西藏拉萨地块阿翁错北二长花岗岩成因：锆石U-Pb年代学、岩石地球化学及Hf同位素制约

西藏拉萨地块阿翁错—盐湖岩浆弧的成因是解决班公湖—怒江特提斯洋俯冲极性和时限的关键。本文选取阿翁错北岩体中的二长花岗岩进行岩相学、锆石U-Pb年代学、岩石地球化学与Hf同位素研究。结果表明：阿翁错北二长花岗岩的锆石²⁰⁶Pb/²³⁸U加权平均年龄为(107.0±0.5) Ma,MSWD＝2.6,属早白垩世晚期。样品表现为高硅、富钾的高钾钙碱性岩石系列,A/CNK值介于1.006～1.019之间,属弱过铝质;微量元素富集大离子亲石元素Rb、K、U、Th及轻稀土元素（LREE）,亏损Nb、Ta、P、Ti等高场强元素,具中等至弱的负Eu异常（δEu=0.55~0.78）,属弱过铝质未分异的I型花岗岩。二长花岗岩样品锆石初始Hf同位素ε_Hf(t)值除1颗锆石达11.1外,其他17颗锆石介于3.7~6.3之间,平均值5.0,Hf同位素二阶段模式年龄变化于928~765 Ma之间。基于同位素以及岩石地球化学数据,阿翁错北岩体很可能是新生地壳熔融产生的长英质岩浆与镁铁质岩浆发生不均一混合作用形成,并有少量幔源物质的参与。结合拉萨地块中北部岩浆岩Hf同位素研究分析,阿翁错—盐湖岩浆弧形成于班公湖—怒江特提斯洋后退式俯冲的构造体制下,阿翁错北岩体的形成时代（107~104 Ma）代表了由断离板片俯冲末期向碰撞环境转化的时限。     

西藏拉萨地块革吉石英闪长岩及镁铁质微粒包体成因及其对班——怒特提斯洋晚侏罗世构造演化的启示

在班公湖—怒江结合带南缘,新识别出一条中-晚侏罗世岩浆岩带,其形成时代并非前人所认为的早白垩世,这一发现为重新认识班公湖—怒江特提斯洋俯冲极性与过程提供了新的资料。笔者以西藏阿里地区革吉岩体为研究对象,开展岩体地质及岩相学、锆石U--Pb年代学、地球化学及全岩Sr--Nd同位素研究。研究表明,革吉岩体岩性主要为石英闪长岩,岩体中普遍发育暗色的镁铁质微粒包体。革吉石英闪长岩及镁铁质微粒包体的锆石U--Pb年龄分别为153±2 Ma和150±2 Ma,两者在误差范围内一致,代表了同期岩浆事件的产物。地球化学特征显示革吉石英闪长岩属于钙碱性准铝质岩石系列,镁铁质微粒包体属于拉斑系列的准铝质岩石。镁铁质微粒包体与寄主石英闪长岩具有相似的稀土、微量元素配分曲线,但总体上微粒包体Nb、Ta、Zr和Hf等元素含量更低,而稀土总量更高。石英闪长岩样品(~(87)Sr/~(86)Sr)_i值介于0. 714 2～0. 715 2之间,ε_(Nd)(t)值为-12. 0～-14. 0;镁铁质微粒包体样品(~(87)Sr/~(86)Sr)_i值为0. 712 9～0. 714 4,ε_(Nd)(t)值介于-11. 4～-13. 8之间。综合岩相学、地球化学的特征,革吉岩体中的寄主石英闪长岩及其镁铁质微粒包体很可能是富集地幔来源的镁铁质岩浆与古老地壳物质熔融形成的长英质岩浆发生不同比例混合作用的结果。结合区域构造演化的研究,革吉岩体可能形成于班公湖—怒江特提斯洋南向俯冲的体制下,狮泉河多岛弧盆系统与古拉萨地块发生弧-陆软碰撞的构造背景。     

