Genesis of Kupferschiefer

ECONOMIC GEOLOGY AND THE

BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

VOL. 81 DECEMBER, 1986 NO. 8

Genesis of Kupferschiefer Cu-Ag Deposits by Convective Flow of Rotliegende Brines during Triassic Rifting

E. CRAIG JOWETT* Department of Geology, University of Toronto, Erindale Campus, Mississauga, Ontario, Canada L5L 1C6

Abstract

Extensive and rich Cu-Ag sulfide mineralization occurs in Poland across the contact between the Upper Permian Zechstein restricted marine sequence and the Lower Permian Rotliegende continental volcanic and clastic sequence. Geologic evidence suggests that the mineralization was formed during late diagenesis when metalliferous brines migrated through the Rotliegende, leaching metals from the volcanic detritus, and up the flanks of basement highs, possibly along fracture porosity, to the pyritic Kupferschiefer and Zechstein limestone above. Thick evaporites in the lower Zechstein preclude a vertical flow-through model, but metal zoning attitudes suggest that the brines overturned where the Rotliegende pinched out against the highs and moved laterally along the base of the Zechstein toward the basin centers, presumably to sink back down into the Rotliegende, completing a convection cell. A Middle Triassic palcomagnetic age for the metal zoning and Kimmerian attitudes of dilatant sulfide veinlets indicate that the mineralizing event coincided with continental rifting associated with the opening of the Tethys ocean.

Slow unicellular convection may be commonplace in sandstone basins where small lateral temperature gradients, AT, exist, but greater velocities are necessary to form large orebodies. An anomalous tensional and thermal event, such as rifting, can increase velocities by increasing permeability, slope angle, or AT. The palcothermal structure of southwestern Poland was determined by modeling the conductive heat flow in 14 one-dimensional geologic sections which described the evolution of the basin architecture for 10 m.y. in the Early Triassic. Rifting was simulated by increasing the lower boundary condition from 500 ø to 1,000øC at 25 km. This thermal pulse produced a surprisingly high AT of 25øC across the Lubin ore district solely from differences in thermal conductivities between the basement high (4.2 W/ møC), the sandstone (2.5 W/møC), and the shale basin center (1.25 W/møC). Unicellular convection patterns would be induced by the lateral boundary conditions of warm basement highs and cold shale centers and by the greater horizontal permeability. These cells were 15 to 20 km long, 400 m high, and subhorizontal.

A slope angle of 2ø and a permeability of i D produces a convection velocity of 13 cm/yr which, with a copper solubility of 1,000 mg/kg in 20 to 30 percent Ca-Na-C1 brines in equilibrium with hematite, can form the Lubin deposit in less than 6 m.y. Using a fracture permeability of 2 D and a solubility of 300 mg/kg, the time needed is less than 10 m.y. The Konrad mine can be formed in similar time periods. Without the continuous recycling of the brine inherent in convective flow (20 times or more), the metal solubilities needed to form the Lubin deposits by a flow-through model would be unreasonably high.

Natural gases likely migrated along with the metalliferous brines and helped convection by creating secondary porosity and increasing the buoyancy of the fluids. Because the fluids are recirculated and not expelled, convection provides a way in which secondary migration of methane and petroleum can occur effectively in solution as well as in separate phases.

Introduction

EXTENSIVE Cu-Ag-(Pb~Zn) sulfide mineralization exists across the contact between the Lower Permian

Present address: Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109.

Rotliegende continental volcanic rocks and rift-filling red beds and the Upper Permian Zechstein marine carbonate-evaporite-red bed sequence in central Eu- rope, with the largest and richest deposits being in southwest Poland (Fig. 1). Although the Kupferschie- fer deposits are usually thought to be syngenetic or

0361-0128/86/618/1823-1552.50 1823

1824 E. CRAIG JOWETT

15'E •, 16 17'E 16

•/• /%• (• basement highs • • 1•'•. .... • RF Role F,ule ••.•L • •'• ", • Zechstein basin edge

•:"•::::• • •/ •,,•-•'-, , ,. ,

'

FIG. 1. The hematite (RF), copper, lead, zinc, and pyrite metal zoning in the basal Zechstein and uppermost Rotliegende of southwestern Poland. The rote f•iule (RF) copper zones are associated with underlying basement highs and lower Rotliegende volcanics and are considered to represent the foci of upwelling Rotliegende fluids. Mining areas are: (1) the Lubin district which includes the Malomice, Lubin, Polkowice, Rudna, and Sieroszow- ice mines, each about 50 km•; (2) the Konrad mine; and (3) the Nowy Kosciol and Lena mines. Line A-A' represents location of section in Figure 2. (Palcogeography after Pokorski (1978); metal zoning after Rydzewski (1978).)

early diagenetic (Gregory, 1930; Dunham, 1964; Rentzsch, 1974), this paper will show evidence which suggests that the ore deposits were formed during late diagenesis in the Triassic by metalliferous fluids convecting in a unicellular manner within the Rotlie- gende (Jowett, 1983, 1984).

The criteria for convection in clastic basins will be discussed, and it will be shown that vertical and lateral temperature gradients, AT, caused by the difference in thermal conductivities between the basement highs and the shale basin centers can initiate unicellular

convection. The palcothermal structure of southwest Poland will be reproduced by simulating the Triassic rifting event and modeling the conduction of the thermal pulse through the crust and sediments. In order to form the ore deposits in a geologically reasonable time period, a sufficiently large AT is needed to create adequate convection velocities. It will be shown that a specific regional thermal event, such as rifting, is able to produce this AT.

Evidence for Late Diagenetic Convection

Several large and irregular zones of rote f•iule, 1 within which pyrite has been replaced by hematite, occur in the mineralizing system in Poland (Fig. 1). These zones have low domal shapes and are consid-

] An old miner's term which means "red-colored waste rock."

ered by Rydzewski (1965, 1978), Lisiakiewicz (1969), and Oszczepalski (1980) to be the centers of upwelling of ore-forming fluids. Most workers believe that the source of the metals was the Rotliegende (e.g., Rentzsch et al., 1976), although some (e.g., Wede- pohl, 1971) consider that they were brought in by rivers. (For detailed and comprehensive syntheses of the history of genetic ideas and their geologic basis, the reader is directed to Gregory (1930), Dunham (1964), Oberc and Serkies (1968), Lisiakiewicz (1969), Jung and Knitzschke (1976), and Rentzsch et al. (1976).)

