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Sedimentary fans: A new genetic model for sedimentary exhalativeore deposits
Huan Li a,⁎, Xiao-Shuang Xi b
a Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japanb School of Geosciences and Info-physics, Central South University, Changsha 410083, China
⁎ Corresponding author at: Department of Earth ResEngineering, Kyushu University, 744 Motooka, Nishi-Tel.: +81 928023314.
Sedimentary-exhalative (SEDEX) deposits are one of themost important types ofmetal ore deposits. The genesisof such economic deposits has been problematic; thus a holisticmodel is urgently needed to explain their forma-tion. Based onfield surveys and geochemical analyses of theXitieshan Pb–Zn, the Zhaokalong Fe–Cupolymetallic,and several other typical sedimentary-exhalative deposits in China, this study proposes a “sedimentary fanmodel” to explain such deposits. The results of this research suggest that the ore forming fluid of sedimentary-exhalative deposits can be considered as a kind of turbidity current. These fluids originated from the exhalativeprocess itself, passing through syngenetic faults,flowing into themarine basin, and forming fan-shaped sedimen-tary ore deposits. The ore bodies of sedimentary fans consist of three parts: pipe, central, andmarginal facies. Thefans typically show zonations of mineralization, trace elements, fluid inclusions, isotopes, redox environments,etc. Pipe facies is deeply sourced and proximal to synsedimentary faults, generally constituted by unstratified,altered stockworks, exhalite (e.g., siliceous rocks) and ore bodies, and characterized by high temperatures andsalinities. Central and marginal facies mainly occur as stratiform ore bodies, exhibiting typical characteristics ofsedimentary origin. From the central facies to the marginal facies, increasing seawater components enter intothe ore-forming process, resulting in a gradual change of metallogenic environment. This sedimentary fanmodel is used with satisfactory results to predict metallogenesis in the Xitieshan, the Zhaokalong and othersedimentary-exhalative deposits in China.
Sedimentary exhalative deposits (SEDEX deposits) are ore depositsthat are interpreted to have been formed by release of ore-bearinghydrothermalfluids into awater reservoir (usually the ocean), resultingin the precipitation of stratiform ore (Gu et al., 2007; Heinrich, 2005;Large et al., 2005; Russell, 1996). The SEDEX deposits are widelydistributed, and sedimentary-exhalative processes are consideredto be related to formation of a variety of ore deposits, includingsediment-hosted Pb–Zn–Ag (Decrée et al., 2008; Kawasaki et al., 2010;Large et al., 1998; Paradis et al., 1998; Sangster and Hillary, 1998;Tornos and Heinrich, 2008; Wang et al., 2014), Cu (He et al., 2009;Kampunzu et al., 2009; Li and Xi, 2012; Vishwakarma, 1996), barite(Canet et al., 2014; Clark et al., 2004), Ni–Mo (Lott et al., 1999; Shiet al., 2014; Xu et al., 2013), Sn-polymetallic (Cheng et al., 2012, 2013;Jiang et al., 1999) and, probably, some precious metal deposits (Canetet al., 2004; Emsbo et al., 1999; Gu et al., 2012).
Sedimentary exhalative deposits are huge concentrations of avariety of metals, but the genesis of these sediments has been subjectedto different views (Canet at al., 2004; Gu et al., 2007; Wang et al., 2008;Xue et al., 2007). In recent years, a few studies were concerned withmorphological characteristics, while the rest mainly focused ongeochemistry research (e.g., Cooke et al., 2003; Large et al., 2005).Though several researches investigated the formation environment ofexhalative deposits (e.g., Gu et al., 2012; Tornos, 2006; Yu et al., 2014),research on their genesis is still ongoing.
Determining genetic processes is essential to understanding theoverall morphology in the study of sedimentary-exhalative deposits.The basic forms of ore distribution patterns improve the accuracy offorecasts and effectively evaluate the deposit prospects. Sedimentary-exhalative deposits are formed in the sea, so the genetic study of thiskind of deposit can be combined with the principles of sedimentology(Ruffell et al., 1998; Tang, 2006). According to the mechanism ofsedimentary-exhalativemineralization, the sedimentary layer is formedas seabed sediments. Therefore the fan-shaped layer patterns can benamed “ore-body sedimentary fans.” Sedimentary-exhalative oredeposits usually underwent two major stages: submarine exhalativeore-forming stage and superimposed structural transformation stage(Prokin and Buslaev, 1998; Tang, 2006; Xi et al., 2005). The former
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gives the deposits their basic form and the structural deformationexperienced by the latter stage generally changes the ore body shaperather than positions and trends. Therefore, it is important to studythe earlymineralizationmorphological characteristics of these deposits.
