Zeolite Minerals from Wat Ocheng, Ta Ang, Ratanakiri Province, Cambodia – Occurrence, Composition, and Paragenesis

In the remote northeastern corner of Cambodia, along the border with Laos and Vietnam, lies Ratanakiri province. Its name derives from the Khmer words “ratana” or “gem” and “kiri” or “mountain”. Today, Ratanakiri province is one of the world's foremost sources of gem zircon. Known locally as “t'bong Bokeo” (Bokeo gems) or “t'bong khieu Ratanakiri” (blue Ratanakiri gems), Cambodian zircon is renowned in the gemological world for exhibiting the darkest, most brilliant blue color when heat-treated.

The source of the gem zircon, and the location of the new zeolite deposit, are the Cenozoic basalts of the Ratanakiri Volcanic Province (RVP), part of the Southeast Asian Volcanic Province, which includes basalts in Vietnam, Thailand, and Laos (Barr & MacDonald 1978, Barr 1981). Little detailed research has been conducted on the RVP since Lacombe (1969). Our continued fieldwork in the area is focused on the mineralogy and petrogenesis of the basalts and their relationship to others in the Southeast Asian Volcanic Province, in particular the Cenozoic basalt plateaus in neighboring Vietnam.

Zeolites are a large group of hydrated aluminosilicate minerals containing alkaline and alkaline earth elements. Corner-shared [SiO₄]^4– and [AlO₄]^5– tetrahedra form an open, microporous framework with interconnected negatively charged cages and channels which contain charge-balancing alkali and alkaline earth cations (Na⁺, K⁺, Ca²⁺, NH⁴⁺) as well as water (Armbruster & Gunter 2001, Gottardi & Galli 2012). This open framework structure results in a number of unique physiochemical properties and allows zeolites to be used in many technical applications, including as catalysts, adsorbents, and ion exchangers (Sand & Mumpton 1978, Armbruster & Gunter 2001, Weitkamp & Puppe 2013, Mumpton 2018)

Zeolites are dominantly the result of alteration of primary minerals by aqueous fluids (Gottardi & Galli 2012). In volcanic rocks, zeolites form during burial metamorphism in the lava pile or by hydrothermal alteration in regions of high heat flow (Neuhoff et al. 1999, Neuhoff et al. 2006, Weisenberger & Selbekk 2009, Gottardi & Galli 2012). The mineralogy and geochemistry of secondary mineral assemblages in basaltic rocks can be used as indicators of their thermal history, fluid evolution in the basaltic pile, and the processes under which they were altered following eruption (Neuhoff et al. 1999, Schenato et al. 2003, Mattioli et al. 2016). In particular, the presence of zeolite minerals in vugs, amygdales, and veins can provide information on hydrothermal events, low-grade metamorphism during burial, and the presence or absence of residual or meteoric fluids and their interaction with primary minerals (Neuhoff et al. 1999, Neuhoff et al. 2000, Schenato et al. 2003, Weisenberger & Selbekk 2009, Weisenberger & Spürgin 2009, Neuhoff 2010, Mattioli et al. 2016). The dominance of one zeolite mineral over another is controlled by the temperature and composition of the fluids infiltrating the vesicle, the temperature and porosity of the host rock, the local mineralogy of the basalt adjacent to the amygdale, and the degree of water/rock interaction (Weisenberger & Spürgin 2009, Kónya & Szakáll 2011).

Our research in the RVP is one project which is helping to increase knowledge of a country which has largely been ignored by the geological and mineralogical community. To date, we have added to the general understanding of the geology of the Ratanakiri province, uncovered many new mineral localities, and highlighted some which have only been known to locals. One of these localities is the zeolite-bearing Cenozoic Wat Ocheng basaltic flow. This is the first such zeolite occurrence in the RVP and in Cambodia, producing cavity assemblages that are of interest to micromount mineral collectors. The current study will describe the morphology and chemical composition of zeolite species from this new occurrence, determine their sequence of formation and paragenesis, as well as elucidate the local and regional heat and fluid sources which may be responsible for zeolitization.

