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AN INTRUSIVE ORIGIN FOR THE KOMATIITIC DUNITE-HOSTED MOUNT KEITH DISSEMINATED NICKEL SULFIDE DEPOSIT, WESTERN AUSTRALIA

N. M. Rosengren^,* and S. W. Beresford

School of Geosciences, P.O. Box 28E, Monash University, Clayton, Australia 3800

B. A. Grguric

WMC Exploration, P.O. Box 91, Belmont, Western Australia 6984

R.A.F. Cas

School of Geosciences, P.O. Box 28E, Monash University, Clayton, Australia 3800

Corresponding author: e-mail, Nic.Rosengren{at}wmc.com

	Abstract

Top Abstract Introduction Geologic Setting Contact Relationships Discussion and Conclusions Conclusions References

 
The MKD5 nickel deposit at Mount Keith is hosted within the Mount Keith Ultramafic Complex, a thick komatiitic dunite body previously interpreted as either a large volume lava flow or as a dikelike intrusion. The upper contact relationships of the dunite body are critical for the evaluation of an extrusive versus intrusive origin. New drill core examined during this study has revealed preserved upper contact relationships between the Mount Keith Ultramafic Complex and the enclosing dacitic volcanic rocks. These contacts have lobate geometries with apophyses of the ultramafic material intruding the overlying dacite and dacitic xenoliths within the ultramafic rock along all observed margins. These contact features indicate an intrusive relationship between the Mount Keith Ultramafic Complex and the enclosing stratigraphy, which is consistent with the lack of definitive extrusive features. Our new interpretation of the Mount Keith Ultramafic Complex suggests that thick komatiitic dunite bodies may be regarded as subvolcanic sills emplaced within and below an extrusive komatiite pile. Importantly this model implies that komatiitic dunite bodies are not an integral or even necessary feature of a komatiite flow field.

	Introduction

Top Abstract Introduction Geologic Setting Contact Relationships Discussion and Conclusions Conclusions References

 
Large dunite and peridotite bodies of komatiitic affinity occur in several localities worldwide and are largely restricted to Archean greenstone belt terranes. Examples include the Mount Keith, Perseverance, and Six Mile Well Complexes of the Yilgarn craton in Western Australia (Burt and Sheppy, 1975; Naldrett and Turner, 1977; Dowling and Hill, 1990, unpub. report for CSIRO, 1993), the Dumont sill in the Abitibi province, Canada (Eckstrand, 1975; Duke, 1986), and Hunters Road, Zimbabwe (Prendergast, 2001). All of these bodies host low-grade, disseminated Fe-Ni-(Cu) sulfides. The MKD5 deposit at Mount Keith, which has been mined since 1994, is currently the largest nickel producer in Australia. These low-grade deposits (type IIb in the classification of Lesher and Keays, 2002) are significantly different from komatiite-hosted massive sulfide deposits, as exemplified by the Kambalda ore field system (type I of Lesher and Keays, 2002). The latter are associated with well-developed spinifex-textured komatiites and are interpreted as komatiitic lava flow fields (e.g., Gresham and Loftus-Hills, 1981; Lesher et al., 1984; Cowden, 1988; Hill et al., 1995; Cas et al., 1999; Beresford et al., 2002).

In contrast to type I deposits, the mode of emplacement of type II dunite bodies was originally interpreted as either horizontally emplaced sills, e.g., Dumont (Eckstrand 1975; Duke 1986) and Six Mile Well (Naldrett and Turner, 1977), or vertically emplaced dike complexes, e.g., Mt Keith (Burt and Sheppy, 1975). Later work by Donaldson et al. (1986), Barnes et al. (1988), Dowling and Hill (1990, unpub. report for CSIRO), and Hill et al. (1990) led to a reinterpretation of the dunite complexes of the Agnew-Wiluna greenstone belt as giant lava channels capable of thermomechanically eroding their substrates. The lava channels were interpreted to represent the more proximal facies of Kambalda-style komatiite flow fields (Hill et al., 1995). On this basis these workers reinterpreted the Mount Keith, Six Mile, and Perseverance dunite bodies of Western Australia and associated nickel deposits as products of large-scale volcanism. However, the Agnew-Wiluna greenstone belt is a polydeformed, highly faulted belt that has locally been metamorphosed to middle amphibolite facies (middle greenschist facies at Mount Keith: Barrett et al., 1977), making interpretation of original lithological types and textures difficult and interpretations of emplacement origins even more so. Contact relationships, in particular, are commonly deformed, and primary contacts are rare. Since the original work of Burt and Sheppy (1975) and the more recent work by Dowling and Hill (1990 unpub. report for CSIRO, 1993) and Hill et al. (1990), the drill hole and geochemical database for the Mount Keith area has been augmented by a large number of additional drill holes and more comprehensive whole-rock geochemical data, which resulted from the start of mining of the MKD5 deposit in 1994 and a requirement for a continual refinement of the ore reserve and resource definition.