The mineralization occurs as thin, but laterally very extensive, blankets of sulfides in distinct metal zones above and lateral to the barren rote f•iule. The rote f•iule-ore contacts and other metal zone contacts

transgress lithologic bedding from the Zechstein limestone through the Kupferschiefer shale down to the Weissliegende sandstone of the uppermost Rot- liegende (Fig. 2). The rote f•iule-copper zones are associated with underlying basement highs and lower Rotliegende volcanics (Fig. 1); the metal zoning gen- erally dips away from the highs toward the basin centers (Fig. 2). Copper sulfides commonly replace pyrite and other copper sulfides, quartz grains, lithic frag- ments, and diagenetic calcite cement (Haranczyk, 1972). (Detailed accounts of the metal zoning and sulfide mineralogy can be found in Haranczyk (1972), Rentzsch (1974), and Jung and Knitzschke (1976).) Dilatant sulfide veinlets were formed after lithification

(Salski, 1977) and at the same time as the replacement

SSW tubiu District NNE

/ Zechstein i Evaperite• ,•• ' ,,.• ., ..•,. , '• • '

\ ?, ,, ,. ,, i l //

FIG. 2. Geologic reconstruction across the North Sudetic basin, Fore-Sudetic block, Fore-Sudetic monocline, and Wolsztyn highlands, showing the closed Rotliegende basins and mineralized Kupferschiefer (Ks) and Zechstein limestone (Ca 1) covered by gypsum and anhydrite (A 1) and rock salt (Na 1) of the Zechstein first cycle. In general, the rote f•iule and metal zones (RF, Cu, Pb, Zn, pyrite) dip away from the basement highs toward the basin centers, suggesting that the ores were formed by Rotliegende brines which migrated up along the flanks of the highs, turned over below the Zechstein, and presumably sank back down into the clastic basins, forming a simple convection cell.

GENESIS OF KUPFERSCHIEFER Cu-Ag DEPOSITS 1825

sulfides (Jowett, 1985). These characteristics suggest that the rote fSule and ore-grade sulfides are late diagenetic (Lisiakiewicz, 1969; Niskiewicz, 1980; Jowett et al., 1982, 1987).

The dilatant veinlet orientations are similar to

Kimmerian-age directions (Middle Triassic to Late Jurassic) (Salski, 1977; pers. commun. 1983), and the rote f'•iule hematite carries a stable chemical remanent

magnetization acquired in Middle Triassic time (Jow- ett et al., 1986), suggesting that the mineralizing event occurred in the Triassic.

Most hydrologic models for secondary migration of metalliferous oil field brines invoke movement up along flanks of basement highs or along fracture zones and then through a chemical trap where metals are precipitated (White, 1971; Anderson, 1983; Cathies and Smith, 1983; Goldhaber et al., 1983). Thick beds of salt and anhydrite in the lower Zechstein preclude such a flow-through model. The geologic evidence is compatible, however, with a model whereby Rotlie- gende brines, carrying metals leached from the volcanic detritus, migrated through the red beds and up the flanks of the basement highs to mineralize the overlying pyritic Kupferschiefer and Zechstein limestone. The brines appeared to have overturned along the Zechstein-Rotliegende contact toward the basin centers, presumably to sink down into the Rotlie- gende, completing a simple convection cell (Fig. 2).

Criteria for Convection in Porous Media

Theoretical and practical aspects of hydrothermal convection in porous media have been fully treated by Bories and Combarnous (1973) and Combarnous and Bories (1975) and summarized by Wood and Hewett (1982, 1984) in their model of sandstone cementation. These authors assumed box-shaped basins and homogeneous, isotropic permeabilities, whereas the Rotliegende basins are wedge shaped (thickening toward the centers) and have anisotropic permeabilities caused by interbeds of shale toward the basin centers. Because of the lack of theoretical work which

can be adapted to irregular, anisotropic basins in general, formulas for homogeneous boxes will be used: however, the calculated values must be considered only rough approximations of the real values.

The criteria for the onset of convection is the di-

mensionless Rayleigh number, Ra, which through theory and physical modeling has been shown to con- trol the onset and shape of convection cells in horizontal and sloping layers (Bories and Combarnous, 1973). This number is expressed as:

g. a. (t)C)e' K- AT. H Ra= , (1)

where g is the gravitational acceleration, a is the volumetric thermal expansion coefficient of the fluid, (pC)f is the volumetric heat capacity of the fluid, K is

the permeability of the medium, AT is the temperature difference across the layer, H is the thickness of the porous layer, v is the kinematic viscosity of the fluid, and h* is the effective thermal conductivity of the fluid-filled medium.

For layers with isothermal upper and lower boundaries, and sloping at an angle, 0, simple unicellular convection will always occur until:

Ra. cos 0 k 4•r 2 • 40. (2)

Above this value, the convection cells will be polyhedral if the slope is less than 15 ø, or counterrotating rolls if above 15 ø (Bories and Combarnous, 1973). In basins with irregular boundaries, the shapes of the cells are affected by the basin geometry.

In unicellular convection, fluid velocities are greatest near the upper and lower boundaries, though in different directions; they decrease toward the center of the cell, as in:

. -- ,

where z is the distance from the lower boundary (Bories and Combarnous, 1973).

It is unlikely that world-class orebodies like the Lubin district were created during normal diagenetic fluid flow as common cements were; otherwise, they would be more common. Rather, an unusual event at a specific time is probably necessary. The parameters, which can vary significantly with time and affect the velocity, are the temperature difference, AT, the slope, 0, and the permeability, K. Permeability could be increased by tensional fracturing during tectonism or during rapid unroofing (Narr and Currie, 1982) or through secondary porosity (Schmidt and McDonald, 1979). The slope angle could be increased by differential subsidence between basin margin and basin center, and temperature gradients across the Rotlie- gende might be increased by differential burial or by a thermal event. An anomalous thermal or tensional

event is probably necessary to increase these variable parameters enough to initiate adequate convection to form Lubin, and this event should be reflected in the geologic record.

