Rare earth elements and hydrothermal ore formation processes

Ore Geology Reviews, 7 (1992) 25-41 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

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Rare earth elements and hydrothermal ore formation processes B.G. Lottermoser Institut fur Geowissenschaften, UniversittitMainz, Postfach 3980, 6500 Mainz, Germany (Received February 6, 1991; accepted after revision June 14, 1991 )

ABSTRACT Lottermoser, B.G., 1992. Rare earth elements and hydrothermal ore formation processes. Ore Geol. Rev., 7:25-41. The geochemical behaviour of REE is influenced by nearly all important hydrothermal ore formation processes including fluid-rock interactions, fluid precipitations, adsorption and scavenging onto particles, and changes in fluid temperature, pressure, pH, Eh, alkalinity and ligand concentration. Destabilization of REE complexes in response to these physicochemical changes and possible chemical-crystallographic controls determine the concentration and distribution of REE within hydrothermal minerals, mineraloids and amorphous phases. Alteration assemblages of intrusive related mineralisations may exhibit a wide range of REE distributions and REE fractionation trends. The REE distribution within these hydrothermally altered lithologies will depend on the REE concentration in the rock and the fluid, the partitioning behaviour of the REE between the rock phases and the fluid, and the types of alteration reactions which take place. Reactive rocks such as carbonate-rich lithologies of skarn deposits may gain or loose significant amounts of REE during fluid-rock interactions. REE geochemical investigations combined with fluid inclusion and stable isotope studies may point to the origin of alteration products and the source (s) of ore fluids. Ancient massive sulphide ores commonly exhibit anomalous Eu concentrations similar to many recent submarine hydrothermal precipitates. The recognition of hydrothermal fluid, seawater, sedimentary or volcanically derived REE distributions within ores, chemical sediments and hydrothermally altered lithologies may help to constrain genetic modelling of massive sulphide deposits. Care should be taken with the interpretation of REE distributions from metamorphosed hydrothermal ore deposits because metamorphic and diagenetic alteration reactions may change the REE distributions. Mobility of REE may occur during diagenesis of carbonate-rich rocks and during the development of shear zones and migmatites. In contrast, very large fluid-rock ratios are necessary to cause significant changes in REE patterns of silicate-rich rocks during diagenesis and regional metamorphism.

Introduction

The term "rare earth elements" is variably applied in the chemical and geochemical literature to the group of elements from La (atomic number 57) to Lu (atomic n u m b e r 71 ), from Ce (atomic n u m b e r 58) to Lu, or from La to Lu plus Y (atomic number 39) and Sc (atomic number 21 ). Thus, a general agreement for the usage of the term "rare earth elements" has not been reached. In addition, the term "lanthanons" refers to the elements La to Lu plus Y, and the terms "lanthanides" and "lanthanoids" are used either for the elements Ce to Lu,

or for La to Lu. This paper is concerned with the elements La to Lu where they are referred to as rare earth elements (REE) to conform with the bulk of past and current geochemical literature. The application of REE abundances to petrogenetic problems has centred so far on the evolution of igneous rocks, where such processes as partial melting of crustal or mantle material, fractional crystallization a n d / o r mixing of magmas are involved (e.g., see summary by Henderson, 1984). In addition, REE studies have been used to investigate sedimentary processes and the chemistry of the oceans

0169-1368/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

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B.G. LOTTERMOSER

(e.g., see summary by Fleet, 1984), and to determine the evolution of the continental crust (e.g., see summary by Taylor and McLennan, 1985). In contrast, there have been surprisingly few studies with an emphasis on the potential of REE to provide information about ore formation processes. Mineyev (1963) and Borodin ( 1967 ) pointed to the potential of REE to supplement and to complement geochemical studies of ore deposits. However, only recently their usefulness in areas of ore genesis modelling and mineral exploration has widely been recognized (Whitford, 1983; Ganzeyev et al., 1984; Arvanitidis and Rickard, 1987 ). Detailed REE investigations have been performed in the past on metamorphic Au (Kerrich and Fryer, 1979), Mississippi Valley-type Pb-Zn-Cu (Graf, 1984), weathering REE-Nb-Ta-P-Ti-Zr (Lottermoser, 1990a), and stratabound U, W and Cu-Pb-Zn-Ag-Pt-Pd deposits (McLennan and Taylor, 1979; Fryer and Taylor, 1987; Pagel et al., 1987; Maas et al., 1987; Kwak and Abeysinghe, 1987; Hammer et al., 1988; Lottermoser, 1989a). In addition, the main thrust of the work has been on: (a) Intrusive related hydrothermal U, Sn, Cu, W, Au, Au-Ag, Pb-Zn, Cu-Mo, Cu-Ag, Ni-Cu-Fe, Fe-W-Cu and fluorite deposits (M/511er et al., 1976, 1979; Taylor and Fryer, 1980, 1982, 1983; Chatterjee and Strong, 1984; Wilton, 1985; Palacios et al., 1986; Dill et al., 1986; Ekambaram et al., 1986; Giuliani et al., 1987; Cathelineau, 1987; Fryer and Taylor,

