The role of fluids in partitioning brittle deformation and ductile creep in auriferous shear zones between 500 and 700 °C

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Tectonophysics 446 (2008) 1 – 15 www.elsevier.com/locate/tecto

Review article

The role of fluids in partitioning brittle deformation and ductile creep in auriferous shear zones between 500 and 700 °C Jochen Kolb ⁎,1 Institute for Mineralogy and Economic Geology, RWTH Aachen University, D-52056 Aachen, Germany Received 30 April 2007; received in revised form 1 October 2007; accepted 2 October 2007 Available online 10 October 2007

Abstract The fabric, mineralogy, geochemistry, and stable isotope systematics of auriferous shear zones in various hydrothermal gold deposits were studied in order to discuss the role of fluids in rock deformation at temperatures between 500 °C and 700 °C. The strong hydrothermal alteration and gold mineralization indicates that effective permeability development goes ahead with high-temperature rock deformation. The economic gold enrichment is often hosted by breccias and quartz veins in the ductile shear zones, which either formed at fast strain rates or by low strain continuous deformation at slow strain rates. Both processes require (1) a close-to lithostatic to supralithostatic fluid pressure and/or (2) a strong rheology contrast of the deformed lithologies that is often developed during progressive hydrothermal alteration. Compartments of high fluid pressure are sealed from the rest of the shear zones by high-temperature deformation mechanisms, e.g. intracrystalline plasticity and diffusion creep, and compaction. In contrast, in mylonites with heterogeneous crystal plastic and brittle deformation mechanisms for the various minerals, an interconnected network of a grain-scale porosity forms an effective fluid conduit, which hampers fluid pressure build-up and the formation of veins. The auriferous shear zones of the various gold mines represent fluid conduits in the deeper crust, 100 m along strike and up to 1000 m downdip. The hydrothermal fluids infiltrated may be responsible for low magnitude earthquakes in the Earth's lower crust, which otherwise deforms viscously. © 2007 Elsevier B.V. All rights reserved. Keywords: Hydrothermal fluids; Shear zone; Seismicity; Rheology; Gold mineralization

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Introduction . . . . . . . . . . . . . . . . . . . . . . . Auriferous shear zones in gneisses of the Renco mine . 2.1. Quartz-feldspar-biotite-hornblende mylonites . . 2.2. Lensoid breccia zones . . . . . . . . . . . . . . Auriferous shear zones in amphibolites and gneisses. . 3.1. Mylonites . . . . . . . . . . . . . . . . . . . . 3.2. Gold-quartz veins . . . . . . . . . . . . . . . . Auriferous flexural-slip folds in calcsilicate rocks . . . 4.1. Massive sulfide lenses . . . . . . . . . . . . . . 4.2. Gold-quartz veins . . . . . . . . . . . . . . . . Discussion . . . . . . . . . . . . . . . . . . . . . . . 5.1. Porosity controlling mechanisms . . . . . . . . 5.2. Seismic or slow strain rate brittle deformation .

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⁎ Tel.: +45 38142212; fax: +45 38142050. E-mail address: [email protected]. 1 Present address: Department of Economic Geology, Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. 0040-1951/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2007.10.001

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6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Experimental results and field studies of exhumed shear zones indicate that permeability enhancement to values between 10− 6 and 10− 5 and concomitant fluid flow is commonly associated with macroscopically continuous deformation (Knipe and McCaig, 1994; Géraud et al., 1995; McCaig, 1997; Mancktelow et al., 1998; Bauer et al., 2000; Zhang et al., 2001; Streit and Cox, 2002; Kolb et al., 2004). Fluid flow in hydrothermally mineralized faults and shear zones is typically associated with brittle deformation in a regime of transiently changing fluid pressure associated with an earthquake cycle close to the brittle-ductile transition (Cox et al., 1986; Boullier and Robert, 1992; Sibson, 1994; Kolb et al., 2004). The brittleductile transition is suggested to be located at N 12 km depth and temperatures N 300 °C–450 °C, depending on a quartz-rich or feldspar-rich composition of the crust (Sibson, 1984; Scholz, 1988; Ito, 1999). The Earth's lower crust is often aseismic at depths greater than 15–20 km due to elevated temperatures (Meissner and Strehlau, 1982; Chen and Molnar, 1983; Maggi et al., 2000). This is in agreement with the widely accepted lithospheric strength model, namely the jelly sandwich or pine tree model, of a weak, viscous lower crust between an underlying stronger

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upper mantle and an overlying brittle upper crust. However, lower crustal micro-earthquakes (M b 4) to a depth of 32 km are recorded for various crustal areas and were used to question the established lithospheric strength model (Maggi et al., 2000; Jackson, 2002; Aldersons et al., 2003). In contrast to that, Handy and Brun (2004) argue that lower crustal seismicity reflects transient instabilities of shear zones and cannot be used as a monitor of lithospheric strength. Here, I compare data from the Renco (Zimbabwe), the Navachab (Namibia), the Hutti (India), and the Kochkar (Russia) gold mines about the distribution of alteration and ore minerals with respect to recrystallization structures from auriferous shear zones and, additionally, flexural-slip folds, all of which formed within a temperature range of 500 °C–700 °C (Table 1). The shear zone textures and the geochemical data suggest that hydrothermal fluid flow through these shear zones controlled the formation of cracks in breccias and veins, and most probably seismicity by elevated fluid pressures, the viscosity contrast, and the degree of reactivity of the lithology. 2. Auriferous shear zones in gneisses of the Renco mine The auriferous shear zones of the Renco mine form part of a thrust zone, which juxtaposes the Limpopo Belt against the