General Geologic History

The Carboniferous Hercynian orogeny in central Europe culminated in the deposition of the West- phalian coal-bearing continental clastics in late Car- boniferous times in broad regional basins in the foreland of the young mountain chain (Ziegler, 1978). At the beginning of the Lower Permian (ca. 280 m.y.), strong extensional tectonism with associated continental flood basalts and rhyolites produced a series of linear and isolated closed basins into which Rotlie-

gende clastics were rapidly deposited (Jowett and Jarvis, 1984). The basin-filling sediments of the lower

1826 E. CRAIG IOWETT

and upper Rotliegende consist of a lateral and upward- changing sequence of alluvial fan, braided river, meandering stream, and eolian and saline lake depositional environments laid down in a succession of sedimentary cycles in a semidesert environment (Glennie et al., 1978; Pokorski, 1978, 1981). This results in a basin architecture of coarse clastics ad-

jacent to the interbasin basement highs and basin edges, and of siltstone and shale toward the basin centers. Rapid lateral facies changes indicate that syndepositional tectonics were important in the basin history, especially in the lower section (Tomasik, 1980; Nemec, 1981; Roniewicz et al., 1981).

The Zechstein was deposited on a peneplain as a series of four to five sedimentary cycles (Z1, Z2, Z3, etc.) in a tectonically quiescent period relative to the Rotliegende (Peryt, 1978). The cycles typically consist of coaly carbonate-rich shale (like the Kupfer- schiefer), normal to restricted shallow marine carbonate, and thick beds of sulfate, red shale, and sandstone, often with rock salt and potash (Wagner et al., 1981). The Z1 and Z3 cycles completely covered the Polish Rotliegende basins and the Fore-Sudetic block (Peryt, 1981), whereas the Z2 had a narrower extent. The Z4a and Z4b cycles were deposited in narrow basins in northwestern Poland (Wagner et al., 1981), before the Triassic Buntsandstein covered the whole

Zechstein basin (Senkowiczowa and Szyperko- Sliwczynska, 1975). In general, the Zechstein is thin- ner over basement highs and is thicker over the Rot- liegende basins (Peryt, 1978, 1981; Oszczepalski, 1980). Over the Fore-Sudetic block the Zechstein was likely only 150 m thick (Peryt, 1981); in the Lubin and Fore-Sudetic monocline areas, about 250 to 500 m (Tomaszewski, 1981; Wagner et al., 1981); and it reaches a maximum of 1,500 m in the Polonian basin in northwestern Poland (Wagner et al., 1981).

Following the regular Zechstein sedimentation, a further period of extension and rifting occurred (this time with little igneous activity) across central Europe during the Triassic and Early Jurassic (Ziegler, 1982), likely associated with the opening of the Tethys ocean to the south. Day (1984) found evidence to suggest that extension occurred by reactivation of Hercynian thrust faults. Deposition of the continental to restricted marine Buntsandstein red beds, open marine Muschelkalk carbonates, and restricted marine Keu- per shales was very widespread and rapid, with up to 2,500 m of Triassic sediments deposited in long linear basins (Senkowiczowa and Szyperko-Sliwczynska, 1968).

Continental and shallow marine Jurassic sediments reached a similar thickness in Poland (Dadlez, 1968) but were restricted to narrower basins through the center of Poland, with uplift and erosion alternating with sedimentation in the area of the Fore-Sudetic

monocline (Dadlez and Kopik, 1975). The Late Cre- taceous to Cenozoic Alpine orogeny which accompanied the closing of the Tethys ocean caused inver- sion of these basins and regional uplift in the foreland (Ziegler, 1982). Alpine tectonism disturbed the ore deposits without remobilizing the sulfides (Salski, 1977) and uplifted the Fore-Sudetic block to expose the basement (Fig. 1).

Aspects of the geologic record relevant to convection

This geologic history indicates several aspects important to convection ofRotliegende formational waters. First, in the basin-and-range geomorphology and semidesert environment, concentrated interstitial brines can form during sedimentation and then evolve into Na-Ca-C1 brines (Hardie and Eugster, 1970; Eugster and Hardie, 1978). This evolution can occur simply by precipitation of carbonate, gypsum, and anhydrite (Lerman, 1970), which are common Rot- liegende cements (Brunstrom and Walmsley, 1969; Glennie et al., 1978; Pokorski, 1981). The extremely saline brines in the Rotliegende basins (Bojarska et al., 1981; Laszcz-Filakowa, 1981) are contaminated by surface water only around Cretaceous Alpine faults (Bojarska et al., 1981; Solak and Zolnierczuk, 1981), suggesting that these brines are original and that the basins were isolated until the Alpine orogeny.

Second, the complete covering of the closed Rot- liegende basins by thick Z1 evaporites would disallow any basin dewatering after Z1 time, resulting in over- pressuring of the Rotliegende and basal Zechstein sediments. This precludes a flow-through genetic model after Z1 time. The resulting undercompaction would keep porosity and permeability open during diagenesis, allowing more internal fluid flow. Rotlie- gende brines are often overpressured (Van Wijhe et al., 1980; Bojarska et al., 1981) and the Polish brines are characteristically uniform in composition throughout the basin and have stable pressure gradients, unlike the highly variable Zechstein brines (Bojarska et al., 1981). This uniformity possibly re- flects the open nature of the porosity and flow within the Rotliegende basins.

In addition, the Lower Permian Rotliegende and the Lower Triassic Buntsandstein record two conti-

nental rifting events. Continental extension is gen- erally thought to be caused by stretching and thinning of the lithosphere (Jarvis and McKenzie, 1980), which raises the asthenosphere higher in the section, producing a regional thermal anomaly (Turcotte and Emerman, 1983). This anomaly is dissipated by conduction through the lithosphere when there is no magmatic hydrothermal convection (Turcotte and Ahern, 1977; Beaumont et al., 1982).

