Ore textures and remobilization mechanisms of the Hongtoushan copper–zinc deposit, Liaoning, China

Ore Geology Reviews 57 (2014) 78–86

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Ore textures and remobilization mechanisms of the Hongtoushan copper–zinc deposit, Liaoning, China Yajing Zhang, Fengyue Sun ⁎, Bile Li, Liang Huo, Fang Ma College of Earth Sciences, Jilin University, Changchun 130061, PR China

a r t i c l e

i n f o

Article history: Received 24 May 2013 Received in revised form 6 September 2013 Accepted 6 September 2013 Available online 13 September 2013 Keywords: Phlogopite gneiss Textures Remobilization mechanism Copper–zinc deposit Hongtoushan

a b s t r a c t The Hongtoushan copper–zinc deposit is a volcanic-associated massive sulfide deposit in the Archean greenstone belt in Liaoning, China. Polymetamorphism has resulted in changes to the composition and textures of minerals in the deposit, along with remobilization. During metamorphism, the original alteration minerals that formed with the ore minerals, such as chlorite and sericite, were transformed into cordierite, anthophyllite, and phlogopite. After further remobilization, new minerals, such as gahnite and actinolite, were formed. In this process, the original textures were destroyed and new textures were formed, including recrystallization and growth textures, brittle and ductile deformation textures, durchbewegung textures, replacement textures, chalcopyrite disease, and retrograde textures. The ore-forming components underwent two periods of remobilization. In the first (early) stage, mechanical remobilization was important, and formed a high grade Cu–Zn–Au–Ag “ore pillar” along the vertical hinge of a synformal fold. In the second (late) stage, the mixed hydrothermal–mechanical remobilization affected the ores, and was typically characterized by matrix sulfides, together with silicate minerals, moving from the matrix into individual fractured pyrite metablasts and replacing them to varying degrees. © 2013 Elsevier B.V. All rights reserved.

1. Introduction

2. Geological setting

The final textures and structures of sulfide deposits affected by metamorphism depend on both the physical conditions of metamorphism, such as temperature and pressure, and the initial properties of the materials (Vokes, 1969). The Hongtoushan copper–zinc deposit is an example of a sulfide deposit affected by multi-phase metamorphism and deformation. Located in an Archean greenstone belt in Liaoning, China, the deposit is a volcanic-associated massive sulfide deposit (Sun, 1992; Zhang, 2010; Zhang et al., 1984) of importance both in geological and economic terms. Various processes related to the multiphase deformation and metamorphism have influenced the geometry of the ore bodies and their textures and structures, including the process of remobilization (Liu and Chen, 1982). Although the Hongtoushan deposit has been reasonably well studied (Gu et al., 2004a,b; Liu and Chen, 1982; Zhang et al., 1984), several issues are yet to be resolved. These include: (1) the origin of phlogopite gneiss that is located near the ore bodies; (2) the nature of changes in ore composition, texture, and structure during metamorphism and deformation; and (3) the mobilization mechanism(s) of minerals. This paper combines field geological data, microscope observations, scanning electron microscopy data, and electron probe test data in an effort to provide a clearer understanding of these issues.

The Hongtoushan copper–zinc deposit is located in the Archean granite–greenstone terrane of the northeastern section of the North China Craton, in Qingyuan County, Liaoning Province (Fig. 1). The greenstones in the area of the deposit comprise the Hongtoushan Formation of the Qingyuan Group. The formation comprises mainly hornblende gneiss interbedded with biotite gneiss, and these rocks contain garnet, sillimanite, and anthophyllite. Petrological and geochemical investigations by previous authors indicate that the parent volcanic rocks of Qingyuan Group are continuously differentiated volcanic rocks varying in composition from mafic through intermediate to felsic of both the tholeiitic and calc-alkaline affinities, and are thus the protoliths of lithologies that typically form in an island-arc environment (Li et al., 1995; Zhai et al., 1984; Zhang, 2010; Zhang et al., 1984). In the early Precambrian, the Hongtoushan deposit was affected by the Anshan and Lvliang movements (Gu et al., 2004b) resulting in the Anshan and Lvliang deformation cycles. The Anshan deformation cycle occurred during amphibolite facies regional metamorphism, with medium pressure and temperatures of 500–700 °C, and was characterized by plastic flow deformation. The Lvliang deformation cycle was characterized by brittle deformation of greenstone belts and late injection of mafic dikes. The thinly layered, interbedded Hongtoushan Formation experienced upper amphibolite facies metamorphism, with temperatures of 600–650 °C and pressures of 0.8–1.6 GPa (Yang and Yu, 1984). The gneissic fabric therein is essentially parallel to lithological boundaries, and dips toward the southeast at 70–80°, defining an isoclinal fold

⁎ Corresponding author at: 2199 Jianshe Street, College of Earth Sciences, Jilin University, Changchun 130061, PR China. Tel.: +86 431 88502185; fax: +86 431 88584422. E-mail addresses: [email protected] (Y. Zhang), [email protected] (F. Sun), [email protected] (B. Li), [email protected] (L. Huo), [email protected] (F. Ma). 0169-1368/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.oregeorev.2013.09.006

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Fig. 1. Simplified geological map of the Hongtoushan copper–zinc deposit (B) (modified after Yang and Yu, 1984) and sketch of regional geology and distribution of the Hongtoushan deposits (A).

