geochemistry of associated igneous host rocks ...

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Geological Society, London, Special Publications

Identification of ore-deposition environment from trace-element geochemistry of associated igneous host rocks J. A. Pearce and G. H. Gale Geological Society, London, Special Publications 1977, v.7; p14-24. doi: 10.1144/GSL.SP.1977.007.01.03

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Identification of ore-deposition environment from trace-element geochemistry of associated igneous host rocks J. A. Pearce Department of Earth Sciences, The Open University, Milton Keynes, Buckinghamshire, England G. H. Gale Department of Mines, Resourcesand Environmental Management, Winnipeg, Manitoba, Canada 550.42:546:553.067 Synopsis

The tectonic environment of formation of volcanogenic massive sulphide deposits and porphyry tin and copper deposits can be identified from the geochemical characteristics of the associated igneous rocks. The "stable" trace-element geochemistry (involving Ti, Zr, Y, Nb, Cr and rare-earth elements) and geology of metabasalts related to 12 massive sulphide deposits indicate that the deposits studied fall into four distinct classes. (1) Cyprus-type, including Cyprus, Oman and Betts Cove, possibly formed during the early stages of back-arc basin development; (2) L r ken- type, including Lr and York Harbour, possibly formed at back-arc basin spreading centres; (3) Joma-type, including Joma, Rr and Bidiovagge, possibly formed in a small ocean of Red Sea type; and (4) Gjersvik-type, including Gjersvik, Buchans, Noranda and Lynn Lake, possibly formed during an early stage of island arc evolution (Gjersvik), later during island arc evolution (the Buchans Kuroko-type deposit) or in a Precambrian setting (Noranda and Lynn Lake). Deposits related to major ocean ridge crests appear to be small and relatively uncommon, perhaps because relatively few favourable sites for ore deposition exist in such environment~ The environment of intrusion of acid-intermediate igneous rocks can be deduced by use of diagrams based on the element Nb (e.g. SiO2 versus Nb). On this basis, tinbearing granites can be classified either as within-plate magmas (Nigeria) or as magmas from evolved volcanic arc settings (e.g. Bolivia, Cornwall and Indonesia); the latter group can be further subdivided geologically into postorogenic and back-arc extensional settings. Although the continental crust may play a significant role in the formation of tin deposits, the geochemical data presented here suggest that partial melting of tin-enriched mantle above a subduction zone may be the single most important genetic factor. The purpose of this paper is to show how igneous geochemistry can be used to identify the tectonic environments in which volcanogenic ore deposits are formed. The paper is divided into two parts: the first deals with deposits formed in a submarine environment (i.e. volcanogenic massive sulphide deposits), and the second with deposits formed in a continental environment - in particular, tin deposits. Many works of synthesis have already been published on the relationship between massive sulphide deposits and plate tectonics. 1-s These use mainly geological criteria to point to a variety of possible environments of massive sulphide 14

deposition, at major ocean ridges, in marginal basins, in island arcs and in oceanic islands. In many cases, however, particularly in the deformed and metamorphic rocks of mountain belts, the geological evidence is ambiguous. Geochemical techniques then have a useful application. Because the host rocks are commonly metamorphosed, stable traceelementgeochemistry 6 can be used to deduce their environment of eruption. Elements such as Ti, Zr, Y, Nb, 7.8'9 p,9 Cr 10,11 and the rare earths 12 all exhibit systematic differences between basalts from different tectonic settings while remaining relatively immobile during weathering and metamorphic processes. In this paper we study the concentrations of these elements in the volcanic rocks related to mineralization in the Norwegian Caledonides, Newfoundland, Cyprus and Oman and in Precambrian terrain in Scandinavia and Canada. Ore deposits related to continental magmatism can also be studied by such a geochemical approach. It is fairly well established that porphyry copper deposits are formed above destructive plate margins. TM The environment of formation of tin deposits is much more enigmatic, however. The possibilities suggested 14-18 are post-orogenic, within-plate ('hot-spot') and destructive plate margin settings. Only rarely can the precise setting of tin-bearing granites be identified from geological criteria alone. Geochemical methods are developed in this paper to distinguish between granitic rocks originating in 'within-plate' and 'volcanic arc' settings. These methods are applied to some tin-bearing granitic rocks in Cornwall, Bolivia, Indonesia and Nigeria. Massive sulphide deposits

