Features of the Composition, Release, Localization, and Environmental Effects of Free Gases in the Khibiny Massif (Kola Peninsula, Northwest Russia): A Review

Abstract

The article presents a comprehensive analysis of long-term studies on hydrogen-hydrocarbon free gases (FGs) in the rocks of the Khibiny massif, systematically organized and generalized for the first time. Gasometric observations were predominantly conducted within underground mine workings, with occasional measurements taken during the drilling of exploration boreholes at the surface or in subsurface air within loose sediments. Methane is the primary component of these gases, followed in descending order by hydrogen, ethane, helium, other methane homologs, and alkenes. Nitrogen is also presumed to be present, although its proportions remain undefined. The carbon and hydrogen in FGs exhibit relatively heavy isotopic compositions, which progressively lighten from methane to ethane. The intensity of gas emissions is characterized by a gas flow rate from shot holes and boreholes, reaching up to 0.5 L/min but generally decreasing significantly within an hour of reservoir exposure. Gas-bearing areas, ranging in size from a few meters to tens of meters, are distributed irregularly and without discernible patterns. The FG content in rocks and ores varies from trace amounts to approximately 1 m³ of gas per cubic meter of undisturbed rock. These gases are primarily residual, preserved within microfractures and cavities following the isolation of fluid inclusions. Their distribution and composition may fluctuate due to the dynamic geomechanical conditions of the rock mass. The release of flammable and explosive FGs presents a significant hazard during ore deposit exploration and development, necessitating the implementation of rigorous safety measures for mining and drilling operations. Additionally, the environmental implications and potential applications of gas emissions warrant attention. Future comprehensive studies of the Khibiny gases using advanced methodologies and equipment are expected to address various scientific and practical challenges.

1. Introduction

The widely known Khibiny massif of nepheline syenites and foidolites is located in the west of the Kola Peninsula, the Russian Arctic zone. Being unique in terms of size, mineral diversity, and the presence of large and super-large apatite deposits, the massif, like the adjacent Lovozero one, features an unusually high (for magmatic complexes) saturation with reduced hydrogen-hydrocarbon gases (HHCGs) of different morphological types (e.g., [1,2,3]). Similar gases, predominantly methane, occluded within vacuoles of fluid inclusions in minerals (OGs), have been identified in several other alkaline massifs, primarily those of the agpaitic type. These include the Ilímaussaq massif in Greenland ([4,5]), Strange Lake in Canada ([6,7]), and the Kiya-Shaltyrsky, Korgeredabinsky, and Zaangarsky massifs in Russia ([2]). However, the Khibiny and Lovozero massifs are unique in also containing free gases (FGs), a distinct morphological type. These gases occupy interconnected fracture systems, particularly microfractures, as well as other cavities in the rocks ([2,3,8]). When FG accumulations or migration paths are exposed through surface openings, such as shot holes, boreholes, or mining operations, the gases are released spontaneously from the rock mass. An intermediate category, diffusely dispersed gases, exists between occluded and free gases ([3]). Research into occluded HHCGs in alkaline magmatic complexes is primarily geochemical, focusing on the mechanisms, influencing factors, and potential scale of hydrocarbon generation in such systems. Another important aspect is their role as agents and indicators of rock and ore formation environments. The study of natural free HHCGs also holds considerable practical significance. Their spontaneous emission into mine atmospheres poses risks due to their flammable and explosive nature, complicating underground mining safety ([2,3,8] and references therein). Gas safety protocols are similarly crucial during the drilling of exploration boreholes. Moreover, potential links between Khibiny and Lovozero FGs and other environmental hazards or adverse phenomena have been hypothesized with varying levels of scientific support ([8,9,10,11,12,13]).

Compared to the extensively studied occluded gases (OGs) in the Khibiny massif—whose composition, content, and distribution have been documented in numerous scientific publications—free gases (FGs) have received significantly less attention in the literature. This disparity arises from several factors: the lack of efficient research techniques, the inherent challenges of studying this morphological type of gas within magmatic complexes, and the requirement for resource-intensive field observations. Such studies were typically conducted at the behest of mining and exploration companies to address specific practical issues, with findings often documented in numerous unpublished scientific and production reports, many of which remain inaccessible. Only a limited amount of generalized data on FGs in rocks associated with certain apatite deposits has previously been presented ([2,3]).

This paper compiles, systematizes, revises, and statistically processes all available materials and research findings related to Khibiny free HHCGs since their discovery in 1951, with most data originating from the author’s work. However, it is worth noting that the intensity of such studies has significantly declined in the 21st century. One of the primary objectives of this article is to reignite the interest of researchers across various disciplines, encouraging a collaborative, comprehensive investigation of HHCGs in the Khibiny and Lovozero massifs using modern methodologies and advanced equipment.

2. Brief Geological Background

Many publications, including numerous monographs, focus on the geological structure, mineralogy, and geochemistry of the Khibiny massif and its associated mineral deposits. This article provides only an overview of the massif’s geology, offering sufficient context to support the subject matter discussed.

Based on available data, the Khibiny massif formed at the contact between Archean gneisses and a Proterozoic volcanogenic-sedimentary sequence. It features an asymmetric zonal-concentric structure, primarily composed of nepheline syenites (Figure 1). These nepheline syenites are divided by the rocks of the Main (Central) Ring Structure (CRS) into two nearly equal parts: the external zone (khibinites) and the internal zone (lyavochorrites and foyaites). Foyaites, like khibinites, are sometimes further subdivided—though conventionally—into massive and trachytoid types. Recently, all these nepheline syenite varieties have been reclassified as a single foyaite intrusion ([14,15]). The CRS rocks include foidolites (mainly ijolites and urtites), high-potassium poikilitic (kalsilite)-nepheline syenites (rischorrites), malignites, and ores such as apatite-nepheline and titanite-apatite-nepheline varieties. Vari-grained nepheline syenites (lyavochorrites) are also frequently incorporated into this ore-bearing complex. The CRS exhibits the highest differentiation in the chemical composition of both the rocks and their constituent minerals. Most of the late-stage pegmatite and hydrothermal veins, as well as diatremes and dikes of alkaline and alkaline-ultrabasic rocks, are concentrated in the CRS. A carbonatite complex located near the eastern boundary of the massif is similarly associated with the CRS.

Apatite deposits and ore occurrences in the Khibiny massif are primarily located in the apical parts of the CRS foidolite unit. These deposits occur as compact or stockwork ore bodies, varying in thickness from several tens of centimeters to 200 m and in length from a few meters to 15 km. They are grouped into three main ore fields (sectors): the southeastern, southwestern, and northern sectors. The largest deposits, including Kukisvumchorr, Yuksporr, Apatite Circus, and Rasvumchorr Plateau, are concentrated in the southwestern sector. These represent different parts of what is essentially a single ore body (Figure 1). In the southeastern ore field, the deposits (e.g., Koashva, Nyorkpakhk, and Oleniy Ruchei) are characterized by relatively compact stockworks of lens-veined ore bodies. Meanwhile, the deposits in the northern sector (e.g., Kuelporr and Partomchorr) are composed of individual thin, sheet-like ore bodies.

3. Methods and Materials

Methods for studying the gas-bearing capacity of crystalline rocks and associated ore deposits are underdeveloped compared to techniques used for gas-bearing coal and salt mines. In crystalline rocks, free gases (FGs) form only localized accumulations and are primarily present in a dispersed state. Due to the extremely uneven distribution and high mobility of FGs, determining the gas saturation level of igneous rocks through direct methods remains unreliable. For instance, collecting samples of pure geogases is nearly impossible, as they are inevitably diluted to some degree with atmospheric air during sampling. To address these challenges, various indirect field observation techniques, including modified sampling methods originally designed for other ore-bearing deposits, have been adapted to study the specific characteristics of FGs in the Khibiny and Lovozero massifs ([2,3,8]).