The organic chemistry of the Fore-Sudetic mono-


cline sediments (Gondek, 1980) indicates maximum palcotemperatures of 110 ø to 140øC for the Zech- stein and Triassic rocks, whereas temperatures for the Lower and Middle Jurassic sediments were 100 ø and below 90øC, respectively. These palcotemperatures were corroborated by vitrinite reflectance studies performed in Poland (Gondek, 1980; Majorowicz et al., 1983) This indicates that a thermal anomaly, probably related to thinning of the lithosphere, occurred in the Triassic and was dissipated during the Jurassic. Therefore, the Triassic rifting event represents the thermal and tensional event which can form

the Kupferschiefer deposits by convection.

Sedimentation Rates during the Triassic Rifting Event

Continental shales and sandstones were formed to-

ward the end of the Zechstein and are gradational into the Lower Triassic Buntsandstein (Milewicz, 1968). The uplift and accompanying erosion of the Variscan highlands to the south (Milewicz, 1968) caused the deposition of the Buntsandstein into rapidly subsiding basins over most of Poland. It developed to 600 m maximum in the North Sudetic basin and at least 90 m survives in the Sudeten basin to the south. In the Fore-Sudetic monocline, 600 to 740 m was deposited and a maximum of 800 m occurs in the Central Polish basin (Sokolowski et al., 1976), indicating fairly consistent subsidence over a large area. The westerly to northwesterly depositional strike of the Zechstein and Triassic sediments (Sokolowski et al., 1976) suggests that the Fore-Sudetic block was also covered before the Cretaceous uplift. Some differential subsidence occurred in the North Sudetic basin (Milewicz, 1968), but the consistent 600 to 800 m deposited in only 6 m.y., using Van Eysinga's (1975) time scale, suggests that the continental crust had completely foundered over the whole area between the Variscans and the present-day Baltic coast.

The Middle Triassic Muschelkalk was also fairly consistent in thickness, with 217 to 291 m deposited in the monocline and 112 m in the Central Polish basin (Sokolowski et al., 1976). Although only the lower Muschelkalk survives in the North Sudetic basin, its thickness (145-167 m) is similar to the maximum 169 m of lower Muschelkalk in the monocline.

The ranges of sedimentation rates for the Permian to Cretaceous (Fig. 3) show that the rate jumps to a maximum in the Early Triassic but decreases expo- nentially to zero by the Middle to Upper Jurassic. This is succeeded by uplift and erosion in the Upper Jurassic and slow sedimentation during the Creta- ceous.

The Muschelkalk rate is anomalously low, but the rates of these open marine carbonates cannot be di-

Permian Triassic Jurassic Cretaceous Tertiary

L U L M U L M U L U

::

:-:--:-:_-::-== :___:•

:-=%---:--•_ _5--_E--__-- .:_%•::_• Subsidence ............

-25 280 251 23J25 213 195 176 161 141 100 65

Age (My)

FIG. 3. Sedimentation rates in the Fore-Sudetic monocline

from the Permian to the present, showing the effects of the Triassic continental rifting associated with the opening of the Tethys ocean to the south. The rapid, fault-controlled subsidence of the Triassic gives way to the thermally controlled subsidence of the latest Triassic and Jurassic. (Sources: Dadlez and Kopik (1975), Pokorski and Wagner (1975), Senkowiczowa and Szyperko-Sliwczynska (1975), Sokolowski et al. (1976), Wagner et al. (1981).)

rectly compared with the other Triassic rocks, which are continental and lagoonal clastics. Rapid subsidence during Muschelkalk time would produce a basin filled with water, and the sediment-starved basin would not record rapid subsidence in the rock record. For these reasons the average rate has been drawn above the Middle Triassic rate and below the Upper Triassic rate. The rates chosen for the first 10 m.y. of computer simulation varied with time from 130 to 90 m/m.y. in the Fore-Sudetic monocline, and from 120 to 80 m/m.y. in the North Sudetic basin area.

Comparison of Southwest Poland with the Anadarko Basin

There is a similarity in the geologic histories of the Anadarko basin in Texas and Oklahoma and the Fore-

Sudetic monocline region. The Southern Oklahoma aulacogen was formed by rifting with much igneous activity during the Proterozoic to Cambrian (Hoffman et al., 1974), followed by 20 m.y. of rapid, fault-controlled subsidence and 70 m.y. of slower, thermal subsidence in the Ordovician (Feinstein, 1981). Dur- ing the Pennsylvanian, a second rifting event created fault-controlled subsidence for 20 to 25 m.y. followed by thermally controlled subsidence for 70 m.y. (Gar- ner and Turcotte, 1984), but there was little igneous activity.

In Poland, the Lower Permian rifting event (Jowett and Jarvis, 1984) was marked by the intense bimodal igneous activity (Ryka, 1981) and rapid, tectonically controlled differential subsidence of the Rotliegende (Tomasik, 1980), followed by the regular, presumably


thermal, subsidence of the Zechstein. Similarly, the second rifting event in the Triassic lacks significant igneous activity. The rapid subsidence (100-120 m/ m.y.) during Buntsandstein deposition (Fig. 3) is similar to the fault-controlled, synextensional subsidence of the Anadarko (Garner and Turcotte, 1984). The lower rates from the latest Triassic (30 m/m.y.) to latest Jurassic (0 m/m.y.) are like those of their period of thermal subsidence.

The similar geologic histories of these two areas and the similar subsidence rates over similar time periods of the second rifting events imply that certain of the thermal and mechanical assumptions used and confirmed by Garner and Turcotte (1984) can be ap- plied to the Fore-Sudetic monocline in modeling the thermal structure of the lithosphere during a rifting event. They showed that isostatic subsidence occurred early in the rifting event as a result of tensional normal faulting, producing a lithosphere with low flexural ri- gidity. This was accompanied by differential lithospheric thinning (thinning of the crust from 45 to 32 km and raising of the 1,200øC isotherm of the lithosphere-asthenosphere boundary from 100 to 35 km), and a subsequent increase in heat flow. (This early and differential thinning of the lithosphere will be used in the following modeling.) After the fault-controlled subsidence, thermal contraction and subsidence occurred as the lithosphere returned to thermal equilibrium.

