胶东金矿成矿时代及区域地壳演化——基性脉岩的SHRIMP锆石U-Pb年龄及其地质意义

关于胶东金矿的成矿时代前人已进行了较多研究,但其成矿时限尚没有准确确定。对胶东焦家金矿床成矿前和成矿后的2个基性脉岩进行了锆石SHRIMP年龄测定和锆石氧同位素分析。锆石阴极发光图像显示,样品中有少量岩浆新生锆石,其晶形、环带清晰。在2个成矿前脉岩样品中测得岩浆锆石206Pb/238U年龄最小值为124.9±1.8Ma和124.2±1.1Ma,1个成矿后脉岩样品中测得最小年龄值为112.2±0.7Ma。故成矿前脉岩的侵位年龄小于等于124.9Ma和小于等于124.2Ma,成矿后脉岩的侵位年龄小于等于112.2Ma,金矿的成矿年龄被限制在124~112Ma之间。此外,较多的锆石年龄数据记录了早侏罗世、三叠纪、古生代、新元古代、中元古代和新太古代的岩浆事件和变质事件,指示胶北隆起深部既有华北克拉通新太古代、中元古代和新元古代的地质体或地质事件,也有苏鲁造山带古生代和三叠纪的地质体或地质事件,苏鲁超高压变质带曾俯冲到胶北隆起之下。锆石氧同位素组成变化范围较大,δ18O值为3.86‰~11.37‰,指示了不同时代锆石氧同位素组成的不均一性和物质来源的差异性。     

大别造山带岳西地区中生代岩浆岩年代学研究

北大别岳西地区广泛分布燕山期侵入岩和火山岩。14个岩浆岩样品中锆石LA-ICP-MS U-Pb定年结果显示,这些岩浆岩形成于134~125 Ma之间。以130 Ma为界,岩浆岩可以分为两个期次:早期以石英闪长岩—石英二长闪长岩—似斑状二长花岗岩组合为主;晚期以二长花岗岩—钾长花岗岩组合及脉岩为主。早期岩浆岩含有古元古代(1900~2120 Ma)、新元古代(670~800 Ma)和三叠纪继承锆石;晚期岩浆岩中缺少古元古代继承锆石,以新元古代、三叠纪及140~160 Ma时期的继承锆石为主。高Sr低Y地球化学特征仅出现在早期岩浆岩中,结合继承锆石的年龄特征,表明早期岩浆岩物质来源于加厚的基性下地壳,下地壳的拆沉和岩石圈的减薄发生在约130 Ma;而晚期岩浆岩为以新元古代年龄的中—下地壳在减薄后的环境下发生深熔作用形成。     

右江盆地上二叠统-中三叠统凝灰岩年龄及其地质意义

右江盆地上二叠统-中三叠统地层中常含有多层凝灰岩, 对采自广西河池九圩及田林利周罗楼组和百逢组底部的4件凝灰岩样品进行锆石原位LA-ICPMS方法U-Pb同位素定年, 结果显示凝灰岩的形成年龄分别为252.8±1.8 Ma、253.0±2.4 Ma、247.2±1.5 Ma和239.3±2.7 Ma, 指示晚二叠世末期到中三叠世Bithynian期华南地块西南缘存在多期火山活动, 其可能与华南西南缘陆块的裂解与汇聚有关.凝灰岩中继承或捕虏锆石年龄(260~270 Ma)记录了中晚二叠世峨眉山大火成岩省岩浆活动或华南西南缘岛弧火山活动事件.凝灰岩的岩相学研究表明, 中三叠世特别是Bithynian期之后右江盆地由伸展型向挤压型转换, 盆地进入快速坳陷期.      

云南富宁者桑金矿床与基性岩年代测定——兼论滇黔桂地区卡林型金矿成矿构造背景

素有"金三角"之称的右江盆地广泛发育卡林型金矿,其成因是否与岩浆活动有关颇有争议。本文以云南富宁地区的者桑卡林型金矿床和该区出露的基性岩为研究对象,分别确定了卡林型金矿床和基性岩的年龄。研究表明,矿床中与载金矿物黄铁矿和毒砂共生的热液蚀变成因绢云母的Ar-Ar坪年龄为215.3±1.9Ma,该区晚期基性岩中锆石的U-Pb年龄为215±5Ma,成岩成矿时代高度一致。在此基础上,本文提出了该矿床的成矿动力学模式:伴随古特提斯洋的闭合,在印支运动后碰撞伸展背景下,形成了富宁地区215±5Ma的基性岩。者桑金矿床分布区的深部可能存在该时期的隐伏岩体,深部岩浆活动驱动主要为大气降水成因的流体浸出地层中的金、砷和锑等成矿元素形成了者桑金矿床。     

