Failure mechanisms and development of catastrophic rockslides triggered by precipitation and open-pit mining in Emei,Sichuan, China

Two deadly rockslides, triggered by heavy precipitation and open-pit mining, were reported in Emei County, Sichuan Province, China, from 2011 to 2015. About 6.0 million m³ of rock detached from the upper slopes, pushed the pre-sliding deposits, and hit the opposite mountains at average velocity of 18 to 36 km/h. Detailed field investigation, geological mapping, and UAV aerial photographic interpretation are presented to analyze the failure mechanisms of the events. The results suggest that the high-speed consequent bedding rockslides were triggered by the failure of rock mass, which were influenced by the engineering activities and climate change. Key contributive factors were weathered and fragmented basalts that were affected by open-pit mining and frequent blasting, as well as the weak underlying tuffs with swell-shrink potential. Persistent rainfall was the direct trigger in initiating and reactivating the landslide. Water affected the slope stability by increasing the slope material’s unit weight and penetrating into joints and cracks to make the tuffs degrade and causing a reduction in effective stress. The mechanisms for the two landslide events are a high-speed regressive consequent bedding (RCB) rockslide in 2011 and a reactivated high-speed advancing consequent bedding (ACB) rockslide in 2015. This paper can provide an insight into large-scale consequent bedding rockslides associated with the interaction between the rainfall and open-pit mining slopes instabilities.     

The Oeschinensee rock avalanche,Bernese Alps,Switzerland: a co-seismic failure 2300 years ago?

Landslide deposits dam Lake Oeschinen (Oeschinensee), located above Kandersteg, Switzerland. However, past confusion differentiating deposits of multiple landslide events has confounded efforts to quantify the volume, age, and failure dynamics of the Oeschinensee rock avalanche. Here we combine field and remote mapping, topographic reconstruction, cosmogenic surface exposure dating, and numerical runout modeling to quantify salient parameters of the event. Differences in boulder lithology and deposit morphology reveal that the landslide body damming Oeschinensee consists of debris from both an older rock avalanche, possibly Kandertal, as well as the Oeschinensee rock avalanche. We distinguish a source volume for the Oeschinensee event of 37 Mm³, resulting in an estimated deposit volume of 46 Mm³, smaller than previous estimates that included portions of the Kandertal mass. Runout modeling revealed peak and average rock avalanche velocities of 65 and 45 m/s, respectively, and support a single-event failure scenario. ³⁶Cl surface exposure dating of deposited boulders indicates a mean age for the rock avalanche of 2.3 ± 0.2 kyr. This age coincides with the timing of a paleo-seismic event identified from lacustrine sediments in Swiss lakes, suggesting an earthquake trigger. Our results help clarify the hazard and geomorphic effects of rare, large rock avalanches in alpine settings.     

Meteoric lithification of catastrophic rockslide deposits: Diagenesis and significance

Deposits of catastrophic rockslides composed of lithologies rich in carbonate minerals may undergo precipitation of cements that can be used to proxy-date the rockslide event and/or subsequent geomorphic changes of the rockslide mass.In the Alps, localized to widespread lithification of post-Glacial rockslide deposits is observed in lithologies ranging from limestones and dolostones to metacarbonates to calcphyllites. Lithification of rockslide deposits to breccias may be localized to meteoric ‘runoff-shadows’ below larger boulders, or may comprise a layer of breccia or may affect a rockslide mass down its base. In addition, precipitation of cements and small stalactites may take place in megapores on boulder undersides. Cements found in rockslide deposits comprise skalenohedral calcite, prismatic calcite, blocky calcite, calcitic micrite and micropeloidal calcitic cement and, rarely, botryoidal aragonite. Initial cement formation probably is driven by meteoric dissolution–re-precipitation of (mini-) micritic abrasive rock powder generated by dynamic disintegration during the rockslide event. Preliminary ²³⁴U/²³⁰Th ages of rockslide cements support a concept that cementation starts immediately or early after a rockslide event. In rockslide deposits of calcphyllite with accessory pyrite, oxidation of pyrite probably also propels the process of carbonate dissolution–re-precipitation. Limestone-precipitating springs emerging from rockslide masses, and well-cemented talus slopes and fluvial conglomerates percolated by rockslide-derived groundwaters, indicate that rockslide deposits remain diagenetically active long after emplacement.     

