Aug 11, 2009 - A plot of. Formation of Sedimentary Rock-Hosted Stratiform Copper Deposits through Earth History. MURRAY W. HITZMAN,1,â DAVID SELLEY ...
©2010 Society of Economic Geologists, Inc. Economic Geology, v. 105, pp. 627–639
Formation of Sedimentary Rock-Hosted Stratiform Copper Deposits through Earth History MURRAY W. HITZMAN,1,† DAVID SELLEY,2 AND STUART BULL2 1 Department
of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado 80401
2 Centre
for Ore Deposit Research, University of Tasmania, Hobart, Tasmania Australia 7000
Abstract Sedimentary rock-hosted stratiform copper deposits form by movement of oxidized, copper-bearing fluids across a reduction front that results in the precipitation of copper sulfides. Large-scale production of such oxidized fluids, as well as the formation of mobile hydrocarbons (oil) has probably been common since the formation of the first red beds in the Paleoproterozoic, and deposits of this type occur in rocks from the Paleoproterozoic to the Tertiary. However, supergiant deposits are currently recognized in only three basins: the Paleoproterozoic Kodaro-Udokan basin of Siberia, the Neoproterozoic Katangan basin of south-central Africa, and the Permian Zechstein basin of northern Europe. The paucity of data regarding the Udokan deposit makes understanding this system difficult in terms of Earth history events. Both the Neoproterozoic and the Permian were times of supercontinent breakup with major landmasses at low latitudes. This global tectonic framework favored the formation of failed rifts that subsequently became significant intracratonic basins with basal, synrift red-bed sequences overlain by marine and/or lacustrine sediments and, in some basins located at low latitudes, by thick evaporitic strata. The intracratonic setting of these basins allowed the development of a hydrologically closed basinal architecture in which highly oxidized and saline, moderate-temperature basinal brines were produced that were capable of supplying reduction-controlled sulfide precipitation over very long time periods (tens to hundreds of millions of years). The length of time available for the mineralizing process may be the key factor necessary to form supergiant deposits. However, examination of the absolute ages for the Kupferschiefer (Zechstein basin) and Katangan deposits allows speculation that other factors may also have been important. Both the Neoproterozoic and Permian were times of major glacial events. Glaciation may also be conducive for the formation of supergiant sediment-hosted stratiform copper deposits. Glacial periods correspond to magnesium- and sulfate-rich oceans that could have been responsible for additional sulfur in basinal brines developed during evaporite formation and would then be available during the long mineralization process.
Introduction PRECIPITATION of copper sulfides from moderate-temperature, moderate- to high-salinity, oxidized fluids at oxidationreduction interfaces has been common since the Paleoproterozoic. The largest known deposits occur within sedimentary basins, generally at the contact between subaerial red-bed sequences and overlying marine or lacustrine shales, siltstones, sandstones, or carbonate rocks. Deposits in such settings have been termed sediment-hosted stratiform copper deposits (Kirkham, 1989) or sedimentary rock-hosted stratiform copper deposits (Cathles and Adams, 2005). Sedimentary rock-hosted stratiform copper deposits consist of relatively thin (generally 2 Mt contained Cu; Singer, 1995). The temporal distribution of basins containing giant and supergiant sedimentary rock-hosted stratiform copper deposits is clearly nonrandom. This deposit type only occurs after oxygenation of the atmosphere in the early Paleoproterozoic (Knoll and Holland, 1995; Holland, 2005a, 2006). A plot of
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Submitted: August 11, 2009 Accepted: September 11, 2009
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Redstone
Udokan
Spar Lake
Kupferschiefer
White Pine
Paradox Basin
Dzhezkazgan
Creta, Magnum
Aynak
Central African Copperbelt
FIG. 1. Map of the world showing the location of the sedimentary rock-hosted stratiform copper districts and deposits mentioned in the text.