胶莱盆地东北缘西涝口金矿煌斑岩脉岩石地球化学、锆石U-Pb年龄及其对构造岩浆事件的记录

西涝口金矿是胶莱盆地东北缘成矿区近年在深部勘查找矿工作中取得突破性进展的大型金矿床,金矿体主要赋存于古元古代牧牛山二长花岗岩与中生代鹊山糜棱岩化花岗岩之间的构造破碎带中。区内煌斑脉岩广泛分布并与金矿存在密切的时空关系。为探究其形成时代及成因机制,进行了岩相学、岩石地球化学和锆石U-Pb年龄研究。地球化学结果显示,煌斑岩具有富碱、高钾、低钛、富集大离子亲石元素和轻稀土元素,相对亏损高场强元素和重稀土元素的特征,Nb/Ta及Zr/Hf值与原始地幔十分接近,且远高于大陆地壳值,证明煌斑岩岩浆源区以幔源为主,受地壳混染作用不强烈。锆石U-Pb年龄显示,成矿期脉岩形成时间为118.6±1.5 Ma,成矿前脉岩形成时间为157±4～150.2±2.8 Ma,除以上年轻锆石外,其余为捕获老锆石,老锆石的同位素年龄分别记录了新太古代、古元古代、新元古代及三叠纪的岩浆事件和变质时间,与区内古元古代岩浆侵入、中元古代地壳伸展与变质、三叠纪板块碰撞等事件记录吻合。经钻孔取样分析发现,西涝口金矿煌斑岩脉虽与金矿脉共用同一构造通道,且Au含量普遍不高,但其“褪色”作用为金矿形成提供了部分成矿物质。已有研究证实,...     

新疆卡鲁安矿区伟晶岩锆石U-Pb定年、铪同位素组成及其与哈龙花岗岩成因关系研究

利用LA-ICPMS和LA-MC-ICPMS技术,对卡鲁安-阿祖拜伟晶岩田中4条伟晶岩脉以及哈龙岩体中锆石U-Pb定年、铪同位素组成进行了研究。研究显示,卡鲁安矿区805、806、807号脉的形成时代分别为(216.0±2.6)Ma、(223.7±1.8)Ma和(221±15)Ma,属三叠纪岩浆活动的产物。对于库卡拉盖650号脉,锆石U-Pb定年结果显示,它由形成时代为(227.9±2.6)Ma的早期钠长石伟晶岩与形成时代为(211.3±2.4)Ma的晚期锂辉石-钠长石-锂云母伟晶岩2期伟晶岩构成。哈龙岩体形成时代为400.9~403.3 Ma,由于伟晶岩与哈龙岩体之间存在形成时代上的差异(170 Ma以上),预示着它们之间不具成因上的联系。卡鲁安矿区伟晶岩脉中锆石显示较小的正εHf(t)值(+0.65~+2.50)和较大的模式年龄TDM2(1090~1213 Ma),相似于可可托海3号脉、柯鲁木特112号脉中锆石的铪同位素组成,指示伟晶岩由陆-陆碰撞体制伸展背景下的加厚地壳物质减压熔融所形成。     

滇东南瑶山群SHRIMP锆石U-Pb年代学及其地质意义

为探讨滇东南瑶山群的原岩年龄以及后期经历的岩浆-变质-构造事件年龄,开展了SHRIMP锆石U-Pb定年研究。结果表明,阴极发光(CL)分析和光学显微分析显示瑶山群的锆石组成比较复杂,可以分为继承锆石、变质锆石、深熔锆石/岩浆锆石等几大类。除少量测点207Pb/206Pb年龄为新元古代外,继承锆石206Pb/238 U表面年龄变化范围集中在235±1.7Ma~261±3.6Ma,加权平均值为250.8±9.8Ma(N=4,MSWD=4.1);深熔/岩浆锆石206Pb/238 U表面年龄显示出~85Ma和~75Ma两个峰值;变质增生锆石206Pb/238 U表面年龄为49~30Ma,且主要集中在32Ma、37Ma和42Ma左右。结合区域地质情况,初步推测:250.8±9.8Ma的继承锆石峰值年龄表明,瑶山群主体的原岩时代可能不早于二叠纪;变质锆石、深熔锆石/岩浆锆石的206Pb/238 U表面年龄可分为~85Ma、~75Ma、~42Ma、~37Ma和~32Ma等5组,可能代表了红河断裂南段自燕山晚期以来5期较强烈构造-岩浆-变质事件的时限。     