This paper utilizes the additional drill holes to identify and document previously unrecorded, rare, low-strain primary contact relationships that establish the emplacement mechanism of the Mount Keith Ultramafic Complex. The vast majority of contacts between the Mount Keith Ultramafic Complex and the enclosing units are highly strained and contact relationships have been overprinted. This communication is a preliminary account of work in progress reassessing the contact relationships, internal architecture, and genesis of the Mount Keith nickel sulfide deposit. The aim of this communication is to (1) show that in low-strain domains around lenticular dunite bodies, structurally unmodified primary contact relationships can be identified; (2) document the textural and geometric features of these primary contacts, at micro- and macroscopic scales; and (3) provide an initial interpretation of the described textures, contact relationships, and geometries in terms of the emplacement origin of the Mount Keith Ultramafic Complex.

	Geologic Setting

Top Abstract Introduction Geologic Setting Contact Relationships Discussion and Conclusions Conclusions References

 
The Mount Keith Ultramafic Complex occurs within the Agnew-Wiluna greenstone belt, which is located within the Eastern Goldfields province in the eastern part of the Archean Yilgarn craton of Western Australia (Hill et al., 1990). The Agnew-Wiluna greenstone belt comprises the northern portion of the Norseman-Wiluna greenstone belt, which has a strike length of approximately 650 km. The Agnew-Wiluna greenstone belt trends in a north-northwest–south-southeast direction and is bounded by large- to terrane-scale faults and granitoid bodies (Fig. 1). The Mount Keith-Cliffs-Sarah’s Find area is located in the most attenuated portion of the Agnew-Wiluna greenstone belt, approximately 80 km south of Wiluna, where it has a maximum thickness of about 6 km. The belt is bound to the west and east by voluminous Archean granitoids. The greenstone sequence in the Mount Keith area includes three ultramafic horizons, locally designated the Mount Keith, Cliffs, and Monument ultramafic units (Fig. 1). These three ultramafic horizons can be traced 30 km north of Mount Keith to the Honeymoon Well nickel deposit and 9 km south to the Cliffs-Charterhall nickel deposit. Beyond these limits, complexities in the structural architecture produce uncertainties in the definition of the belts.

View larger version (51K): [in this window] [in a new window]   FIG. 1. Local geology and stratigraphy of the Mount Keith region, outlining the three ultramafic belts and intercalated mafic and/or felsic volcanic rocks. The MKD5 nickel deposit is hosted within the Mount Keith Ultramafic Complex (modified from Dowling and Hill, 1990 unpub. report for CSIRO).

All three ultramafic units dip steeply (and locally subvertically) to the west. Igneous textures and geochemical trends indicate a west-facing orientation for the Mount Keith and Cliffs ultramafic units (Naldrett and Turner, 1977; Dowling and Hill, 1990, unpub. report for CSIRO). Local east-facing orientations and shallow dips within the Monument ultramafic unit are attributed to a shallowly plunging, tight to isoclinal synclinal fold (Bongers, 1994).

The MKD5 nickel deposit is a type IIB nickel deposit as defined by Lesher and Keays (2002) and is hosted in a komatiitic dunite and/or peridotite pod, which forms part of a zone of substantial thickening in the Mount Keith ultramafic unit. The pod was completely serpentinized and altered to talc and carbonate during a retrograde fluid infiltration event that followed metamorphism to middle greenschist facies (Barrett et al., 1977; Rödsjö and Goodgame, 1999). Fe-Ni-(Cu) sulfides occur interstitial to former olivine grains with an average abundance of 3 to 5 vol percent, and the deposit has a current resource of 503 Mt at 0.55 percent Ni. Although the host rocks of the MKD5 mineralization have been extensively serpentinized, the textures and premetamorphic composition of the ores are consistent with a magmatic origin for the mineralization (Groves et al., 1979; Donaldson, 1981). The typical sulfide mineralogy comprises pentlandite, pyrrhotite, and pyrite with minor millerite, hypogene violarite, godlevskite, and heazlewoodite (Grguric, 2002).

