Characterisation and prediction of sediment-hosted ore deposits using sequence stratigraphy

Ore Geology Reviews 12 Ž1998. 207–223

Characterisation and prediction of sediment-hosted ore deposits using sequence stratigraphy A.H. Ruffell ) , N.R. Moles, J. Parnell School of Geosciences, The Queen’s UniÕersity of Belfast, Belfast, Northern Ireland BT7 1NN, UK Received 10 April 1997; accepted 8 December 1997

Abstract The processes that form stratabound ore deposits involve diagenesis, fluid-flow, rock–fluid interaction and metal fixation which, like the maturation, migration and entrapment of hydrocarbons are integral aspects of basin history. Sequence stratigraphy is a method by which the stratigraphic record of sedimentary basins may be analyzed and placed in facies-dependant depositional units related to changes in relative sea level. Here we show how some of the concepts of sequence stratigraphy can be applied in the interpretation and better understanding of known metal generative zones and in the prediction of potential host rocks of sedimentary ore deposits. Sequence stratigraphic concepts are of most use in explaining and predicting the juxtaposition of metal- or sulphur-generative sediments such as evaporites, red-beds and organic-rich shales, and in understanding mineralization associated with black shales, hardgrounds and reworking surfaces Žplacer deposits.. The analysis of depositional sequences may also enable predictions to be made concerning likely ore-hosts, such as MVT mineralization in carbonate reefs and reservoir sands. The final position of a sedimentary ore deposit is dependent on Ž1. the geometry of the sedimentary sequence, Ž2. the depositional environments Ži.e. specific lithologies. within each systems tract, and Ž3. the structure of the basin. We test this model by creating a sequence stratigraphic model for the Precambrian Aberfeldy barite deposit of Scotland and then consider other horizons in the Dalradian succession that may also stratiform mineral deposits. q 1998 Elsevier Science B.V. All rights reserved. Keywords: sequence stratigraphy; maximum flooding surface; Mississippi Valley-type deposits; stratiform ores ŽSedex.; Neoproterozoic; Ba–Pb–Zn mineralization

1. Introduction The importance of ‘basin analysis’ in the exploration for sediment-hosted ore deposits has become increasingly appreciated over the past two decades. Basin analysis is widely used in petroleum exploration, where it comprises integrated studies of the )

Correspondence author. Fax: q44-1232-321280; e-mail: [email protected].

configuration of sedimentary infills primarily based on seismic interpretation ŽPayton, 1977.. Although seismic data is less commonly used in mineral exploration, the principles of basin analysis can still be applied to the analysis of the stratigraphy and facies relationships of sedimentary basins which host, or have potential for hosting stratabound mineralization. Aspects of basin analysis have been used by several authors to analyze and predict the occurrence of phosphate, evaporite, placer, and sedimentary man-

0169-1368r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 1 3 6 8 Ž 9 7 . 0 0 0 2 9 - 2

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ganese deposits Že.g. Sheldon, 1964; Cannon, 1983; Force and Cannon, 1988; Minter, 1990.. Large Ž1988. described the evaluation of sedimentary basins for massive sulphide deposits and emphasised that by studying the complete stratigraphy of an ore-hosting succession, mineralising events may be closely associated with particular phases of basin evolution. Large Ž1988. technique comes close to the sequence stratigraphic method adopted here. The accelerating interest in this approach is indicated by the review volume on ‘Sedimentary and diagenetic mineral deposits: a basin analysis approach to exploration’ ŽForce et al., 1991.. Furthermore, the highly-developed methods employed by the oil industry in assessing rates of burial and tectonic subsidence have recently been employed by Maynard and Klein Ž1995. in characterising sedimentary ore deposits. This paper follows their ethos: for sediment-hosted ores, a concept that finds widespread use in the oil industry Žsequence stratigraphy. may have applicability to the metal industry also. This paper is split into two parts. In the first part, we outline the theoretical constraints implicit in using sequence stratigraphy, review the location of sediment-hosted ore deposits within basins, and then indicate the ways in which sequence stratigraphy may be used to predict the occurrence of metalliferous mineralization. This expands on previously published abstracts by the authors Že.g. Moles and Ruffell, 1993. and on the specific sequence stratigraphic analysis of an ore deposit ŽMount Isa, Australia. by McConachie and Dunster Ž1996. and others Žsee Baker et al., 1996.. In the second part we take the sequence stratigraphic models of stratabound mineralization and apply the relevant features to a known ore-bearing succession in the Dalradian Supergroup ŽNeoproterozoicrPrecambrian. of Scotland. In this succession, the sequence stratigraphic location of a currently exploited ore zone is used as a predictive model for a second horizon that may be worthy of exploration. 2. Sequence stratigraphy Seismic stratigraphy, and its successor, sequence stratigraphy, were developed in the 1970s by the Exxon Production and Research Company ŽPayton, 1977.. This group proposed the concept of a se-

quence as ‘a stratigraphical unit composed of a relatively conformable succession of genetically related strata bounded at its top and base by unconformities or their correlative conformities’. The Exxon group believed that global sea-level fluctuations Žeustasy. were the main control on sedimentary sequence development, as the sedimentary response to gradual sea level change was often the widespread formation of abrupt lithological junctions Že.g. unconformities, transgressive surfaces. which can be recognised in the field and identified by discordant relationships on seismic profiles. Sequences consist of depositional systems tracts which show characteristic geometries, bounding surfaces and internal facies relationships ŽFig. 1.. Systems tracts comprise a predictable succession of facies-dependant depositional units developed as a consequence of sea-level change, and are characterised by cyclical beds termed parasequences Že.g. ‘coarsening-up’ beds of a deltaic or shelf succession. which stack in a characteristic manner Že.g. thickening-upwards, thinning-upwards.. Lowstand, transgressive and highstand systems tracts occur successively after the sequence boundary, and relate to rising then falling patterns of sea-level Žsee Wilson, 1990.. Highstand systems tracts are characterised by sediment progradation during a slowing rate of sealevel rise and regression. Fluvial sediments may characterise the later part of this systems tract which is terminated by an unconformity produced by sealevel fall ŽVan Wagoner et al., 1990.. Lowstand systems tracts develop when sea-levels are falling. If the shelf area becomes subaerially exposed, rivers cut into it and deposit sediment onto the slope as submarine fans and wedges: in clastic-dominated environments these fans will comprise sands and muds whilst in the carbonate system limestone turbidites will be interbedded with oozes. The transgressive systems tract is deposited during a relatively rapid sea-level rise which floods the shelf area, resulting in little sediment reaching the slope, where condensed sections form. As the rate of sea-level rise slows again, another prograding highstand systems tract will develop. A useful concept in sequence stratigraphy is the maximum flooding surface, at which clastic sedimentation reaches its minimum in the basin and marine beds are deposited over a wide area of the shelf and adjoining shoreline. Some

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Fig. 1. Schematic section of part of a basin, relating features of a theoretical depositional sequence to changing sea-level and indicating sources and host rocks for metal enrichments. The relative geometry of systems tracts are depicted. After Moles and Ruffell Ž1993..