(Fig. 1) that plunges at 80° towards the southeast. The shapes of the ore bodies are controlled by this fold with the ore bodies occurring mainly in the hinge. The overall configuration of the ore bodies is in the shape of a horizontal “Y”, with the two arms of the “Y” opening towards the east (Fig. 1). The ore bodies at the junction of the Y-shape attain the maximum thickness and the highest grade in Cu, Au and Ag, and thus form a substantial, vertical “ore pillar”. The axis of the ore pillar points progressively counterclockwise with increasing depth. Below a depth of −467 m (all depths are relative to sea level), the dip angle of the fold axis decreases to less than 30° and in this zone the ore bodies occur mainly in the limbs of the fold. The ore bodies are mainly stratiform or stratiform-like in shape, and have a massive structure. The stratiform ore bodies dip toward the southeast in general, sub-parallel to the gneissosity of the wall rocks. In contrast, the stratiform-like ore bodies, which were affected by deformation and metamorphism, occur mainly as veins and lenticular bodies, and, at small scales, cut across the stratigraphy. Quartz–sulfide veins, composed predominantly of quartz, muscovite, chlorite, carbonate, and sulfides, cut across the wall rocks and ore bodies. In volcanic-associated massive sulfide deposits, alteration pipes initially have a chlorite core that grades laterally and vertically into a sericite-rich outer zone and finally into unaltered rocks (Barrett et al., 2005; Large et al., 2001; Theart et al., 2010; Yao and Sun, 2006). In such deposits, the most characteristic features of the altered rocks, relative to fresh rocks, are increased contents of Mg and Fe, and decreased Ca and Na, giving rise to the formation of chlorite and sericite. Cordierite–anthophyllite gneiss, which is distributed near the Hongtoushan ore bodies, is considered to represent metamorphosed seafloor hydrothermal alteration zone (Zhang et al., 1984; Zheng et al., 2008). Relative to the unaltered protolith, the gneiss is geochemically characterized by strong enrichment in Fe, Mg, and Si, and a corresponding depletion in K, i.e., the same characteristics as chloritic and silicified rocks near unmetamorphosed VMS systems.

Laboratory of the Beijing Research Institute of Uranium Geology, Beijing, China, operated at an accelerating voltage of 30 kv and a beam current of 10 nA, using a focused beam. 4. Minerals of ore-bearing rocks The predominant ore minerals of the Hongtoushan ores are pyrite and pyrrhotite, with less chalcopyrite and sphalerite, and minor cubanite, magnetite, galena, rutile, and electrum. Gangue minerals are dominated by quartz, plagioclase, and biotite, with less biotite, gahnite, cordierite, anthophyllite, muscovite, actinolite, andradite, clinozoisite, chlorite, carbonate, sillimanite, and anhydrite. Pyrite grains and included silicate minerals have been replaced by chalcopyrite and sphalerite. Quartz– sulfide veinlets, if present, fill cracks in the sulfides and contain sulfide, quartz, chlorite, carbonate, and muscovite. 4.1. Ore minerals Here we describe the dominant ore minerals in turn, based on observations of the thin sections and polished sections. Previous studies have also characterized the ore minerals in some detail (Gu et al., 2004a; Yu, 2006).

3. Samples and analytical methods

4.1.1. Pyrite Pyrite crystals in the massive sulfide ores are generally coarse porphyroblasts in a matrix of pyrrhotite, chalcopyrite, and sphalerite. Pyrite porphyroblasts contain inclusions of spherical plagioclase and quartz. Pyrite porphyroblasts are fragmented, and fissures are commonly filled with remobilized chalcopyrite, pyrrhotite, sphalerite, and quartz. In the disseminated ores, pyrite is usually subhedral or anhedral, and is locally elongate within the plane of the gneissosity, and is even boudinaged into multiple sections. In the disseminated ores, chalcopyrite occurs in pressure shadows, and pyrite occurs as a retrograde mineral that replaces pyrrhotite.

