Relationships between gas geochemistry and release rates and the geomechanical state of igneous rock massifs

Tectonophysics 336 (2001) 233±244

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Relationships between gas geochemistry and release rates and the geomechanical state of igneous rock massifs Valentin A. Nivin a, Nikolai I. Belov b,1, Peter J. Treloar c,*, Vladimir V. Timofeyev b a

Geological Institute, Kola Sci. Centre, Apatity 184200, Russia b Mining Institute, Kola Sci. Centre, Apatity 184200, Russia c CEESR, School of Geological Sciences, Kingston University, Penrhyn Road, Kingston upon Thames, Surrey KT1 2EE, UK

Abstract In contrast to sedimentary sequences, the relationships between the stressed state of igneous rocks and the chemistry and physical properties of gases contained within them are not well known. Here, we attempt to ®ll this gap by using, as an example, the apatite±nepheline and rare-metal ore deposits hosted within the Khibiny and Lovozero alkaline nepheline±syenite complexes of the Kola Peninsula, NW Russia. These massifs are characterized by unusually high, for igneous rocks, contents of multi-component, essentially hydrogen±hydrocarbon, gases and also by high hardness, elasticity and unevenly distributed, subhorizontal tectonic stresses. Relationships between the chemical and dynamic characteristics of the gases and the geomechanical properties of the host rocks have been examined using ®eld observations and laboratory experiments. Patterns of gas release variations in time and space, gas emissions from rock pillars during arti®cial loading, variations of gas pressure in sealed shot-holes and changes in liberation rates of gaseous components during experimental rock loading are suggested to result from changes in rock stress and deformation state. Gas compositions in sealed shot-holes in stressed rocks change with time. Partly, this is due to belated release of gases held in ¯uid inclusions and isolated voids and their subsequent mixing with gases held in interconnected fracture systems as the included gases are preferentially released as ¯uid inclusion arrays are opened during later stages of stress build-up. Partly, it may also be because released gases may react with new fracture surfaces to generate enhanced levels of reduced H2 gases. q 2001 Elsevier Science B.V. All rights reserved. Keywords: gas-geochemical characteristics; gas-dynamic parameters; geomechanical properties; tectonophysical state; alkaline rocks; ore deposits

1. Introduction The release of volatiles stored in rocks is an important side effect of any mining or underground construction activity. The dynamic side effects of mining include phenomena such as rock bursts, * Corresponding author. Tel.: 144-181-547-7525; fax: 144-181547-7497. E-mail addresses: [email protected] (V.A. Nivin), [email protected] (P.J. Treloar), [email protected] (V.V. Timofeyev). 1 Deceased

mining-induced tectonic bursts and shallow earthquakes (e.g. Gibowicz, 1990; Wong, 1992; Melnikov et al., 1996). All these are the result of changes in the stressed state of the rock, and some may be accompanied by catastrophic release of over-pressurized volatiles. To forecast these effects requires an evaluation of rock mechanics as well as a quantitative assessment of the gas content of a rock mass and of the gas distribution within it. In addition, frequent emissions of harmful and potentially dangerous gases occur during deep mining (Matvienko, 1983; Bruce and Donley, 1985).

0040-1951/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0040-195 1(01)00104-4

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Gases, in one form or another, are present in varying quantities in rocks of all geological types and, because of their mobility, are sensitive to changes in the physical nature of the host rock. Data from coal and salt mines show clear relationships between the dynamic behaviour of gases and rockbursts (e.g. Proskuryakov, 1980; Petrosyan et al., 1983; Molinda, 1988; Zhou et al., 1993). The release of both active (H2, CH4, N2) and inert (He, Ar) gases has been shown to be the result of microcracking processes during compressional loading of rock samples under laboratory conditions (Giardini et al., 1976; Honda et al., 1982). Coseismic behaviour of gaseous components, including, Ar, He, H2, CH4, CO2, N2, H2S, NH3, Rn, shows them to be released, either freely discharged or, most often, dissolved in groundwater, during tectonic activity along fault zones where their release may often predate earthquake activity (Kita et al., 1982; Barsukov et al., 1985; King, 1986; Shi and Cai, 1986; Wakita et al., 1989; Nagamine, 1994; Tonglei et al., 1995; Sugisaki et al., 1996). However, in most of these cases, the distribution of gases and the precise mechanism of gas release and its controls on changes in gas chemistry cannot be de®ned as the gases under study represent a mixture of gases derived from a large volume of mixed rock formations. To date, there have been few ®eld or in situ studies of gas release mechanisms in magmatic complexes and mineral deposits hosted by them. In this paper, we start to ®ll this gap by reporting data collected in the course of studies of gas behaviour and concentration in the Lovozero rare-metal and Khibiny apatite±nepheline deposits over a 15-year period, and hitherto published only in part (Nivin and Belov, 1993, 1999; Nivin et al., 1993a). Both rockbursts of varying energy, some enhanced by over-pressurized gas, and release of anoxic gases are encountered during mining of apatite±nepheline and rare-metal deposits located within the Khibiny and Lovozero alkaline igneous complexes (Turchaninov et al., 1972; Ikorsky and Nivin, 1984; Nivin, 1991; Kozyrev and Panin, 1993; Kozyrev et al., 1994, 1996) and this is, thus, a sensible region in which to study the relationships between stress state and gas mobility and the mechanisms of gas release in igneous rock masses. 2. Geology and gas geochemistry The Khibiny and Lovozero alkaline igneous