authors Že.g. Galloway, 1989. place more significance on the maximum flooding surface, using it to split sedimentary successions into genetic sequences. The more traditional ‘Exxon’ view is that sequence boundaries or unconformities are more significant and thus these surfaces should be used to split the record into depositional sequences. This concept is not always so simple, as the early workers discovered when they were required by the evidence of the stratigraphic record to differentiate sub-aerially generated unconformities ŽType 1 sequence boundaries. and those generated by other means of erosion ŽType 2 sequence boundaries.. Some authors take a pragmatic line and use whichever feature is most readily identified in the material they are working on, as we have in the case study described below. Both the ‘‘Exxon’’ and ‘‘Genetic’’ sequence stratigraphic models utilise the concept of accommodation space to explain the preservation of depositional sequences. This simple yet useful concept suggests that relative sea-level controls the amount of space available for the deposition of sediments. Increasing sea-level or uninterrupted subsidence aids the creation of accommodation space and thus the likely preservation of stratigraphy. We will comment upon this concept in the examples that follow. Modern sedimentological

and petroleum industry literature abounds with examples of the uses of sequence stratigraphy. The basic citations in this work will suffice as a starting point for those interested in the methods and debates. Sequence stratigraphic models can be applied to carbonate and evaporite settings in addition to siliciclastic deposits. To date the models have been developed mainly for passive continental margin and intracratonic rift settings, although the wider application of the technique is evident from the work of Posamentier and Allen Ž1993.. There has been much debate concerning the relative roles that eustasy and tectonics play in controlling the formation and geometry of depositional sequences. Regardless of this controversy, sequence stratigraphy is now wellestablished as a powerful tool in modelling the three-dimensional geometry of depositional systems to predict their origin, distribution and use as hydrocarbon traps ŽWilliams and Dobb, 1991.. 3. Metal sources in Basins and sequence stratigraphic methods of concentration Sedimentary ore deposits are known from many different types of basin, from the intracratonic rift to the continental margin and thence to compressive

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Fig. 2. Sequence stratigraphic analysis of three environment-based classes of stratiform ore deposits. Logs and environmental interpretation are re-drawn after Morganti Ž1981.; sequence stratigraphic analysis from Moles and Ruffell Ž1993.. SBssequence boundary.

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flysch-filled foreland basins. Although often deformed, the successions from each type of basin lend themselves to sequence stratigraphic analysis ŽPosamentier and Allen, 1993.. The petroleum geologist, when faced with typical stratigraphic successions from each of these three basins Žas described by Morganti, 1981, and displayed in Fig. 2. will recognize the characteristic retrogradation of the transgressive systems tract in intracratonic successions or the progradational units. The common occurrence of trace hydrocarbons in metal ores in sedimentary basins is now widely recognised, particularly in limestone-hosted epigenetic ŽMississippi Valley-type. Pb q Zn " Ba " F ore deposits. Organic-rich shales ŽHanor, 1994; Prosser et al., 1994. and evaporite deposits ŽAyora et al., 1995., which during diagenesis yield hydrocarbons, are also commonly metal-enriched and may act as a minor source of metalliferous basinal brines. Other autochthonous metal sources within sedimentary basins are reviewed by Hanor Ž1994. and include red-bed sediments Žmetals leached from oxide coatings and cements., alkali feldspars Žmetals released during albitisation. and clay minerals Žmetals released during illitisation.. Allochthonous metal sources include the basement hinterland, from which metals may be transported into basins through either mechanical or chemical processes ŽShelton et al., 1995., and igneous material introduced by volcanic eruptions or intruded into the sediment pile. The migration of metal-enriched basinal brines has been extensively documented in the formation of MVT ore deposits, and has been postulated for the formation of sedimentary exhalative ŽSedex. Pb q Zn " Ba deposits and shale-hosted stratabound Cu " Co deposits Žsuch as the Zambian Copperbelt.. Seawater should of course be added to this list as it is the ultimate source of U, Zn, Cu, Pb, Ni, Cr, Mo Žamongst others. in many ore types. Metals may be sufficiently concentrated to form sedimentary ore deposits by mechanical, chemical or biochemical processes. Mechanical reworking is essential in forming placer deposits. Some deposit types are a result of chemical or biochemical precipitation occurrence synchronously to mechanical concentration, for example phosphatic nodule beds and mineralized algal breccias ŽMcConachie and Dunster, 1996.. Chemical processes can be further subdivided according to whether they acted during sedi-

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mentation, or subsequently during diagenesis or post-lithification. Examples of synsedimentary metal sources Žand sometimes hosts. include stratabound ironstones and manganese deposits; metalliferous black shales Že.g. Kupferschiefer.; and metal sulphide–sulphate deposits in sabkhas, all of which are reviewed below. Examples of post-depositional precipitation include pore-filling Pb, Cu and U mineralization in sandstones; and MVT Pb–Zn deposits which form by the replacement of carbonates or in pre-existing permeable zones such as karstic cavities or breccias ŽMcConachie and Dunster, 1996.. Processes controlling the development of sedimentary sequences which have a marked effect on the concentration and fixation of metals include: Ž1. transgressive and regressive reworking of basin margin sediments; Ž2. condensation under maximum flooding; and Ž3. delta, reef and evaporite development and karstification during sea-level fall. We examine each of these scenarios in turn. 3.1. The transgressiÕe basin margin deposits Fluvial, littoral and shallow marine placer deposits are characterised by repeated mechanical reworking due to changes in the base-level of erosion. It can be argued that the frequent changes in sea-level accompanying the Quaternary glaciations were particularly favourable to the development of placer deposits ŽCannon, 1983.. By far the most intensively studied palaeoplacers are the Archaean Witwatersrand Au–U deposits. Two schools of thought have emerged concerning the origin of the mineralization. Sutton et al. Ž1990.; Robb and Meyer Ž1991. and Minter et al. Ž1993. suggest that the deposits are primarily of the placer type, with some modification of Au distribution during metamorphism. Phillips et al. Ž1989. and Phillips and Myers Ž1989. had considered a metamorphic origin likely and detailed petrographic studies now suggest that much mineralization followed a phase of high-temperature deformation. Whatever the origin of the Au and U, the final location of the ore is dependant on one of three stratigraphic locations ŽMinter, 1979.. These are: regressive deposits on unconformities at the base of sedimentary units Žsequence boundaries.; transgressive deposits on angular unconformities Žtransgressive systems tracts.; and terminal deposits on discon-