For this study we collected 550 samples, including ore and wall rocks, from 32 tunnels at seven underground mining levels between depths of −467 and −827 m in the Hongtoushan copper–zinc deposit. Ninety-one rock and ore samples were selected from which thin sections and polished sections were made to examine the minerals, textures, and structures of the ore rocks and wall rocks. Microprobe analyses were performed using a JEOL JXA-8100 Electron Probe housed at the Analytical

4.1.2. Pyrrhotite Two generations of pyrrhotite occur in the massive ores: one occurs mainly among the matrix sulfides, and has straight grain boundaries and no replacement relationship with pyrite; the other generation, together with chalcopyrite and sphalerite, fills fissures in cataclastic pyrite and replaces pyrite porphyroblasts. The appearance of pyrrhotite, chalcopyrite, and sphalerite therefore represents a new phase of

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mineral growth. Pyrrhotite in the disseminated ores is anhedral and occurs among silicate minerals. 4.1.3. Chalcopyrite Chalcopyrite is the most common remobilized mineral in fissures of pyrite in the massive ores. Interestingly, where chalcopyrite, sphalerite, and pyrrhotite all occur in the fissures of pyrite in such ores, chalcopyrite is usually located in the far end of each fissure, followed by pyrrhotite, and finally by sphalerite. This indicates that the relative mobility of the three minerals increases in the sequence sphalerite–pyrrhotite– chalcopyrite, but when quartz–sulfide veins are also present, the far end of the fissure is commonly filled with quartz–sulfide veins. In addition, it is commonly observed that chalcopyrite fills biotite cleavage cracks in the ores. In the disseminated ores, chalcopyrite usually occurs in sites of relatively low pressure, such as in pressure shadows adjacent to pyrite. 4.1.4. Sphalerite In the massive ores, sphalerite occurs in the matrix sulfides, together with chalcopyrite and pyrrhotite, which fill in and replace cataclastic pyrite. To varying degrees, fine-grained, emulsion-like chalcopyrite occurs in sphalerite, and this mode of occurrence is usually referred to as “chalcopyrite disease”, indicating that the minerals have undergone metamorphic recrystallization, probably under prograde conditions. Sphalerites, distributed among silicate minerals in the disseminated ores, are mainly anhedral. 4.1.5. Electrum Gold and silver in massive sulfide ores occur mainly as electrum grains 1 to 20 μm in size. Most of the electrum occurs as inclusions in chalcopyrite, pyrite and pyrrhotite, as well as in gangue minerals. 4.2. Gangue minerals Quartz, which is the main gangue mineral in the Hongtoushan deposit, is usually rounded, occurs as inclusions in pyrite or matrix sulfides, and is locally rimmed by late-stage carbonate minerals. Quartz in the massive ores usually has 120° triple-junction grain boundaries, suggesting annealing. Biotite grains are common in the massive ores (Fig. 2B, C) and they are similar to those in phlogopite gneiss in both their meso- and microscopic characteristics (Fig. 2A) in contrast to the fine grained biotite in the biotite gneiss. Table 1 shows the compositions of mica in massive ores which are close to those in phlogopite gneiss especially in the Mg#(where Mg# = MgO/(MgO + FeO)). However, in the massive ores, most of the biotite grains are kinked (Fig. 2B) and others are fractured (Fig. 2C), and the fissures and cleavages are filled with remobilized chalcopyrite. Some biotite grains are replaced by actinolite, gahnite, and clinozoisite. These silicate minerals generally grew together as aggregates (Fig. 2D). In some samples, chlorite and garnet also occur in aggregates, with the chlorite usually occurring as the outer ring of the aggregates. Electron microprobe analyses reveal that micas in phlogopite gneiss and in the massive ore are rich in Zn, relative to the mica in biotite gneiss. Previous studies have reported gahnite in metamorphosed massive sulfide deposits (Castroviejo et al., 2011; Spry and Scott, 1986; Williams, 1983; Zaleski et al., 1991). It is generally agreed that gahnite has potential