In this study, characteristics of exhalative-sedimentary mineraliza-tion in the Xitieshan Pb–Zn deposit, the Zhaokalong Fe–Cu polymetallicdeposit and other SEDEX type deposits in China are presented. We in-vestigated multi-geological factors such as ore body three-dimensionalshape, ore geochemistry, grade, texture and structure,mineral granular-ity and composition in these deposits. In combinationwith comparativestudies of classical models and theoretical analysis, a sedimentary fanmodel is formulated for these sedimentary-exhalative deposits. Thismodel can be used for analyzing the geological formation mechanismof sedimentary fans of SEDEX deposits, improving metallogenicprognosis and exploration achievements.
2. Methodology
Field investigation and laboratory analysis were combined to studythe ore body characteristics in the Xitieshan and the Zhaokalongdeposit. For other typical SEDEXdeposits in China, previous publicationsthat reported the ore body morphology and geochemistry weresystematically reviewed and selectively used as support evidences inthis study.
For the Xitieshan deposit, geological maps on cross sections andlevel plans have been collected and used to interpret the morphologyof the ore body layered structures. The geological data have beenintegrated to establish an integral space form of the ore body layers.The on-site field surveys were carried out to determine the location ofthe ore layer border and the scope of the ore bodies. In addition, thesulfide mineral granularity (N1 mm) was measured from differentexploration lines in the 3062 m level of the Xitieshan deposit, totallywith 26 measuring sites and more than 30,000 data. Geochemically,fluid inclusion and C–Si isotopic data from the different kinds of oreswere collected fromprevious publications. These datawere recalculatedto reveal the geochemical–physical characteristics (i.e., Eh, pH anddensity) and genesis of the fluids.
For the Zhaokalong deposit, abundant ore grade data was collectedfrom the mine. Systematic geological cross section maps werecombined to reveal the ore body layer characteristics. Field investigationwas carried out to determine the occurrences of the ore bodies. The REEand trace element composition of the different types of ores (whole
Fig. 1. Geological map of the Xitieshan deposit (mod
rock)were determined using ICP-MS. The homogenization temperatureand salinity of the fluid inclusions were determined, and gaseous andaqueous compositions of the fluid inclusions from the representativeore minerals were also analyzed using chromatography. The analyticalmethods followed the procedures described in Li et al. (2013).
3. Sedimentary fan characteristics of the Xitieshan deposit
3.1. Introduction of the Xietieshan deposit
The Xitieshan deposit is located in the northern part of QinghaiProvince, ~700 km away from the provincial capital city of Xining(Fig. 1). It is one of the largest Pb–Zn deposits in China,with ore reservesamounting to 43 million tons and grades of at 3.7% Pb, 5.39% Zn,0.39–1.12 g/t Au, 19.6–46.6 g/t Ag, 14.43–18.4% S, 0.087% Sn, 0.033%Cd and 0.0031% In. Themineralization took place in a back-arc spreadingsetting in theOrdovician, and then subjected to intense tectonic deforma-tion and transformation during the Late Silurian Caledonian orogeny(Tang, 2006). The stratigraphic sequence in the deposit area frombottomto top consists of Lower Proterozoic Dakendaban Group metamorphicrocks, Upper Ordovician Tanjianshan Group chlorite-quartz schist andsandstone, Upper Devonian Armunike Formation conglomerate, LowerCarboniferous Chengqianggou Formation limestone and marble (Fig. 1).Among them, the Upper Ordovician Tanjianshan Group is the ore-bearing strata. Recent research suggested that theXitieshan lead–zinc de-posit is a SEDEX-type deposit, formed in the early stage of the back-arcextensional process during the Ordovician period (Tang, 2006; Wanget al., 2008).