1987; Pagel et al., 1987; Leroy and Turpin, 1988; Baker and Hellingwerf, 1988; Constantopoulos, 1988; Baker et al., 1988; Schneider et al., 1988; Oreskes and Einaudi, 1990; Lottermoser, 1990b; Eppinger and Closs, 1990; Vander Auwera and Andre, 1991 ); and (b) Massive sulphide Cu-Zn, Cu-Fe, Cu-PbZn, Pb-Zn and Mn-Fe deposits (Robertson and Fleet, 1976; Graf, 1977; Thurston, 1981; Campbell et al., 1982, 1984; Crerar et al., 1982; Whitford, 1983; Baker and De Groot, 1983; Strong, 1984; Windrim et al., 1984; Bence and Taylor, 1985; Arvanitidis and Rickard, 1986; Lesher et al., 1986a, b; Colley and Walsh, 1987; Hellingwerf, 1987; Patocka, 1987; Vance and Condie, 1987; Whitford et al., 1988; MacLean, 1988; Wonder et al., 1988; Schade et al., 1989; Lottermoser, 1988, 1989b). The principal objective of this article is to document the REE behaviour during the formation of hydrothermal ore deposits (i.e, ore deposits formed by hot aqueous solutions). The paper will highlight the use of REE distributions and REE patterns as an additional guide to ore genesis and will point to directions for future REE research. Reference will also be made to the general geochemical and mineralogical properties of REE and to the role of diagenesis and metamorphism in the behaviour of REE.

General geochemical properties The REE represent the 4f series of elements with the atomic numbers 57 to 71. After La the

BerndLottermoserreceiveda Ph.D. in geologyfrom the Universityof Newcastle,Australia, in 1990. From 1985, he workedperiodicallyas an explorationgeologistand as a consultingmineralogistfor severalminingcompanieson base-metal,goldand rare-earth element projects in Australia, Papua New Guinea and Brazil. He is currently research fellowat the Institute of Geosciences,Universityof Mainz, Germany.

RARE EARTH ELEMENTS AND HYDROTHERMAL ORE FORMATION PROCESSES

energy of the 4f level is lower than that of the 5d level and thus subsequent electrons are added to the inner shielded 4f orbitals (Moeller, 1963). As there are seven such orbitals, a total of fourteen elements then result before the 5d orbitals are filled. This accounts for the elements Ce to Lu and for their very similar physical and chemical properties. Only one of the REE, promethium (Pm, atomic number 61 ), is unstable and is not observed in the natural environment. The REE have slight differences in crystal field and ligand field energies, and exhibit bipositive (Ce, Nd, Sm, Eu, Tm, Yb), tripositive (all REE) and tetrapositive (Ce, Pr, Nd, Tb, Dy) oxidation states as shown by laboratory studies (Moeller, 1963; Moeller et al., 1965). Ionic radii of the REE decrease with atomic number from 1.03 angstr/Sm for La 3+ (atomic number 57) to 0.86 angstr/Sm for L u 3+ (atomic number 71 ) (for six-fold coordination; Shannon, 1976). Because of this contraction, the heavy REE (HREE, defined herein as Gd to Lu) have larger ratios of charge to atomic radius, possess higher ionization potentials and generally form stronger bonds than the light REE (LREE; La to Eu). Discontinuities in ionization potentials between the third and fourth (Nd-Sm), at the seventh (Gd) and between the tenth and eleventh (Ho-Er) 4f electron fillings are known as tetrad effects (Nugent, 1970; Siekierski, 1971 ). The most distinct discontinuity in the contraction and associated tetrad effect exists between Gd and Tb ( " G d break"). Evidence for the Gd break in geological environments has been found in the form of Gd and Tb anomalies on shale-normalized REE patterns by Roaldset ( 1974 ) and De Baar et al. ( 1985 ). The REE show a similar and coherent geochemical behaviour but with fractionation occurring. This is possibly the result of their particularly stable tripositive oxidation state in geological environments, and the wide range of types and sizes of cation coordination polyhedra in rock-forming minerals (Henderson,

27

1984). However, separation of individual REE occurs during geological processes and this is indicated by Ce and Eu deficiencies or excesses compared to their neighbouring REE concentrations on chondrite-normalized REE patterns. These excesses and deficiencies are commonly explained as reflecting oxidation of Ce to the tetravalent and reduction of Eu to the divalent state. Nucleii which have equal numbers of protons and neutrons are inherently more stable than those which have unpaired neutrons and protons. Therefore isotopes and elements of even atomic number (Ce, Nd, Sm, Gd, Dy, Er, Yb) are generally more abundant than those adjacent with odd atomic number (La, Pr, Eu, Tb, Ho, Tm, Lu). This regularity is referred to as the Oddo-Harkins rule (Oddo, 1914; Harkins, 1917 ). Normalization of determined REE concentrations eliminates the effect of the Oddo-Harkins rule providing that an appropriate standard has been chosen. Methods commonly used include normalization to chondritic meteorites (e.g., Evensen et al., 1978; Boynton, 1984), to a sedimentary average rock (e.g., NASC: North American Shale Composite, Haskin and Paster, 1979; Gromet et al., 1984; ES: European Palaeozoic Shale Composite, Haskin and Haskin, 1966; PAAS: Post-Archean Australian Shale, Nance and Taylor, 1976; YSAB: Younger sediments in the Angola Basin, Wang et al., 1986), or to a specific rock or mineral, which is part of the system under investigation. The normalization and comparison to an average clastic sedimentary rock REE pattern represented by the NASC, ES, PAAS, or YSAB should be treated with caution. Firstly, the different source provenances can have an effect on the REE distribution in clastic sedimentary rocks (cf. Roaldset, 1973; Cullers et al., 1974). And secondly, the Eu depletion of post-Archean clastic sediments is possibly related to the tectonic setting and depositional environment of the lithologies (cf. McLennan and Taylor, 1988; McLennan et al., 1990).