Magmatism in the oceanic environment If (as is widely accepted) hydrothermal circulation of sea water through oceanic crust is the dominant process in the formation of massive sulphide deposits, TM then, given a sufficiently high geothermal gradient and permeable crust, massive sulphides could form at almost any site of submarine igneous activity. So, before studying the lavas related to these deposits, it is first necessary to geochemically categorize lavas from major ocean spreading centres, marginal basin spreading centres, continental margins, island arc seamounts, ocean islands and seismic/aseismic ridges. To do this, three main magma types are first defined: ocean-floor Table 1 Mean concentrations of trace elements (ppm) in three main oceanic magma types (from literature survey made in 1973 33); ocean island basalt column includes both tholeiitic and alkalic types

Ti Zr Y Nb Cr La Ce Yb K Rb Ba Sr U Th Cu Co V Ga

Ocean-floor tholeiites

Island arc tholeiites

Ocean island basalt

8400 83 28 2.5 280 3.0 10 2.5 1300 2.5 8.5 120 0.09 0.16 73 51 23O 17

5000 45 18 1.5 107 3.2 6.5 2.0 2700 4.7 60 175 0.15 0.37 62 30 270 15

13 500 211 27 27 200 27 56 2.3 5450 13 176 415 0.64 0.77 81 60 240 18 .

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basalts (which include tholeiitic and alkalic types), wit:binplate basalts (which also include tholeiitic and alkalic types) and island arc basalts (which include tholeiitic and calcalkaline types). Within-plate tholeiites are distinguished from ocean-floor tholeiites by their higher concentrations of most lithophile trace elements, the few notable exceptions including Y, the heavy rare earths, Sc and Cr. Island arc tholeiites are distinguished from ocean-floor tholeiites by lower concentrations of small ion lithophile elements, including Cr; their concentrations of large ion lithophile elements are typically similar to or greater than those of ocean-floor tholeiites. Table 1 illustrates the nature of these geochemical differences. Submarine basalt types

Within -plate basalts

basin directly overlies a subduction zone, its magma type might be expected to be intermediate between ocean-floor and island arc type; similarly, magmas erupted during the initial stages of sea-floor spreading could be intermediate between ocean-floor and within-plate magma types. Fig. 1 illustrates the probable relationship between magma type and tectonic setting. It emphasizes that geochemical studies will often need to be supplemented by geological evidence for an unambiguous interpretation to be made.

Geochemical characteristics o f the host lavas Stable trace-element geochemistry, coupled with geological information, is used here to characterize metavolcanic rocks related to a variety of massive sulphide deposits. We deal first with the geochemical information, which is summarized in Table 2. 9 Joma type L~kken type 9 Gjersvik type o Cyprus type

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Table 2 Mean concentrations of 'stable' trace elements (ppm) in greenstones used in this study. Lavas from deposits 1, 2, 5, 7, 9 and 10 include samples from actual mines; remainder are samples from same geologic unit containing deposit, but not from deposit itself Deposit

1

Cyprusdeposits 20, 21

Location of lavas

Ti

Zr

Troodos Massif 11, 22 (axis Sequence) Troodos Massif 11, 22 (upper pillow lavas)