3.2. Morphological characteristics of the ore bodies
Themineralization zone in the Xitieshan deposit is 2100m in lengthand 130 m in width, striking from northwest to southeast, with a highinclination in section. The major ore bodies are located between themarble and the phyllite, mostly sheeted and discontinuous in the belt,with average thickness of less than 20 m. The ore bodies can be classi-fied into two types: unstratified and stratified. These two kinds of orebodies are separated laterally and vertically (Wang et al., 2008). Taking2942m level as an example, the boundary between them is near Line 37in planar view (Fig. 2). In cross section, unstratified ore bodies appear atthe upper portionswhereas stratified ore bodies are located at the lowerportions.
ified from Tang (2006) and Wang et al. (2008)).
Fig. 2. Lever plan at the 2942 m elevation of the Xitieshan deposit (after Zhu et al. (2007) and Wang et al. (2008)).
According to the ore boundary delineation results, the ore bodies aremorphologically characterized by fan-shapes. The upper portions ofthese ore bodies are folded, whereas the deep portions are outstretched(Fig. 3). The upper portions of the bodies have two opposite tendencies,east and west. The convergent center lies in Line 50. The ore bodies areasymmetric from the center to their different sides. Ore bodies havelarger widths on the eastern side than that on the western side. Theirboundaries have smaller plunge values on the upper portions, withrelatively flat boundary lines. However, from3100m level to the deeperportions, the boundaries become steeply plunging. Ore bodies outcropat a maximum level of 3500 m on the surface from Line 30 to Line 64,with a width of 850 m. However, at the 3000 m level, the ore bodiesexpand to a range from Line 05 to Line 75, with awidth of approximately2000 m. All of this indicates that the ore bodies are fan-shaped.
3.3. Identification of the fans and their internal characteristics
Based on a detailed analysis of the longitudinal and transversesections of the geological map and from on-site geological survey, theore bodies have been divided by Line 30 into two sedimentary orefans: eastern fan and western fan. The edges of these two fans areoverlapped above the 3182 m level, resulting in significantly thick orebodies in the upper portions. The fans begin to deviate from eachother below the 3182 m level, which form the ore-absent area up to300m inwidth.Meanwhile, with increasing depth, themarble becomes
Fig. 3. Vertical projections of ore body (laye
thinner and inter-layered with schist, showing the characteristics of amarginal zone. The western and eastern fans are combined againbelow the 2942 m level, and the ore-absent areas gradually narrowand vanish. The ore body layers become multi-layered mineralizationin the deeper portions.
Notably, these two fans have their own inner characteristics. A singlefan shows asymmetric characteristics in ore body continuity,mineralization type and marble distribution. Moreover, these two fanshave similar asymmetric trends. For example, the ore bodies arediscontinuous in the western portion of the western fan, extendedshortly and separated by barren areas. From Line 55 to the east, theore bodies become continuous; single ore body extends increasinglyand the barren areas become narrow (Fig. 3). The eastern fan has asimilar character. The ore bodies from the western portion of theeastern fan are scattered, whereas those from the eastern portionare continuous. However, overall, the eastern fan has better continu-ity than the western fan. Sulfide compositions also have similartrends. The central and western ore bodies of the western and theeastern fans have high concentrations of galena and sphalerite,whereas pyrite contents are much higher in their eastern portions.Additionally, marble thickness has an increasing trend in thewestern portions of the fans. Laterally, the marble area is also farbeyond the scope of the ore body layers in their western portions. Asketch illustration is drawn to explain the relationship between thesetwo sedimentary fans (Fig. 4).
Fig. 4. A sketch illustration explaining the relationship between the two sedimentary fansin the Xitieshan deposit.
Fig. 5. Typical ore textures and structures in the Xitieshan deposit. a–c: Pipe facies, d–g: centraexplosive marble breccias; c: explosive siliceous breccias; d: Bouma-type sequence consi2) interbedding formed by fine-grained galena and sphalerite (central), and 3) interbedding fortional bedding indicates that gravity flow was generated when the exhalative fluid met the seasphalerite ore interbedding with country rocks, showing a rhythmic layering characteristics;coarse galena, sphalerite and pyrite; h: interbedding of chlorite-quartz schist and fine grainedrocks; j: laminae galena–sphalerite ore deposited with clastic sediments.