Table 1 Compilation of P–T data for the various locations Petrology Renco

Mylonite N550 °C

Breccia

Method

Isotopes Method

Paragenesis, recrystallization, Qtz texture Grt-Bt thermometry

667– 691 °C

Oxygen isotopes ΔQtz-Pl, ΔQtz-Bt, ΔPl-Bt

Consistent isochors

Au with Bt (b2 g/t)

1, 2, 3

648– 709 °C 530 °C + 20 / − 30 °C –

Oxygen isotopes ΔPl-Ilm, ΔWhr-Grt, ΔQtz-Ilm Oxygen, hydrogen isotopes of various minerals and whole rock –

Consistent isochors –

Au inclusions in Grt (about 20 g/t) Disseminated b6 g/t

1, 2, 3, 4

Free visible Au

8, 9

–

–

510–590 °C; N400 °C, 3–5 kbar –

Disseminated b3 g/t

10

–

–

Free visible Au

10, 11

–

–

310–550 °C, 1–3 kbar –

Barren

12

–

–

–

Free visible Au

13

–

–

N450 °C

Free visible Au

14

580–650 °C, 4 ± 1 kbar HuttiMylonite 575 °C ± 40 °C, Hbl-Pl thermometry; Maski 2 ± 1 kbar; T-X pseudosections greenstone 460–510 °C, 3 kbar belt Quartz N450 °C Paragenesis, vein recrystallization Kochkar

Navachab

Mylonite N550 °C; 500 °C ± 20 °C, 3–5 kbar Quartz N450 °C vein Mylonite 550–650 °C, 3 ± 1 kbar Breccia 570–590 °C, 2–2.5 kbar Quartz N450 °C vein

Paragenesis, recrystallization; Bt-Tm thermometry Paragenesis, recrystallization Geothermobarometry (various) Sph-Apy geothermobarometry Paragenesis, recrystallization

Fluid inclusions Gold mineralization Reference

5, 6, 7, 8

1 (Kolb et al., 2000), 2 (Kolb et al., 2003), 3 (Kolb and Meyer, 2002), 4 (Blenkinsop and Rollinson, 1992), 5 (Kolb et al., 2005a), 6 (Kolb and Meyer, in press), 7 (Rogers, 2004), 8 (Hellmann et al., 2005), 9 (Mishra et al., 2005), 10 (Kolb et al., 2005b), 11 (Ertl unpubl. data), 12 (Puhan, 1983), 13 (Nörtemann et al., 2000), 14 (Wulff unpubl. data).

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Zimbabwe Craton (Fig. 1). The individual shear zones are 1– 3 m wide and have a tabular slightly undulating geometry. They form a conjugate set of shallow S-dipping thrusts and steep Ndipping antithetic shear zones interpreted as a R2-Riedel geometry (Kisters et al., 2000a). They formed at 600 °C– 700 °C during retrograde shearing of granulite facies tonalitic gneisses (Table 1, Kolb et al., 2000). The shear zones are characterized by quartz-feldspar-biotite-hornblende mylonites and locally developed lensoid breccia zones (lithons of Kisters et al., 2000a) of 1–50 cm width and 1–50 m along strike and down-dip (Fig. 2), which host the bulk of the hydrothermal gold mineralization. 2.1. Quartz-feldspar-biotite-hornblende mylonites The quartz-feldspar-biotite-hornblende mylonites show numerous features of mineral deformation typical of upperamphibolite facies mylonites (Fig. 3a–c). Quartz- and feldspar-rich zones alternate in about 1–2 mm wide ribbons. Large, 1–2 mm quartz grains are recrystallized by subgrain rotation and grain boundary migration to grains 0.15–0.25 mm in diameter. Hornblende (b 1 mm) occurs in a fish-texture, is undulose and, locally, has subgrains. Biotite (0.2–0.5 mm) is well aligned parallel to the mylonitic foliation and also shows the fish-texture. The well-developed crystallographic preferred orientation of quartz is consistent with deformation in the amphibolite facies (Kolb et al., 2003). The plagioclase and K-feldspar grains have a core-and-mantle texture with 0.1–0.2 mm recrystallized grains,

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which formed by subgrain rotation. This texture and the formation of myrmecitic quartz-feldspar grains are characteristic features of high-temperature deformation (Simpson, 1985; Simpson and Wintsch, 1989). Lobate quartz-feldspar grain boundaries point to diffusion creep during the deformation (Tullis and Yund, 1985). The mylonites show a localized precipitation of pyrrhotite, chalcopyrite, ilmenite, and traces of gold along (a) narrow, foliation-parallel bands and (b) in pressure shadows of feldspar porphyroclasts associated with recrystallized silicate minerals (Fig. 3b). This suggests that a hydrothermal fluid migrated through the mylonites during deformation. The geochemistry and the density (2.73 g/cm3) of the quartzfeldspar-biotite-hornblende mylonites and the surrounding, undeformed gneisses are similar. Only sulfur is slightly enriched due to the mineralization with sulfides. Mass balance calculations after the method of Gresens (1967) indicate that mass and volume of the mylonites have not significantly changed during ductile deformation compared to the undeformed gneisses. Furthermore, the stable oxygen isotopes vary only over a narrow range of 0.1–0.5‰ (SMOW) for bulk rock and quartz separates, indicating that metasomatic changes are only minor (Fig. 4, Kisters et al., 2000a; Kolb et al., 2000). 2.2. Lensoid breccia zones The breccia zones are characterized by altered wall rock fragments composed of garnet, biotite, quartz, feldspar, and

Fig. 1. Schematic geological map of southern Zimbabwe showing the contact between rocks of the Zimbabwe craton and granulites of the Limpopo Belt in the south (modified after, Berger et al., 1995). The Renco gold mine is situated in a higher order thrust system in the hanging wall of the North Limpopo Thrust Zone (Kolb et al., 2000).