The study of FGs in the Khibiny massif was primarily conducted in shot holes, various boreholes, and the atmosphere of underground mine workings. Less frequently, these studies extended to exploration boreholes drilled from the surface and the subsoil air. Among the methods used, the shot hole technique in underground mines proved to be the most informative. In the early stages of research, gas–air mixture samples were routinely collected for laboratory analysis from unsealed shot holes, typically technological ones, after drilling. However, the time interval between drilling and sampling was often unrecorded, leading to inconsistent results. Over time, shot hole gasometric observations became more systematic. Special shot holes were drilled specifically for such studies, typically 40 mm in diameter and about 2 m deep, in the walls of mine workings that exposed various rock and ore types. Immediately after drilling, the shot holes were sealed at a depth of 20–40 cm from the mouth. Sealing was achieved using a device or a cementing plug, through which one or two thin pipes were inserted. This setup allowed for: (a) periodic measurement of gas component concentrations in the sealed shot hole using portable gas detectors, (b) collection of gas–air mixture samples for laboratory analysis, and (c) measurement of gas flow rates and excess pressure in the gas phase. For low gas flow rates, the flow was estimated by observing the increment in gas component concentrations, usually during the first 10 min after sealing. When the FG flow rate exceeded 10 mL/min, it was measured with a rotameter-type flow meter. Excess gas pressures above 10 kPa were recorded using arrow pressure gauges. Similar gasometric studies were also performed in various boreholes. To evaluate the stability of gas emissions, repeated measurements were conducted in many shot holes and boreholes, albeit irregularly and at varying intervals, to track changes over time.

Mine air samples were collected using a 0.25 L bottle initially filled with a saturated sodium chloride solution, which was emptied during sampling to capture the gas–air mixture. Serial (periodic) sampling of outgoing ventilation jets was performed using the same method, typically over the course of a work shift or day. In these cases, air flow rate measurements were conducted simultaneously with the sampling. By combining the measured air flow rates with the results of laboratory analyses of the gas–air samples, researchers were able to determine the intensity of natural gas emissions in specific areas, such as the face of actively mined workings or sections of the underground mine. Mine rescuers were involved in collecting atmospheric samples immediately following explosions at the faces of mine workings or after mass explosions.

To study the composition of subsoil air in loose sediments, subvertical boreholes approximately 1 m deep were created using a sledgehammer and a pointed metal pipe. Immediately after drilling, a gas sampler was placed into the borehole channel. This sampler consisted of a perforated tube fitted with a sealing rubber cone-shaped plug and a short rubber hose for connection to measuring instruments or a pump. Ten minutes after sealing the borehole, concentrations of methane (CH₄), carbon dioxide (CO₂), and, less frequently, hydrogen (H₂) were measured using a portable gas analyzer. If the concentrations of these components exceeded the detection threshold of the analyzer, additional subsoil air samples were collected for laboratory analysis. These samples were drawn using a hand pump and stored in 0.25 L glass bottles filled with a locking solution to preserve their composition for further study.

Laboratory analyses of all gas samples were performed using chromatographs of various brands, predominantly Russian ([2,8,13]). Helium, argon, and hydrogen served as carrier gases. The number of gas components identified in a single sample ranged from 4 to 17, depending on the type of chromatograph, the columns and sorbents used, the choice of carrier gas, and the overall gas saturation of the rock. Commonly analyzed components included methane (CH₄), hydrogen (H₂), nitrogen (N₂), and oxygen (O₂). Less frequently, homologs of methane (up to pentanes), carbon monoxide (CO), carbon dioxide (CO₂), alkenes, and helium were also measured. If a specific gas component was analyzed using two instruments, the average of the resulting values was taken to ensure accuracy. Calibration of the chromatographs was carried out using sets of standard gas mixtures, encompassing the full range of expected concentrations for the measured gases.

The minimum detectable concentrations of individual gases varied slightly depending on the type of chromatograph and sorbent used, averaging (in percent by volume) 0.0005 and 0.00005 for methane and ethane, respectively, 0.00032 and 0.00045 for hydrogen and helium, and 0.013 and 0.042 for carbon monoxide and carbon dioxide. The mean square error of the analysis of individual components was 0.4–0.8, and the coefficient of variation ranged from 2.7 to 4.6%.

After revision, a collection of unique analyses of gas–air samples, primarily sourced from unpublished scientific reports and working journals, was organized into a specialized data bank comprising 1355 entries. Of these, 684 samples were contributed by the author. The majority of the samples, totaling 977, were collected from shot holes and boreholes, while the remaining samples were obtained from the mine atmosphere.

When samples were repeatedly collected from the same shot hole or at the same depth of a borehole, their average (median) chemical composition or the most methodologically appropriate and informative values were utilized for statistical analysis. Concentration values below the detection threshold were excluded to ensure data reliability. This refinement resulted in a selection of 610 samples for analysis. The empirical density distribution of gas components in samples from shot holes and boreholes suggests that, in most cases, the distribution aligns closely with the lognormal distribution family.

4. Results and Discussion

4.1. Composition of Free Gases

When studying free gases (FGs) in the Khibiny massif, the challenge of their mixing with atmospheric air is significant. The proportions of components in the geogas–air mixture are influenced not only by the characteristics of gas localization and emission from the rock mass but also by various sampling factors. These include the sampling pattern, frequency, the tightness of the shot hole or other reservoir being sampled, and the duration of gas accumulation before sampling.

Table 1. Examples of the chromatographic analysis of compositions (vol.%) of natural FGs minimally contaminated by the atmospheric air in shot holes and boreholes.

CH4	C2H6	C3–C5 1	H2	He	N2	O2
75.0	8.1	0.860	17.6	0.314	1.1	0.1
73.0	6.2	0.629	20.0	0.371	3.8	0.1
73.1	7.5	0.741	18.7	0.359	3.2	0.2
73.4	6.7	0.657	19.6	0.364	4.2	0.2
72.0	3.8	0.340	20.2	2 n.a.	2.6	0.3
72.8	8.9	0.868	19.0	0.307	2.5	0.6
62.3	2.6	0.080	26.5	0.260	7.5	0.8
82.2	5.8	0.826	4.9	0.710	4.7	0.9
65.2	6.4	0.570	19.0	0.332	12.1	1.0
84.1	7.7	0.590	3.1	n.a.2	2.9	1.6
79.7	5.2	0.690	2.9	0.216	9.1	2.1
43.7	1.6	0.105	28.6	1.22	21.6	3.5
43.7	1.7	0.098	31.5	1.08	17.6	3.6
66.2	4.4	0.640	2.8	0.210	20.8	4.0

¹ C₃H₈ + C₃H₆ + nC₄H₁₀ + iC₄H₁₀ + αC₄H₁₀ + βC₄H₁₀ + nC₅H₁₂ + iC₅H₁₂. ² n.a.—not analyzed.

presents examples of analyses of real gas mixtures that were relatively weakly contaminated with atmospheric air.

To compare differently sampled FGs from various manifestations, analytical data are typically recalculated to represent an airless mixture of the studied components. In this context, it is reasonable to exclude not only nitrogen and oxygen but also carbon monoxide and carbon dioxide. The latter are formed here mainly during blasting operations due to the decomposition of explosives, less during the operation of mining machinery and equipment, i.e., they are mainly of anthropogenic origin. This is evidenced by the results of special experiments, data of field observations and the nature of correlation with other components of free gases. Since insignificant amounts of carbon oxides are sometimes found in the composition of occluded gases, a small fraction of endogenous CO and CO₂ may be present in FGs as well. Also, a small fraction of these components in mine air is obviously of atmospheric origin.

A previous analysis demonstrated that the variability in the ratios of constant FG components increases as methane concentrations decrease in gas–air samples, corresponding to higher levels of air dilution [3].

Table 2. Compositions of the Khibiny free gases (vol.%).

	Gas–Air Mixtures				Without Air and Man-Made Components
	n.o.p. 1	Min	Max	Median	n.o.p. 1	Min	Max	Median
H2	390	0.0003	31.5	0.0027	328	0.028	97.7	7.0
CH4	465	0.00009	84.1	0.036	328	1.52	99.9	85.5
C2H6	182	0.00005	8.9	0.0051	108	0.058	30.8	5.09
C2H4	34	0.000007	3.6	0.0003	23	0.0034	4.19	0.104
C3H8	133	0.00002	2.0	0.002	91	0.006	19.1	0.58
C3H6	17	0.000008	0.006	0.0006	9	0.00006	0.779	0.0035
nC4H10	53	0.00002	0.20	0.011	46	0.0146	2.76	0.116
iC4H10	42	0.00007	0.05	0.006	38	0.0005	0.77	0.031
αC4H8	7	0.00001	0.002	0.00004	4	0.0022	0.0062	0.0024
βC4H8	3	0.00010	0.0017	0.0009	3	0.0022	0.0096	0.0031
nC5H12	20	0.00001	0.017	0.004	17	0.0016	0.031	0.009
iC5H12	24	0.00001	0.032	0.004	21	0.0042	0.062	0.016
He	383	0.0007	1.22	0.0011	181	0.047	9.79	1.38
CO2	64	0.011	2.6	0.23
CO	29	0.003	1.09	0.043

¹ The number of observation points (shot holes and boreholes); the results of the analysis were preliminary averaged at 37 observation points with multiple sampling (from 2 to 114 samples at each point).

presents the ranges and average (median) concentrations of gas components in gas–air mixtures, as well as in recalculated compositions that exclude air and man-made components. Airless mixtures provide a more accurate representation of the natural gas composition. However, inconsistencies may still arise due to variations in the completeness and sensitivity of chromatographic analyses conducted at different times. To assess the extent of these discrepancies, 59 analyses were selected where significant concentrations (above the detection threshold) of constant FG components (excluding alkenes) were present. Their variations and medians were evaluated (

Table 3. Composition of FGs (vol.%) according to the results of the most complete analyses (without air and man-made components).