Within the study area, the three ultramafic horizons are enclosed by a sequence of variably deformed and altered felsic and mafic volcanic rocks (Fig. 1). The Cliffs and Monument ultramafic unit horizons are broadly enclosed by mafic units of basaltic composition, which are generally very fine grained and deformed and preserve no obvious igneous features. The footwall to the Mount Keith ultramafic unit comprises monotonous coherent to in situ fragmented, dacitic, phenocryst-rich, and vesicular lithological units. The jigsaw fit of the clasts in the brecciated units and curviplanar nature of their edges indicates that they are hyaloclastites produced by quench fragmentation of lavas upon contact with cold seawater (Pichler, 1965). On this basis the dacitic units are interpreted as submarine, autobrecciated to quench fragmented lavas. Lateral and vertical facies variations have been previously identified by Heptinstall (1991) and Palich (1994).

The hanging-wall lithologies of the Mount Keith ultramafic unit immediately to the north of the MKD5 deposit and in the vicinity of Shed Well and Sarah’s Find consist of dacitic rocks geochemically and texturally identical to those of the footwall sequence. The hanging wall to the Mount Keith ultramafic unit at the MKD5 deposit consists of a thin pyritic chertlike unit overlain by fine-grained foliated mafic units followed by the Cliffs ultramafic unit. The Cliffs ultramafic unit is in faulted contact with the Mount Keith ultramafic unit in the southern portion of the MKD5 pit and at several locations along strike between the MKD5 and Cliffs-Charterhall deposits (Fig. 1).

To the north of the MKD5 deposit the footwall and hanging-wall lithologies are compositionally and texturally identical, representing the same coherent to in situ fragmented dacitic to andesitic phenocryst-rich lithologies present in the footwall. This relationship has also been recognized south of the MKD5 deposit toward Shed Well and Golgotha (Hepinstall, 1991; Palich, 1994). The primary contacts described in this paper indicate that this stratigraphy (dacitic lavas and associated fragmental facies) represents the primary sequence into which the Mount Keith ultramafic unit was emplaced. In the vicinity of the MKD5 pit, the hanging-wall sequence to the Mount Keith ultramafic unit is complicated by faulting, which appears to have juxtaposed the original felsic hanging-wall sequence and the more mafic rocks. The mafic and felsic sequences present in the hanging wall are separated by a layer of highly deformed pyritic chert, apparently the locus of detachment along which faulting occurred.

	Contact Relationships

Top Abstract Introduction Geologic Setting Contact Relationships Discussion and Conclusions Conclusions References

 
In the course of this investigation, 49 diamond drill cores were logged. Several cores between the northern end of the MKD5 deposit and Sarah’s Find preserve primary hanging-wall contacts. All of these contacts occur within the low-strain zones. Figure 2 shows the location of three representative drill holes (MKG47, MKG59, and MKD288) in relationship to the Mount Keith ultramafic unit and hanging-wall sequence. Figure 3 shows generalized lithological logs of the drill holes and a representative geochemical profile of drill hole MKG59. Figure 4 contains detailed photographs of the contact relationships observed in drill core. Each of these key drill holes and relevant contact relationships are discussed below.

View larger version (173K): [in this window] [in a new window]   FIG. 2. A. Schematic architecture and geology of the MKD5 deposit, showing drill hole locations. Boxed area indicates northwest corner, highlighting large-scale ultramafic apophyses in overlying dacite. Dashed lines indicate tectonized contacts around the thickened Mount Keith Ultramafic Complex. B. and C. Photographs of pit exposures of ultramafic apophysis in the dacite hanging wall. Note the lobate contacts and partial enclosure of a dacite xenolith. D. Ultramafic apophyses in dacite footwall to the MKD5 deposit. Their location is indicated by the rectangle on the lower contact of the MKD5 deposit in A.