as an exploration indicator for highly metamorphosed massive sulfide deposits (Spry and Scott, 1986). On the basis of microscopy, scanning electron microscopy (SEM), and electron probe analysis, gahnite in the massive sulfide ores of the present study normally occurs near biotite, actinolite, and sphalerite (Fig. 2D). Gahnite is not found in areas without sphalerite. 5. Interpretation of microscopic features The fabrics of sulfide ores in the Hongtoushan deposit reflect processes related to metamorphism. Original depositional textures and structures were completely obliterated as a result of both recrystallization and deformation. The various microfabrics all formed due to processes related to metamorphism and/or deformation, and are described below in terms of recrystallization and growth textures, brittle and plastic deformation textures, remobilization textures, and retrograde textures. The implications of these textures for the evolution of the ore deposit are discussed. 5.1. Recrystallization and growth textures Metablastic growth in the ores led to two main changes in mineral morphology: (1) changes in the form of minerals, producing metablastic textures; and (2) changes in the grain size of minerals. The two changes are described separately below. 5.1.1. Metablastic textures Recrystallization produces characteristic crystal shapes and intergrowths in sulfides, depending on the relative propensity of individual minerals to crystallize (Vokes, 1969). In massive ores, the general result of recrystallization is to produce fabrics in which pyrite, a mineral with high surface energy, can grow as metablasts, which contrasts with the surrounding low-surface-energy matrix sulfides, chiefly chalcopyrite, sphalerite, and pyrrhotite, which rarely develop crystal faces (Fig. 2E). Pyrite, with an average grain size of 2 mm, forms a mosaic with individual grains meeting with equal interfacial angles at triple-junction points. The triple-junction grain boundaries appear to have opened up, allowing the components of chalcopyrite to move in between the pyrite grains and commonly replace them to varying degrees. Euhedral pyrite metablasts and matrix sulfides always include rounded or ellipsoidal silicate aggregates (Fig. 2F, G); these contain syn-peak metamorphic silicate minerals such as biotite, cordierite and hornblende. The rounded or ellipsoidal textures, which are discussed further below, were formed during remobilization, and cannot occur in massive ores that have not experienced deformation and metamorphism (Vokes, 1968). The rounded or ellipsoidal quartz and plagioclase inclusions in euhedral pyrite indicate textural equilibration and suggest that pyrite metablasts recrystallised or grew during high temperature metamorphism (Vokes and Craig, 1993). 5.1.2. Growth textures Lydon (1984) found that massive ore in weakly metamorphosed deposits is usually a fine-grained mosaic of sulfide grains that increase in coarseness with increasing metamorphic grade. Studies of the size of pyrites in different deposits (Craig et al., 1998; Vokes, 1968, 1969) have found that almost all pyrite particles in high-grade metamorphic

Fig. 2. Photomicrographs of phlogopite gneisses and textures of ores of the Hongtoushan copper–zinc deposit. Mineral abbreviations: Act—actinolite; Bt—biotite; Car—carbonate; Ccp— chalcopyrite; Chl—chlorite; Ghn—gahnite; Gn—gangue mineral; Mc—microcline; Phl—phlogopite; Po—pyrrhotite; Py—pyrite; Qz—quartz; Sp—sphalerite; A) Fractured phlogopite in phlogopite gneisses (transmitted light). B) A kinked phlogopite crystal replaced by actinolite in massive ore (transmitted light with crossed nicols). C) Fractured phlogopites in massive ore. The fissures and cleavages are filled with remobilized matrix sulfides (transmitted light). D) Gahnite in the massive ore occurs near magnesium-rich biotite, actinolite, and sphalerite (transmitted light). E) Coarse grained pyrite metablasts developed euhedral or subhedral shapes. Chalcopyrite, sphalerite, and pyrrhotite distributed around pyrites, forming intergrowth relationships (Polished surface). F) Pyrite metablasts include rounded or ellipsoidal quartz (plane reflected light). G) Intragranular cracks of pyrite filled by chalcopyrite, sphalerite, and silicate minerals (plane reflected light). H) A pyrite crystal in the disseminated ore is elongated parallel to the gneissosity, and even pulled apart into several sections and chalcopyrite filled in the cracks (plane reflected light). I) Spherical quartz and silicate aggregate included in a pyrite porphyroblast. Quartz is enveloped by a rim of carbonate (plane reflected light). J) Fractures in pyrite filled and replaced by quartz–sulfide veinlets, sphalerite and pyrrhotite. The distal far end of the fissure is filled by the quartz–sulfide veins (plane reflected light).

