Sediment-hosted micro-disseminated gold mineralization constrained by basin paleo-topographic highs in the Youjiang basin, South China

Journal of Asian Earth Sciences 20 (2002) 517±533

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Sediment-hosted micro-disseminated gold mineralization constrained by basin paleo-topographic highs in the Youjiang basin, South China Jianming Liu a,*, Jie Ye a, Hanlong Ying a, Jiajun Liu b, Minghua Zheng c, Xuexiang Gu c a

Research Center for Mineral Resources Exploration, Institute of Geology and Geophysics CAS, P.O. Box 9701, 100101 Beijing, People's Republic of China b Institute of Geochemistry CAS, 550002 Guiyang, People's Republic of China c Chengdu University of Technology, 610059 Chengdu, People's Republic of China Received 9 May 2000; revised 20 April 2001; accepted 23 May 2001

Abstract The Youjiang basin is a Devonian±Triassic rift basin on the southern margin of the Yangtze Craton in South China. Strong syndepositional faulting de®ned the basin-and-range style paleo-topography that further developed into isolated carbonate platforms surrounded by siliciclastic ®lled depressions. Finally, thick Triassic siliciclastic deposits covered the platforms completely. In the Youjiang basin, numerous sediment-hosted, micro-disseminated gold (SMG) deposits occur mainly in Permian±Triassic chert and siliciclastic rocks. SMG ores are often auriferous sedimentary rocks with relatively low sul®de contents and moderate to weak alteration. Similar to Carlin-type gold ores in North America, SMG ores in the Youjiang basin are characterized by low-temperature mineral assemblages of pyrite, arsenopyrite, realgar, stibnite, cinnabar, marcasite, chalcedony and carbonate. Most of the SMG deposits are remarkably distributed around the carbonate platforms. Accordingly, there are platform-proximal and platform-distal SMG deposits. Platform-proximal SMG deposits often occur in the facies transition zone between the underlying platform carbonate rocks and the overlying siliciclastic rocks with an unconformity (often a paleo-karst surface) in between. In the ores and hostrocks there are abundant synsedimentary±syndiagenetic fabrics such as lamination, convolute bedding, slump texture, soft-sediment deformation etc. indicating submarine hydrothermal deposition and syndepositional faulting. Numerous ¯uid-escape and liquefaction fabrics imply strong ¯uid migration during sediment basin evolution. Such large-scale geological and fabric evidence implies that SMG ores were formed during basin evolution, probably in connection with basinal ¯uids. It is well known that basinal ¯uids (especially sediment-sourced ¯uids) will migrate generally (1) upwards, (2) towards basin margins or basin topographic highs, (3) and from thicker towards thinner deposits during basin evolution. The isolated carbonate platform (as a basin paleo-high) and related syndepositional fault system, together with the unconformity-related facies succession, may have controlled the migration pathway of ore-forming basinal ¯uids and subsequently determined the location of SMG deposits in the Youjiang basin. Unlike Carlin-type gold deposits, SMG mineralization in the Youjiang basin may represent an integral aspect of the dynamic evolution of extensional basins along divergent continental margins. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Sediment-hosted, micro-disseminated gold; Basin paleo-high; Syndepositional faulting; Basinal ¯uid migration; The Youjiang basin, South China

1. Introduction Over the last two decades, over 120 sediment-hosted, micro-disseminated gold (SMG) occurrences were discovered in four basins in South China (Fig. 1). At present, SMG deposits constitute a signi®cant portion (about 20%) of Chinese total gold reserves (except placer gold) (Li et al., 1999), and are very promising for the future. SMG deposits were ®rst identi®ed in the Youjiang basin, and then in the Songpan, South Qinling, and Xiangguiyue basins in South China. These basins are remarkably all Devonian±Triassic rift basins superimposed upon the continental margin of the * Corresponding author. Fax: 186-10-6488-9849. E-mail address: [email protected] (J. Liu).

Yangtze Craton (Fig. 1) and are related to the rifting event in response to the opening and evolution of Paleo-Tethys (Huang and Chen, 1987; Liu et al., 1993; Chen, 1994; Pan et al., 1997). Our knowledge of Chinese SMG deposits comes mostly from the Youjiang basin, because of its large number of SMG (up to 80) occurrences and relatively minor post-basin modi®cation of rocks and ores compared to other basins. Since 1985, we carried out a systematic investigation of the geological, fabric, and geochemical features of more than 40 Chinese SMG occurrences, ®rst in the Songpan basin, later in the Youjiang basin, and recently in the South Qinling and Xiangguiyue basins (Fig. 1). Our research has revealed a close relationship between ore formation and sedimentary basin evolution. In particular,

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J. Liu et al. / Journal of Asian Earth Sciences 20 (2002) 517±533

Fig. 1. Paleo-tectonic scheme of South China during the Devonian showing the four SMG-hosting rift basins around the Yangtze Craton, as well as the location of Figs. 2 and 3. A, B, C and D ˆ the Youjiang, Xiangguiyue, Songpan and South Qinling basins respectively. (Compiled after Chen, 1994; Zeng et al., 1993a,b; Liu et al., 1993; Chen and Zeng, 1990). The small inset in the upper right corner is a sketch map of China showing the location of Fig. 1 and the plate tectonic framework of China (after Chen, 1994). The small inset in the lower right corner is a paleo-tectonic and paleo-geographic scheme of the Youjiang basin during the Devonian showing the growth faults and related deep-water sediments along the faults (after Chen and Zeng, 1990).

the distribution of SMG occurrences seems to be constrained by basin paleo-topographic highs and related syndepositional faulting. Micro-disseminated gold deposits are known as Carlintype gold in North America. Many researchers regarded Chinese SMG as Carlin-type (e.g. Cunninghan et al., 1988; Ashley et al., 1991; Guo et al., 1992; He et al., 1993; Liu, 1994), and suggested a post-basinal, epigenetic hydrothermal process involving heated and circulating meteoric water in ore-forming process. For a complete genetic discussion on hydrothermal ore deposits, one has to know (1) the origin of metals and ¯uids (where they came from and how), (2) factors controlling ¯uid migration, and (3) parameters controlling metal ®xation from the ¯uids (where and how the ores were deposited). In this paper, we present large-scale geological and fabric evidence, and discuss possible constraints on ¯uid migration related to SMG deposits in the Youjiang basin. The hydrothermal

ore deposits are only found in areas affected by ore-forming ¯uids. 2. The SMG-hosting Youjiang basin Paleo-Tethys was an enormous Archipelagic seaway located between Gondwana and Laurasia during Devonian±Triassic times with numerous micro-plates, rifted blocks, islands and seamounts. The Yangtze Craton in South China was one such micro-plate located in the eastern part of Paleo-Tethys (e.g. Pan et al., 1997; Yin et al., 1999). In response to the opening of Paleo-Tethys, a widespread rifting event occurred around the Yangtze Craton, which produced all four SMG-hosting basins in Fig. 1, including the Youjiang basin (Huang and Chen, 1987; Liu et al., 1993; Chen, 1994). The Youjiang basin contains Devonian to Triassic