5900 3270

61 31

Y

Nb

28 15

1 1 --

Cr

P2Os

No. of analyses

100 405

0.08 0.06

85 45

109

0.05

15

--

--

2

Lasail,Oman 23

Lasail 23

3750

34

16

3

BettsCove,24 Newfoundland

Bale Verte 2s

1950

15

11

4

York Harbour, Newfoundland 26

Bay of Islands complex 27

10700

125

32

160

0.17

7

8550

93

26

4

221

0.09

38

9850 11400

129 171

32 26

35

114 367

0.14 0.19

18 5

--

171

0.09

18

2 --

3

5

L~kken, Norway 2s, 29

L~kken 30

6 7

RePros,Norway 31 Joma,Norway 28, 29

Gjelsvik sequence 32 Joma 3o

8

Bidjovagge,Norway

Finnmark 33

8050

103

24

9

Gjersvik,Norway 28, 29

Gjersvik 3o

8400

73

27

5

24

0.07

16

10 11

Buchans,Newfoundland Lynn Lake, Manitoba 36

Buchans 34

Rusty Lake greenstones 33,3s

3900 5100

75 54

-16

-2

74 139

0.13 0.24

90 24

12

Noranda,Ontario 36

Noranda 37

7150

92

--

--

157

0.13

78 15

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Fig. 3 Mean analyses of metabasalts from various massive sulphide deposit types plotted on Z r / Y - T i / Y discrimination diagram (analogous to T i - Z r - Y diagram of Pearce and Cann 8). Diagram separates within-plate from other magma types is a general increase in Ti and Zr from island arc through ocean-floor to within-plate magma types. Fig. 3 is a plot of T i / Y against Zr/Y. This distinguishes within-plate from other magma types, making use of the distinctive enrichment in Ti and Zr (but not Y) of within-plate basalts; Fig. 4 is a plot of Ti and Cr 11 which separates the 'plate margin basalts' of Fig. 3 into ocean-floor and island arc basalts, since for 9 a 9 o

Joma type Lekken type Gjersvik type Cyprus type

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Island arc basalt

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Cr, ppm Fig. 4 Mean analyses of metabasalts f r o m various massive sulphide deposit types plotted on log T i - l o g Cr 11 discrimination diagram any given Ti concentration island arc basalts will nearly always contain lower Cr concentrations. Fig. 5 shows chondrite-normalized rare-earth patterns, which provide useful additional information on magma type. Ocean-floor tholeiites and island arc tholeiites show slight rare-earth element depletions; ocean-floor alkali basalts and island arc calc-alkaline basalts show slight rare-earth element enrichment; whereas within-plate basalts show moderate to strong enrichment in the light rare-earth elements. The massive sulphide deposits investigated in this paper are associated with various basalt types listed above and can be divided into four categories according to the characteristics of the host lavas. Each of these categories has been named after one of its member deposits, and its inferred tectonic environment is given in parenthesis. 16

La

C'e

Joma York Harbour Gjersvik L~kken Oman Troodos

'

'

Sm ' Eu '

G'dT'b

. . . .

Tm

V'b

Lu '

Fig. 5 Chondrite-normalized rare-earth patterns for typical samples of metabasalt f r o m various massive sulphide deposit types. Cyprus analysis from Kay and Senechal 38

Cyprus-type (?early back-arc) deposits Deposits of this type were identified from the Troodos Massif, Cyprus, 2~ 21 Oman 23 and Betts Cove, Newfoundland, 24 and all contained Cu > Zn. Geologically, these deposits are situated in the lava unit of f u l l y developed ophiolite complexes, from which it is implicit that they were formed at a constructive plate margin. Basaltic pillow lavas form the main host rocks. Cyprus and Oman contain t w o lava units 21,22 _ the lower basaltic, the upper containing rocks of komatiitic composition as well as basalts. The lower unit is thought to have been erupted at the ridge axis; the upper off-axis outside the zone of deep circulation of sea water. Ore deposits in these areas are mostly located at the u p p e r - l o w e r pillow lava boundary, though some (such as Lasail, Oman 2 3 ) O c c u r within the lower sequence and are cut by numerous feeder dykes to the overlying lavas. Geochemically, the lavas are characterized by low concentrations of the small ion lithophile elements, including Cr, and therefore classify as island arc tholeiites in Figs. 2, 3 and 4; the light rare-earth-depleted patterns in Fig. 5 are consistent within this interpretation. Clearly, the geological and geochemical data are at variance. A consistent model can, Table 3

Some important characteristics of Cyprus lavas

1 Two petrologically distinct lava units 2 Concentrations of Ti, Zr and Y decrease up lava sequence 22 3 Overall, Ti, Zr and Y concentrations are low tl, 22, and there are strong light rare-earth element depletions 38 4

Low Cr and MgO concentrations (at given Ti concentration) 11

5 Basaltic komatiites present 6 Orthopyroxene phenocrysts in some lavas tl, 22 7 8"/Sr/S6Sr ~ 0.7036 in rocks apparently unaffected by isotopic exchange with sea water 39 8 Chromite deposits 9 Underlying 'upper mantle' ultramafics very depleted 4o in their basaltic component

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however, be built up by considering a transitional back-arc environment where new oceanic crust is being formed at spreading axes above subduction zones. There is some petrogenetic evidence for this hypothesis. Table 3 lists some of the ways in which lavas of the Cyprus type differ from rocks dredged and drilled from major ocean ridges. Features 3, 5 and 9 in Table 3 can be explained in terms of high degrees of partial melting under hydrous conditions; features 4, 6 and 8 can be explained by crystal fractionation under hydrous conditions; 41 feature 7 can be explained by input of water rich in radiogenic Sr derived from subducted oceanic crust; features 3 - 7 are common characteristics of island arc volcanic rocks. This particular type of back-arc environment is therefore an attractive possibility, although this is still at present only one of several hypotheses proposed.