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Normalized REE data are presented as REE patterns, which are constructed by plotting the normalized concentrations (i.e., concentration of sample/concentration of standard) of each element versus the element name or the ionic radius. The smoothness of the constructed curves is generally used for testing the internal consistency of analytical results and also for interpolation and extrapolation of REE not determined.

General mineralogical properties In most rock-forming minerals the REE occur as minor or trace constituents (ppb to ppm levels), but there are minerals in which REE are essential components (wt% levels). Incorporation of REE into minerals can be described with various models (e.g., Burns and Fyfe, 1967; Burns; 1970; Iiyama and Volfinger, 1976). These models assume that the uptake of trace elements by a mineral is exclusively determined by crystallographic controls involving substitution, couple substitution and/or generation of vacancies. However, REE may not only be present in structural sites, substantial amounts of REE can also be present as surface concentrations on microfractures and grain boundaries (e.g., Suzuki, 1987 ), and within daughter minerals of fluid inclusions (e.g., Kwak and Abeysinghe, 1987; Salvi and Williams-Jones, 1990). In addition, the main thrust of the work has been on magmatically crystallized minerals. Little information is available concerning the REE contents of minerals formed within weathering, metamorphic and hydrothermal environments. Several studies have cast doubt on the influence of crystallographic controls on REE partitioning between aqueous solutions and precipitating minerals (Mineyev et al., 1962; Fleischer and Altschuler, 1969; Mrller et al., 1976; Guichard et al., 1979; Exley, 1980; Joliff et al., 1987; Fryer and Taylor, 1987; Pagel et al., 1987; Terakado and Masuda, 1988 ). These studies have shown that the REE distribution

B.G. LOTTERMOSER

within minerals precipitated from aqueous solutions or formed during fluid/rock interactions are either independent, or only partly dependent on chemical-crystallographic constraints. The interpretation of REE distributions and abundances in low-temperature, low-pressure geological environments is, however, hindered by the fact that partitioning experiments on REE have largely been concerned with magmatic systems at high temperatures and high pressures (e.g., Cullers et al., 1973; Flynn and Burnham, 1978; Wendlandt and Harrison, 1979 ). A broad experimental data base appropriate for hydrothermal and weathering conditions is still lacking. Thus, the interpretation of the geochemistry of REE has largely been based on empirical evidence (e.g., Mineyev, 1963; Borodin, 1967; Kubach et al., 1981 ), on thermodynamic data (e.g., Wood, 1990a, b), and on the limited experimental work performed on the behaviour of REE in high-temperature, high-pressure environments.

Hydrothermal fluids REE are thought to be transported in aqueous solutions as complexes, as most REE salts possess extremely low solubility products. The stabilities of the various REE complexes vary and HREE and Y thereby form more stable carbonate, fluoride, oxalate, chloride, and sulphate complexes than the LREE (cf. Choppin and Unrein, 1963; Moeller et al., 1965; Bilal and Becker, 1979; Ciavatta et al., 1981; Kubach et al., 1981; Spahiu, 1985; Cantrell and Byrne, 1987). REE fluid compositions of early magmatic solutions are related to the composition and physical properties of the crystallizing magma, which controls the partitioning of REE into the evolving fluids (cf. Flynn and Burnham, 1978). Hydrothermal fluids derived from a crystallizing magma or produced by leaching of rocks within hydrothermal systems generally carry low REE contents (Michard and AI-

RARE EARTH ELEMENTS AND HYDROTHERMAL ORE FORMATION PROCESSES

bar6de, 1986; Palacios et al., 1986; Michard et al., 1983, 1987; Sanjuan et al., 1988; Michard, 1989; Honda et al., 1989a, b; Oi et al., 1990) (cf. ppb levels within seawater; e.g., Goldberg et al., 1963; Elderfield and Greaves, 1982; Elderfield, 1988). REE concentrations of hydrothermal solutions generally increase with decreasing pH (Michard, 1989). The REE distribution (i.e., LREE or HREE enrichments; Eu and Ce concentrations) within hydrothermal fluids are dominantly controlled by pH, temperature and the kind of complexing agents present in the solutions (Table 1 ). Destabilization of LREE and HREE complexes by changes in temperature, pressure, pH, alkalinity or complexing agent concentrations will lead to the fractionation and deposition of REE (cf. Kubach et al., 1981 ). Reaction of these fluids with surface waters and wall rocks may change the REE composition of the solutions and of the reacting country rock.

Intrusive-related hydrothermal deposits Rare earth element mobility REE are commonly regarded as insensitive to hydrothermal processes. They have been used as immobile elements for the quantitative evaluation of geochemical alteration processes and for the classification of basic igTABLE 1 REE distributions o f geothermal and submarine hydrothermal fluids (Michard et al., 1983, 1987; Michard, 1988, 1989; Sanjuan et al., 1988; Honda et al., 1989a, b; Oi et al., 1990) Venting pH temperature (° C)