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formities at the top of sedimentary units Žsequence boundaries.. The argument for an epigenetic replacement model for the Witwatersrand gold deposits involves the interaction of Au–S-bearing metamorphic fluids interacting with Fe andror Cu that had collected in pisolites–ferricretes and carbon seams along unconformities ŽPhillips and Myers, 1989.. In either case, sequence boundaries that are more often than not overlain directly by transgressive deposits and possibly the maximum flooding surface are an important stratigraphic control in the Archaean Au– U–S ores. This model may have a strong tectonic component in that Maynard and Klein Ž1995. suggest that detrital Au was introduced at the time of slowest subsidence, which matches when the most sediment reworking Ži.e. transgressive conditions. would be expected. Lead in sandstone and red-bed copper deposits are associated with clastic aquifers deposited in continental-shallow marine settings, with metals usually derived from weathering of a crystalline basement hinterland. Bjorlykke and Sangster Ž1981. reviewed the depositional settings of lead in sandstone deposits and contrasted these with red-bed Cu deposits. They conclude that Cu is released early through mild chemical weathering of mafic minerals in rapidly deposited, immature, rift-generated feldspathic redbed sandstones, whereas prolonged chemical weathering associated with peneplanation and slow transgression onto cratons leads to the release of Pb through the breakdown of feldspar and the deposition of relatively mature quartz sandstones. In the Triassic Oberpfalz Basin of Germany, Pb concentrations in basin margin sandstones are associated with development of Pb-rich clays derived from weathering of the basement. The lead occurs mainly as cerussite cement in sandstones and as galena replacements of wood fragments. The ideal sequence stratigraphic model for such conditions to occur is when shallow marine sands of the lowstand and transgressive systems tract onlap basement rocks. The sequence stratigraphic model also allows for sediments of the transgressive systems tract to directly overlie the sequence boundary, with no lowstand preserved. This commonly occurs in ‘up-dip’ basin margin locations where the first transgressive surface is amalgamated directly onto the sequence boundary, often with basement lying directly beneath. In addi-

tion, conditions of relative sea-level rise create the accommodation space required for the preservation of lead-bearing sandstones. Weathering of the basement hinterland is presumably just as aggressive during deposition of the remaining depositional sequence Ži.e. the highstand systems tract. only the transgressive conditions suitable for preservation Žas in the Oberpfalz Basin situation. are not always in existence. 3.2. Deposits associated with condensed sections (maximum flooding) Relative sea-level rise occurs throughout the late lowstand and transgressive systems tracts. During this time, sediments of the previous depositional sequence may be reworked and at the formation of minor flooding surfaces, some sediment starvation or condensation may occur ŽWilson, 1990.. Such condensed units may reflect minor hiatuses developed as transgressive surfaces above coarsening-up parasequences or parasequence sets. Prolonged hiatuses also occur and reflect times of marine flooding onto sequence sets Žespecially in the transgressive systems tract.. As relative sea level approaches its maximum, the basin becomes more starved of sediment resulting in the formation of the maximum flooding surface or condensed section. This marks the boundary between the transgressive and highstand systems tracts when a variety of deposits including metalliferous black shales, phosphatic nodule beds, ironstones and mineralized algal breccias may be deposited ŽMoles and Ruffell, 1993.. These may be sufficiently metal-rich to constitute ore deposits, they may become source rocks for metalliferous fluids generated during diagenesis, or they may form a cap to later fluid convective cells in the lowstand and transgressive systems tracts. Present-day phosphorites and glauconites are associated with upwelling cold oceanic waters reaching warm continental shelf areas, where chemical and biochemical precipitation occurs ŽFollmi, 1993.. Although similar conditions are often cited when ancient phosphates or glauconites are considered ŽFollmi, 1993., stratigraphically condensed sections well away from sites of upwelling also display high concentrations of these mineral groups, especially through discrete periods of Earth history ŽSheldon,

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1980.. Reworking and concentration by longshore currents are significant in producing economic deposits, as is the volume and rate of terrigenous or carbonate dilutent sedimentation ŽSheldon, 1964; Ruffell, 1990.. In condensed sections radioactive elements such at K, U and Th may be concentrated, leading to the gamma-ray spike so frequently used in sequence stratigraphic studies of wireline logs to identify the flooding surfaces ŽVan Wagoner et al., 1990.. In addition, rare earth elements, chalcophiles and trace elements like Cr and Ir may also be concentrated in condensed, phosphatic beds ŽOrth et al., 1986.. Hallam Ž1992. suggests that these elemental concentrations Žespecially Iridium. may be the result of condensation that occurred during the rapid transgression of previously subaerially-exposed surfaces. In sequence stratigraphic terminology, this scenario represents the amalgamation of one or more flooding surfaces onto a Type 1 sequence boundary Žsee above for explanation.. Oolitic ironstones, such as the Cleveland Ironstone in Yorkshire, UK ŽSlater and Highley, 1977., represent condensation of the sedimentary record with regard to the siliciclastic successions found above and below. Hallam and Bradshaw Ž1979. show that such ironstones are common at the top of shallowing-upward cycles and may either record the end of a regression or the start of a new transgression. Hallam and Bradshaw Ž1979. go on to show that condensation often marks the start of a transgressive episode and thus the latter cause is more likely. In addition, Hallam Ž1992. demonstrates the relationship between black shales and ironstones, further strengthening the transgression link. We may suggest that ironstones are often associated with condensation of the stratigraphic record during the transgressive systems tract in shallow water conditions or with maximum flooding when minimal clastic or carbonate deposition allowed the accumulation of chemical and biochemical precipitates of iron oxides and carbonates. The oceanic origin of Mn and thus sedimentary manganese deposits has been recognised before ŽVarentsov and Rakhmanov, 1977.. However, the introduction of new, manganese-bearing oceanic waters into a basin can now be placed in a sequence stratigraphic context. In many cases, manganese deposits are associated with transgressive conditions

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and more specifically flooding surfaces. This is clearly illustrated by the marine Oligocene Nikopol Basin deposits in the Ukraine which account for some 70% of the world’s reserves of MnO. Carbonate facies of the ore show a clear relationship to water depth with increasing ore-grades occurring through the transgressive systems tract and the highest-grade ore being found just below the maximum flooding surface. The ore horizon is underlain, and overlain, by terrigenous clastic Žred bed. sediments. Palaeobotanical research has shown that ore deposition coincided with a marked climatic change from humid sub-tropical to cold temperate. Force and Cannon Ž1988. suggest a slight variation on this model, concluding that most shallow-marine manganese deposits formed after the point of maximum flooding, during high sea level stands in narrow time intervals when ocean anoxia was widespread, the cold conditions creating a sluggish circulation. Beukes and Gutzmer Ž1995. suggest that the early diagenetic manganese ores they describe from Andhra Pradesh in India are restricted to one stratigraphic horizon, deposited immediately following a major transgression. This may be analogous to the postmaximum flooding surface model of Force and Cannon Ž1988. mentioned above. Metal-enriched black shales are frequently underlain by red-bed clastics and overlain by evaporites, being characteristic of anoxic shallow seas associated with marine inundations of continental areas ŽCannon, 1983; Force and Cannon, 1988.. This cyclicity may also be explained by sequence stratigraphical models. Classic examples are the European Kupferschiefer, and the Zambian Copperbelt, both of which have had refinements made to the original syngenetic origin of the ore, yet still find application to the sequence stratigraphic model. The Kupferschiefer stratiform Cu deposits that are found throughout central Europe and the North Sea have long been regarded as an example of syngenetic metal enrichment of shales ŽDunham, 1964.. Jowett et al. Ž1987. challenged this view, providing a model whereby convective cells in the reddened Rotliegende sands below were capped by impermeable Zechstein salts, causing the stripping of metals from the sands and ore precipitation at the sand–salt contact, or roughly the stratigraphic horizon of the Kupferscheifer. In this case, although mechanisms of sea-

