Presentations

Yardley Chemistry

Description
publicacion
Categories
Published
of 10
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
Share
Transcript
  Yardley, B.W.D. et. al., 2000 - The Chemistry of Crustal Brines: Tracking Their Origins; in Porter, T.M. (Ed.), Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective,  Volume 1; PGC Publishing, Adelaide, pp 61-70. 61 THE CHEMISTRY OF CRUSTAL BRINES: TRACKING THEIR ORIGINS 1 Bruce W.D. Yardley, 1 David A. Banks & 2 Andrew C. Barnicoat 1 School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK  2  Rock Deformation Research, School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK  Abstract   - Brines may be generated in sedimentary, magmatic or metamorphic settings, and they changechemistry extensively as they move through rocks and interact with them. The primary constraint on their metal carrying capacity is their salinity, but they may carry very variable amounts of S in solution, dependingon their source and the rocks that they have encountered. Sulphur availability and oxidation state are alsomajor controls on which metals will be transported and which precipitated. Availability of fluid inclusion brine analyses is making possible the characterisation of a much wider range of brine types than was hitherto possible, and providing information about metal contents in a wide range of settings, as well as tracer analyses. Iron contents of brines are broadly temperature dependent, and are much higher in magmatic brines than in sedimentary ones, but basinal fluids may still carry sufficient Fe in solution to precipitate ironoxides at an oxidation front, and may be much more voluminous. Brines of different origins can often bedistinguished on the basis of conservative halogen tracers unaffected by wall rock interactions: Br/Cl ratiosused in conjunction with I/Cl ratios or d 37 Cl values separate residual bittern fluids from re-dissolved evaporites,with igneous brines forming an intermediate, but somewhat distinct, grouping. Introduction The significance of brines and evaporites in the genesis of ore deposits has been recognised increasingly in recentyears (Warren, 1999), and they have been implicated inthe formation of a wide range of deposits from base metalsto emeralds. In general terms this is not surprising; manymetals are complexed in aqueous solution by chloride, andso relatively small amounts of brine can transport relativelylarge masses of metal. However chloride is extremelysoluble in aqueous solution, only entering into rock-formingminerals under exceptional conditions, and as a result its behaviour in the Earth is not well understood. Untilrecently, brine research has concentrated almost exclusivelyon oil field waters (e.g. Carpenter et al. 1974, Rittenhouse1967, Hanor 1994, Land 1995), however the advent of newanalytical techniques for fluid inclusions have enabledmany of the geochemical techniques for interpreting oilfield brine srcins, to be applied to ore deposit brines(Bohlke & Irwin 1992, Kessler et al. 1995, Campbell et al.1995). In the process, it has become apparent thatcomparable mechanisms to those that generate brines inthe near-surface, may operate at greater depths and atelevated temperatures, in addition to derivation formmagma. New models for the genesis of brine-relateddeposits require both an understanding of how brines candevelop and interact with rocks, and the development of tracer techniques for distinguishing brines of differentsrcins.Highly saline fluids can evolve in four distinct geologicalsettings, the first three of which are of potential economicsignificance:i)Exsolution from crystallising magma.ii)Evaporation of seawater, leading to the formation of  bittern brines, enriched in Br, once halite begins to precipitate.iii)Re-dissolution of halite deposits.iv)Removal of water from chloride-bearing solutionsduring metamorphic hydration reactions within thecrust.This contribution is concerned with two aspects of crustal brines: the ways in which their chemical composition may be modified as a result of reactions with the rocks throughwhich they pass, and the application of tracers to determinethe srcins of particular brines. Primary Controls on Brine Chemistry The initial chemical and isotopic composition of a brinegenerated by magmatic or sedimentary processes may bechanged rapidly by interaction with rocks along its flow path. As a result, common chemical characteristics canappear in brines of very different srcins. The fundamentaldistinction to be made is between conservative  components,i.e. those that are partitioned strongly into the fluid phaseand are therefore little affected by interactions with rocks,and non-conservative  components that are readily  62 General Papers exchanged with wall rocks and therefore lose any distinctivesignature of brine source. The best known of theconservative components, that retain the longest memoryof the srcins of the brine, are Cl and Br, while δ D and δ 37 Cl are conservative isotopic tracers. Cations, manyanions, and most isotopic tracers such as δ 18 O or Sr isotoperatios are less conservative; some