	H2	CH4	C2H6	C3H8	nC4H10	iC4H10	nC5H12	iC5H12	He
Min	7.58	72.5	3.28	0.221	0.0417	0.0097	0.0018	0.0045	0.133
Max	20.8	83.8	8.21	0.411	0.6830	0.1134	0.0146	0.0308	0.830
Median	10.5	82.2	6.36	0.439	0.1347	0.0183	0.0066	0.0094	0.621

). A comparison of Table 2 and Table 3 reveals no substantial differences between the two approaches, suggesting that both methods yield comparable results for estimating the composition of natural gases.

As in other forms of occurrence, the primary component of FGs is methane (CH₄), followed in descending order by hydrogen (H₂), ethane (C₂H₆), helium (He), methane homologs, and alkenes. Compared to OGs, FGs contain slightly lower concentrations of methane, significantly higher concentrations of hydrogen (four to six times greater) and methane homologs (on average), and an order of magnitude higher concentration of helium. Positive correlations are observed among all geocomponents (hydrocarbons, H₂, and He), whereas their correlations with atmospheric components (N₂ and O₂) are negative (

Table 4. Correlation matrix (Spearman’s pair correlation coefficients), based on symmetric balances, for concentrations of the FG components ¹ (vol.%).

	H2	CH4	C2H6	C3H8	C4H10	C5H12	He	N2
H2		0.69	0.67	0.53			0.71	−0.70
		0.84	0.71	0.32	0.35	0.64	0.73	−0.67
CH4	0.69		0.93	0.79			0.57	−0.87
	0.84		0.83	0.57	0.58	0.59	0.79	−0.79
C2H6	0.67	0.93		0.86			0.54	−0.89
	0.71	0.83		0.82	0.76	0.84	0.53	−0.96
C3H8	0.53	0.79	0.86				0.38	−0.73
	0.32	0.57	0.82		0.89	0.69	0.30	−0.70
C4H10
	0.35	0.58	0.76	0.89		0.64	0.38	−0.69
C5H12
	0.64	0.59	0.84	0.69	0.64		0.26	−0.68
He	0.71	0.57	0.54	0.38				−0.33
	0.73	0.79	0.53	0.30	0.38	0.26		−0.39
N2	−0.70	−0.87	−0.89	−0.73			−0.33
	−0.67	−0.79	−0.96	−0.70	−0.69	−0.68	−0.39
O2	−0.70	−0.88	−0.9	−0.70			−0.31	1.00
	−0.71	−0.79	−0.94	−0.58	−0.63	−0.78	−0.38	0.97

¹ Statistical samples for the correlation matrix are significantly limited in volume due to the need to form them without missing numerical values, which is dictated by the specific transformations of the one-to-one log ratio family. The upper value of the correlation coefficient (sample size n = 66, critical value of r_S = 0.242 at two-tailed α = 0.05) corresponds to the composition of seven components, and its lower value (n = 20, critical value of r_S = 0.447 at two-tailed α = 0.05) corresponds to the composition of nine components.

). Among hydrocarbons, the strength of correlation increases as their molecular masses become closer. FGs show stronger correlations between H₂ and He and between these components and hydrocarbons compared to OGs. For H₂ and He paired with hydrocarbons, the correlation strength tends to weaken as the molecular weight of the hydrocarbons increases. Notably, hydrocarbons correlate more closely with H₂ than with He. Correlations of CO₂ with other components are as follows: −0.29 for H₂, −0.47 for CH₄, −0.68 for C₂H₆, −0.64 for C₃H₈, and 0.61 for N₂, 0.59 for O₂ (these correlations were derived from a separate statistical sample. The full correlation matrix is not presented here due to the method’s high data requirements).

In the Khibiny occluded gases (OGs), the ratios of individual components vary depending on the rock type and the total specific gas content within the rock ([2]). When recalculating the ranges and median contents of OG components in the rocks of the Khibiny deposits, as reported in [3], the resulting gas composition of this morphotype (in vol.%) is as follows: methane (CH₄)—89.2, hydrogen (H₂)—5.4, ethane (C₂H₆)—4.7, heavier hydrocarbons (C₃–C₅)—0.5, and helium (He)—0.1. Insignificant amounts of carbon dioxide (CO₂) and carbon monoxide (CO) were sometimes detected, as well as nitrogen (N₂) and oxygen (O₂), though their origin remains unclear due to methodological peculiarities in OG analysis. A similar (except for helium) recalculation of median specific OG contents in different rock types [17] revealed the following variations in concentrations of CH₄, H₂, C₂H₆, and C₃–C₅ (vol.%), respectively: 81.5–93.3, 3.7–8.9, 1.8–5.4, and 0.2–0.7. A comparison of these data with those in Table 1, Table 2 and Table 3 shows that, in general, the compositions of FGs and OGs are identical. FGs differ only by higher average hydrogen concentrations.

The isotopic composition of the primary gas-forming elements, carbon and hydrogen, in the Khibiny free gases (FGs) was studied only to a limited extent and some time ago (

Table 5. Isotope compositions of carbon and hydrogen in the Khibiny free gases.

Component	Sampling Location	Isotope Ratios 1	Reference
δ 13CCH4	Deposits of southwestern group	−19.3 ÷ −11.8 (5)	[18]
δ 13CCH4	Shot-hole No. 7, Apatite Circus deposit	−10.6 ÷ −6.5 (2)	[19]
δ 13CC2H6	The same	−11.7	The same
δ 13CC2–C5	― «―	−23.9	― «―
δ DCH4	― «―	−82 ÷ −56 (2)	― «―
δ DC2H6	― «―	−173 ÷ −144 (2)	― «―
δ 13CCH4	Borehole No.360, Apatite Circus deposit	−16.5 ÷ −7.7 (26)	[10]
δ 13CC2H6	The same	−24.0 ÷ −12.1 (3)	The same

¹ ‰, relative to the PDB (δ ¹³C) and SMOW (δ D) standards (number of samples in parentheses).

).

In general, the concentrations of δ ¹³C and δ D in the Khibiny FGs are comparable to those observed in the better-studied OGs (see the overview [3]) and differ only in slightly narrower variation ranges. In contrast to gases from sedimentary rocks and oil and gas fields, both morphological types of gases are characterized by an inverse trend of the carbon isotope distribution in individual hydrocarbons, as well as by isotopically lighter hydrogen in ethane compared to methane.

4.2. Pattern, Amount, and Dynamics of Gas Emission

Gas emission from the rocks may be characterized by the gas outflow type, intensity, and duration. These characteristics are determined by the gas pressure in a reservoir, its volume and morphology, fluctuations in atmospheric pressure and temperature, and some other factors, including man-made ones.

Due to the predominance of microfracture-type reservoirs, a combination of quiet filtration outflow (jet-type) and diffusion outflow of gases from the rock mass is common for the Khibiny FGs. Gas bubbles passing through water at the bottoms of mine workings or in the water-filled channels of vertical boreholes are rarely observed. On exceptional occasions, high-yield emissions of gas and flushing fluid have been reported, including unverified accounts of drill tube ejections from boreholes. In mines, free gases (FGs) are primarily released in areas of active mining operations during all stages of the technological cycle. To a lesser extent, gases may also emit from the walls of previously mined workings and exposed rock surfaces. The intensity of gas emissions depends on the gas saturation of the rock and the magnitude of excess pressure. Most often, gas release diminishes sharply within the first tens of minutes after a reservoir is exposed by a shot hole, borehole, or mine working (Figure 2). After this initial decrease, gas emissions typically taper off more slowly and unevenly, often fluctuating over time. In most cases, this gradual attenuation lasts from several hours to several days. However, in some shot holes and boreholes, pulsating emissions with variable intensity have been observed to persist for months or even years. The longest recorded duration of such emissions exceeded 15 years ([9,19]). The rarity of documenting such prolonged emissions may partly be due to their relatively low intensity. Degassing of blasted rock occurs quickly during excavation or after large-scale ore destruction caused by “massive explosions”, often lasting only tens of minutes to a few hours. Estimating the proportion of gases released from the destroyed rock mass compared to those emitted from the mine walls is practically impossible. Natural gases released during these processes are carried away by ventilation systems along with the gaseous byproducts of the explosions (Figure 3).

