Basic Geological Characteristics of Volcanic Beryllium Uranium Deposits

Volcanic beryllium-uranium deposits were initially grouped with skarn, greisen, carbonate, and alkaline beryllium deposits under the category of non-pegmatite beryllium deposits to distinguish them from pegmatite beryllium deposits. At that time, pegmatite rare metal deposits were an important source of metallic beryllium. With a deeper understanding of the genesis and formation environment of volcanic beryllium-uranium deposits, Foley et al. (2017) classified deposits associated with volcanic beryllium-uranium deposits into four categories: (1) breccia pipe-type fluorite deposits; (2) shallow low-temperature beryllium deposits hosted in carbonate rocks; (3) volcanic uranium deposits; and (4) magmatic exhalative beryl deposits.

The main understanding of volcanic beryllium-uranium deposits is that metallic beryllium and uranium primarily originate from the host tuff, with subsequent epithermal formation leading to beryllium re-enrichment into industrial-grade ore. The discovery of the Spor Mountain beryllium deposit in the United States in 1959 was due to the recognition that beryllium is mainly associated with highly differentiated felsic rocks and fluorine-rich hydrothermal systems, and benefited from the ability of neutron source gamma-ray spectrometry to rapidly and semi-quantitatively analyze the beryllium content in rocks. The mineralization control conditions of volcanic beryllium-uranium deposits are complex; therefore, a comprehensive and in-depth discussion of their basic geological characteristics, such as mineralization conditions, ore-controlling factors, alteration types, and mineral assemblages, is still difficult. The following is a brief introduction to the characteristics of the host volcanic rocks, including rock type, rock geochemistry, ore-controlling structures, and alteration types.

1. Types of Volcanic Rocks

Most discovered volcanic-type beryllium-uranium deposits and mineralization points occur in Mesozoic and Cenozoic volcanic rocks, such as Spor Mountain, Honey Comb Hills, Wah Wah Mountains, Apache Warm Springs, and Sierra Blanca in the United States. Some volcanic-type beryllium deposits also occur in older volcanic rocks, such as the Brockman volcanic-type beryllium deposit in Western Australia, which occurs in Paleoproterozoic volcanic rocks. In terms of the types of rocks that bear the ore, the volcanic rocks associated with volcanic-type beryllium-uranium deposits include rhyolitic vitreous tuff, welded tuff, ash flow tuff, porphyritic lava, and volcanic breccia; subvolcanic rocks include rhyolite porphyry, felite porphyry, granite porphyry, and topaz rhyolite porphyry.

2. Geochemical Characteristics of Volcanic Rocks

The volcanic rocks associated with volcanic-type beryllium-uranium deposits are products of the late stages of magmatic differentiation and evolution. Therefore, chemically, they are often characterized by high silica, high alkali (Na₂O+K₂O), high Mn, and high F, while being low in TiO₂, CaO, MgO, and P₂O₅. They are enriched in elements such as Li, Be, Rb, Cs, Mo, Nb, Zn, Pb, Ga, and Sn, with relatively low Zr/Hf and Nb/Ta ratios, and relatively high Rb/Sr ratios. However, the enrichment capacity of these volcanic-type beryllium-uranium deposits for rare earth elements (REEs) varies significantly; some volcanic rocks are enriched in REEs, while others are depleted. However, from the perspective of REE distribution patterns, regardless of whether they are enriched or depleted, they all exhibit a gull-shaped distribution or a relatively flat right-dipping curve, with a significant negative Eu anomaly.

The granites associated with volcanic beryllium-uranium deposits are all alkaline series highly differentiated granites. For example, the topaz rhyolite and granite porphyry of the Spor Mountain deposit in the United States have undergone varying degrees of partial melting and extreme fractional crystallization. These granites exhibit significant radioactivity and are rich in elements such as F, Li, Cs, Nb, Mn, Ga, Sn, and Zn. Diagenetic and metallogenic geochronological studies indicate that the formation of these deposits presupposed a long history of magma fractional crystallization. Although these volcanic or subvolcanic rocks exhibit high differentiation and evolution, with relatively low Nb/Ta and Zr/Hf ratios, beryllium enrichment does not increase with increasing differentiation. Compared to rare metal granites, the enrichment of beryllium and uranium in volcanic rocks does not require the same degree of differentiation and evolution. Based on the metallogenic geological characteristics of the Spor Mountain deposit in the United States, the Brockman deposit in Western Australia, the superposition of late-stage hydrothermal fluids from deep magma chambers plays a crucial role in the reactivation and re-enrichment of uranium and beryllium in volcanic rocks. The presence of hydroxysiliceous beryllium in the Spor Mountain tuff, occurring as submicroscopic grains and associated with fine-grained fluorite, opal, and calcite to form layered nodules, primarily due to the replacement of carbonate fragments in the tuff, provides strong evidence for this. The beryllium content in the unaltered volcanic glass melt of the Spor Mountain tuff (59 x 10⁻⁶) is eight times that of strongly altered glass (7 x 10⁻⁶), indicating that almost 90% of the original beryllium in the tuff matrix volcanic glass was activated and migrated, precipitating and enriching in carbonate fragments.

