化学地层学研究中的若干注意事项

化学地层学因其交叉学科特点和研究对象的复杂性使得研究过程中任何一个环节的不足都将造成测试出现偏差,成果的可信度也将出现问题。高昂的成本也要求研究者把有限的资源集中到正确的目标上。近20年研究历程却恰恰将此类性质的问题陆续暴露了出来。如采样阶段获取原生性状考虑不充分或依据不足、测试阶段缺乏校验、数据分析阶段挖潜和综合利用有待完善以及化学岩、碎屑岩和生物岩各自的基础性和针对性研究有待提高等。有些问题带有相当程度的普遍性,有些甚至是严重的核心技术问题。笔者结合最新相关研究成果对上述问题进行了逐一分析和系统修正。     

Rare Earth Deposits of North America

Rare earth elements (REE) have been mined in North America since 1885, when placer monazite was produced in the southeast USA. Since the 1960s, however, most North American REE have come from a carbonatite deposit at Mountain Pass, California, and most of the world’s REE came from this source between 1965 and 1995. After 1998, Mountain Pass REE sales declined substantially due to competition from China and to environmental constraints. REE are presently not mined at Mountain Pass, and shipments were made from stockpiles in recent years. Chevron Mining, however, restarted extraction of selected REE at Mountain Pass in 2007. In 1987, Mountain Pass reserves were calculated at 29 Mt of ore with 8.9% rare earth oxide based on a 5% cut‐off grade. Current reserves are in excess of 20 Mt at similar grade. The ore mineral is bastnasite, and the ore has high light REE/heavy REE (LREE/HREE). The carbonatite is a moderately dipping, tabular 1.4‐Ga intrusive body associated with ultrapotassic alkaline plutons of similar age. The chemistry and ultrapotassic alkaline association of the Mountain Pass deposit suggest a different source than that of most other carbonatites. Elsewhere in the western USA, carbonatites have been proposed as possible REE sources. Large but low‐grade LREE resources are in carbonatite in Colorado and Wyoming. Carbonatite complexes in Canada contain only minor REE resources. Other types of hard‐rock REE deposits in the USA include small iron‐REE deposits in Missouri and New York, and vein deposits in Idaho. Phosphorite and fluorite deposits in the USA also contain minor REE resources. The most recently discovered REE deposit in North America is the Hoidas Lake vein deposit, Saskatchewan, a small but incompletely evaluated resource. Neogene North American placer monazite resources, both marine and continental, are small or in environmentally sensitive areas, and thus unlikely to be mined. Paleoplacer deposits also contain minor resources. Possible future uranium mining of Precambrian conglomerates in the Elliott Lake–Blind River district, Canada, could yield by‐product HREE and Y. REE deposits occur in peralkaline syenitic and granitic rocks in several places in North America. These deposits are typically enriched in HREE, Y, and Zr. Some also have associated Be, Nb, and Ta. The largest such deposits are at Thor Lake and Strange Lake in Canada. A eudialyte syenite deposit at Pajarito Mountain in New Mexico is also probably large, but of lower grade. Similar deposits occur at Kipawa Lake and Lackner Lake in Canada. Future uses of some REE commodities are expected to increase, and growth is likely for REE in new technologies. World reserves, however, are probably sufficient to meet international demand for most REE commodities well into the 21st century. Recent experience shows that Chinese producers are capable of large amounts of REE production, keeping prices low. Most refined REE prices are now at approximately 50% of the 1980s price levels, but there has been recent upward price movement for some REE compounds following Chinese restriction of exports. Because of its grade, size, and relatively simple metallurgy, the Mountain Pass deposit remains North America’s best source of LREE. The future of REE production at Mountain Pass is mostly dependent on REE price levels and on domestic REE marketing potential. The development of new REE deposits in North America is unlikely in the near future. Undeveloped deposits with the most potential are probably large, low‐grade deposits in peralkaline igneous rocks. Competition with established Chinese HREE and Y sources and a developing Australian deposit will be a factor.     

Petrogenesis of the Ulsan carbonate rocks from the south-eastern Kyongsang Basin, South Korea

Abstract The petrogenesis of the Ulsan carbonate rocks in the Mesozoic Kyongsang Basin of South Korea, which have previously been interpreted as limestone of Paleozoic age, is reconsidered in the present study. Within the Kyongsang Basin, a small volume of carbonate rocks, containing a magnetite deposit and spatially associated ultramafic rocks, is surrounded by sedimentary, volcanic and granitic rocks of the Mesozoic age. The simple cross‐cutting relationships and other outcrop features of the area indicate that the carbonate rocks are an intrusive phase and younger than the other surrounding Mesozoic rocks. The Ulsan carbonates have low concentrations of rare earth elements (REE) and trace elements with the carbon and oxygen isotope values in the range of δ¹³C_PDB = 2.4 to 4.0‰ and δ¹⁸O_SMOW = 17.0 to 19.5‰. Outcrop evidence and geochemical signatures indicate that the Ulsan carbonates were formed from crustal carbonate melts, which were generated by the melting/fluxing of crustal carbonate materials, caused by the emplacement‐related processes of alkaline A‐type granitic rocks. Compared to typical mantle‐derived carbonatites associated with silica‐undersaturated, strongly peralkaline systems, the relatively small size and geochemical characteristics of the Ulsan carbonates reflect carbonatite genesis in a silica‐saturated, weakly alkali intrusive system. Major deep‐seated tectonic fractures formed by the collapse of the cauldron or the rift system associated with the opening of the East Sea (Japan Sea) might have facilitated the ascent of the crustal carbonate melts.     