Lr (evolved back-arc) deposits L~kken, Norway, 28,29 and York Harbour, Newfoundland,26, 27 fall into this category. These deposits contain Cu > Zn and Cu < Zn, respectively. The York Harbour deposit is found in the lavas of the Bay of Islands ophiolite complex; the Lr deposit occurs within an inverted lava sequence with associated gabbros but no exposed ultramafics. Both Lr and York Harbour apparently contain two basaltic lava units, one above and one below the ore deposit. Geochemically, the lavas contain higher concentrations of all the small ion lithophile elements in comparison with Cyprus-type lavas, although the upper lava units again appear more depleted in these elements than the lower units. These lavas classify as ocean-floor basalts in Figs. 2, 3 and 4 and this is supported by light rare-earth element depletion In Fig. 5. The question still remains whether the setting was in a major or marginal ocean basin; lavas of similar character have been found in both environments. The geological setting of the L~kken lavas 42 and their high concentration of large ion lithophile elements (which cannot be accounted for solely by alteration) both favour a back-arc setting similar to the Marianas basin at the present day. The situation for York Harbour is ambiguous in view of the conflicting interpretations of Newfoundland geology, 43 and both a major or marginal basin environment are possible. Joma-type (continenta/ margin) deposits

This category includes Palaeozoic deposits from Joma, Norway, 26, 29 Rr Norway, 31 and the Proterozoic deposit at Bidjovagge in Finnmark, Norway. 30 These deposits are all related to greenschist facies metabasalts and more silicic tuffs; metagabbros and serpentinites of uncertain association are also found in these areas. Metamorphosed sedimentary rocks overlie or are interbedded with the volcanics, and these include argillaceous and arenaceous rocks in the Rr area, and quartzites and limestones at Joma and in Finnmark. Geologically, the environment appears to resemble that of 'Sullivan-type' 5 deposits. Geochemically, the Joma samples contain high T i / Y and Zr/Y ratios (Figs. 2 and 3) and light rare-earth-elementenriched rare-earth patterns (Fig. 5), both indicative of a within plate origin. The Rr and Finnmark lavas had lower Ti and Zr concentrations, and this, coupled with high Cr concentrations, points to an ocean-floor basalt character. Geochemically, they closely resemble the Lr type of greenstone. If the geology and the geochemistry are combined, a model can be built up to suggest that these deposits were formed at a continental margin, in a small ocean of Red Sea type. At Joma both the geology (shelf sedimentation) and geochemistry (the within-plate character) support this. At R~ros and Finnmark the lavas

have an ocean-floor basalt character; but, as Fig. 2 shows, they are chemically somewhat transitional towards withinplate basalts; the presence of silicic tuffs and interbedded sediments also argues against a normal oceanic ridge setting. Their geochemistry and the geological settings are therefore both consistent with their formation in a continental margin environment.