Dominating REE enrichment ( + ) / complexing REE depletion ( - ) agent

> 230 43-97 12-16 60 68-81 43-97

C1SO ] CO 2CO 2CO 2CO 2-

<6 <4.2 <7 6 7.7-7.9 6.7-9.5

+ LREE, + Eu +LREE,-Eu +HREE,-Eu + LREE, - Eu +LREE,-Eu +LREE,-Eu,-Ce

29

neous rocks in order to determine the magmatic affinity of igneous lithologies (e.g., Winchester and Floyd, 1977; Floyd and Winchester, 1978). However, there are numerous studies which do show that REE can be mobilized during hydrothermal alteration (e.g., Martin et al., 1978; Taylor and Fryer, 1980; Baker and De Groot, 1983; Baker, 1985; Gier6, 1986; Andersen, 1986; Vocke et al., 1987; Dickin, 1988; Baker and Hellingwerf, 1988; Tullborg et al., 1988; Morogan, 1989; Oreskes and Einaudi, 1990), low-grade regional metamorphism (e.g. Hellman et al., 1979; Nystrrm, 1984), and palagonitization of basic lavas (e.g., Wood et al., 1976; Ludden and Thompson, 1979 ). Factors favouring REE mobility upon fluid/rock interactions appear to be long fluid residence times, high fluid/rock ratios and abundant REE complexing agents in the solutions (cf. Taylor and Fryer, 1980; Staudigel and Hart, 1983 ). Also the stability of the reacting, REEbearing phases and the location of the REE in either crystallographic sites, or more accessible grain boundaries (Suzuki, 1987 ) and glassy materials (Wood et al., 1976; Humphris et al., 1978 ) seems to be important for a possible liberation from the host mineral or amorphous substance. The REE content of the incoming fluid, REE adsorption during alteration and the pH conditions during alteration can influence the transport of REE groups (LREE, HREE) away from their original locations, whereas changing Eh and temperature conditions may especially determine the mobility of Eu and Ce (cf. Alderton et al., 1980; Taylor and Fryer, 1980; Sverjensky, 1984).

Hydrothermal alteration assemblages Studies on the REE distributions of alteration assemblages enclosing and/or hosting a variety of intrusive-related mineralizations revealed REE mobility and a complex REE behaviour with changing fluid conditions. In a system with low fluid/rock ratios, a high

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pH and abundant chloride ions in solution, magmatic-hydrothermal fluids will produce early-stage alteration assemblages with some REE mobility (cf. Alderton et al., 1980; Taylor and Fryer 1980, 1982, 1983; Chatterjee and Strong, 1984; Palacios et al., 1986; Cathelineau, 1987; Schneider et al., 1988; Lottermoset, 1990b). These early-stage alterations include K-feldspathization, albitization, biotitization and chloritization. The development of late-stage hydrothermal fluids and the production of sericitization, argillization, tourmalinization, epidotization, silicification and chloritization is characterized by increasing fluid/rock ratios, abundant REE complexing agents (CO 2-, F-, Cl-, PO 3- ), decreasing temperature and pH conditions and pronounced REE mobility (cf. A1derton et al., 1980; Taylor and Fryer, 1980, 1982, 1983; Chatterjee and Strong, 1984; Palacios et al., 1986; Cathelineau, 1987; Lottermoser, 1990b). These fluids either produce a progressive leaching of all REE (Alderton et al., 1980; Taylor and Fryer, 1980, 1982, 1983; Chatterjee and Strong, 1984; Schneider et al., 1988) or an enrichment of all REE with increasing alteration intensity (Palacios et al., 1986; Cathelineau, 1987; Lottermoser, 1990b). Hydrothermal alteration profiles may exhibit distinct vertical zonations, whereby leached REE from the lower part of the profile are redeposited at the top of the alteration sequence (Palacios et al., 1986; Lottermoser, 1990b ). Thereby, mobility of REE appears to increase with the change from early-magmatic to late-stage hydrothermal fluids (cf. Taylor and Fryer, 1980, 1982, 1983; Chatterjee and Strong, 1984; Palacios et al., 1986; Lottermoser, 1990h ). This is likely caused by increasing fluid/rock ratios associated with the formation of late-stage hydrothermal alterations. REE distributions of monomineralic alteration assemblages have been related to chemical-crystallographic controls of the alteration minerals (Corey and Chatterjee, 1990). In contrast, REE distributions of polymineralic

B.G. LOTTERMOSER

alteration assemblages have been related to changing physico-chemical conditions during alteration including pH, Eh, fluid/rock ratios, and complexing agent concentration (cf. AIderton et al., 1980; Taylor and Fryer, 1980, 1982, 1983; Chatterjee and Strong, 1984; Palacios et al., 1986; Cathelineau, 1987; Baker and Hellingwerf, 1988; Leroy and Turpin, 1988; Lottermoser, 1990b). Chemical-crystallographic controls are generally thought to have no influence on the REE distributions of polymineralic alteration products. Detailed studies of individual minerals from a range of hydrothermal alteration types are clearly required to support such a general assumption.

Hydrothermal veins Studies of intrusive related vein mineralizations have commonly been combined with fluid inclusion or stable isotope investigations (cf. M611eret al., 1979; Ekambaran et al., 1986; Dill et al., 1986; Giuliani et al., 1987; Constantopoulos, 1988). Hydrothermal vein minerals may show a wide range of REE distributions and REE contents. Based on these compositional differences and combined with microthermometric and isotopic data, the formation of hydrothermal veins can be assigned to specific mineralizing events or ore fluids. Wall rocks enclosing vein assemblages may have experienced hydrothermal alteration during vein formation and thereby associated mobility and fractionation of REE (Wilton, 1985 ).

Skarns Reactive rocks such as carbonate-rich lithologies may loose or gain significant amounts of REE during fluid/rock interactions (cf. McLennan and Taylor, 1979; Graf, 1984; Lottermoser, 1989b; Vander Auwera and Andre, 1991 ). Variations in REE distributions within skarn parageneses have been related to changing parameters such as pH, temperature and water/rock ratios prevailing during ore for-

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RARE EARTH ELEMENTS AND HYDROTHERMAL ORE FORMATION PROCESSES

marion (Baker and Hellingwerf, 1988). The REE distributions were thought to be independent of major element variations and mineral phases. In contrast, Giuliani et al. (1987) attributed the REE concentrations within scheelite-bearing skarn parageneses to the scheelite/ fluid REE partition coefficient. In general, the studies have shown that the REE patterns of mineralized/altered rocks reflect those of the host carbonates and of any additions/losses of REE occurring during postdepositional ore formation.