皖南青阳-泾县地区中酸性脉岩锆石特征、U-Pb年龄及地质意义

文章在对皖南青阳-泾县地区与金-锑多金属矿化有关的中酸性脉岩进行系统调查的基础上,开展了锆石特征及U-Pb定年研究,识别出新生锆石、变质重结晶锆石、捕获锆石和继承锆石共4类锆石,并相应获得一系列年龄值。新生锆石和变质重结晶锆石年龄(125～124 Ma)代表中酸性脉岩形成时代,该年龄值与皖南地区燕山期晚阶段A型花岗岩(130～123 Ma)年龄基本一致,指示二者为同一期岩浆热事件的产物。微量元素特征显示中酸性脉岩源区存在石榴子石残留,而A型花岗岩源区存在斜长石残留,指示源区具双峰式岩浆组合特征,表明该时期皖南地区处于强烈的拉张构造背景下。捕获锆石给出148～142 Ma和136～132 Ma 2组年龄,对应于皖南燕山期早阶段和晚阶段早侵入岩的年龄,暗示脉岩出露区的深部存在隐伏的燕山早期岩浆岩,脉岩岩浆侵位过程中同化混染了早期岩浆岩。继承锆石给出的年龄与扬子板块地壳增生和再造事件中的3期模式增长的年龄相对应,反映与江南过渡带可能有相同的基底物质组成。中酸性脉岩中锆石具有较低的Ce4+/Ce3+比值,表明其岩浆结晶时具有相对低的氧逸度,有利于金、锑的硫化物的形成,进而发育金-锑矿化。区内多金属矿床(点)具有较为明显的分带性,近岩基的中酸性岩株及其接触带发育斑岩-矽卡岩型矿床,成矿元素以钨、钼、铜为主;远离岩基的岩枝或岩脉状中酸性脉岩中及旁侧断裂带内发育脉状矿化,成矿元素以金、银、锑为主,显示出区域成矿作用受断裂构造系统、岩浆活动期次和成矿元素活动性的共同制约。     

黔西南白层超基性岩墙锆石SHRIMP U-Pb年龄和Hf同位素组成研究

文章对黔西南白层地区出露的燕山期超基性岩墙进行了单颗粒锆石SHRIMP U-Pb年龄和Hf同位素组成研究,获得A、B两组不同的锆石年龄.A组锆石呈不规则粒状,具不完整的宽大条带,12颗锆石的 SHRIMP U-Pb年龄加权平均值为(84±1)Ma,代表了超基性岩墙的侵位年龄;锆石Hf原位分析表明,176Hf/177Hf从0.282561到0282719,εHf(84 Ma)平均为-3.61,表明岩浆源区主要以富集地幔为主,并受到部分地壳物质的混染.B组锆石晶形完整,具典型的中酸性岩浆型振荡环带,3颗锆石的SHRIMP U-Pb模式年龄为409～450 Ma.176Hf/177Hf从0282379到0282440,εHf(t)平均为-3.77,均低于A组锆石,属于捕获锆石.据此认为84 Ma左右由于华南岩石圈伸展,软流圈地幔上涌,导致岩石圈富集地幔部分熔融,熔体上升过程中与409～450 Ma左右形成的少量地壳岩石发生混染,随后快速侵位冷凝而形成基性-超基性岩墙.白层地区84 Ma的超基性岩浆活动是整个华南西部燕山晚期(80～90 Ma)岩浆活动的一部分.右江褶皱带周缘的燕山晚期岩浆活动主要与大规模的Sn、W、Ag、Cu、Pb、Zn等矿床有关,但右江褶皱带内部的燕山晚期岩浆活动是否与以卡林型金矿为代表的低温热液矿床有成因上的联系,尚需更多矿床年代学资料的证实.     

