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The role of sedimentary and tectonic brines in the Damara Orogen, Namibia
Applied Geochemistry,Vol.
2, pp. 535-542, 1987
0883-2927/88 $3.00 + .00 Pergamon Press plc
Printed in Greal Britain
The role of sedimentary and tectonic brines in the Damara Orogen, Namibia H . - J . BEHR a n d A . SCHMIDT-MUMM Institut ftir Geologie und Dynamik der Lithosphtire, UniversittR G6ttingen, Goldschmidtstr. 3, D-3400 G6ttingen, F.R.G.
Abstract--Along the southern margin of the Upper Proterozoic Damara Orogen, Namibia, an accumulation of extraordinarily large megacrystalline quartz-dolomite bodies occur. They were emplaced in tectonically controlled positions during an early deformational phase of the Damara Orogeny, where hot nappes were thrust over a fluvial-lacustrine and evaporitic metaplaya sequence (Kamtsas and Duruchaus Formations) which was deposited on the faulted EW-trending continental margin of the Kalahari craton. Individual occurrences of the quartz-dolomite bodies often cover several hundred square meters. Characteristic for the quartz-dolomite bodies is a zoning with an outer shell of giant milky quartz crystals, some more than 15 m long along the c-axis, tightly intergrown or twinned (Brazilian twins) with a perfect cleavage parallel to the positive rhombohedral faces {1011 ) ; there is an inner shell of crystalline dolomite and a central pipe of dolomitic breccia. Based on fluid inclusion studies the formation fluids of the quartz-dolomite bodies can be related to the mobilization of interstitial fluids and to dehydration and leaching of evaporitic hydrate minerals of the metaplaya sequence. The fluids are characterized by extremely high salinities of up to 68 wt % total salt content. Minimum temperatures of formation, as determined in fluid inclusion studies, ranged from 150 to 250°C. At a later stage CO2 derived from decarbonatization reactions was dissolved in the fluids. Changes in pressure and temperature led to effervescence and the formation of a quartz stockwork in the surrounding country rock. During the main phase of the progressive Damara Orogeny, the southward advancing accretionary nappe pile of the Khomas trough drove ahead large amounts of tectonometamorphic fluids, characterized by intermediate salinity and high CO 2 contents. When these fluids met with the previously established hypersaline fluid system, large amounts of CO2 were released due to the mixing of the two fluids; if there is no mixing, each fluid then maintains its salinity and there is no CO2 degassing. The CO 2 from this mixing is now present as secondary, high-density inclusions not only in the quartz-dolomite bodies but also in the surrounding country rock. Pressure estimations indicate at least 100-600 MPa as a minimum pressure of formation for these inclusions. The remaining aqueous fluid phase has produced local alterations and Cu. Pb, and Au mineralization.
INTRODUCTION THIS paper gives a summary description of the megaquartz bodies located along the southern margin of the U p p e r Proterozoic D a m a r a Orogen, Namibia. Fluid inclusion and o t h e r data are used to infer the chemical composition and conditions of formation of these unusual geological features. The chemical characteristics of the fluid systems which were active at the southern margin of the D a m a r a O r o g e n have been outlined by BEHR et al. (1983). Two distinctly different fluid sources have been determined, that is, a high-salinity system and a low-salinity system related to the Duruchaus metaevaporites and the K h o m a s metapelites, respectively. The chemical variation within the high-salinity system was given by SCHMIDT-MUMM et al. (1987) using a native-S-bearing quartz-dolomite. The megaquartz bodies and their fluid content were first described by BEHR and HORN (1982) who recognized three generations of saline fluid inclusions in the Hakos megaquartz body with decreasing temperatures of homogenization; SCHMIDT-MUMM and BEHR (1987) also d e t e r m i n e d three phases of fluid activity that led to the formation and alteration of the mega-
quartz bodies and also affected the surrounding country rock. The formation and subsequent alteration of the quartz-dolomite bodies can be related to the various phases of the evolution of the D a m a r a Orogen.