Directions for future research The fluid dynamic model of Henley and McNabb (1978) for porphyry copper emplacements describes the evolution of a magmatic fluid and its interaction with groundwater. Magmatic fluid rises, cools, and condenses upon pressure release into a two-phase plume comprising a high-salinity liquid and a low-salinity vapour (boiling). Cool groundwater is subsequently entrained into the rising boiled fluids. Steam escape from such two-phase fluids commonly occurs at higher levels. The effect of such differential flow rates of liquid and vapour phases on the partitioning behaviour of trace and major elements, including REE, has been neglected by previous investigations into intrusive related hydrothermal mineralizations. Varying liquid/vapour ratios and the gas composition of the vapour phases could be important in the partitioning of REE between liquid and vapour, also between hydrothermal fluid and alteration or vein assemblage. Further geochemical investigations combined with fluid inclusion studies of intrusive related mineralizations could reveal that REE patterns and concentrations, and also the trace element, precious-and base-metal distributions, are influenced by differential flow rates of the individual hydrothermal fluid phases. Also, combined geochemical and stable isotope studies may help to evaluate the origin of hydrothermal alteration products and

the source(s) of the hydrothermal fluids (cf. Morteani et al., 1986; Vander Auwera and Andre, 1991 ).

Submarine hydrothermal deposits Recent metalliferous sediments Recent hydrothermal fluids discharged from submarine vent sites contain mantle-derived (e.g., He) and crustal-derived (e.g., Cu, Zn), i.e., basalt-derived elements (Rona, 1984), and precipitate metal-rich sediments within the discharge zone of the fluid conduit. Submarine hydrothermal fluids show a pronounced enrichment in LREE and Eu (Michard et al., 1983; Michard and Albar~de, 1986; Michard, 1988, 1989; Fig. l ). Also, pure hydrothermal precipitates of the Red Sea are strongly enriched in LREE and Eu (Courtois and Treuil, 1977) and hydrothermal sediments from the Green Seamount (Alt, 1988 ) are distinctly enriched in Eu suggesting a hydrothermal origin of the REE distributions (Fig. 2 ). The primary mechanism responsible for the acquisition of REE by the metalliferous sediments involves adsorptive scavenging (cf. Ruhlin and Owen, 1986 ). Elderfield ( 1988 ) highlights the Eu en-

0.01 W I-123 Z 0 ~ 0.001 0 .d b_

L'oC'ePr Nd ' SmE'u 6~dT'bDy Ho E~rT'mYb lJu ATOMIC NUMBER

Fig. 1. Rangeof REE contents within hydrothermalfluids from the East PacificRise (Michard et al., 1983;Michard and Albar~de, 1986). Chondritevaluestaken from Boynton (1984).

32

B.G.

LOTTERMOSER

lC Iii I---

v

100 W

Z 0 I C)

12]

1

Z 0 "r-

10

C) 0 5~ 0 n.-

L'a['eP'rN'd '

S'm E'u B'd l~b Dy Ho ErT'm Yb

iA

Lu

ATOMIC N U M B E R

Fig. 2. Rangeof REE results on hydrothermalmetalliferous sediments (Courtois and Treuil, 1977). Chondrite values taken from Boynton (1984).

Iii I-01~ 0.0001 Q Z 0 "1Q.) CD _.~ 0.00001 J LL i

i

i

Lo Ce Pr Nd

ATOMIC

i

,

i

i

i

i

i

Sm Eu Gdl~b DyHo Er Tm Yb Lu

NUMBER

Fig. 3. Rangeof REE contentswithin seawater (Goldberg et al., 1963; Hogdahlet al., 1968). Chondritevaluestaken from Boynton (1984). richment of hydrothermal fluids compared to seawater and suggests that the E u / S m ratio could be used as a hydrothermal tracer in order to evaluate possible hydrothermal inputs. In contrast, metalliferous Fe-Mn sediments precipitated everywhere on the sea floor have REE distributions very similar to that of seawater (e.g., Bender et al., 1971; Piper and Graef, 1974; Dymond et al., 1977; Figs. 3 and 4). The similar shape of the REE patterns including negative Ce and Eu anomalies are taken as indicating a seawater source for the REE. The Ce depletion of seawater is commonly interpreted as the oxidation of Ce to the tetravalent state and its incorporation into Mn nodules formed on the sea floor.

L'aI Pr I ' I EuIT'blH'olT1m r Lu Ce Nd Sm Gd Dy Er

ATOMIC

Yb

NUMBER

Fig. 4. Range of REE results on hydrogenousmetalliferous Fe-Mn sediments (Piper and Graef, 1974; BonnotCourtois, 1981; Dymond et al., 1977). Chondrite values taken from Boynton (1984). Thus, there appear to be two fluid phases, seawater and submarine hydrothermal fluids, with distinctly different REE distributions competing for the scavenging mechanism in metalliferous sediments. Hydrogenous Mn-Fe sediments formed on the seafloor scavenge large amounts of trace elements, base metals, and seawater REE distributions. In contrast, hydrothermal sediments in and around submarine vent sites accumulate precious metals, base metals and associated elements (As, Sb, T1, Ba, Co, Se, Cd), and hydrothermal fluid REE distributions. The time of exposure of a hydrothermal sediment to seawater, the degree of subaqueous oxidation and the degree of mixing between seawater and hydrothermal fluid will also influence the trace element and REE distribution of a metalliferous sediment (cf. Bonatti et al., 1976; Marchig et al., 1986). Courtois and Treuil ( 1977 ) and Michard et al. ( 1983 ) discounted the basalts present at the recent hydrothermal vent sites and in the suboceanic lithosphere as the REE sources for both the submarine hydrothermal fluids and the hydrothermal precipitates. These authors showed that (a) the low water/rock ratio at the vent sites could not change the basalt REE pattern,

