Occurrence and Distribution of Moganite and Opal-CT in Agates from Paleocene/Eocene Tuffs, El Picado (Cuba)

Jens Götze,Tanja Mohr-Westheide,Klaus Stanek,Gerardo Orozco,Moritz Liesegang

doi:10.3390/min11050531

Jens Götze, Tanja Mohr-Westheide + Show 3 more

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https://doi.org/10.3390/min11050531

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Abstract

Agates in Paleocene/Eocene tuffs from El Picado/Los Indios, Cuba were investigated to characterize the mineral composition of the agates and to provide data for the reconstruction of agate forming processes. The volcanic host rocks are strongly altered and fractured and contain numerous fissures and veins mineralized by quartz and chalcedony. These features indicate secondary alteration and silicification processes during tectonic activities that may have also resulted in the formation of massive agates. Local accumulation of manganese oxides/hydroxides, as well as uranium (uranyl-silicate complexes), in the agates confirm their contemporaneous supply with SiO2 and the origin of the silica-bearing solutions from the alteration processes. The mineral composition of the agates is characterized by abnormal high bulk contents of opal-CT (>6 wt%) and moganite (>16 wt%) besides alpha-quartz. The presence of these elevated amounts of “immature” silica phases emphasize that agate formation runs through several structural states of SiO2 with amorphous silica as the first solid phase. A remarkable feature of the agates is a heterogeneous distribution of moganite within the silica matrix revealed by micro-Raman mapping. The intensity ratio of the main symmetric stretching-bending vibrations (A1 modes) of alpha-quartz at 465 cm−1 and moganite at 502 cm−1, respectively, was used to depict the abundance of moganite in the silica matrix. The zoned distribution of moganite and variations in the microtexture and porosity of the agates indicate a multi-phase deposition of SiO2 under varying physico-chemical conditions and a discontinuous silica supply.

Highlights

Cuba is a country that is famous for its enormous deposits of Ni ores.Huge surficial laterite horizons in the eastern part of the Caribbean island belong to the most important nickel deposits in the world and contain considerable amounts of other valuable chemical compounds of, e.g., Sc, Co, or rare earth elements (REE) [1]
These results document that the primary mineral composition of the volcanic rocks is strongly modified during the alteration processes, resulting in the release of remarkable amounts of SiO2
The present study provides first detailed mineralogical data of agates from El Picado/Los Indios in the Eastern part of Cuba (Moa region)

Summary

Introduction

Cuba is a country that is famous for its enormous deposits of Ni ores.Huge surficial laterite horizons in the eastern part of the Caribbean island belong to the most important nickel deposits in the world and contain considerable amounts of other valuable chemical compounds of, e.g., Sc, Co, or rare earth elements (REE) [1]. Different gemstones were exploited between 1989 and 1998 in Cuba, especially silica minerals and rocks such as chalcedony, opal, jasper or silicites [2]. The geology of Cuba isand dominated metamorphic and sedimentary rocks. The geology is dominated metamorphic sedimentary rocks. Cenozoic times along trending fault systems, resulted in a geologic trisection [5]. Central part (Escambray) consists of ophiolites, lifted and metamorphic western part of theThe island is mainly characterized by carbonate sediments widespread rocksformation. Central part (Escambray) ophiolites,by lifted metamorphic iolite complexes of the Caribbean, K-poor volcanic rocks (and associated intrusive rocks) rocks and carbonate sediments. The eastern part of Cuba is dominated by the largest of the Paleogene to Eocene island arc volcanism, as well rocks as overlaying calcareousintrusive sediophiolite complexes of the Caribbean,.

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