THE MEGAQUARTZ BODIES
Megaquartz bodies are widespread along the southern margin of the D a m a r a Orogen. The largest known exposure is situated approximately 160 km southwest of W i n d h o e k , Namibia, in the Hakos Mountains (see Fig. 1). It was first described by DE WAAL (1966) and covers an area of about 0.15 km 2 at a m a x i m u m height of 80 m (Fig. 2). Single crystals can reach up to 15 m in length along the c-axis (Fig. 3), and in one case a single crystal of 50 m could be traced along its crystal faces. O t h e r occurrences of similar genesis are shown in Fig. 1. A m a j o r feature of the megaquartz bodies is a well-developed zoning (SCHMIDT-MUMM and BEI.IR, 1987) (Fig. 4). In most cases the central part is formed by a silicified dolomitic breccia pipe with clasts of crystalline dolomite, quartz and wall rock. This pipe 535
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is surrounded by a shell of crystalline dolomite, tightly intergrown with an outer shell of megacrystalline quartz. The size of the dolomite crystals can reach 20-30 cm, whereas individual quartz crystals often are several meters long and sometimes twinned (Brazilian twinning). Flakes or clusters and even large packets of talc are present in the alteration halo of the quartz bodies, resulting from dedolomitization of the above-mentioned dolomitic breccia. A later generation of needlelike quartz and some minor pyrite are commonly found associated with the talc. The dolomitic parts of the quartz-dolomite bodies underwent karstification during the Karroo Period (Carboniferous to Jurassic). Large caves of unknown extent (more than 50 m), some with stalagtites and stalagmites, formed along the contact of quartz and dolomite. Field studies show that a close relation exists between the quartz-dolomite bodies and the mobilization of fluids from the highly hydrous Duruchaus Formation. Fluid channelways and intrusive carbonate breccia with alteration haloes, accompanied by minor quartz-dolomite plugs and veins, are a wide-
spread field pattern. The relation to the evaporitic sequence is also strongly supported by fluid inclusion studies and trace element analyses of the quartz. The mean contents (ppm) of several important elements in the megaquartz from the evaporitic environment (Duruchaus Formation) and in quartz segregation from the pelitic metamorphites of the tectonically overthrust Damara nappe were determined with neutron activation analysis and are given in Table 1. In the metamorphic quartz there is a mixing with solution from the evaporitic environment (secondary fluid inclusion generation); in the evaporite related quartz there is a secondary mixing with fluids from the metamorphic sources. The right column in Table 1 shows analyses of the typical quartz mobilizations of the pelitic metamorphites without the mixing of saline solutions. The extremely high CI/Br ratio in Duruchaus quartz, which is of evaporitic origin, is particularly conspicuous. The low K/Na ratios and low K, Rb, Cs, Sc, and R E E contents also support an association with evaporites and contradict a magmatic or metamorphic origin of the fluids or indicate an intensive hydrothermal alteration of feldspar-rich
Role of sedimentary and tectonic brines, Oamara Orogen, Namibia
FIG. 2. Megaquartz body as described in text, dark central part is crystalline dolomite; exposure in Hakos Mountains (Fig. 1). FIG. 3. Large single quartz crystal on Hakos megaquartz. Note hammer for scale (approx. 30 cm).
537
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FIG. 4. Formation and alteration of the megaquartz bodies: (I) Mineralization from highly saline, evaporite-related fluid system in open cavities. Homogenization temperatures 150-200°C, locally max. 350°C; NaCI + KCI + (CaC12 + MgCl2) saturated; 65 wt % total salt content. Estimated pressure 80-120 MPa. (II) Increasing CO2 content in the fluid phase from decarbonatization reactions. Increasing mean homogenization temperature (180-230°C), NaC1 + KC1 but decreasing CaC12 + MgC12 content. Effervescence of CO2-rich phase from decreasing tectonic stress and formation of quartz stockwork. Partial talc formation in dolomitic breccia and crystalline dolomite. This is followed by tectonic and metamorphic fluids driven ahead with the progressive Damara Orogeny. Fluids are CO2-saturated and undersaturated with respect to NaC1 + KC1. Immiscibility of fluid system releases large amounts of CO 2. Estimated pressure 400-600 MPa.
Table 1. Results of neutron activation analyses of quartz from the Duruchaus Formation, Khomas Formation, and pelitic metamorphites (ppm) Megaquartz from the Duruchaus Formation
Quartz mylonites from the Khomas Formation
Quartz mobilizates from pelitic metamorphites
Na AI K Ca Mn Fe Ge As Rb Sr Sb Cs La Au U
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15.6 146 3.9 13.4 0.20 3.4 1.0 0.045 -(0.28) 0.047 9.5 • 10 3 0.011 1.6. 10 -5 4.2. 10-4
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m e t a m o r p h i c rocks. A s a result of the mixing of fluids f r o m b o t h sources in the quartz of the D u r u c h a u s and K a m t s a s F o r m a t i o n s , the differences are b a l a n c e d out for the m o s t part. FLUID INCLUSION STUDIES
Analytical methods To master the large number of samples for regional fluid inclusion studies on the quartz-dolomite bodies a decrepitometer was developed at the IGDL (Institut fiir Geologie und Dynamik der Lithosphiire, Universitiit G6ttingen). The instrument was designed for mobile use and provides high-resolution decrepitograms which reflect the statistical distribution of the fluid inclusion generations in a given sample. All samples were thus first analyzed by decrepitometry and selected samples were further analyzed by microthermometry to determine the physicochemical characteristics of the trapped fluid inclusions. Figure 5 shows the correlation of the results of decrepitometry with microthermometric data. In some cases, single phases of individual fluid inclusions were analyzed by Raman microprobe to obtain information on their chemical composition. Three generations of fluid inclusions were distinguished, which all had their effects on the formation and alteration of the quartz-dolomite bodies.
Role of sedimentary and tectonic brines, Damara Orogen, Namibia tounfs,
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FLUID GENERATIONS High-sallnity, evaporRe-related fluid inclusions (phase I} Saline CO2-bearing fluid inclusions (phase II) Low-sallnlty, high-density fluid inclusions related to tectonic and metamorphic brines (phase III)
FIG. 5. Correlation of decrepitometric data (a, b) with results from microthermometry (c). Fluid inclusion generations can be clearly distinguished by their different decrepitation temperatures. Height of peaks (counts/10°C) represents frequency distribution of fluid inclusion generations. Figure 5a shows decrepitometric patterns from undisturbed sample. In Fig. 5b, which is markedly different from Fig. 5a, primary inclusions are nearly completely absent due to natural decrepitation.