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Fig. 2. Simpli®ed geological map of the Youjiang region, showing the SMG distribution and the association of SMG deposits with isolated carbonate platforms. All of the 35 SMG deposits in Table 1 are presented by their names, and all the related carbonate domes (platforms) are located by number: (1) Nibao; (2) Getang; (3) Laizishan; (4) Mashan; (5) Xiongwu; (6) Anran; (7) Banqi; (8) Leye; (9) Tian-e; (10) Longlin; (11) Longhuo; (12) Lingyun: (13) Gaolong; (14) Longtian; (15) Lingma. (The geological map was compiled mainly after Guizhou Bureau of Geology and Mineral Resources, 1987; Bureau of Geology and Mineral Resources of Guangxi Zhuang Autonomous Region, 1985).

sedimentary rocks (Fig. 2). The Devonian rifting event in the Youjiang region began with a series of NW- and NEstriking extensional faults (growth faults), probably in response to local mantle doming that resulted in a complex basin-and-range style topography (graben and horst structures) (Zeng et al., 1993a,b; Liu et al., 1993) (see Fig. 1, particularly the lower-right inset in Fig. 1). Two types of sediments were deposited: (1) shallow-water carbonates on a submarine horst (platform); (2) deep-water mudstone, sandstone, micrite, and chert in the graben (depression). With the progressive development of extensional tectonics, the depressions enlarged, while the platforms subsided and gradually became isolated. The carbonate platforms were constrained by growth faults and surrounded by second order detrital basins (Figs. 2 and 3) (Wang et al., 1994a;

Feng et al., 1994b). The basin-and-range topography occurred in the central basin undergoing major subsidence, which was surrounded by a narrow slope and vast epicontinental carbonate platforms further craton-ward (Fig. 3a). South China experienced maximum ¯ooding during the Early Permian with widespread shallow-water carbonate deposition covering almost the entire Yangtze Craton as shown in Fig. 3a (Wang et al., 1994a; Feng et al., 1994b). A regional unconformity developed between the Lower\Upper Permian in connection with an orogenic event principally around the southeastern basin margin (Fig. 3b). A second rifting phase during the Late Permian induced a new stage of basin evolution where siliciclastic deposits dominated and volcanic activity increased. Extensive ¯ood basalts (the so-called `Emeishan basalt') erupted at the

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Fig. 3. (a) Lithofacies paleogeography of the Early Permian in the Youjiang basin and its peripheries. There are two types of carbonate platforms. Small isolated carbonate platforms occur in the central Youjiang basin undergoing major subsidence while the vast epicontinental carbonate platforms surround the central basin. Note that the location of Fig. 2 is consistent with the central basin, and there is a narrow slope between the central basin and the epicontinental platforms. (after Feng et al., 1994b). (b) Lithofacies paleogeography of the Middle Triassic in the Youjiang basin and its peripheries. Note the widespread clastic deposition while most of the former isolated carbonate platforms have disappeared (overlain by clastics). (after Feng et al., 1994a).

beginning of the Late Permian in the western Yangtze Craton. During Early to Middle Triassic time, intermediate to acidic volcanic materials accumulated along the southern part of the Youjiang basin. In response to renewed tectonic subsidence, many welldeveloped isolated carbonate platforms appeared. As basin paleo-topographical highs, they were quite sensitive to sealevel change and often experienced episodic subaerial exposure (during lowstand of sea-level) that resulted in karst dissolution unconformities (Fig. 5a and b). We identify a platform burial event where a carbonate platform (with or without a karst dissolution unconformity surface) is overlain by siliciclastic sediments in response to sea-level rise and/or tectonic subsidence related to syndepositional faulting of the platform-marginal faults. A burial sequence refers to the deepening-upward sequence directly overlying the platform, namely, the lower part of the onlapping sequence (Fig. 6a). SMG ores are often located in or above such burial sequences (see Figs. 4 and 6). The related facies transition between the underlying shallow-water platform carbonates and the onlapping siliciclastic deposits is analogous to a type I SB (Sequence Boundary). The Lingyun dome, for example, is a large carbonate platform with a number of SMG occurrences along its margins and ¯anks (Fig. 2, Fig. 4e, and no 15, 17, 18, 20, 34 in Table 1). Carboniferous and Permian carbonate rocks make up the dome core surrounded by Triassic deep-water siliciclastic sediments. There are three depositional discontinuities between the Lower\Upper Permian, Upper Permian\Lower Triassic, and Lower\Middle Triassic sequences with the former two being paleo-karst surfaces. The Lingyun dome is well known for its syndepositional slumping and faulting along its ¯anks (Wang et al., 1990; Guo et al., 1992).

As a result of the Paleo-Tethyan closure during the Middle Triassic, the Youjiang basin evolved into a foreland basin in response to uplift around the western and southeastern margins. A thick Middle Triassic sequence of interbedded mud/sandstone turbidite (up to several thousand meters thick) accumulated widely and overlapped most of the former isolated carbonate platforms (Fig. 3b). Late Triassic termination of deposition in the Youjiang basin was accompanied with compressional deformation. Many of the former growth faults were inverted and reactivated to reverse faults to varying degrees. Two types of folds developed within the two depositional facies. Tight and steep folds formed within rocks of the depression facies, while broad and gentle folds occur within rocks of the platform facies. Strong deformation (including folding, fracturing and faulting) occurred along the contact zone between the two facies. Thus the above-mentioned ore-hosting burial sequence and related unconformities were often strongly modi®ed during basin inversion. There are generally two groups of regional faults in the Youjiang region, NW- and NE-striking faults (Figs. 2 and 1). Many were regarded as long-lived faults that controlled the basin paleo-topography, basin-®lling geometry and subsequent post-basinal deformation during basin inversion. Small-scale faults often developed around the platform margins (platform-marginal faults). Around such platforms syndepositional slumping, soft-sediment deformation, and debris ¯ows developed in response to syndepositional faulting (Chen and Zeng, 1990; Wang et al., 1990; Liu et al., 1993; Feng et al., 1994a,b; Chen et al., 2000). The study of sequence stratigraphy revealed two depositional mega-cycles, corresponding to the two rifting events (Early Devonian and Late Permian, respectively), with the

J. Liu et al. / Journal of Asian Earth Sciences 20 (2002) 517±533

above-mentioned regional unconformity between the Lower\Upper Permian as type I SB (Sequence Boundary) (Wang et al., 1994a; Chen et al., 2000). In accordance with the two depositional facies (platform and depression), two SB groups were observed: (1) type I SB in the form of a regional unconformity or paleo-karst surface with platform facies, as well as a scour erosional surface or gravity-¯ow layer on the platform ¯anks; (2) type II SB in the form of a facies transition zone or turbidity-current scour surface within the graben facies (Tian et al., 2000). The former group includes the above-mentioned karst unconformity and is often associated with SMG mineralization. Chen et al. (2000) and Tian et al. (2000) identi®ed more than 12 SB's from Upper Devonian to Middle Triassic sequences in the Youjiang basin. There is no evidence of magmatic intrusive rocks in or near the SMG ore districts in most cases. A possible genetic connection between SMG mineralization and post-basinal magmatic processes cannot be demonstrated.