黔西北及邻区安尼阶底界锆石U- Pb定年及对生物复苏的启示

我国西南地区广泛分布着中-下三叠统海相(包括盆地相、斜坡相及台地相)沉积地层和多层凝灰岩。通过采用锆石LA-ICP-MS U-Pb测年方法,对黔西地区沙窝剖面和彝良柳溪剖面中/下三叠统界线上凝灰岩首次进行了锆石微区同位素定年,分别获得界线年龄为247.2±2.4Ma (2σ, MSWD=0.85)和247.4±1.4Ma (2σ,MSWD=0.51),界定了"黔中隆起"及北部坳陷区中/下三叠统界线时限,此年龄结果与国际地层年代表(2018版)建议的奥伦尼克阶/安尼阶(Olenekian-Anisian)界线年龄247.2Ma以及贵州关刀剖面Olenekian-Anisian界线附近凝灰岩锆石TIMS年龄247.2±0.1Ma高度吻合。凝灰岩岩石地球化学特征显示出富集K_2O、MgO,具明显负铕异常的特征,其物源可能来自古特提斯洋俯冲作用形成的大陆岩浆弧。幕式的火山活动可能是导致早-中三叠世生态环境间歇性恶化和生物迟滞复苏的重要原因,而深化对三叠纪幕式岩浆活动的认识,为我国西南地区三叠纪地层划分对比、构造演化以及探索三叠纪生物复苏-辐射与生态系统协同演化等提供依据。     

Wavelength-dispersive X-ray fluorescence spectrometric determination of Sc, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba, Pb, and Th in komatiites

Komatiites are mantle-derived ultramafic volcanic rocks. Komatiites have been discovered in several States of India, notably in Karnataka. Studies on the distribution of trace-elements in the komatiites of India are very few. This paper proposes a simple, accurate, precise, rapid, and non-destructive wavelength-dispersive x-ray fluorescence (WDXRF) spectrometric technique for determining Sc, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba, Pb, and Th in komatiites, and discusses the accuracy, precision, limits of detection, x-ray spectral-line interferences, inter-element effects, speed, advantages, and limitations of the technique. The accuracy of the technique is excellent (within 3%) for Sc, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Zr, Nb, Ba, Pb, and Th and very good (within 4%) for Y. The precision is also excellent (within 3%) for Sc, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba, Pb, and Th. The limits of detection are: 1 ppm for Sc and V; 2 ppm for Cr, Co, and Ni; 3 ppm for Cu, Zn, Rb, and Sr; 4 ppm for Y and Zr; 6 ppm for Nb; 10 ppm for Ba; 13 ppm for Pb; and 14 ppm for Th. The time taken for determining Sc, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba, Pb, and Th in a batch of 24 samples of komatiites, for a replication of four analyses per sample, by one operator, using a manual WDXRF spectrometer, is only 60 hours.     

硅酸盐类样品中硅、铁、铝、钛、锰、钙、镁、钾、钠和磷的x射线荧光光谱测定

本文拟定了一种以熔融法制备样片,用X射线荧光光谱测定硅酸盐类样品中Si、Fe、Al、Tj、Mn、Ca、Mg、K、Na、P等元素的分析方法。在对不同靶材X光管和分光晶体实验对比的基础上,选择了最佳的测量条件。该法具有快速、准确,测量范围广,检测限低,价格便宜等优点。经过近百个各种类型标样或管理样品的分析对比表明,本法不仅适用于硅酸盐类岩石样品的分析,还适用于铁矿、铝土矿、碳酸盐类岩矿样品以及水泥、耐火材料等样品的分析。     

高压微波消解法测定醋酸纤维过滤器采集的微尘粒子中的砷镉钴铬铜铁锰镍铅钒锌

最新的流行病学研究表明,空气中较高浓度的悬浮细颗粒可能对人类的健康有不利的影响。根据该项研究显示,由于心脏病、慢性呼吸问题和肺功能指标恶化而导致死亡率的升高与细尘粒子有关。这些研究结果已经促使欧盟于1999年4月出台了限制空气中二氧化硫、二氧化氮、氧化氮、铅和颗粒物含量的法案(1999/30/EC),对各项指标包括对可吸入PM10颗粒的浓度提出了新的限制性指标。PM10颗粒是指可以通过预分级器分离采集的气体动力学直径小于10μm的细颗粒。目前研究的兴趣重点逐步偏向PM2.5这些更细微颗粒物,PM2.5这种颗粒物对健康有明显的不利影响。在欧盟指令2008/50/EC中,对PM10和PM2.5都提     

Isotopic systematics of He,Ar, S,Cu, Ni,Re, Os,Pb, U,Sm, Nd,Rb, Sr,Lu, and Hf in the rocks and ores of the Norilsk deposits

