Zeolites May Mask Recognition of Alkaline-Carbonatite Abundance

In a recent paper, researchers suggest that rapid and progressive reaction of low-silica alkaline–carbonatitic volcanic tuffs with magmatic and crustal fluids produce bedded zeolite deposits - reactions that disguise the initial character and origin of the rocks.

Zeolites

A novel hypothesis links minerals of the zeolite group with carbonatitic magmatism through their shared associations with alkaline igneous rocks and alkaline fluids. In a recent paper Campbell et al. suggest that rapid and progressive reaction of low-silica alkaline–carbonatitic volcanic tuffs react rapidly with magmatic and crustal fluids to produce bedded zeolite deposits  reactions that disguise the initial character and origin of the rocks [1]. This hypothesis was collectively suggested by: (1) literature on zeolite formation from volcanic glass precursors and alkaline fluids, (2) mineralogical characteristics of specific zeolite species, (3) comparative review of global occurrences of alkaline carbonatite suites and of zeolite minerals, and (4) the authors’ trace element data from zeolite samples.
 
The authors hypothesize that a variety of zeolite deposits can result from progressive fluid reactions in extrusive alkaline volcanics, sometimes leading to observed zoned sequences with silica enrichment, depending on fluid compositions. A conceptual model based on tectonic and geological settings, hydrological regime, and mineralogy was developed to explain the phenomenon of this zeolite “masquerade.” The five main controls in the model are: 1) rift or extensional tectonics, 2) original glass/magma composition, 3) hydrological regime, 4) trace element partitioning and mobility, and 5) zeolite mineral properties.
 
Linking rare earth element patterns of zeolites with geological criteria shows how the regional hydrological regime impacts on interpretation of altered mineral compositions.  Rare earth patterns in zeolites in the study coupled with zeolite mineral assemblage data suggest that pathfinder criteria based on mineral occurrences and their major and trace element signatures could be useful in the search for new alkaline carbonatite suites and their potential mineral resources. Subsequently, the hypothesis and model also will help address larger questions regarding the origin of carbonatite magmas and carbon cycling deep in Earth.

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