View larger version (53K): [in this window] [in a new window]   FIG. 3. Details of drill holes described in the text. A. Schematic log for drill hole MKG47. B. Schematic log for drill hole MKD288. C. Geochemical profile and schematic log and photos for drill hole MKG59. Note that the peaks in Al2O3 correspond with bastite (after oikocrystic pyroxene-rich orthocumulates). OAD= olivine adcumulate, Omc = olivine mesocumulate, Ooc = olivine orthocumulate.

View larger version (173K): [in this window] [in a new window]   FIG. 4. A. Ultramafic-dacite contact from drill hole MKG47. Note the planar contact between the ultramafic and dacite units and the inclusion of a dacitic pendant within the ultramafic rock. B. Contact between ultramafic and dacite in drill hole MKD288. Note the angular sawtooth texture of the contact. C. Dacitic xenolith below the contact in drill hole MKD288. Note the resorbed edges and crosscutting ultramafic material. D. Entire contact between ultramafic and dacite in drill hole MKG59. Note the lobate apophysis of ultramafic material to the left of the photograph and the apparent zone of mixing between dacite and ultramafic material.

Macroscopically, the hanging-wall contact is a very sharp, slightly undulating surface, with a thin selvage (2–3 mm) of extremely fine grained chlorite and/or serpentine against the dacite surface and some microveinlets of serpentine penetrating a short distance into the dacite (Fig. 4A). Although the main ultramafic and/or dacite contact appears relatively planar, the presence of a dacitic pendant or xenolith within the ultramafic unit in the section of drill core examined (Fig. 4A) indicates that in three dimensions the contact surface is highly irregular. This pendant is rimmed by a similar thin selvage of fine-grained chlorite and/or serpentine. Although the contact appears macroscopically sharp, thin section examination reveals numerous small apophyses of ultramafic material in the dacite. The dacitic pendant below the contact is mineralogically identical to the dacite hanging-wall material and is composed of variably saussuritized plagioclase phenocrysts in a fine-grained quartz and feldspar groundmass. No evidence of a shear fabric or foliation was observed.

Drill hole MKD288
Drill hole MKD288 penetrates three lithological intervals: an upper mafic unit overlies a sequence of coherent and clastic dacitic and/or andesitic material that is in contact with the Mount Keith ultramafic unit. The upper mafic unit is a very fine grained, high Fe tholeiite. The mafic unit is featureless apart from a moderate foliation. It is separated from the underlying dacitic sequence by a sheared 5.5-m-wide variably pyritic chert horizon that can be traced south along the hanging-wall contact in the main area of the MKD5 deposit. The dacitic horizon underlying the pyritic chert is a dominantly coherent, porphyritic unit with fragmental facies developed at certain points within the unit. This unit is equivalent to the dacitic unit present at the hanging-wall contact in drill holes MKG59 and MKG47. The hanging-wall–Mount Keith ultramafic unit contact occurs at 345.6 m, followed by 165 m of underlying ortho- to mesocumulate-textured ultramafic rock.

Macroscopically and microscopically, this contact is completely unstrained. The dacite is coherent and porphyritic with phenocrysts of plagioclase and rare actinolite pseudomorphs after igneous pyroxene. The contact has a serrated form with wedgelike fingers of serpentinite invading the dacitic host unit (Fig. 4B). The upper portion of the contact has a sawtooth appearance, defined by veins of serpentinite, surrounded by alteration halos that penetrate the dacitic unit (Fig. 5A). Approximately 20 cm below the main contact is a dacitic xenolith, which is cut by veins of serpentinized ultramafic material. The xenolith has resorbed edges and igneous plagioclase crystals are still faintly visible (Fig. 4C).

View larger version (84K): [in this window] [in a new window]   FIG. 5. A. Photomicrograph of the ultramafic-dacite contact in drill hole MKD288, showing small-scale lobate interfingering between dacite and ultramafic material. Note the feldspar phenocryst in the dacite unit. B. Photomicrograph of stellate actinolite within the ultramafic apophysis, which is part of the drill hole MKG59 contact.