This paper reports the first results of a study of 11 isotope systems (³He/⁴He, ⁴⁰Ar/³⁶Ar, ³⁴S/³²S, ⁶⁵Cu/⁶³Cu, ⁶²Ni/⁶⁰Ni, ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ^206–208Pb/²⁰⁴Pb, Hf–Nd, U–Pb, and Re–Os) in the rocks and ores of the Cu–Ni–PGE deposits of the Norilsk ore district. Almost all the results were obtained at the Center of Isotopic Research of the Karpinskii All-Russia Research Institute of Geology. The use of a number of independent genetic isotopic signatures and comprehensive isotopic knowledge provided a methodic basis for the interpretation of approximately 5000 isotopic analyses of various elements. The presence of materials from two sources, crust and mantle, was detected in the composition of the rocks and ores. The contribution of the crustal source is especially significant in the paleofluids (gas–liquid microinclusions) of the ore-forming medium. Crustal solutions were probably a transport medium during ore formation. Air argon is dominant in the ores, which indicates a connection between the paleofluids and the atmosphere. This suggests intense groundwater circulation during the crystallization of ore minerals. The age of the rocks and ores of the Norilsk deposits was determined. The stage of orebody formation is restricted to a narrow age interval of 250 ± 10 Ma. An isotopic criterion was proposed for the ore-bearing potential of mafic intrusions in the Norilsk–Taimyr region. It includes several interrelated isotopic ratios of various elements: He, Ar, S, and others.     

Viscous, cohesive, non-Newtonian, depositing, radial slurry flow

Mining induced subsidence can significantly affect mining costs where major surface facilities and natural environment need to be protected. Overburden grout injection is a technology used to control coal mine subsidence by injecting the mine waste material extracted from the coal back into the inter-burden rock during longwall mining. The flowing slurry is here categorised as a nonlinear viscous cohesive (Bingham plastic) fluid. During longwall mining the grout slurry is pumped into the separated beds of the rock mass through a central vertical borehole, which is drilled deep into the inter-burden rock strata above the coal seam. However, a blockage can occur in the injection system when the slurry velocity falls below a certain critical threshold velocity, indicating a material phase change from cohesive-viscous to cohesive-frictional. In situ field injection tests through boreholes have been simulated at a smaller scale at the CSIRO laboratory in Brisbane by pumping the slurry through a radial disk (gap = 4 mm) from its centre. Laboratory experiments indicate a general, nonlinear, cohesive, viscous, frictional model for shear behaviour of the slurry, in which the material shear parameters are functions of the disk radial distance. Complete dimensional and dimensionless analytical solutions have been developed based on an approach related to Bingham–Herschel–Bulkley fluid mechanics. The derived formulae include relations for minimum pump pressure, local pressure and pressure gradient, wall shear stress, volume rate, velocity and velocity gradient. The theoretical results match the experimental measurements. The experiments covered slurries with maximum particle sizes of 0.5 to 2 mm with about 50% being larger than 100 µm. The viscosities at the various solids concentrations were measured with a standard torsion viscometer. This study differs from the previous research in several distinct aspects, namely, consideration of the variable shear parameters rather than fixed values, inclusion of total nonlinear behaviour, and implementation of a friction function to mimic behaviour of the deposited and consolidating stiff slurry, which can cause a significant pressure rise as a result of the increased shear resistance.     

硅酸盐中铁、锰、钛、钙、钾、磷、硅、铝、镁、钠、锶、铷、钡的X-射线荧光光谱定量测定

本文介绍样品经四硼酸锂熔融制成玻璃小饼。采用Lachance模式和理论a系数来校正元素间的效应,由3080E型X-射线荧光谱仪和DF-350B数据处理系统完成硅酸盐中十三个项目的测定。     