RARE EARTH ELEMENTS AND HYDROTHERMAL ORE FORMATION PROCESSES

(b) the temperature-pressure dependence of silica solubility indicated a depth of hydrothermal alteration greater than 1 km, and (c) the REE patterns of the basalts did not show any similarity to the REE patterns of the hydrothermal fluids or the hydrothermal sediments. Courtois and Treuil ( 1977 ) and Michard et al. (1983) also pointed out that the general shape of the REE distribution curves and the excess Eu content are quite similar to those observed in alkali peridotites. These authors proposed that submarine hydrothermal fluids inherit their REE patterns by deep-seated alteration of peridotite in the suboceanic lithosphere.

Massive sulphide deposits REE studies have been conducted in and around ancient massive sulphide deposits. Several studies detected REE mobility (especially of Eu ) during mineralization (e.g., Graf, 1977; Thurston, 1981; Baker and De Groot, 1983; Windrim et al., 1984; Bence and Taylor, 1985; Vivallo, 1985; Hellingwerf, 1987; Patocka, 1987; MacLean, 1988; Whitford et al., 1988; Schade et al., 1989), whereas other investigations found little or no evidence for REE mobility (e.g., Gulson and Rankin, 1977; Strong, 1984; Vance and Condie, 1987). Mobility of REE has been detected in alteration halos of large massive sulphide deposits and may reflect large fluid volumes and thus high fluid/rock ratios favourable for the mobility of REE (Campbell et al., 1982, 1984; Whitford et al., 1988). Volcanics from pervasive alteration halos and alteration pipes associated with massive sulphide deposits may possess REE enrichments or depletions (cf. Campbell et al., 1982, 1984; Bence and Taylor, 1985; Lesher et al., 1986a; Patocka, 1987; Hellingwerf, 1987; MacLean, 1988; Whitford et al., 1988 ). Eu depletions of felsic volcanics could be a primary magmatic feature (Campbell et al., 1982, 1984;

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Lesher et al., 1986a), whereby ore-associated felsic volcanics are generated by partial melting in the lower crust or upper mantle and subsequent fractional crystallization produces a Eu anomaly. In contrast, Whitford et al. (1988) and Lesher et al. (1986b) interpreted the Eu depletion as a secondary hydrothermal feature; the depletion is thought to be due to hydrothermal alteration processes. Mobilization of REE during hydrothermal alteration has been related to the destruction of parent rock minerals and the removal of REE by ligand complexation. REE precipitation is seen as a result of changing fluid temperature or ligand concentration which would destabilize REE complexes (cf. Campbell et al., 1982, 1984; Lesher et al., 1986a; Patocka, 1987; Hellingwerf, 1987; MacLean, 1988; Whitford et al., 1988 ). Changing Eh conditions during alteration may result in anomalous Ce concentrations within the alteration products (cf. Patocka, 1987). Positive Eu anomalies on chondrite-normalized REE patterns are commonly found in ores and chemical precipitates associated with massive sulphide deposits (e.g., Graf, 1977; Arvanitidis and Rickard, 1986; Lottermoser, 1989b). This Eu enrichment has been interpreted as reflecting low-temperature, rock/ water interaction between Eu-enriched feldspar-bearing volcanic piles and hydrothermally convecting seawater which would mobilize Eu and thus cause a pronounced increase of Eu in the ore-bearing hydrothermal fluids and their precipitates (Graf, 1977; Bence and Taylor, 1985). The Eu enrichment reflects high-temperature ( > 250 ° C) and/or reducing fluid conditions as Eu occurs within such solutions as divalent species (cf. Sverjensky, 1984). It has also been suggested that the Eu enrichment of ores and chemical precipitates could be related to the Eu depletion in the hydrothermally altered country rocks (Graf, 1977; Bence and Taylor, 1985; Whitford et al., 1988). Large volumes of hydrothermal fluids would leach and mobilize Eu from the vol-

34

cano-sedimentary pile and redeposit it in ores and exhalites. Alternatively, the ore fluids could gain Eu by deep-seated alteration of Euenriched lithologies in the suboceanic lithosphere (cf. Courtois and Treuil, 1977; Michard et al., 1983). A distinct decrease of the Eu content in chemical precipitates has been detected with distance from the Broken Hill orebodies (Lottermoser, 1988, 1989b). The decrease in the Eu concentration is thought to be due to decreasing fluid temperature and/or changing oxidation-reduction conditions prevailing during formation of the exhalites. Such changes in REE patterns of exhalites with distance from the mineralization could provide a powerful tool for the exploration of massive sulphide deposits as changing REE patterns of chemical sediments reflect changing physico-chemical parameters of the precipitating fluids. Chemical compositions of ancient metalliferous sediments have been interpreted by various authors to reflect contributions from biogenic, hydrothermal, seawater or detrital sources (e.g., Robertson and Fleet, 1976; Crerar et al., 1982; Colley and Walsh, 1987; Wonder et al., 1988). The recognition of hydrothermal fluid, seawater, sedimentary or volcanically derived REE patterns within ores, chemical precipitates and hydrothermally altered lithologies has the potential to constrain genetic modelling of massive sulphide ore deposits.