滇中昆阳群火成碳酸岩的发现及其意义

在武定西矿带发现的已大理岩化的滇中昆阳群（Pt2k)火成碳酸岩富含钠长石、更长石、金云母、黑云母等硅酸盐矿物，发育火成结构构造，如斑状结构、隐晶结构，流纹构造、带状构造、气孔、杏仁构造等，与沉积围岩相比，岩石高Si、Al、Fe、Mg、Mn、P，富K2O Na2O，低钙，轻稀土富集，富含Nb、Zr、Hr、U、Th、Sr、Ba、稀土等不相容元素，与粗面质、粗安质等碱性硅酸盐岩共生。（铁）白云石（＞５０％）内，含有粗面南、碳酸质熔融包裹体；流体包裹体均一温度可达４５０℃以上。其发现证明滇中元古代曾有碳酸质岩浆活动。     

生物礁岩分类方案

本文系统地提出了一个生物礁岩分类的新方案。生物礁岩首先根据次生组分的有无分为原生礁岩和次生礁岩。次生礁岩根据次生组分来源分为骨源次生礁岩和礁源次生礁岩。次生礁岩的二级分类根据次生组分的含量进行。骨源次生礁岩分为骨屑岩和含骨屑礁（灰）岩、倒骨央和含倒骨礁（灰）岩；礁源次生礁岩分为礁屑岩和含礁屑礁（灰）岩。骨屑岩根据次生组分的粒度分为巨骨砾（屑灰）岩、骨砾（屑灰）岩、细骨砾（屑灰）岩、骨砂（屑灰）岩；     

Continuous Gradations among Primary Carbonatitic, Kimberlitic, Melilititic, Basaltic, Picritic, and Komatiitic Melts in Equilibrium with Garnet Lherzolite at 3-8 GPa

Multianvil melting experiments in the system CaO–MgO–Al₂O₃–SiO₂–CO₂(CMAS–CO₂) at 3–8 GPa, 1340–1800°C, involvingthe garnet lherzolite phase assemblage in equilibrium with CO₂-bearingmelts, yield continuous gradations in melt composition betweencarbonatite, kimberlite, melilitite, komatiite, picrite, andbasalt melts. The phase relations encompass a divariant surfacein P–T space. Comparison of the carbonatitic melts producedat the low-temperature side of this surface with naturally occurringcarbonatites indicates that natural magnesiocarbonatites couldbe generated over a wide range of pressures >2·5 GPa.Melts analogous to kimberlites form at higher temperatures alongthe divariant surface, which suggests that kimberlite genesisrequires more elevated geotherms. However, the amount of waterfound in some kimberlites has the potential to lower temperaturesfor the generation of kimberlitic melts by up to 150°C,provided no hydrous phases are present. Compositions resemblinggroup IB and IA kimberlites are produced at pressures around5–6 GPa and 10 GPa, respectively, whereas the compositionsof some other kimberlites suggest generation at higher pressuresstill. At pressures <4 GPa, an elevated geotherm producesmelilitite-like melt in the CMAS–CO₂ system rather thankimberlite. Even when a relatively CO₂-rich mantle compositioncontaining 0·15 wt % CO₂ is assumed, kimberlites andmelilitites are produced by <1% melting and carbonatitesare generated by even smaller degrees of melting of <0·5%. KEY WORDS: carbonatite; CO₂; kimberlite; melilitite; melt generation     

Cenozoic Crustally-derived Carbonate-rich Magmatic Rocks in West Junggar,North Xinjiang,Western China: Geochronology,Geochemistry and Tectonic Implications

The carbonate-rich magmatic rocks of West Junggar are distributed in the Baijiantan and Darbut ophiolitic mélanges in the forms of extrusive rocks overlying the mélanges and dykes, either along the margins of the mélange or cross-cutting components of mélanges. Chilled margin and flow structures are present. A SHRIMP zircon U-Pb age of 39.7 ± 1.3 Ma indicates that these carbonate-rich rocks in West Junggar were formed during the Eocene. They have low concentrations in REEs, Th, U, Nb, Ta and are characterized by extremely low εN d(t), high(87 Sr/86 Sr)i ratios, relatively high δ18 OV-SMOW values and high δ13 CV-PDB values, which is similar with most sedimentary carbonates. Furthermore, no contemporaneous mantle-derived silicate rocks have yet been found in West Junggar. The carbonate-rich rocks in West Junggar are thus distinct from mantle-derived carbonatites and are interpreted to result from melting of the Carboniferous sedimentary carbonates at crustal levels, these rocks therefore being referred to as 'crustal carbonatites'. The Eocene crustal carbonatites in West Junggar and other Cenozoic magmatic rocks in North Xinjiang are generally situated along regional strike-slip faults or fault intersections. Therefore, we propose that the reactivation of the Darbut and Baijiantan crustal-scale strike-slip fault zones(ophiolitic mélanges), due to the far-field effects of the Indian-Eurasian collision, enables decompression melting of the underlying continental lithospheric mantle. These resulting melts ascended to the lower crust through the strike-slip faults, causing partial melting of the Carboniferous carbonaceous sediments. The crustal carbonatites in West Junggar provide a new piece of evidence for Cenozoic magmatism in North Xinjiang and are also significant for the investigation of tectono-magmatic relations in North Xinjiang and the Central Asian Orogenic Belt.     