The hanging-wall contact zone starts at 298.6 m and in contrast to drill hole MKG47 consists of a 20-cm zone, showing a mutual lobate boundary texture between Mount Keith ultramafic unit and the dacite. There is no evidence of any structural modification or overprint. The upper limit of the contact is the last appearance of serpentinite and occurs at 298.8 m, where there is a globular apophysis of ultramafic material partially enclosed by dacite (Fig. 4D). The remainder of the contact zone consists of a continuous body of dacite, penetrated by lobes of serpentinite. Microscopically, the ultramafic rock consists of finely intergrown lizardite and actinolite laths with patchy segregations of chlorite flakes. The upper globular zone contains abundant stellate actinolite laths (after pyroxene), indicative of a chilled margin (Fig. 5B). Within the lower portions of the contact no unequivocal primary textures were identified, however, the vaguely rectangular shape of chlorite-rich domains suggests that they are pseudomorphs after pyroxene. As is the case for drill hole MKG47, no microscopic evidence of a shear fabric or foliation was observed in thin section.

	Discussion and Conclusions

Top Abstract Introduction Geologic Setting Contact Relationships Discussion and Conclusions Conclusions References

 
The Mount Keith Ultramafic Complex has been interpreted as a dikelike intrusion (Burt and Sheppy 1975), a sill-like intrusion (Naldrett and Turner, 1977), and more recently as a giant lava river by Donaldson et al. (1986), Dowling and Hill (1990 unpub. report for CSIRO), and Hill et al. (1995). The interpretations of an extrusive origin are based on two essential points: the Mount Keith ultramafic unit adcumulate pod exhibits both a vertical and lateral gradation to spinifex-textured komatiite flows (Hill et al., 1990), and the lensoidal thickening of the Mount Keith Ultramafic Complex represents thermal erosion of the underlying felsic stratigraphy (Hill et al., 1990).

Upper contact relationships have not been considered in previous work, possibly due to lack of drill core at the time of investigation and poor preservation and shearing evident in the available drill core. Extensive core logging in this study has established that nowhere within the Mount Keith area does the Mount Keith ultramafic unit show a gradational transition to spinifex-textured komatiite units; however, in places to the south of Mount Keith (eg., southern end of the MKD5 to the Cliffs deposit; Fig. 1) the spinifex-bearing Cliffs ultramafic unit is in faulted contact with the Mount Keith ultramafic unit. Detailed examination of the geometry of the MKD5 deposit and its enclosing units as part of this study indicates that there is little evidence for downcutting into the footwall stratigraphy by the Mount Keith ultramafic unit. A similar lack of evidence for this feature has been noted for the Perseverance dunite to the south (Trofimovs et al., 2003).

We propose that the contact relationships outlined here for the Mount Keith ultramafic unit are indicative of an intrusive relationship between it and the enclosing stratigraphy. There are numerous features of each contact which are consistent with this interpretation: (1) the irregular geometry of the upper contact (in both drill holes MKD288 and MKG59 highly irregular upper contacts are preserved; in drill hole MKG47 the upper contact is essentially planar, but the presence of a dacitic pendant suggests that it is highly irregular on a larger scale); (2) xenoliths of dacite occur within the Mount Keith ultramafic unit along upper contacts, (i.e., hanging-wall material has been incorporated); (3) all three cores have definite graded, finer grained margins developed in the ultramafic material at the contact, indicating that the Mount Keith ultramafic unit chilled against the dacitic unit; (4) small-scale apophyses of ultramafic material project into the overlying dacite along the upper contact, and these could only have formed by upward intrusion of the Mount Keith ultramafic unit into the dacite; (5) no definitive extrusive textures have been observed (i.e., no contacts exhibit development of spinifex textures or flow top breccias). The last observation is true for all contacts (sheared and unsheared) and the Mount Keith ultramafic unit in general. All contacts described here show no evidence of structural modification, as they have been shielded from deformation due to the strain shadow created by the thickened Mount Keith Ultramafic Complex (Fig. 2). The contact in drill hole MKG59 (Fig. 3D) is a particularly good example of an intrusive geometry and it is difficult to envisage how such a lobate geometry could have been produced by extrusive flow processes. Each contact has a unique geometry; however, in all cases the geometry indicates that the hanging-wall material was enveloped by the intruding ultramafic material.