西藏班公湖-怒江缝合带中段去申拉组火山岩锆石U-Pb年龄及Hf同位素特征

西藏班公湖-怒江缝合带广泛分布中生代岩浆活动,对于认识特提斯洋的演化有重要的启示。班公湖-怒江缝合带中段东卡错微陆块去申拉组安山岩的锆石U-Pb测年和Hf同位素测试结果显示,其中21个测点给出的锆石~(206)Pb/~(238)U年龄加权平均值为110.3±0.7Ma;ε_(Hf)(t)值为-5.66～1.05,二阶段Hf模式年龄为1020～1448Ma,表现出壳幔混源的特征。综合区域构造沉积演化及前人研究成果,认为东卡错微陆块内发育的去申拉组火山岩很可能与早白垩世班公湖-怒江洋壳南向俯冲消减引起的板片断离有关,并形成于板内伸展环境,其岩浆来源于聂荣微陆块成熟地壳物质发生深熔或重熔作用形成的酸性熔体与古老岩石圈地幔部分熔融产生的基性熔体的混合。     

青藏高原南羌塘荣玛晚侏罗世闪长岩:班公湖——怒江特提斯洋向北俯冲产物

班公湖—怒江特提斯洋的演化一直以来备受关注,对于其俯冲极性存在不同认识。报道在青藏高原南羌塘荣玛地区新发现的晚侏罗世闪长岩年代学、地球化学和锆石Hf同位素特征。LA-MC-ICP-MS锆石U-Pb年龄结果表明,岩石形成于149.7±0.8Ma,时代为晚侏罗世。岩石呈富碱,低Mg O、TFe O、Ca O,富集轻稀土元素和Rb、Th、U、K等元素,亏损Ta、Nb、P、Ti等元素。锆石εHf(t)值为正值,并且具有较大的变化范围(+4.26~+9.29),指示岩浆来源于幔源和壳源岩浆的混合。结合区域地质背景认为,荣玛地区晚侏罗世闪长岩为班公湖—怒江特提斯洋向北俯冲所引起的幔源和壳源岩浆混合作用的产物。     

班—怒成矿带西段江玛南铜银矿区石英闪长玢岩锆石U-Pb年龄、Hf同位素及地球化学特征

西藏革吉县江玛—赛登地区是班公湖—怒江成矿带内近两年发现的成矿潜力较好的地区之一。本文对江玛南铜银矿点的石英闪长玢岩进行锆石U-Pb年代学、全岩地球化学和锆石Hf同位素组成研究。利用LA-ICP-MS测得206Pb/238U年龄为(118.6±2.8)Ma(N=13,MSWD=3.8),此年龄可解释为石英闪长玢岩成岩年龄。全岩主量和微量元素分析显示,石英闪长玢岩属于中钾钙碱性准铝质I型花岗岩。岩石轻、重稀土元素分馏明显(LREE/HREE=9.37～9.76),富集轻稀土元素;δEu值为0.94～0.9, Eu负异常不甚明显。岩石相对富集Rb、Th、U、K等大离子亲石元素(LILE),亏损Nb、Ta、Zr、Ti等高场强元素(HFSE),具有岛弧岩浆岩的岩石化学属性。锆石Hf同位素分析结果显示,锆石εHf(t)值为+7.9～+11.2,平均值为+9.5,两阶段模式年龄TDMC=456～675Ma,表现出明显的幔源成岩物质来源特征。通过年代学、岩石地球化学、Hf同位素及区域综合对比研究,认为江玛南石英闪长玢岩的形成与班公湖—怒江特提斯洋南向俯冲有关。     