copper metal contained in these deposits versus age of host rock (Fig. 2) shows distinct spikes in the Neoproterozoic and the Permian. The Udokan deposits represent a spike in the Paleoproterozoic, although the relatively poor age constraints of the host rocks, and even poorer age constraints for the pe-
riod of mineralization, makes temporal definition of these deposits difficult. The temporal spike in the Neoproterozoic is primarily due to the Katangan basin deposits, with a contribution from the Aynak deposits of Afghanistan and a lesser contribution from
250 Kupf
CACB ( Aynak, Red)
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50 Dzh
Udokan WP/PI
0 T
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P
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Rev 1500
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Geological Time ( Ma) and Period FIG. 2. Contained tons of copper for sediment-hosted stratiform copper deposits by age of the host rocks. Whereas hostrock ages are relatively well constrained for the Mesoproterozoic to Tertiary, significant uncertainties exist for the exact ages of host rocks for deposits in the Paleoproterozoic sequences. Data is taken from Hitzman et al. (2005). Kupferschiefer (Kupf), Dzhezkazgan (Dzh), Redstone (Red), Central African Copperbelt (CACB), White Pine/Presque Isle (WP/PI), Revett deposits (Rev). 0361-0128/98/000/000-00 $6.00
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the Redstone deposit of northern Canada (Chartrand et al., 1989). The spike in the Permian is almost entirely caused by the Kupferschiefer deposits of Europe, although a number of much smaller deposits of Permian age, such as Creta and Magnum (Hagni and Gann, 1976; Johnson, 1976; Smith, 1976), are recognized in North America. Good age control for both host rocks and the duration of mineralization in the Keweenaw, Katangan, and Zechstein basins allows analysis of specific Earth history events that may be important for the genesis of post-Paleoproterozoic giant and supergiant examples of this deposit type. Utilization of these concepts suggests that the older Udokan ores may define similar geologic events. Basin Architecture of the Sedimentary Rock-Hosted Stratiform Copper Deposits Sedimentary rock-hosted stratiform copper deposits are the products of evolving basin-scale, or at least sub-basin-scale, fluid flow systems. Similar to all ore systems, these deposits can be thought of as the end products of processes involving a source(s) of metal and sulfur, a source(s) of metal-transporting fluid, transport paths, possible mobile reductants, a thermal or hydraulic pump, and the chemical and physical factors that result in precipitation (“trapping”) of the sulfides. Whereas the general nature of the ore fluids responsible for the formation of these deposits is clear (Rose, 1976; Brown,
1992), exact fluid compositions in terms of metal and sulfur contents are not known due to a lack of fluid inclusion data (Hitzman et al., 2005). This lack of data is the result of the absence of suitable coarse minerals, containing workable fluid inclusions, intergrown with sulfides in the vast majority of deposits. A wide variety of basin architectures and processes can lead to the formation of sedimentary rock-hosted stratiform copper deposits (Hitzman et al., 2005). However, supergiant deposits must have formed in basins with unique conditions that allowed for the accumulation of large amounts of metal-bearing fluid, sufficient reduced sulfur, large amounts of reductants, and focusing of fluid movement into relatively small areas. The key to forming the required basin architecture is deposition of a specific sedimentary sequence within an intracratonic basin that becomes hydrologically closed (Hitzman et al., 2005). The general stratigraphic sequence observed in productive basins for both giant and supergiant deposits is a basal sequence of synrift red beds, often with mafic or bimodal volcanic rocks, that serve as a source for rock-buffered oxidized fluids, as well as a potential source for metals, particularly copper (Fig. 3). This oxidized sedimentary package is overlain by marine to lacustrine sediments deposited during rift climax and postrift phases of sedimentation that contain, or can generate through burial, areally extensive zones with large
Sediment-hosted, stratiform copper deposits
Sandstones, siltstones, and shales
Evaporites
SEAL RESIDUAL BRINES
Carbonates Sandstones, siltstones Basement
Red beds
METAL SOURCE
Bimodal volcanic rocks