Fluid systems and geological evolution of the Damara Orogen The evolution of the fluid inclusion generations in the megaquartz has to be seen in relation to the general geological evolution of the Damara Orogen and the special setting of the quartz body. Figure 6 shows the pressure-temperature path determined with fluid inclusions• The earliest generation (I in Fig. 6) is characterized by extremely high salinity. It is present as primary inclusions in the main part of the body and is therefore regarded as being representative of the genetic fluid system. Intact inclusions of this type are rare due to secondary alteration and natural decrepitation (Fig. 7). Based on the homogenization temperatures of the always present KCI and NaCI daughter minerals the (KCI + NaCI) content of the solution was estimated to be approximately 35-38 wt %. The CaCI 2 and MgCI z ratios were estimated from the eutectic and melting temper-
atures of ice and hydrate phases (after CRAWFORDet al., 1979). They are 60-70% CaCIE and 40-30% MgC12. The total salt content is approximately 67 wt %. These values are very unusual and indicate extreme conditions. From the chemical characteristics of these primary inclusions as well as field evidence, a relation between the fluid systems and the evaporitic Duruchaus Formation is implied. This fluid generation occurs not only in the megaquartz but also, above all, in the main mineralizations of the metaevaporitic rocks (dolomite, albite, dravite, scapolite and quartz). During the initial phase of the Damara Orogeny a convective system was established, and the evaporitic units of the Duruchaus Formation were dehydrated and leached by basinal brines from the thick sedimentary sequence of the Nosib Group (BEHR et al., 1983), which was deposited on the faulted basement of the southern margin of the intracontinental branch of the Damara rift system (PORADA, 1983).
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Thc second phase of fluid activity (II in Fig. 6) is closely related to the first phase, but can be clearly distinguished by the increasing CO2 content in the fluid inclusions. The CO2 is likely derived from decarbonatization of the dolomitic parts of the Duruchaus Formation and the mobilized dolomitic breccia, where it resulted in extensive formation of talc. Another characteristic is the decreasing content of Ca -"+ + Mg 2+ as determined from the eutectic tem-
peratures (Fig. 8). The extremely low eutectic temperatures of the above-mentioned first two phases of fluid activity (in some cases as low as -78°C) indicate the presence of further dissolved species. In some inclusions SrCO 3, carbonates, anhydrite, S, and HSwere identified with Raman spectroscopy. This has to be taken into consideration when interpreting the low eutectic temperatures. Homogenization temperatures of the vapor phase indicate an increase in the
FIG. 7. Primary fluid inclusion showing effects of natural decrepitation (circles).
Role of sedimentary and tectonic brines, Damara Orogen, Namibia
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temperatures of fluid inclusion formation to 250°C. Estimated formation pressure is approximately 80120 MPa. The very variable gas-liquid ratios indicate true effervescence of the fluids (separation of a CO2rich phase) (RoEODER, 1984). This led to the formation of a quartz stockwork in the surrounding country rock. Effervescence of the fluid content in the thrust planes would also have further enhanced nappe movement. Thus, the Naukluft Nappe Complex was transported at least 80 km (MARTIN et al., 1983). The source of the third fluid system present in the megaquartz bodies (III in Fig. 6) is related to the main phase of the progressive Damara Orogeny. Large quantities of metamorphic and tectonogenic fluids were driven ahead with the accretionary nappe pile of the metasedimentary Khomas units approaching from the northwest. The fluids are characterized by moderate salinity, mostly undersaturated with respect to NaC1 and KC1, but CO2-saturated. The previously established high-salinity, evaporiterelated fluid system acted as a chemical barrier to these fluids. Some mixing of the two fluids resulted in decreased solubility of CO2 due to the high contents of dissolved salts, and a CO2-rich phase separated. High-density CO2 inclusions in the megaquartz bodies and the country rock give evidence of this reaction. Pressure estimations from these inclusions indicate at least 500-600 MPa as the minimum formation pressure, corresponding to the thick pile of overriding nappes. The remaining aqueous phase further advanced at the orogenic front and, because of the varied physiocochemical properties, was now capable of dissolving and transporting other ele-
ments. In the foreland of the southern margin of the Damara Orogen there are numerous minor occurrences of Cu and Pb sulfides occasionally associated with considerable amounts of A u and a number of hydrothermal Au-vein deposits. These deposits were probably formed from these residual fluids and are present in late shear zones in the pre-Damara basement.