西藏拉萨地块西段狮泉河地区晚侏罗世花岗岩年代学、地球化学与岩石成因

目前关于拉萨地块西段狮泉河地区中生代岩浆岩的报道相对较少,限制了对该地区中生代岩浆作用的认识.对狮泉河地区石英闪长岩和闪长质包体的锆石U-Pb年龄、岩石学特征与元素地球化学进行了研究.结果显示,寄主石英闪长岩的年龄为161.1±1.7 Ma,闪长质包体的年龄为159.8±1.6 Ma和157.0±1.3 Ma,两者为同期形成.寄主石英闪长岩为I型准铝质中钾-高钾钙碱性系列岩石,具有富集大离子亲石元素、亏损高场强元素的特征.闪长质包体为准铝质中钾-高钾钙碱性系列岩石.岩石学、地球化学特征研究表明,该套岩石可能与中侏罗世班公湖-怒江特提斯洋南向俯冲有关,班公湖-怒江特提斯洋南向俯冲引起幔源物质发生熔融,上涌的幔源岩浆与拉萨地块古老基底重熔形成的酸性岩浆混合,形成了含闪长质包体的晚侏罗世岩体.      

西藏赛角铜金矿闪长岩LA-ICP-MS锆石U-Pb年龄、Hf同位素和地球化学特征及成矿意义

赛角铜金矿是西藏多龙超大型铜金矿集区的组成矿床之一,目前对其成岩成矿时代、矿床成因等方面的认识程度还很低,制约了对多龙矿集区成矿潜力的全面认识。对赛角闪长岩进行LA-ICP-MS锆石U-Th-Pb同位素、全岩主量和微量元素、锆石Hf同位素测试。测得闪长岩中锆石U-Pb年龄为120.5±1.2Ma,该年龄被解释为岩浆结晶年龄。全岩主量和微量元素分析显示,闪长岩为中钾—高钾钙碱性岩石,富集轻稀土元素和大离子亲石元素,亏损重稀土元素和高场强元素,具有活动大陆边缘弧岩浆岩的特征。锆石Hf同位素分析结果表明,锆石εHf(t)值为+5.4~+12.1,平均值为+9.0,二阶段Hf模式年龄为373~743Ma,平均值为545Ma,表现出明显的幔源特征。赛角铜金矿的形成受控于班公湖—怒江洋陆俯冲体系,成岩成矿岩浆是板片流体交代的地幔楔部分熔融形成的基性母岩浆与新生下地壳部分熔融形成的酸性岩浆混合的产物。     

西藏班公湖-怒江缝合带中段侏罗纪高镁安山质岩石对中特提斯洋演化的制约

班公湖-怒江缝合带中段地区南北向分布了三条分支蛇绿岩亚带,它们记录了该地区中特提斯洋复杂的构造演化过程。目前对于该地区洋盆俯冲消减动力学过程一直缺乏有效制约。为探讨这一问题,本文对班公湖-怒江缝合带中段新近厘定的安山岩和闪长岩开展了系统的野外、岩石地球化学和锆石U-Pb年代学研究。安山岩主要呈不整合覆盖于晚三叠世沉积地层之上,或与侏罗纪俯冲增生杂岩和橄榄岩以断层接触,闪长岩主要呈岩脉体侵入于橄榄岩中。锆石U-Pb定年表明,安山岩和闪长岩均形成于中晚侏罗世(165～161Ma)。安山岩和闪长岩地球化学组成类似,它们大都具有高的MgO含量和Mg#值,这与高镁安山岩相类似。稀土和微量元素组成显示出典型的岛弧岩浆岩特征,富集轻稀土(LREE)和Rb、Th、U、Pb等元素,亏损Ba和高场强元素(HFSE; Nb,Ta和Ti)。同时,样品还显示出较低的Ba/Th和较高的(La/Sm)N比值,以及负的锆石εHf(t)值和古老的锆石Hf模式年龄。这些特征表明这些高镁安山岩和闪长岩是大洋板片俯冲沉积物部分熔融的熔体交代地幔楔的产物。结合区域地质和前人研究,认为这些岩石可能形成于靠近海沟的大陆边缘环境,是班公湖-怒江中特提斯洋中段北拉-拉弄分支洋盆初始俯冲消减的产物,该初始俯冲作用可能与安多微陆块和南羌塘地块碰撞导致的俯冲南向跃迁有关。     