Cambodia, along with Laos, the southern parts of Vietnam, and the eastern part of Thailand, is part of a larger tectonic unit called the Indochina Block. This block of continental crust has remained rigid, intact, and stable since the end of the Indosinian orogeny (210 Ma) as younger fold belts formed around its margins, resulting in extensive deformation of the older marine sedimentary basins (Workman 1977, Ridd et al. 2011, Searle & Morley 2011). In Cambodia, the Indochina Block contains Precambrian and Paleozoic gneisses of the Kontum massif, as well as younger marine, lagoonal, and continental sediments associated with uplift and continental–continental collision during the final stages of the Indosinian orogeny (Searle & Morley 2011). Northeastern Cambodia has been described as a broad shallow basin, underlain by the westward extension of the Kontum massif in Vietnam (Workman 1977). Folding in the sediments is not uniform across the region, a function of both the depth of the basement and the presence or absence of local fault lines (Workman 1977).

The subsequent Cenozoic tectonic evolution of the Southeast Asian Volcanic Province is related to the collision between the Indian and Eurasian plates during the Himalayan Orogeny (55–45 Ma; Khain 2001). Uplift, extensional tectonics, and rifting throughout Southeast Asia resulted in lithospheric thinning, which triggered decompression melting and upwelling of the underlying asthenosphere along deep-seated faults to form late Cenozoic basalts (Mukasa et al. 1996, Fedorov & Koloskov 2005). These basalts occur throughout Southeast Asia and range in age from 24 Ma to as recent as a 1923 eruption off the Vietnamese coast near the island of Poulo Cécir (Patte 1925, Saurin 1967, Hoang & Flower 1998, Hoàng et al. 2013, Nguyen & Kil 2019). In the eastern region of Cambodia, Quaternary basalts cover ∼10,000 km² and lie unconformably on Quaternary alluvium and Mesozoic sediments. Small basalt flows and vents can also be found in the west near Pailin and Poipet and in the central northern province of Preah Vihear. Two distinct basalt plateaus are found in the Ratanakiri region: (1) the Ratanakiri Volcanic Province located between the Tonlé Srepok and the Tonlé San (tonlé = river), and (2) the Ban Chay plateau, northeast of Bokeo, north of the Tonlé San near the Vietnamese border (Lacombe 1969).

Two separate periods of volcanism are noted: (1) an early phase consisting of SiO₂-rich, Fe- and Ti-depleted quartz and olivine tholeiites and rare trachyandesites, and (2) a later, highly alkaline phase consisting of low SiO₂, high Fe and Ti olivine tholeiites, alkali basalts, basanites, nephelinites, and rare trachybasalts and trachytes (Hoang & Flower 1998, Fedorov & Koloskov 2005). The RVP (Fig. 1) is a 1500 km² area located between the Tonlés Sre Pok and Sesan and comprises both alkaline and transitional basalts (olivine basalts, alkali basalts, basanites, nephelinites, trachyandesites). Lacombe (1969) noted that early flows preceded the last paleomagnetic inversion (0.7 Ma) and the later, more explosive basaltic volcanism occurred during and after the geomagnetic reversal. The two phases of volcanism are separated by variably altered paleosols and alluvial sediments. Tektites from the 0.79 Ma Indoaustralian Strewn Field are found associated with the altered paleosols derived from the older, fissure-controlled basalts, but not with the younger volcanic cones or pyroclastics (Lacombe 1969). The age of the RVP falls within the range (17.6–0 Ma) established by Hoang & Flower (1998) for related basaltic volcanism in Vietnam and throughout the northwestern Pacific continental margin.

Although the focus in the RVP has been on gem minerals, in particular zircon, the lack of extensive mineral exploration and mineral collection in Cambodia means that there are ample opportunities for the discovery of new mineral localities. Our fieldwork has discovered a new source of zeolite minerals in a basalt flow on the outskirts of Ban Lung in the town of Ta Ang. This flow is part of the larger RVP, but it has generally been overlooked by local miners and exploration geologists because it does not contain gem zircon, nor is it a potential source of base or precious metals.

Located at the intersection of Highway 78 and the Lumphat road in the town of Ta Ang, in the far southwest corner of the RVP, sits Wat Ocheng, a new temple being constructed on a high point in the landscape. During fieldwork, a large basalt boulder was discovered in a field across the road from the Wat Ocheng construction site. Further examination of this boulder revealed it to contain mineralized vesicles with abundant zeolite minerals (Fig. 2). After a search through the roadside brush and jungle as well as the construction site, the source outcrop was found. The exposed part of the flow is 150 m long and 65 m wide, striking in a NE–SW direction across Highway 78 and into the Wat Ocheng property. No other exposure of the flow was found in the area. Although vesicular basalts are common in the RVP, predominantly scoria and cinders, no other zeolite-bearing basalt flows have been discovered, which suggests unique conditions of formation for the Wat Ocheng flow, or that other zeolite-bearing flows are hidden beneath the later-stage volcanics. This is the first extensive occurrence of zeolite-group minerals in the RVP basalts and in Cambodia.