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level change may not have caused the initial metal concentration Žas in placer deposits., the geometry of the depositional sequences, and most importantly the location of a shale at the maximum flooding surface, has still controlled the location of the Cu ore. This is a theme which we will see repeated in the description of MVT deposits below. During deposition of the 900-million year old Roan Series which host the Zambian Copperbelt deposits, black, carbonaceous, metal-rich shales were deposited diachronously as transgression occurred followed by regression. A distinctive zonation of metal distribution and mineralogy in the marine sediments has been known for some time and is consistent with water depth and distance from the shoreline ŽBjorlykke and Sangster, 1981.. More recently however, Wendorff Ž1994. has summarised the two main refinements made to this simple model. Firstly, Sweeney and Binda Ž1994. have demonstrated the importance of bacterially-controlled early diagenesis in fixing the Cu and Co derived from basement rocks. Secondly, Binda Ž1994. has examined the whole Roan Supergroup stratigraphy and suggests that the ore, although concentrated by early diagenesis, is clearly related to sedimentary facies. Binda Ž1994. sub-divisions appear to conform to a simple sequence stratigraphic interpretation, with the lower division Žnon-marine Siliciclastic Unit. comprising the lowstand; the middle division Žcarbonate marine Mixed Unit. comprising the transgressive systems tract and the upper division ŽCarbonate or Upper Roan. comprising the highstand. Binda Ž1994. recognises the Mixed Unit as a transgressive deposit, reflected in many places by the transition from the topmost Siliciclastic Unit ŽMufulira arenites. to the early Mixed Unit Že.g. Chambisi silty ore shale.. Undetected ore zones may occur in the Siliciclastic Unit below, probably as a result of diagenetic remobilisation. In summary, the greatest copper concentrations coincide with maximum flooding of the Archaean granitic and gneissic basement. At this time, there was minimal clastic deposition so that the chemical precipitates were least diluted. Current models for these deposits envisage metal precipitation at redox boundaries from oxidised meteoric waters or basinal brines moving through the underlying red beds. Sedimentary exhalative Pb–Zn–Ba deposits are often hosted by black shales deposited during maxi-

mum flooding, but formed in relatively deep marine basins often where periods of oceanic anoxia resulted in the deposition of carbon- and sulphur-rich shales Že.g. Large, 1988.. Exploration for this type of deposit involves the correlation of ‘fertile’ horizons, as the stratigraphical position of one deposit is a good guide to finding others ŽForce et al., 1991.. Targets can be narrowed by locating syndepositional normal faults, which often control the location of orebodies. Differentiating syngenetic from epigenetic mineralization in stratabound ores is often problematic as the chronology of fluid movement and remobilisation of metals is not always clear. Bierlein et al. Ž1995. suggest models for the origin of both syngenetic and epigenetic ores, both of which are located in carbonates that are clearly above quartzose sands typical of the lowstand to transgressive systems tracts yet below the pelites Žoriginally mudrocks. of the highstand. The compartmentalisation of the original, deep seated ‘reservoir’ that Bierlein et al. Ž1995. envisage would have been facilitated by basement rocks below and pelites above. In his review of sedimentary stratiform ore deposits, Morganti Ž1981. classified them according to three tectonic settings: intracratonic, platform-marginal and flysch ŽFig. 2.. Morganti included the Kupferschiefer and the Zambian Copperbelt deposits in his Intracratonic category, together with the giant McArthur River and Mount Isa lead–zinc deposits. As we have already noted, the first two deposit types coincide with marine flooding of continental areas. The McArthur River and Mount Isa deposits occur in non-marine, playa lake settings where one might not expect sequence stratigraphy to be applicable. Early models of sequence stratigraphy were developed for application to the shallow marine setting ŽVan Wagoner et al., 1990.. In the past four years however, many studies have been made of the sequence stratigraphy of foreland and lacustrine basins Že.g. Posamentier and Allen, 1993; Tang et al., 1994. where maximum flooding is associated with highstands of lacustrine waters. The non-marine sequence stratigraphic models can be applied to the Mount Isa deposit where, although the lead–zinc mineralization is regarded as post-sedimentary in origin, there is a clear correlation to horizons reflecting lake highstands ŽLawrie, 1991..

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Morganti’s flysch and deep marine basin deposits are hosted by deep-water turbidite sequences and characterised by the presence of barite, alterationfeeder zones, and location in fault-bounded subbasins that have undergone significant post-depositional deformation. Nonetheless, as we demonstrate in our Neoproterozoic case study Žsee below., depositional sequences can be recognised in structurally complex areas. To confirm this, Morganti Ž1981. shows pre-deformation stratigraphic examples from the Meggen–Rammelsberg, Sullivan, and the Macmillan Pass deposits in the Selwyn Basin. Note that in Morganti’s representative log ŽFig. 2., mineralization is associated with shales deposited in the transgressive systems tract. Morganti’s platform marginal deposits occur in areas immediately seaward of carbonate shelves adjacent to large, deep basins. He illustrated this type with examples from the Howards Pass, also in the Selwyn Basin. Once again, mineralization occurs in shales and cherts of the lowstand and transgressive systems tract, often below the maximum flooding surface. 3.3. Deposits associated with the highstand systems tract As relative sea level begins to fall Žsubsequent to maximum flooding. carbonate reefs or deltasrfluvial sands may prograde towards the basin centre. These facies of the highstand systems tract are common petroleum reservoirs and hosts for Mississippi Valley-type ore deposits Žsee Baines et al., 1991.. During a fall in sea-level, the accommodation space available for thick delta sands or reefs diminishes, so the basin is rarely starved of sediment. At this time, immediately prior to the formation of the next sequence boundary the late highstand deposits may become temporarily exposed. The occasional fluctuations of sea level allow both the preservation of coals, evaporites and weathering Žincluding karstification. to occur. Baines et al. Ž1991. show the relationship between MVT mineralization and the highstand systems tract. However, the ultimate source of MVT type ores is subject to some debate, with basement, sedimentary basins and local sedimentary sources all being invoked by different authors ŽRavenhurst et al., 1989; Fowler and Anderson, 1991; Schmitt et al., 1991.. Evaporite dissolution has been

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invoked to explain ore-hosting porosity in several MVT districts Že.g. Kyle, 1991.. In this model, ore deposition may be due to mixing of evaporite-derived saline fluids with metalliferous formation waters expelled from shales undergoing compaction and diagenesis. A direct spatial association with evaporites is shown by the Gays River carbonate-hosted Pb–Zn deposit in Nova Scotia ŽAkande and Zentilli, 1984.. The capping evaporitic strata provided an aquitard that controlled the local circulation of metal-bearing fluids and probably served as the sulphur source for sulphide precipitation ŽRavenhurst et al., 1989.. Subaerial exposure of parts of the highstand systems tract during the subsequent lowstand may result in karstification of carbonates and meteoric water flushing of sandstones, which also increase porosity. This may modify or destroy previously formed orebodies Žas in at Gays River., or enhance the potential for ore deposition during subsequent basin development Že.g. Olson, 1984.. In some MVT districts, e.g. Southeast Missouri, sandstone-hosted lead mineralization overlies the main carbonate host; similar stratigraphic distributions of MVT ores have been noted elsewhere and require further sequence stratigraphic analysis in order to test whether the clastics resulted from progradation. Other possible explanations include transgressive migration of a clastic shoreline across an older carbonate ramp or lowstand fans following drowning of a carbonate platform. Models for the deposition of MVT deposits generally involve long-distance, channelled fluid flows through aquifers. In this scenario, metalliferous formation waters migrate due to compaction or tilting of the basin sediments, into older, shelf carbonates where metal sulphide deposition occurs. Fluid migration may be due to tectonic overpressuring, as described by Garven Ž1985. for the Pine Point deposit in western Canada. We have annotated his diagram ŽFig. 3, after Moles and Ruffell, 1993. to show a sequence stratigraphical interpretation. Schmitt et al. Ž1991. have satisfactorily explained many features of the northern Morocco MVT deposits by modelling convective fluid flow through Triassic evaporites and basement rocks, which is driven by the presence of the basin-marginal horst structure ŽFig. 4.. The Schmitt et al. Ž1991. model is at the scale of a whole basin, rather than certain sequences within it as in