班-怒结合带西段革吉地区帕阿岩基的成因:元素地球化学、锆石U-Pb年代学及Hf同位素约束

班公湖-怒江结合带是藏北一条重要的特提斯主洋残余,在洋盆消减过程中发生了强烈岩浆作用,在其南缘形成了沿狮泉河-永珠缝合带两侧广泛分布的白垩纪大规模岩浆岩和火山岩,为了解班公湖-怒江特提斯洋俯冲和闭合过程提供了良好窗口。本文对出露于革吉地区帕阿花岗闪长岩岩基进行了详细的野外地质填图、全岩元素地球化学、锆石U-Pb年代学和Hf同位素组成研究。研究结果表明,花岗闪长岩和其闪长质包体的锆石206Pb/238U年龄分别为155. 7±1. 0Ma(谐和年龄,MSWD=0. 26,N=18)和156. 5±2. 3Ma(年龄加权平均值,MSWD=2. 6,N=16),岩浆侵位于晚侏罗世,不是以往所认为的白垩纪岩浆产物。花岗闪长岩主要属于高钾钙碱性系列,高铝饱和指数(A/CNK=0. 91～1. 04),显示了过铝质特征。花岗闪长岩富集轻稀土元素(LREE/HREE)=5. 51～8. 54,(La/Yb)N=6. 27～20. 2)和大离子亲石元素,亏损Nb、Ta、Sr、Ti等高场强元素,具有中等至弱Eu负异常(δEu=0. 61～0. 85)。锆石Hf同位素分布集中,花岗闪长岩和闪长质包体的锆石εHf(t)均为负值,分别为(-9. 4～-6. 3)和(-10. 9～-8. 6),Hf地壳模式年龄分别为(1. 32～1. 48Ga)和(1. 44～1. 56Ga),指示了帕阿花岗闪长岩岩基主体系古老地壳基底部分熔融产物,存在少量地幔物质的加入,形成于弧环境,可能属于班公湖-怒江洋盆南向俯冲到拉萨地块古老结晶基底重熔形成的一套具弧属性的未分异I型花岗岩。     

新疆谢米斯台地区晚古生代俯冲作用:来自塞勒肯特岩体的岩石学、年代学及地球化学证据

塞勒肯特岩体出露于谢米斯台中东部,岩性主要为二长花岗岩、石英闪长岩及花岗闪长岩。本文通过锆石U-Pb年代学、岩石地球化学和Sr-Nd-Hf同位素等研究,探讨其形成的构造环境及岩石成因。结果表明,二长花岗岩(400.9±4.3Ma)和石英闪长岩(398.1±4.5Ma)形成于早泥盆世,花岗闪长岩(381.7±2.9Ma)形成于晚泥盆世。岩体整体富碱,属于准铝质-弱过铝质高钾钙碱性花岗岩类。轻重稀土分馏较明显且富集轻稀土((La/Yb)_N=5.09～9.22),Eu异常不明显,相对富集Rb、Th、U、K等元素,亏损Nb、Ta、Ti等高场强元素;二长花岗岩和石英闪长岩具有低的(~(87)Sr/~(86)Sr)_i值(0.7040～0.7043),正ε_(Nd)(t)值(+4.85～+6.18),年轻的t_(DM1)(Nd)年龄(663～732Ma),二长花岗岩锆石ε_(Hf)(t)值为+7.94～+12.12,t_(DM2)(Hf)=648～889Ma;花岗闪长岩也具有低的(~(87)Sr/~(86)Sr)_i值(0.7045～0.7046),正ε_(Nd)(t)值(+4.61～+4.80),年轻的t_(DM1)(Nd)年龄(731～749Ma),花岗闪长岩锆石εHf(t)值为+4.26～+11.69,tDM2(Hf)=631～1103Ma。综合研究表明,塞勒肯特岩体形成于俯冲背景下的大陆边缘弧环境,可能是俯冲板片脱水交代地幔楔产生的玄武质岩浆上涌,导致新生下地壳发生部分熔融。二长花岗岩及石英闪长岩均来源于新生下地壳的部分熔融;花岗闪长岩主要来源于新生下地壳的部分熔融,并有少量幔源物质的加入,花岗闪长岩中的暗色微粒包体可能是幔源物质与新生下地壳部分熔融的岩浆未发生完全混合,最终冷凝结晶的产物。谢米斯台地区与俯冲相关的中酸性岩浆活动至少从晚奥陶世一直延续至晚泥盆世。     