Quantitative estimation of natural gas emissions in mines can only be approximated. This can be achieved by analyzing the increase in concentrations of primary gas components in the mine atmosphere over time, determining specific gas emissions (specific gas flow density) from exposed mine surfaces, or measuring the flow rates from shot holes and boreholes. More reliable and representative data on the intensity of FG emissions are obtained through daily monitoring of outgoing ventilation streams in the mine. This monitoring involves periodic sampling of air emitted after forced ventilation of a mine face (Figure 2) or specific sections of the underground mine (Figure 3). The sampled air is then analyzed in a laboratory to determine its composition. By combining air composition analysis with air flow rate measurements, it becomes possible to calculate the volume of free gases released per unit of time.

For newly exposed surfaces (typically mine walls) of the Khibiny rocks, the average specific release of methane and hydrogen combined is 47 cm³ per minute per 1 m², reaching 120 cm³min⁻¹m⁻² for CH₄ and 1700 cm³min⁻¹m⁻² for H₂ ([10] and the author’s unpublished data). During the first 10 min after the drilling, the flow rate of gasometric shot holes (1.8–2 m deep and 40 mm in diameter) usually varies from 0.01 to 10 cm³/min, occasionally peaking at 0.5 L/min. For boreholes, the maximum recorded flow rate does not exceed 6 L/min. In one instance, when a vertical ore pass was driven through a relatively gas-saturated zone with a vertical extent of about 30 m, the average rate of flammable gas entry into the face area of the mine working (V = 40 m³) was 4.3 L/min.

Less than a half of the sealed shot holes drilled in the walls of the underground mines indicated the presence of a residual excess gas pressure varying from 0.1 to 8.5 kPa. In contrast, a pressure rarefaction of up to 2.5 kPa was frequently observed within a few tens of minutes after drilling and sealing a shot hole. Unfortunately, these observations did not include measurements of atmospheric pressure. Estimates suggest that the pressure within some gas accumulations can reach values of several megapascals [20].

The following are examples of several documented geogas occurrences described mainly in unpublished reports.

The occurrence of free hydrogen-hydrocarbon gases (HHCGs) in the Khibiny massif was first documented in 1951 at the mine face of a transport tunnel in the ijolite-urtites of the Yuksporr deposit. This event resulted in a fire triggered by an open flame, causing burns to several miners. The gas released from cracks in the rock burned with pulsating blue flames, reaching lengths of 7–8 cm. A day later, gas emissions intensified after drilling and blasting operations, becoming strong enough to be felt by hand near the mine face. The analysis of selected gas samples indicated the presence of CH₄ and its homologs (81.4), H₂ (13.9), N₂ and inert gases (4.3), and O₂ (0.4) (vol.%).

At the Kukisvumchorr mine, high concentrations of flammable gases were recorded in shot holes drilled in the walls of one of the cross gangways. Over the next 10 months, 50 samples of the gas-air mixture were taken from the sealed channel of one of these shot holes (No. 19). Irregular and non-synchronous fluctuations in concentrations (vol.%) of CH₄ (29–69), N₂ (32–63), O₂ (0.8–8), H₂ (>0.05–1.8), and CO₂ (>0.05–1.0) were registered. Generally, low concentrations of H₂ relative to CH₄ (below the sensitivity level of the employed equipment in some samples) and greatly varying ratios of these components are noteworthy here (Figure 4a).

In 1987, during additional exploration of the Kukisvumchorr deposit, a drilling crew encountered free gas (FG) emission from borehole No. 1253, drilled from the surface. At the time, the borehole depth was 293 m, and the section exposed consisted of lyavochorrites. Samples collected a few days after the gas emission revealed a combined CH₄ and H₂ concentration of approximately 14 vol.%. The borehole was later equipped with an exhaust fan, producing an outgoing airflow containing flammable gases in concentrations of tenths of a percent by volume. Using these concentrations and the air consumption rate, the combined flow rate of CH₄ and H₂ was calculated to vary between 3.5 and 5.8 L/min. When the borehole was sealed, no excess gas pressure was detected. Over the subsequent two months, the concentration of flammable gases in the borehole decreased by about 10 times. During the following year, episodic sampling showed that the total methane and hydrogen content in the borehole atmosphere fluctuated between tenths of a percent and 16 vol.%, with a general downward trend. In terms of an airless mixture, the composition of the released natural gas included 51–63, 33–47, and 5–7 vol.% of CH₄, H₂, and C₂H₆, respectively. The ratio of the main components changed slightly (Figure 4b). Seventeen months after the discovery, the borehole flow rate did not exceed 4 cm³/min, and in the next four months, gas emissions ceased completely.

At the Rasvumchorr mine, which was developing the Apatite Circus deposit, gas was found bubbling through the water in a 55 m deep subvertical borehole drilled in the rischorrites at the bottom of an underground mine shaft. The gas emission rate was about 4 L/min. The components of the gas phase composition were determined as follows (vol.%): CH₄ (65.1), H₂ (18.4), N₂ (8.6), C₂H₆ (3.4), O₂ (2.23), He (0.65), and C₃–C₅ (0.26). After nine days, the gas flow rate and the concentration of hydrocarbons in the gas composition slightly decreased, and the air component in the gas mildly increased. Furthermore, this trend continued, and within 2.5 months, the flow rate dropped to 2 L/min, and the compositions (C₃–C₅ concentrations were not measured) were as follows (vol.%): CH₄ (45.9), H₂ (16.6), N₂ (27.3), O₂ (6.7), C₂H₆ (1.5), and He (0.67).

A significant free HHCG manifestation was registered during the drilling of exploration borehole No. 539 in lyavochorrite near the Eveslogchorr deposit, when the borehole bottom was at a depth of 1580 m. The gas emission interval could not be determined. When the borehole head was sealed, the gas pressure rose sharply and stabilized at about 3.9 kPa. The gas flow rate was 4–6 L/min. Over the following six months—four of which occurred after borehole drilling was completed to a depth of 2000 m—irregular observations recorded pulsating changes in gas excess pressure, ranging from 1 to 8.5 kPa, and in flow rates, which varied from 0.03 to 5 L/min. The concentrations of methane (CH₄) and hydrogen (H₂) in the gas–air mixture fluctuated between 8 and 73 vol.% and 3 and 18 vol.%, respectively, while maintaining a relatively constant ratio (Figure 4c). Subsequently, another borehole was drilled at the same site, during which short-term gas emissions were detected at depths of 268 m and 451 m. When this borehole intersected a zone of heavily fractured rocks at depths of 780–810 m, the gas emissions in the neighboring borehole No. 539 decreased sharply.

An outburst and flash of flammable gases occurred in one of the exploration boreholes at the Koashva deposit when the borehole intersected urtite at a depth of 205–210 m. This incident caused damage to the drilling equipment.

The variability of the FG content in the gas–air mixture of each individual shot hole (borehole) is determined by the duration of its sealing, intensity of gas emission, pressure in a reservoir, time and volume of sampling, etc. Recalculated for an airless mixture, the gas compositions remain quite constant over time in most cases, including ratios of CH₄ and H₂ in the same gas manifestation (Figure 4), which are important for monitoring the composition of the mine atmosphere.

Various scientists have proposed multiple factors to explain the instability of the Khibiny FG parameters, including geodynamic, seismic, hydrogeological, space, meteorological, and man-induced influences. However, due to the irregularity of measurements, only one trend has been reliably established: gas emission tends to decrease significantly within the first tens of minutes or hours after a reservoir is opened. The duration of excess gas pressure in shot holes and boreholes, as well as the intensity of gas emissions, appears to be primarily influenced by fluctuations in atmospheric pressure, changes in the stress–strain state of localized rock areas, and, to some extent, by technological explosions of varying power and at different distances. Unfortunately, these factors and parameters, which are known to significantly affect the dynamics of gas emissions in the Lovozero massif, have not been systematically recorded during observations of the Khibiny FGs.