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rocks are coarse grained, whereas those in low-grade metamorphic rocks are fine grained. The size of pyrites appears to be closely related to the grade of metamorphism, with greater size being associated with higher metamorphic grade (McClay and Ellis, 1983). Sulfide minerals in the Hongtoushan deposit are generally coarse grained, and the size of pyrite particles is particularly coarse (Fig. 2E), commonly larger than 5 mm, and occasionally reaching up to 50 mm. The size of pyrrhotite is 2–5 mm, and it can reach up to 10 mm (Gu et al., 2004a). The occurrence of coarse-grained ores in the Hongtoushan deposit indicates that the sulfides grew by recrystallization in the upper amphibolite facies. 5.2. Deformation textures 5.2.1. Brittle deformation textures Evidence of brittle deformation is well preserved in pyrite porphyroblasts, which show fracturing ranging from hairline cracks to extensive pulverization (Figs. 2F–J, 3A–D). These intragranular cracks are filled by both silicate minerals and matrix sulfides such as chalcopyrite, pyrrhotite, and sphalerite. Pyrite crystals can remain brittle to high temperatures, and they are largely resistant to deformation until brittle failure occurs. This is in stark contrast to the behavior of the matrix sulfides, which undergo ductile deformation, solid-state or chemical remobilization, and annealing under greenschist facies conditions and above (Craig et al., 1998). 5.2.2. Ductile deformation textures In the highly deformed disseminated ores of the Hongtoushan deposit, fine-to-medium elongated pyrite aggregates resemble boudins that have been stretched along the plane of the gneissosity and even pulled apart into several sections (Fig. 2H). Pyrite also shows evidence of ductile deformation when the surrounding minerals, such as quartz and garnet, are relatively resistant to deformation. This phenomenon has also been observed in the Deri ore of India (Tiwari et al., 1998). In contrast, chalcopyrite occurs in pressure shadows about pyrite in the disseminated ores (Fig. 2H), a fabric that develops because the pyrite crystals behave in a brittle fashion while the chalcopyrite deforms in a ductile fashion during deformation. 5.3. Remobilization textures Most massive sulfide ores worldwide have undergone regional metamorphism and deformation, and consequently experienced various degrees of remobilization (e.g., Archean Abitibi Belt, Canada: Sangster, 1972; Black Angel Mine, Greenland: Pedersen, 1980; Broken Hill, Australia: Barnes, 1987). In the Hongtoushan deposit, deformation has resulted in elongation, chaotic folding, the development of gneissosity, flow structures, and the ductile injection of sulfides. Many such structures are, however, the product of deformational events concurrent with the waning part of the metamorphic cycle. This is commonly indicated by an intense retrograde schistosity in wall rocks adjacent to sulfide piercement structures. 5.3.1. Durchbewegung textures In the Hongtoushan deposit, spherical and ellipsoidal silicate minerals such as plagioclase and quartz are always found as inclusions in pyrite porphyroblasts and matrix sulfides (Figs. 2F, I, 3A). In contrast, the Mg-rich biotite, which are kinked (Fig. 2B) or fractured (Fig. 2B, C), only occur in the matrix sulfides. Vokes (1963, 1968, 1969) first introduced the German term “durchbewegung” to the English literature to describe deformation in which internal rotational movements dominated. This phenomenon of “durchbewegung” has since been studied in detail (e.g., Klemd et al., 1987; Maiden et al., 1986; Marshall and Gilligan, 1989; McQueen, 1987; Vokes, 1968, 1969). Vokes noted that it was difficult to find a satisfactory English translation of durchbewegung, which he equated with a progression of processes

Table 1 Composition (%) of three kinds of mica in the Hongtoushan copper–zinc deposit. Mineral