Lake sediments deposited on the Flims rockslide mass: the key to date the largest mass movement of the Alps

Long piston cores taken from the subsurface of two mountain lakes of the eastern Swiss Alps recovered sediments, which overlie the Flims rockslide deposits. These sediments provide new information on the chronology of the largest known Alpine rockslide and can be used to reconstruct the post landslide environmental evolution. The oldest ¹⁴C date of the lake sediments yields a minimum age of the rockslide at 9660–9430 cal. yr BP. In addition, the dating of a wood fragment contained in the rockslide deposits directly below the lake sediments shows a maximum age of 9480–9120 cal. yr BP. The overlap of the maximum and minimum ages, 9480–9430 cal. yr BP, approximates the age of the Flims rockslide. This early Holocene range coincides with a period of higher frequency of large mass movements observed in the Alps, which could be related to climatic changes.     

Successive Holocene rock avalanches at Lake Coleridge, Canterbury, New Zealand

At Lake Coleridge, Canterbury, New Zealand, at least three rock avalanches have been released from a single source area during the Holocene. The first of these was of 10⁷ m³ volume and dates to about 9,750 BP, and two with volumes 5 × 10⁵ and 4 × 10⁴ m³ occurred about 700 BP. All three crossed the course of the Ryton River; the latter two were emplaced within the part of the first that had subsequently been eroded by the Ryton River. All three were most likely triggered by, or related to, seismicity. The first rock avalanche formed a long-lived landslide dam, and no evidence remains to indicate whether its eventual failure was catastrophic. The second formed a correspondingly smaller dam, but there is no evidence that its lake was long-lived; however, a set of anomalously steep outwash terraces downstream of the landslide deposits show that it failed catastrophically. A camping ground is sited about 1 km downstream of the landslide deposits, and proposals to develop it further risk potentially severe hazards from future rock avalanche activity at the site.     

Formation mechanism and evolution process of the Chada rock avalanche in Southeast Tibet,China

The Chada rock avalanche is a prehistoric high-elevation giant rock landslide located in the Boshula Mountains, Lhorong County, Southeast Tibet. It is composed of conglomerates with a volume of 6.62?×?10⁶ m³ and has a height difference of 1450 m and a transport distance of 3155 m. The accumulational landform shows characteristics indicating rock avalanches. With a unique red conglomerate as the marker of landslide movement, we combined the results of geological surveys, aerial surveys, and engineering geological drilling to determine the entrainment and geomorphic features of the rock avalanche. The rock avalanche was divided into the main scarp, entrainment zone (residual deposit, mixed deposit, and impact fragmentation areas), transport zone (compressed, local landslide, and longitudinal ridge areas), and deposit zone. The sequence of deposits in the valley indicates that the rock avalanche formed before the first-stage terrace and after the second-stage terrace. Combined with 3D numerical simulation, four movement stages were obtained: (1) the rock mass was broken and disintegrated due to progressive failure, initiating high-speed sliding; (2) the sliding mass scraped the thick previous slope material and formed oblique ridges by forward extrusion and lateral friction; (3) the 4.95?×?10⁶ m³ sliding mass was compressed and decelerated to form bending ridges, and the 1.67?×?10⁶ m³ sliding mass continued to move through the channel; and (4) the sliding mass extended to form longitudinal ridges in the channel and hummocks in the valley. The rock avalanche accelerated three times and decelerated three times during its motion.

     

Rainfall-induced landslide event of May 2010 in the eastern part of the Czech Republic

More than 150 landslides originated in the eastern part of the Czech Republic (region of the Flysch Outer Western Carpathians—hereinafter, OWC) due to soil saturation caused by antecedent precipitation and long lasting and intensive rainfalls on 16–18 May 2010 (>300 mm as measured by some stations). As a consequence, a multitude of small failures originated 88% of which was smaller than 10⁴ m². Most landslides are characterised as shallow (<10 m) or middle–deep (10–30 m) incipient (rather short travel) landslides, debris slides and soil slips spatially clustered to a geological domain underlain by rather weak thin-bedded flysch and unconsolidated Quaternary deposits. An exception to this is represented by a kilometre-long rockslide (∼2–3 mil m³) affecting tectonically weakened and weathered claystone/mudstone-dominated flysch on the southern slope of Mt. Girová (the Beskydy Mountains). The rockslide is one of the largest long runout landslides in the territory of the Czech Republic activated over the past few decades as it reaches the dimensions of the largest documented Holocene long runout landslides in the Czech part of the OWC. A majority of the May 2010 landslide events developed inside older (Holocene or historic) landslide terrains, which points to their spatial persistency and recurrent nature. In spite of the fact that the May 2010 landslide event was not as destructive as some previous landslide activisation in the OWC region (e.g. July 1997 event), it left many slope failures at the initial stage of their potential future reactivation.     