CONCLUSIONS A thick sequence of metaplaya evaporites and fluvial to lacustrine sediments was deposited during the rifting stage of the Upper Proterozoic pan-African belt system along the southern margin of the intracontinental branch of the Damara Orogen (MAR~N and PORADA, 1977; PORADA, 1983, 1985; L6FFLER and PORADA,in press). During this orogeny alpinotype deformation of the intracontinental branch of the Damara Orogen occurred (MILLER, 1983; PrURR et al., 1987). The high fluid content of this sequence played a major role during the subsequent tectogenesis of the Damara Orogen. During the initial phase of deformation the fluids and later even a highly mobile dolomitic mush were mobilized. This mush intruded into thrust planes, where it acted as a lubricant for the overriding nappes. Chemical reactions as well as temperature and pressure variation resulted in boiling and degassing of the fluids and thus a decrease in the effective normal stress which further enhanced nappe movement. Consequently, stress was also released in the nappe units
542
H.4. Behr and A. Schmidt-Mumm
a n d fractures o p e n e d into which the fluids f u r t h e r a d v a n c e d . T h e large m e g a q u a r t z crystals were precipitated f r o m t h e s e fluids in tectonically c o n t r o l l e d low-pressure situations (pressure s h a d o w s of c o m p e t e n t units, fold hinges). This shows h o w a special s e d i m e n t a r y situation n o t only h a d influence on, b u t even directed the entire geological e v o l u t i o n of the southern Damara Orogen.
Acknowledgements--We are very thankful to the Geological Survey of South West Africa/Namibia who supported our work on the Damara Orogen through the project "Fluid Systems associated with the Duruchaus Formation". Our thanks are also due to Ralph Phillips and Dr E. E. Horn for carefully revising the manuscript.
REFERENCES
BEHR H.-J. and HORN E. E. (1982) Fluid inclusion systems in metaplaya deposits and their relationships to mineralization and tectonics. Chem. Geol. 37,173-189. BEHR H.-J., HORN E. E. and PORADA H. (1983) Fluid inclusions and genentic aspects of the Damara Orogen. In lntracontinental Fold Belts (ed. H. MARTIN and F. W. EDER) pp. 611~o54. Springer. CRAWEORDM. L., KRAUSD. W. and HOLLISTERL. S. (1979) Petrologic and fluid inclusion study of calc-silicate rocks, Prince Rupert, British Columbia. Am. J. Sci. 9, 11351159. DE WAALS. A. (1966)The Alberta Complex, a metamorphosed layered intrusion, north of Nauchas, S.W.A. Unpubl. Ph.D. thesis, Faculty of Science, University of Pretoria, Pretoria, S.A. LOFFLER TH. and PORADA H. (1987) Fazielle Untersuchungen an Metapsammiten der Duruchaus Formation ("Geelkop Dome", siidliches Damara Orogen, Namibia/ South West Africa). Comm. Geol. Surv. S.W.A./ Namibia, 3, in press.
MARTIN H. and PORADAH. (1977) The intracratonic branch of the Damara Orogen in South West Africa. I. Discussion of geodynamic models. II. Discussion of relationships with the Pan-African Mobile Belt System. Precamb. Res. 5,311-338 and 339-357. MARTINH., PORADAH. and WlrrIG R. (1983) Where lies the root zone of the Naukluft Nappe Complex? Spec. Publ. Geol. Soc. S. Africa 11, pp. 185-199. MILLER R. McG. (1983) The Pan-African Damara Orogen of South West Africa/Namibia. Spec. Publ. Geol. Soc. S. Africa 11, pp. 431-515. PFURR N., WISSMAN K., AHRENDT H., HILL R. S. and WEBER K. (1987) Alpinotype thrust tectonics and basement-cover relationships in the southern margine zone of the Pan-African Damara Orogen, Rostock area. Comm. Geol. Surv. S.W. Africa/Namibia 3, in press. PORADA H. (1983) Geodynamic model for the geosynclinal development of the Damara Orogen, Namibia, South West Africa. In Intracontinental Fold Belts (ed. H. MARTIN and F. W. EDER) pp. 503--542. Springer. PORADA H. (1985) Stratigraphy and facies in the upper Proterozoic Damara Orogen, Namibia, based on a geodynamic model. Precamb. Res. 29,235-264. ROEDDER E. (1984) Interpretation and utilization of inclusion measurements: temperature, pressure and density at trapping. In Reviews in Mineralogy 12, Fluid Inclusions (series ed. P. R. RmBE) pp. 251--291. Mineral. Soc. Amer., Washington. ROEDDER E. and BODNAR R. J. (1980) Geologic pressure determinations from fluid inclusion studies. Ann. Rev. Earth Planet. Sci. 8,263-302. SCHMIDT-MUMM A., BEHR H.-J. and HORN E. E. (1987) Fluid systems in metaplaya sequences in the Damara Orogen (Namibia): evidence for sulfur-rich brines--general evolution and first results. Chem. Geol. 61,135-145. SCHMIDT-MUMM A. and BLUR H.-J. (1987) Fluid inclusion studies on hydrothermal mineralizations associated with the Duruchaus Formation--genetic aspects of large quartz-dolomite bodies and associated alteration. Comm. Geol. Surv. S.W.Africa/Namibia, in press. South West Africa/Namibia Geological Map (1980) 1:1,000,000. Geol. Surv. Rep. South Africa and South West Africa/Namibia.
2, pp. 535-542, 1987
0883-2927/88 $3.00 + .00 Pergamon Press plc
Printed in Greal Britain
The role of sedimentary and tectonic brines in the Damara Orogen, Namibia H . - J . BEHR a n d A . SCHMIDT-MUMM Institut ftir Geologie und Dynamik der Lithosphtire, UniversittR G6ttingen, Goldschmidtstr. 3, D-3400 G6ttingen, F.R.G.