retain a memory of theimmediate environment through which a brine was finallyemplaced, while others may give an intermediate memoryof features along the flow path. We can also distinguish between brine components that occur in solution in amanner that is coupled to the availability of Cl-, and thosewhose solubility is, to a first approximation, independentof salinity. Many metals occur in solution predominantlyas either individual ions, whose abundance is limited bythe availability of anions, or as ion pairs with one or moreCl- ions, some may be significantly complexed by other ligands such as fluoride, sulphate or bisulphide, if present,while others, most notably silica, form hydrated speciesapproximately equivalent to an un-dissociated weak acidmolecule. Clearly, the distinctive role of brine in ore genesisresults largely from the major role that chloride plays inthe solubility of many elements. Examples of a range of natural brine compositions are listed in Table 1; the way inwhich the compositions of these brines reflects their srcinwill be returned to below. Note that many of the analyseslisted in Table 1 have been obtained by analyses of fluidinclusions. These analyses have been selected because theyappear to be of high quality, with good charge balance andwith no evidence for contamination by solid phases (for example Mn/Fe is generally much higher than in coexistingFe-bearing minerals). Details of the analytical methodsand their validation is given in the srcinal references citedwith the Table.Brines of sedimentary srcin differ subtly in their composition according to whether they are bittern brinesor are produced by re-dissolution of halite (Rittenhouse1967, Fontes & Mattray 1993). While bittern brinesconcentrate a range of soluble elements, notably Br, relativeto Na and Cl as halite precipitates, re-dissolution of halite produces brines dominated by NaCl itself. Thus thedifferences between them are in the concentrations of awide range of elements initially, with bittern brines enrichedin a range of components relative to seawater, while brines produced by dissolution of halite exhibit complementarydepletion in many trace elements. Seawater itself has adistinctive chemistry resulting from its highly oxidisedcharacter; hence sulphate levels are high, as is Mg, whereasFe is present only at very low levels.Magmatic brines are best known from porphyry-Cudeposits, where they may have been modified by boiling, but data also exists for deeper, supercritical brines (Table1). They are likewise dominated by NaCl, but a wide rangeof elements may be enriched. In particular, levels of K anda range of transition metals, including Fe, are often higher than in sedimentary brines, while Ca is generally lower.Whereas the total salinity of sedimentary brines is generallyclose to halite saturation at surface temperatures, magmatic brines show a much wider range of salinities. Many are of distinctly lower salinity (c. 15 eq. wt. % NaCl), but othersrange up to wet salt melts; salinity variations in evolvingmagmatic fluids may be a complex function of the depthof crystallisation (Cline & Bodnar 1991). A key feature of  brines released at magmatic temperatures is that they havea relatively high proportion of HCl available for acidleaching of cations from wall rocks: according to pressurethis may occur as HCl ion pairs (shallow levels) or asdissociated ions. Although dominated by chloride,magmatic brines may contain significant concentrations of other ligands that are themselves effective at transportingcertain metals, for example fluoride and borate. Evolution of Brine Chemistry in theCrust In the course of fluid-rock interaction, a series of factorsmay influence the final brine chemistry, but in most settingsthe Cl-content remains relatively constant, so that the loadof dissolved metals must satisfy charge balance. The principal factors controlling the final brine chemistryattained due to fluid-rock interaction will have the sameeffects irrespective of the srcinal source of the brine:i)temperature (to a lesser extent pressure, except at veryshallow levels)ii)salinityiii)wall rock lithology and mineralogy, especially ã redox environmentã presence or absence of evaporite minerals Temperature The exchange of cations between minerals and solution isstrongly temperature dependent, and this provides a basisfor the use of mineral geothermometers in geothermalexploration. Most conspicuously, K/Na ratios increaserapidly with temperature relative to the very low values inseawater. In addition to ion exchange, other aspects of mineral solubility are temperature dependent. For exampleheating of seawater leads to precipitation of anhydrite, whilequartz becomes more soluble with increased temperature.In the case of Fe in particular, there is good evidence fromthe analyses presented in Table 1 to suggest that itsconcentration in rock-buffered brines is also temperaturedependent. Concentrations are generally low in oilfield brines, while in low-T shield brines it is present only attrace levels (Fritz & Frape 1987). Magmatic brinestypically have much higher concentrations of Fe, althoughit is clear from the different examples, discussed below,that other factors also exert important controls on Fe-levelsin solution. In general however, it is worth emphasisingthat many metals are very soluble as chlorides over a verywide range of geological conditions, and temperature isnot directly a constraint on their solution and transport. Thisis clearly seen from the formation of ores of a wide rangeof metals at temperatures of just a few hundred degrees.  