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3.4. Ore structure and texture
3.4.1. Ore classificationAccording to the ore occurrences in the sedimentary fan model, we
classified the ores into three facies: 1) exhalative vent pipe facies,2) central fan facies, and 3) marginal fan facies. The pipe facies oresare characterized by an explosive brecciated structure, hosted byaltered-mineralized stockwork rocks. The veins in the rocks consist ofquartz and carbonate (Fig. 5a), and the mineralized breccias arecomposed of marble (Fig. 5b) and cherts (Fig. 5c). The central faciesores are dominated by massive structures, followed by bandedstructures. The banded ores show a Bouma-type sequence (Fig. 5d),layered with country rocks (Fig. 5e). The massive ores are banded inmineral assemblages (Fig. 5f), composed of coarse galena, sphaleriteand pyrite (Fig. 5g). The ores that form the marginal fan facies arecharacterized by banded structures (Fig. 5h and i), and sometimesshow laminae fragmental sedimentary texture (Fig. 5j).
l facies, h–j: marginal facies. a: Stockwork rocks consist of quartz and carbonate veins; b:sts of 1) massive bedding formed by coarse-equigranular mineral assemblage (left),med by thin pyrite layers, country rocks (schist) and siliceous laminae (right). This grada-water, and evolved into drag flow during the transportation processes. e: Banded galena–f: interbedding of galena–sphalerite layers and pyrite layers; g: massive ore composed ofpyrite layers; i: wavy beddings of ore layer skew across horizontal beddings of country
3.4.2. Mineral granularityThe mineral granularity data was used for the second-order
(quadratic curves, Fig. 6a, b and c) and third-order (cubic curve,Fig. 6d) calculations. The observed data displays that the maximumgranularity was measured near Line 30 where the two fans overlapped(Fig. 6b). Therewere also two secondary level peaks observed in Line 14and Line 68 (Fig. 6a and c). As per the trend analysis (cubic curve), it wasalso found that therewere two peak values (Fig. 6d), with the boundarylines in the vicinity of Line 30. Mineral granularity trend curves suggesta typical structure for the sedimentary-exhalative fans. Grain size withpeak values indicates the centers of the exhalative mineralization. Thedecrease in grain size from the centers to the sides corresponds to thedecline of the trend curves. In the 3062 m level, mineral granularity ofthe two peaks corresponds to the center portions of the western andeastern fans. The centers are located near Line 20 and Line 50,respectively.
3.5. Geochemical characteristics of the ore body fans
3.5.1. Characteristics of fluid inclusionsThe homogenization temperatures and calculated salinities of the
fluid inclusions from the different kinds of ores vary differently(Table 1). The preliminary study showed that the fluid in the quartz-hosted inclusions from the altered stockwork rocks that represent thepipe facies was very active (Wang et al., 2008). In addition, fluidinclusions in carbonates near the unstratified ore bodies hosted in thethick-bedded marble, which represents vent-proximal facies, werealso investigated. The inclusions were found to be large in size andhave moderate to high temperatures. The pipe facies has the highesttemperature, followed by the central facies and then themarginal facies(Table 1). Meanwhile, compared to the pipe facies and marginal facies,the central facies have higher salinity values. In the sedimentary ore
Fig. 6.Mineral granularity trend curves at the 3062 m level (a
body fans, massive ores from the central facies have higher pH yetlower Eh values in the inclusions when compared to the banded oresfrom the marginal facies (Table 1). Additionally, the central facies hasthe highest fluid density (1.04–1.12 g/cm3), followed by the marginalfacies (0.91–0.99 g/cm3) and the pipe facies (0.76–0.88 g/cm3).
Aqueous compositions in the fluid inclusions of different ores fromdifferent facies also vary differently (Table 2). Fluids in the alteredstockwork rocks and carbonates in pipe facies have a similarH2O–NaCl–CO2 system, enriched in deeply sourced fluid compositionssuch as CO2, CH4, C2H2, Ar, N2 and Na+ (Wang et al., 2008, 2009).Compared to the pipe facies, the central and especially the marginalfacies absorbed more compositions associated with seawater, such asCa2+, Mg2+, SO4
2– and other ions.