Conductive Heat Flow Modeling of the Triassic Rifting Event

Thermal and mechanical assumptions

Only about 50 m.y. separated the starting points of the rifting periods (280 and 230 m.y.), so the lithosphere would still have been anomalously hot and thin, assuming 90 to 95 m.y. for thermal equilibrium. For this reason and because the Fore-Sudetic mono-

cline presently has a thermal gradient of 20 ø to over 30øC/km (Znosko, 1981; Downorowicz, 1983) with a palcothermal gradient of over 40øC/km (Majoro- wicz et al., 1983), an average 20øC/km gradient over 25 km was chosen as the prerift condition. The sim- ulations and calculations of Beaumont et al. (1982) and Garner and Turcotte (1984) show that depth-dependent thinning of the lithosphere more closely approximated the geologic and geophysical observations in these rifting events than if the crust and lower lithosphere were uniformly thinned. Therefore, a temperature of 1,000øC at 25 km, as a result of differential thinning, is reasonable. Observed heat flow patterns are better predicted by rapid, rather than slow, thinning models (Morgan, 1983), and Jarvis and McKenzie (1980) concluded that an instantaneous thinning model is a good approximation of an event of less than 20 m.y. duration. Therefore, in this study,

a rifting event was simulated by instantaneously increasing the temperature at 25 km depth to 1,000øC from the assumed prerift temperature of 500øC and by allowing the thermal pulse to conduct upward through the crust with time.

The simulated rifting period is a 10-m.y. event beginning at the Zechstein-Buntsandstein boundary and ending 4 m.y. into Muschelkalk deposition, a period of rapid, presumably fault-controlled, subsidence. The computer program maintained the lower boundary temperature at 1,000øC throughout the time studied after rifting (10 m.y.); an artificial condition unless the stretching mechanism were also maintained. The lack of igneous activity imposes slow lithospheric cooling by conduction alone, and the thermal structure of only the upper few kilometers are of interest in this study. Thus this lower boundary condition is a reasonable approximation for the 10-m.y. period.

Computer model and physical parameters A computer program which models the thermal

structure of a one-dimensional geologic section by using a finite difference model of time-dependent conductive heat flow was supplied by Ross Boutilier of the University of Toronto. (A more extensive ver- sion was used by Beaumont et al. (1982) in their thermo-mechanical modeling.) In this study a series of 14 columns, representing one-dimensional geologic sections, were used to define the palcothermal structure of southwest Poland (Fig. 4). The columns were spaced across the North Sudetic basin in the south, through the Fore-Sudetic block and Fore-Sudetic monocline to the Wolsztyn Highlands in the north, to reflect the architecture of the Rotliegende basins. Alluvial fan and braided river sandstone occurs next

to the basement highs and in the entire North Sudetic basin: meandering stream and saline lake shale occurs in the basin centers in the monocline. The Zechstein

and Triassic sediment types were made the same as the Rotliegende sediments because of computer program limitations.

After including the Rotliegende and Zechstein sediment thicknesses, the columns were allowed to "equilibrate" for 10 m.y. using 0.1 m.y. time steps to approximate the thermal structure at the end of the 20 m.y. Zechstein deposition period. The prerift upper and lower boundary conditions (20øC at the surface and 500øC at 25 km) were then changed to 20 ø and 1,000øC to simulate the thinning of the lithosphere at the beginning of the fault-controlled Buntstandstein sedimentation. The thermal structure

was calculated for each 1-m.y. interval, using 0.1 m.y. time steps and a 0.1-km grid size, as the thermal pulse migrated through the section. Horizontal heat flux was assumed to be zero. Triassic sediments at 20øC

were added at various rates each 1-m.y. interval for the 10-m.y. period.

GENESIS OF KUPFERSCHIEFER Cu-Ag DEPOSITS 18 2 9

Physical parameters used for basement (crust), sandstone, and shale were: specific heats of 1,375, 1,088, and 837 J/kgøC and radioactive heat produc- tions of 6.276, 12.552, and 12.552 t•W/m a over a 7.5-km depth. The latter are relatively high because of the intruded granites and locally derived sediments. The thermal conductivity of the basement (4.2 W/ møC) represents a composition of 40 percent schist, 40 percent quartzite, and 20 percent granite (from Znosko and Pajchlowa, 1968) with conductivities of 3.6, 5.2, and 3.2 W/møC, respectively (Clark, 1966; Kristiansen et al., 1982). The conductivity of the sandstone (2.5 W/møC) represents a composition of 45 percent quartz sandstone, 30 percent conglom- erate, and 25 percent water with conductivities of 4.0, 2.0, and 0.7 W/møC, respectively (Clark, 1966; Downorowicz, 1983; Andrews-Speed et al., 1984); that of the shale (1.25 W/reøC) represents shale (1.5 W/møC) with 25 percent water (Clark, 1966; Kris- tiansen et al., 1982; Andrews-Speed et al., 1984).

Triassic Palcothermal Structure of Southwest Poland

The prerift temperature array (Fig. 4A) shows that the isotherms are depressed somewhat within the sediments but that in general they cut through the basins. In the shale basin center (column 11) a low thermal gradient characterizes the structure from the surface down to the base of the shale, below which there is a buildup of heat and a high gradient. Sedi- ments of low thermal conductivity act as a thermal blanket; the heat cannot rise easily and is trapped. Sandstone is not as effective a thermal blanket as is

shale. The basement highs with little sediment cover (columns 1 and 6) are slightly lower in temperature than the adjacent basins; however, this does not apply to basement highs with significant sediment cover (column 14 is hotter than 13). In a horizontal section at 500-m depth, the overall effect in the prerift condition is one of a cold shale basin, a warm sandstone basin, and either a cooler or warmer basement high region. (The cold shale basin is caused presumably by the trapping of heat below the shale; the cool highs are probably the result of no sediment cover, whereas the warmer highs have the thermal blanket of sediment cover.) At greater depths (e.g., 1,500 m) the

shale center is at a higher temperature than the sandstone basins and the same as the basement highs.