Salinity Understanding how salinity influences the metal contentsof brines is central to understanding their role in oreformation. There are two types of effects to be taken intoconsideration: firstly the proportion of mono-valent to  The Chemistry of Crustal Brines - B.W.D. Yardley et. al.  63 divalent (or trivalent) cations in a fluid in equilibrium witha fixed mineral assemblage is influenced by the totalchloride content; secondly, for certain metals, complexing by chloride can become much more effective withincreasing chloride activity.The first effect has been discussed in some detail by Eugster & Gunter (1981) and can be illustrated very simply withreference to plagioclase ion exchange in saline solution:2NaAlSi 3 O 8  + Ca 2+  = CaAl 2 Si 2 O 8  + 4SiO 2  + 2Na + for which at quartz saturation: logK = log ( a CaAl 2 Si 2 O 8 / a 2  NaAlSi 3 O 8 )  + log ( a 2  Na + / a Ca 2+ ) It follows that, for fluids in equilibrium with quartz and agiven composition of plagioclase at a fixed P and T, it isthe ratio of the activity of Na +    squared   to Ca 2+ , not the Na + to Ca 2+  ratio, that is buffered. As the salinity of a fluidincreases, so too does the concentration, and hence activity,of Na +  and Ca 2+ . This means that at progressively higher salinities the Ca 2+  activity must increase with the square of the Na +  activity, leading to a considerable increase in theconcentration ratio of Ca/Na. This is an important factor in accounting for the very high Ca-contents of manyconcentrated brines.The second way in which metal contents change withsalinity is through the formation of a range of chloridecomplexes. In sufficiently dilute solution, metals occur asfree ions, with chloride complexes developing as theavailability of chloride ions is increased:Me 2+  + Cl- = MeCl-MeCl- + Cl- = MeCl 2 oMeCl 2 o + Cl- = MeCl 3+ and so on. The larger complexes are only likely to becomesignificant at high salinities, where their abundanceincreases as a power of the chloride activity, leading torapid increases in metal levels in solution. This effect isdocumented for Pb, for example, a metal which is knownto be able to reach exceptional levels in chloride-richsolutions in the right circumstances (e.g. Svensen et al.1999). Wall Rock Lithology and Mineralogy The influence of wall rocks on the composition of crustalfluid cannot be underestimated in all but the most openenvironments of rapid fluid flow. For example, Banks etal. (1991) found that many aspects of the chemistry of brineinclusions in quartz veins around the Pic de Port Vieuxthrust, central Pyrenees, reflected equilibrium with localwall rocks hosting the vein in a lithologically variedsequence, whereas the chloride and bromide contents of the inclusions showed that in each case the vein fluids camefrom a common reservoir. In both deep diagenesis andactive geothermal fields, fluids in rock-dominated systemsequilibrate with their hosts, and in particular with new-formed minerals grown from the fluid (Hanor 1994, Land1995). A common consequence of this is the removal of SO 4  and Mg from brines in the crust (below), and their enrichment in Ca. While salinity plays a role in determiningthe Ca content of brines, as outlined above, an additionalfactor is the instability of plagioclase at low temperatures.Few plagioclase compositions are stable below about500˚C, and albitisation is a common feature of hydrothermalalteration. Anorthite component recrystallises intorelatively soluble phases such as epidote. Hence while athigh temperatures solutions in equilibrium with plagioclasemay be rich in Na (Orville 1972), at low temperatures evenlow salinity fluids have significantly enhanced Ca contents,often limited by precipitation of calcite.The most dramatic effects of wall rock reaction take placewhere evaporite minerals are present, and lead toenhancement of the dissolved load of the fluid by simpledissolution. While this is a process that is best known atshallow levels in sedimentary basins, it can also occur athigher temperatures, giving rise to correspondingly moresaline brines. Columbian emerald fluids reach salinitescorresponding to halite saturation at temperatures close totheir peak metamorphic conditions.Redox environment plays a crucial role in the solubility of many metals in brines through both its control of sulphur speciation and its direct influence on the solubility of metalswith variable valency. The transition from sulphate- tosulphide-dominated solutions takes place around thehematite-magnetite buffer, in terms of oxygen fugacity, anda range of metals change solubility markedly near this boundary