Influences of the unsaturated,saturated, and riparian zones on the transport of nitrate near the Merced River,California, USA

Transport and transformation of nitrate was evaluated along a 1-km groundwater transect from an almond orchard to the Merced River, California, USA, within an irrigated agricultural setting. As indicated by measurements of pore-water nitrate and modeling using the root zone water quality model, about 63% of the applied nitrogen was transported through a 6.5-m unsaturated zone. Transport times from recharge locations to the edge of a riparian zone ranged from approximately 6 months to greater than 100 years. This allowed for partial denitrification in horizons having mildly reducing conditions, and essentially no denitrification in horizons with oxidizing conditions. Transport times across a 50–100-m-wide riparian zone of less than a year to over 6 years and more strongly reducing conditions resulted in greater rates of denitrification. Isotopic measurements and concentrations of excess N₂ in water were indicative of denitrification with the highest rates below the Merced River. Discharge of water and nitrate into the river was dependent on gradients driven by irrigation or river stage. The results suggest that the assimilative capacity for nitrate of the groundwater system, and particularly the riverbed, is limiting the nitrate load to the Merced River in the study area.     

山西五台山区滹沱群变质玄武岩岩石学、地球化学特征及其成因意义

在野外地质调查的基础上,选取滹沱群豆村亚群底部四集庄组玄武岩、顶部青石村组玄武岩和东冶亚群中下部河边村组玄武岩,进行岩石学和地球化学研究。分析结果表明：玄武岩主量元素高TiO2、TFeO,低MgO,具有演化的基性岩浆的特征。稀土元素含量相对较高,具有弱—中等程度的轻稀土元素富集,中等程度的轻重稀土元素分异,轻微的Eu异常;玄武岩中Cr、Ni含量变化较大,但与Mg#具有很好的正相关性。微量元素配分图解中具有明显的Sr负异常,部分样品具有Nb、Ta和Ti负异常,无Zr、Hf负异常,与岛弧火山岩不同;同时Nb、Ta无分异,Zr/Hf值较高,具有板内玄武岩浆的特征。综合分析认为,滹沱群玄武岩可能形成于板内裂谷环境。在岩浆上升过程中发生橄榄石与尖晶石的分离结晶作用,同时不同程度地受到地壳的混染。     