FIG. 3. Schematic cross section across an intracratonic, hydrologically closed basin that is typical of those hosting giant and supergiant sediment-hosted stratiform copper deposits. Synrift red beds and minor bimodal volcanic rocks floor the basin. Marine sandstones, siltstones, and shales, which may locally be organic rich, transgressively overlie this red-bed sequence. This siliciclastic sequence grades upward into marine carbonates that contain a thick evaporite sequence. In most productive basins, the evaporites contain significant halite and may have evolved to magnesium and potassium salts. The upper portion of the basin contains shallow marine to continental siliciclastic sediments. The total thickness of the sedimentary sequence may range from several to more than 10 km. The sediment-hosted stratiform copper system consists of residual brines or brines from evaporite dissolution that move downward into the basal, oxidized red-bed sequence. Heat from burial and, in some cases, high heat flow and/or igneous activity initiate convection of these highly saline brines, which are capable of leaching metals from both the red-bed sediments and the basement. Oxidized, metal-rich brines circulate upward to the top of the red-bed sequence, where they encounter organic-rich sediments that provide the reductants necessary to precipitate copper sulfides. Fluids may also utilize fault architecture within the basin to escape to higher levels and precipitate sulfides when they encounter significant zones of either in situ or mobile (natural gas, petroleum) reductants. The evaporite beds provide an effective top seal to the hydrologic system, whereas the basin edges themselves provide lateral containment. 0361-0128/98/000/000-00 $6.00
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amounts of contained reductant. The reductant can be either in situ organic matter or hydrocarbons that have migrated within the basin. These reduced facies can have large lateral extents allowing for the formation of supergiant deposits with great strike extent. In the Katangan and Zechstein basins, evaporitic conditions were initiated during the rift-climax phase and persisted throughout the subsequent postrift phase of marine and/or lacustrine carbonate sedimentation. Thick, laterally persistent evaporite strata play a key hydrologic role as effective seals to underlying rift-related permeable units. Equally significant, dense residual brines, generated during precipitation of evaporitic sediments, sink and may ultimately convect through, in some instances, enormous metal reservoirs within the rift-related sediments and potentially the basement (Fig. 3). Capping carbonate or clastic sequences, such as the Nonesuch Shale at White Pine, can provide hydrologic seals sufficient to form giant deposits. Major sediment-hosted stratiform copper deposits form when the evolved basinal fluids leach significant amounts of copper and other metals from the basal synrift sediments and volcanic rocks, and possibly from structurally controlled fluid pathways within the upper portions of the basement, and move upward toward a hydrologic seal to intersect oxidationreduction interfaces in marine and/or lacustrine sediments. In both the Katangan and Kupferschiefer basins, there is evidence of long periods (hundreds of millions of years) of mineralization that suggests enormous volumes of fluids were involved. Thus, to produce the supergiant deposits, it is necessary either to have large volumes of fluids introduced into a basin over time or to recycle fluids through convective flow. Fluid flow of both types is recognized geologically (Cathles and Adams, 2005). In both the Katangan and Kupferschiefer intracratonic basins, it appears most likely that the basin architecture was relatively hydrologically closed and that basinal fluids circulated through the systems over a protracted period. Smaller, but still significant deposits, such as the giant White Pine-Presque Isle system in the United States (~400 Mt of 1.1% Cu; Hitzman et al., 2005), formed primarily from a temporally limited, single pass, mechanical loading-driven compaction hydrologic system (Swenson et al., 2004). In many sediment-hosted stratiform copper systems, and particularly well illustrated in the Zechstein, Katangan, and Keweenaw basins, ore-grade mineralization is best developed along basin margins adjacent to basement highs where the basal red beds are relatively thin or pinch out (Jowett, 1986; Selley et al., 2005). From a hydrological perspective, these are sites where basinal brines are overpressured (Cathles and Adams, 2005), and where fluid flow is concentrated and forced to interact with the overlying reduced strata (Swenson et al., 2004). Host Rock Ages Sediment-hosted stratiform copper deposits are recognized from the middle Paleoproterozoic (e.g., Udokan, Russia: Volodin et al., 1994) to the Tertiary (e.g., Corocoro, Bolivia: Flint, 1989, Ljunggren and Meyer, 1964; Fig. 2). A key component for the formation of this deposit type is the presence of oxidized rock masses, generally composed of red beds, capable of buffering basinal fluids to an oxidized state and providing 0361-0128/98/000/000-00 $6.00