Interactions between rock avalanches and glaciers in the Mont Blanc massif during the late Holocene

Rock avalanches are common in the Mont Blanc massif, which is bordered by valleys with large resident and tourist populations and important highways. This paper combines historical data with detailed geomorphological mapping, stratigraphic observation, and absolute and relative dating, to interpret several deposits resulting from rock avalanching onto glaciers.Nineteen rock falls and rock avalanches are described, ranging in volume from 10,000 m³ to 10 × 10⁶ m³. They occurred between 2500 BP and AD 2007 at six sites. The events at three sites (Miage and Drus Glaciers, and Tour des Grandes Jorasses) are characterised by short travel distances; those at Brenva, Triolet, and Frébouge Glaciers exhibit excessive travel distances.Interactions between rock avalanches and glaciers are of four types: (i) rock-avalanche triggering, where glacial and paraglacial controls include debuttressing of rockwalls due to glacier thinning and retreat, oversteepening of rock slopes by glacial erosion, and effects of glaciers on permafrost; (ii) rock-avalanche mobility, in which mobility and travel distance are modified by channelling of rock-avalanche debris by moraines and valleys, incorporation of ice and snow (often >50% for large events), and irregularities on the glacier surface; (iii) deposit sedimentology, where melting of incorporated ice transforms the final deposit by reducing its thickness typically to <5 m, and debris of variable thicknesses is juxtaposed in a hummocky deposit with chaotic piles of angular rock debris; and (iv) glacier dynamics where insulating debris deposited upon a glacier produces a debris-covered glacier of different dynamics, and high elevated scars can favour the formation of small glaciers.     

Monitoring and early warning of the 2012 Preonzo catastrophic rockslope failure

In this paper, we describe the investigations and actions taken to reduce risk and prevent casualties from a catastrophic 210,000 m³ rockslope failure, which occurred near the village of Preonzo in the Swiss Alps on May 15, 2012. We describe the geological predisposition and displacement history before and during the accelerated creep stage as well as the development and operation of an efficient early warning system. The failure of May 15, 2012, occurred from a large and retrogressive instability in gneisses and amphibolites with a total volume of about 350,000 m³, which formed an alpine meadow 1250 m above the valley floor. About 140,000 m³ of unstable rock mass remained in place and might collapse partially or completely in the future. The instability showed clearly visible signs of movements along a tension crack since 1989 and accelerated creep with significant hydromechanical forcing since about 2006. Because the active rockslide at Preonzo threatened a large industrial facility and important transport routes located directly at the toe of the slope, an early warning system was installed in 2010. The thresholds for prealarm, general public alarm, and evacuation were derived from crack meter and total station monitoring data covering a period of about 10 years, supplemented with information from past failure events with similar predisposition. These thresholds were successfully applied to evacuate the industrial facility and to close important roads a few days before the catastrophic slope failure of May 15, 2012. The rock slope failure occurred in two events, exposing a compound rupture plane dipping 42° and generating deposits in the midslope portion with a travel angle of 39°. Three hours after the second rockslide, the fresh deposits became reactivated in a devastating debris avalanche that reached the foot of the slope but did not destroy any infrastructure. The final run-out distance of this combined rock collapse–debris avalanche corresponded to the predictions made in the year 2004.     

