Grade-tonnage and other models for diamond kimberlite pipes

Grade-tonnage and other quantitative models help give reasonable answers to questions about diamond kimberlite pipes. Diamond kimberlite pipes are those diamondiferous kimberlite pipes that either have been worked or are expected to be worked for diamonds. These models are not applicable to kimberlite dikes and sills or to lamproite pipes. Diamond kimberlite pipes contain a median 26 million metric tons (mt); the median diamond grade is 0.25 carat/metric ton (ct/mt). Deposit-specific models suggest that the median of the average diamond size is 0.07 ct and the median percentage of diamonds that are industrial quality is 67 percent. The percentage of diamonds that are industrial quality can be predicted from deposit grade using a regression model (log[industrial diamonds (percent)]=1.9+0.2 log[grade (ct/mt)]). The largest diamond in a diamond kimberlite pipe can be predicted from deposit tonnage using a regression model (log[largest diamond (ct)]=–1.5+0.54 log[size (mt]). The median outcrop area of diamond pipes is 12 hectares (ha). Because the pipes have similar forms, the tonnage of the deposits can be predicted by the outcrop area (log[size (mt)]=6.5+1.0 log[outcrop area (ha)]). Once a kimberlite pipe is identified, the probability is approximately .005 that it can be worked for diamonds. If a newly discovered pipe is a member of a cluster that contains a known diamond kimberlite pipe, the probability that the new discovery can be mined for diamonds is 56 times that for a newly discovered kimberlite pipe in a cluster without a diamond kimberlite pipe. About 30 percent of pipes with worked residual caps at the surface will be worked at depth. Based on the number of discovered deposits and the area of stable craton rocks thought to be well explored in South Africa, about 10^–5 diamond kimberlite pipes are present per square kilometer. If this density is applicable to the South American Precambrian Shield, more than 70 undiscovered kimberlite pipes are predicted to be present.     

国外金刚石矿床与江苏北部金刚石找矿前景

金刚石是发展国民经济和国防尖端工业不可缺少的战略矿产。多年来，我国经济建设所需的天然金刚石70％依靠进口。随着我国经济建设的快速发展，今后对金刚石的需求必将与日俱增。因此，通过对世界金刚石矿床勘查现状的了解和分析，可以为我国在一些有找矿前景的地区如江苏北部地区，加强对大型宝石级金刚石矿床的勘查，产生积极的指导意义。     

模糊数学方法在金刚石原生矿预测中的应用

在金刚石原生矿预测研究中应用模糊数学中的相似性优选和综合评判方法取得了好的效果。相似性优选主要利用定量数据并可直接用汉明距离求出规格化对比方阵,综合评判方法可以同时把定量和定性数据都纳入四个综合评判模型,若有必要还可以继续进行二级综合评判。在进行综合评判时必需考虑元素在岩体中的分布模型,若使用常量元素分析结果,则直接用原始数据,若为微量元素分析结果则先对原始数据先取对数。再根据样品数的多少分别选用格贴近度(N>5)和中间型隶属函数(N≤5)求值,若为其它的定性数据则采用其它相应的隶属函数求值,在此基础上再进行综合评判。笔者在实际应用时以综合结果为主,参考相似性优选结果进行选点,只利用岩石化学分析结果就获得了十分理想的效果,选出的第一点取样经鉴定即为钾镁煌斑岩。     

Patterns and controls on the distribution of diamondiferous intrusions in Australia

The distribution of kimberlite, lamproite and related alkaline volcanism in Australia can be broadly related to the structure of the Australian continent and lithosphere. Diamondiferous kimberlites and lamproites, with the apparent exception of the weakly diamondiferous Orrorro kimberlites in the Adelaide Fold Belt, lie within the large Precambrian shield where seismic tomographic models and heat flow data indicate the presence of relatively cold, high seismic wave speed lithosphere (tectosphere) typically some 200 km thick or more beneath the Archaean cratons and up to 300 km in parts of central Australia. Many of the diamondiferous intrusions appear to lie at the margins rather than in the centre of the lithosphere domains. The highest concentration of diamondiferous intrusions (kimberlites and lamproites) is on and around the Kimberley Craton where seismic data indicate crustal thicknesses of 35–40 km and a lithosphere up to 275 km thick that is distinct from Proterozoic northern Australia.

Many, but clearly not all, of the intrusions show evidence of regional and local structural controls. Some are spatially associated with known crustal structures, especially regional faults. Others are aligned, either singly or in clusters, along or near discontinuities and/or gradients evident in regional scale potential field data, especially the total horizontal gradients of gravity data continued upward tens to hundreds of kilometres. Many of these features are not evident in the original datasets as their signatures are masked by shorter wavelength (near surface) anomalies. In some cases, the kimberlites and associated rocks lie within crustal blocks and domains defined by discontinuities in the potential field data rather than at domain boundaries.