Powder X-ray diffraction data were collected with a Bruker D8 Discover microdiffractometer equipped with a DECTRIS EIGER2 R 500K detector and IμS microfocus Cu X-ray source (CuKα = 1.54184 Å) running at 50 kV and 1 mA. The instrument was calibrated at a sample-to-detector distance of 175 mm for optimal resolution using a statistical calibration method (Rowe 2009). The average exposure time per analysis was of 300 s for ∼200 μm powder balls mounted on 20 μm fiber pin mounts. Samples were analyzed with continuous φ rotation and 10° rocking motion along the ψ axis of the Centric Eulerian Cradle stage. Mineral identifications were performed with Bruker's DIFFRAC.EVA software using the ICDD PDF-4+ 2020 database.

Back-scattered electron images (BSEI) were obtained using a JEOL 6610Lv scanning electron microscope (University of Ottawa) operating at 20 kV in low-vacuum mode (30 pascals) with a working distance of 20 mm. Chemical analyses of the zeolites were obtained with a JEOL Superprobe 8230 instrument operating in wavelength-dispersive mode using Probe for EMPA Extreme Edition software (https://www.probesoftware.com/) at the University of Ottawa. In order to avoid sample damage and cation migration during analysis, several mitigation strategies were used. Prior to collecting compositional data, intensity over time for various analytical conditions was plotted. These tests showed that for chabazite-phillipsite, intensities were uniform during the entire analysis time using an accelerating voltage of 15 kV, beam current of 10 nA, and a 20 μm spot size. For natrolite, the optimal conditions were 15 kV, 10 nA, and a 10 μm spot size. The intensity data were corrected for Time Dependent Intensity (TDI) loss (or gain) using a self-calibrated correction for K Kα, Fe Kα, Si Kα, Na Ka, and Sr Lα. In order to decrease the amount of time the beam remained on the sample, backgrounds were calculated using a Mean Atomic Number correction (Donovan et al. 2016). A total of 10 elements were sought and the following standards and X-ray lines were employed: sanidine (K Kα, Si Kα, Al Kα, Ba Lα), albite (Na Kα), synthetic diopside (Mg Kα, Ca Kα), tephroite (Mn Kα), hematite (Fe Kα), and celestine (Sr Lα). Count times were 20 s on peak for all elements. Raw intensities were converted to concentrations using the default φρZ corrections of the Probe for EMPA software package (Armstrong 1988). Water contents were calculated from stoichiometry and included in the matrix correction. Elemental interferences were corrected using empirical overlap corrections. The charge balance error (E%), used when the water content can only be determined by difference and as a check for possible light element volatilization, was calculated as E% = 100[(Al+Fe)–(Na+K)–2(Mg+Ca+Sr+Ba)]/[(Na+K)+2(Mg+Ca+Sr+Ba)] (Gottardi & Galli 1985, Rogers & Hudson 1992). Analyses with E% > 10% were discarded. All zeolite compositions were calculated on an anhydrous basis and can be found in Tables 1–5. Figure 3 shows the chemical composition of all of the zeolites in the R²⁺ (Ca+Sr+Ba+Mg+Mn+Fe)–Na–K ternary diagram (3A) and T_Si (Si/Si+Al) versus K+Na/K+Na+Ca (3B).

Samples of the Wat Ocheng basalt were obtained from the upper and lower parts of the flow along its exposed length. It is a fine-grained, vesicular to amygdaloidal tholeiite with a primary assemblage of 30% lath-like plagioclase (average: Ab₅₁An₄₅Or₄) and 35% interstitial augite (average: Wo₄₄En₃₅Fs₂₁), with secondary chabazite-Ca and phillipsite-Ca (25–30%) and minor skeletal ulvöspinel (Fig. 4A–B). The plagioclase and volcanic glass have been extensively altered and replaced by a mixture of chabazite and phillipsite (Fig. 4A). Figures 4C–H show X-ray maps for the major elements (K, Na, Ca, Al, Fe, and Ti) in oxide%. Zoning of K, Na, and Ca is clearly visible from the core to the rim of the amygdales. Based on major and trace-element whole-rock analyses, the basalt is classified as a subalkaline to transitional alkaline intraplate tholeiite with ocean island basalt affinities (Fig. 5).