One rather surprising effect is the large, lateral temperature gradient (12ø-14øC across about 20 km) which exists in the prerift, quasiequilibrium condition between the base of the Rotliegende in column 8 and its top in column 11. This suggests that slow convection of interstitial fluids, possibly causing quartz and calcite cementation (Wood and Hewett, 1982, 1984; Rabinowicz et al., 1985), might be commonplace.

During rift simulation, Triassic sediments at 20øC were added at the surface and the thermal pulse rose through the section. No effect of the pulse was apparent in the upper 3 km until after 2 m.y. (Fig. 4C). Between i and 5 m.y. (Fig. 4B-E), the temperature at 3-km depth consistently rose 10øC/m.y. After 5 m.y. (Fig. 4E) the temperatures below the Triassic sediments approached an approximate equilibrium condition, and between 8 and 10 m.y. (Fig. 4F), they were essentially at equilibrium, possibly because the cooling from above tended to balance the heating from below. The overall effect was one of decreasing the vertical thermal gradient between the surface and the top of the Rotliegende, and of increasing it from there down.

During this heating period, the blanketing effect increased to produce a very cool basin center above and within the shale, and a high thermal gradient at the base of and below the shale. More dramatic is the

change in the basement highs (columns 1 and 6). Al- though the thinly covered highs (columns 1 and 6) were cooler than the adjacent basins before rifting and at 1 m.y. after, the thermal pulse rose through the conductive basement more quickly and caused the highs to become the hottest regions at 5 m.y., both above the Rotliegende (500 m) and below (2,000 m). The most deeply buried basement high (column 14) always remained a thermal high compared to the adjacent basin.

The horizontal temperature gradient between columns 8 and 11, a distance of only 20 km, almost dou- bled, from 12 ø to 14øC (at a 700-m depth) to 22 ø to 24øC (at 1,700 m), by 8 to 10 m.y. after rifting (Fig. 4F). The vertical gradient across the 400-m average Rotliegende thickness similarly increased. The high lateral temperature gradient could be further in-

FIG. 4. Palcothermal structure of the surficial 3-km crust in southwest Poland demonstrating the changing isotherms due to differential thermal conductivities during a simulated rift (detailed explanations in text). Stippled pattern = sandstone, dashed pattern -- shale. Top stippled pattern (B-F) denotes cold Triassic sediments deposited on the surface at each 1-m.y. time step. A. Quasiequilibrium stage at the end of Zechstein sedimentation with upper boundary condition of 20øC at surface and lower boundary condition of 500øC at 25 km. B. One million years after rift simulation with new boundary condition of 1,000øC at 25 km. C. Two million years after rifting; thermal pulse is now beginning to heat the upper 3 km. D. Three million years after rifting. E. Five million years after rifting. A quasiequilibrium stage has been reached in which rate of cooling from above by sedimentation matches the rate of heating from below. F. Ten million years after rifting, the end of the rift simulation.


A

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Depth (m)

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1500

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NORTH SUDETIC FORESUDETIC FORESUDETIC MONOCLINE BASIN BLOCK

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1832 E. CRAIG ]OWETT

creased by imposing differential sedimentation during the Triassic, or by keeping in mind that poorly con- ducting Paleozoic sediments underlie the Rotliegende basins (Znosko and Pajchlowa, 1968). Both of these changes would add to the thermal blanketing effect in the basins. Also, fluid convection itself would increase the lateral temperature gradient by increasing heat flow in areas of upwelling, thus heating the flanks of the highs, and of decreasing it where fluids are descending, thus cooling the basin centers (Sass and Sammel, 1976; Andrews-Speed et al., 1984). Assum- ing, therefore, that the temperature gradients calculated in this study are minimum values, then a thermal gradient of 25øC is reasonable and will be used in the convection calculations to follow. The vertical

temperature differences in columns 8, 9, 10, and 11 between the top and bottom of the Rotliegende are 12 ø, 20 ø, 30 ø, and 45øC over œ00, 350, 500, and 600 m of sediment. This represents a constant gradient of 60øC/km for the sandstone and about 75øC/ km for the shale.

The maximum temperature attained by the Kup- ferschiefer was 50øC in columns 1, 5, and 7 and close to 60øC in column 14, all near the basement highs. This is far below the temperatures of 110 ø to 140øC noted above for the Fore-Sudetic monocline Kupfer- schiefer. That the maximum temperature was not attained in this conduction study suggests that the maximum came later when the rate of cold sedimentation

decreased and the Kupferschiefer had time to equilibrate at a greater depth of burial (2,500 m in the monocline at the beginning of the Lower, and 3,000 m by the Upper Jurassic). As the thermal structure approached thermal equilibrium after rifting, likely near the Jurassic-Cretaceous boundary, the isotherms in each column would become more equidistant and a temperature of 110 ø to 130øC at 2,500 to 3,000 m would not be unreasonable (see Fig. 4F). In addition, fluid convection would increase the overall heat flux, possibly in a heterogeneous pattern (Andrews-Speed et al., 1984) and thus raise the temperature of the basal Zechstein, at least locally, above that predicted by this conduction model. Otherwise, the difference could be the result of additional factors not considered in this study; e.g., the existence of deep tensional fractures which permitted the advection of heat from below.

Brine Convection Calculations

Styles and velocities of convection

To calculate the Rayleigh number and velocities, the following values for brines and undercompacted clastics would be reasonable: a = 4.0 X ]0-4C-1; •oCf = 4.0 X 106 W sec m-aC-•; H = 400 m; AT = 25øC (for Lubin); AT = 15øC (for Konrad); v = 4.0 X 10 -7 m2sec-•; )•* = 2.5 W m-•C-•; K = 1D = 1 X 10 -12 m 2 (for 25% porosity in undercompacted siliciclastics which are 40-70 m.y. in age); K = œD -- œ X 10 -•2

m 2 (for fractured rock); 0 = 2'•0 ø slope (for Lubin at 10 m.y. after rifting); 0 -- 1.3 ø slope (for Konrad at 10 m.y. after rifting).