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Fig. 3. Schematic diagram of cross-formational, long-distance fluid migration model for the formation of the Pine Point MVT deposit Žmodified from Garven, 1985., with our sequence stratigraphic interpretation. Note that the lowstand systems tract may not be preserved in a continental shelf succession.

the previous discussion. Nonetheless, the usefulness of sequence stratigraphy in characterising such deposits is that the models contain a variety of operating scales. Thus the Moroccan example ŽFig. 4. shows a thick megasequence-scale succession, with convective flow restricted by the limestones of the

late transgressive systems tract to the lowstand deposits. Fowler and Anderson Ž1991. have argued that relatively short migration paths for fluids depositing MVT deposits in shale-rich sequences can be invoked due to the development of sealed, geopressured zones near basin margins. Such sealed zones

Fig. 4. Schematic cross-section through the Touissit–Bou Beker Horst MVT deposit, northern Morocco. Section and fluid-flow model after Schmitt et al. Ž1991.; sequence stratigraphic analysis after Moles and Ruffell Ž1993.. The Tiouli Graben is thought to contain 4–5,000 m of Triassic–Jurassic sediment, hence there is no accurate vertical scale available.

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may have the same attributes as the lowstand shown in Fig. 4, sealed above by deposits of the maximum flooding surface, and below by the pre-lowstand.

4. A summary of the sequence stratigraphic models of stratiform ore deposition Sequence stratigraphic concepts are of use in explaining and predicting the juxtaposition of oregenerative sediments such as evaporites, red-beds and organic-rich shales, and in understanding mineralization associated with black shales, hardgrounds and reworking surfaces Žplacer deposits.. They may also enable predictions to be made concerning likely ore-hosts in sediments, such as transgressive surfaces, condensed beds, carbonate reefs and permeable sandstones. The final position of any sedimentary-hosted ore deposit is dependent on Ži. the geometry of the sedimentary sequence, Žii. the depositional environments Ži.e. specific lithologies. within each systems tract, and Žiii. the structure of the basin. While structural controls are undeniably important in channelling fluid flow and localising ore deposition for many types of deposit, the concepts outlined here could enhance exploration by targeting particular basins or areas within basins for more detailed appraisal. Similarly, the preceding discussions have included examples of ore deposition that was synchronous with deposition Že.g. ironstones, placers. as well as those that are patently diagenetic or even epigenetic Že.g. MVT deposits.. In each case, however, we have attempted to bring out the features that are of sequence stratigraphic significance, in order to demonstrate the flexibility of the model. In many such cases, although sequence stratigraphy may explain the final location of the ore, its previous origin may be less clear. For instance, sea-level rise may be controlled by melting of the ice-caps, or by an increase in the volume of the mid-ocean ridges. In the latter case, increased mantle activity, and volcanism may provide the ultimate source for the metalenrichment we observe Žin this paper. at each transgressive or maximum flooding surface. Thus, for sediment-hosted deposits, studied on a regional scale Ži.e. one basin, or a series of linked basins., sequence stratigraphy is a very powerful tool in modelling ore distributions. For ore deposits that are hosted in

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fractured or igneous rocks, then sequence stratigraphy will only be relevant in considering any source for the ore rather than its final location. As Lawrie Ž1991. points out, sedimentary facies analysis and palaeogeographical interpretations are also relevant to exploration for syntectonicrsynmetamorphic mineral deposits, even though sedimentary processes are not the primary ore-concentrating mechanisms. The Aberfeldy deposit in Scotland is considered to be an example of the sedimentary exhalative Pb–Zn–Ba type, discussed above ŽMaynard and Klein, 1995.. Isotopic measurements have indicated bacterial reduction and anoxic conditions during deposition of the black shale Žnow schist. ore-bearing strata ŽHall et al., 1991.. Conditions most conducive to syn-depositional and early diagenetic mineralization occurred during periods of oxygen and clastic sediment starvation ŽLarge, 1988.. This is envisaged for the Aberfeldy succession, where conditions of sediment starvation occurred during the maximum rate of rise in sea-level Žmaximum flooding surface.. In the Exxon model, this point in the sea-level history of a basin will result in a cut-off in sediment supply, the formation of a chrono- and lithostratigraphically condensed section, and the concentration of rare metals.

5. A Test of the Models from the Precambrian Cambrian Dalradian Supergroup of Scotland Whilst each of the various sequence stratigraphic scenarios developed above have an actual example or series of examples to demonstrate their worth, there is some danger of a circular argument developing. To test the theoretical concepts outlined in the first part of this paper against a real example, we have interpreted the sequence stratigraphic location of the Aberfeldy Ba-Pb-Zn deposit in Scotland with a view to making stratigraphic predications in similar Neoproterozoic successions along strike from the deposit. 5.1. Background information Stratiform mineralization in the Aberfeldy area is controlled by lithological, stratigraphic and structuralrmetamorphic variation in the host rocks. We need to condense the abundant knowledge of the

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area into the key features of the stratigraphy required for our analysis ŽFig. 5.. The host rocks comprise mica schists with minor quartzites, dolostones and metabasites ŽCoats et al., 1980.. The ore and host lithologies occur within the Ben Eagach Schist and at the transition to the stratigraphically overlying Ben Lawers Schist. Both of these formations are within the late Precambrian Easdale Subgroup ŽArgyll Group. of the Dalradian Supergroup ŽHarris et al., 1994.. In the Foss deposit, which is the westernmost section of the 7-km strike length mineralized zone, most of the mineralization occurs in the uppermost, barium-enriched, graphitic quartz-mica schists of the Ben Eagach Schist ŽFig. 5., although some extends up into the non-graphitic and calcareous Ben Lawers Schist above. In contrast to the uppermost schists, the lower unmineralized part of the Ben Eagach Schist is characterised by lower graphite contents and absence of cryptic barium enrichment, and by the presence of garnet and chlorite and a greater

Fig. 5. Pre-deformation Žrestored. log of the Precambrian metasedimentary host to the Ba–Pb–Zn deposit at Aberfeldy ŽPitlochry., Scotland. The approximate thickness of the whole succession depicted is 200 to 300 m, this variation being dependant on original sedimentary thickness variations Žsee Fig. 6.. The succession is part of the Easdale Subgroup, Argyll Group, Dalradian Supergroup.

