minerals Article Occurrence and Distribution of Moganite and Opal-CT in Agates from Paleocene/Eocene Tuffs, El Picado (Cuba) Jens Götze 1,*, Klaus Stanek 2, Gerardo Orozco 3 , Moritz Liesegang 4 and Tanja Mohr-Westheide 5 1 Institute for Mineralogy, TU Bergakademie Freiberg, Brennhausgasse 14, 09599 Freiberg, Germany 2 Institute for Geology, TU Bergakademie Freiberg, Cottastraße 4, 09599 Freiberg, Germany; [email protected] 3 Departamento de Geología, Universidad de Moa, Avenida Calixto Garcia Iñiguez 15, Moa, Holguín 83330, Cuba; [email protected] 4 Institute for Geological Sciences, FU Berlin, Malteserstraße 74–100, 12249 Berlin, Germany; [email protected] 5 Institute for Evolution and Biodiversity Science, Museum für Naturkunde, Invalidenstraße 43, 10115 Berlin, Germany; [email protected] * Correspondence: [email protected] 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 Citation: Götze, J.; Stanek, K.; of the agates is characterized by abnormal high bulk contents of opal-CT (>6 wt%) and moganite Orozco, G.; Liesegang, M.; (>16 wt%) besides alpha-quartz. The presence of these elevated amounts of “immature” silica phases Mohr-Westheide, T. Occurrence and Distribution of Moganite and emphasize that agate formation runs through several structural states of SiO2 with amorphous Opal-CT in Agates from silica as the first solid phase. A remarkable feature of the agates is a heterogeneous distribution of Paleocene/Eocene Tuffs, El Picado moganite within the silica matrix revealed by micro-Raman mapping. The intensity ratio of the main −1 (Cuba). Minerals 2021, 11, 531. symmetric stretching-bending vibrations (A1 modes) of alpha-quartz at 465 cm and moganite https://doi.org/10.3390/min11050531 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 Academic Editor: Galina Palyanova a multi-phase deposition of SiO2 under varying physico-chemical conditions and a discontinuous silica supply. Received: 27 April 2021 Accepted: 12 May 2021 Keywords: agate; alpha-quartz; chalcedony; moganite; opal-CT; Raman spectroscopy Published: 18 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- iations. Cuba is a country that is particularly 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]. However, occurrences of other mineral deposits are rare and restricted to a few locations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. For instance, different gemstones were exploited between 1989 and 1998 in Cuba, This article is an open access article especially silica minerals and rocks such as chalcedony, opal, jasper or silicites [2]. Remark- distributed under the terms and able are chalcedony from Palmira near Cienfuegos, opal from Loma de los Ópalos and conditions of the Creative Commons Pontezuela (near Camagüey) as well as silicites from Corralillo on the northern coast of Attribution (CC BY) license (https:// Cuba (Figure1). Silicified wood occurs in Upper Cretaceous volcanic sequences of the creativecommons.org/licenses/by/ Sierra de Najasa SE of Camagüey (Sibanicu) and above ophiolites in the Sierra de Cubitas 4.0/). north of Camagüey (Miocene) [2,3]. Minerals 2021, 11, 531. https://doi.org/10.3390/min11050531 https://www.mdpi.com/journal/minerals Minerals 2021, 11, 531 2 of 13 Minerals 2021, 11, x FOR PEER REVIEW 2 of 13 Figure 1. A geomorphological map of Cuba showing occurrences of different SiO2 mineralization. Figure 1. A geomorphological map of Cuba showing occurrences of different SiO2 mineralization. Agates were sporadically found in Upper Cretaceous rocks overlaying ophiolites of Agates were sporadically found in Upper Cretaceous rocks overlaying ophiolites of the Camagüey fault zone NE of Camagüey. In addition, chalcedony, jasper and silicified the Camagüey fault zone NE of Camagüey. In addition, chalcedony, jasper and silicified corals are reported from the Rio Yumuri region near Matanzas [2]. In the eastern part of corals are reported from the Rio Yumuri region near Matanzas [2]. In the eastern part of Cuba, silica mineralization occurs in particular in the surrounding area of Moa. Jasper was Cuba,found silica in altered mineralization serpentinites occurs of the inCosta