Using the Lubin parameters, the Rayleigh number, Ra, is 157 and Ra. cos 0 at 0 = 1.3 ø is well above the critical value of 40, indicating that polyhedral convection shapes should prevail (Bories and Combar- nous, 1973). However, greater horizontal permeability would tend to force the flow along the flanks of the basement highs to produce a unicellular shape, and the geologic evidence supports this. As well, the boundary conditions of a warming effect on one side (basement high) and a cooling effect on the other (shale center) will tend to direct the convective flow up the highs and down toward the basin centers in a unicellular pattern. In this case, the basin can be viewed as a combination of a Bories and Combarnous

(1973) box at 0 ø slope (in which polyhedral flow occurs) and one at 90 ø slope (in which unicellular convection always occurs). The convection values calculated here must be assumed to be only rough approximations.

The velocity along the 2 ø slope at Lubin, 5 m above the lower contact, is 0.132 m/yr. Thus the brine would take 303,000 yr to complete a full 40-km convection cell, equivalent to 3.3 cycles/m.y. For the Konrad basin edge, with a slope of 1.3 ø, the velocity is 0.05 m/ yr, equivalent to 600,000 yr for a 30-km cell, or 1.67 cycles/m.y.

Time needed for mineralization

The time needed for the migrating fluid to form an orebody can be approximately determined by cal- culating a mass balance between the metalliferous brine and a slice of ore parallel to the flow. The Lubin deposit can be approximated by a slice of 2 percent Cu ore which is œ m high, 1 m wide, and 6 km long, and which transects the lithology over a 10-m vertical height (Fig. 5). This slice contains 2.15 million kg Cu using a density of 8,950 kg/m 3 for copper metal. In the 10 m by 1 m by 40 km volume of rock in one convection cycle, with 25 percent porosity, there are 1 X 105 m 3 of brine, or 1.15 X l0 s kg at 1,150 kg/ m a density. For a very concentrated, chloride-rich brine in equilibrium with hematite, a solubility of 1,000 mg Cu per kg solution is reasonable (Rose, 1976; Barnes, 1979; Roedder, 1979), which works out to 1.15 X 105 kg Cu in one cycle of solution.

A concentrated brine close to equilibrium with hematite would have very low sulfide activities and would carry metals as chloride complexes (Barnes, 1979). The efficiency of sulfide precipitation is close to 100 percent in a situation such as this where metal chlorides in solution are brought into contact with reduced sulfur (pyrite in the Kupferschiefer) (Barnes, 1979). Assuming this, the brine would need to complete about 19 convection cycles in order to carry all

GENESIS OF K UPFERSCHIEFER Cu-Ag DEPOSITS 1833

the metal through the slice of ore. Therefore, the convection cell would have to endure for 5.75 m.y. to form the orebody. Using the fracture permeability of 2 D, and a more conservative 300 mg/kg copper solubility, the orebody could be formed in 9.6 m.y. If intense fracturing occurred, increasing permeabilities to 10 D, the mineralizing process, using the lower solubility, could be completed in less than 2 m.y.

Similarly, the Konrad deposit can be represented by a slice of 1.1 percent Cu ore which is 1.2 m high, 1 m wide, and 4 km long, but which transects 5 m of section instead of 10 m. The time needed for min-

eralization is 6.6 m.y. using 1-D permeability and 1,000-mg/kg solubility, or 11 m.y. using 2 D and 300 mg/kg.

In these calculations, the brine is recycled within the Rotliegende basins a minimum of about 20 times (at 1,000 mg/kg solubility) in order to bring enough metal through the ore deposits. In a flow-through model, where the brine is constantly being expelled from the basin, metal solubilities might have to be tens of thousands of mg/kg in order to form the deposits--a difficult value to justify.

These simple mass balance approximations indicate that convection of brines in this environment can

readily produce an orebody within geologically reasonable time but that the metal solubilities necessary for a flow-through model may be unreasonably high.

Movement through Kupferschiefer shales

The presence of ore several meters thick in the basal Zechstein indicates that mineralizing fluids migrated into the shales from the Rotliegende basins, although the 6-km lateral extent of the Lubin district suggests that the flow was more horizontal than vertical. This migration into relatively impermeable sediment need not present a problem if the shales were geopressured relatively soon after deposition as discussed earlier. The resulting delay in sediment dewatering during burial would preserve porosity and permeability to a greater extent than if dewatering were allowed. As well, the mineralized dilatant veinlets in the Kupferschiefer (discussed earlier) indicate that a system of horizontal and vertical fractures was open to mineralizing fluids after the shale was lithified. The Zechstein mineralization (or alteration) is thickest in areas where the rote f•iule is developed, and thins away from these zones until the mineralization is pre- dominantly in the sandstone below, suggesting that the vertical component of fluid flow was greatest in areas near the rote f•iule zones, the presumed sites of upwelling.

Increase in velocity

This modeling shows that adequate velocities for mineralization could be maintained in the Early to Middle Triassic. However, with more rapid subsi-

, HOle FIG. 5. Sectional, 1-m-thick slice of rock, parallel to presumed

fluid flow, representing the Lubin ore district. Fluid flow was probably greater in the more porous Weissliegende, tilting the oxidation-reduction front to near horizontal. Both the Konrad and Lubin deposits could be formed in 5 to 10 m.y. by fluid convection.

dence occurring to the north in the basin center (Peryt et al., 1978) and a subsequent increase in slope angle, the velocity fluid motion on the northern flanks of the basement highs (including the Lubin district) could actually increase with time if other factors remained constant. Similarly, the velocities on the southern flanks of the basement highs (including the Konrad district) should decrease with time.