Abstract--Along the southern margin of the Upper Proterozoic Damara Orogen, Namibia, an accumulation of extraordinarily large megacrystalline quartz-dolomite bodies occur. They were emplaced in tectonically controlled positions during an early deformational phase of the Damara Orogeny, where hot nappes were thrust over a fluvial-lacustrine and evaporitic metaplaya sequence (Kamtsas and Duruchaus Formations) which was deposited on the faulted EW-trending continental margin of the Kalahari craton. Individual occurrences of the quartz-dolomite bodies often cover several hundred square meters. Characteristic for the quartz-dolomite bodies is a zoning with an outer shell of giant milky quartz crystals, some more than 15 m long along the c-axis, tightly intergrown or twinned (Brazilian twins) with a perfect cleavage parallel to the positive rhombohedral faces {1011 ) ; there is an inner shell of crystalline dolomite and a central pipe of dolomitic breccia. Based on fluid inclusion studies the formation fluids of the quartz-dolomite bodies can be related to the mobilization of interstitial fluids and to dehydration and leaching of evaporitic hydrate minerals of the metaplaya sequence. The fluids are characterized by extremely high salinities of up to 68 wt % total salt content. Minimum temperatures of formation, as determined in fluid inclusion studies, ranged from 150 to 250°C. At a later stage CO2 derived from decarbonatization reactions was dissolved in the fluids. Changes in pressure and temperature led to effervescence and the formation of a quartz stockwork in the surrounding country rock. During the main phase of the progressive Damara Orogeny, the southward advancing accretionary nappe pile of the Khomas trough drove ahead large amounts of tectonometamorphic fluids, characterized by intermediate salinity and high CO 2 contents. When these fluids met with the previously established hypersaline fluid system, large amounts of CO2 were released due to the mixing of the two fluids; if there is no mixing, each fluid then maintains its salinity and there is no CO2 degassing. The CO 2 from this mixing is now present as secondary, high-density inclusions not only in the quartz-dolomite bodies but also in the surrounding country rock. Pressure estimations indicate at least 100-600 MPa as a minimum pressure of formation for these inclusions. The remaining aqueous fluid phase has produced local alterations and Cu. Pb, and Au mineralization.
INTRODUCTION THIS paper gives a summary description of the megaquartz bodies located along the southern margin of the U p p e r Proterozoic D a m a r a Orogen, Namibia. Fluid inclusion and o t h e r data are used to infer the chemical composition and conditions of formation of these unusual geological features. The chemical characteristics of the fluid systems which were active at the southern margin of the D a m a r a O r o g e n have been outlined by BEHR et al. (1983). Two distinctly different fluid sources have been determined, that is, a high-salinity system and a low-salinity system related to the Duruchaus metaevaporites and the K h o m a s metapelites, respectively. The chemical variation within the high-salinity system was given by SCHMIDT-MUMM et al. (1987) using a native-S-bearing quartz-dolomite. The megaquartz bodies and their fluid content were first described by BEHR and HORN (1982) who recognized three generations of saline fluid inclusions in the Hakos megaquartz body with decreasing temperatures of homogenization; SCHMIDT-MUMM and BEHR (1987) also d e t e r m i n e d three phases of fluid activity that led to the formation and alteration of the mega-
quartz bodies and also affected the surrounding country rock. The formation and subsequent alteration of the quartz-dolomite bodies can be related to the various phases of the evolution of the D a m a r a Orogen.
THE MEGAQUARTZ BODIES
Megaquartz bodies are widespread along the southern margin of the D a m a r a Orogen. The largest known exposure is situated approximately 160 km southwest of W i n d h o e k , Namibia, in the Hakos Mountains (see Fig. 1). It was first described by DE WAAL (1966) and covers an area of about 0.15 km 2 at a m a x i m u m height of 80 m (Fig. 2). Single crystals can reach up to 15 m in length along the c-axis (Fig. 3), and in one case a single crystal of 50 m could be traced along its crystal faces. O t h e r occurrences of similar genesis are shown in Fig. 1. A m a j o r feature of the megaquartz bodies is a well-developed zoning (SCHMIDT-MUMM and BEI.IR, 1987) (Fig. 4). In most cases the central part is formed by a silicified dolomitic breccia pipe with clasts of crystalline dolomite, quartz and wall rock. This pipe 535
536
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FIG. 1. Regional geology of South West Africa/Namibia. Exposures of the largest megaquartz bodies are marked by @ (modified after South West Africa/Namibia Geological Map, 1980).