Holocene palaeoflood events recorded by slackwater deposits along the middle Beiluohe River valley,middle Yellow River basin,China

Palaeohydrological investigations were carried out in the middle Beiluohe River valley in the middle Yellow River basin. Palaeoflood slackwater deposits (SWDs) are natural records of overbank flooding and are often identified in the Holocene aeolian loess–soil profiles along the river valley. Three bedsets of palaeoflood SWDs were found within the mid‐Holocene Climatic Optimum palaeosol (S₀) on the right riverbank. Both the sedimentary and the analytical results, including magnetic susceptibility, grain‐size distribution and concentrations of chemical elements, indicate that these palaeoflood SWDs were deposited from the suspended sediment load of overbank flooding. We identified three episodes of extraordinary overbank flooding events. These hydro‐climatic events were dated to 7600–7400, 5800–5000 and 4200–4000 a BP, from optically stimulated luminescence (OSL) dating and pedostratigraphical correlation. The minimum flood peak discharges were estimated to have been between 12 600 and 14 100 m³ s^?1 using the slope–area method. These hydro‐climatic events are considered to be a regional expression of known climatic events and demonstrate that the mid‐Holocene climate was far from stable. These results show that the magnitudes of floods are highly sensitive to climate change in semi‐arid and sub‐humid regions with a monsoonal climate.     

A Discontinuous Approach to the Numerical Modelling of Rock Avalanches

Summary. The runout of dry rock avalanches produced by planar rockslides affecting a limestone formation with clayey interbeds is analysed by means of distinct element modelling. Potential and past rock avalanche events are described with reference to the geotechnical and structural conditions of the slope, typical of several Alpine valleys. Runout prediction analyses of potential rock avalanches performed with the PFC^2D code are based on independent measurement of strength, energy dissipation and stiffness parameters of the rock mass and are validated by means of the back analysis of a historical rockslide occurred in the investigated area. Physical aspects of the avalanching process evidenced by modelling are also discussed. Author’s address: Paolo Tommasi, CNR – Institute for Geo-Engineering and Environmental Geology, c/o Facoltá di Ingegneria, Via Eudossiana 18, 00184 Rome, Italy     

Reconstructing 4000 years of mass movement and tsunami history in a deep peri‐Alpine lake (Lake Geneva,France‐Switzerland)

The study of mass movements in lake sediments provides insights into past natural hazards at historic and prehistoric timescales. Sediments from the deep basin of Lake Geneva reveal a succession of six large‐scale (volumes of 22 × 10⁶ to 250 × 10⁶ m³) mass‐transport deposits, associated with five mass‐movement events within 2600 years (4000 cal bp to 563 ad ). The mass‐transport deposits result from: (i) lateral slope failures (mass‐transport deposit B at 3895 ± 225 cal bp and mass‐transport deposits A and C at 3683 ± 128 cal bp ); and (ii) Rhône delta collapses (mass‐transport deposits D to G dated at 2650 ± 150 cal bp , 2185 ± 85 cal bp , 1920 ± 120 cal bp and 563 ad , respectively). Mass‐transport deposits A and C were most probably triggered by an earthquake, whereas the Rhône delta collapses were likely to be due to sediment overload with a rockfall as the external trigger (mass‐transport deposit G, the Tauredunum event in 563 ad known from historical records), an earthquake (mass‐transport deposit E) or unknown external triggers (mass‐transport deposits D and F). Independent of their origin and trigger mechanisms, numerical simulations show that all of these recorded mass‐transport deposits are large enough to have generated at least metre‐scale tsunamis during mass movement initiation. Since the Tauredunum event in 563 ad , two small‐scale (volumes of 1 to 2 × 10⁶ m³) mass‐transport deposits (H and I) are present in the seismic record, both of which are associated with small lateral slope failures. Mass‐transport deposits H and I might be related to earthquakes in Lausanne/Geneva (possibly) 1322 ad and Aigle 1584 ad , respectively. The sedimentary record of the deep basin of Lake Geneva, in combination with the historical record, show that during the past 3695 years, at least six tsunamis were generated by mass movements, indicating that the tsunami hazard in the Lake Geneva region should not be neglected, although such events are not frequent with a recurrence time of 0·0016 yr^?1.     