abundance of quartzitic beds. In the Foss deposit, the barium-enriched graphitic schist zone is 20 to 110 m thick, and within this zone up to seven laterally-discontinuous, stratiform mineralized horizons were identified from drillhole intersections by Moles Ž1985.. Each mineralized horizon comprises bedded barite, cherts andror carbonate rocks variably enriched in sulphides. Moles Ž1985. found evidence for marked lateral variations in thickness and facies of both mineralized horizons and enclosing metasediments, which he explained by syndepositional faulting. These fault movements controlled the locus of hydrothermal vents, the pulsatory exhalative activity, and the palaeotopography of the marine basin ŽFig. 6.. Shallow-water environments on horst blocks are suggested by thick Žlocally ) 20 m. composite barite units, with little detrital input but with evidence of erosive reworking between and during successive hydrothermal emanations. Elsewhere, anoxic basinal environments with local brine pool development are indicated by chert- and sulphide-dominated mineralization enclosed within thick, turbiditic sediments with evidence of slump-thickening. Sulphur isotope data from sulphides and baryte support these interpretations ŽHall et al., 1991; Moles et al. in prep... Sub-economic barium-enriched stratiform mineralization within the Ben Eagach Schist was discovered in the 1980s by the British Geological Survey at several other localities in the Scottish Highlands ŽCoats et al., 1984; Hall, 1993, see Fig. 8.. In the Loch Lyon area about 50-km southwest of Aberfeldy, the mineralized zone is 1–2 m thick and lies about 4 m below the stratigraphic top of the Ben Eagach Schist. A similar distance along strike to the northeast of Aberfeldy, between Glenshee and Braemar, chemically similar mineralization was found in the Glas Maol Graphitic Schist which is a stratigraphically equivalent formation to the upper graphitic member of the Ben Eagach Schist ŽFortey et al., 1993.. Rapid lateral changes in facies and thickness of the host sediments also characterise this area, and are again interpreted in terms of syndepositional faulting and subsidence. As with the Foss deposit, lateral compositional variation in the mineralization Žfrom barite to sulphidic chert to barium silicate rocks. suggests distinct geochemical environments related to water depth and rates of discharge of the hydrothermal fluids.

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Fig. 6. Simplified reconstruction Žcross-section prior to deformation. of the Aberfeldy barite deposit, Pitlochry, Scotland. After Moles Ž1985.. Dashed lines indicate location of possible synsedimentary faults.

5.2. Interpretation Glover and McKie Ž1996. and Goodman et al. Ž1997. have both completed sequence stratigraphic analyses of the Dalradian by considering both the vertical Žlogged. and horizontal geometry of the successions. From the vertical succession at Aberfeldy ŽFig. 5. we characterise the Ben Eagach to Ben Lawers stratigraphic boundary as a candidate maximum flooding surface and thus a useful datum. With this model we have then made a simplified reconstruction of the pre-deformational geometry of the Foss succession ŽFig. 6.. This is essentially similar to the larger-scale, pre-deformation reconstruction made by Coats et al. Ž1984. to demonstrate the topography of the succession immediately after deposition. Together, the log of the succession ŽFig. 5. and the reconstructions ŽFigs. 6 and 7. show that the sequence stratigraphic analysis of the ore zone must also take into account sedimentary events below, including the lower quartzitic parts of the Ben Ea-

gach Schist. These quartz schists are transitional from the Carn Mairg Quartzite below. With its pelite rip-up clasts, remnant cross-stratification and highly variable thickness distribution, the Carn Mairg Quartzite is interpreted here as a shallow marine succession deposited in the lowstand or possibly transgressive systems tract. The Ben Eagach Schist also shows great variation in thickness, probably as a result of the infilling of fault-generated grabens ŽFig. 7.. The mineralized zones at Foss, Loch Lyon and northeast of Glenshee occur within the black shale Žpelite. host of the maximum flooding surface which marks the top of the Ben Eagach Schist. As mentioned above, barite mineralization at Foss occurs locally upward into the calcareous Ben Lawers Schist, in the vicinity of the main exhalative centres during the waning phase of the hydrothermal system. Coats et al. Ž1984. interpret the Ben Lawers Schist as representing a rapidly deposited, current-sorted, calcareous clastic sediment. The absence of graphite and sulphides, and low zinc and lead contents, sug-

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Fig. 7. Schematic section of the Scottish Dalradian ŽNeoproterozoic. intracratonic basin during deposition of the Ben Eagach Schist, showing stratiform mineral deposits and development of sub-basins. After Coats et al. Ž1984. and Fortey et al. Ž1993..

gests deposition in an oxygenated environment. We interpret the Ben Lawers Schist as a highstand deposit following maximum transgression ŽFig. 5.. 5.3. PredictiÕe Use of the Dalradian Model Our analysis of the Aberfeldy succession suggests that the original sedimentary exhalative ore was generated during the maximum stand of relative sea-level

in this area, or at the point of maximum flooding. Depositional sequences are naturally repetitive and thus we may consider other horizons where the stratigraphic conditions conducive to Aberfeldy type mineralization may be repeated. In a folded and metamorphosed succession like the Dalradian, this is no straightforward task: indeed, so labour-intensive are the processes required to map out correlative surfaces and reconstruct the pre-deformational Žde-

Fig. 8. Dalradian lithostratigraphy of the Aberfeldy ŽAgyll Group. maximum flooding surface and the predicted maximum flooding surface of the Appin Group below. Schematic key and no vertical scale. After Harris et al. Ž1994. and Goodman et al. Ž1997..

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positional. geometries of Dalradian successions that we here use a recently published example ŽGoodman et al., 1997. as a template for our predictive analysis. The Aberfeldy mineralization is interpreted to occur in the maximum flooding surface of the Argyll Group. Below the Argyll is the Appin Group, which contains some comparable lithologies and thus a possibility of secondary ore-generating zones. Goodman et al. Ž1997. studied the Appin Group stratigraphy in the eastern Highlands of Scotland. Here, deformation has frequently been invoked in explaining rapid lithological changes within mapped formations. Goodman et al. Ž1997. utilised a ‘‘key surface’’ sequence stratigraphic approach to explain such variations in part of their stratigraphy without recourse to large tectonic slides. This approach led to an analysis of the whole Appin Group stratigraphy, where the mid-Blair Atholl Sub-group was identified as a time of maximum flooding ŽFig. 8.. In the SW Highlands, this maximum flooding surface is marked by the transition from Cuil Bay Slate to Lismore Limestone which is similar in lithology and sequence stratigraphy to the Aberfeldy succession. In this test of the predictive capacity of sequence stratigraphy in ore mineralization, this area and horizon Žtop Cuil Bay Slate of the SW Highlands. would be our target for further exploration. As secondary considerations, we might also examine the transgressive horizons in the Pitlochry area where the Dark Schists pass into Dark Limestones of the Blair Atholl Subgroup.

Acknowledgements We are grateful to all those attendees at Geofluids ’93 and Geofluids II who gave us critical feedback. Graham Leslie and Sally Goodman are gratefully acknowledged for their help in expaining Dalradian stratigraphy to us. John Dunster and Barry Maynard both gave us excellent reviews. Maura Pringle and Gill Alexander helped with the artwork.

References Akande, S.O., Zentilli, M., 1984. Geologic, fluid inclusion, and stable isotope studies of the Gays River lead–zinc deposit, Nova Scotia, Canada. Econ. Geol. 79, 1187–1211.