complexes are located in the Kola Peninsula in the NE part of the Baltic Shield. The geology, petrology and mineralogy of these c. 370 Ma, nepheline±syenite complexes and ore deposits hosted by them have been summarized elsewhere (Arzamastsev, 1994; Kogarko et al., 1995). The alkaline complexes are intrusive into Archaean gneises and Proterozoic sedimentary± volcanic suites. The Khibiny apatite±nepheline deposits occur within a ring zone of an urtite± juvite±rischorrite complex. The ore bodies, which are up to 200 m thick, dip toward the centre of the massif and can be traced to a depth of 2 km. Apatite and nepheline are the main economic minerals. The Lovozero rare-metal deposits occur within a differentiated complex composed of thin, rhythmically alternating layers of lujavrite, foyaite and urtite. Juvite and malignite are less common petrographic varieties of nepheline syenites. The main ore mineral is loparite, a composite oxide of Ti, REE and Nb. It has long been known that these alkaline plutons contain high concentrations of hydrogen and hydrocarbon gases (Petersilie, 1962). Similar gases, mainly held in ¯uid inclusions, have subsequently been described from other alkaline complexes in Greenland, Siberia and Canada (Petersilie and Sorensen, 1970; Konnerup-Madsen et al., 1985; Ikorsky, 1991; Salvi and Williams-Jones, 1992). The origin of multicomponent, hydrocarbon-bearing gas phases in alkaline igneous complexes has been the focus of much research. An abiogenic origin, the result of late- to post-magmatic sub-solidus reactions, is indicated for the gases (Ikorsky et al., 1992, 1993; Nivin, et al., 1993b, 1995; Potter et al., 1998; Nivin and Ikorsky, 1999) with the hydrocarbon gases most likely produced during late-stage hydration of primary igneous mineral assemblages through a Fisher± Tropsch reaction (Potter et al., 1998). There are two basic morphological gas types present in the Khibiny and Lovozero complexes. Included gas (IG) ®lls mineral micro-cavities, mainly in ¯uid inclusions, and intergranular pores. Free discharged, or fracture gas (FG), is present in cracks, principally micro-cracks, of different length, opening and connection. IG can be extracted from a rock only by ®ne grinding and heating to high temperatures, whereas FG can move within rock units along rheologically weak boundaries such as bedding planes and fractures, and will be released either when such zones