Mica in biotite gneiss

Mica in phlogopite gneiss

Mica in massive ores

Sample

HTS-5

HTS-4

HTS-19

HTS-15

HTS-16

HTS-12

HTS-13

HTS-9

SiO2 TiO2 Al2O3 FeO MnO MgO Na2O K2O F Cl CuO V2O3 ZnO CaO Mg# Total

35.12 3.32 16.83 19.05 0.29 10.36 0.08 10.5 – – 0.04 – – – 0.49 95.59

35.59 3.51 16.57 20.37 0.27 9.82 0.07 10.58 – 0.02 – – 0.13 – 0.46 96.93

35.35 2.8 15.98 19.83 0.12 11.01 0.06 10.71 – 0.03 – – – – 0.50 95.89

37.85 1.96 16.51 5.66 0.15 21.09 0.11 11.08 0.33 – – – 0.25 – 0.87 94.99

38.03 2.23 16.8 8.83 0.2 18.62 0.14 10.87 – 0.02 0.06 0 0.23 – 0.79 96.03

37.45 1.22 16.1 14.68 0.14 15.85 0.42 9.64 – 0.02 – – 0.2 – 0.66 95.72

37.17 1.55 16.28 14.46 0.11 16.83 0.3 10.1 – – – – 0.24 – 0.68 97.04

37.45 0.28 17.17 14.72 0.14 16.15 0.34 9.82 0.11 – 0.06 – 0.2 – 0.66 96.44

involving disruption, separation, kneading, milling, and rotational movement of competent silicate minerals and competent ore minerals within an incompetent sulfide matrix. The resultant rock would include rootless fragments and rounded clasts of competent components studded throughout an incompetent matrix (Vokes, 1963, 1968, 1969). This process and resulting structures usually occur during the formation of folds and shear zones (Klemd et al., 1987; Maiden et al., 1986; Marshall and Gilligan, 1989; McQueen, 1987). In contrast to the competent silicates, most metal sulfides are incompetent; under conditions of moderate temperature and pressure, mechanical remobilization of the sulfides can occur, but in different ways according to the particular mineral involved. Whereas pyrite and arsenopyrite are deformed and remobilized through cataclasis, the remaining incompetent sulfides are deformed by plastic flow. Because they are relatively ductile (Marshall and Gilligan, 1993), most sulfides are strain partitioned during deformation, causing massive bodies of sulfide to become the focus of deformation. Like any mobile phase, these ductile deforming bodies will migrate from regions of high pressure to domains of low pressure. This is known as solid state remobilization. Subsequently, pyrite porphyroblasts and matrix sulfides recrystallize, and rootless fragments and rounded clasts of competent components are retained as isolated fragments therein. In the present study, it appears that under the influence of late metamorphism and deformation, such structures were extensively deformed, but the resistance nature of pyrite to deformation has allowed it to preserve the features of its metamorphic origin, including features that formed in the early stages of metamorphism (e.g., see McClay and Ellis, 1983). Evidence of late metamorphic remobilization is widespread in the Hongtoushan ores. Cracks in pyrite porphyroblasts, were filled and replaced by matrix sulfides that contain inclusions of spherical silicate minerals (Fig. 4). The surfaces of the cataclastic pyrite contain features that indicate the former existence of spherical silicate minerals. Such features have been observed in various deposits (Klemd et al., 1987; Maiden et al., 1986; Marshall and Gilligan, 1989; McQueen, 1987), and those in the Hongtoushan deposit may have formed by the following sequence of events: ductile sulfides again invade brittle fractures in pyrite, and break off pieces of pyrite; the sulfides then capture pyrite debris and separate the spherical silicate minerals from the cataclastic pyrite. Subsequently, the spherical silicate minerals experience another episode of kneading, milling, and rotation. Finally, during decreasing temperature and pressure, matrix sulfides recrystallize again, with the separate silicate minerals scattered among them. Fig. 4 shows that edges of pyrite grains are smooth rather than brecciated, indicating that fluid may have been involved in the sequence of events outlined above.

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Fig. 3. Photomicrographs of textures of ores of the Hongtoushan copper–zinc deposit. Mineral abbreviations are the same as Fig. 2. A) Globule quartz half included in a pyrite crystal. Fractures in pyrite was filled and replaced by remobilized chalcopyrite sphalerite and pyrrhotite (plane reflected light). B) Chalcopyrite fills fractures in pyrite, whereas at the outer edges of the pyrite grains, chalcopyrite is rare (plane reflected light). C) Remobilized chalcopyrite is located in the far end of the fissures in pyrite, followed by pyrrhotite (plane reflected light). D,E) Chalcopyrite disease textures. Chalcopyrite show fined-grained, blebs, dots, and dust-like texture in sphalerite (plane reflected light). F) Pyrrhotite replaced by pyrite in the disseminated ore (plane reflected light).

5.3.2. Replacement textures In the massive ores of the Hongtoushan deposit, textural evidence of matrix sulfide remobilization is seen in many highly fractured individual pyrite metablasts, where chalcopyrite, sphalerite, pyrrhotite, and silicate minerals, or their components, have moved from the matrix into the fractures and have replaced to varying degrees not only the fracture walls but also the outer edges of the pyrite grains (Figs. 2G, J, 3A–E). There seems to be a systematic behavior among the matrix sulfides regarding the order of mobilization with respect to the pyrite. The highly fragmented pyrite grains appear to have been “invaded” and partly replaced by some of the matrix sulfides, such as chalcopyrite, sphalerite, and pyrrhotite, apparently depending on their relative abundance in a particular volume of ore. But in all cases where matrix sulfides have filled the pyrite fractures, chalcopyrite has preceded sphalerite and pyrrhotite (Figs. 2G, 3B, C). This apparent order of remobilization corresponds with the general order of mobility of sulfide minerals, which increases in the order of pyrite–sphalerite–pyrrhotite–chalcopyrite (Vokes, 1969). Fig. 3B shows an interesting phenomenon in massive ores with

high sphalerite content, only chalcopyrite fills fractures in pyrite, whereas at the outer edges of the pyrite grains, chalcopyrite is rare. Not only have the matrix sulfides followed chalcopyrite into pyrite fractures, but in the end of the fractures, vein-like silicate minerals have also filled and replaced pyrites, together with sulfides, in the fractures (Fig. 2G, J). Regarding the vein-like silicates, the case for fluidphase mobilization is much more convincing, as that sulfides deform by ductile flow more easily and rapidly than the silicates, and so they partition the strain, leaving the silicates unaffected where abundant sulfides are present, i.e., the silicates remain as sub-spherical inclusions in the sulfides rather than becoming elongated as they would in a more typical shear fabric (Fig. 2G, J). 5.3.3. Chalcopyrite disease Chalcopyrite disease in matrix sulfides is commonly observed in the Hongtoushan deposit samples. Sphalerite, with chalcopyrite inclusions, fills fractures in cataclastic pyrite (Fig. 3D, E) and occurs as intergrowths with other matrix sulfides. Chalcopyrite occurs as blebs, dots, and dust-

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were formed substantially during peak and post-peak metamorphism, e. g. durchbewegung, metablastic textures, growth textures in massive sulfide ores and ductile deformation textures in the disseminated ores.