Gjersvik-type (island arc) deposits

This type of deposit, defined by the peculiar geochemical characteristics of its related basic lavas, encompasses several sub-classes of very different geological character. With regard to the Gjersvik type, the Gjersvik C u - Z n deposit itself 28,29 lies within a pile of predominantly basic metavolcanic rocks, although some andesitic and silicic lavas, agglomerates and dykes are present. Geochemically, the lavas contain moderate concentrations of Ti, Zr and Y (Figs. 2 and 3), but very low Cr (Fig. 4), which classifies them as island arc basalts; flat rare-earth patterns are consistent with this interpretation. The nearby Z n - C u deposit at Skorovas, described elsewhere in this volume, 44 has a similar setting, but with more andesites and fewer basalts in the volcanic pile; the geochemistry of the basic lavas from Gjersvik and Skorovas is very similar. 44 The deposits seem on this evidence to have formed during submarine volcanism in an island arc setting. Kuroko-type deposits have been described in detail from Japan and elsewhere, 45,46 where the geological evidence favours an island arc origin. There are, however, several geological differences between these deposits and the Gjersvik type of deposit. These differences include the nature of the lavas (Kuroko depositsare mainly related to acid pyroclastic rocks), and the ore mineralogy and texture (Kuroko deposits contain Cu, Zn and Pb minerals with barite and gypsum). Kuroko-type deposits appear to be genetically related to small acid intrusions. Nevertheless, published geochemical data on andesites from the Buchans deposit, Newfoundland, 34 closely resemble data on the andesites from the Gjersvik area. The Noranda type of Z n - C u deposit occurs within Precambrian greenstone terrain in association with acid pyroclastic rocks. 47 Geochemical data on the more basic rocks from the Noranda 37 and Lynn Lake 33,35 regions revealed low to moderate Ti, Zr and Y concentrations (Figs. 2 and 3) and low Cr (Fig. 4) - characteristics very similar to those of basic lavas from the Gjersvik region. The Lynn Lake lavas appeared to be tholeiitic in character, the Noranda lavas more calc-alkaline. Thus, although we do not yet know sufficient about tectonic processes in the Archaean to interpret these data in terms of tectonic environment, we can say that the lava geochemistry most closely resembles that from present,lay island arcs.

Tectonic setting of massive sulphide deposits The main conclusion that can be drawn from this study is that massive sulphides can form in a variety of environments. Most of the examples used here appeared to have formed in a back-arc setting or during incipient formation of an ocean basin; Fig. 6 illustrates the possible environments of formation of the various types of massive sulphide deposit in a typical western Pacific arc-trench system. The results also raise the important question: do all settings in which submarine volcanism takes place have an equal massive sulphide forming potential? The evidence from this paper suggests that perhaps they do not. With the possible exception of the small deposit at York Harbour, no good examples have been found of massive sulphide deposits related to the major ocean ridge types of volcanic 17

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Continental margin

Marginal basin

Third arc

Inter-arc basin

Island arc

~

. 9 ,

~

Arc --trench gap

Trench Sea-Leve[

a ~ j ~

5 10

km

15 /"

~

/,

I M ,,~

s

20

f

J i

1 2 3 4 5

Joma type Lc~kken type Gjersvik type Cyprus type Kuroko type

Fig. 6

Possible distribution of massive sulphide deposit types in marginal basin setting. Section taken from Karig 8o

rock. This might be explained in terms of the absence of true mid-ocean ridge ophiolites in the geologic record. A few probable examples do exist, however, in Macquarie Island, 4 the western Alps 49,50 and St~ren and Stavenes 30 in the Norwegian Caledonides. These do contain tiny deposits, confirming that ore-forming processes do take place in this environment; but there are no known deposits of any major economic value. If this apparent absence of deposits in major oceans is real, it can be explained in several ways. Magmatic fluids could be more important than has hitherto been considered; perhaps magmatic fluids in a back-arc setting contain a source of reduced sulphur derived from the breakdown of pyrite to form pyrrhotite in subducted oceanic crust. Perhaps the passage of the fluids through evaporites at some continental margins yields an important source of sulphur. Both these possibilities can be tested by thermodynamic calculations. The most probable explanation may, however, lie in the necessity to have a site for ore deposition. The highly faulted ocean crust that exists during early spreading and in back-arc settings could provide fault-bounded troughs where low Eh and restricted sea-water circulation would provide a favourable environment for ore deposition; these conditions are less likely to be met in an evolved ocean.

Tin deposits Magmatism in the continental environment Tin deposits are related to acid-intrusive rocks emplaced into continental crust - at 'hot spots' within plates, at destructive plate margins and as a result of c o n t i n e n t continent collision. These rocks belong to two main magma types: volcanic arc magmas (which include rocks of the tholeiitic, calc-alkaline and shoshonitic series) and withinplate magmas (which include rocks of the tholeiitic and alkalic series). There are substantial geochemical variations both within and between these magma types, whose relationship with tectonic setting is illustrated in Fig. 7. Acid and intermediate within-plate magmas tend to have high concentrations of both large and small ion lithophile elements, with the exception of a few elements, e.g. Ti and Sr, which have not behaved incompatibly during magma genesis. The concentrations of the incompatible elements increase from tholeiitic to alkali magma types. Volcanic arc magmas also contain high conentrations of the incompatible large ion l ithophile elements - concentrations which

18

increase from tholeiitic through calc-alkaline to shoshonitic types. By contrast, however, concentrations of small ion lithophile elements are low, particularly in the island arc tholeiitic series. Continental magma types Withinplate magmas