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Fig. 2. Schematic sketch of (a) the geometry and (b–d) the internal fabrics of the auriferous shear zone in the Renco mine. Breccia zones form lensoid bodies in the ductile shear zones (modified after, Kisters et al., 2000a). (b) Initial stage of breccia formation with well defined dilational jogs and veins. (c) Propagated stage of breccia formation with closly-spaced sulfide-filled veins. (d) Late stage of breccia formation with rounded and lobate fragments, showing evidence of wear and chemical abrasion (cf., Fig. 3d).

hornblende (Fig. 3a and c). These fragments show ductile deformation textures such as alignment of biotite into a foliation and rotated garnet porphyroclasts. The breccia matrix is represented by abundant microscopic and macroscopic cracks of b1 cm length and b 3 mm width. They are filled by the hydrothermal ore paragenesis, comprising pyrrhotite, chalcopyrite, ilmenite, and gold. The vein material can make up about 10–80 vol.% of the rock, which results in massive sulfide rocks containing rounded to subrounded fragments of quartz and, to a lesser extent, altered wall rock up to 5 cm in diameter (Fig. 3d). The breccias show different fabrics that resulted from different mechanisms, which dominated during their formation. Various crack geometries are developed locally, including extension veins, extensional shear veins, shear veins, and dilational jogs (Figs. 2b,c and 3a). The sulfides have, locally, syntaxial idiomorphic textures and the irregularities in the opposite vein selvages match well. The wall rock fragments itself are often not rotated and the fragment morphology yields an overall puzzle-like texture. These breccias formed by fluid-assisted brecciation related to temporal variations in fluid pressure (Sibson, 1990; Jébrak, 1997; Kisters et al., 2000a). Locally, implosion breccias are observed in the veins and along the vein selvages (Fig. 3c), which formed due to fluid pressure drops caused by rapid opening of the crack (Jébrak, 1997; Kisters et al., 2000a).

The breccias with the rounded fragments formed by a combination of wear abrasion and corrosive wear (Jébrak, 1997). The fragments show evidence for rotation and frictional sliding of the fabrics. Locally, the fragments display dissolution features at their rims (Figs. 2d and 3d). This suggests that the hydrothermal fluid corroded some fragments and may explain the dominance of quartz fragments in such breccias, where minerals like feldspar, biotite, and garnet are probably completely dissolved. The lensoid breccia zones represent the various stages of breccia evolution (Jébrak, 1997): (1) The initial propagation stage, where the crack geometry is still recognized (Figs. 2b and 3a); (2) the wear stage with rotated fragments (Figs. 2c and 3c); and (3) the dilation stage represented by the massive sulfide breccia (Figs. 2d and 3d). The geochemistry and density of the breccia (up to 4.17 g/cm3) is completely different to the precursor rock, the granulite facies gneiss (2.73 g/cm3). Major elements like Fe and S are significantly enriched during the hydrothermal overprint. Mass balance calculations indicate major mass and volume gain between 50%–300% in the breccia zones, which is mainly the result of adding the vein material (Fig. 4). This suggests that volume expansion due to explosion processes partly contributed to the breccia evolution (cf., Jébrak, 1997). Similarly, bulk rock and quartz δ18O values are significantly enriched in the breccia by up

J. Kolb / Tectonophysics 446 (2008) 1–15

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Fig. 3. (a) Hand specimen showing the distribution of mylonite and breccia zones in the auriferous shear zones in the Renco mine. The cracks in the breccia are healed by hydrothermal sulfides. These veins have various orientations, including extension veins (ev), shear veins (sv), and extensional shear veins (esv, locally in R1 geometry). (b) Photomicrograph of a quartz-feldspar-biotite-hornblende mylonite. The sulfides (mainly pyrrhotite: Po) and biotite (Bt) occur (1) in the pressure shadow of the feldspar porphyroclast (Fs1), and (2) areas in which smaller, recrystallized feldspar (Fs2) dominates. (c) Photomicrograph of the contact between mylonite and the breccia. Note the garnet(Grt)-biotite alteration and the fractured feldspar associated with the sulfides. (d) Polished section of a massive sulfide breccia, representing a late-stage breccia with advanced fracture propagation and strong chemical and physical abrasion of the fragments.

to 1.5‰ (SMOW), which points to major metasomatic changes in these breccia zones (Kisters et al., 2000a; Kolb et al., 2000). 3. Auriferous shear zones in amphibolites and gneisses A number of amphibolite facies shear zone hosted lode-gold deposits in meta-mafic rocks were studied, including Kochkar (Urals, Russia) and those in the Hutti-Maski greenstone belt (Fig. 5; Dharwar Craton, India). The shear zones, gold-quartz veins, disseminated gold mineralization, and alteration formed at temperatures between 500 °C and 600 °C, and are described together in this paragraph (Table 1, Kolb et al., 2004, 2005b;

Hellmann et al., 2005). The approximately 1–10 m wide shear zones represent near vertical strike-slip or reverse deformation zones and are characteristically zoned on an m-scale (Fig. 5b). In the center of the auriferous shear zones, biotite schists have a closely spaced, mylonitic foliation, b 10 vol.% porphyroclasts and are, based on this, classified as ultramylonite (Passchier and Trouw, 1996). The outer shear zone parts, which can be correlated with a weaker fabric development, consist of chlorite-biotite schists with 30–60 vol.% porphyroclasts and are classified as mylonite to protomylonite (Passchier and Trouw, 1996). Notably, the chlorite in these altered mafic rocks formed at temperatures of 500 °C–600 °C as suggested by its

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Fig. 4. Sketch of the auriferous shear zone in the Renco mine with the metasomatic changes in the mylonites and breccia zones (Kisters et al., 2000a; Kolb et al., 2000).

