芬兰科密铬铁矿床研究新进展

芬兰科密铬铁矿床是欧洲规模最大的铬铁矿床。对该矿床的地质背景、矿床特征、矿床成因及找矿标志进行了总结。结果表明,该矿床与古元古代层状超镁铁质杂岩体具有密切的时空分布关系,杂岩体内的铬铁矿床具典型的层状堆积特征,矿层延伸稳定。由于Cr/Fe值较低,该矿床矿石品位在世界同类矿床中属偏低水平。同位素年代学证据表明,其形成于古元古代早期(2.44 Ga),是由卡累利阿造山作用诱发多期次岩浆侵入活动并与新太古代基底岩层发生混染作用形成的。科密铬铁矿床属于层状铬铁矿床,其地质特征与中国目前已发现的豆荚状或似层状铬铁矿床存在一定区别。     

Exploring for kimberlites in glaciated terrains using chromite in quaternary till—a regional case study from northern Finland

Prospecting for kimberlites and related rocks in till-covered terrains requires a methodology for recovering a few small grains within tens of kilograms samples, necessitating 1 ppb sensitivity or better. As part of reconnaissance survey for the kimberlite indicator minerals, i.e. pyrope garnet, picroilmenite, chromite and chromian diopside, the Geological Survey of Finland (GTK) developed such a system by significantly modifying and augmenting a 3″ Knelson Concentrator that accomplishes nearly complete recovery of moderately heavy minerals (>0.25 mm) from till samples.Diamondiferous kimberlites occur in the eastern Finland around the Kaavi–Kuopio and Kuhmo areas and much of the rest of the Karelian craton remains prospective based on the empirical evidence necessary for diamond preservation: thick (>200 km) lithospheric mantle, low heat flow and Archaean age rocks. A target area in Lapland, 20×50 km in size, was selected for a pilot study to test extraction of chromite for the (1) discrimination of regionally and locally derived populations, and (2) recognition of possible kimberlitic/lamproitic chromites. Area selection was based on the regional occurrence of a variety of mantle-derived rocks, the recovery of a chromian pyrope grain from till in 1996 and most importantly, the well-established Quaternary stratigraphy in the region. The sample material consisted of sixty-two 80-kg excavator and 40-kg shovel samples. Approximately 1000 chromite grains, almost exclusively 0.25–0.5 mm in diameter, were recovered and analysed by electron microprobe.Tills in the sampling area proved to contain at least two compositional populations of chromite. The first is present in almost every sample and is apparently derived from layered mafic intrusions distal to and up-ice from the study area. The second population consists of chromites with low Ti, high Cr and Mg similar to inclusions in diamond. It is present in approximately one third of the samples, concentrated in a couple of clusters within the target area and is therefore considered to be of more local derivation. Since no high-Ti, high-Cr chromites diagnostic for kimberlites and lamproites were present in the samples, the source for the low-Ti, high-Cr, high-Mg chromite grains remains uncertain, but is probably not kimberlitic. Although this apparently is a negative outcome for diamond exploration in the target area, the main goal of the study was realised by showing the applicability of the system to heavy mineral separation from Quaternary glacial deposits.     

芬兰北部克维特斯塔镍-铜-铂族元素矿床地质特征及矿床成因

位于芬兰北部中拉普兰绿岩带的克维特斯塔(Kevitsa)镍-铜-铂族元素矿床是世界上主要的岩浆镍-铜硫化物矿床之一。该矿床储量大,含丰富的镍-铜硫化物和铂族元素。对矿床产出环境、地质特征、矿床成因等进行了总结,结果表明:矿体主要赋存于克维特斯塔基性—超基性层状侵入体的超基性单元中。主要矿石类型为普通型和镍-铂族元素型2种,其中镍-铂族元素型矿石内橄榄石具极高的Ni含量。主要矿石矿物为磁黄铁矿、镍黄铁矿、黄铜矿、黄铁矿、针硫镍矿、红砷镍矿、砷镍矿、辉砷镍矿等,绝大部分铂族矿物包含在硅酸盐中和附着在硫化物颗粒边界。Re-Os、Sm-Nd、Sr及S同位素特征显示成矿岩浆为幔源,但受到地壳物质的混染作用。Pb同位素年代学结果表明克维特斯塔侵入体形成于古元古代。