The AD 1717 rock avalanche deposits in the upper Ferret Valley (Italy): a dating approach with cosmogenic 10Be

On 12 September AD 1717, a rock volume larger than 10 million m³ collapsed onto the Triolet Glacier, mobilized a mass composed of ice and sediment and travelled more than 7 km downvalley in the upper Ferret Valley, Mont Blanc Massif (Italy). This rock avalanche destroyed two small settlements, causing seven casualties and loss of livestock. No detailed maps were made at the time. Later investigators attributed accumulations of granitic boulders and irregular ridges on the upper valley floor to either glacial deposition, or the AD 1717 rock avalanche, or a complex mixture of glacial deposition, earlier rock avalanche and AD 1717 rock avalanche origin. In this study, we present cosmogenic ¹⁰Be exposure ages from nine boulders in the extensive chaotic boulder deposit with irregular ridges, two from Holocene glacier‐free areas, and one from a Little Ice Age moraine. Exposure ages between 330 ± 23 and 483 ± 123 a from eight of nine boulders from the chaotic deposit indicate that at least seven were deposited by the AD 1717 rock avalanche. The other three boulders yielded ¹⁰Be exposure ages of 10 900 ± 400, 9700 ± 400 and 244 ± 97 a, respectively. Our results are in good agreement with the existing chronology from dendrochronology and lichenometry, and radiocarbon analysis of wood samples, but not with older ¹⁴C ages from a peat bog in the upper part of the valley. Based on the new age control, the rock avalanche deposits cover the whole bottom of the upper Ferret valley. Copyright © 2012 John Wiley & Sons, Ltd.     

Cassiterite LA-MC-ICP-MS U/Pb and muscovite 40Ar/39Ar dating of tin deposits in the Tengchong-Lianghe tin district,NW Yunnan,China

The Tengchong-Lianghe tin district in northwestern Yunnan, China, is an important tin mineralization area in the Sanjiang Tethyan Metallogenic Domain. There are three N–S trending granite belts in the Tengchong-Lianghe area, with emplacement ages ranging from Early Cretaceous to Late Cretaceous and Early Cenozoic. Tin mineralization is spatially associated with these granitic rocks. However, the petrogenetic link between the tin deposits and the host granites is not clear because of the lack of age data for the tin mineralization. We investigate the possibility of direct dating of cassiterite from three typical tin deposits in the Tengchong-Lianghe tin district, using laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). In situ LA-MC-ICP-MS dating of seven cassiterite samples from the Lailishan (LLS-1 and LLS-2), Xiaolonghe (XLH, WDS, DSP, and HJS), and Tieyaoshan (TYS) tin deposits yielded well-defined ²⁰⁶Pb/²⁰⁷Pb–²³⁸U/²⁰⁷Pb isochron ages. To assess the accuracy of the in situ U/Pb dating of cassiterite, ⁴⁰Ar/³⁹Ar dating of coexisting muscovite (in samples LLS-1, DSP, and TYS) was also performed. The cassiterite in situ U/Pb ages (47.4?±?2.0, 71.9?±?2.3, and 119.3?±?1.7 Ma, respectively) are in excellent agreement with the coexisting muscovite ⁴⁰Ar/³⁹Ar ages (48.4?±?0.3, 71.9?±?1.4, and 122.4?±?0.7 Ma, respectively). The U/Pb ages of cassiterite combined with the ⁴⁰Ar/³⁹Ar ages of muscovite indicate that there are three tin mineralization events in this district: the Lailishan tin deposit at 47.4?±?2.0 to 52?±?2.7 Ma, the Xiaolonghe tin deposit at 71.6?±?2.4 to 3.9?±?2.0 Ma, and the Tieyaoshan tin deposit at 119.3?±?1.7 to 122.5?±?0.7 Ma. These ages are highly consistent with the zircon U/Pb ages of the host granites. It is su.ggested that the Cretaceous tin mineralization might have taken place in a subduction environment, while the Early Tertiary tin metallogenesis was in a postcollisional geodynamic setting.     

Reconstruction of a large runout landslide in the Krušné hory Mts. (Czech Republic)

The aims of this study were to summarize current knowledge of a large runout prehistoric landslide, critically review all of the existing data and, in particular, gather new data in order to estimate the age of the accumulation and reveal the movement mechanism. The reconstruction of a large rockslide-rock avalanche in the NW part of the Czech Republic was supported by the analysis and interpretation of 216 boreholes and by GIS analysis of the original 1950s pre-mining surface using digitized old military topographic maps. For the age estimation, we used the Schmidt hammer test. The total volume of the quaternary deposit was calculated to be between 25.4 and 27.4 mil m³, occupying an area of 778,000 m² and consisting of six to eight generations of colluvial sediments. Three main landslide events were identified based on extensive Schmidt hammer sampling, and the approximate age was established using a regression equation assembled by Engel (2007). All three of the documented events occurred around the time of significant climate change. The oldest event occurred due to the Oldest Dryas warming, the largest event probably occurred at the end of the Younger Dryas (11,700 yBP), and the youngest of the documented events was purely of a Holocene age, with the highest landslide frequency being during the Atlantic temperature fluctuations (approximately 8200 yBP). The slope deformation occurred on a fault slope with a relative height of over 400 m and in tectonically weakened rocks. Sediments in the Most Basin were weakened from meltwater during rapid warming periods, which allowed mobilization of rockslide deposits and runout of up to 1000 m from the mountain foothills.     