paragenetic occurrence with high-temperature minerals and pseudosection modeling (Kolb and Meyer, in press). In mafic metavolcanic rocks, chlorite is commonly stable in the amphibolite facies, especially in Mg-rich rocks similar to those in the studied deposits (Spear, 1993). In the Hira Buddini prospect, the shear zone is developed at the contact of amphibolites with gneisses, which form mylonites at the contact (Fig. 6). In contrast to the other studied examples, the

near vertical shear zone records a reverse sense of movement (Hellmann et al., 2005). About 0.2–0.5 m wide, 2 m in strike length, near horizontal gold-quartz tension gashes are formed in this shear zone (Fig. 6b). In the Kochkar gold mine, the strike-slip shear zones are also developed at the lithological contact between amphibolites and orthogneisses (Kolb et al., 2005b). The gneisses are deformed by ductile shearing and have a strong mylonitic foliation close

Fig. 5. (a) Geological map of the Hutti-Maski greenstone belt in the Dharwar craton, India (modified after, Ashok Kumar and Dhar, 1994). The Hutti mine and various prospects (Wandalli, Hira Buddini, and Uti) are situated in amphibolite wall rocks. (b) Schematic block diagram of the auriferous shear zones in all the different mines and the distribution of the characteristic alteration zones. The strike of the shear zone is NNW–SSE in Hutti and Uti, and WNW–ESE in Wandalli and Hira Buddini.

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Fig. 6. (a) Geological map of the Hira Buddini deposit. The auriferous shear zone is developed at the amphibolite-gneiss contact on the deposit scale. For location of Hira Buddini see Fig. 5a. (b) Photograph of the hydrothermally altered reverse shear zone with subhorizontal, sigmoidal auriferous quartz veins (sm, underground, 2nd level).

to the lithological contact. Massive to laminated, 0.2–1 m wide gold-quartz veins are developed at this contact to up to 500 m along strike (Fig. 7). 3.1. Mylonites The amphibolites in the outer shear zone parts contain up to 60 vol.% hornblende and plagioclase porphyroclasts (b 0.5 mm in diameter) in a fine-grained plagioclase matrix, which points to relative low strain in this protomylonite. The plagioclase is recrystallized by subgrain rotation and grain boundary bulging. Its grain size is reduced by at least one order of magnitude. The small plagioclase grains show a strong zoning pattern with a lowered anorthite component at the rims, which is interpreted to reflect changes in fluid composition during recrystallization by grain boundary migration under retrograde conditions (Fig. 8a, cf.,

Stünitz, 1998). In contrast, hornblende shows abundant cracks and evidence of only weak intracrystalline plasticity (twinning) and diffusive mass transfer (dissolution-precipitation creep), which are typical features in hornblende deformed at or above 500 °C in the presence of an aqueous fluid (Imon et al., 2004). Alteration minerals, comprising biotite, chlorite, feldspar, quartz, tourmaline, sulfides, and traces of gold, are restricted to the fine (about 50 μm), recrystallized plagioclase matrix and the rims and/or cracks of hornblende porphyroclasts (Fig. 8b). The alteration mineral content increases with proximity to the shear zone center. The dominance of biotite and chlorite in the rock results in a lepidoblastic strongly foliated fabric of an ultramylonite. The original hornblende and plagioclase of the precursor amphibolite are completely replaced, which resulted in the formation of a biotite schist containing a disseminated gold-sulfide mineralization in all studied examples (Kolb et al., 2004, 2005b; Hellmann et al., 2005).

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Fig. 7. Geological map of the Kochkar mine and surroundings in the East Uralian Zone, Russia (modified after, Borodaevsky, 1952). The gold mineralization is hosted in near vertical shear zones in the amphibolite.

The geochemistry of the biotite schists strongly differs from that of the amphibolites, involving the hydrothermal addition of major elements like Si, K, C, and S. Gold is enriched by the hydrothermal fluids at 2–4 orders of magnitude (Kolb et al., 2004, 2005b). Mass balance calculations point to a significant mass and volume gain of 11% and 8% increasing to values of 16% and 12% in the central shear zones of the Hutti mine (Fig. 9; Kolb et al., 2004). The stable isotope systematics of these shear zones show a 0.5–1‰ (SMOW) decrease in δ18 O and a 1–5‰ (SMOW) increase in δD, additionally pointing to strong metasomatic changes (Fig. 9, Kolb et al., 2004). The gneisses are characterized by a weak foliation defined by biotite and muscovite. About 0.5 mm quartz grains and up to 2 mm feldspar grains are recrystallized by subgrain rotation and grain boundary migration (Kolb et al., 2005b). The alteration is weak and restricted to the formation of muscovite, epidote, and calcite at the expense of feldspar. This alteration paragenesis is stable at temperatures above 500 °C, especially in the presence of an aqueous fluid (Spear, 1993). No major differences in the major and trace element geochemistry between precursor granitoids and mylonitic gneisses are detected (Kolb et al., 2005b). 3.2. Gold-quartz veins Auriferous quartz veins are commonly made up of quartz (70–98 vol.%) with minor calcite, muscovite, scheelite, biotite, actinolite, tourmaline, pyrite, chalcopyrite, and gold. The vein texture is blocky. The 0.3–0.5 mm quartz grains show evidence of recrystallization, progressive deformation, and later, retrograde overprint by undulose extinction, subgrain rotation, grain boundary migration, and brecciation. The tension gashes in the Hira Buddini mine are extension veins, which form en echelon arrays of several tens of meters. The foliation parallel shear veins in the Kochkar mine are laminated with 1–3 cm wide

Fig. 8. (a) BSE image from the feldspar matrix of a deformed amphibolite (Wandalli, India). The small recrystallized plagioclase (Pl) is zoned with An60 composition in the core and An15 at the rims, which points to changing fluid compositions during grain boundary migration recrystallization. Fluid migration is, therefore, considered to be focused into the feldspar matrix of the amphibolites. Note the small pores at feldspar grain boundaries and triple junctions. (b) Photomicrograph of an altered amphibolite (Wandalli, India). Hornblende (Hbl) porphyroclasts are altered by biotite (Bt) and pyrite (Py) along internal fractures and grain boundaries. The occurrence of hydrothermal alteration minerals is concentrated in the plagioclase groundmass.