is surrounded by a shell of crystalline dolomite, tightly intergrown with an outer shell of megacrystalline quartz. The size of the dolomite crystals can reach 20-30 cm, whereas individual quartz crystals often are several meters long and sometimes twinned (Brazilian twinning). Flakes or clusters and even large packets of talc are present in the alteration halo of the quartz bodies, resulting from dedolomitization of the above-mentioned dolomitic breccia. A later generation of needlelike quartz and some minor pyrite are commonly found associated with the talc. The dolomitic parts of the quartz-dolomite bodies underwent karstification during the Karroo Period (Carboniferous to Jurassic). Large caves of unknown extent (more than 50 m), some with stalagtites and stalagmites, formed along the contact of quartz and dolomite. Field studies show that a close relation exists between the quartz-dolomite bodies and the mobilization of fluids from the highly hydrous Duruchaus Formation. Fluid channelways and intrusive carbonate breccia with alteration haloes, accompanied by minor quartz-dolomite plugs and veins, are a wide-
spread field pattern. The relation to the evaporitic sequence is also strongly supported by fluid inclusion studies and trace element analyses of the quartz. The mean contents (ppm) of several important elements in the megaquartz from the evaporitic environment (Duruchaus Formation) and in quartz segregation from the pelitic metamorphites of the tectonically overthrust Damara nappe were determined with neutron activation analysis and are given in Table 1. In the metamorphic quartz there is a mixing with solution from the evaporitic environment (secondary fluid inclusion generation); in the evaporite related quartz there is a secondary mixing with fluids from the metamorphic sources. The right column in Table 1 shows analyses of the typical quartz mobilizations of the pelitic metamorphites without the mixing of saline solutions. The extremely high CI/Br ratio in Duruchaus quartz, which is of evaporitic origin, is particularly conspicuous. The low K/Na ratios and low K, Rb, Cs, Sc, and R E E contents also support an association with evaporites and contradict a magmatic or metamorphic origin of the fluids or indicate an intensive hydrothermal alteration of feldspar-rich
Role of sedimentary and tectonic brines, Oamara Orogen, Namibia
FIG. 2. Megaquartz body as described in text, dark central part is crystalline dolomite; exposure in Hakos Mountains (Fig. 1). FIG. 3. Large single quartz crystal on Hakos megaquartz. Note hammer for scale (approx. 30 cm).
537
538
H.-J. Behr and A. Schmidt-Mumm
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FIG. 4. Formation and alteration of the megaquartz bodies: (I) Mineralization from highly saline, evaporite-related fluid system in open cavities. Homogenization temperatures 150-200°C, locally max. 350°C; NaCI + KCI + (CaC12 + MgCl2) saturated; 65 wt % total salt content. Estimated pressure 80-120 MPa. (II) Increasing CO2 content in the fluid phase from decarbonatization reactions. Increasing mean homogenization temperature (180-230°C), NaC1 + KC1 but decreasing CaC12 + MgC12 content. Effervescence of CO2-rich phase from decreasing tectonic stress and formation of quartz stockwork. Partial talc formation in dolomitic breccia and crystalline dolomite. This is followed by tectonic and metamorphic fluids driven ahead with the progressive Damara Orogeny. Fluids are CO2-saturated and undersaturated with respect to NaC1 + KC1. Immiscibility of fluid system releases large amounts of CO 2. Estimated pressure 400-600 MPa.
Table 1. Results of neutron activation analyses of quartz from the Duruchaus Formation, Khomas Formation, and pelitic metamorphites (ppm) Megaquartz from the Duruchaus Formation
Quartz mylonites from the Khomas Formation
Quartz mobilizates from pelitic metamorphites
Na AI K Ca Mn Fe Ge As Rb Sr Sb Cs La Au U
18.7 177 4.8 16.1 0.020 2.4 2.2 0.057 0.0081 0.12 0.043 7.5 • 10 3 4.1 - 10 4 1.1 • 10-5 1.1 • 10 3
15.6 146 3.9 13.4 0.20 3.4 1.0 0.045 -(0.28) 0.047 9.5 • 10 3 0.011 1.6. 10 -5 4.2. 10-4
27.7 318 25.4 12.5 0.33 10.4 1.2 0.040 0.12 0.37 0.047 0.045 2.8- 10 .3 2.2- 10.5 5.0. 10 .4
CI/Br
731
313
128
m e t a m o r p h i c rocks. A s a result of the mixing of fluids f r o m b o t h sources in the quartz of the D u r u c h a u s and K a m t s a s F o r m a t i o n s , the differences are b a l a n c e d out for the m o s t part. FLUID INCLUSION STUDIES
Analytical methods To master the large number of samples for regional fluid inclusion studies on the quartz-dolomite bodies a decrepitometer was developed at the IGDL (Institut fiir Geologie und Dynamik der Lithosphiire, Universitiit G6ttingen). The instrument was designed for mobile use and provides high-resolution decrepitograms which reflect the statistical distribution of the fluid inclusion generations in a given sample. All samples were thus first analyzed by decrepitometry and selected samples were further analyzed by microthermometry to determine the physicochemical characteristics of the trapped fluid inclusions. Figure 5 shows the correlation of the results of decrepitometry with microthermometric data. In some cases, single phases of individual fluid inclusions were analyzed by Raman microprobe to obtain information on their chemical composition. Three generations of fluid inclusions were distinguished, which all had their effects on the formation and alteration of the quartz-dolomite bodies.
Role of sedimentary and tectonic brines, Damara Orogen, Namibia tounfs,
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FLUID GENERATIONS High-sallnity, evaporRe-related fluid inclusions (phase I} Saline CO2-bearing fluid inclusions (phase II) Low-sallnlty, high-density fluid inclusions related to tectonic and metamorphic brines (phase III)
FIG. 5. Correlation of decrepitometric data (a, b) with results from microthermometry (c). Fluid inclusion generations can be clearly distinguished by their different decrepitation temperatures. Height of peaks (counts/10°C) represents frequency distribution of fluid inclusion generations. Figure 5a shows decrepitometric patterns from undisturbed sample. In Fig. 5b, which is markedly different from Fig. 5a, primary inclusions are nearly completely absent due to natural decrepitation.