221

Ayora, C., Taberner, C., Pierre, C., Pueyyo, J.J., 1995. Modelling the sulfur and oxygen isotopic composition of sulphates through a halite–potash sequence: Implications for the hydrological evolution of the Upper Eocene Southpyrenean Basin. Geochim. Cosmochim. Acta 59, 1799–1808. Baines, S.J., Burley, S.D., Gize, A.P., 1991. Sulphide mineralization and hydrocarbon migration in North Sea oilfields. In: M. Pagel, J.L. Leroy ŽEds.., Source, Transport and Deposition of Metals. Balkema, Rotterdam, pp. 507–510. Baker, T., Rotherham, J.F., Richmond, J.M., Mark, G., Williams, P.J. ŽEds.., 1996. New Developments in Metallogenic Research: The McArthur–Mount Isa–Cloncurry Minerals Province, Extended Abstracts, Townsville, MIC ’96. Beukes, N.J., Gutzmer, J., 1995. Composition and origin of the Neoproterozoic Penganga manganese deposits of India revisited. In: J. Pasava, B. Kribek, K. Zak ŽEds.., Mineral Deposits. Balkema, Rotterdam, pp. 527–529. Bierlein, F.P., Plimer, I.R., Ashley, P.M., 1995. An integrated model for sulphide mineralization in the Willyama Supergroup, South Australia. In: J. Pasava, B. Kribek, K. Zak ŽEds.., Mineral Deposits. Balkema, Rotterdam, pp. 31–34. Binda, P.L., 1994. Stratigraphy of Zambian Copperbelt orebodies. J. Afr. Earth Sci. 19, 251–264. Bjorlykke, A., Sangster, D.F. 1981. An overview of sandstone lead deposits and their relation to red-bed copper and carbonate-hosted lead–zinc deposits. Econ. Geol. 75th Anniversary volume, pp. 179–213. Cannon, W.F., 1983. Some climatic and oceanographic controls on the time and place of mineralization. In: T.M. Cronin, W.F. Cannon, R.Z. Poore ŽEds.., vol. 822, Paleoclimate and Mineral Deposits, US Geological Survey Circular, pp. 24–25. Coats, J.S., Smith, C.G., Fortey, N.J., Gallagher, M.J., May, F., McCourt, W.J., 1980. Strata-bound barium–zinc mineralization in Dalradian schist near Aberfeldy, Scotland. Trans. Inst. Miner. Metal. B 89, 110–127. Coats, J.S., Fortey, N.J., Gallagher, M.J., Grout, A., 1984. Stratiform barium enrichment in the Dalradian of Scotland. Econ. Geol. 79, 1585–1595. Dunham, K.C., 1964. Neptunist concepts in ore genesis. Econ. Geol. 59, 1–21. Follmi, K.B., 1993. Phosphorus and phosphate-rich sediments, an environmental approach. Chem. Geol. 107, 375–378. Force, E.R., Cannon, W.F., 1988. Depositional model for shallow-marine manganese deposits around black shale basins. Econ. Geol. 83, 93–117. Force, E.R., Eidel, J.J. and Maynard, J.B. ŽEds.., 1991. Sedimentary and Diagenetic Mineral Deposits: A Basin Analysis Approach to Exploration. Reviews in Economic Geology, vol. 5, Society of Economic Geologists. Fortey, N.J., Coats, J.S., Gallagher, M.J., Smith, C.G., Greenwood, P.G., 1993. New stata-bound barite and base metals in Middle Dalradian rocks near Braemar, northeast Scotland. Trans. Inst. Min. Metall. B102, 55–64. Fowler, A.D., Anderson, M.T., 1991. Geopressure zones as proximal sources of hydrothermal fluids in sedimentary basins and the origin of Mississippi Valley-type deposits in shale-rich sequences. Trans. Inst. Miner. Metal. B100, 14–18. Galloway, W.E., 1989. Genetic stratigraphic sequences in basin

222

A.H. Ruffell et al.r Ore Geology ReÕiews 12 (1998) 207–223

analysis I: Architecture and genesis of flooding-surface bounded depositional units. Bull. Am. Assoc. Pet. Geol. 73, 125–142. Garven, G., 1985. The role of regional fluid flow in the genesis of the Pine Point deposit, Western Canada sedimentary basin. Econ. Geol. 80, 307–324. Glover, B.W., McKie, T., 1996. A sequence stratigraphical approach to the understanding of basin history in orogenic Neoproterozoic successions: an example from the central highlands of Scotland. In: S.P. Hesselbo, D.N. Parkinson ŽEds.., vol. 103, Sequence Stratigraphy in British Geology. Spec. Publ. Geol. Soc. Lond., pp. 257–269. Goodman, S., Crane, A., Krabbendam, M., Leslie, A., Ruffell, A., 1997. Correlation of depositional sequences in a structurally complex area: the Dalradian of Gleann Fearnach to Glen Shee, Scotland. Scott. J. Geol. 33, in press. Hall, A.J., 1993. Stratiform mineralization in the dalradian of Scotland. In: R.A.D. Pattrick, D.A. Polya ŽEds.., Mineralization in the British Isles. Chapman and Hall, London, pp. 38–101. Hall, A.J., Boyce, A.J., Fallick, A.E., Hamilton, P.J., 1991. Isotopic evidence of the depositional environment of Late Proterozoic stratiform barite mineralisation, Aberfeldy, Scotland. Chem. Geol. ŽIsotope Geoscience Section. 87, 99–114. Hallam, A., 1992. Phanerozoic Sea-level Changes. Colombia University Press, p. 266. Hallam, A., Bradshaw, M.J., 1979. Bituminous shales and oolitic ironstones as indicators of transgressions and regressions. J. Geol. Soc. London 136, 157–164. Hanor, J.S., 1994. Origin of saline fluids in sedimentary basins. in: J. Parnell ŽEd.., vol. 78, Geofluids: Origin and Migration of Fluids in Sedimentary Basins. Geol. Soc. Spec. Publ., pp. 151–174. Harris, A.L., Haselock, P.J., Kennedy, M.J., Mendum, J.R., 1994. The Dalradian Supergroup in Scotland, Shetland and Ireland. In: W. Gibbons, A.L. Harris ŽEds.., vol. 22, A Revised Correlation of the Precambrian Rocks in the British Isles. Spec. Rep. Geol. Soc. London, pp. 33–53. Jowett, E.C., Rydzewski, A., Jowett, R.J., 1987. The Kupferschiefer Cu–Ag ore deposits in Poland: a re-appraisal of the evidence of their origin and presentation of a new model. Can. J. Ear. Sci. 24, 2016–2037. Kyle, J.R., 1991. Evaporites, evaporitic processes and mineral resources. In: J.L. Melvin ŽEd.., Evaporites, Petroleum and Mineral Resources. Developments in Sedimentology, vol. 50, Elsevier Science, pp. 477–533. Large, D., 1988. The evaluation of sedimentary basins for massive sulfide deposits. In: Base Metal Sulfides in Sedimentary and Volcanic Environments, Spec. Pub. no. 5, Society for Geology Applied to Mineral Deposits, Springer-Verlag, New York, pp. 3–11. Lawrie, K.C., 1991. Facies analysis and exploration models for sediment-hosted, syntectonic, gold and base metal deposits. In: A.H. Bouma, R.M. Carter ŽEds.., Facies Models in Exploration and Development of Hydrocarbon and Ore Deposits. VSP, Utrecht, pp. 3–18. Maynard, J.B., Klein, G.D., 1995. Tectonic subsidence analysis in