3. Geological and fabric features of SMG deposits in the Youjiang basin 3.1. Distribution of SMG occurrences In the Youjiang basin, SMG occurrences are highly concentrated in the central basin undergoing major tectonic subsidence, where strong basin topographic relief and welldeveloped isolated carbonate platforms were present, and relatively thick siliciclastic materials accumulated (compare Figs. 2 and 3). Some SMG deposits occur along or near the continental slope between the central basin and the vast epicontinental carbonate platforms (Fig. 3). Few SMG occurrences, however, were found in the epicontinental platforms. The SMG occurrences are often distributed around the isolated carbonate platforms (Figs. 2 and 4). These platforms resulted from extensional faulting that constrained the evolution of the rifted Youjiang basin and acted as basin topographic highs affecting the distribution of the depositional facies. Although they have different shapes and variable sizes from ,3 km (e.g. the Gaolong dome in Fig. 4 and the Banqi dome in Table 1) to .20 km (e.g. the Lingyun dome in Fig. 2) in diameter, they show a similar litho-stratigraphic con®guration. Older carbonate rocks (Lower Permian, Carboniferous and sometimes Devonian or Upper Permian in age) make up the platform core that is surrounded and overlain by younger (mainly Triassic and Upper Permian) detrital rocks (a burial sequence). In between, an unconformity often occurs with beautiful paleo-karsti®cation as in the case of the Getang and Gaolong dome (Fig. 4a and b, Fig. 5a and b). In some cases, there may be two or three unconformities on the same platform (e.g. the Laizishan, Longhuo and Lingyun domes in Table 1), but usually only one of them is associated with SMG

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ores. In the underlying carbonate sequence, reef complexes along platform margins, and fore-reef slump breccias down the platform slopes (often in response to syndepositional faulting) were widely observed (Fig. 5d and e). Between the overlying detrital sequence and the underlying carbonate rocks, there is often a chert interval characterized by welldeveloped breccias, underwater erosion (scour-and-®ll) and banded structure (Fig. 5c). SMG ores occur only above the unconformity in the cherty interval and in the lower part of the detrital sequence, seldom in the carbonate rocks beneath the unconformity. It is obvious that the carbonate platform is one of the primary controls in determining the location of SMG mineralization. Two end types of SMG mineralization can therefore be identi®ed, namely platform-proximal and platform-distal. The former is always situated directly on or near carbonate platforms and the latter can be found either near or at some distance from the platforms. As described later, these two types distinguish themselves in many respects. For our discussion, we summarized in Table 1 the most important features of 22 platform-proximal and 13 platform-distal SMG occurrences in the Youjiang basin. However, the situation is much more complex. In Table 1, only the few SMG deposits with ` p' belong to typical examples of the two end types. All the others more or less show transitional features between the two end types and thus might be called `transitional type' (Fig. 6b and c). 3.2. Ore-hosting sequences and stratigraphic positions of the auriferous horizons As shown in Table 1, most of the SMG deposits occur in the Upper Permian or Lower±Middle Triassic rocks. Although auriferous horizons are always overlain by siliciclastic rocks, they consist of different rock types and show a different relationship to the underlying carbonate platform and related karst unconformity. Accordingly, there are in general four types of ore-hosting litho-stratigraphic sequences as shown in Fig. 6. Apart from the two end members for typical platform-proximal and platform-distal SMG deposits, respectively (Fig. 6a and d), there are two transitional sequence types re¯ecting the whole transitional span in between (Fig. 6b and c). Five platform-proximal SMG deposits are shown in Fig. 4. For typical cases (the Getang and Gaolong deposits in Fig. 4a and b), auriferous chert directly overlies a paleokarst surface on top of a carbonate platform (Fig. 6a). The lowest part of the auriferous chert section in the Getang deposit is rich in rounded clasts (pebbles) and red Fe-hydroxides (limonite) (Fig. 5a) that decrease upward while pyrite replaces limonite. Black shale with coal intercalates then appears. This indicates a normal transgressive sequence (lower part of a TST, Transgressive System Tract). The typical auriferous burial sequence includes the chert section and overlying siliciclastic section (Fig. 6a). For several years, investigators were puzzled because oxidized ore (with limonite) was found in the deeper zone and unaltered

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ore (more refractory with pyrite and organic materials) near the modern surface. In some platform-proximal SMG deposits, ore-horizons are not located directly upon the karst unconformity. Instead, there is a section of marl and/ or silty mudstone located between the ore-horizon and carbonate platform, as shown in Fig. 6b (see also Fig. 4d and e). Chen et al. (2000) and Tian et al. (2000) identi®ed more than 12 unconformities over the entire Devonian to Triassic succession. Therefore, several ore-hosting horizons also exist for platform-proximal SMG with the Upper Permian and Middle Triassic intervals being the most popular (Table 1). In a few typical cases of platform-distal SMG, the auriferous horizon is part of a very thick turbidite sequence corresponding to a part of a Highstand Sequence Tract (HST) or TST, and shows no direct relationship with carbonate platforms and unconformities (Fig. 6d). Often, however, ore-horizons were located in a transitional section between a thick siliciclastic sequence and an underlying sequence of inter-bedded marl, lime/dolostone and silt/ mudstone that in turn onlap a carbonate platform (Fig. 6c), just like the Zimudang deposit (Fig. 7). Ore-hosting rocks are mainly Lower±Middle Triassic (partly Upper Permian) ®ne-siliciclastic rocks like siltstone and mudstone, often with high carbonate (both dolomite and calcite) contents, as well as marl and silty dolomite. Auriferous chert is seldom observed (Table 1). In general, the immediate host rocks could be classi®ed into two main groups. (1) Chert and various cherty/quartz breccias in a standard platform-proximal SMG as shown by Fig. 6a, that directly overlies a karst unconformity on a carbonate platform and represents the lowest part of a burial sequence. (2) Fine siliciclastic calcareous and carbonaceous rocks, including silt/mudstone, marl, and silty dolomite that occur stratigraphically higher than the cherty unit. From standard platform-proximal to standard platform-distal (from Fig. 6a±d), the ore-bearing rock types change from chert dominated to siliciclastic dominated and the orebodies change from dominantly stratiform to discordant forms. The ore-hosting horizons, in turn, take higher stratigraphic positions. 3.3. Characteristics of SMG ores and orebodies In the SMG ore districts, ore-controlled faults or fracture zones can be classi®ed into four groups based on their relationship to the carbonate platform and bedding. (1) Platform marginal faults that acted as growth faults (normal faults) at