Mineralized vesicles range in size from 2 mm to 5 cm (Fig. 2B). Nine mineral species have been identified in the Wat Ocheng flow: analcime, chabazite-Ca, gonnardite, natrolite, phillipsite-Ca, and thomsonite-Ca, along with clays, aragonite, calcite, and pyrite. The distribution of the zeolite minerals is not homogeneous throughout the basalt flow.

Analcime, Na(AlSi₂O₆)·H₂O, occurs as water-clear, colorless, subhedral to euhedral trapezohedra lining the vesicles. It occurs in a lower zone of the flow, which is generally devoid of other zeolite minerals. Amygdales containing analcime are generally smaller than those containing a variety of zeolites and appear to be less porous and interconnected. Crystals range in size from 0.2 to 1 mm, with larger euhedral crystals up to 2 mm (Fig. 6A, B). In rare cases, late-stage chabazite-Ca II crystals occur with the finer-grained analcime. Surfaces of larger analcime crystals exhibit pitting which appears to be crystallographically controlled (Fig. 6C). Compositionally, analcime from Wat Ocheng is almost pure endmember (Fig. 3) with only trace concentrations of Ca (0.01 apfu), Sr (0.12 apfu), and Ba (0.04 apfu). The T_Si ratio (Si/Si+Al) ranges from 0.67–0.68.

Chabazite-Ca, Ca₂(Al₂Si₄O₁₂)₂·12H₂O, is one of the most abundant zeolites at Wat Ocheng. It occurs in three habits: (1) a drusy amygdale lining as subhedral to euhedral, colorless, milky white to orange-beige, domed, lens-shaped or blocky crystals up to 0.75 mm, intergrown with phillipsite-Ca; (2) in spherical aggregates at the base of sprays of natrolite/thomsonite-Ca; and (3) later-stage single euhedral phenocrysts up to 1.6 cm of the variety “phacolite”, the complicated result of multiple penetration twins of convex rhombohedral crystals (Fig. 7A–D). Early stage chabazite-Ca I is coeval with phillipsite-Ca, the earliest minerals to form in the crystallization sequence (Fig. 7E–F).

Chemical analyses of chabazite-Ca indicate Ca to be the dominant extra-framework cation, with minor Na, K, Sr, and Mg (Fig. 3). The large phenocrysts (chabazite-Ca II) show minor enrichments in K, Mg, and Na relative to the early drusy vug coatings, with maximum values of 0.30, 0.12, and 0.09 apfu, respectively. No regular compositional zoning was observed. The T_Si ratio ranges from 0.65 to 0.68, comparable to hydrothermal chabazite-Ca from other alkaline basalt localities (Gottardi & Galli 1985, England 1992, Kónya & Szakáll 2011).

Gonnardite occurs as a minor phase intergrown with natrolite and thomsonite-Ca. It is not visible in hand sample or under the microscope and was only identified by pXRD. No chemical analyses were possible.

Natrolite, Na₂(Al₂Si₃O₁₀)·2H₂O, is a common zeolite at Wat Ocheng, crystallizing early in the paragenetic sequence. It occurs as water-clear, prismatic euhedral needles up to 0.3 cm long in radiating and hemispherical aggregates up to 0.8 cm in diameter (Fig. 8A). Natrolite also occurs in a unique epitaxial intergrowth with thomsonite-Ca (Fig. 8B), a relationship which has been noted from very few localities including alkaline volcanic rocks in Bourgas, southeast Bulgaria (Petrov et al. 2019) and the Deccan Traps, western India (Filippidis et al. 1996). Intergrowths of zeolites are not uncommon, as their basic chemistry and structure is similar and allows for extensive substitutions. However, in most cases, natrolite is found intergrown with mesolite or scolecite (Rychlý & Ulrych 1980, Ulrych & Rychlý 1980, 1983, Gunter et al. 1993). Natrolite has been observed growing epitaxially on thomsonite at other localities, but at Wat Ocheng, euhedral, prismatic natrolite forms the core of the epitaxy and is overgrown by oriented thin blades of thomsonite-Ca. The two minerals are oriented such that their c axes are parallel. Thomsonite-Ca epitaxially overgrows the (111) face of natrolite via its (010) or (100) face (Fig. 9), as predicted by Ross et al. (1992).