V.A. Nivin et al. / Tectonophysics 336 (2001) 233±244

come to the surface or when they are opened by drilling or mining. A portion of the FG gas may be retained in systems of conjugate microcracks after they have been opened. This retained ®ssure gas (RFG) can, if extracted by thermal±vacuum desorption, be used to characterize the FG composition and content. On the whole, both gas types contain the same components although their proportions vary with gas type and are dependent on the total gas content of a rock and the extent of atmospheric contamination or mixing. Approximate estimates of average gas component proportions, in vol.%, in airfree mixtures are as follows: Khibiny IG: CH4, 93, H2, 4, C2H6, 2.7, He, 0.05; Khibiny FG: CH4, 74, H2, 20, C2H6, 5, He, 0.8; Lovozero IG: CH4, 76, H2, 20, C2H6, 4, He, 0.07; Lovozero FG: CH4, 60, H2, 35, C2H6, 3.2, He, 2 (Ikorsky et al., 1992). CO and CO2 are rarely present as minor components. Compositional and isotopic similarities between the IG and FG gases at Khibiny and Lovozero suggest that they share a primary origin. IG and RFG gas contents in rocks vary from 0 to 150 ml/kg. Initial FG emission in 42 mm diameter shot-holes 1.8 m long varies from 0.01 to 260 ml/ min (Ikorsky et al., 1992). Usually, gas release rates are at a maximum immediately after opening of a system, before decreasing signi®cantly during the ®rst minutes or hours, and sometimes days, after opening. After that, gas ¯ow, with at times sizeable ¯uctuations, continues for several weeks or even years. In some shot-holes, discharge lasted for up to 6 years with gas ¯ow rates near 200 ml/h. In rare instances, gas gauge pressure in sealed shot-holes was as high as 0.1±0.3 MPa. During rare outbursts of drilling liquid from boreholes, the resultant liquid column reached a height of hundred of metres. These data indicate the presence of signi®cant volumes of FG gas con®ned at high pressure, not only in microfractures, but also in isolated macro-cracks and other cavities. The distribution of IG and, especially, FG is uneven. IG distribution is partly a function of the distribution of rocks that contain minerals likely to host ¯uid inclusions. FG distribution may be a function of an ill-de®ned relationship between rheology and fracture development. Some spatial relationships are revealed between these two gas types. Enhanced FG release was usually observed in rocks with

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relatively high IG concentrations (Ikorsky et al., 1992). This could suggest release of included gases into sealed fractures during strain, but may also imply that IG volumes are greater than FG volumes. Generally, ore bodies in both the Khibiny and Lovozero complexes are characterized by comparatively low gas contents with zones of elevated gas content located above and beneath the ore bodies (Nivin, 1985; Ikorsky et al., 1993; Nivin et al., 1993a). FGsaturated rock units are typically water free. As groundwater in fractured tectonic zones and discrete fractures rarely contains more than minor quantities of dissolved hydrogen and hydrocarbon gases, it is probable that rocks open to groundwater migration are rapidly ¯ushed of their primary gas components. The analytical methods used in this study cannot ensure no contamination by some arti®cial hydrocarbon gases or H2 (the so called `bit-metamorphic' gases of Faber et al., 1988) in the measured IG and FG gas contents. However: (1) correlation between gas quantities extracted from ¯uid inclusions in nepheline by milling and acid dissolution (Ikorsky et al., 1992); (2) direct non-destructive measurements of gas components in ¯uid inclusions (Kogarko et al., 1987; Potter et al., 1998); (3) carbon and hydrogen isotope compositions (Voitov et al., 1990, 1992; Nivin et al., 1995), particularly, the similarity of the isotope compositions of the Lovozero and Khibiny gases to those from the Zambales ophiolite, Ilimaussaq alkaline complexes and East Paci®c Rise basalts (Nivin et al., 1995); (4) high (20±10 3) C2H6/C2H4 ratios in IG and FG gases; and (5) the FG gas release pattern suggest only minor contamination, if any, by the arti®cial gases. In the rest of this paper, the following abbreviations are used: HHCG, hydrogen±hydrocarbon gas; FG, freely discharged, fracture gas; IG, included gas, mainly in ¯uid inclusions; and RFG, FG gas retained in opened microfractures. 3. Geomechanical features and seismicity of the rock massifs The Khibiny and Lovozero complexes and related ore deposits are composed of rocks with different fracture geometry, matrix permeability, hardness and elasticity and are distinguished by, rather unevenly