西昆仑阿巴列克铜铅矿床黄铜矿Re-Os定年及地质意义

除了传统的测试Rb-Sr、Sm-Nd、U-Th-Pb、K-Ar或Ar-Ar等方法来讨论碳酸盐岩层控型铅锌矿的成矿年龄外,利用硫化物矿物Re-Os分析成矿过程,在矿床成矿系统上也是十分重要而有效的工具.从新疆西昆仑阿巴列克铜铅矿床的主矿体中,选取7个黄铜矿和1件黄铁矿样品,对其进行了Re-Os定年,结果表明它们分别含有626×10-9～14533×10-9 Re和0.026×10-9 ～0.36×10-9Os,给出海西期的331.3±5.2Ma等时线年龄.样品的高Re/Os比值、低含量普通Os和高放射成因Os的组成特性支持这组黄铜矿样品为表壳构造成因.构造演化历史表明,塔里木地块西南缘在晚古生代属于被动大陆边缘,赋矿地层霍什拉甫组属于台地相碳酸盐岩-碎屑岩建造,地层时代与上述等时年龄几乎相近.本次工作提供了一个新的证据,支持阿巴列克矿床在海西期经历了同生成矿作用,矿床类型主要为沉积层控型,成矿物质主要来源于地层,矿化发生在上泥盆统与下石炭统的结合部位,岩性以砂岩与碳酸盐岩组合为主,具有特定的层位控制特点.从中可以看出,碳酸盐岩层控型铅锌铜矿床成因具有一定的复杂性,具有深入研究的广阔空间.     

The Mogangling giant landslide triggered by the 1786 Moxi M 7.75 earthquake,China

A good understanding of seismic giant landslides could provide favourable guidance for seismic stability evaluation of nearby slopes. Here, an excellent example of a catastrophic seismic landslide named the Mogangling giant landslide (MGL), located upstream along the Dadu River and triggered by the 1786 Moxi M 7.75 earthquake, is analysed for its deposit characteristics, failure mechanism and dammed lake. The MGL, with a volume of approximately 4500?×?10⁴ m³, 450 m long and 1000 m wide, blocked the Dadu River completely and caused over 100 000 deaths when the landslide dam broke. The MGL occurred on the upper part of a narrow granite ridge; a potentially unstable wedge-shaped rock mass was separated from the remaining massif by unloading fissures and an active fault (Detuo fault) that just crossed the slope foot. The Moxi earthquake coupled with strong site amplification triggered the MGL, which blocked the Dadu River; the elevation of the dam crest was approximately 130 m higher than the present river level. The dammed lake had a volume of approximately 9.504?×?10⁸ m³, an area of 19.91 km² and a length of approximately 31 km; the peak flow of the outburst flood was larger than 7100 m³/s. After hundreds of years of concave bank erosion, the deposit is divided into the right bank deposit (main deposit) and left bank deposit (residual deposit).

     

In, Sn, Pb and Zn Contents and Their Relationships in Ore-forming Fluids from Some In-rich and In-poor Deposits in China