I

Volcanic magmas

arc

Thoteiitic~ ~ Atkatic Continental rifts "hot-spot = volcanoes Tholeiitic ~ C a t c - a t k a l i n e ~ S h o s h o n i t e s ~ island a r c s Andeon-type continental arcs collision

ttMay include crustal melts Fig. 7 Distribution of magma types within continental environment. Magma types transitional between withinplate and volcanic arc types do occur, but only in rare tectonic settings The most effective elements for identifying whether an acid igneous rock has a within-plate or volcanic arc character are therefore those with small highly charged ions. Nb is particularly useful, and diagrams such as SiO2-Nb, K 2 0 - N b and Z r - N b are all valuable in the identification of magma type. The S i O 2 - N b diagram presented in Fig. 8 separates low Nb volcanic arc magmas from high Nb withinplate magmas. An overlap occurs on this diagram between tholeiitic within-plate magmas and high K calc-alkaline or shoshonitic magmas from volcanic arc and post-orogenic settings. K20 and Rb can be used to partly eliminate this overlap, provided that K-silicate alteration has not taken place. Acid within-plate magmas range from about 100 ppm (at Nb = 15 ppm) to 300 ppm Rb (at Nb = 500 ppm), whereas acid volcanic arc magmas range from about less than 50 ppm Rb (at Nb = 5 ppm) to greater than 200 ppm (at Nb :> 20 ppm). This rough guide suggests that acid rocks which contain very high Rb and K20 concentrations and which plot within the overlap field have a volcanic arc origin. Because so little is known about geochemical and mineralogical variations in the lower crust and upper mantle it is impossible to give a precise explanation for this diagram. A few general points can, however, be made. It is apparent from the magnitude of the Nb variation ( 5 - 5 0 0 ppm in acid rocks) that variations in the degree of

Downloaded from http://sp.lyellcollection.org/ at Dalhousie University on January 9, 2013

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Estimated fietd of "-- crustal melts

Fig. 8 Nb-SiO 2 diagram showing data from volcanic rocks of known tectonic settings. Single diagonal line gives lower limit for within-plate magma type; double diagonal line gives upper limit for volcanic arc magma type. In general, Nb content of within-plate magmas increases from tholeiitic to alkalic types, and Nb content of volcanic arc magmas increases from island arc to shoshonitic types. Estimate of Nb-SiO 2 range of crustal melts derived from sediments metamorphosed up to and including amphibolite facies superimposed on diagram

partial melting and/or crystal fractionation cannot alone explain why this element does discriminate between the various tectonic environments. The nature and composition of the source rock may be the single, most important factor. Partition coefficients for Nb 33 between crystal and melt are probably low (< 0.2) for olivine, orthopyroxene, garnet and plagioclase, moderately low (< 0.6) for clinopyroxene, moderately high (> 1.0) for amphibole and high for micas ('> 2.5), magnetite, apatite and zircon. As a result, the presence of minor phases, particularly minerals such as phlogopite,52,,s3 in the source rock and their behaviour during melting will play an important part in determining the concentration of Nb in the melt. Because of their high ionic potential, Nb 5+ ions will not be strongly partitioned into aqueous fluids. Ions of low ionic potential, such as K +, will, however, be readily transported in these fluids. This last point may partially explain the geochemical characteristics of volcanic arc magmas. The mantle wedge above the subduction zone will be modified by aqueous fluids derived from dehydration of oceanic lithosphere, s4, 55 These fluids should contain high K + concentrations and relatively low Nb s+ concentrations. Thus, any acid magma derived from partial melting of this mantle followed by crystal fractionation will not be enriched in Nb. By contrast, heterogeneities in within-plate settings involve pro-