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Fig. 9. Schematic block diagram of the auriferous shear zone in the Hutti mine with the metasomatic changes in the high-strain central part and the lower-strain distal part of the shear zone (Kolb et al., 2004).

quartz veins alternating with mm-scale slices of altered wall rock, forming composite vein systems up to 1 m wide.

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2000; Kisters et al., 2004; Wulff et al., 2004). Stratiform mineralization is located at the contact of layered calcsilicatemarble rocks and biotite schists in the steep limb of the dome (Fig. 10a). Gold is concentrated in cigar-shaped, b40 m wide, about 20° NNE-plunging massive sulfide lenses, which can be followed over 800 m along strike (Fig. 10, Steven et al., 2003). These prolate bodies form an en echelon geometry parallel to the fold axis of the dome (Fig. 10b). Currently three bodies are exposed in the open pit and by drilling. A second type of gold mineralization occurs in variably oriented quartz veins, which formed at various stages during the progressive folding of the Karibib dome (Kisters, 2005): (1) A conjugate set of up to 50 cm wide veins with two maxima dipping moderately to the NW and SE, respectively. Their intersection is oriented broadly parallel to the fold axis of the dome. This suggests that they formed as extensional shear veins during the formation of the Karibib dome. (2) A conjugate set of near vertical quartz veins is observed, which are a few centimeter to some meter long and usually have a sigmoidal shape. They reach a maximum thickness of 5 cm. Three main strike orientations are distinguished: (i) N–S, (ii) ENE–WSW, and (iii) WNW–ESE trending veins. This may be explained as a conjugate set, which formed as extensional shear veins (types i and ii) and extension veins (type iii) during WNW–ESE compression and folding of the Karibib dome.

4. Auriferous flexural-slip folds in calcsilicate rocks 4.1. Massive sulfide lenses Gold at Navachab (Damara Orogen, Namibia) is hosted by a regional scale, NW-verging, non-cylindrical anticline, the Karibib dome, which formed under amphibolite facies conditions (550 °C–650 °C, Table 1, Puhan, 1983; Nörtemann et al.,

The massive sulfide lenses comprise up to 50 vol.% hydrothermal pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, bismuth, gold, and Bi-tellurides. The original calcite and

Fig. 10. (a) Geological map of the Navachab mine showing the NNE–SSW trending, near vertical stratigraphy of the steep limb from the Karibib dome (modified after, Steven and Badenhorst, 2002). Gold mineralization is hosted by the layered calcsilicate-marble rocks of the Okawayo Formation. (b) Geometry of the cigar-shaped gold mineralized zones in a NNE–SSW section. Gold grades in meter g/t Au (modified after, Steven et al., 2003).

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calcite-clinopyroxene layers of the calcsilicate-marble host rock are variably replaced by a peak metamorphic, high-temperature hydrothermal alteration assemblage, comprising garnet, biotite, and garnet, clinopyroxene, K-feldspar, and quartz, respectively (Wulff et al., 2005). The fabric of the garnet-biotite alteration shows a foliation mainly developed at the contact of the various calcsilicate layers, which points to layer-parallel shearing during flexural-slip (Fig. 11a). The strong metasomatic changes are related to reactions of the host rocks with a hydrothermal fluid. Calcite is released from the altered rocks increasing the permeability drastically. The layers dominated by clinopyroxene and/or garnet are deformed by brittle deformation. The b 3 cm long and b1 cm wide cracks are mainly filled by the hydrothermal sulfide ore assemblage (Fig. 11a). The sulfides show, locally, syntaxial growth fabrics and the irregularities of the opposite vein selvages match well. The massive sulfides form a matrix with rounded to subrounded fragments of altered wall rock up to 2 cm in diameter (Fig. 11b). The fragments, locally, display corrosion features (Fig. 11b), which suggests

that the massive sulfide rocks represent breccias in the dilation stage with abraded and chemically decomposed fragments (cf., Jébrak, 1997). In the matrix, the sulfides all show triple junctions due to post-deformational static recrystallization. 4.2. Gold-quartz veins The veins are invariably composed of quartz with rare sulfides comprising pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, bismuth, gold, and Bi-tellurides, similar to the paragenesis in the massive sulfide lenses. The alteration assemblage comprises garnet, clinopyroxene, K-feldspar, biotite, quartz, and actinolite in marbles and calcsilicate rocks, and garnet, quartz, and actinolite in biotite schists. The alteration zones form about 1 m wide halos around the veins and are particularly developed along the calcsilicate-marble contacts, which acted as conduits for the hydrothermal fluids. This suggests that the veins and the massive sulfide lenses formed by a similar hydrothermal fluid during brittle deformation in the amphibolite facies. 5. Discussion The studied gold deposits show various internal structures of the mineralized shear zones over a range of wall rock types including amphibolites, gneisses, calcsilicate rocks, and marbles. The metasomatic changes are spatially closely associated with mineral deformation and recrystallization. In most of the deposits, economic gold mineralization, and, thus, strong hydrothermal fluid flow, is hosted in gold-quartz veins and massive sulfide breccias, which developed at temperatures N500 °C (Table 1). This indicates that in lower-crustal deformation zones effective hydrothermal fluid flow is controlled by brittle deformation. Under reasonable geological strain rates, this requires intergranular cementation, compaction, and healing and sealing of pores to provide fluid pressures high enough to facilitate the formation and propagation of cracks. The formation of macroscopic cracks in shear zones is either episodic by seismically generated fractures (Etheridge, 1983; Etheridge et al., 1984; Sibson, 1986, 1994; Kisters et al., 2000a; Cox et al., 2001; Kolb et al., 2004) or more continuous by brittle-viscous deformation at low strain rates (Blenkinsop and Sibson, 1992; Streit and Cox, 2002). 5.1. Porosity controlling mechanisms

