Metasomatic Interaction Research Articles

Metamorphic carbonate–mafic amphibolite interactions from Arinteiro, northwest Spain

AbstractSome contacts between interlayered marbles and amphibolites at the Arinteiro copper mine are associated with zoned metasomatic calc‐silicate assemblages. Textures indicate that formation of the calc‐silicate zones involved mass transfer over distances up to 15 cm and that pore fluid solution (infiltration) may have been a significant reaction mechanism. The sequence of minerals is very similar to previously described products of metasomatic interactions between marbles and pelites. Similar metasomatic processes involving primary concentrations of carbonates can be invoked to explain the common occurrence of zoned calc‐silicate patches and folia in metaigneous amphibolites both near Arinteiro and in other terrains.

Mantle metasomatism: The REE story

Refractory rocks with light REE/heavy REE ratios greater than chondrite are common as xenoliths in basalts and kimberlites and are found in some oceanic peridotite massifs. This has led to the supposition that large parts of the upper mantle have been metasomatically altered by addition of light REE and other hyperfusible constituents. Structural and major-element geochemical evidence from xenoliths and alpine peridotites, however, suggest that the metasomatic effects are local and are related to emplacement of partial melts. The melts are represented by dikes of pyroxenite, hydrous minerals, and gabbro that occur in mantle peridotites of all origins and were emplaced in them in the same sequence as indicated by crosscutting relations. REE distributions in both the peridotite and the dikes may be explained as a result of metasomatic interaction between dikes and peridotite wall rock in which the peridotite is enriched in light REE and the dikes depleted in light REE relative to the original liquid. Differentiation of the intrusions and separation of residual liquids may further reduce the light REE/heavy REE ratio in pyroxenite dikes; these residual liquids (hydrous mineral veins) enriched in light REE extend the volume of metasomatized peridotite as they too interact with their wall rocks. Differences in the relative abundances of altered peridotite (reacted wall rock) in xenoliths and massifs are seen as a sampling problem rather than a difference in process.

The geology and geological development of the broken hill lode in the new broken hill consolidated mine, Australia Part III: Petrology and petrogenesis

Abstract Ten ore types, of three genetic types, are recognized in the mine: those formed by the recrystallization of ore layers rich in carbonates and sulphides, those formed by the diffusion of Ca, Mn, and Fe from ore layers into adjacent weakly mineralized siliceous rocks, and those ore layer rocks modified by the loss of Ca, Mn, and Fe. Some constituents moved at least tens of metres. The method of transport probably was by diffusion through, and/or transport by, a gas phase which was being expelled as the system dehydrated, decarbonated, and desulphidized during metamorphism. Most of the ‘metasomatic interaction’ between ore lodes and wall rocks occurred early in the history of the area, but some occurred after earliest metamorphic foliation, had formed, and resulted in the formation of ‘epigenetic’ vein‐like bodies of lode rock ('droppers'). Chemically similar but quantitatively minor migration took place during the high‐grade metamorphism and subsequent retrograde metamorphic events.

The geology and geological development of the broken hill lode in the new broken hill consolidated mine, Australia part I: Structural geology

Abstract Three age groups of structures are recognized regionally; the oldest of these can be subdivided into at least two age groups within the NBHC Mine. The oldest structures, which are defined as those which occur in rocks affected by the highest grade regional metamorphism, formed before the final high‐grade equilibration of the rocks. In the last phase of the oldest deformation, and in the second deformation, which was accompanied by amphibolite‐facies retrograde metamorphism, shear‐zone‐like structures formed, whose orientation and character indicate that the rocks were compressed horizontally and extended vertically. Foliation within the ‘shear zones’ is inclined to the attitude of the zone containing it; so folds within ‘shear zones’ are en‐echelon and do not plunge parallel to large folds formed on the boundaries of zones. Most of the lode and associated rocks must have existed as mechanically distinctive units at the time of the youngest phase of the oldest period of deformation, since the style of folds formed in this deformation are related to present differences in the lithology of the folded rock units. However, stratigraphic relations indicate that some of the rocks with a low Pb‐Zn content and at least some garnet quartzite formed by metasomatism which accompanied the regional metamorphism, i.e. by metasomatic interaction between adjacent rocks during metamorphism.