in S-bearing systems. Conventionally, metalsolubility is often contoured onto log  f  O 2  - pH plots drawnup for a total S concentration specified irrespective of itsspeciation. While this may be appropriate for fluid systemswith an external control on S-level, such as volcanic gasses,it is not obviously applicable to a rock-buffered brine for which redox state and pH may be limited by oxide andsilicate equilibria and the amount of S entering solution isdictated by saturation with pyrite or another S-mineral inthe rock.Figure 1 is a log  f  O 2  - pH plot contoured to show theconcentration of S in solution in equilibrium with pyriteand an iron oxide phase (hematite or magnetite, accordingto the value of  f  O 2 ). The calculations were constructedusing the EQ3 code (Wolery et al. 1983), for a temperatureof 300˚C and a pressure of 0.1GPa, and demonstrate that,far from being constant, total S in solution varies over 2orders of magnitude within the range of log  f  O 2  and pHthat is likely to represent reasonable upper crustalconditions. These variations have significant implicationsfor transport and precipitation of Fe and Au.Dissolution of iron is strongly dependent on redox state, because Fe 2+  is much more soluble than Fe 3+ . For example,it was noted by Bottrell and Yardley (1991) that the Mn/Feratio of fluids in equilibrium with vein chlorite from ahematite-bearing host rock was much greater (and in excessof 1) than the value obtained for fluids in equilibrium witha similar composition chlorite from a graphitic host. Thiseffect is illustrated in Table 1 by the examples of brines  64 General Papers Table 1 .Compilationofanalysesofcrustal brinessampledbydrillingorasfluidinclusions  1 2 3 4 5 6 7 8 9 10 11 12 13 Approx.Temp.(C)        2       2       1       2       9        1       4       3        2       5        0        2       6        0        2       8        0        3        1       0        3        2       0        3        2       0        5        5        0        6        0        0        6        0        0        6        0        0      N    a         1       8        9        0        6        1       1       0        0        6        3        0        0        0        6        1       4       6        2       3        2       7        0        0        2       9        1       3        5        6        2       2       1       3        1       3        0        9        4       6        5        4       5        9        0        7        8        0        0        0        3        9        5        2       0        1       8        2       4       3        8        1       7        8        6        3        2     K        4       3        8        5        4       6        1       5        0        3        0        4       8        6        5        4       0        0        1       2       1       8        1       4       5        6        4       5        7        9        5        1       1       6        2       2       3        7        0        0        0        1       3        9        6        0        6        1       4       0        9        7        4       8        6        8      C    a         6        3        8        0        2       8        8        0        0        4       4       6        0        0        1       9        3        5        4       4       3        4       6        3        9        0        0        4       4       7        6        3        7        6        7        4       6        4       4       0        2       7        3        2       0        0        9        6        2       0        3        0        7        2       3        4       2       3        3        6      M   g          7        8        1       8        3        0        2       7        7        0        8        9        1       3        4       7        1       1       3        3        1       2       0        7        1       0        6        2       6        3        9        1       2       0        3        4       3        2       6        8      F   e         2 .        0        7        3        3        8        3        2       0        1       7        2       7        3        4       7        2       3        7        2       2       8        6        8        4       3        0        0        2       3        7        9        1       6        0        0        0        0        1       1       9        5        0        1       6        3        1       0        1       3        9        1       5      M   n        4 .        5        7        6        5        6        0        7        8        0       <       1       0        2       2       4       1       7        7        3        4       2       0        3        3        0        8        1       8        0        0        0        3        2       4       0        2       1       5        6        4       1       7        4       1       7      L    i        0  .        