Fig. 11. (a) Polished thin section showing the alteration and mineralization in the layered calcsilicate-marble layers. The rheologically weaker biotite-rich central layer shows a mylonitic texture with a closely spaced foliation. The clinopyroxene (Cpx)-rich layer is deformed by brittle deformation indicated by the development of sulfide-filled veins in the massive sulfide lens. The lack of cracks and veins and, therefore no gold mineralization, point to dominating crystal-plastic deformation in the weak biotite-rich layer. Note, that the Po-Cal filled horizontal crack is only developed in the rheologically stronger Cpx-rich layer. (b) Polished section showing the contact of a massive sulfide lens with a garnet-biotite schist. Note the fabric of the feldspar (Fs) and clinopyroxene (Cpx) fragments in the pyrrhotite-dominated sulfide matrix, which suggest brittle deformation of the precursor rocks and chemical corrosion at the fragment rims.

The porosity in metamorphic rocks comprises: (1) intergranular, primary pore space and/or (2) grain-scale to macroscopic fractures. The pore connectivity and, therefore, the permeability are not only controlled by the total porosity of a rock, but also by different wetting characteristics of mineral surfaces by fluids, especially at a low porosity (Holness and Graham, 1995; Jamtveit and Yardley, 1997; Cox et al., 2001). The permeability of metamorphic rocks, determined in the laboratory, lies within the range from 10− 23 to 10− 18 m2 (Brace, 1980). The weak connectivity of pore space at a porosity b 3–6% results in the decline of the permeability below the percolation threshold

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(Manning and Ingebritsen, 1999). Therefore, fluids in metamorphic rocks are at or close to the lithostatic pressure (Etheridge et al., 1984). Experimental results and field studies of exhumed shear zones indicate that permeability enhancement to values between 10− 6 and 10− 5 m2 is commonly associated with deformation (Knipe and McCaig, 1994; Géraud et al., 1995; McCaig, 1997; Mancktelow et al., 1998; Bauer et al., 2000; Zhang et al., 2001; Streit and Cox, 2002). The processes of the formation of connected pore space by deformation at temperatures N 500 °C are, however, only poorly understood. The high-T shear zones at Renco display a heterogeneous permeability structure. Locally developed lensoid breccia zones characterize the ductile shear zones, which acted as pathways for the hydrothermal fluids. Only limited porosities are developed in the mylonitic rocks as indicated by only weak hydrothermal alteration (Figs. 3 and 4). The recrystallization of minerals during viscous deformation leads to a grain size reduction, which increases porosity by the formation of new grain boundaries, i.e. primary porosity, and, in turn, decreases the flow strength of the mylonite. The observation of the active diffusion processes suggests that grain-scale micro fractures can heal relatively fast. Experimental data are scarce, showing that micro cracks may form due to high-T deformation, but are, especially in the case of quartz, relatively short-lived (Tullis, 1983; Tullis and Yund, 1985; Brantley et al., 1990; Kruse et al., 2001, 2002). Therefore, the interconnected porosity is easily destroyed and the permeability of the shear zone will be reduced aided by the precipitation of minerals from the hydrothermal fluid (Fig. 3) and the compaction of the shear zone. These processes may effectively close up mylonites containing fluid in intergranular pores from the rest of the shear zone. Compaction during intracrystalline plastic deformation promotes the sealing of parts of the shear zone (Sleep and Blanplied, 1992). In these compartments, the fluid pressure can reach lithostatic to supralithostatic levels. The massive sulfide lenses in Navachab are texturally similar to those in Renco (Figs. 3 and 11) and, therefore, most probably formed by similar processes. The crystal-plastic deformation and the dynamic calcite recrystallization in the marble-layers during flexural-slip have sealed fluid compartments, leading to a fluid pressure build-up to close-to lithostatic and probably also supralithostatic values. Due to the different lithology at Renco (gneiss) and Navachab (marble, calcsilicate rock), viscous deformation and diffusion creep can act in Navachab at temperatures some 100 °C lower than those at Renco (Kern and Wenk, 1983; Rutter, 1995; Ulrich et al., 2002). The porosity in the deformed rocks is, furthermore reduced by the precipitation of minerals from the hydrothermal fluid and by fluid-rock reaction. The low viscosity of the marble and the garnet-biotite schists as alteration product (Fig. 11) in conjunction with the progressive compaction during doming of the Karibib dome represent effective processes for the formation of the large, cigar-shaped massive sulfide lenses, which represent about 800 m long compartments up to 40 m in diameter. At lower temperatures, intracrystalline plastic deformation is still a vigorous mechanism, however, might not be active for all