All the indium-rich deposits with indium contents in ores more than 100×10- 6 seems to be of cassiterite-sulfide deposits or Sn-bearing Pb-Zn deposits, e.g., in the Dachang Sn deposit in Guangxi, the Dulong Sn-Zn deposit in Yunnan, and the Meng'entaolegai Ag-Pb-Zn deposit in Inner Mongolia, the indium contents in ores range from 98×10-6 to 236×10-6 and show a good positive correlation with contents of zinc and tin, and their correlation coefficients are 0.8781 and 0.7430, respectively. The indium contents from such Sn-poor deposits as the Fozichong Pb-Zn deposit in Guangxi and the Huanren Pb-Zn deposit in Liaoning are generally lower than 10×10-6, i.e., whether tin is present or not in a deposit implies the enrichment extent of indium in ores. Whether the In enrichment itself in the ore -forming fluids or the ore-forming conditions has actually caused the enrichment/depletion of indium in the deposits? After studying the fluid inclusions in quartz crystallized at the main stage of mineralization of several In-rich and In-poor deposits in China, this paper analyzed the contents and studied the variation trend of In, Sn, Pb and Zn in the ore-forming fluids. The results show that the contents of lead and zinc in the ore-forming fluids of In-rich and -poor deposits are at the same level, and the lead contents range from 22×10-6 to 81×10-6 and zinc from 164×10-6 to 309×10-6, while the contents of indium and tin in the ore-forming fluids of In-rich deposits are far higher than those of In-poor deposits, with a difference of 1-2 orders of magnitude. Indium and tin contents in ore-forming fluid of In-rich deposits are 1.9×10-6-4.1×10-6 and 7×100-6-55×10-6, and there is a very good positive correlation between the two elements, with a correlation coefficient of 0.9552. Indium and tin contents in ore-forming fluid of In-poor deposits are 0.03×10-6-0.09×10-6 and 0.4×10-6--2.0×10-6, respectively, and there is no apparent correlation between them. This indicates, on one hand, that In-rich ore-forming fluids are the material basis for the formation of In-rich deposits, and, on the other hand, tin probably played a very important role in the transport and enrichment of indium.     

西藏洛巴堆矽卡岩型铁多金属矿床石榴子石和锆石U-Pb测年及地质意义

石榴子石是矽卡岩型矿床中最常见的蚀变矿物之一,因此,对石榴子石进行年代学研究能够准确限定矽卡岩型矿床的成矿时代。青藏高原冈底斯成矿带中部发育众多矽卡岩型多金属矿床,由于缺乏精确的成矿年代学数据,制约着对这些矿床成因和动力学背景的深入认识。因此,文章以该成矿带具有代表性的洛巴堆矽卡岩型铁多金属矿床为研究对象,通过对赋矿矽卡岩中石榴子石和花岗闪长岩中锆石分别进行LA-ICP-MS U-Pb测年,以期能够准确限定该矿床的成矿时代。背散射图像和电子探针分析显示,洛巴堆矽卡岩矿床中石榴子石普遍发育环带结构特征,端员组成上以钙铝-钙铁榴石为主,w(U)为0.4×10^-6~28.1×10^-6,LA-ICP-MS U-Pb测年数据显示石榴子石形成时代为(62.7±2.3)Ma(n=94)。同时,与矽卡岩密切接触的花岗闪长岩的锆石U-Pb年龄为(62.6±0.8)Ma(n=30),与石榴子石形成时代一致。这一结果说明,洛巴堆矽卡岩型铁多金属矿床形成于古新世,与同期的花岗闪长岩具有密切的成因联系。结合区域65~50 Ma时的印度-欧亚大陆碰撞事件,该测年结果显示了洛巴堆矽卡岩型铁多金属矿床形成于印度-欧亚大陆初始碰撞的构造背景。此外,通过本次研究显示,相对于云母等Ar-Ar测年,石榴子石U-Pb测年体系受青藏高原剥蚀隆升作用的影响较小,能够更好的约束成矿时代,具有广泛的应用前景。     

辽宁小佟家堡子金矿床成矿时代探讨

小佟家堡子金矿床位于辽东金矿集中区,为产于古元古代变质岩系中的微细粒浸染型金矿,矿床类型独特。文章采用了绢云母^40Ar/^39Ar法对样品从450℃进行了8个阶段的加热分析。从绢云母石英交代岩型金矿石中挑选出与金矿化密切共生的绢云母单矿物,通过^40Ar/^39Ar快中子活化阶段加热法获得坪年龄为167Ma。绢云母^40Ar/^39Ar法测年结果与矿床的控矿构造特征、区域岩浆活动的演化规律相吻合,进一步证明了该金矿床为燕山期成矿。同时,绢云母^40Ar/^39Ar的马鞍形年龄谱还表明,在绢云母石英交代岩形成过程中,可捕获部分“过剩剩氩”,这可能是造成K－Ar法年龄值高、范围较大的原因之一。