Fluid systems and geological evolution of the Damara Orogen The evolution of the fluid inclusion generations in the megaquartz has to be seen in relation to the general geological evolution of the Damara Orogen and the special setting of the quartz body. Figure 6 shows the pressure-temperature path determined with fluid inclusions• The earliest generation (I in Fig. 6) is characterized by extremely high salinity. It is present as primary inclusions in the main part of the body and is therefore regarded as being representative of the genetic fluid system. Intact inclusions of this type are rare due to secondary alteration and natural decrepitation (Fig. 7). Based on the homogenization temperatures of the always present KCI and NaCI daughter minerals the (KCI + NaCI) content of the solution was estimated to be approximately 35-38 wt %. The CaCI 2 and MgCI z ratios were estimated from the eutectic and melting temper-
atures of ice and hydrate phases (after CRAWFORDet al., 1979). They are 60-70% CaCIE and 40-30% MgC12. The total salt content is approximately 67 wt %. These values are very unusual and indicate extreme conditions. From the chemical characteristics of these primary inclusions as well as field evidence, a relation between the fluid systems and the evaporitic Duruchaus Formation is implied. This fluid generation occurs not only in the megaquartz but also, above all, in the main mineralizations of the metaevaporitic rocks (dolomite, albite, dravite, scapolite and quartz). During the initial phase of the Damara Orogeny a convective system was established, and the evaporitic units of the Duruchaus Formation were dehydrated and leached by basinal brines from the thick sedimentary sequence of the Nosib Group (BEHR et al., 1983), which was deposited on the faulted basement of the southern margin of the intracontinental branch of the Damara rift system (PORADA, 1983).
H.-J. Behr and A. Schmidt-Mumm
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in g/cm 3
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400
600
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FIG. 6. Pressure-temperature path of the megaquartz bodies and surrounding country rock as determined in fluid inclusion studies. Isochores are taken from ROEDDERand BODNAR(1980). Fluid generations as in Fig. 5.
Thc second phase of fluid activity (II in Fig. 6) is closely related to the first phase, but can be clearly distinguished by the increasing CO2 content in the fluid inclusions. The CO2 is likely derived from decarbonatization of the dolomitic parts of the Duruchaus Formation and the mobilized dolomitic breccia, where it resulted in extensive formation of talc. Another characteristic is the decreasing content of Ca -"+ + Mg 2+ as determined from the eutectic tem-
peratures (Fig. 8). The extremely low eutectic temperatures of the above-mentioned first two phases of fluid activity (in some cases as low as -78°C) indicate the presence of further dissolved species. In some inclusions SrCO 3, carbonates, anhydrite, S, and HSwere identified with Raman spectroscopy. This has to be taken into consideration when interpreting the low eutectic temperatures. Homogenization temperatures of the vapor phase indicate an increase in the
FIG. 7. Primary fluid inclusion showing effects of natural decrepitation (circles).
Role of sedimentary and tectonic brines, Damara Orogen, Namibia
541
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temperatures of fluid inclusion formation to 250°C. Estimated formation pressure is approximately 80120 MPa. The very variable gas-liquid ratios indicate true effervescence of the fluids (separation of a CO2rich phase) (RoEODER, 1984). This led to the formation of a quartz stockwork in the surrounding country rock. Effervescence of the fluid content in the thrust planes would also have further enhanced nappe movement. Thus, the Naukluft Nappe Complex was transported at least 80 km (MARTIN et al., 1983). The source of the third fluid system present in the megaquartz bodies (III in Fig. 6) is related to the main phase of the progressive Damara Orogeny. Large quantities of metamorphic and tectonogenic fluids were driven ahead with the accretionary nappe pile of the metasedimentary Khomas units approaching from the northwest. The fluids are characterized by moderate salinity, mostly undersaturated with respect to NaC1 and KC1, but CO2-saturated. The previously established high-salinity, evaporiterelated fluid system acted as a chemical barrier to these fluids. Some mixing of the two fluids resulted in decreased solubility of CO2 due to the high contents of dissolved salts, and a CO2-rich phase separated. High-density CO2 inclusions in the megaquartz bodies and the country rock give evidence of this reaction. Pressure estimations from these inclusions indicate at least 500-600 MPa as the minimum formation pressure, corresponding to the thick pile of overriding nappes. The remaining aqueous phase further advanced at the orogenic front and, because of the varied physiocochemical properties, was now capable of dissolving and transporting other ele-
ments. In the foreland of the southern margin of the Damara Orogen there are numerous minor occurrences of Cu and Pb sulfides occasionally associated with considerable amounts of A u and a number of hydrothermal Au-vein deposits. These deposits were probably formed from these residual fluids and are present in late shear zones in the pre-Damara basement.