Fig. 4. Second stage of the formation of durchbewegung texture (photomicrograph, reflected light). Dashed circles indicate silicate minerals, and arrows show the possible direction of migration. An early-formed spherical silicate mineral was separated from fragmented pyrite and transported by remobilized sphalerite and pyrrhotite. During this process, the silicate mineral became smaller. The circle at top right indicates the original site of the silicate mineral, which migrated in the direction indicated by the arrows. Refer to Figs. 2 and 3 for explanation of mineral abbreviations.

like inclusions in sphalerite, and the amount of chalcopyrite can reach up to 10%. Barton and Bethke (1987) referred to chalcopyrite inclusions in sphalerite as “chalcopyrite disease”, and concluded that such inclusions result mostly from the replacement of the iron-bearing sphalerite via the reaction of iron with copper ions transported in hydrothermal solutions. This mechanism is supported by the results of experiments undertaken by Eldridge et al. (1988). Bortnikov et al. (1991) considered that the reaction typically involves an addition of Cu and Fe, and a loss of Zn. 5.4. Retrograde textures Retrograde metamorphism results in changing mineralogy in sulfide ores (Vokes, 1969). Such effects in the Hongtoushan ores are rather weak, and are by no means universally observed or recognized. Pyrrhotite is replaced by pyrite in the disseminated ores of Hongtoushan (Fig. 3F), and such replacement is generally considered to be the product of retrograde sulfidation (Vokes, 1969). 5.5. Discussion On the basis of our study of the compositions and microscopic textures of Hongtoushan deposit ores, it can be concluded that the deposit experienced two episodes of remobilization, corresponding to two periods of deformation and metamorphism. The specific processes involved are as follows. 5.5.1. First deformation phase The first phase of deformation occurred under medium-pressure conditions of the upper amphibolite facies. The original alteration zone was replaced by cordierite–anthophylite gneiss and phlogopite gneiss. Remobilization migration of ores occurred during the deformation, with the oreshoots forming, and an increase in copper, zinc, gold and silver grades from limbs to fold hinges. The present textures

5.5.2. Second deformation phase The second phase of deformation and metamorphism was characterized by conditions of relatively low temperature and pressure (Li et al., 1995; Zhai et al., 1984; Zhang et al., 1984). Fluids carrying the remobilized sulfides filled fissures cutting through the wall rocks and ore bodies, and formed quartz–sulfide veins (Gu et al., 2004a). As to biotite grains, those in massive ores probably have the same origin as those in phlogopite gneiss. Phlogopite gneiss is rich in Mg and K, and poor in Na, and this rock may represent the metamorphic product of chloritic and sericite rocks in the zone of alteration. Theart et al. (2010) came to a similar conclusion regarding a high-grade metamorphic terrain in South Africa. During high-grade regional metamorphism, such chloritic precursor rock types are replaced by an unusual mineral paragenesis, typically containing cordierite, phlogopite, orthoamphiboles or orthopyroxenes, and aluminium-rich minerals such as sillimanite and corundum. Relative to the unchanged phlogopite grains in phlogopite gneiss, the biotite grains in massive ores were replaced by mineral aggregates including actinolite, gahnite, clinozoisite, chlorite, biotite, and carbonate minerals and turn out to be low Mg# and rich in Zn in composition, reflecting that they were influenced by fluids after they have formed. Pyrite metablasts underwent severe cataclastic deformation, while the remobilized matrix sulfides, lubricated by fluids, were again injected into fractures in pyrite, resulting in further fracturing and even disaggregation of pyrite. The sulfides captured the pyrite fragments and separated the spherical silicate minerals from the cataclastic pyrite, after which the spherical silicate minerals experienced another episode of kneading, milling, and rotation (as shown in Fig. 4). During the process, pyrite was replaced to a certain extent, and chalcopyrite disease was formed. Finally, with decreasing temperature and pressure, matrix sulfides recrystallized again among scattered silicate minerals. 6. Remobilization mechanisms Previous authors have investigated different aspects of ore remobilization in different deposits (Bailie and Reid, 2005; Barnes, 1987; Frost et al., 2002, 2011; Mavrogenes et al., 2001; Pedersen, 1980; Sangster, 1972; Sparks and Mavrogenes, 2005). Ore remobilization is clearly important to exploration geologists, because it promotes, in many deposits, the localization of larger and sometimes richer volumes of ore material. In some deposits, ore remobilization has transformed what would otherwise be an uneconomic deposit into an economic one (Tomkins, 2007). The Hongtoushan deposit was metamorphosed under upper amphibolite facies conditions and concurrently underwent complex multi-phase deformation. This combination of higher temperatures and deformation promoted the remobilization of ore constituents. Information on remobilization at Hongtoushan should therefore assist exploration in the deposit. Early discussions of the mechanism involved in remobilization centered on two main types: solid-state and fluid-state mobilization. Based on the earlier proposals, Marshall and Gilligan (1987) subsequently classified the mechanism into two main types, chemical (the contemporary term is hydrothermal dissolution and reprecipitation; Tomkins, 2007) and, mechanical, recognizing that in many cases these two processes occur together, producing a mixed signature of remobilization. Most recently, a third mechanism of ore remobilisation has been recognized in the highly metamorphosed sulfide ore deposits; that of metamorphic sulfide melting and consequent melt mobilization (Bailie and Reid, 2005; Frost et al., 2002, 2011; Mavrogenes et al., 2001; Sparks and Mavrogenes, 2005; Tomkins, 2007; Tomkins and Mavrogenes, 2002; Tomkins et al., 2004, 2006, 2007).