4.3. Features of the Gas Seep Localization

The intensity and duration of geogas emissions in the Khibiny massif are primarily influenced by the collecting properties of rocks, including porosity, permeability, and sorption capacity, as well as by the gas pressure, volume, and morphology of the reservoirs. Historically, studies of the Khibiny FGs have concentrated on their composition, with little information available about the shape or size of the gas reservoirs. Like other igneous rocks, the Khibiny rocks exhibit poor collecting properties. Laboratory measurements of rock samples show low effective porosity (3–7%) and extremely low permeability (in the range of hundredths to thousandths of millidarcy). However, the gas permeability of the rock mass as a whole, which is strongly influenced by microfracturing, is generally higher than that of individual samples. Field observations in mine workings qualitatively confirm a dependence of gas emission intensity and duration on rock permeability. For this purpose, an excess pressure of 6–7 atmospheres was artificially created in sealed individual shot holes by pumping the compressed air. No change in pressure was observed for at least 30–60 min in the shot holes that did not penetrate the gas-saturated zone, whereas in the shot holes drilled in areas with geogas manifestations, the pressure drop began in the very first minutes of observation. Areas with no gas emission indicated no gas pressure in a sealed shot hole (receiving hole) in case the adjacent shot hole (injection hole) was supplied with compressed air and constant pressure was maintained in it (the distance between the two holes was not more than 0.2 m). On the contrary, an increase in excess pressure was observed inside the gas-saturated zones in the receiving (one or several) shot holes. The distance between the injection and receiving shot holes was 0.2–0.8 m. In a number of cases, differences in the gas permeability of rocks in different directions were revealed. The relatively increased permeability of the massif rocks within the zones of gas manifestations is also evidenced by the inability to create or maintain artificial vacuum or excess pressure in some sealed shot holes. Open fracturing provides degassing, characterizing mostly the gas-permeable properties of rocks. The sorption capacity of the Khibiny rocks is also extremely low, thousands of times less than, for example, that of coal [20]. In general, gas manifestations here gravitate towards unwatered blocks of the rock mass with a developed system of microfractures, surrounded by either practically impermeable monolithic rocks or rocks with isolated, unconnected micro- and macrocavities.

Observations in underground mines suggest that most individual rock areas with increased gas saturation are less than a few meters thick. For example, breaker boreholes drilled in a 0.7 × 0.7 m grid failed to fully degas the outlined rock blocks over a period of more than a month. This was evident as gas combustion continued at the mine face for 15–20 min following the blasting of these boreholes. Shot holes spaced 2–3 m apart were drilled into the walls of underground mine workings in areas where gas manifestations had previously been recorded. These workings were mined along the strike and dip of the rocks. Sharp differences in the flow rate of free gases were observed even between closely spaced shot holes, with no degassing effect detected between them. This indicates that gas inflow and concentration in one shot hole did not influence the gas behavior in neighboring shot holes. Larger, more extended gas-saturated zones, spanning the first tens of meters are encountered much less frequently during the mining operations. The results of long-term observations in some shot holes and boreholes may evidence the existence of larger gas reservoirs of unknown configuration These cases may represent the opening of narrow gas-permeable zones or individual fractures that intersect areas of varying gas saturation and exhibit differing ratios of gas components.

The uneven distribution of FGs over scales of hundreds of meters is depicted in Figure 5. Apatite-nepheline ores and tectonic zones generally exhibit low gas content. The most frequent and intense FG manifestations during deposit mining occur in urtite, followed less commonly by other foidolites and rischorrite. Similarly, gas manifestations during the drilling of exploration boreholes have been predominantly observed in lyavochorrite and urtite. No clear patterns have been identified regarding the distribution of gas-saturated areas and zones. Contrary to expectations, increased gas emissions have not been detected in tectonic zones intersected by mine workings or boreholes. Additionally, no elevated concentrations of geogases have been recorded in the subsoil air above fault zones on the massif surface ([13,16]).

On a scale of the entire Khibiny massif, all known significant emissions of FGs are mostly identified within and in close proximity to the CRS. One of the reasons for this is that the structure is much better studied in relation to the FGs, since the underground mine workings and most of the boreholes relatively convenient for such studies are concentrated here. Nevertheless, this localization of the FG manifestations is consistent with the results of the subsoil air survey [13]. In particular, the highest concentrations of subsurface CH₄ are found in the loose sediments that cover the CRS rocks and the trachytoid khibinites adjacent to the CRS (Figure 6).

For a long time, it was believed that the distribution of FGs could be reliably inferred from the content of OGs in rocks, as a genetic and spatial connection between them was assumed ([1,2,21]). The high content of OGs in rocks penetrated by exploration boreholes was used as the sole criterion for predicting increased FG emissions and the gas abundance of mines, specifically concerning flammable and explosive natural gas components during deposit development. However, subsequent studies showed that this connection in the rocks of the Khibiny and Lovozero deposits is relatively weak and manifests only in certain areas ([3,21]). This observation is also illustrated in Figure 5.

Comparing the concentrations of gases released into sealed shot holes in the first 10 min after drilling with the OG content in the surrounding rocks reveals only a moderate correlation between these parameters, even when evaluated on a logarithmic scale (Figure 7a). The connection between these morphological types of gases is even weaker in terms of the composition expressed through the CH₄/H₂ ratio (Figure 7b).

In spite of having more or less common genesis, these morphological gas types have not proven to be closely spatially interconnected. Due to their mobility, FGs can respond to the constantly changing stress–strain state of the rock mass (e.g., [22,23]) and move considerable distances from the place of initial conservation [8,11].

Therefore, it must be acknowledged that making a reliable local forecast of intense or increased gas emissions during the exploitation of these deposits, as the mining front expands and moves deeper, is not yet feasible.

4.4. On the Origin of Free Hydrogen-Hydrocarbon Gases

The origin of HHCGs in nepheline–syenite complexes has been a subject of debate for over half a century, with a particular focus on the mechanisms, conditions, and relative timing of OG formation, primarily concerning their hydrocarbon components. Reviews of the various hypotheses regarding the origin of these gases—ranging from biogenic to mantle-derived—are provided in detail in works such as [2,3,17,24]. Combined data from multiple studies include observations on the distribution of gases in rocks and minerals, isotopic compositions of carbon, hydrogen, and noble gases, molecular weight distribution of alkanes, thermobarometric and Raman spectroscopy analyses of fluid microinclusions, and thermodynamic calculations. These findings suggest a polygenic, heterogeneous, and diverse origin for the Khibiny OGs, with their formation and transformation occurring at all stages of mineral development, starting from early magmatic processes but predominantly during late- and postmagmatic stages [17]. The most possible processes of the OG formation are the Fischer–Tropsch-type reactions (nCO₂ + (3n + 1)H₂→C_nH_2n+2 +2nH₂O), polymerization of primary methane (nCH₄→C_nH_2n+2 + (n − 1)H₂), and oxidation of hydrocarbon gases (4CH₄ + O₂→2C₂H₆ + 2H₂O). A slight addition of hydrocarbons of biogenic origin is possible at low-temperature stages of the gas phase evolution. Being a product of the magmatic fluid evolution, the OG molecular hydrogen may have probably been generated during various processes. Besides oxidation and dehydrogenation of HCGs, these could be the processes of interaction of iron-bearing minerals and aluminosilicates with water (for example, in the course of formation of aegirine, magnetite, cancrinite, and zeolites), and radiolysis of water or (less often) hydrocarbons [25,26,27].