copper and other metals. Red beds first appeared near the Archean-Paleoproterozoic boundary, coincident with the oxygenation of the atmosphere (Knoll and Holland, 1995; Holland 2005a, 2006). In addition, reductants within the rock mass stratigraphically above the synrift sequence are necessary to form the chemical trap that allows copper sulfides to precipitate. These could be either in situ reductants or reduced material that has also migrated into the trap area. The latter commonly may be hydrogen sulfide, probably with petroleum and/or natural gas, as has been recognized in Dzhezkazgan, parts of the Zambian Copperbelt, and the Kupferschiefer. The capacity to produce major amounts of mobile hydrocarbons capable of forming such chemical traps was apparently first generated in the early Paleoproterozoic (Melezhik et al., 1999a, b). Whereas significant sediment-hosted stratiform copper deposits are recognized in the Paleoproterozoic (e.g., Udokan deposits), absolute ages of the sequences hosting these deposits are poorly constrained between 2200 and 1800 Ma (Abramov, 2008). In the Mesoproterozoic, the age of the Revett Formation, which hosts deposits such as Spar Lake in the northwestern United States, is constrained by underlying 1468 Ma lower Belt sequence (Anderson and Davis, 1995) and the overlying 1454 Ma carbonate sequence (Evans et al., 2000). The absolute age of host rocks for the Mesoproterozoic deposits in the White Pine district, Unites States, is approximately 1080 Ma (Davis and Paces, 1990). The Redstone, Canada, and Aynak, Afghanistan, deposits are Neoproterozoic age. The Redstone deposit host rocks are 100 m.y.) than the host rock, whereas the age of mineralization in the White Pine/Presque Isle (WI/PI) system is early relative to host-rock age. Host-rock age and tonnage data are taken from Hitzman et al. (2005). Mineralization age data is mainly from Kulig et al. (1994), Michalik (1997), Bechtel et al. (1999), Swenson et al. (2004), and Selley et al. (2005). 0361-0128/98/000/000-00 $6.00
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inversion (Selley et al., 2005). Subsequent postpeak-metamorphic, fracture-hosted, Cu-Mo-U mineralization occurred at the Kansanshi deposit at ~512 to 502 Ma (Torrealday et al., 2000), whereas a postorogenic extensional phase of mineralization at ~450 Ma is recorded by the breccia-related Cu-ZnGa ores of Kipushi (Schneider et al., 2007). Uranium-Pb geochronology of uraninite, a phase intimately associated with, but commonly postdating, stratiform copper mineralization in the Central African Copperbelt, also reveals a protracted history of fluid flow, with ages between 670 and 503 Ma (Darnley et al., 1961; Cahen et al., 1971, 1984; Meneghel, 1981; Richards et al., 1988; Loris et al., 1997). Abundant evidence also exists for protracted sulfide precipitation in the Permian Kupferschiefer (Jowett, 1992; Large et al., 1995; Michalik and Sawłowicz, 2001; Fig. 4). Potassium-Ar dating in the Kupferschiefer and underlying Weissliegendes sandstones suggests that authigenic illites associated with mineralization formed over a broad time range, from early diagenesis to synorogenic Alpine uplift and inversion (Kulig et al., 1994; Michalik, 1997; Bechtel et al., 1999). The relative amounts of sulfides precipitated throughout the long time intervals of mineralization in the sediment-hosted stratiform copper deposits remains speculative. At White Pine, late sulfides appear to represent a very small fraction of the total ore. Textural evidence in the Kupferschiefer indicates that much of the sulfide was deposited relatively early, although sulfide precipitation clearly continued, perhaps sporadically, over an extended period with a probable spike in mineralization intensity during Alpine orogeny. In the Central African Copperbelt, geochronologic constraints, coupled with the need to have generated and mobilized significant volumes of hydrocarbon and/or sour gas, suggest that much of the classical stratiform mineralization was likely to have been formed late in the basin’s history. Postpeak metamorphic copper mineralization typically forms a relatively minor component of sediment-hosted stratiform ores, but in the cases of Kansanshi (Broughton et al., 2002) and Kipushi (Intiomale and Oosterbosch, 1974) forms large economic orebodies. The combination of textural, isotopic, and geochronologic data provides compelling evidence that the mineralization processes responsible for the formation of sediment-hosted stratiform copper deposit systems may occur from near the time of sediment deposition to late in a basin’s history (Fig. 4). Significant deposits, such as White Pine, can form from single-pass, mechanical load-driven compaction (Swenson et al., 2004). However, the evidence indicates that two of the basins containing supergiant deposits underwent very prolonged periods of mineralization (>100 m.y.), which suggests that convection must have been established to recycle mineralizing fluids. This implies that these basins were relatively tectonically quiescent for long periods. Critical Factors for the Formation of Sediment-Hosted Stratiform Copper Deposits Sediment-hosted stratiform copper deposits require oxidized metal source beds (red beds), reduced facies to serve as metal traps, and saline brines capable of leaching and carrying metals. To form significant deposits, mineralizing fluids must be confined within the red beds and expelled through relatively focused zones, often in areas of stratal pinch out or 0361-0128/98/000/000-00 $6.00
