岩浆岩特征更为明显,集安地区则显示为变质热液改造特征。辽东地区斜长角闪岩的稀土地化显示为热液改造的幔源岩浆特征,集安地区的斜长角闪岩的稀土地化显示为壳源沉积岩特征。含硼岩系斜长角闪岩与浅粒岩稀土配分模式相似,表明部分斜长角闪岩与浅粒岩具有岩浆成因特征,而电气变粒岩、黑云变粒岩与混合岩稀土富集模式显示海相沉积泥岩变质成因,混合岩化及变质热液对原岩具有明显改造作用。斜长角闪岩石的地球化学特征表明原岩环境为富钠、富硼的海相环境,原岩主要是海相富镁拉斑玄武岩和海相泥质岩。     

In situ LA–MC–ICP–MS boron isotope and zircon U–Pb age determinations of Paleoproterozoic borate deposits in Liaoning Province,northeastern China

A large number of Paleoproterozoic borate deposits are hosted by the lower units of a volcanic-sedimentary sequence in Liaoning Province, northeastern China, and are a major source of boron in China. The ore-bearing wall rocks in the deposits are serpentinized ultrabasic rocks and carbonates, with layered leptynites, leptites, amphibolites, and migmatites adjacent to the ore. Both the borate ores and country rocks contain tourmaline, although the country rocks have much lower abundances of the mineral. Based on in situ boron isotope measurements using laser ablation–multi-collector–inductively coupled plasma–mass spectrometry (LA–MC–ICP–MS), boron isotope data show that: (1) δ¹¹B values of borate ores range from + 6.8‰ to + 13.9‰ (mean + 10.8‰); (2) tourmalines from the borate ores have δ¹¹B values from + 9.5‰ to + 12.7‰; and (3) the wall rocks within the borate ores yield slightly lower δ¹¹B values ranging from + 5.7‰ to + 7.6‰, and those outside the deposits from − 9.9‰ to − 5.9‰. Positive δ¹¹B values in borates as well as in tourmalines inside the mining area indicate that boron in these Paleoproterozoic borate deposits was derived from marine evaporites. δ³⁴S_V-CDT (where V-CDT is Vienna Canyon Diablo Troilite) values of borate ores, serpentinized marbles, and anhydrites range from + 16.1‰ to + 24.7‰, whereas δ¹³C_V-PDB (where V-PDB is Vienna Pee Dee Belemnite) values of marbles range from + 3.2‰ to + 5.9‰. These isotopic characteristics are interpreted to reflect formation in a marine evaporative environment. LA–MC–ICP–MS zircon weighted²⁰⁷Pb/²⁰⁶Pb ages of leptite and serpentinized olivine basalt from the hanging wall of the borate deposits are 2139 ± 13 Ma and 2130 ± 19 Ma, respectively. Therefore, the (~ 2.2 Ga) borate deposits may have originated from marine evaporative boron-bearing sediments, which were interbedded within bimodal volcanic rocks during the early stages of development of the Liaoji rift.     

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.     

Gold mineralization in China: Metallogenic provinces,deposit types and tectonic framework