3.5.2. Isotopic constraintsWang et al. (2009) carried out fluid isotopic analysis on quartz-
hosted inclusions in different kinds of ores from the unstratified (pipefacies) and the stratified ore bodies in the Xitieshan deposit. The resultsshowed that the δ13CCO2 value of the quartz-hosted fluid from the pipefacies is −5 ± 2‰, indicating deep origin characteristics. The δ13CCO2values of the quartz-hosted fluid inclusions proximal to the stratifiedore bodies also have a similar range, indicating the migration of thefluid from the pipe facies. On the other hand, the δ13CCO2 values of thequartz-hosted inclusions from the stratified ore bodies are around 0‰,indicating the genesis features of ore-bearing carbonate strata.Additionally, δ30Si values of the quartz in the pipe facies rangefrom −0.1 to −0.4‰, and the δ13C and δ30Si values of the marblelocated in the unstratified ore body flanking are −5.76‰ and −0.5‰,respectively. All of this suggests that the pipe facies may be influencedby deeply sourced (magmatic?) fluid, whereas the stratified ore bodiesmay be hydrothermal exhalative sedimentary in origin.
, b and c: partial quadratic curves; d: overall cubic curve).
Note: The calculation of density follows the methods in Liu et al. (2000); the calculations of pH and Eh use the methods mentioned in Wang and Wang (2011).
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4. Sedimentary fan characteristics of the Zhaokalong deposit
4.1. Introduction of the Zhaokalong deposit
The Zhaokalong Fe–Cu polymetallic deposit lies in the northwestportion of the Jiangda island arc uplift, Sanjiang ore belt, NorthwestChina. Geographically, it is located in the southern part of QinghaiProvince, ~900 km away from the provincial capital city of Xining. TheJiangda island arc is located to the southwest of the famous Jinshajiangsuture zone, which is surrounded by the Yangtze Block, Dege Block andJiangtang–Qamdo Block (Fig. 7; Chen et al., 2008). This deposit occurredin a shallow water environment in the extensional basins of the lateTriassic arc rift, with Fe–Cu–Au–Pb–Zn and other metal mineralization.According to the latest exploration results, it was determined as a large-scale polymetallic deposit, with total proven ore reserves of more than30 Mt and average grades at 33.50% Fe, 0.70% Cu, 0.91% Pb, 0.71% Zn,25.47 g/t Ag and 0.41 g/t Au. The rift basins were filled by a sequenceconsisting of siliceous rock, laminar fine-crystalline dolomite and dolo-mitic limestone, and host most of exhalative-sedimentary polymetallicdeposits (Hou et al., 2007). Strata from the deposit are not complicated,exposed Triassic Batang group (including Upper Clastic Formation, TopCarbonate Formation) and Quaternary Alluvium. The Upper ClasticFormation can be further subdivided into three members, that is,Upper, Middle and Lower Members. The Lower Member is composedof sandstone, siltstone, argillite, quartz conglomerate, pebbly ooliticlimestone, etc. The Middle Member is formed by siltstone, silty slate,and feldspar–quartz sandstone. The Upper Member is the mainore-bearing strata, which is composed of andesite, limestone, argillite,carbonaceous slate, feldspathic-quartz sandstone and dolomite, with amild chloritization (Li et al., 2013).Wall rocks of ore bodies are andesite,limestone, carbonaceous slate and argillite. The main wall-rockalteration is sericitization, chloritization, silicification, pyritization andcarbonation. We believe the Zhaokalong deposit belongs to SEDEX
Table 2Aqueous compositions of the quartz-hosted fluid inclusions in the Xitieshan deposit (ppm).
type deposits, but some researchers (e.g., Hou et al., 2007) classifiedthis deposit as a volcanic hosted massive sulfides (VHMS) type due toits close relationship with the volcanic rocks.
4.2. Morphological characteristics of ore bodies
Major ore bodies in the Zhaokalong deposit are strictly controlled bythe stratigraphy and lithology, and the ore belt has a strike length of2400 m, and a horizontal width of 200–800 m. According to the orebody distribution, the ore belt is divided into seven mineralizationzones (Fig. 7), and each zone has characteristic mineralizing types andore grades (Table 3). The ore bodies are characterized by layered,lens-shaped or quasi-lamellar structures. The orientation of ore bodiesand host rocks is basically the same, trending NE–SE with high dipangles. Exceptionally, the ore bodies from Mineralization Zone V arelenticular in shape, having a flat dipping occurrencewith a NE–SE trend.