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the beginning but were often reactivated and inverted into reverse-thrust faults during basin inversion as for example in the Banqi and Gaolong domes (Fig. 4b). (2) Interlayered faults that developed along the interface between rocks of different rheology (e.g. mud/sand, mud/chert, ore-layer/ sand-layer etc.), for example in the Zimudang (Fig. 7) and Longhuo deposits. (3) Reversed-slip faults that strike parallel to the fold axes and bedding, but have different dip angles than the bedding, as for example in the Jinya, Yata and Zimudang deposits (Figs. 4e and 7). (4) Transverse faults that crosscut the bedding and fold axes, often postdating and crosscutting SMG ores, as for example in the Lannigou and Langquan deposits (Fig. 4d). Some transverse faults host small but rich orebodies near the present surface, probably due to reactivation of pre-existing ores. Sometimes a reversed-slip fault near the surface gradually changes into an interlayered fault at depth, as in the case of the Zimudang deposit (Fig. 7). Two types of orebodies were identi®ed: (1) concordant stratiform auriferous layers and lenses, and (2) discordant orebodies like mineralized fracture zones, stockwork and veins. In many cases, both types co-exist in the same deposit, and discordant orebodies occur at higher stratigraphic positions as for example in the Zimudang deposit (Fig. 7). Stratiform orebodies clearly exhibit the same type of folding and other deformational features as the nonmineralized surrounding sedimentary rocks. SMG orebodies show very different sizes, usually with a stretching length of 30±500 m, a lateral extent of 20±300 m, and a thickness of 1±30 m. Stratiform orebodies are usually more continuous than discordant ones. SMG ores exhibit low-grade feature with an average grade between 2 and 6 ppm Au. SMG ores are often auriferous sul®de-bearing rocks, as usually described `look like rock' (Li and Perters, 1998). An orebody is part of the auriferous sequence or auriferous fracture zone, therefore SMG ores are predominantly composed of rock-forming minerals with one to several percent so-called `hydrothermal' minerals, including pyrite, arsenopyrite, stibnite, realgar, cinnabar, marcasite, as well as quartz and carbonates. Base-metal sul®de minerals are rare or absent in most deposits. Gold is invisible (submicron size) in SMG ores. There are mainly two oretypes, siliceous (cherty) and siliciclastic, corresponding to the two groups of ore-bearing rocks. The siliciclastic ores have pyrite and arsenopyrite as typical sul®des while stibnite is the dominant sul®de in the siliceous ores. The auriferous rocks are often fractured and faulted to varying

Fig. 4. Geological maps and crosssections of ®ve platform-proximal SMG deposits. Stratigraphic position of auriferous horizons and related karst unconformities are shown within the stratigraphic legends. Stratigraphic labels are the same as in Table 1. The location of the geological crosssection is indicated by a line `fÐ Ð Ðf'. All ®ve SMG deposits and their related platforms (called `dome' in the maps) can be found in Table 1 and in Fig. 2. (a) The Getang deposit on the Getang dome. Note the auriferous burial sequence with a karst unconformity in the stratigraphic legends. (b) The Gaolong deposit on the Gaolong dome. Note the auriferous burial sequence with a karst unconformity in the stratigraphic legends. Note also the platform marginal faults and the change of rock attitudes around the dome. (c) The Longtian dome and related SMG mineralization. (d) and (e) The Langquan deposit on the Leye dome and the Jinya deposit on the Lingyun dome respectively. Note that there are two unconformities. The auriferous horizon is not located on the ®rst unconformities, but has a higher stratigraphic position. (References are the same as in Table 1).

Jinya/Lingyun Donglan/Tian-e Louluo/Lingyun Mingshan/Lingyun Gaolong p/Gaolong Siling/Lingyun Longtian p/Longtian Nashen p/Lingma

T2/T1/P2b/P2a//P1 T2/T1/P2 T2/T1/P2b/P2a//P1 T2 T2 T2b/T2a//T1//P2 T2b/T2a//T1//P2 T2b/T2a//T1//P2 T2b/T2a/P2 T2b/T2a/T1 T2 T2b/T2a/T1/P2b //P2a T2c/T2b/T2a/T1/P2

T1/P2/P2a//basalt //P1 T1/P2/P2a//P1 T1/P2/P2a//P1 T1//P2/P2a//P1 T2b/T2a/T1//P2 T1/P2/P1/C T2/T1//P2 T2b/T2a//P2/P1 T2b/T2a/T1//P2/P1 T2b/T2a/T1//P2 T2/T1b//T1a//P2 T2/T1b/T1a//P2/P1 P/C2/C1//D T2/T1/P2b/P2a//P1/C2/C1b/ C1a//D3 T2c/T2b/T2a//T1//P2//P1 T2b/T2a/T1//P2 T2b/T2a//P1 1 C T2c/T2b/T2a//P1 T2b/T2a//P2 1 P1 T2b/T2a//P T2b/T2a//P1 T/P//C 1 D

Stratigraphy with uncomformity b

c c c d d c c c d d d c d

b a b b a a a a

a a a b b b a a a, b a a b a b

clast, clast, clast, clast clast clast clast clast clast clast clast, marl, clast

clast chert, clast clast chert, chert, chert, chert,

chert clast

marl marl marl

clast clast clast clast

clast

chert, clast chert, clast chert, clast clast chert, clast clast chert, clast clast clast, chert clast, chert chert, clast clast clast clast, chert

Sequence type c Hostrock d

1 2

2

2

2, 1 2 2 2 2 2 2 2 2 2, 1 2 2, 1 2

2, 1 1 ? 1 1 1 1, 2 1

1 1 1 1 1, 1 1, 1 2, 1, 1 1 1 1

clast, v,f clast, v clast, v clast, v, f clast, v, f clast, v, f clast, v, f clast, v, f v, clast, f clast clast, f, v clast, f clast, f, v

bulk,v si, clast clast, si clast, v si, clast si, clast si, clast si, clast f, liq, v v, liq, f v, liq, f Lam, liq, f, diag Lam, liq, f, diag f, diag, v f, diag, v f, diag, v f, v lay, f brec, lay, f brec, lay f, brec,

lam, f lam, bre lam, bre lam, v, bre bre, lam, lam, lay, brec brec, lay lam, lay, brec

si, clast bre si, clast bre, lam, si, clast bre Clast lam, v clast, si, bre, lam, f clast f si, clast, f lam, lay, f clast, f lam, brec, lay clast, si, brec, lay clast, si lam, brec, f clast, si, bre, lam, clast lay, lam clast, si brec, lay clast, si, lam, diag

Orebody e Ore-type f Fabrics g

carb, si 2 carb, si 2 carb, si 2 si, carb Si carb, si 2 carb, si 2 si, carb si, decarb si 2 si, arg Si si, arg

arg 2, si 2 si 1 si si, carb, arg si 1, f 2, si 1, arg 1 si 1 si

si 1 si 1, ¯u 2, si 1 si 2, arg 2 si 1 si, carb si 1 si, carb, arg si, arg si 1 si, arg 2 si, arg si, arg si 2, arg 2

Alteration h

apy, clay, qz, clay qz, clay apy, real, clay, qz, dol stb, apy, qz, ch hem, qz, ch, clay stb, ch, qz, clay qz, ch

py, apy, real, carb, clay py, apy, real, cin, carb, clay py, apy, real, carb, clay py, apy, real, stb apy, py, stb py, apy, real, stb py, apy, real, stb py, apy, real, stb py, stb, hem, apy, qz, clay py py, qz, clay py, stb, apy py, hem, qz, clay

py, py, py, py, py, py, py, py,

py, apy, stb,qz, ch, ¯ Qz, py, apy, stb, ch, ¯ py, apy, qz, stb, ch, ¯ py, apy, clay, qz, dol py, stb, real, qz, clay py, clay, cc, qz py, stb, ch, qz, clay py, hem, qz, carb, clay py, qz, ch, cc py, apy, stb, qz, clay py, stb, ch, clay, qz py, qz, clay py, qz, clay py, apy, clay, qz

Mineral assemblage i

Au±As Au±As±Hg Au±As Au±As±Sb Au±As Au±As±Sb Au±As±Sb Au±As±Sb Au±Sb±As Au Au Au±As±Sb Au

Au±As Au±Si Au±Si Au±As Au±Sb±Si Au Au±Sb Au

Au±Sb±Si Au±Sb±Si Au±Sb±Si Au±As Au±Sb±As Au Au±Sb Au Au Au±Sb± (As)-Si Au±Sb±Si Au Au Au±As

Element assemblage j

2 2 2 2 2 2 2 2 2 2 2 ?