cesses that affect K + and Nb 5+ to an approximately equal extent. These could be caused by migration of small amounts of incompatible rich melt 56 or represent variations in the melting--recycling history of the mantle. In either case partial melting of mantle enriched in this way followed by crystal fractionation should produce acid melts rich in Nb. There are also other factors that could also partially explain the Nb variations - in particular, the behaviour of some of these Nb-bearing minor phases in the mantle. For example, residual mica phases will reduce the Nb content of the melt in equilibrium with them. The arguments outlined above apply to magmas derived from the mantle; but, of course, some acid magmas may have had a crustal source. A field for crustal melts has therefore been superimposed on this diagram. This was delineated by explaining the geochemistry of concordant granitic veins in Precambrian terrains (see, for example, Bowes 57 and Drury 58) and by carrying out petrogenetic calculations on the melting of metasedimentary rocks. Nb-bearing phases, hornblende, biotite, muscovite or phlogopite, are usually residual phases during partial melting of pelitic and siliceous sediments 59,6o,61 and this constrains the melt to low Nb concentrations in most models. There are, however, some exceptions to this: partial melting of eclogite and some granulites will leave pyroxene, garnet and plagioclase as residual phases, and high Nb melts can therefore be produced. 19

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Most granites produced by melting of continental crust at depths where amphiboles and micas are residual phases should, however, plot in or close to the area shown and will geochemically overlap (and perhaps include) thole9149 within-plate magmas and magmas from volcanic arcs, particularly as far as their Nb and SiO 2 concentrations are concerned. Geochemical characteristics of tin-bearing magmas Table 4 summarizes some of the important geochemical characteristics of tin-bearing granites from Nigeria, 62, 63, 64 Cornwall,17,18 Bolivia 65 and Indonesia. 66 These data have been plotted on to the SiO2-Nb diagram (Fig. 9). The common characteristic of these four granite provinces is their very high concentration of K 2 0 and Rb. There are, however, significant differences in the concentrations of other elements. The Nigerian granites contain very Table 4 Mean concentrations (ppm) of some trace elements in tin-bearing granites (note that Nigeria data include all Younger Granites; tin-bearing biotite granites have Slightly lower concentrations of incompatible trace elements) Nigeria SiO2, %

74.5

Zr Y Nb

650 220 170

La Sm Yb

185 57 34

Rb

330

500

Bolivia 67.1 173 19 21 41 5.2 1.6 310

[]

Cornwall

9

Indonesia

o

Bolivia

II

Nigeria

x

Wales

Indonesia

70.8

Cornwall

72.4

130 64 15

78 13 15

68 11.2 4.7

33 7.0 0.9

405

580

high concentrations of Nb, Zr, Y and the rare earths and plot in the within-plate magma field in Fig. 9. This interpretation is consistent with geological evidence for a 'hot-spot' origin for these granites. 15,61,62 The other granites contain much lower concentrations of these elements and, in Fig. 9, plot in the overlap zone between volcanic arc and withinplate magmas; the very high Rb concentrations, however, point to a volcanic arc magma type. Geological criteria are then needed to distinguish between an active volcanic arc and a post-orogenic setting. The Bolivian granites were clearly related to subduction processes; however, their setting in the Eastern Cordillera of the Andes argues in favour of an origin associated with tensional stressesand back-arc rifting rather than with the processes that produced magmas in the Western Cordillera (the volcanic arc sensu stricto). The Cornish granites are geochemically almost identical to the Bolivian granites, although geological opinion is divided between a post-orogenic and a volcanic arc setting.i7,18 The Indonesia granites contain concentrations of Rb, Zr and Nb similar to those of the Bolivian and Cornish granites, but significantly higher concentrations of Y and the heavy rare earths. This does not indicate any obvious difference in tectonic environment (a back-arc rifting setting is geologically likely 16), but it does suggest significant differences in the role of garnet during the genesis of the magma. To illustrate the composition of intrusive rocks related to porphyry copper deposits some analyses 60 from the region of the Coed-y-Brenin porphyry copper deposit 67 in Wales have been plotted in Fig. 9. They plot, as expected 68,69 in the field of volcanic arc magmas, their very low Nb concentrations favouring an island arc rather than continental margin setting.

H i g h Rb II

m

u9 99

100

9

9149

i

9

m9

Ill

9

5O E

Within-plate

EL EL

_

~

/

Z Z

High Rb

L w Rb

9

10

Volcanic arc

~x Crustal x

x

\\

$ I

metts

/I

\

Low Rb 45

50

55

60

65

70

75

S i 02, % Fig. 9 SiO 2 and Nb data from some tin-bearing granites plotted on to N b - S i O 2 diagram of Fig. 8. Also plotted for comparison are analyses of intrusive rocks from Coed-y-Brenin p o r p h y r y copper district, Wales