冕宁-德昌稀土成矿带碳酸岩流体研究

通过对四川冕宁-德昌稀土矿带主要矿物中包裹体岩相学、显微测温分析与包裹体成分分析,指出由岩浆碳酸岩分异出的成矿流体为富含高密度CO2、K、Na、SO42-和多种成矿元素的超临界流体,流体以高温、高压、超高盐度、富CO2为特征,从早期到晚期流体中CO2含量增加。结合前人对碳酸岩流体、稀土矿带周边新生代盆地中无机成因CO2气藏、富钾卤水、稳定同位素和隋性气体同位素研究成果,从碳酸岩流体的性质、流体体系、碳、氦同位素组成,初步探讨了碳酸岩流体与周边新生代盆地中CO2气藏、富钾卤水的成因联系。     

Genetic and ore-forming ages of the Fe–P–(Ti) oxide deposits associated with mafic–ultramafic–carbonatite complexes in the Kuluketage block,NW China

Abstract

During the past 50 years, many geological and ore-deposit investigations have led to the discovery of the Fe–P–(Ti)-oxide deposits associated with mafic–ultramafic–carbonatite complexes in the Kuluketage block, northeastern Tarim Craton. In this paper, we discuss the genetic and ore-forming ages, tectonic setting, and the genesis of these deposits (Kawuliuke, Qieganbulake and Duosike). LA-ICP-MS zircon U–Pb dating yielded a weighted mean ²⁰⁶Pb/²³⁸U ages of 811?±?5?Ma, 811?±?4?Ma, and 840?±?5?Ma for Kawuliuke ore-bearing pyroxenite, Qieganbulake gabbro and Duosike ore-bearing pyroxenite, respectively. The CL images of the Kawuliuke apatite grains show core–rim structure, suggesting multi-phase crystallisation, whereas the apatite grains from Qieganbulake and Dusike deposits do not show any core–rim texture, suggesting a single-stage crystallisation. LA-ICP-MS apatite ²⁰⁷Pb-corrected U–Pb dating provided weighted mean ²⁰⁶Pb/²³⁸U ages of 814?±?21?Ma and 771?±?8?Ma for the Kawuliuke ores, and 810?±?7?Ma and 841?±?7?Ma for Qieganbulake and Duosike ores, respectively. The core–rim texture in apatite by CL imaging as well as two different ore-forming ages in the core and rim of the apatite indicate two metallogenic events for the Kawuliuke deposit. The first metallogenic period was magmatic in origin, and the second period was hydrothermal in origin. The initial ore-forming age of the Kawuliuke Fe–P–Ti mineralisation was ca 814?Ma and the second one was ca 771?Ma. On the other hand, the ore-forming ages of the Qieganbulake and Duosike deposits were ca 810?Ma and ca 841?Ma, respectively. Qieganbulake and Duosike deposits were of magmatic origin. Combined with previous geochronological data and the research on the tectonic background, we infer that the Kawuliuke, Qieganbulake and Duosike Fe–P–(Ti)-oxide deposits were formed in a subduction-related tectonic setting and were the product of subduction-related magmatism.     

热水沉积岩及矿物岩石标志

热水沉积物不同于普通沉积物 ,主要与热水流体类型有关。文中把热水流体划分为中高温热水流体与中低温热水流体。中高温热水沉积岩包括钾长石岩、硅质岩、电气石岩、钠长石岩、萤石岩 ;中低温热水沉积岩包括碳酸盐、硫酸盐等岩石。钾长石岩是文中确定的一种标准高温热水沉积岩 ,热水沉积钾长石以冰长石和钡长石为主 ;热水沉积碳酸盐矿物一般为铁、镁、锰、钙碳酸盐 ,碳酸盐的形成与CO2 和H2 O的不混溶温度有关 ,一般在不混溶温度 ,即 2 66℃以下生成 ,或在海水补偿线以上形成。热水沉积岩中有热水交代蚀变岩夹层 ,尤其是在高温热水活动区 ,可以交代泥质、钙泥质沉积物形成热水交代沉积岩 ,包括方柱石黑云母岩、透辉石透闪石岩、夕卡岩、绿泥石岩等。根据对霍各乞铜多金属矿床的研究 ,热水交代透辉石透闪石岩的稀土总量较低 ,表现为轻稀土富集 ,重稀土亏损 ,稀土配分模式表现为正Eu异常