扬子北缘马元铅锌矿床闪锌矿微量元素及S-Pb-He-Ar-C同位素地球化学研究

马元铅锌矿床是近年扬子陆块北缘铅锌找矿的新突破。矿体呈层状、似层状产于碑坝隆起翼部震旦系灯影组角砾状白云岩层间构造带中,围岩蚀变很弱。矿石中硫化物以闪锌矿、方铅矿为主,中粗粒晶质结构,充填于白云岩角砾间。闪锌矿富集Cd、Ge、Ag,贫In、Tl、Se,Ga/In为6~132,Ge/In多1000,成矿温度以中-低温为主。金属硫化物ε~(34)S值相对集中,为12.9‰~19.4‰,平均为17.4‰,来自于海相硫酸盐的还原。铅同位素组成稳定,~(206)Pb/~(204)Pb为17.858~17.918:~(207)Pb/~(204)Pb为15.603~15.694;~(208)Pb/~(204)Pb为37.756~38.046,具有造山带和上地壳铅的特征,震旦系可能提供了金属成矿物质。闪锌矿中流体包裹体的~3He/~4He为0.03Ra~1.05Ra,~(40)Ar/~(36)Ar为326.1~765.1,~(38)Ar/~(36)Ar为0.183~0.204,表明成矿流体主要为地壳流体和饱和大气水(大气降水或海水)的混合。闪锌矿内流体包裹体挥发分δ~(13)C_(CH_4)值为-36.01‰~-28.80‰,δ~(13)C_(C_2H_6)值为-27.72‰~-22.44‰,δ~(13)C_(CO_2)值为-23.24‰~-9.68‰,表明有机流体参与了成矿作用。石英、方解石的H-O同位素结果表明具有海水和有机水混合的特征。可见,成矿流体具有两种流体混合的特征,一为蒸发海水与围岩反应所形成的盆地卤水,二为有机流体。推测矿区可能存在一个古油气藏,由于TSR生成一高硫气藏,为区内还原性有机流体的主要来源。当富含Pb、Zn等成矿物质的成矿流体运移至富含CH_4和H_2S的还原性流体的矿区角砾岩带时,两种流体混合,Pb、Zn等遇到H_2S发生反应而沉淀成矿,并伴生热液白云石等,形成了马元铅锌矿床。综上所述,我们认为马元矿床属MVT型铅锌矿床。     

贵州岑巩羊桥罗家塘杷榔动物群的发现及其后生动物早期演化的意义

岑巩县羊桥乡罗家塘杷榔组仅出露中-上部地层,岩性单一,均为青灰、灰绿色粉砂质泥、页岩.在其上部层位发现了俞氏贵州始海百合(Guizhoueocrinus yui Zhao,Parsley et Peng,2007),计有棘皮动物始海百合、腕足动物、软体动物-软舌螺、节肢动物-三叶虫等8属9种,包括了2个未定名的新种和1...     

深港西部通道填海区地下水铜、钒、铊、镉、钨迁移的实验研究

填海工程使滨海地区的地下水物理化学条件改变,导致该区域地下水中重金属浓度升高。本文以深港西部通道填海区为例,采用实验室模拟填海条件的办法,初步探讨了影响填海区地下水重金属迁移的关键因素。模拟实验结果表明,填海区地下水中钨元素含量升高与地下水淡化、淤泥氧化过程以及填料风化有关;铜元素含量升高则与淤泥氧化过程以及填料风化有关;钒元素含量升高与地下水淡化和淤泥氧化过程有关,而其他元素含量升高则主要与填料风化释放有关。     

西藏南木林县秦马弄地区二长花岗岩体岩石成因

以出露在西藏南木林县秦马弄地区二长花岗岩体为研究对象,从野外地质调查、岩石学、LA-ICP-MS锆石U-Pb定年、元素和Lu-Hf同位素地球化学等方面对其进行了分析研究。结果显示：秦马弄地区二长花岗岩体类似于A型花岗岩,形成于古近纪(E1),其LA-ICP-MS锆石U-Pb年龄为(62.0±0.5)~(63.0±0.5) Ma;岩体形成的温度较高(锆石饱和温度为745~805 ℃,平均766 ℃);岩石富SiO2(72.6%~75.0%)、Na2O(2.86%~3.59%)和K2O(4.12%~5.25%),贫CaO(0.72%~1.39%)和MgO(0.26%~0.36%),高FeOT/MgO值(7.26~10.3),属高钾钙碱性系列,弱过铝质;岩石具明显的负Eu异常(Eu/Eu*=0.40~0.70),普遍表现出Ba、Sr、P、Ti元素的亏损和Rb、Th、K、Zr、Hf元素的富集。εHf(t)值为-4.7~-1.8,其两阶段地壳Hf模式年龄tDM2为(1.1~1.3) Ga。秦马弄A型花岗岩体是冈底斯基底地壳物质部分熔融的岩浆产物,形成于古近纪印度板块与欧亚板块俯冲碰撞引起的总体为岛弧背景下的局部拉张构造环境。     

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.     