Field surveys revealed that the ore bodies from the Zhaokalongdeposit were strongly transformed by tectonic deformation, wrinkledbut synchronized with the host rocks. Mineralization zones thatoutcropped on the surface are connected at depth, and the folds ofthis deposit controlled the basic sketch of ore body distribution. Orebody layers responded to the strong fold deformation, forming thepresent-day distribution characteristics with multiple mineralizationzones.
4.3. Zonation of the mineralization systems
Based on field investigation, the strata correlation between thecentral portion and the eastern portion of the deposit was discoveredto be unintelligible, especially in ore-bearing strata. The mineralizationtypes in the central and eastern portions of the deposit are alsodifferent: Mineralization Zone I and II are mainly high-grade Fe–Cuoxide mineralization hosted by andesite, whereas Zone III and V are
dominantly high-grade Pb–Zn sulfide mineralization in the sideritelayer (Table 3). On the other hand, drilling data suggests that theoxide and sulfide layers do not entirely correspond with the formationstrata. For example, iron oxide layers can occur in the upper part of thesiderite formation, and sulfide layers can also be found in the andesiteand exhalative rock formations. Hence, there should be a syngeneticfault between the central and eastern mineralization.
Based on the survey of stratigraphic and magmatic rocks, theZhaokalong deposit appears to have a double-vent sedimentary-exhalative system, which is corroborated by the mineralized zonation(Fig. 8). Volcanic hydrothermal activity in the central area caused theformation of Mineralization Zone I, II, VI, and VII. On the other hand,hydrothermal activity in the eastern area resulted in the formation ofMineralization Zone III and V. Mineralization Zone I and II from thecentral mine are enriched in Fe–Cu, indicating they are the core of thefan. Zone I, II and Zone VI, VII are connected and belong to the samesedimentary-exhalative system, showing the transition relation fromthe central facies to the marginal facies. On the contrary, Zone III andV are dominated by Pb–Zn–Ag mineralization, showing characteristicsof marginal facies of the other fan (Fig. 8). The Zhaokalong depositshows significant mineralization zoning in the single ore-formingsystem. Vertically, metallic elements are distributed from the bottom
Table 3The mineralized type and ore grade of different ore-forming zones in the Zhaokalong deposit.
Mineralization zone Mineralized type Ore grade (%)
Fe
Major Minor Maximum General Average
I Fe, Cu 43.08 28–39 33.87II Fe, Cu Pb, Zn 39.93 25–36 31.32III Fe, Pb, Zn Cu 36.56 20–33 30.29IV Pb, Zn Fe, Cu 32.73 28–32 32.73V Fe, Pb, Zn, Cu 36.42 29–36 26.57VI Pb, Zn Fe, Cu 33.54 27–34 28.73VII Pb, Zn, Cu Fe 35.2 30–32 29.09
to top with an order of Cu, Au–Fe, Cu, Au–Fe–Fe, Pb, Zn, Ag–Pb, Zn andAg. Laterally, elements also display trends of mineralization zoning: FeandCuaremainlymineralized in the central portion of themine,whereasZn, Ag and other relatively low-temperatureminerals primarily occurredin the western portion of the mine (Fig. 9).
The Fe–Cu ores from the pipe facies are characterized by massivestructures, formed by explosive breccias (Fig. 10a) and exhalativesilicalites (Fig. 10b). The central facies contains the ores of banded andmassive structures (Fig. 10c), occasionally with mixed sedimentary-exhalative components (Fig. 10d). The ores that form the marginalfacies are characterized by bedding structures (Fig. 10d), casuallycomposed of hematite layers (Fig. 10f).
4.4. Geochemistry of the ore-body sedimentary fans
We carried out trace element (including REE) and fluid inclusionanalysis on different ore types from the Zhaokalong deposit. Magnetiteores from Zone I and II, chalcopyrite ores from Zone VII, and sphalerite–galena ores from Zone III and V represent pipe facies, central fan faciesand marginal fan facies, respectively. The detailed analytical resultscan be found in Li et al. (2013).