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minerals in a gneissic mylonite, which causes strain partitioning into the weaker matrix. In the granitic rocks in Kochkar and Hira Buddini, weak quartz and muscovite form an interconnected matrix and, therefore, no significant permeability for hydrothermal fluids was developed as reflected by the only weak hydrothermal changes of the deformed lithology. In contrast, in the sheared amphibolites, plagioclase is plastic, while amphiboles act as porphyroclasts and are brittle (cf., Imon et al., 2004). Dislocation creep of plagioclase generates dislocation-induced micro cracks and new grain boundaries, which contribute to the total porosity of the shear zone. Grain boundary migration under retrograde conditions forms smaller, new plagioclase grains with lower anorthite-component and also lower molar volume, additionally increasing the porosity (Fig. 8b). Amphiboles are affected by dissolution-precipitation processes, forming actinolite, biotite, and chlorite under retrograde conditions, which is likely to increase the porosity significantly (cf., Cox and Etheridge, 1989). The metasomatic changes additionally lower the flow strength of the mylonite and progressively partition ductile deformation into the weakened rock. The pervasive and disseminated style of alteration suggests a mesh-like grain-scale permeability structure in the shear zone. The usually observed alteration zoning in these shear zones as well as mass balance calculations and stable isotope systematics vary systematically with the strain, from strongly deformed central parts to weakly deformed distal parts (Fig. 9, Cox et al., 2001; Kolb et al., 2004). Experimental high strain deformation of plagioclase-diopside aggregates produced abundant cavities that evolved with strain and resulted in a significant porosity (Dimanov et al., 2007). This may explain the relatively high permeability in the amphibolites at Hutti and the lateral variation of alteration over the shear zone profile from protomylonites to ultramylonites. The deformation constantly develops new interconnected porosity, i.e.; permeability, and causes porous fluid flow through the sheared amphibolites, promoted by high-strain deformation, the significantly different flow strength of the constituting minerals (i.e., hornblende and plagioclase, Handy and Zingg, 1991) and the reactivity of mineral and fluid. 5.2. Seismic or slow strain rate brittle deformation The sheared rocks at Renco and Navachab are invariably characterized by a low viscosity, due to the high deformation temperatures. This promotes slow strain rates at a relatively low shear stress during deformation. Unfortunately, clear strain markers are absent due to subsequent deformation and overprinting alteration. Experimental work by Streit and Cox (2002) has defined high fluid pressure, slow strain rates, low shear stress and low strain deformation as critical parameters for the formation of fractures in ductile shear zones. The experimentally produced fracture network with Riedel and jog geometry is similar to the texture of the sulfide-filled cracks in the breccia zones in Renco and Navachab (Figs. 3 and 11, cf., Kisters et al., 2000a; Streit and Cox, 2002). The macroscopic veins of these lensoid bodies are, therefore, most probably

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formed by continuous brittle-viscous deformation in the auriferous high-temperature shear zones. The critical feature is the effective sealing of pore volume by diffusion and recrystallization-precipitation in order to keep the fluid pressure close to lithostatic. At Renco, in particular the locally observed implosion breccias point to deformation at fast strain rates (Fig. 3, Sibson, 1986; Jébrak, 1997). The shear zones in the compressive regime acted as fault-valves in the sense of Sibson (1990), sealing fluid compartments so effectively that the fluid pressure increased above the sum of the regional least principle stress and the tensile strength of the rocks (Pf N σ3 + T: Fig. 12a). The lensoid breccia zones with the matrix of the hydrothermal ore paragenesis formed due to critical fracturing during fluidassisted brecciation in the ductile thrust zones. Locally, the initial stages of the breccia evolution with the various crack

geometries are still preserved probably due to the episodic deformation style caused by the fluid pressure fluctuation. This is at variance with the typically observed progressive deformation in ductile shear zones. The massive sulfide breccias represent later stages of the breccia formation where significant crack propagation and abrasion occurred. The massive sulfide lenses at Navachab closely resemble these massive sulfides at Renco (Figs. 3 and 11). However, the initial stages are not preserved, most probably, due to an advanced evolution of the breccias and the more reactive nature of the calcsilicate wall rocks. This facilitated fluid-wall rock reactions and corrosive wear of the fragments in the breccia. Clear markers of fast strain rate deformation such as implosion breccias are lacking at Navachab, however, the close textural similarity between the hydrothermal breccias at Renco and Navachab may suggest similar processes of their formation.

Fig. 12. (a) Sketch of the auriferous shear zones at Renco and the probable orientation of the principle stress axes during deformation. Supralithostatic fluid pressure (Pf N σ3 + Tgneiss) is reached in lenses sealed from the ductile shear zone, resulting in seismic and aseismic brittle deformation and the formation of the auriferous lensoid breccia zones. (b) Sketch of the auriferous shear zones at Hira Buddini and the probable orientation of the principle stress axes during deformation. Supralithostatic fluid pressure (Pf N σ3 + Tbiotite schist) is reached aided by the strong rheology contrast (Tgneiss ≫ Tbiotite schist) promoted by progressive alteration of amphibolite to biotite schist. The rheology contrast causes strain incompatibilities by strain partitioning into the weaker layer; the orientation of the resulting opening vector (Vopening) can be estimated and is consistent with fracture orientation. (c) Sketch of the auriferous shear zones at Hutti and the probable orientation of the principle stress axes during deformation. The shear zone is preferably oriented for ductile creep and the mylonites are progressively weakened by hydrothermal alteration of amphibolite to biotite schist and fabric development. In this scenario, the fluid pressure (Pf) remains between hydrostatic and lithostatic values not promoting fracture formation during the ductile deformation. (d) Sketch of the auriferous shear zones at Kochkar and the probable orientation of the principle stress axes during deformation. Shear zoneparallel fractures (gold-quartz veins) formed due to the strong rheology contrast (Tgneiss ≫ Tbiotite schist) promoted by progressive alteration of amphibolite to biotite schist. The orientation of the resulting opening vector (Vopening) can be estimated and is consistent with fracture orientation.

