Downhole mineralogy logs: mineral inversion methods and the problem of compositional colinearity
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Abstract The inversion of chemical data derived from geochemical and related nuclear spectroscopy tools to provide mineralogy logs is now a widespread approach to the interpretation of these data. The most valuable of such transforms attempt to provide the percentages of actual phases present (minerals and fluids) at each depth interval rather than the ideal minerals occurring in simple theoretical models. Of the numerous problems involved in the inversion for a particular phase assemblage, the most serious is probably that of compositional colinearity in which three or more of the phases sought lie on, or close to, the same compositional plane. Depending upon the algorithm used for the inversion the effects of such compositional constraints may vary between a failure to find any (numerical) solution, failure to find a unique solution, or a solution which may be significantly in error for only a very small deviation in the chemical log or phase compositions. These effects are illustrated using geochemical logs from sedimentary environments together with examples from laboratory derived and numerically simulated geochemical data.Protolith
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The Rae craton on Melville Peninsula, Nunavut, comprises several lithotectonic domains, including a structurally and lithologically distinct yet poorly known crustal terrane, the Repulse Bay block (RBb). This study presents new lithological and petrographic observations, combined with U–Pb zircon data, to better understand the Archean and Paleoproterozoic crustal evolution of the RBb. The new data demonstrate that the central-eastern RBb consists of the following: (i) upper amphibolite- to granulite-facies, ca. 2.73–2.71 Ga intermediate granitoid gneisses and gabbroic sheets; (ii) ca. 2.69 Ga two-pyroxene charnockite to enderbite intrusions; and (iii) thin slivers of both Archean and Paleoproterozoic supracrustal rocks. Inherited zircon also attests to the presence of a Mesoarchean to Paleoarchean substrate. A semi-pelitic gneiss from one of the Paleoproterozoic supracrustal panels was deposited sometime after ca. 1.89 Ga and shows a similar detrital zircon age profile to a <1.92 Ga semi-pelitic gneiss from the Lyon Inlet boundary zone at the northern extent of the RBb. Zircon in most rocks analyzed record metamorphism related to the Trans-Hudson orogeny between ca. 1.84 and 1.82 Ga. Results from this study are most consistent with the RBb, representing a piece of lower to middle crustal level of the Rae craton, rather than a distinct and separate crustal entity (i.e., an exotic block).
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东秦岭地区是我国重要的花岗伟晶岩区及稀有金属成矿区。电气石在东秦岭各类花岗伟晶岩中广泛发育,通常在无矿化伟晶岩、铍矿化及锂矿化伟晶岩中呈黑色-深蓝色。本文旨在通过各类伟晶岩中电气石的对比研究揭示电气石地球化学特征对东秦岭伟晶岩矿化类型的指示作用。本文所研究电气石为作为东秦岭各类伟晶岩贯通矿物的黑电气石系列。在双峰村、碾盘及风原无矿化伟晶岩中,黑电气石生长环带发育,单偏光下呈蓝绿色-棕色,具有较高的Mg、Ti、Sc含量,较低Li、Mn、Zn含量,δ11B值变化较大(约-19.00‰~-12.00‰)。街子沟富铍伟晶岩中黑电气石单偏光下呈蓝色-草绿色,Ti、V、Sc含量低于无矿化伟晶岩中黑电气石,δ11B值约-16.00‰~-12.00‰。丰庄伟晶岩具弱铌钽矿化,黑电气石具有核-边结构,单偏光下呈蓝色-深蓝色,比无矿化和铍矿化伟晶岩中黑电气石含有更高Li、Zn含量,δ11B值-21.92‰~-14.75‰。在蔡家沟Li矿化伟晶岩中,黑色-深蓝色电气石Li2O含量达到约1.0%,成分上过渡至黑电气石-锂电气石系列,具有最高Li、Mn、Zn含量,而Mg、Ti、Sc、V含量极低,其δ11B值-18.96‰~-16.89‰。硼同位素分析揭示碾盘、风原及丰庄伟晶岩中存在两类电气石,Ⅰ类电气石富重B同位素,可能为较早从伟晶岩熔体中结晶形成;而Ⅱ类电气石具有更负的δ11B值,可能为伟晶岩流体出溶时由流体中晶出。流体出溶导致围岩中形成热液电气石,其δ11B值与伟晶岩中Ⅱ类电气石相似,表明出溶流体与围岩的作用并未导致显著的B同位素分馏。但伟晶岩与围岩之间的作用,使得Mg、Ti、V进入伟晶岩,同时Li、B、Al进入围岩。在研究区内,从无矿化至锂矿化伟晶岩,随伟晶岩分异程度增加,岩浆成因的黑电气石中Li、Mn、Zn、F含量升高而δ11B值降低,表明黑电气石的Li、Mn、Zn及F含量和B同位素组成可以有效指示伟晶岩矿化类型。
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Pegmatite
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Carbonatite
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Triassic magmatism and associated rare metal deposits are increasingly being identified within the Central Tianshan Massif in NW China. A lack of geochronological studies on these mineralized plutons, however, has limited our understanding of their origins and tectonic setting. The Guobaoshan pluton, situated in the eastern segment of Central Tianshan Massif, contains ca. 281,000 tons of Rb2O resource and may be the largest Rb deposit in the world. The pluton exhibits a gradual lithological zonation from muscovite granite to amazonite(-bearing) muscovite granite from west to east; pegmatite veins of amazonite occur within the pluton and around its periphery. In this contribution, LA–ICP–MS U–Pb age dating and trace-element analysis have been conducted on zircon, monazite, cassiterite, and columbite to address the timing of magmatism and Rb mineralization, and the processes that generated the mineralization. Internal textures and trace-element contents suggest that these minerals crystallized during the magmatic–hydrothermal transition stage over a period of ca. 10 million yeas from the Early to Middle Triassic (240–249 Ma). Combined with available data, it is suggested that the Triassic was an important period for granitic magmatism and rare metal metallogeny in the Central Tianshan Massif, which may be a response to this region being dominated by an intra-continental extensional setting during this time period.
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Metallogeny
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Dike
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Metasomatism
Greenschist
Orogeny
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Cassiterite
Pegmatite
Migmatite
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Dike
Greisen
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