1 ? 1 1 1 ? 1 1

1 1 ? 1 ? ? 1 1 1 1 ? ? 1 1

1, 3, 4 1, 2 1, 2 1, 2 2 1, 4 1, 2 1, 2 2 1, 2 2 2 2

1, 4, 8, 9 2, 10 2, 8, 9 1, 8, 9, 11 1, 4, 8 2 1, 2 1, 2

1, 2 1, 3, 4 2 1, 5, 6 2 1, 2, 5, 2 2 2 1, 2, 5 1, 2 2 2 1, 2, 7

Paleokarst k References l

a Deposit/related carbonate dome. All the 35 deposits and their related domes were labeled in Fig. 2 by their name and number respectively. Those SMG deposits with ` p' represent the typical end type of platform-proximal or platformdistal SMG, respectively, and the others are transitional SMG occurrences in between. b SMG-hosting stratigraphy and ore-occurring stratigraphical position: Stratigraphical labels in the table, in all the ®gures and sometimes in texts are as conventional: D ˆ the Devonian; C1 and C2 ˆ the Lower and Upper Carboniferous; P1 and P2 ˆ the Lower and Upper Permian; P2a ˆ the lowest part of the Upper Permian; T1 and T2 ˆ the Lower and Middle Triassic; T2a, T2b, T2c, to T2d ˆ sequential upwards sub-division of the Middle Triassic. `/' ˆ continuous depositional contact; `//' ˆ unconformity (type I SB and/or karst unconformity), directly under ore body or nearby. Ore-hosting horizon was shown by overstriking. c Four types of ore-hosting sequences as shown in Fig. 6 and as described in the related text. d Directly ore-bearing rock: chert ˆ chert and cherty breccias; clast ˆ siliciclastic rocks; marl ˆ marl and silty dolomite. e Attitudes of orebodies in relationship to bedding: ` 1 ' means concordant stratiform and ` 2 ' discordant. f Main ore types: `si'-siliceous ore; `clast'-siliciclastic ore; `v'-stockwork and vein type ore; `f'-fractured ore. g Main fabrics: bre ˆ breccias; liq ˆ liqifaction; lam ˆ lamination; lay ˆ layer; d ˆ diagenetic; v ˆ stockwork and vein; f ˆ fractured. h Alterations: si ˆ silici®cation; carb ˆ carbonatization, mainly calcitization, dolomitization and ankeritization; arg ˆ argillization; f ˆ ¯uoritization; decarb ˆ decarbonitization; ` 1 ' ˆ strong, ` 2 ' ˆ weak. i Mineral assemblage: apy ˆ arsenopyrite; cc ˆ calcite; ch ˆ chalcedony; cin ˆ cinnabar; dol ˆ dolomite; ¯ ˆ ¯uorite; hem ˆ hematite; py ˆ pyrite; qz ˆ quartz; real ˆ realgar; sid ˆ siderite; stb ˆ stibnite. j Paleo-karst surface on carbonate platforms: ` 1 ' ˆ observed; `?' ˆ unknown; ` 2 ' ˆ no. k Reference: 1. Personal investigation; 2. Unpublished local exploration reports; 3. He et al., 1993; 4. Liu, 1994; 5. Wang et al., 1994b; 6. Dong, 1997; 7. Zhang et al., 1996; 8. Guo et al., 1992; 9. Wang et al., 1990; 10. Zhang, 1998; 11. Wang, 1998. (The SMG deposits presented in Figs. 4 and 7 have the same references as here in Table 1).

Platform-distal type 23 Zimudang/Huijiabao 24 Xiangbahe/Huijiabao 25 Shanchahe/Huijiabao 26 Yata p/27 Baidi p/28 Lannigou/Laizishan 29 Lintan/Laizishan 30 Niluo/Laizishan 31 Lianglong/Leye 32 Baile/Tian-e & Fengshan 33 Mahao p/34 Moutun/Lingyun 35 Mazhuang/Leye

15 16 17 18 19 20 21 22

Platform-proximal type 1 Nibao/Nibao 2 Getang p/Getang 3 Xiongwu/Xiongwu 4 Tangxinzhai p/Laizishan 5 Daguan/Mashan 6 Yangyou/Laizishan 7 Longfeng /Leye 8 Xiangbo/Anran 9 Zhebao/Anran 10 Banqi p/Baqi 11 Langquan/Leye 12 Yanpeng/Leye 13 Zhe-ai/Longlin 14 Longhuo p/Longhuo

No. Deposit/carbonate dome a

Table 1 Summary of important features of 35 SMG deposits in the Youjiang basin

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degrees. In the case of strong fracturing, there are two subordinate ore-types: (1) Stockwork ore refers to realgar± stibnite±quartz±carbonate veins (usually 0.2±5 cm thick) and related breccias (typical open-space ®lling). (2) Fractured ore develops along fracture zones in siliciclastic rocks, with strong schistosity and other deformation. Pyrite and arsenopyrite are the most abundant sul®des, both in siliciclastic ores and in non-mineralized hostrocks, either in the form of ®ne concordant laminae and disseminated grains in the rock matrix (Fig. 8a and b), or as lenses, concretions, irregular aggregates and discordant veins. Finegrained stibnite with minor pyrite occurs in siliceous and fractured ores, while coarse stibnite crystals are often associated with realgar in stockwork ores or in some reworked ores. Realgar is often observed together with carbonate and quartz in the form of stockwork and breccia cement. Besides the strong chalcedonization associated with siliceous ores, there are usually moderate to weak alterations with the other ore types, including silici®cation, argillization (mainly sericite- and kaolinite-forming), as well as carbonatization (calcitization, dolomitization, and ankeritization) mainly in the form of carbonate veins and stockwork. Usually the more strongly deformed the orehosting rocks, the more strongly altered they were, especially with regard to argillization. The low-temperature mineral and alteration assemblages of SMG ores described above are consistent with the observed low homogenization temperatures (150±2408C) of ¯uid inclusions in quartz and carbonates in SMG ores (e.g. Liu, 1994). 3.4. Fabric features of SMG ores and related hostrocks In both siliciclastic and siliceous ores, there are abundant laminae and beds (0.1±30 mm thick) composed of sul®des (pyrite and arsenopyrite), quartz and carbonate, from monomineralic to multi-mineralic, with a concordant attitude to bedding (Fig. 8a and b). Fig. 8a shows a concordant depositional succession, from bottom to top: (1) calcite-quartzpyrite layer composed of about ten ®ne laminae with internal lamina bending; (2) silt layer; (3) ®ne calcite±pyrite layer; (4) silt layer with disseminated ®ne pyrite decreasing upwards; (5) and (6) clay/silt layer. A remarkable submarine erosion pit occurs in layer 2 (see the inset in Fig. 8a). As shown in Fig. 8c, these laminae/layers are often folded together with the intercalated silt/mud layers. Many soft deformational textures in the form of soft-sediment bending (Fig. 8c), slump folding (Fig. 8d), sand pillows (Fig. 8e) and brecciation were observed. In the Jinya deposit, two slumping events were identi®ed and most of the ore bodies occur in the ®rst slump zone (Fig. 4e), where mud¯ows with sand-pillows (Fig. 8e) were often observed. Such slumping events were usually related to syndepositional faulting around carbonate platforms (Wang et al., 1990; Guo et al., 1992; Liu, 1994). In siliciclastic rocks of the ore-hosting sequences, there