In contrast to the origin of OGs, which was almost always associated in some way with the massif formation, FGs were often thought to have later (or even recently) flowed, at least in part, from the deep, for example, from long-lived intermediate magma chambers and even from the mantle or at least from the deep parts of the massif (e.g., [10,11,21,28]). However, the above-mentioned results of systematization and generalization of data from the long-term studies of FGs in underground mines and during the drilling of exploration boreholes, as well as the results of near-surface gas surveys, do not contain any convincing evidence of their inflow, at least modern and significant, from great depths. For example, (a) there is no recorded influx of gases into shot holes and boreholes crossing tectonic zones, including those that do not reach the surface; (b) no increased concentrations of geogases have been detected in the air of the near-surface soil layer above such zones; (c) the expected increase in the FG saturation of rocks with depth has not yet been confirmed; (d) in most cases, gas emission is greatly reduced within the first tens of minutes and/or hours since the opening of a reservoir; (e) the size of individual local rock areas with increased gas saturation in most cases does not exceed a few meters across; (f) typically, gas manifestations tend to gravitate towards blocks of the rock mass with a developed system of microfractures, surrounded by either practically impermeable monolithic rocks or rocks with isolated, unconnected micro- and macrocavities. A number of researchers consider the most FG-saturated CRS to be a percolation cluster through which the endogenous energy was unloaded in the form of fluid and heat flows [14,29]. Hence, it may be assumed that, along with other fluids, the C-bearing volatile components could also rise from the deep and mix with the residual magmatic fluid phase at the postmagmatic stages of the massif formation.

One of the indicators of formation conditions for the Khibiny occluded hydrocarbon gases (alkanes) is the pattern of their molecular weight distribution (MWD) [17]. As in the Lovozero massif [8], the MWD characteristics of alkanes of the Khibiny FGs have turned out to be close to those of OGs in the same rock complexes (Figure 8). These are some discrepancies with the classical Anderson–Schultz–Flory distribution, i.e., (i) relatively increased concentrations of butanes and (ii) coefficients of determination for log-linear dependence graphs and significantly steeper slopes of these graphs compared to thermogenic gases. Along with the similarity in the chemical composition and carbon isotopic composition of these morphological types of gases, this similarity makes it possible to suggest a common origin of gases of both morphological types.

The most noticeable difference in the composition of these gases is a higher proportion of H₂ in FGs. Given the high modern geodynamic and seismic activity of the massif [22,23], an additional new formation of free hydrogen may occur as a result of mechanochemical (hydromechanochemical, mechanoradical) reactions [30,31,32]. The value of such an indicator ratio as CH₄/C₂H₆ in FGs is on average close to that in OGs in albitites and hydrothermalites, where the formation of occluded hydrocarbons was completed at the lowest temperatures [13,17]. One of the reasons for the occurrence of isotopically lighter carbon in FGs compared to OGs may be the partial oxidation of HCGs, which also takes place at relatively low-temperature post-magmatic stages of gas phase transformation [17,33].

Figure 8. Molecular weight distribution of alkanes in the Khibiny, Lovozero, and thermogenic gases. 1 and 2—occluded gases of the Khibiny foidolites and nepheline syenites (rischorrites and lyavochorrites), respectively [17]; 3 and 4—the Khibiny free gases from foidolites and nepheline syenites, respectively; 5 and 6—free gases of the Lovozero massif [8]; 7—gas fields of Western Siberia [34]; 8 and 9—oil-type and coal-type gases in the northern Dongpu Depression, Bohai Bay Basin, China [35].

Figure 8. Molecular weight distribution of alkanes in the Khibiny, Lovozero, and thermogenic gases. 1 and 2—occluded gases of the Khibiny foidolites and nepheline syenites (rischorrites and lyavochorrites), respectively [17]; 3 and 4—the Khibiny free gases from foidolites and nepheline syenites, respectively; 5 and 6—free gases of the Lovozero massif [8]; 7—gas fields of Western Siberia [34]; 8 and 9—oil-type and coal-type gases in the northern Dongpu Depression, Bohai Bay Basin, China [35].

Thus, (a) the proximity of the chemical composition and isotopic compositions of carbon and hydrogen, as well as the character of MWD of alkanes of FGs and OGs, (b) the character of localization in rocks and emission of FGs, and(c) the absence of signs of their inflow from the deep after the formation of the massif all allow for the assumption that the Khibiny FGs are mostly residual in nature, preserved in intermineral pores, microfractures, and other cavities during the consolidation of the massif after their capture by fluid inclusions and losses during the degassing process. Subsequently, under the conditions of a constantly altering stress–strain state of local areas of the rock massif, the distribution, concentration, and composition of FGs appear to have varied somewhat as a result of partial releasing and concentrating of occluded and diffusely dispersed gases, mechanochemical reactions, and degassing (emanations into the atmosphere).

4.5. Obvious and Possible Environmental Consequences and Applications

In contrast to OGs, which are primarily of scientific interest, the study of free HHCGs in the Khibiny has always had a practical purpose due to the need to provide the gas-safe development of apatite-nepheline deposits. However, as data accumulated, other possible aspects, spheres of influence, and application of features of FG emission have been identified, requiring further research at the modern level.

4.5.1. Influence of Natural Free HHCGs on the Atmosphere of Underground Mines and Gas-Safe Mining

Of the three forms of gas occurrence in rocks, FGs are the main source of dangerous concentrations of flammable and explosive components in the mine atmosphere. A pervasive uneven, but generally weak emission of some diffusely dispersed gases determines only the increased background content of these components in the mine air, which is slightly higher than it is on the surface. A partial release of OGs during rock crushing (e.g., shot hole drilling or blasting) has an even lesser effect on the composition of mine air. Objective indicators that determine the possibility of gas-safe mining are the relative and absolute gas abundances, as well as the content of flammable and explosive gases (FEGs) in the mine atmosphere. The latter also reflects the state between the supply of gases from rocks and the amount of air supplied to ventilate the mine spaces. The relative gas abundance (RGA) means the amount of natural gases released when a unit volume of a rock pillar is crushed during blasting. In the considered case, RGA primarily reflects the content of FGs in the undestroyed rock mass. An integral indicator of the intensity of gas emission into the mine atmosphere is the absolute gas abundance (AGA) of a block, horizon, section of a mine, or a mine as a whole. This is the amount of gases released from all possible sources per unit of time.

If there are no highly FG-saturated rocks in the mining areas and the accepted ventilation standards are met, the FEG concentrations in the majority of mine air samples are below the detection threshold or do not exceed thousandths of a volume percent. As should be expected, the increased FEG contents are detected on the following occasions: during the driving of mine workings (especially rising ones) through an area of the FG-saturated rocks; when opening a gas-conducting zone; near the mine faces immediately after blasting; and in dead-end drives or other dead-end mine spaces not equipped with the forced ventilation system (ventilated only due to the general mine air depression). The average FEG concentrations in mine workings made in the host rock turned out to be 2–4 times higher than those in an ore body. The composition of mine air in dead-end unventilated mine workings is weakly dependent on rock type.

Variations in the FEG composition in the mine atmosphere significantly exceed those in shot holes and boreholes. The CH₄/H₂ ratio changes especially widely, which on average is much lower in the mine air. Hydrogen in the mine air is detected more often than methane. These facts may indicate both a wider distribution of H₂ in rocks and its possible new formation during the rock blasting due to mechanochemical reactions and decomposition of the explosive material.

A close positive relationship between the concentrations of CO and CO₂, which are predominantly of explosive origin, and H₂ and CH₄ (

Table 6. Correlation matrix of the mine atmosphere components based on paired Spearman correlation coefficients ¹ (the number of paired analyses is given in parentheses).

	H2	CH4	C2H6	C3–C5	CO	CO2
H2		0.74	0.44	0.73	0.95	0.49
		(185)	(25)	(11)	(92)	(151)
CH4	0.74		0.88	0.74	0.87	0.7
	(185)		(31)	(13)	(91)	(148)
C2H6	0.44	0.88		0.76	0.22 *	0.15 *
	(25)	(31)		(11)	(7)	(6)
C3–C5	0.73	0.74	0.76		−0.36 *	0.67 *
	(11)	(13)	(11)		(6)	(3)
CO	0.95	0.87	0.22 *	−0.36 *		0.78
	(92)	(91)	(7)	(6)		(77)
CO2	0.49	0.70	0.15 *	0.67 *	0.78
	(151)	(148)	(6)	(3)	(77)

¹ Significant (statistical significance level p ≤ 0.05) and insignificant * (p > 0.05).

) confirms that the latter are mainly released immediately after the explosion.

Estimates of the relative gas abundance of various parts of the mines, given in a number of unpublished reports, vary in a fairly wide range, i.e., from 0.03 to 1.5 m³ of gas per 1 m³ of pillar rock. Such significant variations in this indicator correspond to the peculiarities of localization of gas-saturated areas.