CONCLUSIONS A thick sequence of metaplaya evaporites and fluvial to lacustrine sediments was deposited during the rifting stage of the Upper Proterozoic pan-African belt system along the southern margin of the intracontinental branch of the Damara Orogen (MAR~N and PORADA, 1977; PORADA, 1983, 1985; L6FFLER and PORADA,in press). During this orogeny alpinotype deformation of the intracontinental branch of the Damara Orogen occurred (MILLER, 1983; PrURR et al., 1987). The high fluid content of this sequence played a major role during the subsequent tectogenesis of the Damara Orogen. During the initial phase of deformation the fluids and later even a highly mobile dolomitic mush were mobilized. This mush intruded into thrust planes, where it acted as a lubricant for the overriding nappes. Chemical reactions as well as temperature and pressure variation resulted in boiling and degassing of the fluids and thus a decrease in the effective normal stress which further enhanced nappe movement. Consequently, stress was also released in the nappe units
542
H.4. Behr and A. Schmidt-Mumm
a n d fractures o p e n e d into which the fluids f u r t h e r a d v a n c e d . T h e large m e g a q u a r t z crystals were precipitated f r o m t h e s e fluids in tectonically c o n t r o l l e d low-pressure situations (pressure s h a d o w s of c o m p e t e n t units, fold hinges). This shows h o w a special s e d i m e n t a r y situation n o t only h a d influence on, b u t even directed the entire geological e v o l u t i o n of the southern Damara Orogen.
Acknowledgements--We are very thankful to the Geological Survey of South West Africa/Namibia who supported our work on the Damara Orogen through the project "Fluid Systems associated with the Duruchaus Formation". Our thanks are also due to Ralph Phillips and Dr E. E. Horn for carefully revising the manuscript.
REFERENCES
BEHR H.-J. and HORN E. E. (1982) Fluid inclusion systems in metaplaya deposits and their relationships to mineralization and tectonics. Chem. Geol. 37,173-189. BEHR H.-J., HORN E. E. and PORADA H. (1983) Fluid inclusions and genentic aspects of the Damara Orogen. In lntracontinental Fold Belts (ed. H. MARTIN and F. W. EDER) pp. 611~o54. Springer. CRAWEORDM. L., KRAUSD. W. and HOLLISTERL. S. (1979) Petrologic and fluid inclusion study of calc-silicate rocks, Prince Rupert, British Columbia. Am. J. Sci. 9, 11351159. DE WAALS. A. (1966)The Alberta Complex, a metamorphosed layered intrusion, north of Nauchas, S.W.A. Unpubl. Ph.D. thesis, Faculty of Science, University of Pretoria, Pretoria, S.A. LOFFLER TH. and PORADA H. (1987) Fazielle Untersuchungen an Metapsammiten der Duruchaus Formation ("Geelkop Dome", siidliches Damara Orogen, Namibia/ South West Africa). Comm. Geol. Surv. S.W.A./ Namibia, 3, in press.
MARTIN H. and PORADAH. (1977) The intracratonic branch of the Damara Orogen in South West Africa. I. Discussion of geodynamic models. II. Discussion of relationships with the Pan-African Mobile Belt System. Precamb. Res. 5,311-338 and 339-357. MARTINH., PORADAH. and WlrrIG R. (1983) Where lies the root zone of the Naukluft Nappe Complex? Spec. Publ. Geol. Soc. S. Africa 11, pp. 185-199. MILLER R. McG. (1983) The Pan-African Damara Orogen of South West Africa/Namibia. Spec. Publ. Geol. Soc. S. Africa 11, pp. 431-515. PFURR N., WISSMAN K., AHRENDT H., HILL R. S. and WEBER K. (1987) Alpinotype thrust tectonics and basement-cover relationships in the southern margine zone of the Pan-African Damara Orogen, Rostock area. Comm. Geol. Surv. S.W. Africa/Namibia 3, in press. PORADA H. (1983) Geodynamic model for the geosynclinal development of the Damara Orogen, Namibia, South West Africa. In Intracontinental Fold Belts (ed. H. MARTIN and F. W. EDER) pp. 503--542. Springer. PORADA H. (1985) Stratigraphy and facies in the upper Proterozoic Damara Orogen, Namibia, based on a geodynamic model. Precamb. Res. 29,235-264. ROEDDER E. (1984) Interpretation and utilization of inclusion measurements: temperature, pressure and density at trapping. In Reviews in Mineralogy 12, Fluid Inclusions (series ed. P. R. RmBE) pp. 251--291. Mineral. Soc. Amer., Washington. ROEDDER E. and BODNAR R. J. (1980) Geologic pressure determinations from fluid inclusion studies. Ann. Rev. Earth Planet. Sci. 8,263-302. SCHMIDT-MUMM A., BEHR H.-J. and HORN E. E. (1987) Fluid systems in metaplaya sequences in the Damara Orogen (Namibia): evidence for sulfur-rich brines--general evolution and first results. Chem. Geol. 61,135-145. SCHMIDT-MUMM A. and BLUR H.-J. (1987) Fluid inclusion studies on hydrothermal mineralizations associated with the Duruchaus Formation--genetic aspects of large quartz-dolomite bodies and associated alteration. Comm. Geol. Surv. S.W.Africa/Namibia, in press. South West Africa/Namibia Geological Map (1980) 1:1,000,000. Geol. Surv. Rep. South Africa and South West Africa/Namibia.