525

are abundant textures with complexly folded (ptygmatic) layers (Fig. 9a), called liquefaction fabrics in the present paper, referring to complex ¯ow deformation of layered sediments. They were probably formed when ¯uid-saturated (with abnormally high pore-¯uid pressure) and lique®ed layers ¯owed like a viscous ¯uid (Owen, 1987). In fact such ¯uid-saturated layers, usually in connection with over pressured regimes, are very unstable in the basin. Various triggers may disturb them and induce very complex ¯ow deformation. Many `¯uid-migration fabrics' (Fig. 9b) were also observed and probably formed during ¯uid migration similar to water-escape structures in sedimentary petrography. There are also abundant fabrics of veinlet, stockwork, breccias (epigenetic open-space ®lling) in stockwork and fractured ores, obviously associated with later diagenetic to post-basin epigenetic processes as described by Ashley et al. (1991), Guo et al. (1992), He et al. (1993), Liu (1994) and others. 4. Discussion 4.1. Implications from fabric evidence The abundant sul®de±quartz±carbonate laminae/layers in SMG ores that are concordant to the bedding and often exhibit soft-sediment deformation apparently predate lithi®cation of the sediments. They may be readily interpreted as syndepositional textures (Jones and Preston, 1987), demonstrating that at least part of the SMG ores may have formed during sedimentation processes on the sea¯oor in a similar manner as sedex-type ores. The abundant liquefaction and ¯uid-migration fabrics in the ore-hosting sequences imply that parts of these sequences may have once been ¯uid-rich or even ¯uid-saturated before lithi®cation. This may be connected with frequent episodes of overpressure or ¯uid compartmentation that are often found in basins throughout the world, particularly in extensional basins with mixbedded mud/sandstone sequences (Ortoleva, 1994; Law et al., 1998). Fluid compartmentation could profoundly affect ¯uid movement in basins. The episodic expulsion of sealed ¯uids in a compartment was important for oil accumulation, or even for some metal mineralization (Fertl, 1976; Ortoleva, 1994; Fowler, 1994; Swarbrick, 1997; Liu et al., 1997, 2000; Law et al., 1998). The deep-water facies of the Youjiang basin includes thick turbidite deposits of interbedded mudstone and silt/sandstone. Well-developed ¯uid compartmentation systems would be logically expected. 4.2. Implications from ore-hosting carbonate platforms and syndepositional faulting Our research indicates that SMG ores are closely associated with isolated carbonate platforms constrained by platform-marginal faults. SMG ores and related alteration, however, were scarcely found in the platform

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Fig. 5. Some features of SMG-hosting isolated carbonate platforms with karst unconformity. (a) Paleo-karst surface (unconformity) on Upper Permian bioclastic limestone with a beautiful Karst tower that were covered by auriferous chert and are now exposed by miners. The auriferous chert was completely dug out and limestone with Karst tower remained. (Gaolong deposit, Guangxi). (b) Paleo-karst surface (unconformity) on Lower Permian limestone with chalcedonized regolith, rounded clasts and red Fe-hydroxides (limonite) in the lowest part of the Upper Permian sequence in the Getang dome. (Getang deposit, Guizhou). (c) A submarine erosional surface in the auriferous cherty horizon within the Getang deposit. Note that the down-cutting relief was ®lled and then covered by broken fragments of the underlying chert followed by deposition of new chalcedony with clear lamination. (Getang deposit, Guizhou). (d) Bioclastic reef limestone on the margins of isolated carbonate platforms. (Longtian dome, Guangxi). (e) Fore-reef slump breccias down slope of the Banqi dome in response to syndepositional faulting around the dome. (Banqi deposit, Guizhou).

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Fig. 6. Four types of generalized ore-hosting, litho-stratigraphic sequences of SMG deposits in the Youjiang basin, showing the auriferous horizons (dark color), the related carbonate platforms and unconformities. Note that the typical auriferous burial sequence of the platform-proximal type (the ®rst column) includes both the cherty section and the lowest section of the overlying siliciclastic sequence, and there are both stratiform and discordant orebodies (black). Note also the transition from the platform-proximal to platform-distal type (a ! d). All the examples can be found in Table 1 and Fig. 2 (see text for details).

carbonate rocks. Instead they occur in the overlying cherty\clastic sequence (burial sequence) above the carbonate rocks. North (1985) states that syndepositional faults are an intrinsic feature of sedimentary basins, particularly for extensional basins (Conteras et al., 1997; Gupta et al., 1999). Syndepositional faulting was not only closely connected with basin paleo-topographic relief, differential compaction, soft-sediment deformation, slumping, syndepositional brecciation, mud¯ow, episodic ¯uid eruption etc., but was also coupled with local tectonic subsidence and related platform emersion and drowning. These faults are therefore considered as an important control and indicator of sedex-type mineralization (e.g. Large, 1980), and often regarded as partly responsible for hydrocarbon migration (e.g. North, 1985; Hooper, 1991). Growth faults can act as both ¯uid migration pathways and barriers (e.g. Hooper, 1991). When faults are active, they can concentrate ¯uid ¯ow primarily during dilation, seismic pumping, and reopening of sealed ®ssures. When faults are inactive, however, the ¯uid ¯ow is restricted. Because of this, ¯uid migration up the fault is episodic. The growth fault opening may be partly connected with overpressure disturbances in the sedimentary column. A sedimentary basin can be envisioned as a huge chemical reactor containing multi-components of inorganic/ organic solids/¯uids. Fluid process in the basin, which may be active from the beginning of sediment deposition until long after the deposition ended, represents a very important aspect of the dynamic evolution of sedimentary basins. We generally regard basinal ¯uids as complex

aqueous/hydrocarbon ¯uids, including sediment-derived compaction-driven ¯uids (aqueous/hydrocarbon), penetrating gravity-driven meteoric/oceanic water and rising ¯uids from the basement. These ¯uids may take part in diagenetic±epigenetic processes to a variable extent during basin evolution (Liu et al., 2000). It is well known that basinal ¯uids are characteristic of low-temperatures typically between 120±2208C, consistent with the homogenization temperature (150±2408C) of ¯uid inclusions in quartz and carbonates in SMG ores (e.g. Liu, 1994). Large (1980) and Lydon (1983), among others, have related basin evolution and basinal ¯uid migration to the formation and distribution of sedex-type deposits. Several types of sediment-hosted mineral deposits involving diagenesis, brine ¯ow, and metal ®xation, are considered to be integral aspects of basin evolution, just like maturation, migration and entrapment of hydrocarbons (Large, 1988; Force et al., 1991; Fowler, 1994; Garven and Raffensperger, 1997; Liu et al., 1997, 2000; Ruffell et al., 1998). These include MVT type, red-bed type and sedex-type deposits. Similar examples involving sediment-hosted micro-disseminated gold (SMG), however, were rarely reported. Besides the standard Carlin-type gold, Emsbo et al. (1999) interpreted syngenetic gold on the Carlin trend as sedex origin, and suggested: `the transport of gold in basinal ¯uids, and its subsequent deposition in the sedex environment, can be signi®cant'. The basinal ¯uids, especially the sediment-sourced ¯uids, will migrate generally (1) upward, (2) towards basin margins or basin topographic highs, (3) from thicker towards thinner deposits during basin evolution (Magara,