linear structural zones. Potentially productive host rock sequences and geometric configurations are commonly found in rift basins. Supercontinent breakup is ideal for producing rift basins within continental masses that may have had a connection to world oceans, but which did not evolve into passive margin basins. Passive margin basins are inherently leaky with dense fluids able to migrate seaward out of the sedimentary pile and seawater able to infiltrate the basinal sediments thus diluting the basinal brines (Fig. 5A). Large, sediment-hosted stratiform copper deposits appear to be most likely to form in intracratonic basins that are capable of containing moderate- to high-salinity basinal fluids for long time periods without significant leakage or dilution (Fig. 5B). Both the Katangan and Zechstein basins formed in an intracratonic, rather than a passive margin, setting. Strakov (1962) and Kirkham (1989) noted that most sediment-hosted stratiform copper deposits occur in sedimentary rocks that were deposited within 20° to 30° of the paleoequator. Continental areas in such low latitudes are amenable to having a desert environment, and rift basins produced by continent breakup in such settings typically have basal sequences composed of red beds. Overlying marine or lacustrine sequences are likely to include a significant proportion of evaporitic strata that are common in many, but not all, basins containing these types of deposits (e.g., Kirkham 1989, 2001). Evaporite deposition is important for the geohydrology of sediment-hosted stratiform copper systems because it allows for the formation of dense, high-salinity residual brines that can sink into the lower portion of the sedimentary sequence (Hitzman et al., 2005). These dense brines, once present, are capable of leaching metals from the basal, oxidized red-bed sequence. The evaporites themselves are important in establishing a regional aquiclude, or seal, within the basin stratigraphy and allowing for the possibility of establishing a longlasting intrabasinal fluid reservoir within which convective cells can develop (Hitzman et al., 2005). Rupturing of the evaporite seal during basin-inversion–driven halokinesis may allow upward escape of potentially mineralizing basinal brines to higher stratigraphic levels, where they could interact with zones containing reductants and cause cross-stratal sulfide precipitation, such as occurred at the Kansanshi and other deposits in the Central African Copperbelt (Hitzman et al., 2005). The area of present-day Europe best illustrates the tectonic configuration required to form an intracratonic basin suitable for the generation of large sediment-hosted stratiform copper deposits during the Late Permian (Fig. 6). During this period, northern Europe occupied the southern margin of the Pangean continent and was located at low latitudes (Fig. 6A). Incipient rifting, to north of the Tethys Ocean, led to the formation of several major intracratonic basins likely intermittently connected through narrow rift basins to the Tethys Ocean to the south and the Panthalassic Ocean to the north (Fig. 6B). Progressive filling of the Zechstein basin began with deposition of synrift siliciclastic and volcanic rocks. These were later transgressed by marine-derived sediments, including the organic-rich, marine Kupferschiefer Formation immediately overlying the red beds, and subsequent Zechstein carbonate and evaporite sediments, setting the favorable
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A.
Sandstones, siltstones Sandstones, siltstones, and shales
BASINAL FLUID ESCAPE
Carbonates
Evaporites
Deep water carbonates
Red beds
Basement
B. Sandstones, siltstones, and shales
Carbonates Evaporites
Sandstones, siltstones
BASINAL FLUID CIRCULATION
Red beds
Basement FIG. 5. Schematic cross sections of sedimentary basins. A. Cross section of a passive margin basin with a thin rift sequence overlain by continental siliciclastic sediments (sandstones and siltstones) that grade seaward into evaporates and then carbonates. The carbonate sediments prograde seaward and overlap deeper water carbonates. This carbonate-rich sequence is covered by deltaic to slope basin siliciclastic sediments (sandstones, siltstones, and shales). Basinal fluids in this system are able to escape from the sedimentary sequence vertically and laterally. B. Cross section of an intracratonic rift basin with a basal red-bed sequence and overlying siliciclastic, carbonate, and evaporite sediments. Basinal fluids in this system are trapped by confining basement and lateral sediment pinch-outs.