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distributed, subhorizontal tectonic stresses linked to natural and mine-induced seismicity (Turchaninov et al., 1972; Kozyrev and Panin, 1993; Kozyrev et al., 1994, 1996; Melnikov et al., 1996). The full range of different ore and host rock types present in the complexes have the following average physical parameters: density, 2.6±3.1 cm 3/g; Young's modulus (4±10) £ 10 4 MPa; axial compressive strength, 80±250 MPa; tensile strength, 4.5±10.2 MPa. Experimentally supported horizontal stresses as high as 60±80 MPa exceed the vertical lithostatic load 10±20 times and these can increase two- to threefold in the vicinity of mine workings and tectonic faults. The present day geodynamic behaviour of the apatite±nepheline and rare-metal deposits under consideration has been affected by the large volumes of rock that have been extracted. Interaction between far ®eld tectonic stresses and local stress concentrations near to underground mines and deep pits result in mining-induced seismicity. Local `bumps' and rock bursts are recorded in separate mines, together with relatively weak dynamic forms of rock pressure manifestation such as ¯aking and spalling. More dangerous tectonic bursts and mining-induced earthquakes may release 10 6 ±10 12 J energy. One such event, accompanied by enhanced release of HHCG, occurred in the Lovozero mine on August 17, 1999, resulting in destruction of underground workings over an area .0.5 km 2 and signi®cant economic damage. Mineinduced earthquakes differ from natural ones through seismic energy release due to anthropogenic factors (Kozyrev and Panin, 1993). For them to occur, requires `favourable' geomorphological conditions (mountainous relief in this case) together with high horizontal stresses and the presence of large workings. 4. Experimental techniques In situ observations were carried out in underground workings in rare-metal and apatite±nepheline mines within the Khibiny and Lovozero massifs. Freely discharged (FG) gases were primarily examined by shot-hole drilling after the methods of Ikorsky and Nivin (1984). Shot-holes 1.8±2.0 m long and 40± 42 mm in diameter were drilled. For short-term measurements, these were sealed by an air-®lled rubber cuff or by a special steel±rubber seal. For

long-term observations, the shot-holes were sealed by a cement plug. In each case, two thin tubes were passed through the seal for gas sampling, discharge and gas pressure measurements. The length of all the plugs was 20±30 cm. Hermetic sealing of the shothole mouth, involving washing with water and compressed air, was carried out shortly after drilling. A sealing device of two air-®lled rubber cuffs separated by a 20 cm gap was used to measure gas ¯ow rate and radon emission in separate sections of a sealed shot-hole. Gas samples were taken from isolated parts of the shot-holes for chromatographic analysis. CH4 and H2 concentrations were sometimes determined by a portable gas detector. In both ®eld and laboratory, an EM-6P emanometer was used to measure Rn emission and a SRP-68-03 radiometer to measure g radiation levels. Column and membrane manometers were used to measure gas pressure. Gas ¯ow rate was directly measured using a rotameter or a graduated glass burette or, in the case of low ¯ow rates, incremental analysis of intrinsic gas component concentrations in known volumes of sealed shot-holes. To measure the RFG, samples taken from the drill core were placed immediately into hermetically sealed steel containers with the RFG removed under laboratory conditions by thermal±vacuum desorption. IG was extracted from rock samples under laboratory conditions by means of vacuum ball mill and mercury pump. All forms of gas phase were analysed by chromatography (Ikorsky et al., 1992). The geodynamic state of the rock mass was evaluated using local measurement techniques (ultrasonic, acoustic and electromagnetic emissions, core disking, off-loading, destruction observation in workings) as well as regional scale seismic and deformation monitoring, mathematical and physical modelling. Experiments included measurements in horizontal boreholes drilled into a rock pillar 1 m high with length and breadth of 3 m. The pillar was under arti®cial loading at the time due to transfer on to it of the weight of overlying rocks after blasting of adjacent pillars. He and Rn emissions and acoustic emissions were recorded. The latter were measured by piezoaccelerometer and pulse counter with ampli®er and threshold facility in the frequency band 0.05±1.0 MHz. Compressional loading experiments were carried out in the laboratory using nepheline±syenite core

V.A. Nivin et al. / Tectonophysics 336 (2001) 233±244

Fig. 1. Generalized pattern of response of HHCG release into sealed shot-holes in response to blasting. Gas discharges were recorded in 12 cases using a rotameter. Conventional moment of blasting at a distant section of the mine is shown under the horizontal axis as an arbitrary time. C.u., conventional units.

samples 132 mm in diameter and 300 mm long. For each sample, a 250 mm long, 50 mm diameter hole was drilled along the axis of the core and the sample placed into a three-axial compression device within a work chamber isolated from the atmosphere. Each sample was face (s z) and side (s w ) loaded. During loading of one sample, Rn emission and g radiation in the working chamber were measured. During loading of the second sample, acoustic emissions were recorded and portions of air±intrinsic gas mixture were periodically taken from the working chamber for chromatographic analysis. Elastic properties of the rock samples were measured using ultrasonic equipment in the frequency band 0.4±0.6 MHz. Young's modulus and Poisson's ratio were calculated using measured velocities of longitudinal and shear waves. A pulse counter with amplitude discrimination and high-pass ®lters for noise reduction of the press equipment was applied to measure the acoustic emissions. Sample strength was determined using a uniaxial compression test on core samples and a tension test on disk shaped samples (the Brazilian test). A brittleness value was determined using the method of Hoek (1977). 5. Results and discussion In both the Khibiny and Lovozero massifs, CH4/H2 ratios are higher in the IG than in the FG with CH4 contents lower and H2 contents higher in the FG than