The average chemical composition of natrolite from Wat Ocheng is close to endmember, (Na_1.85Ca_0.09)_Σ1.94Al_2.00(Si_2.99O₁₀)·2H₂O (Fig. 3), and the T_Si ratio ranges from 0.59 to 0.61 (ideal T_Si = 0.60).

Phillipsite-Ca, (Ca_0.5,K,Na,Ba_0.5)_x[Al_xSi_16–xO₃₂]·12H₂O, is the earliest mineral to crystallize in the vesicles, most commonly as a thin vug lining intergrown with chabazite-Ca. It forms as subhedral to euhedral, blocky to prismatic crystals ranging in size from 25 to 100 μm (Fig. 10A–C). Later-stage phillipsite-Ca II occurs as subhedral to euhedral, colorless to milky, blocky, pseudo-orthorhombic twinned crystals, generally <0.5 mm wide. Complex penetration twins of phillipsite-Ca are common at other alkaline localities (Tschernich 1992, Kónya 2006, Kónya & Szakáll 2011), where a number of twin types have been observed. At Wat Ocheng, the dominant twin type is the Marburg twin. Monomineralic vugs (0.5 cm) containing euhedral, untwinned blue-tinged phillipsite-Ca crystals up to 1.5 mm are common (Fig. 10D).

Phillipsite-Ca shows a narrow range of compositions, regardless of the morphology, with Ca–(K+Na) and Al–Si substitutions responsible for the compositional variations (Fig. 3). The average composition is (Ca_1.86K_1.56Na_0.20Ba_0.01Sr_0.01)_Σ3.63(Al_5.51Si_10.49O₃₂)·12H₂O, with T_Si ratios ranging from 0.62 to 0.67 (average 0.66). Larger euhedral phillipsite-Ca II crystals show compositional zoning with rims enriched in K+Na (1.84 apfu) and depleted in Ca (1.82 apfu) relative to the cores (K+Na 1.50 apfu, Ca 1.85 apfu). This is consistent with chabazite-Ca where later-stage crystals also show enrichments in K+Na relative to drusy vug coatings.

Thomsonite-Ca, Ca₂Na(Al₅Si₅O₂₀)·6H₂O, is a late-stage mineral, occurring both epitaxially on natrolite and as acicular sprays or incomplete spheres associated with drusy phillipsite-Ca. In some cases, the thomsonite-Ca sprays are nucleated on a sphere of very fine-grained chabazite-Ca (Fig. 11AB), although it also nucleates directly on the phillipsite-Ca vug lining (Fig. 11C).

As with the other zeolites at Wat Ocheng, thomsonite-Ca shows limited compositional variance between samples (Fig. 3). The average composition is (Ca_6.27Sr_0.01)_Σ6.28Na_5.64(Al_17.96Si_21.96O₈₀)·24H₂O with T_Si = 0.55. The ideal composition of thomsonite-Ca is Ca₈Na₄(Al₂₀Si₂₀O₈₀)·24H₂O with a T_Si of 0.5 (Gottardi & Galli 1985). However, natural thomsonite trends from ideal compositions toward a mesolite composition (T_Si = 0.60) as a result of the presence of extra-framework cations where the Ca/Ca+Na ratio deviates from the ideal 0.67 toward 0.62 (Wise & Tschernich 1978, Weisenberger & Selbekk 2009).

Both aragonite and calcite occur in the basalt vugs. Aragonite is a minor phase in the vugs and occurs as white to colorless prismatic sprays. Calcite is the more common carbonate mineral and two generations have been noted. Calcite I occurs as euhedral, colorless to white, translucent scalenohedral (up to 1.2 cm long) and rhombohedral (0.1–0.4 cm) crystals (Fig. 12A). It forms late in the crystallization sequence after chabazite-Ca and phillipsite-Ca and is coeval with natrolite. The crystals are commonly coated with dark red-brown clays and yellowish Mn-Fe oxides, discoloring the calcite surface, and rarely with secondary calcite.

Calcite II is a late-stage phase found overgrowing chabazite-Ca, phillipsite-Ca, and thomsonite-Ca. It occurs as lustrous, colorless to white, anhedral fine-grained coatings resembling flat rosettes (Fig. 12B).