Caused mainly by hydrogen, the average AGA of active faces of mine workings in rocks with a relatively low FG content usually does not exceed several liters of gases per minute in terms of the amount of flammable gases. As shown in Figure 2, the AGA daily variations depending upon the specific stage of technological operations in such mine faces are typical of the Khibiny mines. The maximum gas emission (up to 25 L/min) is expectedly observed immediately after the explosion. Although in such cases, AGA may irregularly change even with one type of mining operations, the gas abundance of H₂ increases during and after the mine face drilling, and that of CH₄ increases at the time of rock removal.

According to the observations of outgoing ventilation flows, the AGA value of some blocks and sections of a mine varies within tens of cubic meters, and for a mine as a whole, it varies within a few hundred cubic meters of FEG per day. The principles of gas-safe development of the Khibiny gas-bearing deposits were formulated, and appropriate recommendations were given according to the results of studies. Based on these recommendations and in accordance with the requirements of regulatory documents, special measures for gas-safe mining in underground mines were developed and implemented with the assistance of specialists from mining enterprises. Approved by the federal state supervising authorities, these measures are periodically reviewed and adjusted according to the new gasometric data and changes either in geological settings or in the system and technology of mining operations.

The introduction of a fairly flexible and adjustable set of such measures has provided gas-safe underground mining operations during the development of the Khibiny apatite deposits. Having been investigated, different accidents (including injuries and deaths of miners) and emergencies caused by combustion and explosions of natural flammable gases in mine workings showed that they all occurred as a result of non-compliance with or major violation of the measures.

4.5.2. Gas Emanations from the Khibiny and Lovozero Massifs as Potential Agents, Indicators, and Precursors of Dangerous Natural Phenomena and Processes

Along with the accumulation of explosive concentrations of gases in mine workings, there can be other various effects on the environment, produced by emanations of reduced gases from the considered complexes. Mostly negative, these effects demand specially focused studies. Taking into account the size of the massifs, available estimates of the specific density of gas flow from their surface [2,9], data from profile surveys of subsurface air [13,16], and the gas abundance of mines, the current integrated flow of CH₄ and H₂ from the massifs into the atmosphere can be n × 10⁷ m³ per year or more. Meanwhile, methane is known to be the second most abundant (after CO₂) greenhouse gas, whose concentrations in the Earth’s atmosphere continue to increase. It is also known that the maximum concentrations of CH₄ in the atmosphere are observed in the Arctic/Subarctic region, where its anthropogenic component is minimal. It seems that the CH₄ emanations from the alkaline massifs may make their contribution to the CH₄ balance of the high latitude atmosphere along with the other more powerful sources. Though not a decisive one, this contribution deserves attention and further study.

An urgent problem of the present time is the global loss of stratospheric ozone and the formation of the so-called ozone holes, i.e., localized reductions in total ozone content. Additional solar energy comes through the ozone holes to the Earth’s surface in the form of ultraviolet and infrared radiation. The first one can negatively affect biological objects, for example, reduce the immunity of the population and contribute to the emergence and spread of epidemics, and the second one is the energy source for typhoons, hurricanes, melting of snow and glaciers in the mountains, etc. Ozone photochemistry in the stratosphere is known to be closely related to the behavior of hydrogen and methane [36]. Some authors (e.g., [12] and references therein) suggest that the formation of a strong negative anomaly of the total stratospheric ozone content, periodically occurring over the territories of the Kola Peninsula and the White Sea, is largely due to the release of H₂ and CH₄ from the surface of the Khibiny and Lovozero alkaline massifs.

Taking into account that (a) the rocks of nepheline–syenite complexes feature a relatively high content of natural radionuclides, including radon isotopes, (b) hydrogen and methane are recognized as the main carrier gases of natural radon, both in underground mines, where it poses an immediate danger to people, and in the surface atmosphere, where radon is the only air ionizer, and (c) knowledge of the structure and dynamics of atmospheric fields of ionizing radiation is necessary for radioecology, radiobiology, and atmospheric physics, identifying the nature of the relationship between the emanations of H₂, CH₄, and Rn may be of scientific and practical interest.

Emitted hydrocarbon gases are also capable of reacting with some gaseous components of anthropogenic origin (explosive gases, industrial and transport emissions) to form ozone, which in ground conditions is considered to be one of the most dangerous air pollutants.

Another possible consequence of the HHCG emissions from the considered complexes is periodically occurring mass fishkills in water bodies located within these massifs. There are no convincing explanations for this phenomenon yet. Meanwhile, it has been established that even very small concentrations of methane in water have various (direct and delayed) harmful effects on fish [37]. Indirect evidence of such impact was collected at Lake Seidozero in the center of the Lovozero massif [13]. These were (a) anomalously high concentrations of methane in the subsurface air on the shore of the lake, (b) gas bubbles of up to two meters in diameter observed by the inspectors of the nature reserve, rising from the bottom during one of the large-scale fishkills, and (c) the high seismicity of the massif registered by the geophysical service during the same period, which could have triggered gas emissions.

Besides the HHCG emission, the development of the Khibiny apatite deposits has been complicated by another, perhaps even more serious problem. This is the need to estimate the stress–strain state of rocks and predict dangerous dynamic manifestations of the rock pressure ([22,23] and references therein). According to the long-term studies (see the links above), the massif (including associated deposits) composed of rocks of various fracturing, permeability, hardness, and elasticity, features unevenly expressed tectonic stresses, as well as noticeable natural and induced seismicity. Horizontal stresses in the rock mass are 10–20 times higher than the vertical stresses produced by the weight of the overlying rocks, increasing by 2–3 times near the mine workings and tectonic faults. The general effect of tectonic stresses and their local concentrations near the underground mine workings determines the induced seismicity of the massif and leads to the destruction of these workings and boreholes. Alongside such relatively weak forms of the rock pressure manifestations as peeling, flaking, and “shooting”, far more dangerous events, including rock bursts, mountain-tectonic impacts, and small-focus (natural)-man-induced earthquakes with a power of 10⁶–10¹² Joules are recorded. The methods used to diagnose the tectonophysical state of a rock mass are either labor intensive or not reliable and efficient enough. The problem of short-term forecasting of rock bursts is still far from being solved not only in Khibiny but all over the world [38]. Therefore, the task of searching for new indicators of changes in the stress–strain state of rocks and precursors of dangerous geodynamic phenomena remains relevant.

It seems that gases may become the indicators and precursors to be used during the mining operations within the Khibiny and Lovozero massifs, since gas components are sensitive to any changes in the state of their host environment due to their mobility. The co-seismic behavior of natural gases (Rn, He, Ar, H₂, CH₄, CO₂, etc.), whether dissolved in groundwater or free, has long been used to study the tectonic activity of fault zones and search for gas-geochemical precursors of earthquakes and other geodynamic processes (e.g., recent reviews by Zhou et al., Martinelli, Martinelli and Tamburello, Szkacs ([39,40,41,42]). However, most researchers agree that, despite certain achievements, no significant progress has been made in this field yet. Among the reasons for this are the lack of a long-term research strategy and large-scale international cooperation, and the use of one or two signals for earthquake prediction rather than a complex of precursor phenomena. In addition, direct links between the gas geochemical parameters, tectonophysical state of rocks, and mechanism of formation of gas anomalies in most cases remain unclear, primarily due to the fact that the integral effect of many factors has been studied within the large areas and blocks of the Earth’s crust composed of rock complexes different in many respects. It can be expected that the relationship between the stress–strain state of limited and localized areas of a rock massif and its gas-metric features will be more definite, and the estimates and forecast of the seismic hazard in such cases will therefore be more realistic. To some extent, these expectations are justified by studying and comparing these parameters during the laboratory experiments and in situ observations in mine workings at the Khibiny apatite-nepheline and Lovozero rare-metal deposits [3,8,9,10,11,13,43]. For example, statistical relationships of different strengths and trends have been established between some components and their ratios in the compositions of diffusely dispersed gases and OGS, on the one hand, and elastic and strength properties of rocks and ores, on the other hand. An experimental loading of nepheline syenite samples in a laboratory setting has revealed that as the axial and tangential stresses grow, the intensity of radon emanations and HHCG emission increases. The maximum gas release was recorded immediately before the destruction of the sample. These observations correspond with the results of a field experiment in an underground mine, where in the process of increasing the load on the rock pillar, the dependence of the release of volatile components on the stress–strain state of the rock has been established. Only tectonic and seismic activity of massifs and changes in the stress–strain state of local areas of rocks may cause some spatial and temporal variations in their gas-geochemical and gas-dynamic parameters, e.g., concentrations of individual gas components and their ratios, excess gas pressure and its build-up rate, and gas emission rate. Observed on a short-period (fractions and first seconds) time scale, an irregular and impulsive gas emission from shot holes drilled in the walls of underground mine workings may also indicate the continuous seismicity of the rock massif. This correlation is confirmed by the increased amplitude and frequency of the gas flow rate fluctuations that occur immediately after the remote explosions. It is known that the seismic effect of the explosions is equivalent to the impulse compression of the medium, which leads to changes in pores and fractures in rocks. Even some minor violation of the integrity of local areas of the massif (e.g., shot hole drilling) causes their activity (the so-called “breathing” of the massif), which is indicated by observations of changes in pressure and atmospheric composition in the shot hole channels after the mouth sealing. Changes in stresses and, consequently, in the permeability of individual sections and zones of the massif are assumed to be the main cause of long-term variations in the dynamics of FG emission.