stratigraphy for sediment-hosted stratiform copper mineralization. The Zechstein basin underwent progressive sag phase deposition into the Mesozoic, with basin inversion during the Alpine orogeny at ~120 Ma; mineralization appears to have occurred throughout this >100-m.y. period. Unlike the White Pine system, where Swenson et al. (2004) were unable to demonstrate convection, geologic evidence in the Kupferschiefer suggests that convection cells were established in basal red-bed−filled depocenters (Jowett, 1986), perhaps 0361-0128/98/000/000-00 $6.00
through both mechanical loading and differential heat input, and later due to basin inversion associated with the Alpine orogeny. Formation of Supergiant Deposits— Unique Attributes of the Permian and Neoproterozoic The available evidence suggests that for at least two of the supergiant sediment-hosted stratiform copper districts (Kupferschiefer and Katangan basins) a unique attribute is the lengthy
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A.
general area of ice cap
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Sahara Platform
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tin con
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FIG. 6. Paleoreconstructions for the Late Permian (~255 Ma). A. World view showing the location of present-day Europe, just north of the equator and on the southern edge of the Pangean continent. Gray areas indicate areas of land. The thick black lines are subduction zones. The pink oval represents the approximate location of the Zechstein basin. The figure is modified from Scotese (2004). B. Paleogeographic map showing the location of the intracontinental Zechstein basin, to the northwest of the Tethys Ocean, and the outlines of present-day continental areas. The Zechstein basin is the largest of a number of rift basins formed in the area of the present-day North Atlantic and Western Europe. The Zechstein basin had intermittent connection with both the Tethys Ocean to the southeast and to the Panthalassic Ocean to the north through rift troughs. This intermittent influx of marine water, combined with the location near the equator, allowed for the development of a significant thickness of evaporites within the basin. The locations of the two major known areas of Kupferschiefer mineral deposits in Germany and Poland, along the southwestern edge of the basin, are illustrated. This figure is modified from Ziegler (1982, 1988).
time span of mineralization compared with districts containing smaller deposits. Is time the only critical factor for generation of supergiant deposits? Are there other features related to the age of the Kupferschiefer and Katangan basins that could be important for the generation of such large deposits? Whereas evaporites are undoubtedly important for the generation of supergiant sediment-hosted stratiform copper deposits, there is a poor correlation between the age of host rocks for the deposits and the amount of preserved halite in the geologic record (Knauth, 2004; Hay et al., 2006; Fig. 7). Evaporites are a key feature of the basins hosting supergiant 0361-0128/98/000/000-00 $6.00
deposits, but it nevertheless does not appear that sedimenthosted stratiform copper deposit form at times of maximum evaporite deposition. The late Neoproterozoic and the Permian were both times of supercontinent breakup and evaporite deposition. They were also periods during which major glacial events occurred (Fig. 7). The triggers for these glacial events remain poorly known, although eruption of large amounts of subaerial basalt at low latitudes has been implicated (Goddéris et al., 2003), along with differences in sensitivity of the carbon cycle to loss of shallow-water environments and resulting
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Geological Time (Ma) and Period FIG. 7. Correlation of the ages of host rocks and mineralization for sediment-hosted stratiform copper deposits (1100 Ma–present), as well as the ages of major glacial events (in blue; Eyles and Young, 1994), ages of preserved halite accumulations (in yellow) [modified from fig. 2 of Knauth (2004) and including data compiled by Hay et al. (2006) and evidence for significant ~750–800 Ma evaporites in the Katangan and Central Australian basins (e.g., Lindsay, 2002; Jackson et al., 2003)], and variation in seawater chemistry as portrayed by the m(Mg2+)i/m(Ca2+)i ratio (purple dashed line). The seawater chemistry data during the Phanerozoic are based on analyses of fluid inclusions in halite (Horita et al., 2002) and from before 500 Ma, they are based on the results of Hardie (2003). The horizontal line at the ratio of approximately 1.5 shows the break between the aragonite (>1.2) and calcite (