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in the IG (Ikorsky et al., 1992; Nivin et al., 1995). Although this difference might be a function of initially different gas compositions, it is more likely to be the result of a long-term modi®cation of FG compositions through the generation of secondary H2 by chemical reactions involving H2O and Si z and Si±O z type radicals on rock surfaces formed during stress redistribution and the regrowth of secondary fractures. Kita et al. (1982) documented this reaction both experimentally and during mapping of fault zones. During mining, most of the FG is released into underground workings over a period of several hours or days after opening of a gas reservoir. The rate of this release is a function both of initial fracture connectivity and gas content, and the rate at which sealed fractures are opened and connected to the external open network. After that, gas emission reduces while the rock mass relaxes and seismic events occur, together with changes in the stressed state of the rocks and the formation of permanent new surfaces. Regular increases in the H2/CH4 ratio in mine air were reported in a study of combustible and potentially explosive gas concentrations in the Lovozero mine during its movement through underground workings from operating faces to surface (Ikorsky and Nivin, 1984). This could be the result of the hydrogen generation mechanisms outlined above. It cannot, though, be the direct result of releasing IG gases with high CH4/H2 ratios into the opened fracture system. Readjustment of the local stress ®eld, in particular as a result of blasting, is apparently responsible for both seismo-acoustic emission and release of FG in the Lovozero and Khibiny massifs (Voitov et al., 1990, 1992). It seems likely that continuous rock mass seismic activity controls the pulsed patterns of gas release from boreholes and shot-holes that open gas-rich zones. Seismic vibration can also liberate gas components adsorbed on surfaces of mineral grains and fractures (King, 1986; Sugisaki et al., 1996). Clear, short-lived oscillations in gas release rate were repeatedly observed immediately after relatively close blasting (Fig. 1). This blast seismic effect is the equivalent of a `pulsed in¯uence' on the environment which causes changes in rock porosity and crack opening (Ospanov, 1985). When a rock mass is disturbed by shot-hole drilling, activity and `breathing' of the rock volume can

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Fig. 4. Temporal changes in gas discharge with depth along a shothole. The measurements were carried out at 15 (1), 53 (2), 54 (3) and 91 (4) days after drilling the shot-hole.

Fig. 2. Variations of gas pressure in sealed shot-holes. Time from drilling a shot-hole to sealing and commencing measurement: (1) 9 months; (2) 8 days; and (3±9) 1±2 min.

control changes in shot-hole atmosphere composition and gas pressure changes (Fig. 2). Around some of the shot-holes, zones of rock fracturing and decompression appear through opening of isolated cavities and pores, leading to an increase in the effective fracture and pore space volume. When FG contents are high, this normally results in a rapid initial increase in HHCG content in the shot-hole. The HHCG content then plateaus at a level which is a function of the

Fig. 3. Correlation between HHCG (continuous line) and radon (dashed line) emissions with depth along horizontal shot-holes drilled in urtite (1, 2) and foyaite (3, 4) in the Lovozero mine. The measurements were carried out 2±3 months after drilling. C.u., conventional units.

volume of interconnected fracture space and the initial FG content (Fig. 2, cases 1, 3±5). In these cases, fracturing near shot-holes followed by migration of FG HHCG from surrounding rocks through new cracks swamps any potential air leakage. Where air leakage is low, the gas volume curves suggests that gas pressures equilibrate on time scales of less than 2 h. When FG contents are low, this interconnected fracture volume is ®lled with air from the isolated shot-hole leading to an initial reduction in HHCG content and gauge pressure in the sealed shot-hole (Fig. 2, cases 7±9). After between 5±10 min and several hours, slow compression starts, involving closure of cracks and expulsion of air back into the shot-hole after which no further changes are observed. Gas analysis shows that the atmospheric composition in isolated shot-hole chambers was unchanged in these cases. That gas release into shot-holes is not instantaneous (Fig. 2, cases 1, 2), suggests that gas pressure can act as an additional long-term in¯uence on the rock dynamics of local rock volumes. Formation of leak zones does not always occur, but depends on variations in natural stress patterns which determine patterns of FG release. That HHCG release depends on the behaviour of the rock mass, is shown by an almost complete correspondence between the emission rates of radon and the HHCG with shot-hole length (Fig. 3). A redistribution of stresses, and the resultant change in rock permeability, is a primary cause for variations in FG discharge rates in both space and time. Fig. 4 shows changes in gas discharge rate over a period of 91 days within a shot-hole 1.8 m long. Quite clearly, discharge rates change both