particular de Moa in region, the surrounding whereas agates/chalcedony area of Moa. Jasperoc- wascurfound in altered in altered volcanic serpentinites rocks in the of region the Costa of El dePicado, Moa W region, of Moa whereas as well agates/chalcedony as near Baracoa occuron the in river altered Toa, volcanic E of Moa rocks (Figure in 1). the region of El Picado, western Moa as well as near BaracoaBecause on the of river the lack Toa, of eastern mineralogical Moa (Figure data,1 ).the agates of the Moa region were in the focusBecause of the present of the lack study. of mineralogicalMineralogical and data, geochemical the agates investigations of the Moa region were wereaimed in to the focuscharacterize of the present the silica study. minerals Mineralogical in the agates and and geochemical to provide data investigations for the reconstruction were aimed of to characterizeagate formation the silicaprocesses minerals in Moa. in On the the agates other and hand, to the provide results data of the for study the reconstruction should pro- ofvide agate additional formation general processes arguments in Moa. for Onthe theongoing other discussion hand, the concerning results of the the study enigma should of provideagate genesis additional [3,4]. generalEmphasis arguments was placed for on the the ongoing detection discussion and distribution concerning of the the mono- enigma ofclinic agate silica genesis polymorph [3,4]. moganite Emphasis and was opal-CT, placed which on the both detection occur in andconsiderable distribution amounts of the monoclinicin the agates silica from polymorph the surroundings moganite of Moa. and opal-CT, which both occur in considerable amounts in the agates from the surroundings of Moa. 2. Geological Background and Sample Material 2. GeologicalThe geology Background of Cuba is and dominated Sample by Material metamorphic and sedimentary rocks. The ge- ographicThe geology position ofin Cubaa tectonically is dominated active byregion, metamorphic with plate andmovements sedimentary in Mesozoic rocks. to The geographicCenozoic times position along in NE a tectonically trending fault active systems, region, resulted with platein a geologic movements trisection in Mesozoic [5]. The to Cenozoicwestern part times of alongthe island NE is trending mainly characterized fault systems, by resulted carbonate in asediments geologic and trisection widespread [5]. The westernkarst formation. part of the The island central is mainly part (Escambray) characterized consists by carbonate of ophiolites, sediments lifted andmetamorphic widespread karstrocks formation. and carbonate The sediments. central part The (Escambray) eastern part consistsof Cuba ofis dominated ophiolites, by lifted the largest metamorphic oph- rocksiolite andcomplexes carbonate of the sediments. Caribbean, TheK-poor eastern volcanic part rocks of Cuba (and isassociated dominated intrusive by the rocks) largest ophioliteof the Paleogene complexes to Eocene of the Caribbean,island arc volcanism, K-poor volcanic as well rocksas overlaying (and associated calcareous intrusive sedi- rocks)ments of [5]. the Paleogene to Eocene island arc volcanism, as well as overlaying calcareous sedimentsThe investigated [5]. agate material derives from El Picado near the village of Los Indios, ca. 15The km investigated west of the agate city of material Moa (Figure derives 1). from The Elhost Picado rocks nearare strongly the village altered of Los Paleo- Indios, ca.cene/Eocene 15 km west tuffs of the(ca. city55 Ma) of Moalocally (Figure intercal1).ated The with host limestones. rocks are stronglyThe altered altered volcanic Pale- ocene/Eocenerocks are fractured tuffs (ca.and 55contain Ma) locallynumerous intercalated fissures and with veins limestones. mineralized The by altered quartz volcanic and rockschalcedony, are fractured indicating and a contain secondary numerous silicification fissures of the and pyroclastic veins mineralized material during by quartz tectonic and chalcedony,activities (Figure indicating 2). a secondary silicification of the pyroclastic material during tectonic activitiesXRD (Figure studies2). of