Search

Geogas-carried metal prospecting, an integral part of deep-penetrating geochemistry, is potentially effective in the geochemical exploration of concealed ore deposits. However, its principles and applicability remain controversial. This study summarizes and discusses the progress in geogas-carried metal prospecting in China. The method comprises three constituents: geogas, nanoparticles, and their vertical transportation. Researchers have failed to determine the exact contributions of different sources of geogas. Studies on Pb isotopes, rare earth element patterns of geogas, the comparisons between metals in soil, geogas, and ore geochemistry, and characteristics of nanoscale metals in earthgas (NAMEG), confirmed the relationship between NAMEG and concealed ore deposits. A statistical analysis of field experiments and applications showed that geogas-carried metal prospecting is applicable for the geochemical exploration of magmatic and hydrothermal Cu, Au, Zn, Pb, U, Sn, and Ag deposits and is suitable for most geochemical landscapes except deserts and cold swamps. Finally, genetic models of NAMEG anomalies were constructed. High-permeability migration channels are critical in the formation of NAMEG anomalies over concealed ore deposits. Future work entails applying geogas-carried metal prospecting to certain types of ore deposits and geochemical landscapes and studying NAMEG to provide quantitative information for targeting concealed ore deposits. [ABSTRACT FROM AUTHOR]

There are many Late Triassic mafic--ultramafic intrusions in the Hongqiling magmatic Ni--Cu sulfide deposit, North-east China. Research on magma evolution leading to formation of these mafic--ultramafic intrusions is of great significance for understanding the mantle beneath Northeast China and associated Ni--Cu mineralization. A trace element study of the No. 1, 3, and 7 intrusions in the Hongqiling deposit reveals that these mafic--ultramafic intrusions are characterized by enrichment of incompatible elements, which however cannot be interpreted by subduction modification. Furthermore, model of batch partial melting of depleted mantle accompanied by upper crustal contamination can simulate the trace element patterns of these mafic--ultramafic intrusions, but partial melting of depleted mantle accompanied by lower crustal contamination model cannot work. In addition, Sr--Nd isotopic compositions of the Hongqiling No. 1, 3, and 7 intrusions also indicate that crustal contamination could have occurred mainly during the magma ascent. Consequently, a possible scenario for the magma evolution is that the primary mafic--ultramafic magma was derived from batch partial melting of a depleted mantle, and then contaminated by Cambrian--Ordovician metamorphic rocks of the Hulan Group during ascent. We conclude that the mantle source contained no significant crustal component in the Late Triassic and was also independent of substantial contribution from subducted material, and therefore the Mesozoic large-scale lithospheric delamination beneath eastern China may happen after a period of time of the Late Triassic.De nombreuses intrusions mafiques--ultramafiques du Trias tardif sont presentes dans le gisement magmatique de sulfures de Ni--Cu de Hongqiling, du nord-est de la Chine. La recherche sur l'evolution du magma ayant mene a la formation de ces intrusions est d'une grande importance pour la comprehension du manteau qui sous-tend le nord-est de la Chine et la mineralisation associee en Ni--Cu. Une etude des elements traces des intrusions no 1, 3 et 7 du gisement de Hongqiling revele qui ces intrusions mafiques--ultramafiques sont caracterisees par un enrichissement en elements incompatibles, qui ne peut toutefois etre interprete comme etant le resultat de la subduction. En outre, un modele de fusion partielle par lot de manteau appauvri accompagnee par une contamination par la croute superieure peut reproduire les distributions d'elements traces de ces intrusions, alors que la fusion partielle de manteau appauvri accompagnee d'une contamination par la croute inferieure n'y arrive pas. La composition des isotopes de Sr et Nd des intrusions no 1, 3 et 7 de Hongqiling indique egalement qu'une contamination crustale pourrait s'etre produite principalement durant l'ascension du magma. Par consequent, un scenario possible pour l'evolution du magma est que le magma mafique--ultramafique primaire est derive de la fusion partielle par lot d'un manteau appauvri et a ensuite ete contamine par des roches metamorphiques cambro--ordoviciennes du Groupe de Hulan durant son ascension. Nous concluons que la source mantellique ne contenait pas de composante crustale significative au Trias tardif et etait independante d'une contribution substantielle de materiel subduit et, donc, que la delamination lithospherique a grande echelle au Mesozoique sous l'est de la Chine pourrait s'etre produite apres un certain temps au Trias tardif. [Traduit par la Redaction], IntroductionNumerous mafic--ultramafic intrusions are exposed in the Hongqiling area, and many of them are ore-bearing intrusions of considerable economic value. Previous studies indicate that these mafic--ultramafic intrusions were emplaced in [...]