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Previous studies have investigated remobilization in the Hongtoushan copper–zinc deposit (Gu et al., 2004b; Liu and Chen, 1982), but the exact nature and effects of the process are still debated. On the basis of our detailed study of Hongtoushan ore deposit, no strong evidence was found to prove that partial melting had modified the ores, yet we distinguish two mechanisms of metamorphic remobilization: mechanical and mixed hydrothermal–mechanical. 6.1. Mechanical remobilization The massive sulfide ore bodies at Hongtoushan are clearly deformed, as they have been locally thickened and thinned by folding (Fig. 1). Mechanical remobilization is important for the translocation of ore bodies; it is particularly effective in redistributing massive sulfides. And in disseminated ores it occurs only at a small scale, usually over centimeters, whereas in massive sulfides it can occur at a scale of tens of meters (Tomkins, 2007). Mechanical remobilization mechanisms occurs by both plastic and cataclastic flow (Marshall and Gilligan, 1987); both are active at the same time within a deforming, heterogeneous assembly of sulfides, because of the differing mechanical properties of each constituent sulfide. For example, when pyrite is deforming and being remobilized through cataclasis, the minerals sphalerite, pyrrhotite, and chalcopyrite may deform by plastic flow. The apparent order of migration during solid state ductile transfer corresponds with the mobility of sulfide minerals, which increases in the order of pyrite–sphalerite–pyrrhotite– chalcopyrite (Vokes, 1969). During metamorphic remobilization, the greater the mobility of a sulfide mineral, the more quickly and further it travels. Under the upper-amphibolite-facies conditions at Hongtoushan, massive sulfides in the fold limb regions experienced shear deformation, partitioning strain away from the adjacent silicates and becoming areas of compression, whereas the fold hinge acted as a dilatational zone. Therefore, sulfides migrated from the limbs via plastic and cataclastic flow into the fold hinge, causing thickening at the hinge. The grade of Cu, Zn, Au and Ag ores is higher in the hinge than in the limbs indicating that deformation was more effective in remobilizing chalcopyrite, sphalerite and electrum than other minerals. 6.2. Mixed hydrothermal–mechanical remobilization Chalcopyrite, pyrrhotite, and sphalerite are commonly observed filling and replacing cataclastic pyrite metablasts in the Hongtoushan ores, with chalcopyrite having first entered the fissures, followed by pyrrhotite, and then sphalerite. This sequence of migration reflects mineralrelated differences in the propensity for mechanical remobilization, and indicates that mechanical remobilization was still occurring during the late stages of deformation. The appearance of quartz–sulfide veinlets in the deposit together with the changed composition of biotite in ores, reflected that fluids had engaged in the remobilization in massive ores. The role of fluids co-existing with submarine exhalative ores during deformation is unclear. However, Plimer (1987) argued that fluids would lubricate the ductile and cataclastic flow of sulfides and that water may weaken sulfide structures during deformation. 7. Conclusion Our analysis of ore and mineral compositions, and ore textures in the Hongtoushan deposit yielded the following conclusions. (1) Phlogopite grains that formed during metamorphism of the upper amphibolite facies, including the transition of chlorites to phlogopites under the influence of fluids, were replaced by silicate aggregates composed of actinolite, gahnite, clinozoisite, and chlorite during upper greenschist facies retrogression.

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(2) Ore bodies in Hongtoushan underwent two periods of metamorphic remobilization. The first period was accompanied by intense deformation and formed a vertically plunging fold that defines the shape of the deposit. Sulfides migrated from the limbs through plastic and cataclastic flow into the fold hinge, causing thickening at the hinge. This process resulted in higher grades of copper– zinc–gold–silver ore in the hinge than in the limbs by mechanical remobilization, and in the formation of oreshoots. The second period was relatively weak, characterized by low temperatures and pressures. Pyrite metablasts underwent severe cataclastic deformation, while the remobilized matrix sulfides, lubricated by fluids, were injected into the fractures in pyrites and replaced the pyrites to some extent, forming chalcopyrite disease. Acknowledgments This work was funded by the Countrywide Critical Mine Backup Resources Prospecting Program (20089940). 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