Directions for future research Anomalous Eu behaviour has been detected in rocks closely associated with recent and ancient mineralizing submarine hydrothermal environments. This suggests that anomalous Eu behaviour (i.e., its depletion or enrichment compared to the neighbouring REE) within submarine hydrothermal environments may represent an indicator which could assist in finding sought-after hydrothermal mineralizations. However, as with most

B.G. LOTTERMOSER

other geochemical techniques, caution will be necessary in applying the results from one area to other hydrothermal systems. The behaviour of REE within metal-barren hydrothermal systems is at present unknown and investigations are needed in order to evaluate the potential of Eu as an indicator element for hydrothermal mineralizations. Such studies should be conducted on hydrothermal systems of different sizes in order to evaluate the factors necessary for possible REE mobility. The studies would be expected to reveal REE fractionation and mobility within extensive, unmineralized hydrothermal alteration zones formed under high fluid/rock ratios, abundant REE complexing agents and long fluid residence times. However, the reduction of Eu to the divalent state, and its preferential mobility and deposition, could be restricted to highly mineralized hydrothermal environments characterized by reducing and/or hightemperature fluids.

Role of diagenesis and metamorphism Most massive sulphide deposits have undergone some degree of metamorphism, consistent with their formation in environments subsequently incorporated in active tectonic zones. Diagenetic and metamorphic overprinting of ores and country rocks may be accompanied by the mobilization of REE. Diagenesis of cherts may result in the decrease of REE abundances due to an increasing crystallinity and an accompanied loss of REE (Shimizu and Masuda, 1977). Also, diagenesis of carbonate-rich lithologies may lead to the mobilization of REE (e.g., Schieber, 1988). However, previous studies of a range of silicate-rich lithologies from prograde amphibolire to granulite and retrograde metamorphic terrains did not detect REE mobility during metamorphism (e.g., Green et al., 1972; Cullers et al., 1974; O'Nions and Pankhurst, 1974; McGregor and Mason, 1977; Muecke et al., 1979; Pride and Muecke, 1981; McLennan et

RARE EARTH ELEMENTS AND HYDROTHERMAL ORE FORMATION PROCESSES

al., 1984; Taylor et al., 1986; Arvanitidis and Rickard, 1987 ). Extremely large fluid/rock ratios appear to be necessary to cause significant changes in REE patterns during regional metamorphism of silicate-rich lithologies (Taylor and McLennan, 1985 ). Retrograde shear zones are commonly characterized by abundant hydrous minerals suggesting large fluid volumes and long fluid residence times. Such conditions appear to be favourable for REE mobility. Thus except where partial melting or intense retrograde metamorphism occurred, metamorphism does not effect the primary REE patterns of silicate-rich lithologies. In contrast to the above stated immobility of REE during metamorphism of silicate-rich rocks, Kerrich and Fryer (1979) concluded that metamorphic hydrothermal fluids carried REE during the metamorphic outgassing of amphibolite and greenschist facies lithologies. Further research may show that metamorphic veins contain REE which have been mobilized into the solutions during the metamorphic processes. Conclusions

The incorporation of REE into minerals is commonly thought to be controlled by chemical-crystallographic constraints. This school of thought is primarily based on investigations of magmatically crystallized phases. However, studies of hydrothermal minerals already indicate that their REE distributions are also influenced by the composition of the hydrothermal fluids. Normalization and comparison of rock REE patterns from hydrothermal ore deposits to an average sedimentary rock represented by the NASC, PAAS, YSAB or ES should be treated with caution. The REE distributions of clastic sedimentary rocks appear to be a function of source provenance and depositional environment. An REE experimental data base appropriate for hydrothermal conditions is still lacking.

35

Further experimental research on the complexing of REE could indicate possible transport of REE under geochemical relevant conditions by complexes other than carbonate, chloride, sulphate and fluoride. Also, such experiments may reveal that destabilization of complexes which do not carry REE, may have an indirect influence on the stability of REE complexes also present in the solutions. Hydrothermal alteration assemblages may have a wide range of REE distributions, whereby REE mobility generally increases with increasing fluid/rock ratios and alteration intensity. Possible gains or losses of REE during alteration are also influenced by the reactivity of the host rocks. The REE pattern of an alteration assemblage can be related to the mineralogical siting of the REE within the parent rock minerals, to the partitioning behaviour of REE between the alteration phases and the solution, and to the REE distribution within the hydrothermal fluid. Destabilization of REE complexes is due to changing ligand concentrations, temperature, pressure, alkalinity, pH, and Eh conditions. REE distributions of polymineralic alteration products are generally thought to be independent of chemical-crystallographic controls, whereas REE distributions of monomineralic alteration products are interpreted as dependent on mineral/fluid partition coefficients. Many hydrothermal precipitates formed on the seafloor and ancient massive sulphide ores are enriched in Eu reflecting Eu-enriched reducing/high-temperature ore fluids. This Eu enrichment is either related to the mobilization of Eu from alteration halos enclosing the massive sulphide deposits or due to deepseated alteration of Eu-enriched source rocks in the lithosphere. Ancient and recent metalliferous sediments may exhibit a wide range of REE distributions which can be derived from the hydrothermal fluid, seawater, or a mixture thereof. Volcanic or sedimentary detritus may also influence the REE patterns of metalliferous sediments.