Red-brown, Fe-dominant clay is one of the first minerals to crystallize and occurs as botryoidal masses (20–50 μm wide) lining the base of the vugs (Fig. 13). The balls are comprised of very fine-grained, platy, mica-like crystals. The clay is a minor phase at Wat Ocheng; pXRD indicates the masses are amorphous but are likely a smectite-group mineral (saponite).

In addition, a blue-green clay is present underneath the water-clear analcime crystals lining the base of the amygdale. It was not possible to analyze this material, but given the coloration, it is suggested that this is celadonite.

Two filiform pyrite wires (0.35 mm long, <5 μm wide) were found growing on a calcite rhombohedron in a single vesicle. The pyrite needle is impaling a sphere of an unknown yellow mineral.

The geochemical conditions which result in the crystallization of zeolites in basalt have been discussed in numerous studies of both natural and experimental systems (Barth-Wirsching & Holler 1989, De'Gennaro et al. 1999, Chipera & Apps 2001, Utada 2001). Most zeolites crystallize in water-rich environments at low temperatures (40–250 °C), the result of either burial diagenesis or low-grade hydrothermal alteration (Kristmannsdottir & Tomasson 1978, Mattioli et al. 2016). Pressure-temperature conditions, porosity and permeability of the basalt, the pH and geochemistry of the groundwater and hydrothermal fluids, along with the composition of the primary minerals and the volcanic glass, specifically the Si/Si+Al ratio, all contribute to the reactions that take place during zeolitization (Browne 1978, Weisenberger & Selbekk 2009, Weisenberger & Spürgin 2009, Kónya & Szakáll 2011). Examination of the mineralogy and compositional variations between the zeolites in the Wat Ocheng basalt allows us to determine their conditions of formation.

The zeolite and hydrothermal mineral assemblages observed at Wat Ocheng are similar to those reported from other alkaline basalt localities including the Cenozoic Xuan Loc quarry, Đõng Nai province, and Buon Ma Thuot, Đak Lak localities (Weisenberger & Spürgin 2009, Kónya & Szakáll 2011, Triana et al. 2012, Spürgin et al. 2019). The Cenozoic basalts in Vietnam are identical in age to those in Cambodia and display the same two phases of volcanism. However, detailed studies of the zeolite minerals have not been conducted for these localities. The RVP basalts are an extension of the Pleiku plateau in Gia Liu province, Vietnam (Hoang & Flower 1998, Hoàng et al. 2013, Nguyen & Kil 2019).

Variations in the zeolite assemblages are observed between samples, suggestive of localized variations in heat flow, host rock permeability, and fluid compositions (Kristmannsdottir & Tomasson 1978, Pe-Piper 2000, Triana et al. 2012). Early formation of clay minerals and fine-grained zeolites may also reduce the porosity within the basalt, restricting the ability of fluids to circulate and resulting in the formation of restricted domains between individual vesicles (Kristmannsdottir & Tomasson 1978, Pe-Piper 2000).

The zoning and textural relationships between the secondary minerals at Wat Ocheng indicate four distinct paragenetic sequences:

1. Smectite – chabazite-Ca I + phillipsite-Ca I – natrolite – phillipsite-Ca II – chabazite-Ca II – thomsonite-Ca – calcite I – calcite II – pyrite

2. Phillipsite-Ca I + chabazite-Ca I – thomsonite-Ca – calcite – pyrite

3. Phillipsite-Ca II (monomineralic)

4. Smectite – celadonite – analcime – chabazite-Ca II

Based on these assemblages, four distinct stages of secondary mineral and zeolite formation are recognized (Fig. 14).

The earliest-formed alteration products are Fe-rich clays and celadonite, common alteration products of basalt from zeolite and sub-greenschist facies metamorphism during regional burial (Neuhoff et al. 1999, Weisenberger & Selbekk 2009). These have been shown to form at temperatures of 150–200 °C (Barth-Wirsching & Holler 1989, Larsen et al. 1991, Eshaghpour 2003). Of note is the lack of early stage SiO₂ minerals (quartz, chalcedony, opal, cristobalite or tridymite) common in other basalt amygdales (Triana et al. 2012), indicative of reduced Si activity at Wat Ocheng and reflecting the SiO₂-undersaturated nature of the basalt.