岩石密度和超高压岩石折返速率

在常温常压条件下对中国大陆科学钻CCSD主孔100-3000米岩心样品进行了密度测量,建立了密度连续剖面,并界定了不同超高压岩石的密度值。通过对比高温高压物性实验资料,岩石密度随着退变程度增强而降低,榴辉岩密度变化序列为3.52g/cm3→3.49g/cm3→3.07g/cm3→2.93g/cm3。超高压长英质岩石密度变化序列为3.00g/cm→2.80g/cm3→2.65g/cm3。上述实验资料是讨论不同折返阶段岩石所受浮力的基础,为研究折返速率大小提供了基本参数。本文通过折返板块运动平衡时,上浮力与粘滞力平衡这一关系式,定量研究了大陆俯冲板块的折返速率,认为密度差产生上浮力从而引起折返,温度对板块折返速率的影响最为显著;密度差大小、折返角度、折返板块大小对折返速率也有直接的影响。定量模拟分析表明,在温度高于850℃时,板块的折返速率可以超过100mm/a;当温度降至700℃时,折返速率则低于1.5mm/a。作者认为在折返早期,温度较高,板块快速折返至60-70km榴辉岩相深度;随着传导散热,温度降低,板块以较慢的速率折返至中下地壳。折返速率的估算表明,浮力是板块折返第一阶段(从>100km深部折返至<40km的中下地壳)的主要驱动力。     

Mercury,arsenic, antimony,and selenium contents of sediment from the Kuskokwim River,Bethel, Alaska,USA

The Kuskokwim River at Bethel, Alaska, drains a major mercury-antimony metallogenic province in its upper reaches and tributaries. Bethel (population 4000) is situated on the Kuskokwim floodplain and also draws its water supply from wells located in river-deposited sediment. A boring through overbank and floodplain sediment has provided material to establish a baseline datum for sediment-hosted heavy metals. Mercury (total), arsenic, antimony, and selenium contents were determined; aluminum was also determined and used as normalizing factor. The contents of the heavy metals were relatively constant with depth and do not reflect any potential enrichment from upstream contaminant sources.     

滇东南花岗岩与锡、银、铜、铅、锌多金属矿床的成因关系

滇东南锡、银、铅、锌多金属矿床主要分布在个旧、白牛厂、都龙3个矿区,而滇东南较大的3个花岗岩体又分别出露在3个矿区或附近.花岗岩体均形成于燕山晚期,岩石化学、矿物成分相似,富含锡、银、铅、锌、钨、铜等成矿元素.矿床主金属元素组合与岩体之间的距离由远而近出现由锡、钨、(铍、铌、钽)→锡、钨、(铁)、铜、铟→锡、铅、锌、银、铟、(锑)有规律的变化.花岗岩浆是成矿物质来源和成矿热源.岩浆侵入初期使上覆地层隆起产生背斜(凹陷部位成向斜);同时使地层产生一系列裂隙、压性断层或层间断裂;岩浆侵入晚期,这些断层再次活动,表现为张性,岩浆产生的含矿热液沿由其活动所造成的断裂迁移、充填、交代、沉淀而形成矿床.     

Bellechester,Minnesota, USA,lagoon collapses

Bellechester, Minnesota, is a small community of approximately 155 residents located on the county line between Goodhue and Wabasha counties in southeast Minnesota's karst region. Bellechester is served by a 21-year-old wastewater treatment facility (WWTF) consisting of three waste-stabilization ponds. On 28 April 1992 six sinkholes were discovered to have drained cell 2 of the WWTF resulting in the loss of approximately 8.7×10⁶ 1 of partially treated effluent and about 600 m³ of soil into previously undetected subsurface voids of unknown dimensions. In the week following the collapse, approximately 200 water wells located within a 5-km radius of the WWTF were sampled in an after-the-fact, emergency sampling program. Twelve samples with elevated fecal coliform levels, 18 samples with nitrate-nitrogen greater than the 10 mg/1 standard, and no samples with elevated chlorides were found. However, the elevated levels could not be unambiguously attributed to the WWTF collapse. This is the third WWTF to fail by sinkhole collapse in southeast Minnesota since 1974. All three collapsed lagoons have been located in similar geomorphic and stratigraphic settings. However, at least two lagoons have collapsed in the adjacent area in northeast Iowa, and these lagoons are located at different stratigraphic positions. Twenty-two WWTFs constructed in southeast Minnesota's karst region in the last 25 years have been identified as subject to potential sinkhole collapse. An unknown but significant number of manure storage lagoons, flood control structures, etc., have also been constructed in the karst region and are at risk. Public agencies are beginning to develop plans to deal with the risk associated with existing and future waste lagoons in this environment. The critical hydrogeologic parameters that can be used to prioritize the risk of collapse at existing facilities include: (1) the lithology of the first bedrock beneath each lagoon, (2) the thickness of surficial materials between the lagoon and the bedrock surface, (3) the presence and construction of liners (seepage rate), and (4) the proximity to existing sinkholes.