Editorial for the Special Issue of Minerals, “Advances in Low-Temperature Mineralogy and Geochemistry”

Low-temperature geochemical processes are ubiquitous in geological areas related to weathering, diagenesis, very low-grade metamorphism, and post-magmatic processes, and provide an explanation of the P–T–X history of the Earth’s crust. These processes may provide challenging topics of the broad qualitative questions related to near-surface Earth processes involving chemical transfer, mineralogical reactions, recycling of chemical elements, mineralization, etc.

In this way, light (δ⁷Li; δ¹¹B), stable (δ¹⁸O, δD), and radiogenic isotope (¹⁴⁴Nd/¹⁴³Nd, ²⁰⁷Pb/²⁰⁶Pb, ¹⁷⁶Hf/¹⁷⁷Hf, etc.) ratios are potentially powerful tracers of fluid–mineral interactions, while thermo-barometer indicators (e.g., chlorite thermometer, phengite barometer, phengite–chlorite pairs, muscovite-paragonite solvus, etc.) based on fine mineralogical compositions are appropriate probes to reconstruct pro- and retrograde mineralogical reactions. Furthermore, remarkable advances have recently been made in understanding chemical element recycling within the diagenesis and low-grade metamorphism environment, hydrothermal systems, and fluid circulation. Over the past decade, analytical techniques (e.g., X-ray diffraction, secondary-ion mass spectroscopy, laser-ablation mass spectroscopy, high-resolution transmission electron microscopy, etc.) have also advanced and offered a new vision for scientists in mineralogy and geochemistry, making data available with an unparalleled analytical and spatial resolution.

Contributions on low-temperature mineralogy and geochemistry from a broad range of fields were selected and published. The Special Issue begins with a review of chlorite geothermometry and recent related analytical advances [1]. Chlorite, a ubiquitous clay mineral in metamorphic, diagenetic, and hydrothermal systems, exhibits a chemical composition partly dependent on temperature (T) and pressure (P) conditions. Recent analytical developments that allow for circumventing or responding to certain criticisms regarding the low-temperature application of thermometers are shown. Furthermore, new perspectives in terms of analysis (e.g., Mn redox in Mn-chlorite) and geothermometers (molecular solid-solution model, oxychlorite end-member) are addressed.

The ore-forming processes and subsequent processes of alteration–mineralization explain the role of fluids during mineralogical reactions and element recycling. A very interesting mineralogical reaction from hübnerite to scheelite and Fe,Mn-chlorite was studied in the magmatic–hydrothermal system of Borralha (Portugal) [2]. Replacing wolframite with scheelite is common among tungsten deposits where these two tungstates may coexist. Additionally, a genetic linkage between W (Cu, Mo) and chlorite minerals was recognized in this ore-system deposit [3,4]. The replacement of wolframite with scheelite in CO₂-rich fluids was identified in the Borralha magmatic–hydrothermal system, where fluid pressure decreases (or simple cooling) and the ratios of total Ca²⁺ (from feldspar albitization) to total Mn²⁺ (from hübnerite dissolution) contents increase in fluids. A decrease in fluid pressure from lithostatic to hydrostatic levels at a depth of 3–8 km causes CO₂ escape and pH rise and precipitates over 70% of tungsten [5]. Scheelite was formed from a low salinity CO₂-H₂O fluid dominated by CO₂, where the homogenization temperature decreased from 380 °C to 200 °C (average of 284 °C). The δ¹⁸O_Fluid values calculated for quartz–water and wolframite–water fractionation fall within the calculated magmatic water range. The ∆_{quartz-scheelite} fractionation occurred at approximately 350–400 °C. The ∆_{chlorite-water} fractionation factor calculated was approximately +0.05‰ for 330 °C, dropping to −0.68‰ and −1.26‰ at 380 °C and 450 °C, respectively. In addition, estimated crystallizing temperatures based on semi-empirical chlorite geothermometers [6,7,8] show a narrower temperature range of 375 °C to 410 °C estimated for Fe,Mn-chlorite crystallization.