Speculation on the Effects of Natural Gas Generation

Additional porosity and permeability Secondary porosity is created during the early

stages of hydrocarbon generation when organic mat- uration produces carbon dioxide which in solution leaches carbonate cements (Schmidt and McDonald, 1979). Some of the porosity in the North Sea Rotlie- gende is thought to be secondary in origin (Schmidt and McDonald, 1979; Shanmugam, 1985). The source of the natural gases at the top of the Rotliegende is considered to be the Westphalian coals which generated gases during the Triassic (Calikowski et al., 1971; Calikowski and Glogoczowski, 1976) or Jurassic (Van Wijhe et al., 1980). The rate at which hydrocarbons are generated from kerogen accelerates rapidly above 80øC (Laplante, 1974). This temperature could easily be obtained in the basement during the Triassic rifting event (see 80øC isotherm in the basement 3 m.y. after rifting in Fig. 4D). Migration of hydrocarbons through the Rotliegende during the Triassic and Jurassic indicates that permeability was quite high at that time.

Certain rote f•iule-copper zones are associated with west-northwest-trending fault zones which may have been the foci for ascending oxidizing and metalliferous solutions (Lisiakiewicz, 1969; Oszczepalski, 1980). Since the convection probably occurred during the Triassic, a period of extensional tectonism, and since these faults were active even during Z 1 sedimentation (Oszczepalski, 1980), it seems reasonable to assume that fracture porosity, caused by rejuvenation of fault zones, played a role in the internal migration of Rot-


liegende brines. Fault rejuvenation by crustal failure during extension is implied by the fault-controlled subsidence of the Buntsandstein (Fig. 3).

Additional buoyancy Organic matter derived from terrestrial sources (as

in the Westphalian coals and the Kupferschiefer) produces carbon dioxide, water, methane, and nitrogen during thermal metamorphism (Rohrback and Kaplan, 1978; Rohrback et al., 1984; Shanmugam, 1985). Hydrocarbons in the Rotliegende are almost completely methane, and the high nitrogen content indicates both a high temperature at the source (De- powski, 1981) and a terrestrial source (Barker, 1979). In the source rock, as hydrocarbons are produced, the pore water is likely to be saturated with CO2, and methane solubility also increases with the increase in CO2 (Bray and Foster, 1980). When the methane and CO2 are expelled into the Rotliegende, the methane can exsolve into a separate phase because of the increase in salinity (McAuliffe, 1980) from 7 percent or less in the source rock (Schmidt, 1973) to the 20 to 30 percent salinity (Bojarska et al., 1981) and because of the loss of CO• used in leaching carbonate cement (Schmidt and McDonald, 1979). This gas phase should increase the overall buoyancy and increase the velocity of the metalliferous brine. As the fluid convects up the basement high flank, the decreasing temperature and pressure will exsolve more gas (McAuliffe, 1980).

McAuliffe (1980) concluded that the principal mechanism for secondary migration of natural gas was by buoyant flow as a separate phase, because the vol- umes of water necessary for migration in solution were unreasonable. This water supply restriction is avoided, however, if convection occurs, and methane should be able to migrate effectively in solution, as well as by buoyant flow, and collect in the upper Rotliegende by exsolution as long as the convection cell was maintained. Wood and Hewett (1984) and Rabinowicz et al. (1985) suggest that convection plays a role in hydrocarbon migration.

Conclusions

Geologic evidence suggests that unicellular convection of fluids up along the flanks of basement highs and down toward the shale centers of the Rotliegende basins formed the Kupferschiefer Cu-Ag deposits in Poland and that this migration occurred in the Triassic. Heat flow modeling of a simulated rift supports this evidence and shows that, given reasonable geologic conditions, the Lubin deposits could have been formed within 5 to 10 m.y.

The difference in thermal conductivity between the basement highs and the shale basin centers created the lateral temperature gradients which initiated convection. Low velocity convection, possibly with

diagenetic cementation, probably occurred during periods of normal heat flow, but the Triassic rifting event, associated with the opening of the Tethys ocean and indicated by rapid Buntsandstein sedimentation, provided the tensional and thermal event necessary for unusually high convection velocities and the formation of the Lubin district ore deposits.

Natural gas, generated in the Carboniferous basement, probably migrated with the mineralizing fluids up the flanks of the basement highs and helped convection by providing secondary porosity and buoyancy to the fluids. Convection may also provide a way in which secondary migration of natural gases can occur in solution as well as in separate phases.

Abnormal tensional and thermal events, such as rifting, can provide the driving force for the generation of hydrocarbons, the migration of metalliferous oilfield brines, and the formation of kupferschiefer- type ore deposits.

Acknowledgments

The support of Z. Dembowski, President, Central Board of Geology of Poland, W. Ryka, Director, and A. Rydzewski of the Instytut Geologiczny, K. Dubin- ski, Lubin District Chief Geologist, and their permis- sion to undertake this and related studies are grate- fully acknowledged. As well, thanks go to T. Kowal of Lubin, H. Flak and J. Zanko of Polkowice, and A. Cholesiak and T. Klos of Konrad, for their aid in vis- iting the mines, and special thanks to C. Skowronek for discussions concerning the Konrad, Nowy Kosciol, and Lena mines. Discussions with J. Bojarska, Z. Cwierz, S. Downorowicz, I. Grotek, B. Laszcz-Fila- kowa, S. Oszczepalski, T. Peryt, W. Salski, and E. Zu- rawek in Poland and G. T. Jarvis, P.-Y. F. Robin, A. J. Naldrett, and J. B. Currie in Canada proved use- ful in the formulation of these ideas. In particular, I would like to thank R. Boutilier for providing the heat flow modeling program and follow-up advice on specific problems. I appreciate the financial support through the Natural Sciences and Engineering Re- search Council grants to A. J. Naldrett and G. W. Pearce of the Department of Geology at Toronto.

August 1, 1985; January 16, 1986

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GENESIS OF KUPFERSCHIEFER Cu-Ag DEPOSITS 18 3 7

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Genesis of Kupferschiefer

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