Zeolite mineralization commenced following deposition of celadonite and Fe-rich clay minerals. Stage II zeolites include fine-grained, drusy analcime and coeval chabazite-Ca + phillipsite-Ca, as well as slightly later-stage euhedral, prismatic natrolite. Amygdales containing only analcime are indicative of formation in a closed environment at temperatures of 200–250 °C (Barth-Wirsching & Holler 1989) by direct alteration of the Na-rich plagioclase (Ab₅₁An₄₅Or₄). At these temperatures, Na is the most mobile of alkali cations during the alteration process, with volcanic glass and plagioclase (in this case, Na-dominant plagioclase) being the first components of the basalt to be altered (Eggleton et al. 1987).

In lower-temperature Stage II vesicles (100 °C), porosity appears to have been greater, allowing for relatively constant cation exchange between zeolite species and the circulating fluid (Weisenberger & Selbekk 2009), resulting in homogeneous compositions of chabazite-Ca, phillipsite-Ca, and natrolite. In general, Ca-dominant zeolites such as chabazite and phillipsite form at higher temperatures than Na-dominant zeolites (Chipera & Apps 2001, Pe-Piper & Miller 2002). Stage II zeolites follow this trend, with chabazite-Ca I and phillipsite-Ca I crystallizing directly on the amygdale lining, followed by radiating sprays of natrolite as the temperature dropped, the Ca content of the fluid was depleted, and the fluids evolved toward compositions in which Na-dominant zeolites were stable. Interestingly, the trend at Wat Ocheng appears to be toward a decrease in the Si/Al content of the fluid and resultant zeolite compositions at the end of Stage II (Fig. 3B), rather than an increase, which is more the norm in other basaltic environments (Kónya & Szakáll 2011, Triana et al. 2012, Mattioli et al. 2016).

In contrast to Stage II, Stage III is characterized by the growth of coarse-grained, euhedral Ca-dominant zeolites (chabazite-Ca II, phillipsite-Ca II), the epitaxial growth of platy thomsonite-Ca on natrolite later in the stage, and by large euhedral calcite scalenohedra. Stage III represents the termination of crystallization of Na-dominant zeolites, reflecting the lack of easily altered Na-bearing minerals, resulting in late-stage fluids enriched in Ca. The presence of the coarse-grained calcite I also indicates that the fluids were enriched in CO₃^2–. Although Ca is dominant in all the zeolites in Stage III, chabazite-Ca and phillipsite-Ca are compositionally zoned with rims which are enriched in both K and Na relative to the Stage II phases. Thomsonite-Ca, with the lowest T_Si (0.55), is the last zeolite mineral to crystallize in Stage III. The trend of decreasing Si/Al within the crystallization sequence at Wat Ocheng from Stage I to Stage III may be a function of the slightly SiO₂-undersaturated nature of the host basalt and possibly of the infiltrating fluids.

The last stage of mineralization in the Wat Ocheng amygdales is characterized by the cessation of thomsonite-Ca growth, the crystallization of crusts of fine-grained calcite II on previously formed minerals, and of rare filiform pyrite. The formation of calcite II may represent remobilized Ca from earlier-formed zeolites during the final cooling stages of the system.

The Wat Ocheng flow is located on the far western margin of the RVP and represents one of the earliest Stage 2 flows. Temperature gradients and the presence of groundwater have played a significant role in the formation of zeolites at this locality. Given that it is a localized flow, and that other zeolite-bearing localities have not yet been uncovered in the RVP, a detailed discussion on the regional geothermal or geobarometric conditions requires further field work. However, given the local geology and tectonic environment at the time, it is possible to speculate on a heat source and the source of the mineralizing fluids.

Wat Ocheng is underlain by a basaltic andesite from Stage 1, described by Lacombe (1969) as fissure-controlled flows that filled the existing topography and which are underlain by alluvial clays and gravel, shallow-water sediments of the lower Jurassic “Terrain Rouge” (red sandstones and conglomerates) and the middle Jurassic “Grès supérieur” (sandstones). All of these units lie unconformably on Permo-Carboniferous and Triassic marine sediments (Workman 1977). The presence of Stage 1 columnar basalts in the southern and western parts of the RVP confirm the presence of surface waters in the region around the Wat Ocheng flow.