The presence of hanging-wall xenoliths of dacitic clasts is a consistent feature and several other cores examined (drill holes MKD313, MKD 256, MKD260) have contact zones up to 20 m wide containing clasts of dacitic hanging-wall material in varying states of assimilation. This indicates that the ultramafic material was hot enough to melt and partially consume the enclosing dacitic unit and that some degree of thermal and/or mechanical incorporation occurred at the upper contact of the Mount Keith ultramafic unit. This interaction with the dacite at the upper contact indicates an intrusive origin for the Mount Keith ultramafic unit. The incorporation of felsic material into ultramafic magmas is considered to be an important part of ore genesis (Lesher and Burnham, 2001), which may assist in driving the system to sulfur saturation. An intrusive origin for the MKD5 deposit may have allowed it to more efficiently interact with the country rock at both the upper and lower contacts.

The large-scale geometry of the northwest corner of the MKD5 deposit is also indicative of an intrusive origin. Figure 2 illustrates the complex geometry of this area, which comprises numerous large-scale apophyses of ultramafic rock (all samples analyzed fall between 15–20% MgO) into the dacite unit. These apophyses are fine grained and show no extrusive textural features and often contain dacitic xenoliths. The contacts are variably preserved; some are slightly sheared, but the majority are undeformed.

New observations of preserved upper contact relationships outlined in this paper, combined with the numerous occurrences of dacitic xenoliths along the upper margin of the Mount Keith ultramafic unit, indicate that the ultramafic complex was emplaced as a large sill into a felsic-intermediate lava and volcaniclastic sequence. A reinterpretation of the Mount Keith Ultramafic Complex as an intrusive body has implications for the stratigraphic order in the area and opens the way for reevaluation of the larger scale processes involved in the assembly of the Agnew-Wiluna greenstone belt. As most deposits in the Agnew-Wiluna greenstone belt are structurally and/or stratigraphically controlled, understanding the stratigraphic order and assembly of the belt is of paramount importance for exploration. The geometry and internal complexity of the Mount Keith ultramafic unit varies along strike, between the MKD5 and Sarah’s Find deposits (Fig. 2). The Mount Keith ultramafic unit becomes thinner and has less internal complexity, which, along with lithological indications of a period of static crystallization following emplacement (i.e., pyroxenites and gabbros), indicates a transition from a dynamic to a tranquil sill. This represents an excellent opportunity to investigate the dynamics and processes involved in the assembly of a large sill complex.

If the large sill model for the Mount Keith Ultramafic Complex applies elsewhere, this significantly changes interpretations of the way in which komatiite provinces are reconstructed. In an extrusive model, dunite units are interpreted as the proximal equivalents of both thick and thin compound flow units (e.g., Kambalda) in an extensive komatiite flow field (e.g., Hill et al., 1995). However, in an intrusive model the dunitic bodies are subvolcanic sills, possibly comagmatic with the overlying komatiite flow field. Our new interpretation of the Mount Keith Ultramafic Complex suggests that the thick komatiitic dunite was emplaced as a subvolcanic sill, within and below an extrusive komatiite pile. Similar facies associations have been documented by Prendergast (2001, 2003) for Zimbabwean komatiite provinces and are common in flood basalt provinces (e.g., Noril’sk, Naldrett et al., 1992). An intrusive model for komatiitic dunites indicates they are not an integral or even necessary feature of a komatiite flow field, raising important questions about the architecture and emplacement styles of such flow fields.

	Conclusions

Top Abstract Introduction Geologic Setting Contact Relationships Discussion and Conclusions Conclusions References

 
The Mount Keith dunite is reinterpreted as a large intrusive sill, based on the irregular geometry of primary upper contacts, xenoliths of hanging-wall material along upper contacts, apophyses of ultramafic rock intruding into the overlying dacite along the upper contact, and the lack of definitive extrusive textures (i.e., no contacts exhibit development of spinifex textures or flow-top breccias). As a consequence of this work the origin of other komatiitic dunite bodies may need to be reassessed.

	Acknowledgments

 
This work forms part of an Australian Research Council funded large grant to R. Cas to investigate the nature of komatiite-felsic associations and interaction. Particular thanks to WMC Resources Ltd. and the staff at Mount Keith Operations for logistical and scientific support during field work.

January 5, July 7, 2004

	Footnotes

 
^* Present address: WMC Exploration, P.O. Box 91, Belmont, Western Australia 6984.

January 5, 2004; July 7, 2004

	References

Top Abstract Introduction Geologic Setting Contact Relationships Discussion and Conclusions Conclusions References

 

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