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with features typical of submarine hydrothermal deposition. These deposits are always mixed with normal siliciclastic deposits to variable degrees and may exhibit a typical spatial distribution pattern around the platforms. Continued tectonic subsidence and progressive platform drowning results in thick siliciclastic deposits that cover the former platforms and depressions. A temporary ¯uid trap (like an oil trap) may form under a seal sequence resulting in lateral ¯uid migration and subsequent formation of some transitional SMG ores. Upward and lateral migration of ¯uids along faults that crosscut the seal sequence may produce some platform-distal SMG ores with more epigenetic features (predominantly siliciclastic, fractured and stockwork ores as well as discordant orebodies). Consequently, the platform-distal SMG deposits occur far away from the carbonate platform and at a higher stratigraphic position. Most of the 35 SMG occurrences in Table 1 support this preliminary and generalized model. 4.3. Comparison with Carlin-type deposits

Fig. 7. Geological map and cross section of the Zimudang deposit as an example of platform-distal SMG deposits in the Youjiang basin. Note the stratiform orebodies in P2b and the discordant orebodies along fractured zone (a reversed-slip fault) from P2b up to T1f.

1978; North, 1985; England and Fleet, 1991). This is con®rmed not only by studies in petroleum geology, basin modeling, and basin hydrogeology, but also by studies on modern active basinal ¯uids and some basin-¯uid related ore deposits (Large, 1980, 1988; Bjorlykke, 1993; Garven and Raffensperger, 1997; Xie et al., 1999). Thus the isolated platforms (acted as basin paleo-highs) and related syndepositional faults in the Youjiang basin may have together controlled the migration pathway and migration process of ore-forming basinal ¯uids. We can therefore construct a generalized geological and paleo-geographic model for SMG deposits in the Youjiang basin (Fig. 10). Basinal ¯uids may migrate towards isolated platforms constrained by growth faults and characterized by an unconformity. A platform burial/drowning event is constrained by syndepositional faulting, and may therefore be associated with the expulsion of basinal ¯uids. If basinal ¯uids reach the platform before it is completely covered by siliciclastic deposits, the ¯uid may be directly discharged onto the sea¯oor and form platform-proximal SMG ores

SMG deposits in the Youjiang basin are similar to Carlintype gold deposits in many aspects, including sedimenthosted micro-disseminated gold, low-temperature mineral assemblages, geochemistry, alteration minerals, 150± 2408C homogenization temperatures, auriferous cherty bodies (jasperoid) etc. Unlike Carlin-type gold, however, SMG mineralization in the Youjiang basin shows no relationship to post-basin magmatic processes (a key genetic factor for Carlin-type deposits) but instead exhibits a close connection with sedimentary basin evolution such as basin ®lling and diagenesis, basin topography, syndepositional faulting and facies distribution. The auriferous cherty strata always overlie a karst unconformity and exhibit sedimentary features. Some of them can be followed over tens of kilometers, as for example the lowest part of the Upper Permian in the northwestern part of the basin, well-known as the `Dachang stratum' that hosts numerous SMG deposits (No. 1 to No. 4 in Table 1) as well as stibnite and pyrite deposits. They are not comparable with the jasperoid body in Carlin-type deposits as some researchers suggested. Platform-proximal SMG ores often occur along the contact zone between platform carbonates and the overlying detrital rocks with an unconformity in between. Ores and related alterations were found only in the detrital rocks, rarely in the carbonates. In terms of chemical activity, carbonate rocks are more reactive than detrital rocks and more sensitive to hydrothermal solutions, as in the case of Carlin-type deposits. This further demonstrates that platform-proximal SMG deposits were not formed by epigenetic ¯uids along the fractured contact zone. We thus believe that the platform-proximal SMG deposits in the Youjiang basin are fundamentally different from Carlin-type gold. Some local geologists recently called them ªYoujiang-type goldº.

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Fig. 8. Some fabric features of SMG ores and SMG-hosting rocks. (a) Concordant bedding structure in siliciclastic ore. A depositional succession from bottom to top: (1) calcite±quartz±pyrite layer composed of about ten ®ne laminae with internal lamina bending; (2) silt layer; (3) ®ne calcite±pyrite layer; (4) silt layer with disseminated ®ne pyrite decreasing upwards; (5) and (6) silty mud layer. The inset in the upper middle is from another location in the same layers to show a sub-marine erosion pit (arrow) in layer 2, which was ®lled with the same composition as overlying layer 3 (pyrite±calcite) and siltstone fragments from layer 2. (Yata deposit, Guizhou). (b) Under the microscope, pyrite and arsenopyrite were concentrated in ®ne sand laminae, and neighboring mud laminae contain few sul®des. Note the needle-like arsenopyrite crystals. (re¯ected light micrograph. Langquan deposit, Guangxi). (c) Soft deformation of ®ne sul®de±quartz± calcite laminae in siliciclastic ore. (Banqi deposit, Guizhou). (d) Small-scale slump fold in siliciclastic ore. Note the longitudinal scours on the bedding surface. (Jinya deposit, Guangxi). (e) Sand-pillow in mud¯ow in the ®rst slump zone of the Jinya deposit. Note pyrite around the pillow margin (parallel to bedding!) (Jinya deposit, Guangxi).

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Fig. 9. Liquefaction and ¯uid-migration fabrics in SMG ores and hostrocks. (a) Liquefaction texture in form of complex ¯ow deformation with mix-bedded ®ne sandstone (dark) and silt/mudstone (light) in siliciclastic ore. Two parts were enlarged. Part I shows pyrite occurring mainly in a silty/mudy matrix. Part II shows the complex ¯ow folding of the laminae. (Zimudang deposit, Guizhou). (b) Fluid-migration structure. Fluid migrated from black mud layer (lower part, rich in organic matter) into the neighboring silt layer, and black organic matter pigmented the ¯uid migration pathway. (Yata deposit, Guizhou).