Geology, mineralogy, and PGE mineralization of the copper-nickel occurrences of the Kvinum ore field, Sredinny Range, Kamchatka

The geology and mineralogy of host metamorphic rocks, the mineralogy of sulfide ores, and the distribution of PGE mineralization were studied in detail for the Kvinum-1 and Kvinum-2 copper-nickel occurrences of the Kvinum ore field, which are the most promising targets for the copper-nickel-PGE mineralization of the Sredinny Range of Kamchatka. It was established that stringer-disseminated and massive copper-nickel ores are localized in amphibole peridotites, cortlandites, and form ore bodies varying from tens of centimeters to 5–20 m thick among the layered cortlandite-gabbroid massifs. The massive sulfide ores were found only at the bottom of cortlandite bodies and upsection grade into stringer-disseminated and disseminated ores. Pyrrhotite, chalcopyrite, and pentlandite are the major ore minerals with a sharply subordinate amount of pyrite, sphalerite, galena, arsenopyrite, and löllingite. Besides pentlandite, the Ni-bearing minerals include sulforasenides (gersdorffite), arsenides (nickeline), and tellurides (melonite) of nickel. It was found that PGE mineralization represented by antimonides (sudburyite) and tellurobismuthides (michenerite) of Pd with sharply subordinate platinum arsenide (sperrylite) is confined to the apical parts of massive sulfide zones and the transition zone to the stringer-disseminated ores. Ore intervals enriched in arsenides and tellurides of Ni, Pd, and Bi contain high-purity gold. In the central parts of the orebodies, the contents of PGE and native gold are insignificant. It is suggested that the contents of major sulfide minerals and the productivity of PGE mineralization in the cortlandites are defined by combined differentiation and sulfurization of ultramafic derivatives under the effect of fluids, which are accumulated at the crystallization front and cause layering of parental magmas with different sulfur contents. The fluid-assisted layering of mafic-ultramafic massifs resulted in the contrasting distribution of PGM in response to uneven distribution of sulfur (as well as As, Te, and Bi) during liquid immiscibility. The productivity of PGE mineralization significantly increases with increasing contents of S, As, Te, and Bi (elements to which Pt and, especially, Pd have high affinity) in fluids.     

Depth of the base of the Jackson aquifer, based on geophysical exploration, southern Jackson Hole, Wyoming, USA

 A geophysical survey was conducted to determine the depth of the base of the water-table aquifer in the southern part of Jackson Hole, Wyoming, USA. Audio-magnetotellurics (AMT) measurements at 77 sites in the study area yielded electrical-resistivity logs of the subsurface, and these were used to infer lithologic changes with depth. A 100–600 ohm-m geoelectric layer, designated the Jackson aquifer, was used to represent surficial saturated, unconsolidated deposits of Quaternary age. The median depth of the base of the Jackson aquifer is estimated to be 200 ft (61 m), based on 62 sites that had sufficient resistivity data. AMT-measured values were kriged to predict the depth to the base of the aquifer throughout the southern part of Jackson Hole. Contour maps of the kriging predictions indicate that the depth of the base of the Jackson aquifer is shallow in the central part of the study area near the East and West Gros Ventre Buttes, deeper in the west near the Teton fault system, and shallow at the southern edge of Jackson Hole. Predicted, contoured depths range from 100 ft (30 m) in the south, near the confluences of Spring Creek and Flat Creek with the Snake River, to 700 ft (210 m) in the west, near the town of Wilson, Wyoming. Received, May 1997 · Revised, February 1998 · Accepted, April 1998