V.A. Nivin et al. / Tectonophysics 336 (2001) 233±244

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Fig. 5. Acoustic emission (A.e.), helium and radon emissions from a rock pillar 1 m high with basal section 3 m 2 during arti®cial loading. Arrows are for moments of blasting (B) and rockbursts (RB). C.u., conventional units.

Fig. 7. Variations of gas component concentrations and acoustic emission in a sealed hole during arti®cial loading of a foyaite sample. Axial loading remained constant (s z ˆ 35 MPa) while side loading stresses (s w ) were increased step by step.

along the length of the shot-hole and with time. This suggests that interconnected fracture networks may have relatively small volumes. A clear relationship between the stressed state of a rock and release of volatile components is inferred from in situ experiments during arti®cial loading of rock pillars by removal of adjacent, supporting, pillars during a series of four blast events. As the loading on an individual pillar was increased, a sporadic increase of acoustic emission implying crack formation was observed together with an increased rate of release of helium and radon (Fig. 5). Maximum helium emission occurred after the second blast and decreased thereafter, whereas maximum radon emission was greatest after the fourth blast. Just before partial, or total, pillar failure, fracturing and a rapid, avalanchelike (Gorbatsevich, 1996), increase in the interconnection of microcracks occurred. It is this event which stimulated volatile liberation.

Field observations were supplemented by laboratory experiments on arti®cially loaded nepheline syenites from the Khibiny and Lovozero complexes. In the ®rst experiment, both axial (s z) and side (s w ) loading were gradually increased. Both radon and g radiation emission were measured during loading (Fig. 6). Maximum Rn emission occurred at s w ˆ 180 MPa, which is close to the compressive strength limit of nepheline±syenite. As changes in g radiation are a function of the radioactive decay of radon isotopes, the peak of g radiation emission should postdate that of Rn emission. g radiation remained constant as Rn emission increased until the point where, after the onset of sample destruction, continued loading led to closure of gas escape pathways after which Rn emission decreased to zero and g radiation emission increased to a maximum. In a second experiment, axial loading (s z) was held constant at 35 MPa while side loading (s w ) was increased in a series of steps (Fig. 7). During loading, acoustic emission was continuously recorded and aliquots of air±gas mixture were taken periodically for chromatographic analysis. As both acoustic factors and gas behaviour are associated with crack formation, they depend on loading values. Although gas was sampled only periodically rather than continuously, it is clear that the highest gas release rate takes place prior to complete failure of the sample, when dilatancy (prefailure inelastic volume increase due to development of cracks) occurs (Honda et al., 1982; King, 1986). This was observed at s w ~300 MPa. During this period, CH4/H2 and CH4/C2H6 ratios increased, possibly due to release of IG with high

Fig. 6. Changes in Rn emission and g radiation with increasing side (s w ) stresses during laboratory loading of nepheline±syenite in a three-axial compression device.