8        1       5        2       2       9        5        2       7        8        0        2       1       5        1       8        7        9        9        4       4       2       0        3        1       2       1       5        1       2       8        5        4     S    r        1       5        8        1       8        2       0        1       7        7        0        7        5        0        1       1       4       1       1       4       5        3        4       2       1       1       4       4       1       6        0        5        3        0        1       7        5        7        8        4       1       8        2       2     B   a         5        6        8        9        2       5        2       6        8        3        7        0        1       1       5        7        1       1       8        1       0        6        4       1       3        7        3        4       5        9        8        2     P    b         3  .        0        3        4       4       3        1       7        8        1       4       3        0        1       3        1       3        3        0        0        8        0        3        5        3        6      Z   n        0  .        5        2       2       1       7        1       8        8        0        3        0        7        7        6        3        9        2       1       9        6        5        7        3        5        2       0        0        1       5        6        9        2       2       0        7        1       3        4       0      C    u         0  .        3        3        2       5        1       3        8        3        3        4       8        3        9        3        1       1       5        9        0        0       <       1       0       <       1       0      A   s         8        6        7        8        1       7        4       8        0        5        9      B        6  .        2       1       6        0        6        2       3        3        2       0        3        1       5        6        3        9        7        5        4       3        8        2       1       4     F        7        4       5        0        3        2       0        3        8        1       7        1       7        7        0        3        5        7      C     l        1       6        2       7        0        1       5        0        7        0        0        2       0        0        4       0        0        1       3        8        4       0        1       2       0        8        5        3        1       2       4       8        7        5        1       8        7        4       0        5        2       3        0        0        8        2       2       1       1       0        9        1       2       6        6        0        0        0        1       0        6        3        6        0        3        9        8        9        3        7        4       2       9        5        0        4     B   r        1       2       5        1       0        7        0        2       3        4       0        5        2       9        2       5        1       8        0        0        2       7        9        3        3        6  .        7        1       0        0  .        6        4       2       0        1       5        0        1       4       6        2       5        0      I        7        1 .        9        3        2 .        7        2 .        2       0  .        3        0  .        2       4 .        8        0  .        9        1       0  .        5        4     S     O   4        2       2       3        8        0        1       2       8        1       4       5        7        1       5        3        2       5        8        2       1       8        4       4       6        3        1       1       8        9        6        0        7        5        2       4       2       1       0        2       1       4       7        2 1.  Shieldbrine,Sudbury,Canada.Fritz & Frape (1987),sample N3646A 2.  Oilfieldbrine,C.Mississippi,USA.Carpenteret al.(1974),sample 39 3. Oilfieldbrine,C.Mississippi,USA.Carpenteret al.(1974),sample 3 4.  Variscanlowgrade metamorphic brine,Allihies,SW Ireland.Meere & Banks(1997),vein7 5.  Quartz-hematite veinacidsulphate fluid,OuroFino,Brazil.Boironet al.(1999) 6.  Lowgrade metamorphic brine from Alpine vein,Pic de Port Vieux,central Pyrenees.Bankset al.(1991),sample 50177(reducedwall rocks) 7.  Lowgrade metamorphic brine from Alpine vein,Neouville massif,central Pyrenees.McCaiget al.(2000),sample 55141(oxidisedwall rocks) 8.  Brine inclusionsinemerald,Coscuez,Columbia.Bankset al.(2000b),sample GG-6 9.  Brine inclusioningangue quartz,Coscuez emeralddeposit,Columbia.Bankset al.(2000b),sample GG-5 10.  Granite-derivedbrine,quartz-cassiterite veinfluid,Mole granite,Australia.Heinrichet al.(1992) 11.  Granite-derivedbrine,quartz-tourmaline-topaz rock,Cornwall,U.K.Bottrell & Yardley(1988) 12.  Granite-derivedhypersaline brine,quartz-fluorite vein,Capitanpluton,NewMexico.Campbell et al.(1995),sample CPU-2(also1290ppm REE) 13.  Granite-derivedhypersaline brine,quartz-fluorite vein,Capitanpluton,NewMexico.Campbell et al.(1995),sample MTE (also193ppm REE)
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks