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Volume 31 Issue 3
Jul.  2020
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Huanlong Hu, Shuangliang Liu, Hong-Rui Fan, Kuifeng Yang, Yabin Zuo, Yachun Cai. Structural Networks Constraints on Alteration and Mineralization Processes in the Jiaojia Gold Deposit, Jiaodong Peninsula, China. Journal of Earth Science, 2020, 31(3): 500-513. doi: 10.1007/s12583-020-1276-z
Citation: Huanlong Hu, Shuangliang Liu, Hong-Rui Fan, Kuifeng Yang, Yabin Zuo, Yachun Cai. Structural Networks Constraints on Alteration and Mineralization Processes in the Jiaojia Gold Deposit, Jiaodong Peninsula, China. Journal of Earth Science, 2020, 31(3): 500-513. doi: 10.1007/s12583-020-1276-z

Structural Networks Constraints on Alteration and Mineralization Processes in the Jiaojia Gold Deposit, Jiaodong Peninsula, China

doi: 10.1007/s12583-020-1276-z
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  • Structural network studies could give appropriate opportunities to understanding structural/hydrothermal events, transportation of ore-forming fluids and water/rock interaction process. Four structural deformation/hydrothermal events have been identified in the Jiaojia fault zone according to microtexture and deformation of quartz and feldspars. Plagioclase experienced ductile deformation period with bended polysynthetic twin stripes (> 450 ℃) in the early stage, followed by K-feldspar alteration period with ductile-brittle deformation and subgrain rotation recrystallization of quartz (380-450 ℃). Then, sericitization period occurred extensive ductile-brittle deformation (350-420 ℃) and extensive subgrain rotation recrystallization with a little bulging recrystallization in quartz. In the last, gold precipitation-related pyrite-sericite-quartz alteration was dominated by brittle deformation (300-380 ℃) and total bulging recrystallization of quartz. From the K-feldspar alteration zone and sericitization zone to pyrite-sericite- quartz alteration zone, fractal dimension values of dynamically recrystallized quartz grains increase from 1.07 and 1.24 to 1.32, the calculated paleo strain rate values of dynamically recrystallized quartz range from 10-10.7 (380 ℃)-10-9.6 (450 ℃) and 10-9.3 (350 ℃)-10-8.2 (420 ℃) to 10-9.5 (300 ℃)-10-8.0 (380 ℃), and the paleo differential stress values increase from 36.9 and 39.3, to 121.3 MPa. The increase of fractal dimension values and decrease of grain size from pyrite-sericite-quartz alteration zone and sericitization zone to K-feldspar alteration zone decreased average water/rock ratio values, which could lead to different acidity and redox conditions of ore-forming fluids and mineralization differences. Two kinds of ore- controlling fractures have been distinguished which include the gentle dip types (18°-50°) with NW (315°-355°) and SW (180°-235°) dip hosting No. I orebodies and the steep dip types (74°-90°) with NE (45°-85°) and SE (95°-165°) dip hosting No. III orebodies. These faults/fractures crosscut altered Linglong granite of footwall of the Jiaojia fault zone as rhombohedrons that promoted the connection between fractures in the K-feldspar alteration zone and fluid flow passages near the main fault face. Research results indicate No. I and No. III orebodies should be derived from the same mineralization event and belong to different orebody types in different mineralization sites under the same structural networks.
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Structural Networks Constraints on Alteration and Mineralization Processes in the Jiaojia Gold Deposit, Jiaodong Peninsula, China

doi: 10.1007/s12583-020-1276-z

Abstract: Structural network studies could give appropriate opportunities to understanding structural/hydrothermal events, transportation of ore-forming fluids and water/rock interaction process. Four structural deformation/hydrothermal events have been identified in the Jiaojia fault zone according to microtexture and deformation of quartz and feldspars. Plagioclase experienced ductile deformation period with bended polysynthetic twin stripes (> 450 ℃) in the early stage, followed by K-feldspar alteration period with ductile-brittle deformation and subgrain rotation recrystallization of quartz (380-450 ℃). Then, sericitization period occurred extensive ductile-brittle deformation (350-420 ℃) and extensive subgrain rotation recrystallization with a little bulging recrystallization in quartz. In the last, gold precipitation-related pyrite-sericite-quartz alteration was dominated by brittle deformation (300-380 ℃) and total bulging recrystallization of quartz. From the K-feldspar alteration zone and sericitization zone to pyrite-sericite- quartz alteration zone, fractal dimension values of dynamically recrystallized quartz grains increase from 1.07 and 1.24 to 1.32, the calculated paleo strain rate values of dynamically recrystallized quartz range from 10-10.7 (380 ℃)-10-9.6 (450 ℃) and 10-9.3 (350 ℃)-10-8.2 (420 ℃) to 10-9.5 (300 ℃)-10-8.0 (380 ℃), and the paleo differential stress values increase from 36.9 and 39.3, to 121.3 MPa. The increase of fractal dimension values and decrease of grain size from pyrite-sericite-quartz alteration zone and sericitization zone to K-feldspar alteration zone decreased average water/rock ratio values, which could lead to different acidity and redox conditions of ore-forming fluids and mineralization differences. Two kinds of ore- controlling fractures have been distinguished which include the gentle dip types (18°-50°) with NW (315°-355°) and SW (180°-235°) dip hosting No. I orebodies and the steep dip types (74°-90°) with NE (45°-85°) and SE (95°-165°) dip hosting No. III orebodies. These faults/fractures crosscut altered Linglong granite of footwall of the Jiaojia fault zone as rhombohedrons that promoted the connection between fractures in the K-feldspar alteration zone and fluid flow passages near the main fault face. Research results indicate No. I and No. III orebodies should be derived from the same mineralization event and belong to different orebody types in different mineralization sites under the same structural networks.

Huanlong Hu, Shuangliang Liu, Hong-Rui Fan, Kuifeng Yang, Yabin Zuo, Yachun Cai. Structural Networks Constraints on Alteration and Mineralization Processes in the Jiaojia Gold Deposit, Jiaodong Peninsula, China. Journal of Earth Science, 2020, 31(3): 500-513. doi: 10.1007/s12583-020-1276-z
Citation: Huanlong Hu, Shuangliang Liu, Hong-Rui Fan, Kuifeng Yang, Yabin Zuo, Yachun Cai. Structural Networks Constraints on Alteration and Mineralization Processes in the Jiaojia Gold Deposit, Jiaodong Peninsula, China. Journal of Earth Science, 2020, 31(3): 500-513. doi: 10.1007/s12583-020-1276-z
  • Studies of ore-controlling structural networks are a critical part for understanding structure-fluid coupling processes (Deng et al., 2019, 2000; Yang et al., 2014; Zhai et al., 2002). And uncovering transportation processes of ore-forming fluids in structural networks of ore deposits is beneficial for illuminating ore genesis and exploration. For example, steeply dipping fractures which are controlled by deep metallogenic magma result in shell-like alteration and mineralization zones (Sillitoe, 2010). Epithermal deposits are thought to be derived from mixing of evolutionary ore-forming fluids which were differentiated from deep magma with addition of meteoric water at shallow ground, and locations of alteration and mineralization zones could occur with large lateral drift relative to main fluid channels due to shallow structure and ground water flow or meteoric circle (Hedenquist and Taran, 2013; Sillitoe, 2010). Mineralization of orogenic gold deposits is usually controlled by bending or two wings of folds (Goldfarb et al., 2015; Groves et al., 1998). Hence, detailed studies on the distribution characteristics of macroscopic-microcosmic structures could help to reveal patterns of the alteration and mineralization in structural networks.

    The Jiaodong Peninsula is famous for its world-class giant gold deposits (including structure-alteration-type and quartz- vein-type) which are strictly controlled by secondary or lower level faults (Deng et al., 2019; Fan et al., 2016; Zhu et al., 2015; Goldfarb and Santosh, 2014). Geochronology studies indicate all these gold deposits formed at ~120 Ma (Feng et al., 2019; Li X H et al., 2018; Ma et al., 2017; Li L et al., 2015; and references therein). Microthermometry researches of fluid inclusions in ore-related quartz indicate ore-forming fluids in all these gold deposits show similar homogenization temperature, salinity, pressure and physicochemical evolution processes (Cai et al., 2018; Li et al., 2018; Ma et al., 2018; Wen et al., 2016; Wang et al., 2015; Mao et al., 2008; Fan et al., 2003). C-H-O-S stable isotope analyses suggest that they all share similar sources of ore-forming fluids and materials (Feng et al., 2019, 2018; Yang K F et al., 2018; Yang L Q et al., 2017; Wen et al., 2016, 2015; Deng et al., 2015; Mao et al., 2008). All these gold deposits were suggested to be formed in the same mineralizing-geodynamic circumstance, and related with Mesozoic tectonic transition in eastern China (Fan et al., 2005). Structure-alteration-type gold deposits, which provide the main gold reserves in the Jiaodong Peninsula, show the multistage alteration zonation with regular overprint and strict control by fault zones (Cai et al., 2018; Li X C et al., 2013). Exploring history of structure-alteration- mineralization and structural networks characteristics of alteration-type gold deposits could facilitate to reveal the processes and mechanisms of giant amounts of gold precipitation. The Jiaojia Gold Deposit is the prototype of structure-alteration- type gold deposits in the Jiaodong Peninsula. It contains intense and regular alteration zonation in wall rocks that recorded multistage structure-hydrothermal alteration-mineralization activities (Yu et al., 2019; Zhang et al., 2014; Song et al., 2010), which provides a chance to study complex structure-alteration- mineralization evolutionary processes and structural networks for giant gold deposits. Ding and Zhai (2000) investigated fractal characteristics of ore-controlling fractures (including orebody scale faults and microfractures in feldspar grains) and gave detailed calculations for fractal dimension of fractures in the Jiaojia Gold Deposit. The relatively large fractal dimension of fractures was thought to be beneficial for gold mineralization in network system, and the critical fractal dimension (D≈1.30) for ore- controlling fractures was suggested to account for the formation of giant gold (Ding and Zhai, 2000). However, detailed studies of macroscopic/microcosmic deformation and fracture characteristics in different alteration zones are absent, which could hinder the understanding for structural evolution/networks, water/rock interaction processes, and alteration-mineralization processes for the giant Jiaojia Gold Deposit. In this study, attitude measurements have been carried out for ore-controlling fractures, and observations and measurements have been worked on microstructures of quartz (including micro deformation observations of quartz, measurements and calculations of grain size and fractal dimension values of dynamically recrystallized quartz), which could give implications to the structural evolution/networks, water/rock interaction processes, and alteration-mineralization processes for the Jiaojia Gold Deposit.

  • The Jiaodong Peninsula is located in the southeastern margin of the North China Craton and consists of Jiaobei terrane and Sulu ultrahigh metamorphic belt (Fig. 1). The Tanlu fault zone is the western boundary of the Jiaodong Peninsula and locates on the west of the Jiaobei terrane (Fig. 1). The Jiaobei terrane is subdivided into the Jiaobei uplift in the north and the Jiaolai Basin in the south, and the Sulu ultrahigh pressure (UHP) metamorphic belt locates on the southeast of the Jiaobei terrane (Fig. 1). Precambrian metamorphic basement rocks, UHP metamorphic rocks and Mesozoic magmatic rocks are main components for the Jiaodong Peninsula which belongs to a world-class and structure- controlled gold province with hydrothermal genesis.

    Figure 1.  The regional geological map of the Jiaodong gold province (modified after Fan et al., 2003). CS-SSD fault. Cangshang-Sanshandao fault; JJ-XC fault. Jiaojia-Xincheng fault; ZP fault. Zhaoyuan-Pingdu fault.

    Regional strata in the Jiaobei uplift mainly contain Late Archean Jiaodong Group with biotite granulite, biotite gneiss and plagioamphibolite, Paleoproterozoic Jinshan Group, Paleoproterozoic to Mesoproterozoic Fenzishan Group and Neoproterozoic Penglai Group (Fig. 1). And Early Cretaceous sediments/volcanic rocks distribute widely in the Jiaolai Basin (Fig. 1).

    The NNE-NE-striking fault zones are the main faults in the region of the Jiaodong Peninsula (Fig. 1). The NS-striking Tanlu fault zone crosscutting through crust into lithospheric mantle, belongs to the first-order fault in the Jiaodong Peninsula. Secondary fault zones with NNE-NE strike in the Northwest Jiaodong (i.e., the west of Jiaobei uplift, Fig. 1) include Sanshandao- Cangshang, Jiaojia-Xincheng and Zhaoyuan-Pingdu fault zones from west to east (Fig. 1), and they all control the lower level faults. In general, secondary fault zones control large to giant gold deposits, and lower level faults control middle to small gold deposits in the Jiaodong Peninsula (Fig. 1).

    Most magmatic emplaced during the Mesozoic except for Archean TTG (trondhjemite-tonalite-granodiorite) granites in the Jiaodong Peninsula. Syn-collision granitoids emplaced in the Late Triassic (Guo et al., 2005), then calc-alkaline granitoids which were derived from remelting of thickened lower crust at ~160 Ma (Yang et al., 2012; Guo et al., 2005) intruded widely in the Jiaodong Peninsula such as Linglong granite in the west and part of Kunyushan granitoids in the east (Fig. 1). After that, high-K calc-alkaline granites that were derived from mixing of lower crust-derived melts with the involvement of mantle components during 130-126 Ma (Jiang et al., 2016; Yang et al., 2012) intruded into Sanshandao, Shangzhuang, Beijie, Congjia and Guojialing regions. Then, alkali granites were formed by mixing of melting of Neoarchean- Paleoproterozoic crust with limited crust-mantle interaction during 120-110 Ma (Li et al., 2019), and mafic dikes being related with partial melting of lithospheric mantle emplaced during 121-87 Ma (Ma et al., 2014; Cai et al., 2013).

    The Zhaoyuan-Laizhou, Penglai-Qixia and Muping-Rushan gold belts have been divided in the Jiaodong Peninsula (Fan et al., 2003; Fig. 1). The Jiaojia Gold Deposit is located in the Zhaoyuan-Laizhou gold belt of the Northwest Jiaodong.

  • Late Archean Jiaodong Group and Late Jurassic Linglong biotite granite are exposed in the Jiaojia gold camp (Fig. 2). The NE-striking (~30°) and NW-dipping Jiaojia-Xincheng fault zone comprises Archean metamorphic rocks in the hanging wall and Linglong biotite granite in the footwall (Figs. 2, 3). Altetration zones and gold orebodies are hosted in altered Linglong granite of the footwall in this fault zone (Fig. 3).

    Figure 2.  Geologic map of the Jiaojia gold field (modified after Song et al., 2010).

    Figure 3.  The vertical geologic cross sections of orebodies of the line 112 at the Jiaojia Gold Deposit (modified after Song et al., 2010)

    Intense and regular wall rock alteration zones containing orebodies distribute along the Jiaojia fault zone, i.e., K-feldspar alteration zone, sericitization zone and pyrite-sericite-quartz alteration zone occur progressively from fresh Linglong granite to the main fault face (Figs. 3, 4). Observations in mine indicate that sericitization zone is overprinted by pyrite-sericite-quartz alteration zone, and overprints on K-feldspar alteration zone (Figs. 4, 5a-5d). Thin pyrite veinlets and stockworks crosscut in sericitization and pyrite-sericite-quartz alteration zones (Figs. 5a, 6c-6d). Three kinds of mineralization veins were distinguished in K- feldspar alteration zone. The first type is thin pyrite veins/ stockworks that contain a little quartz (i.e., quartz-pyrite veins/ stockworks) and distributes widely in the K-feldspar alteration zone. The second one is that thick white quartz veins contain a little pyrite (pyrite-white quartz veins) and show consistent attitude with the main fault face, which were crosscut by late thin quartz-pyrite veins (Fig. 5c) The last type is composed of thick white quartz veins containing a little pyrite (pyrite-white quartz veins) which possessed consistent strike and steep or inverse dip relative to the main fault face and were also crosscut by late thin quartz-pyrite veins (Fig. 5d).

    Figure 4.  The sketch map of alteration and mineralization in the ~ -600 m tunnel of the Jiaojia Gold Deposit. A-C. Pyrite-sericite-quartz, sericitization, and K-feldspar alteration zones and sample locations.

    Figure 5.  Observations of alteration and mineralization characteristics of the Jiaojia Gold Deposit. (a) Boundary between sericitization and pyrite-sericite- quartz alteration zones. (b) K-feldspar alteration zone was crosscut by pyrite stockworks, and K-feldspar was replaced by sericite. (c) K-feldspar alteration zone was crosscut by thick pyrite-white quartz veins which showed the same attitude as that of the main fault face, and these thick pyrite-white quartz veins were crosscut by late quartz-pyrite veins. (d) K-feldspar alteration zone was crosscut by thick pyrite-white quartz veins which possessed steep and inverse dip relative to the main fault face, and these thick pyrite-white quartz veins were also crosscut by late quartz-pyrite veins.

    Figure 6.  Samples of wall rocks and ores from the Jiaojia Gold Deposit. (a) The Late Jurassic Linglong biotite granite. (b) K-feldspar alteration rocks contain lots of newborn K-feldspar, quartz in these rocks occurs with extensive enlargement, and pyrite stockworks crosscut these alteration rocks. (c) Sericitization rocks were crosscut by late quartz-pyrite stockworks. (d) Pyrite-sericite-quartz alteration rocks contain amounts of disseminated pyrite and micro pyrite stockworks. Bt. Biotite; Kf. K-feldspar; Pl. plagioclase; Py. pyrite; Qtz. quartz; Ser. sericite.

    Pink K-feldspar alteration zone is the widest zone and overprints on fresh Linglong biotite granite (Figs. 3, 5b-5d, 6a-6b, 7a-7b). Plagioclase was replaced extensively by new K-feldspar (Figs. 6b, 7b) and new quartz enlarged extensively (Fig. 6b). Quartz and feldspars show normal extinction characteristics and crystal form in fresh Linglong biotite granite (Fig. 7a). In K-feldspar alteration zone, smooth and bended ductile deformation in polysynthetic twin stripes could be observed in residual plagioclase (Fig. 7b). This kind of residual plagioclase was mainly replaced by newborn K-feldspar along cleavage plane and then experienced brittle deformation (Fig. 7b). Polysynthetic twin stripes of small crushed plagioclase grain (profile) occurred with anticlockwise rotation relative to polysynthetic twin stripes of large parental plagioclase grain, which was consistent with stress direction indicated by bended stripes of adjacent quartz with ductile deformation (Fig. 7b). Quartz grains with ductile deformation experienced extensive dynamic recrystallization, which formed smaller polygonal quartz subgrains that kept similar form with parental quartz grains without obvious interfingering sutures (i.e., subgrain rotation dynamic recrystallization, SGR, see Fig. 1b in Stipp et al., 2002) (Fig. 7b). Quartz with ductile deformation stripes was crosscut by late pyrite-sericite-quartz veins, which led to further bulging dynamic recrystallization (BLG, i.e., bulging and recrystallized quartz grains were formed along boundaries of parental quartz grains, along microcracks in interior of quartz, or as total bulges and recrystallization in grains, see Fig. 1a in Stipp et al., 2002) of quartz to multiply smaller recrystallized quartz subgrains along two sides of microfractures (Figs. 7b, 7d).

    Figure 7.  Altered mineral assemblages, dynamic recrystallization of quartz and brittle-ductile deformation observations in different wall rocks and ores. (a) Undeformed quartz and plagioclase in Linglong biotite granite. (b) Microtextures in K-feldspar alteration zone include: feldspars and quartz experienced multistage deformation; residual plagioclase experienced ductile deformation with smooth and bended polysynthetic twin stripes; residual plagioclase was mainly replaced by newborn K-feldspar along cleavage plane and experienced brittle deformation; polysynthetic twin stripes of small crushed plagioclase grain occurred with anticlockwise rotation relative to those of large parental plagioclase grain, which was consistent with stress direction indicated by bended stripes of adjacent quartz that experienced ductile deformation along shear direction showed by arrow; quartz grains experienced extensive dynamic recrystallization, which formed smaller polygonal quartz subgrains that kept similar form with parental quartz grains (i.e., subgrain rotation dynamic recrystallization, SGR); quartz with ductile deformation stripes was crosscut by late pyrite-sericite-quartz veins, which led to further bulging dynamic recrystallization (BLG) to form smaller recrystallized quartz subgrains along two sides of microfractures. (c) Microtextures in sericitization zone include: feldspars were replaced by sericite completely, leaving behind pseudomorphse; quartz experienced intense ductile deformation and extensive subgrain rotation dynamic recrystallization, resulting in smaller polygonal grains; local bulging bulgily dynamically recrystallized quartz grains occurred around margin of parental quartz with subgrain rotation dynamic recrystallization. (d) Microtextures near quart-pyrite veins in K-feldspar alteration zone include: quartz-pyrite veins crosscut K-feldspar alteration zone; quartz precipitating from ore-forming fluids in pyrite stockworks shows euhedral to subhedral crystal form without any dynamic recrystallization; K-feldspar alteration rocks experienced intense pyrite-sericite-quartz alteration beside pyrite stockworks, and quartz in this pyrite-sericite-quartz alteration zone also experienced extensive bulging recrystallization that led to smaller polygonal subgrains. (e) Microtextures in pyrite-sericite-quartz alteration zone include: feldspars were replaced by sericite completely without the texture of previous minerals being kept; all quartz grains experienced intense bulging recrystallization and transformed into smaller polygonal subgrain; some profile of previous quartz could be observed locally. (f) The latest quartz-calcite veinlets crosscut previous pyrite-sericite-quartz alteration rocks, and euhedral quartz and calcite inlaid with each other without any dynamic recrystallization. Bt. Biotite; Cal. calcite; Kf. K-feldspar; Pl. plagioclase; Py. pyrite; Qtz. quartz; Ser. sericite; SGR. subgrain rotation dynamic recrystallization; BLG. bulging dynamic recrystallization.

    Grey sericitization rocks mainly consist of quartz and sericite with rare disseminated pyrite, which are characterized by crushed mineral grains with uneven size and locally crosscut by quartz-pyrite veins (Figs. 6c, 7c). Feldspars were replaced by sericite completely, leaving behind pseudomorphs (Fig. 7c). Quartz experienced intense ductile deformation along with extensive subgrain rotation dynamic recrystallization, resulting in smaller polygonal grains that were self-similar to their parental quartz grains (Fig. 7c). Local bulging recrystallization was also observed around margin of parental quartz (Fig. 7c).

    Dark gray pyrite-sericite-quartz alteration zone with intensely cataclastic structure and homogeneous fine grains distributes near the main fault face with gouge width of ~5 cm (Figs. 4a, 6d, 7d-7f). Feldspars were replaced by sericite completely without texture of previous minerals being kept (Figs. 7d-7f). Euhedral to subhedral pyrite containing gold inclusions occurs as dissemination or stockwork styles with quartz and sericite (Figs. 6d, 7d-7f, 8a-8d). All quartz grains experienced intense bulging dynamic recrystallization and transformed into smaller polygonal subgrains (Figs. 7e-7f). In K-feldspar alteration zone, quartz grains in intense pyrite-sericite-quartz alteration zone beside quartz-pyrite veins also experienced extensive bulging recrystallization which led to smaller polygonal subgrains (Fig. 7d). Quartz precipitating from ore-forming fluids in pyrite veins shows euhedral to subhedral crystal form without any dynamic recrystallization (Fig. 7d). Pyrite-sericite-quartz alteration rocks and quartz-pyrite veins were crosscut by polymetallic veins/stockworks (electrum, chalcopyrite, galena, sphalerite and siderite) (Figs. 8c-8d). The latest quartz- calcite veinlets crosscut all previous alteration rocks and sulfide veins, and euhedral quartz and calcite inlaid with each other. Four mineralization stages are distinguished through all above macroscopical and microscopical observations. Stage I (quartz-K- feldspar-sericite stage) is characterized by occurrence of K-feldspar alteration, sericitization, quartz enlargement and precipitation of white quartz veins. Stage II (quartz-gold-pyrite stage) precipitated amounts of pyrite with first crystallization of gold, and stage III (quartz-gold-polymetallic stage) formed polymetallic veins with enrichment of gold. Stage IV (quartz-calcite stage) marked the end of hydrothermal activities for gold mineralization. All minerals with corresponding alteration and mineralization stages have been compiled as mineral paragenesis figure in Fig. 9.

    Figure 8.  Observations of metal mineral assemblages in ores. (a) Disseminated pyrite in pyrite-sericite-quartz alteration rocks. (b) Quartz-pyrite veins and gold inclusion being contained in pyrite. (c) Quartz-pyrite veins were crosscut by late polymetallic veins containing electrum, chalcopyrite and galena. (d) Quartz-pyrite veins were crosscut by late polymetallic veins containing electrum, chalcopyrite, galena, sphalerite and siderite, and siderite was crosscut by late quartz-calcite veins. Cal. Calcite; Cpy. chalcopyrite; El. electrum; Gn. galena; Py. pyrite; Qtz. quartz; Sd. siderite; Ser. sericite; Sp. sphalerite.

    Figure 9.  The mineral paragenesis for the Jiaojia Gold Deposit.

  • Sampling for this study was carried out along ~30 m long profile at depth of 600 m under surface. The sampling profile spans pyrite-sericite-quartz, sericitization and K-feldspar alteration zones and sample locations are marked as A-C on Fig. 4. Attitude measurement for ore-controlling fractures was carried out in mining roadways at depth of 300, 450, 498 and 600 m under surface. Profile width for attitude measurement at depth of 450 and 498 m under surface spans between 100 and 200 m.

    The fractal dimension is an important parameter in geometry and could be used to quantitatively describe random shape and phenomenon of self-similarity for studied objects (Kruhl and Nega, 1996). Two methods of closed polygon and area-perimeter are applicable to calculate fractal dimension of dynamically recrystallized quartz grains. Area-perimeter method is chosen for fractal dimension calculation of dynamically recrystallized quartz because area and perimeter of quartz grains are the most direct presentation through optical microscope and area-perimeter method is also executed easily. The area-perimeter method for mineral fractal dimension is derived from logarithm comparison between perimeter of analyzed irregular mineral grain and diameter of circle which has equal area to the analyzed irregular mineral grain (e.g., Mamtani, 2010). The fractal dimension of dynamically recrystallized quartz grains could be calculated through area-perimeter method in this study. Firstly, area (S) and perimeter (P) of dynamically recrystallized quartz in alteration rocks could be measured on optical photomicrographs. Then, diameter (d, grain size) of circle that has equal area to analyzed dynamically recrystallized quartz grain could be calculated. Lastly, all logarithms of perimeter (log (P)) and diameter (log (d)) are plotted in log (P)-log (d) diagram and solve the slope of all these data by least square method. The slope is the fractal dimension of dynamically recrystallized quartz grains in the same deformation event. In the Jiaojia Gold Deposit, more than 40 dynamically recrystallized quartz grains were selected and vectorized and measured for their area and perimeter in every alteration zone. Then fractal dimension values were acquired for corresponding alteration zones by area-perimeter method.

  • Ore-controlling fractures are divided into two types according to dip angle (Table S1, Fig. 10). Gentle dip types show dip angle of 18º-50º with NW dip (315º-355º), SW dip (180º-235º) and a small part of NE dip (0º-45º) fractures. Steep dip types have dip angle range of 74º-90º with SE dip (95º-165º) and NE dip (45º-85º) fractures.

    Figure 10.  Stereonet of attitude measurement (strike and dip) of Jiaojia fault face, No. I and No. III orebodies (the lower hemisphere for stereographic projection).

  • All measured area, perimeter, calculated diameter (grain size, d) and logarithms for dynamically recrystallized quartz grains have been summarized in Table S2. Forty-two dynamically recrystallized quartz grains chosen from K-feldspar alteration zone show a grain size range of 8-118 μm with a concentration of 10.3-48.4 μm, and present a calculated fractal dimension value of 1.07 (correlation coefficient R2=0.94, Figs. 11a-11b). Fifty-two dynamically recrystallized quartz grains selected from sericitization zone show a range of 8-90 μm for grain size with a concentration of 8.2-39.7 μm, and give a calculated fractal dimension value of 1.24 (correlation coefficient R2=0.95, Figs. 11c-11d). And fifty dynamically recrystallized quartz grains taken from pyrite-sericite-quartz alteration zone show a grain size range of 4-11 μm with a concentration of 4.1-9.6 μm, and possess a calculated fractal dimension value of 1.32 (correlation coefficient R2=0.88, Figs. 11e-11f).

    Figure 11.  The frequency distribution of particle size of recrystallized quartz and log-log plots of perimeter-diameter of dynamically recrystallized quartz in different alteration zones in the Jiaojia Gold Deposit. The letter d represents grain size of dynamically recrystallized quartz and the letter P represents perimeter of dynamically recrystallized quartz.

  • Smooth and bended ductile deformation characteristics of polysynthetic twin stripes of residual plagioclase in K-feldspar alteration zone (Fig. 7b) indicate that the Jiaojia fault zone experienced ductile deformation at the least temperature of 450 ℃ in the early (Scholz, 1988).