3. Hydrothermal Alteration Types

There is still no unified metallogenic model for volcanic-type beryllium-uranium deposits. The hydrothermal alteration types vary among different volcanic beryllium-uranium deposits. In the Brockman deposit in Western Australia, the main hydrothermal alterations include fluoritization, ferrodotization, silicification, carbonatization, and albite alteration. Metallic sulfides such as sphalerite, pyrite, chalcopyrite, galena, and nickel arsenopyrite are developed in the hanging wall and footwall of the vein. In the Spor Mountain volcanic beryllium-uranium deposit in the United States, dolomite fragments in the tuff undergo hydrothermal alteration to form layered nodules of calcite, opal, fluorite, and hydroxysilicified beryllium, with hydroxysilicified beryllium closely associated with fluorite. Alterations in the tuff and surrounding volcanic rocks mainly include clay alteration, zeolite alteration, sericitization, lithium-montmorillonite alteration, and potassium feldspar alteration, as well as unique hydrothermal halos of fluorine, lithium, molybdenum, niobium, tin, and tantalum that are abnormally developed around the beryllium mineralization. A key reason for the precipitation of hydroxysiliceous beryllium in volcanic beryllium deposits is the abundance of carbonate clasts in the host tuff. When fluorine-rich beryllium-bearing fluids encounter these clasts, the fluorine reacts with calcium to form fluorite, leading to the instability of the beryllium-bearing fluorine complexes and the precipitation of beryllium as hydroxysiliceous beryllium. Therefore, the properties of the host rock are also a crucial factor controlling hydrothermal alteration, mineral precipitation, and mineralization type in volcanic beryllium deposits.

4. Petrogenes and Mineralization Background

The genesis of the ore-bearing volcanic rocks in volcanic-type beryllium-uranium deposits is a hot topic of interest. Burt et al. (1982) argued that these ore-bearing volcanic rocks were the partial melting of relatively ancient continental crustal material under high heat flow conditions, resulting in a significantly higher fluorine content in the minerals compared to H2O. The mafic magma associated with mineralization provided heat for the melting of crustal material and further magmatic differentiation, including: (1) zonal differentiation during magma ascent and emplacement; (2) extreme fractional crystallization; (3) dehydration of the magma chamber caused by early volcanic activity; and (4) fluid exsolution of the melt, leading to its enrichment at the top of the near-surface magma chamber. Christiansen et al. (2007) proposed that topaz rhyolite is not a melting product of mid-crust granodiorite, nor is it solely derived from previously dehydrated or melted felsic crust, but rather formed by the fractional crystallization of siliceous magma. Siliceous magmas primarily originate from low-level melting of complex continental crust, which may contain abundant newly formed mantle components originating from intraplate mafic intrusions. Christiansen et al. (2007) further suggest that the formation of mafic mantle-derived magmas may be related to decompression caused by lithospheric extension, or convection caused by the subsidence of subducting lithospheric plates.

The volcanic rock belt of the Middle Gobi Province in Mongolia contains volcanic rocks such as ongolite (rhyolite containing topaz), beryllium-rich tuff, and rare earth element-rich alkaline volcanic-intrusive complexes. These rocks are important ore-forming volcanic rocks in the Late Mesozoic rare metal metallogenic belt of the Transbaikal-Mongolian Rare Metals Province. In Siberia, most mineralized deposits consist of Mesozoic granitic rocks and subvolcanic rocks, with skarn or greisen deposits mainly formed at depths due to carbonate rock alteration. For example, the Yermakovskoye fluorite-beryllium deposit in Russia occurs within a large carbonate block within a terrigenous sequence of carbonate rocks. This region is intruded by relatively young Mesozoic alkaline granites and leucogranites. The beryllite-microcline-fluorite ore, primarily formed during the Triassic period by the replacement of fractured limestone, has an average grade of 1.5% BeO. The Orot bertrandite deposit is a large, low-grade deposit associated with alkaline granites and Orot paleovolcanic rocks within the Malokunaley Complex. The Orot volcanic structure is 236.4 Ma old, while the granites (granites or shallow rhyolite) within the Malokunaley Complex are 224.8 Ma old. The ore is mainly veinlet-like, formed by the replacement of dickite and hydroxyl beryllite, with a grade of 3500 x 10⁻⁶ BeO. Studies have shown that the West Transbaikal beryllium metallogenic province and its known beryllium deposits and occurrences may be related to intracontinental rifting; the formation of the Brockman deposit in Australia is related to rift extension. Therefore, an extensional setting is a favorable metallogenic setting for volcanic beryllium-uranium deposits.