We present a review of major gold mineralization events in China and a summary of metallogenic provinces, deposit types, metallogenic epochs and tectonic settings. Over 200 investigated gold deposits are grouped into 16 Au-metallogenic provinces within five tectonic units such as the Central Asian orogenic belt comprising provinces of Northeast China and Tianshan-Altay; North China Craton comprising the northern margin, Jiaodong, and Xiaoqinling; the Qinling-Qilian-Kunlun orogenic belt consisting of the West Qingling, North Qilian, and East Kunlun; the Tibet and Sanjiang orogenic belts consisting of Lhasa, Garzê-Litang, Ailaoshan, and Daduhe-Jinpingshan; and the South China block comprising Youjiang basin, Jiangnan orogenic belt, Middle and Lower Yangtze River, and SE coast. The gold deposits are classified as orogenic, Jiaodong-, porphyry–skarn, Carlin-like, and epithermal-types, among which the first three types are dominant.The orogenic gold deposits formed in various tectonic settings related to oceanic subduction and subsequent crustal extension in the Qinling-Qilian-Kunlun, Tianshan-Altay, northern margin of North China Craton, and Xiaoqinling, and related to the Eocene–Miocene continental collision in the Tibet and Sanjiang orogenic belts. The tectonic periods such as from slab subduction to block amalgamation, from continental soft to hard collision, from intracontinental compression to shearing or extension, are important for the formation of the orogenic gold deposits. The orogenic gold deposits are the products of metamorphic fluids released during regional metamorphism associated with oceanic subduction or continental collision, or related to magma emplacement and associated hydrothermal activity during lithospheric extension after ocean closure. The Jiaodong-type, clustered around Jiaodong, Xiaoqinling, and the northern margin of the North China Craton, is characterized by the involvement of mantle-derived fluids and a temporal link to the remote subduction of the Pacific oceanic plate concomitant with the episodic destruction of North China Craton. The Carlin-like gold metallogenesis is related to the activity of connate fluid, metamorphic fluid, and meteoric water in different degrees in the Youjiang basin and West Qinling; the former Au province is temporally related to the remote subduction of the Tethyan oceanic plate and the later formed in a syn-collision setting. Porphyry–skarn Au deposits are distributed in the Tianshan-Altay, the Middle and Lower Yangtze River region, and Tibet and Sanjiang orogenic belts in both subduction and continental collision settings. The magma for the porphyry–skarn Au deposits commonly formed by melting of a thickened juvenile crust. The epithermal Au deposits, dominated by the low-sulfidation type, plus a few high-sulfidation ones, were produced during the Carboniferous oceaic plate subduction in Tianshan-Altay, during Early Cretaceous and Quaternary oceanic plate subduction in SEt coast of South China Block, and during the Pliocene continental collision in Tibet. The available data of different isotopic systems, especially fluid D–O isotopes and carbonate C–O systems, reveal that the isotopic compositions are largely overlapping for different genetic types and different for the same genetic type in different Au belts. The isotopic compositions are thus not good indicators of various genetic types of gold deposit, perhaps due to overprinting of post-ore alteration or the complex evolution of the fluids.Although gold metallogeny in China was initiated in Cambrian and lasted until Cenozoic, it is mainly concentrated in four main periods. The first is Carboniferous when the Central Asian orogenic belt formed by welding of micro-continental blocks and arcs in Tianshan-Altay, generating a series of porphyry–epithermal–orogenic deposits. The second period is from Triassic to Early Jurassic when the current tectonic mainframe of China started to take shape. In central and southern China, the North China Craton, South China Block and Simao block were amalgamated after the closure of Paleo-Tethys Ocean in Triassic, forming orogenic and Carlin-like gold deposits. The third period is Early Cretaceous when the subduction of the Pacific oceanic plate to the east and that of Neo-Tethyan oceanic plate to the west were taking place. The subduction in eastern China produced the Jiaodong-type deposits in the North China Craton, the skarn-type deposits in the northern margin (Middle to lower reaches of Yangtze River) and the epithermal-type deposits in the southeastern margin in the South China Block. The subduction in western China produced the Carlin-like gold deposits in the Youjiang basin and orogenic ones in the Garzê-Litang orogenic belt. The Cenozoic is the last major phase, during which southwestern China experienced continental collision, generating orogenic and porphyry–skarn gold deposits in the Tibetan and Sanjiang orogenic belts. Due to the spatial overlap of the second and third periods in a single gold province, the Xiaoqinling, West Qinling, and northern margin of the North China Craton have two or more episodes of gold metallogeny.     

Origin of the late Cretaceous potash-bearing evaporites in the Vientiane Basin of Laos: δ11B evidence from borates

The evaporites on the Khorat Plateau comprise one of the largest potash deposits in the world, and their origin has long been a controversial problem. Based on boron isotope measurements from borate as a good indicator to distinguish marine and nonmarine evaporites, the borates of potash layers were identified and the results were used to indicate the source of evaporites in the study area as representative of the whole Khorat Plateau. The results show that the main borates include boracite and hilgardite. The boracites occur as crystals and ooids with minor amounts of hilgardite aggregates. The range of δ¹¹B values is from +21.30‰ to +32.94‰ (averaging +29.74‰) and falls in the range of marine borates. The δ¹¹B values in the potash layer are seemingly variable because of the influence of fluvial influx when salts were precipitating. Although the composition of seawater has possibly been modified, the evidence from boron isotopic composition implies that the evaporites in the study area or in the whole Khorat Plateau are marine deposits.     

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.