    During K-feldspar alteration period, K-feldspar replaced plagioclase along cleavage plane, quartz enlarged, and minerals deformation prevailed when hydrothermal fluids interacted with wall rocks (Figs. 6b, 7b). Previous microtextures of primary quartz crystals were reset during hydrothermal activities and quartz enlargement processes in K-feldspar alteration period. Therefore, ductile deformation of quartz should be formed during or after quartz enlargement processes. Residual plagioclase grains with ductile deformation experienced overprint of brittle events during or after K-feldspar alteration period (Fig. 7b). The polysynthetic twin stripes of small crushed plagioclase grain occurred with anticlockwise rotation relative to those of large parental plagioclase grain, which was consistent with stress direction indicated by bended stripes of adjacent quartz with ductile deformation (Fig. 7b). This phenomenon implies that a brittle-ductile deformation event occurred during ductile deformation of enlarged quartz. Considering the lack of sericitization alteration and ductile deformation of K-feldspar during this period, and fluid activities coupling with structure events, it is possible that the brittle-ductile tectonic event occurred approximatively simultaneously with or a little bit later than K-feldspar alteration. Quartz grains experienced extensive subgrain rotation dynamic recrystallization during the brittle- ductile deformation event, leading to produce smaller polygonal quartz subgrains (Fig. 7b). Quartz grains with bulging or grain boundary migration recrystallization were not observed in K-feldspar alteration. In general, quartz grains experience grain boundary migration recrystallization at the least temperature of 480 ℃; they transform into complete subgrain rotation dynamic recrystallization at the temperature range of 420-480 ℃; and coexistence of subgrain rotation and bulging dynamic recrystallization will occur at the temperature range of 380-420 ℃ (Stipp et al., 2002). Hence, temperature of K-feldspar alteration period and approximatively simultaneous brittle-ductile event is suggested to be in the range of 380-450 ℃ basing on dynamically recrystallization of quartz grains, deformation types of feldspar and quartz, and possible coupling relationship of fluid-structure activities during K-feldspar alteration period.

    Quartz in sericitization zone experienced intense ductile deformation and subgrain rotation dynamic recrystallization. Quartz grains recrystallized to smaller polygonal subgrains (Fig. 7c). Coexistence of these subgrain rotation and bulging dynamic recrystallization (Fig. 7c) indicates that hydrothermal fluid temperature was 380-420 ℃ (Stipp et al., 2002). On the other hand, hydrothermal fluid temperature should be higher than 350 ℃ which corresponded to transformation temperature of brittle-ductile deformation of quartz (Scholz, 1988). Therefore, hydrohteramal fluid temperature for sericitization is constrained in the range of 350-420 ℃ (Stipp et al., 2002).

    The latest brittle deformation event overprinted previous brittle-ductile deformation alteration rocks with wide and intense gold-bearing ore-forming fluid activities, leading to extensive pyrite-sericite-quartz alteration rocks (Figs. 5a-5d, 6c-6d, 7d-7f, 8a-8d). In K-feldspar alteration zone, brittle fractures crosscutting feldspars and quartz with ductile deformation, fine quartz subgrain derived from bulging dynamic recrystallization and simultaneous precipitation of sericite and pyrite were observed at both sides of brittle fractures (Fig. 7b). Previously existed quartz grains suffered intense bulging dynamic recrystallization beside quartz-pyrite veins in K-feldspar alteration zone (Fig. 7d). Almost all quartz experienced intense bulging dynamic recrystallization (Fig. 7e) with simultaneous precipitation of disseminated pyrite (Fig. 6d) in pyrite-sericite- quartz alteration zone. Bulging dynamic recrystallization of quartz grains indicates that temperature of ore-forming fluids and brittle tectonic activities was in the temperature range of 300-380 ℃ (Stipp et al., 2002), which was consistent with ore-forming temperature obtained from microthermometry studies of fluid inclusions in mineralization quartz veins (270-360 ℃, unpublished data).

    All above researches indicate the Jiaojia fault zone experienced complex tectonic activity history evolving from ductile deformation before K-feldspar alteration period, brittle- ductile deformation during K-feldspar alteration and sericitization period, to brittle deformation during main gold-precipitating period. Fluid temperature decreased from early to late period, while dynamic recrystallization types of quartz grains transferred from middle to high temperature type (subgrain rotation) to middle to low temperature type (bulge). Ore-forming fluid activities were closely related to brittle deformation events and bulging dynamic recrystallization of quartz grains.

  • Fractal dimension of quartz (D), deformation temperature T (K) and strain rate ε′ (s-1) follow the next function according to Takahashi et al. (1998)

    where parameters ψ=9.34×10-2, ρ=6.44×102. This function could be applied to approximate calculation of strain rate of quartz in the alteration zones of the Jiaojia Gold Deposit combining with above constrained temperature and fractal dimension values (Li Z S et al., 2013). Corresponding temperature range, grain size, fractal dimension values and calculated strain rate values for above three alteration zones are compiled in Table S3 and Fig. 12. In general, normal strain rate range for natural quartzite/quartz reef is 10-15-10-12 s-1 (Mamtani, 2010; Twiss and Moores, 2007; Scholz, 1988). However, strain rate ranges of quartz in above three alteration zones are higher than general strain rate range of natural quartzite/quartz reef for 1-7 orders of magnitude (Fig. 12). The large deviation of strain rate of quartz in alteration zones was likely related with intense tectonic activities during alteration and mineralization period (Scholz, 1988). It's worth noting that the largest deviation of strain rate, which could be 7 orders of magnitude higher than general strain rate range of natural quartzite/ quartz reef, occurred during pyrite-sericite-quartz alteration period. Therefore, the most intense tectonic activities were coupled with extensive brittle deformation and ore-forming fluid activities. It is proposed that ore-forming fluids of the Jiaojia Gold Deposit were transferred from depth to mineralization sites corresponding to the most intense tectonic activity period.