The monitoring of the dynamics of molecular hydrogen in the subsurface air within the area of the Kukisvumchorr and Yuksporr fields showed that most of the relatively large seismic events that occurred here were preceded by the local minima of gas emission. Such H₂ behavior may be explained by the fact that some parts of the massif experiences compression when the load increases. In this case, the volume of fractures in the massif and, consequently, its permeability decreases and the gas emission reduces. After the stress is released in one form or another, the opening of old fractures and the formation of new ones in the massif contribute to the increase in gas emission. A similar scenario is proposed when the dynamics of volumetric radon activity are interpreted as a potential indicator of geodynamic processes in deep shafts of a bauxite mine in the Urals [44].

The geomechanical state of rocks obviously influences the composition, distribution, and dynamics of gas emission, but the opposite effect is also possible [45,46,47]. For example, the formation of zones of relative extension in a massif enables the gases from adjacent zones of compression to seep into the extension zones with the fluid pressure in microfractures increased and the strength characteristics of rocks decreased. The presence of fluid microinclusions (in this case, gaseous ones) in rocks as concentrators of internal stresses favors the formation of fractures and weakened zones [45,48]. The features of the distribution of secondary gas microinclusions formed in the field of unevenly acting paleo-stresses may indicate their orientation. This orientation is important for evaluating the present-day stress–strain state of the massif, whose changes under the influence of tectonic and erosional processes, mining operations, and natural and anthropogenic seismicity, in turn, cause redistribution and spatial and temporal variations in composition, pressure, character, and intensity of gas emission.

The study of free HHCGs in the Khibiny has so far been determined only by the need to provide the gas-safe mining of apatite-nepheline deposits. Therefore, the main attention was paid to the issues of the composition of gases, the peculiarities of their localization in a rock massif, the features of their emission into mine workings, and their influence on the atmosphere of underground mines. Data on other aspects and consequences of gas emission were obtained only incidentally, undoubtedly deserving special attention and further research. For this purpose, it is necessary to organize a special comprehensive study that would involve scientists of different profiles and utilize modern methods and equipment.

5. Conclusions

The collection, systematization, revision, and statistical processing of all available, mostly previously unpublished data on free HHCGs of the ore-bearing complex of the Khibiny massif has resulted in the following conclusions:

• As in the better studied OGs, the main FG component is methane, followed, in decreasing order, by hydrogen, ethane, helium, methane homologs, and alkenes. Compared to OGs, the composition of FGs usually contains four to six times higher proportions of H₂ and homologs of methane, and an order of magnitude higher concentration of helium. For all pairs of geocomponents (hydrocarbons, H₂ and He), a highly significant strong and medium positive correlation is observed. The most closely interrelated ones are the HHCGs. CO and CO₂ are constantly presented in the gas–air mixture samples; their weak and very weak connections (of varying degrees of significance) with the other components of these mixtures serve as additional evidence of their man-induced origin. The highly significant, moderate, and medium strong correlation of N₂ with HCGs, H₂, and He and its weak correlation with CO and CO₂ suggest a non-atmospheric origin of some part of this component and its presence in the FG composition. In terms of an airless mixture, the composition of FGs in the same gas manifestation in most cases is quite constant over time. Like the OGs, FGs are characterized by an inverse trend in the distribution of carbon isotopes in individual hydrocarbons compared to gases from the sedimentary rocks and oil and gas fields, and by isotopically lighter hydrogen in ethane compared to methane;
• The intensity of gas emission most often decreases quite sharply in the first tens of minutes after the opening of a reservoir with a shot hole, borehole, or mine working. Then, the gas emission usually attenuates slowly and unevenly, often fluctuating. In most cases, this lasts for a period from several hours to several days. More prolonged (months and years) gas emissions of variable intensity (generally weak) into the shot holes and boreholes have rarely been observed. For a relatively recently exposed surface of the Khibiny rocks, the average specific gas release for the sum of methane and hydrogen is 47 cm³ per minute per 1 m², reaching 120 cm³min⁻¹m⁻² for methane and 1700 cm³min⁻¹m⁻² for hydrogen. The flow rate of gasometric shot holes (1.8–2 m deep and 40 mm in diameter) in the first 10 min after the drilling usually varies within the range of 0.01 ÷ 10 cm³/min, sometimes reaching 0.5 L/min. The borehole flow rate does not exceed the latter value either;
• Most often, the areas of rocks with increased gas saturation occur sporadically and do not exceed a few meters in cross-section. Less often, their length reaches tens of meters. In some cases, the presence of more extensive gas reservoirs is assumed, as well as of narrow gas-conducting zones, intersecting areas of different gas saturation with unequal ratios of gas components at different depths. The distribution of gas-bearing areas and zones is uneven (on scales from fractions to hundreds of meters), without obvious patterns. The calculated relative gas abundance varies from hundredths to 1 m³ of gases per cubic meter of undisturbed rock. Apatite-nepheline ores and tectonic zones are characterized by low gas content. Regarding deposit mining, the most frequently revealed and the most intense FG manifestations are those located in urtite; less often they are found in other foidolites and rischorrite. Regarding the drilling of exploration boreholes, FG manifestations occur in lyavochorrite and urtite;
• FGs and OGs are similar in terms of their chemical composition and isotopic compositions of carbon and hydrogen; they also indicate a similar pattern of the MWD of hydrocarbons. Together with the absence of signs of FG inflow from the deep or from host rocks after the formation of the massif, these features make it possible to assume the common abiogenic origin of gases of both morphological types, associated with the formation of the massif. The Khibiny FGs are mostly residual, preserved in intermineral pores, microfractures, and other cavities during the consolidation of the massif after their capture by fluid inclusions and losses during emissions. Subsequently, under the conditions of the constantly altering stress–strain state of local areas of the rock mass, the distribution, content, and composition of FGs have changed as a result of the partial release and concentration of occluded and diffusely dispersed gases, mechanochemical reactions, and emanations into the atmosphere;
• The presence of natural flammable and explosive FGs, such as H₂ and HCGs, in the Khibiny massif poses a danger during the exploration and development (especially by the underground method) of the apatite-nepheline ore deposits. The gas-bearing capacity of these deposits has become an integral part of their mining and geological characteristics. In mine workings, the direct sources of FEGs are shot holes and boreholes that have opened accumulations of FGs, discharges from the surfaces of mine workings, especially at the driving stage, and degassing of blasted rock. The AGA of individual blocks and sections of a mine varies within tens of cubic meters, and for a mine as a whole, it varies within a few hundred cubic meters of H₂ and CH₄ per day. Of the mandatory special measures for gas-safe underground mining operations, the main ones are the constant monitoring of the composition of mine air and compliance with accepted ventilation standards;
• A further comprehensive study of the Khibiny FGs is necessary, involving modern methods and equipment, as well as researchers from different disciplines. It is important, in particular, to detail the spatial distribution of gases in the massif as a whole, determine the specific emission density, and identify temporal variations in the intensity of outgassing in different rock complexes and fault types. Together with data from other studies and observations (geological, geophysical, ecological, and meteorological ones), the obtained results can be used for the following purposes: (a) spatial-temporal forecasting of gas manifestations and optimization of gas-safe mining measures; (b) establishing gasometric precursors of such dangerous geodynamic phenomena as rock bursts and small-focus natural-man-made earthquakes; (c) determining the integral outflow of CH₄ and H₂ from the nepheline–syenite massifs and their possible influence on unfavorable processes in the atmosphere and stratosphere at high latitudes; and (d) estimating the local impact of geogas emanations on the environment.