The next sequence of articles is dedicated to epithermal Au + Sb ores in Southwestern Guizhou (China) and the Baiyun gold deposit in the northeast Hubei Province (China). The Guizhou ore deposit is famous for hosting clusters of Carlin-type Au, Sb, and Hg-Tl deposits [9]. Stibnite, realgar, and orpiment coexist with calcite at the periphery of high-grade orebodies in Au deposits, while stibnite is generally intergrown with fluorite in Sb deposits. The rare earth element (REE) concentrations, Sm/Nd isotope ratios, and Sm–Nd isochron ages were analyzed on separated samples of calcite and fluorite from Au (Zimudang) and Sb (Dachang) deposits. The Sm–Nd isochron ages of 148.4 ± 4.8 and 141 ± 20 Ma obtained on fluorite and calcite are consistent with the age range constrained by the low-temperature thermochronology of zircon (132–160 Ma) and bring new evidence of low-temperature hydrothermal mineralization in Southwestern Guizhou.

In situ LA-ICP-MS was used for trace element and isotope analyses on selected sulfides to evaluate the genesis and evolution of ore-forming fluids from the Baiyun gold deposit in northeastern Hubei, China [10]. Based on in situ Fe and S isotope on pyrite and galena measurements, the authors inferred that the Baiyun gold deposit was formed via magmatic mineralization related to the subduction of the Pacific plate during the Yanshanian plate.

Emphasizing the diagenetic to low-grade metamorphism sequence, phyllosilicate chemistry was used for determining the geological evolution of low-grade metamorphic sequences in early Paleozoic metaclastic rocks in the Eastern Tauride Belt, Türkiye [11]. The authors show that the mineral chemistry of illite/mica and chlorites, together with the evaluation of textural data of low-temperature metaclastic rocks, plays an important role in determining their origin and metamorphic grade.

The kaolinite-to-chlorite conversion is one mineralogical reaction that occurs either in low-temperature diagenetic or hydrothermal systems, where its mechanism is still poorly understood. Direct transformation, conversion via berthierine as an intermediate phase, or direct formation of a berthierine/chlorite mix, either by dissolution–crystallization or by solid-state transformation (or a combination of both), are all hypotheses put forward by Bourdelle et al. [12]. The petrographical, mineralogical, and chemical data suggest that the Fe-chlorite results from the interaction between the shale, providing the Fe,Mg supply, and the Si,Al-rich veins, leading to the chloritization of the kaolinite at a small scale via at least one dissolution–recrystallization step. High-resolution observations highlight that neoformed Fe-rich chlorite contains some 7 Å isochemical layers, as relict of berthierine. The conversion mechanism, suggested by the authors, takes place either through the precipitation of berthierine, followed by a second step involving solid-state berthierine-chlorite conversion, or through the direct precipitation of a chlorite-rich/berthierine-poor mix driven by the Fe/(Fe + Mg) ratio, at low temperature and in reduced conditions. The authors put forward arguments in favor of the second hypothesis.

Active acid-sulfate environments associated with endogenous degassing (i.e., H₂O, CO₂, CH₄, H₂S, SO₂, HCl, HF, etc.) and hot water fluid circulation (hydrothermal/geothermal systems) are described around the world [13,14,15,16,17]. Sulphate efflorescent minerals precipitated on the argillic facies as the result of water–rock interaction and fumarole emission covering the surface of the Donnoe and Dachnoe fields of the Mutnovsky volcano are described by Zhitova et al. [18]. Ca, Ba, (NH₄)⁺, Na-Fe³⁺, (NH₄)⁺-Al, (NH₄)⁺-Fe³⁺, Na-Al, K-Al, and K-Fe³⁺ sulphates were reported, where ammonia species were concentrated around fumaroles.

Finally, basalt-derived carnelian was identified in the Newark basin as medium- to coarse-size pebbles in fluvial gravel and alluvium and colluvium formed from erosion of Lower Jurassic Preakness Basalt [19]. Carnelian contains 97–98 wt.% SiO₂, ~1.0 wt.% Fe₂O₃, and 1.0–1.4 wt.% LOI, and trace element abundances are low except for Y, Nb, Ta, W, Th, and U. Rare earth element (REE) patterns display heavy REE enrichment and large negative Eu anomalies. Si was likely derived from the alteration of basaltic glass. δ¹⁸O_VSMOW values measured on carnelian are high and range from +18.3 to +31.2‰, comparable to global occurrences of volcanic rock-derived chalcedony. According to authors, carnelian was precipitated from the mixing of hydrothermal fluid with meteoric water under conditions of low temperature (20–80 °C) and neutral to slightly alkaline pH.