An erosional hiatus between Stage 1 and Stage 2 resulted in deposition of alluvial gravels and red and brown lateritic soils of variable thickness (Lacombe 1969). Stage 2 basalts were deposited on top of the water-rich alluvial layers. This stage is characterized by increasingly explosive activity concentrated around small volcanic vents/calderas resulting in predominantly welded tufts, pyroclastites, and scoria, indicating that the magmas are interacting with the water table and are of subalkaline to alkaline composition.

Lacombe (1969) indicated a large heat source northeast of the Ban Chay plateau, across the Vietnam border, but did not elaborate further. A large basalt province equivalent to the RVP exists in this region of Vietnam, including the Pleiku plateau and those further south, comprising what has been called a “diffuse igneous province”, the result of collision-induced lithospheric thinning and subsequent volcanism (Hoang & Flower 1998, Nguyen & Kil 2019). This thinned lithosphere with an upwelled, fertile mantle may have generated an extensive, regional heat source for meteoric waters in the east of Cambodia and southeast Vietnam (Hoang & Flower 1998), leading to local zeolitization in the resulting basalts. As mentioned above, there are known zeolite localities in Đõng Nai and Đak Lak provinces. Although many of the basalt plateaus in Vietnam have been drilled to basement and their geochemistry and gem minerals studied extensively, no research has been conducted on the zeolites.

Zeolite mineralization at Wat Ocheng is the result of hydrothermal alteration of the primary basalt due to post-magmatic cooling, with residual heat being supplied locally from the underlying basaltic andesite and regionally from deep-seated, extensional fractures, along with circulation of meteoric waters derived from both the surface and the water-rich alluvial sediments produced from the erosional hiatus. Heating and circulation of meteoric waters enriched in Ca, Na, and Si through the volcanic pile allowed for zeolite formation. Stable isotopic studies would be of interest to determine the source of the Ca, as it is possible it has been partially derived from the underlying sediments. Decreased porosity between the vesicles and variations in the fluid geochemistry and/or temperature are responsible for the heterogeneous mineralogy of the amygdales in parts of the flow, with zeolitization estimated to have occurred between 70 and 200 °C.

The Wat Ocheng alkali basalt in Ta Ang, Ratanakiri province, Cambodia represents the first known zeolite locality in the country. The basalt amygdales contain a total of six zeolite species (analcime, chabazite-Ca, gonnardite, natrolite, phillipsite-Ca, and thomsonite-Ca) in addition to aragonite, calcite, celadonite, pyrite, and a smectite-group mineral. The distribution of zeolites within the amygdales is not homogeneous, indicating (1) decreased porosity between connected vesicles as a result of clay and fine-grained zeolite crystallization and (2) variations in the local fluid geochemistry, temperature, and pH between the vesicles. The chemistry of the host rock minerals (Na-dominant plagioclase and volcanic glass) has had a strong control on the composition of minerals which formed in the amygdales. The unique epitaxial intergrowth of thomsonite-Ca on earlier natrolite is a direct expression of the Na-rich nature of the original basalt and the hydrothermal fluids. Four phases of mineralization are defined, starting with early-stage clay minerals, followed by two phases of zeolite crystallization, and culminating in the last stage, secondary calcite. The sequence of crystallization indicates decreasing temperature and decreasing SiO₂ activity with time, with a shift from Ca-dominated, high T_Si zeolites in Stage II, to Na-Ca-K zeolites with lower T_Si in Stage II and early Stage III, and lastly to a final assemblage of Ca-zeolites and calcite in Stage IV.

The presence of abundant zeolite minerals in the Wat Ocheng basalt is unique within the Ratanakiri Volcanic Province and in other large, Cenozoic basalt provinces in Cambodia. Meteoric waters circulating in the basalt pile, coupled with the burial and the presence of both a regional and local heat source, allowed for zeolite mineralization in the Wat Ocheng flow. Further fieldwork in the RVP, along with additional studies of zeolite-bearing basalts in Vietnam, would be required to determine if other such mineralized flows occur and to shed further light on the geothermal and geobarometric conditions at the time.

The authors thank Associate Editor Henrik Friis and two anonymous reviewers for their comments and suggestions to improve this manuscript. Funding for this project was provided to P.C. Piilonen in the form of a RAC grant from the Canadian Museum of Nature. Support in the field was provided by Angkor Resources Limited – a big thanks to Delayne and Mike Weeks, Samath Ma, Viseth Keo, and the rest of the Khmer staff for all your help. Thanks also go to Reni Barlow and Stephen Stuart for additional macro- and microphotographs of the minerals, respectively.