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CH4 contents into the, previously FG dominated, fracture system. This release re¯ects opening of ¯uid inclusion arrays once stress levels get high enough. The loading pressure in the system subsequently rapidly decreased to zero due to sample failure, partial depressurization of the borehole working chamber occurred and gas concentrations decreased. This increase in CH4/H2 contents with increasing strength is consistent with data from Voitov et al. (1990, 1992) who showed that the chemistry and isotopic composition of the FG gases may change in response to a blast-induced change in rock permeability. In a sealed 20 m long subhorizontal shot-hole, in an apatite±nepheline mine at Khibiny, CH4 and H2 contents changed from 49 to 66% and from 7.9 to 10.3%, respectively, over a period of days and 13C from 27.7 to 214½ PDB over a period of 3 h. In a 1.8 m long sealed shot-hole in the Lovozero mine, CH4 contents increased from 30.7 to 38.3%, H2 contents increased from 54.3 to 62.6%, and 13C composition changed from 27.1 to 215.7½ PDB over a period of 4 days. In both these cases, the CH4/H2 ratio increased, probably due to mixing of different ¯uid types, possibly involving leaking of hitherto sealed IG gases into the fracture system. The change in isotopic composition could be due to mixing gases of different composition or to reaction between gases and freshly fractured wall rocks. These observations allow us to comment on relationships between rock dynamics and gas release characteristics. Unlike hydrous phases, gaseous phases do not transmit stresses from the rock matrix, but, being under pressure, can transfer them to adjacent rock volumes. Zones under tensional stress will permit migration of gases into them from surrounding zones of compressive stress, leading to an increase in gas pressure in microfractures, which will further weaken the rock strength. It is well known that recrystallization of rocks and formation of ¯uid inclusions are controlled by local stresses and lead to an increase in intragranular strains (Shepherd, 1990; Kovderko, 1995). Similarly, the presence of gas-bearing ¯uid inclusions encourages strain concentrations that will favour formation of fractures and zones of ductile weakening (Uspenskaya, 1983; Shepherd, 1990). During fracturing, the IG can be freed from ¯uid inclusions and mix into the FG, although this is most likely to occur in signi®cant amounts only

under conditions of high stress when rapid dilatancy precedes sample failure. As palaeostrains in crystalline rocks can be retained for hundreds of millions of years (e.g. Kovderko, 1995), the distribution and orientation of secondary ¯uid inclusion arrays may be a useful tool in the analysis of palaeostress directions (Shepherd, 1990). Clearly, a knowledge of stress directions indicated by the ¯uid inclusion arrays would be important when modelling likely rockbursts and safety countermeasures. Here, a selection of data, some from Nivin and Belov (1993) and some from this study, are used to document the relationships between IG and RFG chemistries and the elastic-strength properties of the Khibiny and Lovozero rocks. Gas (CH4, H2, C2H6, He and CO2) concentrations and component ratios, and elastic wave data, Young's (elasticity) modulus, Poisson's (lateral deformation) ratio, rock compressive and tensile strength limits, and the brittleness ratio were determined in rock and ore samples. Table 1 summarizes a number of relationships between the contents of different gas types and some physical properties of the rocks. As in other cases (King, 1986; Nagamine, 1994; Sugisaki et al., 1996), individual gas ratios and concentrations of minor gaseous components, although not the overall gas content, appear to be the most sensitive potential indicators of geomechanical state. Correlation between concentrations of different gas components with elastic wave velocities and Young's modulus suggest that a decrease in He contents and IG volumes, and an increase in CH4/He and H2/He ratios are linked to an increase in elasticity and rock strength. High CH4/ C2H6 ratios and low H2 contents in RFG might also re¯ect an increase in rock strength. An increase in rock weakening with increasing compressional strength is matched by decreased levels of CH4 and C2H6 and increased He concentrations in IG gases. An increase in the CH4/C2H6 ratio in IG and RFG, CH4 proportion in RFG and decreases in C2H6 levels in RFG can be related to a reduction in rock tensile strength and a tendency to formation of tension fractures. The most brittle rocks prone to failure in the form of rockburst are characterized by higher CH4 contents and lower H2 contents in IG gases in the Lovozero massif and higher CH4/C2H6 ratios in RFG gases of the Khibiny massif. The data outlined here allow us to suggest the

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241

Table 1 The relationship between gases and geomechanical parameters of rocks from the Khibiny and Lovozero massifs: correlation coef®cients valid at the 5% level Vp Khibiny IG (n ˆ 47) CH4 (c) CH4 (p) H2 (c) 0.31 C2H6 (c) He (c) 2 0.45 He (p) 2 0.61 x CH4/H2 CH4/He 0.62 x H2/He 0.78 x Khibiny RFG (n ˆ 47) 2 0.34 x CH4 (c) CH4 (p) 2 0.35 x H2 (c) H2 (p) C2H6 (c) 2 0.29 x CO2 (c) 0.33 x CH4/C2H6 2 0.36 x CH4/H2 CH4/He CH4/CO2 2 0.33 x H2/He 0.32 Lovozero IG (n ˆ 13) CH4 (p) H2 (p) C2H6 (p) He (c) 2 0.63 x He (p) 2 0.74 x CH4/C2H6 0.70 CH4/He H2/He 0.68 x Lovozero RFG (n ˆ 13) CH4 (p) H2 (c) H2 (p) C2H6 (p) CO2 (c) CO2 (p) CH4/C2H6 0.67 H2/He