    Figure 12.  Ranges of paleo strain rate and temperature recorded by dynamically recrystallized quartz grains in K-feldspar alteration zone, sericitization zone and pyrite-sericite-quartz alteration zone, and the natural strain-rate range of rock approximately varies between 10-15 and 10-12 s-1 (Mamtani, 2010; Twiss and Moores, 2007; Scholz, 1988). Py. Pyrite; Qtz. quartz; Ser. sericite; SGR. subgrain rotation dynamic recrystallization; BLG. bulging dynamic recrystallization.

    Paleo differential stress can be estimated by manometer derived from grain size of dynamically recrystallized quartz

    where σ is paleo differential stress with unit of MPa, d is grain size of dynamically recrystallized quartz with unit of μm, and b is experiment parameter (equal to 3 631) with uint of μm·MPa-R (Stipp and Tullis, 2003). R is also experiment parameter with the range of -1.47- -0.59, and it is taken as -1.26 considering its derivation from higher experiment precision by Stipp and Tullis (2003). Paleo differential stress results are 36.9, 39.3 and 121.3 MPa according to average grain size of dynamically recrystallized quartz in K-feldspar alteration zone, sericitizationzone and pyrite- sericite-quartz alteration zone, respectively.

    Water/rock interaction process plays an important role on precipitation of metallogenic elements and alteration (Xu et al., 2016; Reed et al., 2013). Quartz gives important effect to water/ rock interaction process considering it is one of the main rock-forming minerals in every alteration zone in the Jiaojia Gold Deposit. The larger fractal dimension for quartz means the more complex boundary shape, which indicates the larger specific surface area of quartz in the space. The less grain size of quartz also means the larger specific surface area in the space. Hence, the amount of fractures could vary exponentially when fractal dimension or grain size of quartz changes. In view of these two factors, it is sure that intense dynamic recrystallization of quartz grains in host rocks will result in substantial increase of microfractures, which enhances permeability of host rock and increases average water/rock ratio value. The smallest grain size and the largest fractal dimension of quartz indicate that pyrite-sericite-quartz alteration zone possesses the highest permeability and keeps the largest average water/rock ratio value. The permeability and average water/rock ratio values should decrease from pyrite-sericite- quartz alteration zone, sericitization zone to K-feldspar alteration zone given above variation trend of fractal dimension and grain size of dynamically recrystallized quartz. Previous studies suggest that water/rock ratio values have an important effect on acidity and oxygen fugacity of ore-forming fluids, which leads to differences of mineralization and alteration (Xu et al., 2016; Reed et al., 2013; Wilson et al., 2007; Reed and Palandri, 2006). Therefore, dynamic recrystallization intensity and grain size of quartz could control differences of mineralization and alteration through controlling average water/rock ratio values.

    The distinguished differences of mineralization styles exist between K-feldspar alteration and pyrite-sericite-quartz alteration zones in the Jiaojia Gold Deposit (Figs. 5a-5d). Paleo differential stress in pyrite-sericite-quartz alteration zone was the largest, which likely induced intense dynamic recrystallization and formation of smaller size quartz grains. As a result, these rocks experienced rolling friction to form cataclastic flow, leading to wide formation of micro stockwork-type and disseminated pyrite. The paleo differential stress in K-feldspar alteration zone was lower than that in pyrite-sericite-quartz alteration zone. In this case, the more weakly dynamic recrystallization and larger grain size of quartz occurred in K-feldspar alteration zone. On the other hand, the lower paleo differential stress and existence of feldspar should result in formation of brittle fractures and could not give rise to cataclastic flow further. Hence, ore-forming fluids dispersing in K-feldspar alteration zone could gather into these brittle fractures due to negative pressure, leading to form thick to thin quartz- pyrite veins and stockworks which distributed in K-feldspar alteration zone widely.

  • Attitudes of two types fractures measured in this study is consistent with those of No. I and No. III orebodies of the Jiaojia Gold Deposit summarized by Song et al. (2010). No. I orebodies are approximately parallel to the main fault face and occur as stratiform and lenticular shapes which are located in altered Linglong granite of footwall of the Jiaojia fault zone (Figs. 3, 5c). Pyrite-sericite-quartz alteration zone contains disseminated type orebodies and K-feldspar alteration zone possesses thin quartz-pyrite vein/stockwork type orebodies. Ore-controlling fractures of these orebodies dip to NW and SW with strike ranging in a deviation of 55°-60° between these two types orebodies (Fig. 10). No. III orebodies show inverse dip and consistent strike relative to main fault face, which are located in K-feldspar alteration zone and include thick to thin quartz-pyrite veins/stockworks (Figs. 3, 5d). Ore-controlling fractures of these orebodies show NE and SE dip with strike varying in a deviation of ~70º between these two types orebodies (Fig. 10). All these results of strike and dip indicate that altered Linglong granite was crosscut as rhombohedrons with different size by veins/stockworks (Figs. 13a-13c). Inclined downward axes of these heterogeneous rhombohedrons are basically parallel to the vertical projection line of dip direction onto fault face, and fracture faces around rhombohedrons show different attitude to the main fault face (Figs. 13a, 13b). These characteristics of structural networks are suggested to be derived from local extension in the footwall of Jiaojia fault zone during regional transpression (Deng et al., 2019; Charles et al., 2011). Fracture faces around rhombohedrons could be connected with each other to form connected networks in the space according to observations and measurements of ore-controlling fractures, extensive dynamic recrystallization of quartz and continuity of these fractures in the space. The attitude differences between gentle dip fractures and main fault face (Figs. 5a-5c, 10) indicate that they could intersect with each other, leading to connection of fractures between pyrite-sericite- quartz alteration zone near fault face (or main ore-forming fluid channel) and K-feldspar alteration zone (Figs. 13a, 13b). This connection could facilitate inclined upward and near horizontal transportation of ore-forming fluids from main fluid channel to K-feldspar alteration zone when ore-forming fluids were transported from depths to mineralization sites (Zhu et al., 2015; Li X C et al., 2013; Fan et al., 2003, Fig. 13c). Upright upward distension of steep dip fractures could also facilitate connection of fractures between main ore-forming fluid channel and K-feldspar alteration zone, resulting in horizontal and top-down transportation of ore-forming fluids from main ore-forming fluid channel to K-feldspar alteration zone (Fig. 13c). In addition, gentle dip-type and steep dip-type fractures intersect with each other that enhances connection of structural networks in altered Linglong granite of footwall of the Jiaojia fault zone and is beneficial for flow of ore-forming fluids among those three alteration zones (Figs. 13a-13c). According to above attitude measurement of ore-controlling fractures and studies of grain size, fractal dimension and dynamic recrystallization of quartz, connected fractures in structural networks disperse from thick quartz-pyrite veins/stockworks in K-feldspar alteration zone to dense and thin to micro pyrite stockworks with gradual increase of average water/rock ratio values (Figs. 5a-5d, 13c). In structural networks, mineralization veins in No. I orebodies share the same characteristics as those in No. III, i.e., they all contain thick quartz-white quartz veins that are crosscut by late quartz-pyrite veins along interior of quartz-white quartz veins (Figs. 5c, 5d). And it is about several meters to hundreds of meters from disseminated orebodies near main fault face to orebodies in K- feldspar alteration zone. Disseminated orebodies in pyrite-sericite- quartz alteration zone also show close relationship with vein- to stockwork-type orebodies in K-feldspar alteration zone considering connection of fractures in the structural network space. Therefore, No. I and No. III orebodies could be derived from the same mineralization event and belonged to different orebody types in different mineralization sites under the same structural networks.