20

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Source of tin-rich magmas A number of hypotheses have been proposed to explain why certain magmas should have high tin concentrations: these include the melting of tin-rich detrital sediments 70 or granitic rocks 71 in continental crust; melting of upper mantle; 72 and evolution of fluorine-rich fluids derived from breakdown of apatite in subducted oceanic crust followed by mantle or crustal melting. 16 Here we briefly set out some of the petrological constraints on such processes, emphasizing the deep-seated processes rather than the processes that concentrate and deposit the tin at shallow crustal levels. The following factors appear to be significant. (a) As was stressed in the previous section, the only obviously distinctive features of tin-bearing granites are that they are emplaced into continental rather than oceanic crust and that they are regional rather than local phenomena. They can be formed in most tectonic settings. (b) Examination of patterns of trace-element partitioning 73 and the few available Sn analyses of mineral phases suggests that partition coefficients for Sn between crustal and melt will be low for quartz, feldspar, pyroxene, garnet or olivine, moderate for amphibole, but high (> 1) for micas and other minor phases such as apatite, zircon and sphene. This means that production of tin-rich magmas by partial melting is not possible if micas and amphiboles make up a significant part of the residue. Thus, partial melting of pelitic rocks or of any rock metamorphosed in amphibolite or greenschist facies is not likely to yield tin-rich magmas; partial melting of eclogite and granulite facies metabasic rocks or partial melting of the mantle could yield tin-rich magmas, provided that the tin-bearing phases, such as phlogopite, are completely melted and the source rocks themselves contain sufficiently high tin concentrations. (c) Although there are few thermodynamic data on tin complexes, it is evident that tin has a particular affinity for fluorine (see, for example, Mitchell and Garson 16 and Barsukov 74) and that the partitioning of tin into a melt or aqueous fluid will increase with increasing fluorine concentration. The main fluorine-bearing minerals in the lower crust and mantle are phlogopite and apatite (minerals which also accommodate tin), so the stability of these minerals is likely 88 be particularly important. (d) It is well established 75 that fluorine-rich melts and fluids can leach tin from mica lattices. Reaction between fluorine-rich magma and wallrock may therefore be a potent mechanism for tin enrichment. (e) The small amount of available data shows that tin concentrations in mantle-derived basalts range from 0.2 ppm (see, for example, Gill 76) in island arc tholeiites to > 5 ppm in within-plate alkali basalts. In addition, a recent compilation by Bailey and Macdonald 77 has shown that peralkaline obsidians from continental settings contain significantly higher F/CI ratios than those from oceanic settings. The factors listed above have implications for the origin of the four tin-bearing granite provinces discussed in this paper. The Nigerian granites may give the simplest story, since no subduction zone was involved in their genesis. These granites appear to contain concentrations of Zr, Nb and the rate earths that are much higher than those expected for crustal melts, even when the proposed enrichment or redistribution of these elements during wallrock alteration has been taken into account. 76 Furthermore, their geochemistry resembles that of acid rocks from ocean islands where no continental crust existed. This evidence supports a mantle origin for the granites - by partial melting and subsequent crystal fractionation. Initial 87Sr/86Sr ratios and lead isotope ratios 63 indicate that some crustal contamination may have taken place, although without stable

isotope data this cannot be regarded as unequivocal evidence. The tin itself could have both a mantle component (see above) and a crustal component (tin-bearing pegmatites have been recorded in the Pan-African basement 63, 64). The other tin-bearing granites contain lower concentrations of the small ion lithophile elements and some analyses do overlap the field of crustal melts in Fig. 9; neither a crustal origin nor an origin from mantle enriched by fluids emanating from subducted oceanic crust can therefore be ruled out. We suggest the following multistage model to explain the origin of the Bolivian tin;bearing granites. This model most closely resembles that of Mitchell and Garson, 16 although there are a number of important differences. Stage one involves subduction of oceanic crust beneath the South American continent. During the long time-interval (Late Triassic-Tertiary) 79 between incipient subduction and emplacement of tin-bearing magmas, the composition of the mantle wedge beneath Bolivia is altered by input of small amounts of melt/aqueous fluids derived from eclogite in the subducted oceanic lithosphere. A mantle phase such as phlogopite could accommodate both tin and fluorine (and, of course, other elements, such as potassium) introduced in this way. Stage two involves partial melting of this enriched mantle in a back-arc rifting environment. The tectonic process is probably analogous to that which causes marginal basins to form behind island arcs, but with one important difference - the time