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Macroscopic crack and vein formation is rare in the 500 °C– 600 °C shear zones and was only observed in an environment, where amphibolite is in contact with rheologically weaker rocks (Fig. 12b–d). At the heterogeneity, shearing was initiated increasing the strain incompatibility at the lithological contact (Goldblum and Hill, 1992; Kisters et al., 2000b), which is further promoted by the progressive reaction weakening of the amphibolite to a biotite schist. Lowering the tensile strength of the sheared lithology (TS) results in a situation where the sum of the normal stress (σn) on the shear zone and TS are smaller than the sum of the least principle stress (σ3 + T) of the surrounding lithology (Fig. 12b and d, Kolb et al., 2004; 2005b). Furthermore, the partitioning of ductile strain into the weaker rock leads to the formation of oblique veins in the competent lithology, which accommodated the different amount of strain in the two sheared lithologies (Fig. 12b and d). The close to lithostatic pore-fluid pressure in the shear zone can easily increase above the critical level for failure (Pf ≥ (σn + TS)) and macroscopic veins form in the competent rock due to episodic fluid pressure fluctuation (1) as close-to horizontal vein arrays in compressional regimes (Fig. 12b) or (2) parallel to the lithological contact in transcurrent regimes. However, fluid pressures larger than lithostatic (Pf N σ3 + T) are unlikely due to (1) the relative high permeability in the progressively sheared rocks and (2) a stress field orientation that is preferably oriented for reactivation of the lithological contact (Fig. 12c, Sibson, 2001). At Navachab a set of auriferous quartz veins formed, locally crosscutting the massive sulfide lenses. In the compressional regime during folding of the Karibib dome structure, the formation of the extensional shear veins requires lithostatic to supralithostatic fluid pressures to facilitate fracturing (cf., Sibson, 2004). Several features may be responsible for the low differential stress during deformation, including high pore fluid pressure and/or the progressive reorientation of planar structures: progressive shortening resulted in flexural flow folding and steepening of the NNW limb of the dome. At the lock-up stage, the steep limb is misoriented to be activated by flexural flow, which then favors the formation of the quartz veins. The situation in Navachab is explained by an evolution from (1) a situation akin the formation of the lensoid breccia zones in Renco (Fig. 12a), where sealing of lensoid fluid compartments during ductile deformation is important, to (2) a situation akin the formation of quartz veins at Hira Buddini (Fig. 12b), where the lithological anisotropy is misoriented for reactivation and the tensile strength of the sheared rocks (biotite schist, marble, calcsilicate rock) is different. The laminated texture of the veins in Kochkar and Navachab, and the variably deformed sigmoidal shape of the veins in Hira Buddini point to an episodic deformation style rather than continuous ductile deformation in the N500 °C shear zones. The vein geometry is strongly similar to that described for lower temperatures, which formed by failure of the shear zone rocks analogous to the fault-valve process with fluctuating fluid pressure and distinct fracture (earthquake) events (Sibson, 1990). This suggests that similar processes may be responsible for the formation of the auriferous veins in high-temperature

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settings. However, subcritical vein growth cannot be ruled out, at least for some of the auriferous quartz veins described here. 6. Conclusions A number of gold deposits in different tectonic environments and hosted by different rocks where studied, which show deformation and contemporaneous gold mineralization and alteration at temperatures N500 °C in the lower crust. Three types of structurally controlled hydrothermal mineralization are distinguished: (1) Lensoid breccia zones with a sulfide matrix in mylonites; (2) Auriferous quartz vein arrays associated with ductile shear zones; and (3) disseminated in relatively broad ductile shear zones. This represents a heterogeneous permeability development in shear zones of the lower crust, which is controlled by: (1) The ability of the deformed lithology to seal pressurized fluid compartments by viscous deformation and diffusion creep, which causes episodic hydraulic fracturing and the formation of breccias at fast strain rates. (2) The composition of the rock, inducing brittle deformation of some minerals and mineral-fluid reaction that results in a mesh-like grain-scale permeability. (3) A strong rheology contrast at sheared lithological contacts, enabling brittle vein formation at sublithostatic fluid pressures by the strain incompatibility along the contact. Fluids, thus, play an important role in the style of crustal deformation at temperatures N 500 °C, not only controlling partitioning into viscous deformation and/or dissolution-precipitation creep in mylonites and brittle deformation in breccias and veins but also may induce low-magnitude earthquakes in the lower crust. Acknowledgements The comments by J.-P. Burg, A.-M. Boullier, and J.-P. Bellot helped improving this manuscript. Discussions with A. Dziggel, F.M. Meyer, A. Rogers, K. Wulff, A.F.M. Kisters, and A. Hellmann are gratefully acknowledged. This study was financially supported by the Deutsche Forschungsgemeinschaft grants Me 1425/1-1/2, 1425/2-1/2, 1425/5-1/2, 1425/13-1. References Aldersons, F., Ben-Avraham, Z., Hofstetter, A., Kissling, E., Al-Yazjeen, T., 2003. Lower-crustal strength under the Dead Sea basin from local earthquake data and rheological modeling. Earth and Planetary Science Letters 214, 129–142. Ashok Kumar, J., Dhar, D., 1994. Enhancement of IRS data for structural mapping of Hutti-Maski schist belt, Karnataka, India. In: Muralikrishna, V. (Ed.), Remote Sensing and GIS for Environmental Planning. Tata McGraw Co. Ltd., New Dehli. Bauer, P., Palm, S., Handy, M.R., 2000. Strain localization and fluid pathways in mylonite: inferences from in situ deformation of a water-bearing quartz analogue. Tectonophysics 320, 141–165. Berger, M., Kramers, J.D., Naegler, T.F., 1995. Geochemistry and geochronology of charnoenderbites in the northern marginal zone of the Limpopo Belt, southern Africa, and genetic models. Schweizerische Mineralogisch Petrographische Mitteilungen 75, 17–42. Blenkinsop, T.G., Rollinson, H.R., 1992. North Limpopo Field Workshop Field Guide and Abstracts Volume. Geological Society of Zimbabwe, Harare. 56 pp.

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