The masquerade of alkaline–carbonatitic tuffs by zeolites: a new global pathfinder hypothesis

Abstract

Rapid and progressive reaction of alkaline-carbonatitic tuffs with magmatic and crustal fluids disguises their initial character and origin. This is collectively indicated from (a) the extensive literature on zeolite formation from volcanic glass precursors and alkaline fluids, (b) mineralogical characteristics of specific zeolite species, (c) a comparative review of global distributions of alkaline-carbonatite suites and of zeolite minerals, and (d) new trace element data from zeolite samples. A unifying conceptual model based on tectonic and geological settings, hydrological regime and mineralogy is presented that helps to explain the global distributions and current understanding of occurrences. The model will assist in resource exploration by contributing deeper understanding of the economically important bedded zeolite deposits and further, serve as a guide to the discovery of new alkaline-carbonatitic suites, potentially of economic significance (metallic ores and rare earth elements).It follows that future testing of the hypothesis will impact on models of natural carbon cycling as volcanic contributions of CO 2 are reviewed.

A way to hydrothermal paroxysm, Colli Albani volcano, Italy

“…It is noteworthy that, whereas the observed alteration silicate-mineral assemblages, dominated by clays, feldspars, and SiO 2 phases, are consistent with reactive progression of zeoli tized tuffs analogous to that documented by Hay and Sheppard (2001) and by Campbell et al (2012Campbell et al ( , 2013, the mineral assemblage for the paleosol in the "unaltered" Imater quarry is striking and signifi cant for its marked contrast to the underlying alkaline tuffs (Fig. 5).…”

“…There has been, in particular, a dramatic re-equilibration from strongly alkaline Si-undersaturation to strongly acidic Si-enrichment in all the features observed. The strong alteration and re-equilibrium toward a Si-rich state was driven by the high reactivity of alkaline rocks to acidic hydrothermal fl uids (e.g., de'Gennaro et al, 2000;Chipera and Apps, 2001;Hay and Sheppard, 2001;Rae et al, 2003;Reyes et al, 2003;Mitchell, 2005;Boyce et al, 2007;Campbell et al, 2012). As a rule, Si-rich volcanic products (particularly glasses) are less reactive than intermediate or mafi c products, although chemical-physical properties of altering fl uids and other boundary conditions are all important factors affecting reaction kinetics.…”

Abstracts

“…To demonstrate this, new geochemical data are presented from alkaline hydrothermal assemblages in surface exposure that contain hydrated Al-silicates ± calcite/carbonates (Campbell, Dyer, Williams. and Lythgoe 2014), supported by a novel conceptual model (Campbell et al 2012; Tyrer and Campbell 2012). The data from four continents represent the earliest contributions to a potential global reference framework, supporting exploration with fundamental understandings of mineral–rock–fluid–time compositional relationships in these systems (REE in Alaline Mineral Systems, Year).…”

Determination of zeolite-group mineral compositions by electron probe microanalysis

“…The scientific value of zeolites in the Earth sciences is found partly in their sensitivity to different temperatures and geochemical environments (Chipera and Apps, 2001), enabling predictive deductions to be made about formation conditions from mineral assemblages and compositions (e.g. Neuhoff et al, 2000;Bish and Ming, 2001;Giampaolo et al, 2008;Campbell et al, 2012;Langella et al 2013;Weisenberger et al, 2014;Cappelletti et al, 2015). This has particular relevance to hydrothermal, surface and diagenetic systems, where mineral compositions can track changing fluid regimes (Hay and Sheppard, 2001;Langella et al, 2001;Utada, 2001, Campbell et al, 2012Langella et al 2013;Weisenberger et al 2014).…”

“…Neuhoff et al, 2000;Bish and Ming, 2001;Giampaolo et al, 2008;Campbell et al, 2012;Langella et al 2013;Weisenberger et al, 2014;Cappelletti et al, 2015). This has particular relevance to hydrothermal, surface and diagenetic systems, where mineral compositions can track changing fluid regimes (Hay and Sheppard, 2001;Langella et al, 2001;Utada, 2001, Campbell et al, 2012Langella et al 2013;Weisenberger et al 2014). Where natural mineral reaction paths involving zeolites are combined with other geological criteria, they have potential value in mineral exploration (Campbell et al, 2012(Campbell et al, , 2013(Campbell et al, , 2014, in deepening understanding of global carbon cycling and climate change (Heister et al, 2001;Campbell et al, 2012) and in contributing to the rationalization of volcanic hazard processes (Giampaolo et al, 2008;Bear et al, 2009;Vignaroli et al, 2014).…”