Rare Earth Element Mineralogy, Geochemistry, and Preliminary Resource Assessment of the Khanneshin Carbonatite Complex, Helmand Province, Afghanistan:
Rare Earth Element Mineralogy, Geochemistry, and Preliminary Resource Assessment of the Khanneshin Carbonatite Complex, Helmand Province, Afghanistan:
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阿富汗赫尔曼德省 Khanneshin 碳酸岩杂岩的稀土元素矿物学、地球化学和初步资源评估：
DOI：
10.3133/ofr20111207
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
K. P. Buttleman
中科院分区：
文献类型：
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作者：
R. Tucker;H. Belkin;K. Schulz;S. Peters;K. P. Buttleman
坎尼辛碳酸盐岩是一种第四纪的火成岩，它位于阿富汗赫尔曼德省里格斯坦沙漠新近系沉积岩上方约700米处。该杂岩几乎完全由富含碳酸盐的侵入和喷发的火成岩组成，轮廓粗略呈圆形，在杂岩的东南部露头有三个浅成的白榴石响岩和白榴石小岩塞。火成岩杂岩大致可划分为直径约4公里的中央侵入喷口(或地块)，由粗粒S陨石和角砾状重晶石-铁角闪石钠长花岗岩组成；边缘薄带(1公里宽)，向外倾斜(5°-45°)和碱性交代新近纪沉积地层；以及外围火山岩和火山碎屑地层围裙，距离中央侵入喷口再延伸3-5公里。中央侵入口南缘和东南缘产出直径不超过400m的黑云母-方解石碳酸盐岩和稀有白榴石响岩的小卫星侵入岩。20世纪70年代中期，苏联地质团队圈定了一个轻稀土元素(LREE)远景富集区。轻稀土富集区位于中央喷口外侧广泛脉状、脉状侵入的重晶石-铁闪石-阿尔维花岗岩中，靠近其与新近系沉积岩的东北接触。除了具有非常高的轻稀土元素浓度，重晶石-铁角闪石钠长花岗岩还高度富含钡和锶。美国地质调查局(USGS)的科学家领导了三次对汗尼辛碳酸盐岩的侦察范围任务。其中两项涉及LREE感兴趣的领域，这是本报告的主要主题。轻稀土矿化有两种类型。I型轻稀土矿化由半一致、对称条带状矿脉和不连续矿层组成，厚0.5~0.7m，长几十米。I型带状矿脉和矿层为黄色风化带，围绕深色中心带对称，富含钾盐-(Ce)、重晶石、辉钛矿和次生轻稀土矿物(同辉铜矿-(Ce)和黄铁矿-(Ce))。暗色中心带主要由铁角质白云石、重晶石、磷灰石和辉铜矿组成，也有微量的钾盐-(Ce)。这些1型矿脉和矿层与垂直距离约150米的深色铁角闪石-重晶石-阿尔维石(围岩)层相互交错。在一些矿脉中，轻稀土碳酸盐矿物形成致密的球形集合体(直径100微米)，推测是由不混溶的液滴结晶而成，按体积计占矿脉的30%。类型1矿脉和煤层平均重量百分比为19.92%(重量百分比)，3.61重量百分比锶和2.78重量百分比总轻稀土元素。8个平均全岩的∑轻稀土元素(∑轻稀土元素是La、Ce、Pr、Nd的总和)的值在6.23~1.83wt.%之间。
The Khanneshin carbonatite is a deeply dissected igneous complex of Quaternary age that rises approximately 700 meters (m) above the Neogene sedimentary rocks of the Registan Desert, Helmand Province, Afghanistan. The complex consists almost exclusively of carbonate-rich intrusive and extrusive igneous rocks, crudely circular in outline, with three small hypabyssal plugs of leucite phonolite and leucitite outcropping in the southeast part of the complex. The igneous complex is broadly divisible into a central intrusive vent (or massif), approximately 4 kilometers (km) in diameter, consisting of coarse-grained sövite and brecciated and agglomeratic barite-ankerite alvikite; a thin marginal zone (<1 km wide) of outwardly dipping (5°–45°) and alkali metasomatized Neogene sedimentary strata; and a peripheral apron of volcanic and volcaniclastic strata extending another 3–5 km away from the central intrusive vent. Small satellitic intrusions of biotite-calcite carbonatite and rare leucite phonolite, no larger than 400 m in diameter, crop out on the southern and southeastern margin of the central intrusive vent. A zone of prospective light rare earth element (LREE) enrichment was delineated by Soviet geological teams in the mid-1970s. The area of LREE-enrichment is situated in extensively veined and dike-intruded barite-ankerite alvikite in the outer part of the central vent near its northeast contact with Neogene sedimentary rocks. In addition to having very high concentrations of LREE, the barite-ankerite alvikites are also highly enriched in barium and strontium. Three reconnaissance scoping missions to the Khanneshin carbonatite were led by scientists of the U.S. Geological Survey (USGS). Two of these were to LREE area of interest which is the primary subject of this report. Two types of LREE mineralization occur. Type-1 LREE mineralization consists of semiconcordant, symmetrically banded veins and discontinuous seams, as much as 0.5–0.7 m thick and several tens of meters long. These occur throughout a vertical thickness of at least 150 m. Type-1 banded veins and seams are yellow-weathering zones, symmetric about a dark central zone, that are enriched in khanneshite-(Ce), barite, strontianite, and secondary LREE minerals (synchysite-(Ce) and parisite-(Ce)). The dark central zone, consisting primarily of ankeritic dolomite, barite, apatite, and strontianite, also has trace khanneshite-(Ce). These type-1 veins and seams alternate with dark, meterthick layers of ankerite-barite alvikite (wall rock) over a vertical distance of approximately 150 m. In some veins LREE carbonate minerals form dense spherically shaped aggregrates (100 micrometers diameter), presumably crystallized from immiscible droplets, which constitute as much as 30 percent (by volume) of the vein. Type-1 veins and seams average 19.92 weight percent (wt. percent) Ba, 3.61 wt. percent Sr, and 2.78 wt. percent total LREE. The values of ∑ LREE (∑ LREE is the sum of La, Ce, Pr, and Nd) for eight average whole-rocks range from 6.23 to 1.83 wt. percent.