m

Vs

0.30 0.32 2 0.37 0.33 0.44

0.36

sc

E

0.43 0.42 x 2 0.31 2 0.39 0.38 x

2 0.32 x

2 0.30 2 0.33 0.29 0.40

0.30

2 0.38 x

0.32

s c/s t 0.35 x 0.34

0.41 x

2 0.38 x

2 0.36 x 0.74 0.34 x

0.36 2 0.34 0.29 x

0.34

2 0.62

st

0.61

0.73 2 0.65

2 0.69 x 0.59 0.66

0.63 2 0.57 0.87 0.88 x

0.85

0.84 2 0.83

2 0.91

2 0.73

0.60 x

2 0.82 2 0.79 x 0.71 0.74 2 0.84

Notes and conventional symbols: n ± number of samples; x ± Spearman coef®cient; (c) ± content of a component in a rock; (p) ± proportion of a component in the gas mixture; Vp and Vs ± longitudinal and shear elastic wave velocities; m ± Poisson's ratio; E ± Young's modulus; sc and st ± compressive and tensile strength; sc/st ± brittleness ratio.

following tentative relationships between the gas phase and the geomechanical properties and state of a rock mass. At the immediate, post-magmatic, subsolidus range of alkaline igneous complexes and

associated ore deposits, recrystallization occurred accompanied by hydrogen±hydrocarbon gas phase generation due to a series of sub-solidus Fischer± Tropsch reactions (Potter et al., 1998). Some of

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these gases are contained in ¯uid inclusions and isolated micro-pores (IG). A further part ®lls more extended cavities, mainly microfractures, developed within the massif during cooling (FG). Orientation and alignment of ¯uid inclusion arrays and gas-®lled microfractures probably re¯ect the main stress directions during immediate sub-solidus cooling. Over time, H2 contents likely increased in the FG due to reducing reactions on fracture surfaces. Subsequent to crystallization and cooling, the rock mass was capable of supporting the permanent stresses imposed upon it by the included gas phases. Changes in this long-term state will occur during natural, or mining-induced seismic events, resulting in the, possibly non-uniform, redistribution of zones of compressions and tension. If new fractures form during such events, there will be an enhanced mobility of volatile phases, together with changes in gas chemistry and isotopic composition due to chemical reactions with newly formed rock surfaces. On a local scale, where stresses are high enough to open ¯uid inclusion arrays, IG gases may bleed into fractures and dilute FG gases. On a larger scale, gases from different locations may mix and newly connected fractures will provide pathways for ¯ow of these mixed gases from deeper parts of plutonic complexes. This mobility may be indicated by variations in gas pressure and discharge rates in shot-holes, boreholes and mine workings. 6. Concluding remarks Evidence from in situ observations in apatite± nepheline and rare-metal mines, as well as from laboratory experiments, show clear relationships between the geodynamic characteristics of igneous rock massifs and processes of gas release in response to stress. In the Khibiny and Lovozero massifs, HHCG release is clearly related to the effects of blasting on the stressed state of the rock mass. The relationships between stress state and gas release are non-trivial. Partly, this is because fracture connectivity may be localized with a number of zones of externally closed systems of connected fractures. Partly, it is because released gases may react with new fracture surfaces to generate enhanced levels of reduced H2 gases. However, measurements show that gas compositions do vary with time during stress build-up. That

included gas (IG) compositions are different from fracture gas (FG) compositions, suggests that gas released during early stages of stress build-up on a rock mass will probably have a different composition from gas released during later stages. This is simply because fracture ®lling gas will be released at lower stress levels than gases contained in inclusions and isolated voids, which will only be released when stresses are high enough to open ¯uid inclusion arrays. Therefore, temporal variations in concentrations and proportions of individual gas components and their ratios show future promise as indicators of the stressed state of a rock mass, and may be used as indicators of potentially dangerous, dynamic processes such as spalling, bumps, rockbursts and mine-induced earthquakes. Study of the relationships between gas behaviour and the mechanical state of igneous rocks could, therefore, form an important safety measure during underground mining and construction projects, as well as assisting in the identi®cation and mapping of active tectonic fault zones and in predicting seismic events.

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