the characterization of sedimentary ore deposits: examples from the Witwatersrand ŽAu., White Pine ŽCu. and Molango ŽMn.. Econ. Geol. 90, 37–50. McConachie, B.A., Dunster, J.N., 1996. Sequence stratigraphy of the Bowthorn Block in northern Mount Isa Basin, Australia: Implications for the base-metal mineralization process. Geology 24, 155–158. Minter, W.E.L., 1979. Sedimentological approach to mapping and assessment of the Witwatersrand Sequence. In: J.E. Glover, D.I. Groves ŽEds.., Gold Mineralization. Geology Department, University of Western Australia, Perth, Publ. 3, pp. 89–102. Minter, W.E.L., 1990. Paleoplacers of the Witwatersrand basin. Miner. Eng. 1990, 195–199. Minter, W.E.L., Goedhart, M., Knight, J., Frimmel, H.E., 1993. Morphology of Witwatersrand gold grains from the basal reef: Evidence for their detrital origin. Econ. Geol. 88, 237–248. Moles, N.R., 1985. Geology, geochemistry and petrology of the Foss stratiform barite - base metal deposit and adjacent Dalradian metasediments, near Aberfeldy, Scotland. Unpub. PhD thesis, University of Edinburgh. Moles, N.R., Ruffell, A.H., 1993. Characteristation and prediction of sediment-hosted ore deposits using sequence stratigraphy – transfer of a concept from the oil industry, Unpubl. Geofluids ’93 Extended Abstract, Torquay, 1993, pp. 459–463. Morganti, J., 1981. Sedimentary-type stratiform ore deposits: Some models and a new classification, in: R.G. Roberts, P.A. Sheahan ŽEds.., Ore Deposit Models. Geosci. Can. Reprint Series 3, pp. 67–78. Olson, R.A., 1984. Genesis of paleokarst and stratabound zinc– lead sulfide deposits in a Proterozoic dolostone, northern Baffin Island, Canada. Econ. Geol. 79, 1056–1103. Orth, C.J.L.R., Quintana, J.S., Gilmore, R.C., Westergaard, E.H., 1986. Trace-element anomalies at the Mississippian–Pennsylvanian boundary in Oklahoma and Texas. Geology 14, 900–986. Payton, C.E. ŽEd.., 1977. Seismic stratigraphy – applications to hydrocarbon exploration. Am. Assoc. Pet. Geol., Mem. 26, 516p. Phillips, G.N., Myers, R.E., 1989. The Witwatersrand gold fields: Part 2. An origin for the Witwatersrand gold during metamorphism and associated alteration. Econ. Geol. Monogr. 6, 598– 602. Phillips, G.N., Myers, R.E., Law, J.D.M., Bailey, A.C., Cadle, A.B., Bnenke, S.D., Giusti, L., 1989. The Witwatersrand gold fields: Part 1. Post-depositional history, synsedimentary processes, and gold distribution. Econ. Geol. Monogr. 6, 602–608. Posamentier, H.W., Allen, G.P., 1993. Siliciclastic sequence stratigraphic patterns in foreland ramp-type basins. Geology 21, 455–458. Prosser, D.J., Daws, J.A., Fallick, A.E., Williams, B.J., 1994. The occurrence of d34 S of authigenic pyrite in Middle Jurassic Brent Group sediments. J. Pet. Geol. 17, 407–428. Ravenhurst, C.E., Reynolds, P.H., Zentilli, M., Krueger, H.W., Blenkinsop, J., 1989. Formation of Carboniferous Pb–Zn and barite mineralization from basin-derived fluids, Nova Scotia, Canada. Econ. Geol. 84, 1471–1488. Robb, L.J., Meyer, M., 1991. A contribution to recent debate

A.H. Ruffell et al.r Ore Geology ReÕiews 12 (1998) 207–223 concerning epigenetic versus syngenetic mineralization processes in the Witwatersrand Basin. Econ. Geol. 86, 396–401. Ruffell, A.H., 1990. The mineralogy and petrography of the Sulphur Band phosphates ŽAptian–Albian. at Folkestone, Kent. Proc. Geol. Assoc. 101, 79–84. Schmitt, J.-M., Makhoukhi, S., Goblet, P., 1991. Modelling of structure-induced hydrothermal circulations in a Mississippi Valley Type deposit. In: M. Pagel, J.L. Leroy ŽEds.., Source, Transport and Deposition of Metals. Balkema, Rotterdam, pp. 489–492. Sheldon, R.P., 1964. Paleolatitudinal and paleogeographical distribution of phosphorite. U.S. Geol. Surv. Prof. Pap. 501-C, 106–113. Sheldon, R.P., 1980. Episodicity of phosphate deposition and deep ocean circulation – an hypothesis. In: Y.K. Bentor ŽEd.., Marine Phosphorites, Geochemistry, Occurrence, Diagenesis. Soc. Econ. Paleontol. Mineral., Spec. Publ. 29, pp. 239–247. Shelton, K.L., Burstein, I.B., Hagni, R.D., Vierrether, C.B., Grant, S.K., Hennigh, Q.T., Bradley, M.F., Brandom, R.T., 1995. Sulfur isotope evidence for penetration of MVT fluids into igneous basement rocks, southeast Missouri, USA. Miner. Depos. 30, 339–350. Slater, D., Highley, D.E., 1977. The iron ore deposits in the United Kingdom of Great Britain and Northern Ireland. In: A. Zitzmann ŽEd.., The Iron Ore Deposits of Europe and Adjacent Areas. vol. 1, Hannover, pp. 393–409 ŽSection B: Applied Earth Sciences..

223

Sutton, R.J., Ritger, S.D., Maynard, J.B., 1990. Stratigraphic control of chemistry and mineralogy in metamorphosed Witwatersrand quartzites. J. Geol. 98, 329–341. Sweeney, M.A., Binda, P.L., 1994. Some constraints on the formation of the Zambian Coppperbelt deposits. J. Afr. Earth Sci. 19, 303–313. Tang, Z., Parnell, J., Ruffell, A., 1994. Deposition and diagenesis of the lacustrine – fluvial Cangfanggou Group Župpermost Permian to Lower Triassic., southern Junggar Basin, NW China: a contribution from sequence stratigraphy. J. Palaeolim. 11, 67–90. Varentsov, I.M., Rakhmanov, V.P., 1977. Deposits of manganese. In: V.I. Smirnov ŽEd.., Ore Deposits of the USSR, vol. 1. Pitman, London, pp. 114–178. Van Wagoner, J.C., Mitchum, R.M., Campion, K.M., Rahmanian, V.D., 1990. Siliciclastic Sequence Stratigraphy in Well Logs, Cores and Outcrops: Concepts for High Resolution Correlation of Time and Facies. American Association of Petroleum Geologists, Methods in Exploration, Series 7, p. 55. Wendorff, M., 1994. IGCP Project 302 – The Zambian Copperbelt. Episodes 17, 61–62. Williams, G.D., Dobb, A., 1991. Conference report: Tectonics and seismic stratigraphy. J. Geol. Soc. London 148, 935–937. Wilson, R.C.L., 1990. Sequence stratigraphy: An introduction. Geoscientist 1, 13–23.