4.4. SMG deposits in other basins in China and possibly in other countries Besides the Youjiang basin, SMG mineralization was also found in the Xiangguiyue, Songpan, and South Qinling basins around the Yangtze Craton in South China. It was also discovered recently in the Yanliao basin in the North China Craton in Middle Proterozoic rocks. We have made case studies of SMG mineralization in all these basins. They are all rift basins similar to the Youjiang basin, but show distinct basin-®lling features and different basin evolution histories. In particular, isolated carbonate platforms and related unconformities were not as well developed as in the Youjiang basin, and granitic intrusive rocks may be present either outside or within SMG ore districts. Upper Permian and Lower±Middle Triassic siliciclastic sediments,

the most important ore-hosting sequences in the Youjiang basin, was not widespread in the Xiangguiyue and South Qinling basins. Post-basin orogenic deformation in the South Qinling and Songpan basins was much stronger than in the Youjiang basin and many primary features related to basin evolution were strongly modi®ed. Therefore, a close association between SMG mineralization and a carbonate platform was not widely observed in these basins, except for some deposits in the Songpan basin such as the Dongbeizhai and Qiaoqiaoshang deposits. Instead, SMG occurrences are often located around margins of basement highs that may have played a similar role in constraining basinal ¯uid migration just as the carbonate platforms did in the Youjiang basin. In the Gaolong dome (Fig. 4b), the Carboniferous± Permian platform carbonate rocks are almost horizontal

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531

Fig. 10. Generalized geological and paleo-geographic model for platform-proximal, transitional and platform-distal types of SMG deposits in the Youjiang basin. Basinal ¯uids may migrate towards basin paleo-highs in response to syndepositional faulting of the platform-marginal faults that resulted in the typical spatial distribution pattern (around platform) of platform-proximal and transitional SMG deposits. Lateral migration of ¯uids up faults crosscutting some sealed beds during the later stage of basin evolution may produce platform-distal SMG ores far from the carbonate platform and at a higher stratigraphic position (see text for details).

with weak deformation, but the Triassic siliciclastic rocks around the dome are folded and highly fractured with an outward decrease in deformation intensity. Triassic strata show W±E fold axes north and south of the dome, and N±S axes west and east of the dome (Guo et al., 1992, p. 17±18). A similar situation was often observed in other SMG-hosting domes. This demonstrates different deformational behavior for the two rock groups, and carbonate platforms may have acted as solid masses among the relatively soft, young detrital rocks during basin inversion. Shearing, folding and other deformation are often well developed along the contact zones (also ore-bearing zones) that may have resulted in metal reactivation and epigenetic features in many SMG deposits. At the same time, the karst unconformity was readily replaced by fractures so that only the abrupt facies transition can still be observed today. It is thus very dif®cult to identify the primary features of possible SMG mineralization. In addition to the above mentioned ®ve SMG-hosting basins in China, it is possible that other rift (extensional) basins superimposed upon continental crust, either in China or in other countries, contain similar SMG deposits. Similar gold mineralization has recently been discovered in South Tibet, in Triassic siliciclastic rocks deposited on the passive continental margin of the North India plate (private communication with Li Jingao). Yet similar SMG deposits have not been reported for other basins outside China.

to determine exploration strategy for hydrocarbon in basins. The geological pattern in Fig. 10, which relates SMG mineralization to basin topography, growth faults, basin ®lling and ¯uid migration, may have a more general applicability both for genetic discussion and for exploration. There are still many similar domes in the researched basins of China (carbonate domes in the Youjiang and Songpan basin and basement highs in the other basins). The Longtian dome in NW Guangxi (Fig. 4c and No. 14 in Fig. 2), for example, is a typical isolated carbonate platform with a geological pattern similar to Fig. 10. We have expected to discover SMG ore on its marginal zone in 1994. A geochemical Au anomaly was outlined in 1995, and SMG orebodies were then discovered in 1997 around the dome. More recently (1999) an SMG deposit, called the Nashen deposit (No. 22 in Table 1), was discovered on the northwestern margin of the Lingma dome (No. 15 in Fig. 2) with a geological and paleo-geographic con®guration similar to Fig. 10.

4.5. Implications for exploration

1. SMG mineralization may be genetically associated with basinal ¯uids during the dynamic evolution of extensional basins along divergent continental margins.

It is well known that ¯uid migration pathways can be used

5. Conclusions Although the observations and discussions presented above are far from suf®cient to draw genetic conclusions on SMG deposits in the Youjiang basin, the following preliminary remarks can be made, especially for platformproximal type SMG.

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2. Basin paleo-topographic highs (including carbonate platforms) and syndepositional faulting, together with the unconformity-related burial sequence, may have controlled the migration pathway and migration process of basinal ¯uids to a certain extent and further controlled the distribution of related SMG ores. 3. Despite many similarities, the SMG deposit in the Youjiang basin seems fundamentally different from Carlin-type gold, but shows genetic similarities to sedex-type deposits.

Acknowledgements This work is supported ®nancially by the Ministry of Science and Technology of China (Grand No. 95-Pre-39 and G1999043210), the National Natural Sciences Foundation of China (Project No. 49873022), and the Chinese Academy of Sciences (Project No. KZCX1-07). We are grateful to Li Chaoyang, Hu Ruizhong, G.C. Amstutz, Xue Chunji, O. Schulz, F. Vavtar, Wang Yangeng, Yang Xie, Pan Guangsong, Liu Daoming, Yang Keyou, and Yao Wangxiang for pro®table discussion. We thank the many geological teams and mining companies in the Youjiang region for their support and help during ®eld research. We are particularly indebted to Prof. L. Brigo, Dr M. Moroni and Prof. K. Burke for their careful reviews and useful suggestions. References Ashley, R.P., Cunningham, C.G., Bostick, N.H., Dean, W.E., Chou, I-M., 1991. Geology and geochemistry of three sedimentary-rock-hosted disseminated gold deposits in Guizhou Province, People's Republic of China. Ore Geol. Rev. 6, 133±151. Bjorlykke, K., 1993. Fluid ¯ow in sedimentary basins. Sedim. Geol. 86, 137±158. Bureau of Geology and Mineral Resources of Guangxi Zhuang Autonomous Region, 1985. Regional Geology of Guangxi Zhuang Autonomous Region. Geological Publishing House, Beijing 853 pp., in Chinese. Chen, Y. (Ed.), 1994. Introduction to Regional Geology of China Geological Publishing House, Beijing, pp. 448±476 in Chinese. Chen, H., Zeng, Y., 1990. Nature and evolution of the Youjiang basin. Lithofacies Paleogeogr. 1/1990, 28±37. Chen, H., Qing, J., Tian, J., 2000. Sequence ®lling dynamics of Youjiang basin, South China. Acta Sedimentol. Sinica 18, 165±171 in Chinese. Conteras, J., Scholz, C.H., King, G.C.P., 1997. A model of rift basin evolution constrained by ®rst-order stratigraphic observations. J. Geophys. Res. 102, 7673±7690. Cunningham, C.G., Asshley, R.P., Chou, I-Ming, Huang, Z., Wan, Ch., Li, W., 1988. Newly discovered sedimentary rock-hosted disseminated gold deposits in the People's Republic of China. Econ. Geol. 83, 1462±1467. Dong, J., 1997. Geological±geochemical features of Tangxinzhai gold deposit and its exploration, SW Guizhou. Ore Deposit Geol. SW China 1997-1, 17±25 in Chinese. Emsbo, P., Hutchison, R.W., Hosfstra, A.H., Volk, J.A., Bettles, K.H., Baschuk, G.J., Johnson, C.A., 1999. Syngenetic Au on the Carlin trend: implications for Carlin-type deposits. Geology 27, 59±62.

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