    Figure 13.  The style of ore-controlling structure networks in footwall of the Jiaojia fault zone and the sketch map of average water/rock ratio with relatively high and low comparisons in different alteration zones. (a) Micro unit of structural rhombohedron in altered Linglong granite of footwall of the Jiaojia fault zone, and every face represents brittle fractures. (b) Horizontal cross-section for fault face, orebodies, alteration zones, structural rhombohedrons and brittle stockworks. (c) Vertical cross-section for fault face, alteration zones, structural rhombohedrons and brittle stockworks; different color brightness represents different average water/rock ratio values; and flow directions of ore-forming fluids are also marked in this schematic diagram; Av. w/r. average water/rock ratio values.

    In summary, characteristics of macro and micro structures in structural networks indicate that ore-forming fluids mainly flowed and concentrated near the main fault face to precipitate amounts of pyrite, sericite, quartz with gold and other polymetallic sulfides due to the highest permeability and the largest average water/rock ratio values. In this case, primary physicochemical characteristics (e.g., pH, fO2 of ore-forming fluids could be kept well without large effect from wall rocks. However, the average water/rock ration values or the amount of ore-forming fluids that flowed into sericitization and K-feldspar alteration zones should be less than that in pyrite-sericite-quartz alteration zone. Therefore, mineralization should be weaker in sericitization and K-feldspar alteration zones. Also, primary physicochemical characteristics of ore- forming fluids might be buffered/changed by wall rocks due to the lowest average water/rock ratio value in K-feldspar alteration zone, which could lead to differences in mineralization between K-feldspar alteration and pyrite-sericite-quartz alteration zones. All above transportation processes of ore-forming fluids in structural networks are summarized in Fig. 13.

  • (1) Dynamic recrystallization types of quartz grains transferred from subgrain rotation recrystallization in K-feldspar alteration zone, subgrain rotation recrystallization with a little bulging recrystallization in sericitization zone, to total bulging recrystallization in pyrite-sericite-quartz alteration zone.

    (2) The Jiaojia fault zone experienced complex tectonic activity history evolving from ductile deformation before K-feldspar alteration period (ductile deformation for plagioclase, > 450 ℃), brittle-ductile deformation during K-feldspar alteration (380-450 ℃) and sericitization (350-420 ℃) period, to brittle deformation during pyrite-sericite-quartz alteration period (300-380 ℃).

    (3) From K-feldspar alteration zone and sericitization zone to pyrite-sericite-quartz alteration zone: fractal dimension values of dynamically recrystallized quartz grains increase from 1.07 and 1.24 to 1.32; the calculated paleo strain rate values range from 10-10.7 (380 ℃)-10-9.6 (450 ℃) and 10-9.3 (350 ℃)-10-8.2 (420 ℃) to 10-9.5 (300 ℃)-10-8.0 (380 ℃); and paleo differential stress values increase from 36.9 and 39.3 to 121.3 MPa.

    (4) Gold-bearing ore-forming fluid activities were closely related with brittle deformation event, complete bulging dynamic recrystallization of quartz grains, and the largest paleo strain rate and differential stress.

    (5) Increase of fractal dimension values and decrease of grain size from pyrite-sericite-quartz alteration zone and sericitization zone to K-feldspar alteration zone resulted in decrease of average water/rock ratio values, which could lead to different physicochemical conditions of ore-forming fluids and mineralization/ alteration differences.

    (6) Ore-forming fluids could flow along dispersed gentle and steep fractures (inclined upward, near horizontal and top-down transportation) that crosscut footwall of the Jiaojia fault as heterogeneous rhombohedrons; and No. I and No. III orebodies in these fractures are proposed to be derived from the same mineralization event and belong to different orebody types in different mineralization sites under the same structural networks.

    (7) Intensities of paleo differential stress and fractal dimension of quartz are important factors that control different occurrences of dissemination-type and vein-/stockwork-type orebodies.

  • We are indebted to the local geologists of the Jiaojia Gold Deposit for their sampling helps. We also give sincere thanks to Rong-Xin Zhao from Shandong Gold Group Co., Ltd for his kindly helps of detailed introduction of mine geology materials and attentive arrangement during sampling. Two anonymous reviewers are thanked for their constructive and valuable comments which greatly contributed to the improvement of the manuscript. This study was financially supported by the National Key Research and Development Program (No. 2016YFC0600105) and the National Natural Science Foundation of China (No. 41672094). The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1276-z.

    Electronic Supplementary Materials: Supplementary materials (Tables S1-S3) are available in the online version of this article at https://doi.org/10.1007/s12583-020-1276-z.

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