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Mobility of REE has not been detected under a range of metamorphic conditions. However, mobility may occur during the development of shear zones and migmatites, during diagenesis of carbonate-rich rocks and porous silicate-rich rocks, and under high fluid/rock ratios at the seawater/basalt interface on the seafloor. Metamorphic outgassing may also be accompanied by minor mobilization of REE and their subsequent incorporation onto hydrothermal vein-type mineralizations. References Alderton, D.H.M., Pearce, J.A. and Potts, P.J., 1980. Rare earth element mobility during granite alteration: evidence from southeast England. Earth Planet. Sci. Lett., 49: 149-165. Air, J.C., 1988. The chemistry and sulfur isotope composition of massive sulfide and associated deposits on Green Seamount, Eastern Pacific. Econ. Geol., 83: 1026-1033. Andersen, T., 1986. Compositional variation of some rare earth minerals from the Fen complex (Telemark, SE Norway): implications for the mobility of rare earths in a carbonatite system. Mineral. Mag., 50: 503-509. Arvanitidis, N.D. and Rickard, D.T., 1986. REE-geochemistry of an Early Proterozoic volcanic ore district. Dammberg, Central Sweden; a summary of results. Miner. Wealth, 43: 47-57. Arvanitidis, N.D. and Rickard, D.T., 1987. An evaluation of lanthanide geochemistry in ore petrology. Miner. Wealth, 46:21-28. Baker, J.H., 1985. Rare earth and other trace element mobility accompanying albitization in a Proterozoic granite, W. Bergslagen, Sweden. Mineral. Mag., 49: 107-115. Baker, J.H. and De Groot, P.A., 1983. Proterozoic seawater-felsic volcanics interaction W. Bergslagen, Sweden. Evidence for high REE mobility and implications for 1.8 Ga seawater compositions. Contrib. Mineral. Petrol., 82:119-130. Baker, J.H. and Hellingwerf, R.H., 1988. Rare earth element geochemistry of W-Mo-(Au) skarns and granites from western Bergslagen, Central Sweden. Mineral. Petrol., 39:231-244. Baker, J.H., Anderson, L.-S. and Marinou, A., 1988. Geochemical variations in a Proterozoic hydrothermal mafic breccia dyke related to Ni-Cu-Fe skarn mineralization at Annehill, Bergslagen, Sweden. Geol. Mijnbouw, 67: 363-378. Bence, A.E. and Taylor, B.E., 1985. Rare earth element systematics of West Shasta metavolcanic rocks: petro-

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Elderfield, H. and Greaves, M.J., 1982. The rare earth elements in seawater. Nature, 296:214-219. Eppinger, R.G. and Closs, L.G., 1990. Variation of trace elements and rare earth elements in fluorite: a possible tool for exploration. Econ. Geol., 85:1896-1907. Evensen, N.M., Hamilton, P.J. and O'Nions, R.K., 1978. Rare-earth abundances in chondritic meteorites. Geochim. Cosmochim. Acta, 42:1199-1212. Exley, R.A., 1980. Microprobe studies of REE-rich accessory minerals: implications for Skye granite petrogenesis and REE mobility in hydrothermal systems. Earth Plant. Sci. Lett., 48:97-110. Fleet, A.J., 1984. Aqueous and sedimentary geochemistry of the rare earth elements. In: P. Henderson (Editor), Rare Earth Element Geochemistry. Developments in Geochemistry 2. Elsevier, Amsterdam, pp. 343-373. Fleischer, M. and Altschuler, Z.S., 1969. The relationship of the rare-earth composition of minerals to geological environment. Geochim. Cosmochim. Acta, 33: 725732. Floyd, P.A. and Winchester, J.A., 1978. Identification and discrimination of altered and metamorphosed volcanic rocks using immobile elements. Chem. Geol., 21: 291-306. Flynn, R.T. and Burnham, C.W., 1978. An experimental determination of rare earth partition coefficients between a chloride containing vapor phase and silicate melts. Geochim. Cosmochim. Acta, 42: 685-701. Fryer, B.J. and Taylor, R.P., 1987. Rare-earth element distributions in uraninites: implications for ore genesis. Chem. Geol., 63: 101-108. Ganzeyev, A.A., Sotskav, Y.P. and Lyapunov, S.M., 1984. Geochemical specialization of ore-bearing solutions in relation to rare-earth elements. Geochem. Int., 20:160164. Gier6, R., 1986. Zirconolite, allanite and hoegbomite in a marble skarn from the Bergell contact aureole: implications for mobility of Ti, Zr and REE. Contrib. Mineral. Petrol., 93: 459-470. Giuliani, G., Cheilletz, A. and Mechiche, M., 1987. Behaviour of REE during thermal metamorphism and hydrothermal infiltration associated with skarn and vein-type tungsten ore bodies in central Morocco. Chem. Geol., 64: 279-294. Goldberg, E.D., Koide, M., Schmitt, R.A. and Smith, R.H., 1963. Rare earth distributions in the marine environment. J. Geophys. Res., 68: 4209-4217. Graf, J.L., 1977. Rare earth elements as hydrothermal tracers during the formation of massive sulfide deposits in volcanic rocks. Econ. Geol., 72: 527-548. Graf, J.L., 1984. Effects of Mississippi valley-type mineralization on REE patterns of carbonate rocks and minerals, Virburnum Trend, southeast Missouri. J. Geol., 92: 307-324. Green, T.H., Brunfelt, A.O. and Heier, K.S., 1972. Rare earth element distribution and effects on aqueous par-

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