Our overview suggests that analysis of potential field data, especially horizontal gradients in upwardly continued potential field data, at all scales can assist definition of crustal and, potentially, lithospheric structures that may influence the distribution of diamond pipes. However, more definitive mapping of Australia's diamond prospective regions requires the integration of data on crustal structures, especially trans-lithospheric faults, and geodynamic settings with high resolution tomographic models and other geophysical, petrologic, and isotopic information on the nature of the lithosphere beneath the Australian continent.     

桂北融水地区钾镁煌斑岩Ar- Ar年代学、地球化学特征及其构造意义

桂北地区发育较多的钾镁煌斑岩脉,可能为该区近源金刚石重砂矿的母岩,对于其形成时代一直是长期争议悬而未决的重要地质问题。本文研究表明,融水地区金云钾镁煌斑岩的金云母(斑晶、基质) 40 Ar- 39 Ar冷却封闭年龄为203~204 Ma,表明其形成于晚三叠世。岩石SiO 2为55. 68%~56. 85%,高钾(K 2O/Na 2O=1. 87~3. 06)、高Mg # (71. 90~73. 20)、轻稀土富集、重稀土相对亏损,轻重稀土分异明显,Eu略亏损,富集大离子亲石元素,亏损高场强元素,地球化学特征表明其来源于俯冲流体交代的富集地幔特征。上述研究表明,扬子克拉通东南缘桂北地区中生代岩石圈地幔为经历俯冲流体交代的富集地幔,并于晚三叠世经历了重要的岩石圈伸展减薄构造事件,其形成可能与古太平洋板块平板式俯冲有关。     

Mineral phase compositions in silica-undersaturated ‘leucite’ lamproites from the Bucak area, Isparta, SW Turkey

Ultrapotassic rocks in the Bucak area of Isparta Angle, SW Turkey, show unusually low SiO₂ (46.8–49.2 wt.%) and high MgO (10.4–11.6 wt.%) contents, and lamproitic affinity (K/Na, > 2.5; Mg#, 73–75; Al₂O₃, 9.2–11 wt.%, CaO 7.4–10.6 wt.%, Cr, 525–675 ppm; Ni, 442–615 ppm). They are made up by phlogopite (30–40 vol.%), leucite (25–30 vol.%), olivine (5–20 vol.%), which rarely contain Cr-spinel, clinopyroxene (5–10 vol.%), sanidine (5 vol.%) and richterite, with accessory apatite, magnetite and ilmenite. One sample also include negligible sodalite in groundmass, which is unusual mineral in lamproites. Mineral phase variation and textures record discrete phases of pre-eruptive crystallization: (1) early appearance of (Cr-spinel-bearing) olivine, Ti poor phlogopite ± apatite at pressures of ca. 1.0–2.0 GPa, at or close to the lithospheric Mechanical Boundary Layer (MBL), and (2) later appearance of Ti rich phlogopite, clinopyroxene, richterite, leucite, sanidine, and other minor phases, at pressures of ca. 0.1–1.0 GPa, indicating discrete, pressure-specific fractionation events. The Bucak silica poor ‘leucite’ lamproites were probably generated by partial melting of phlogopite-bearing, refractory peridotite at pressures of ca. 1.5–2 GPa, higher than those proposed for SiO₂-saturated ‘phlogopite’ lamproites (ca. 1–1.5 GPa) from Afyon, to the North. The depth (total pressure) of melt segregation probably dominates over volatile partial pressures (e.g. of CO₂, F, H₂O) in determining the SiO₂-undersaturated character of Bucak magmas.     

塔里木南缘煌斑岩的时代及其地质意义

在塔里木南缘皮山县境内发现东西向煌斑岩带,它们呈脉状产出,围岩为前寒武纪绢云绿泥石英片岩、变质粉砂岩及石炭纪玄武岩、结晶灰岩等。岩石地球化学特征与典型地区的钾镁煌斑岩相似。从煌斑岩中挑选的金云母经ArAr测年,获得理想的坪年龄为217.65±0.39Ma,⁴⁰Ar/³⁶Ar³⁹Ar/³⁶Ar等时线年龄为217.27±1.38Ma,与坪年龄一致。这一年龄值代表了煌斑岩的形成时代,与阿尔金断裂形成时代基本一致,它代表了塔里木南缘在这一时期重要的走滑事件。煌斑岩带的发现,对塔里木南缘的金刚石找矿勘查具有重要意义。     

贵州省施秉大坪钾镁煌斑岩地球化学特征及源区指示

借助矿物学和主微量元素研究手段,本论文对贵州省施秉县大坪钾镁煌斑岩开展了系统地球化学探讨.主元素研究显示,研究区钾镁煌斑岩SiO2含量变化较低(35.0～55.0 wt.％),为超基性岩浆岩;Al2O3、TiO2、CaO、P2O5和Na2O指数表明研究区火成岩属典型的钾镁煌斑岩.另外,研究区钾镁煌斑岩显示出轻稀土元素(...