Fig. 8. Simplified geological sections of the Zhaokalong deposit.
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The magnetite ores are characterized by relatively flat REE patterns,showing a negative Ce anomaly (Fig. 11a). REE compositions of thesulfide ores vary largely, enriched in light rare earth elements withpronounced positive Eu anomalies (Fig. 11a). Additionally, the differenttypes of ore also vary distinctly in metallic elements. As shown inFig. 11b, the sphalerite–galena ores and the chalcopyrite ores havehigher contents of Au, Ag, Hg, Ni, and other indicative elements ofsedimentary processes compared to the magnetite ores. Furthermore,the sphalerite–galena ores have extremely high Mn contents(average 35,000 ppm). On the contrary, the magnetite ore has higherFe3+/Fe2+ and Co/Ni ratios, suggesting exhalative genesis.
The fluid inclusions of the sulfide ores (chalcopyrite and sphalerite–galena) and the magnetite ores also vary from each other (Fig. 11c andd; Li et al., 2013). The magnetite and chalcopyrite ores have highertemperatures and salinities compared to the sulfide ores (Table 4;Fig. 11d). Some components in the sulfide ores (such as SO4
2–, Ca2+)are significantly higher than in the magnetite ores, whereas F–, Na+
and K+ are more abundant in the magnetite ores (Table 5; Fig. 11c).In addition, CH4 compositions in the sulfide ores are much higher thanthat in the magnetite ores, whereas H2 and CO2 contents display
Fig. 9.Mineral zoning characteristics of the ore body sedimentary fans in the Zhaokalongdeposit.
opposite characteristics. Additionally, chalcopyrite ores have thehighest fluid density (0.95 g/cm3), followed by the sphalerite–galenaores (0.94 g/cm3) and the pipe facies (0.93 g/cm3) (Table 4). The Ehvalues have the same trends: chalcopyrite ores have the lowest values,followed by the sphalerite–galena ores and then the magnetite ores.The pH values decrease in facies from pipe to central and then tomarginal (Table 4). All of these may indicate that these ores havedifferent ore-forming environments. The sulfide ores probably absorbedmore components from seawater, and the ore-forming environmentwas partial to a lower temperature, reducing environment. In contrast,the ore-forming elements for the magnetite ores are more likely tohave originated from submarine eruptions of volcanic rocks under ahigher temperature, oxidizing environment, and deeply sourced fluids(probably magmatic) were the main contributions (Li et al., 2013).
5. Discussion
5.1. Theoretical formation process of ore body sedimentary fans
For the SEDEX deposits, the theory of ore genesis by submarineexhalation on the seafloor is now widely accepted (Gu et al., 2012).The SEDEX deposits are distinctive in that it can be shown that the oreminerals were deposited in a marine second-order basin environment,related to discharge of metal-bearing brines into the seawater. Thegenetic model for SEDEX mineralisation is varied, depending on thetype of ore that is deposited by sedimentary exhalative processes(Clark et al., 2004; Tornos and Heinrich, 2008; Yang and Scott, 1996,2005).
Large (1981) pointed out that there were two alternative discussionson sediment-hosted submarine exhalative deposits: a morphologicalclassification upon empirical features, or a genetic one based on a par-ticular interpretation of these features. The main problem of the mor-phological classification of mineral deposits was the impossibility ofcharacterizing a class of ore deposits by one or two features (Large,1981). He suggested that sediment-hosted submarine exhalativemineralization can be explained by the “submarine exhalative” model,and those deposits are formed on the sea floor from hydrothermalsolutions discharged into the sea. Two of the possible sources of themetals were proposed: 1) leached from underlying sediments andvolcanics and 2) juvenile derivation from magmatic processes (Large,1981). Convective circulation of the metal-bearing solution wassuggested to be related to sea water circulation within a great size ofthe convective cell beneath the sea floor. Precipitation of the metalsulfides occurs as a result of four variables: a decrease in temperature,dilution of the solution, an increase in pH towards neutrality, and an
increase in concentration of reduced sulfur. The following representa-tive models and reviews on SEDEX deposits given by Russell et al(1981), Kimberley (1989), MacIntyre (1991, 1995), Gu et al (2003),Seal (2004), Clark et al (2004), Goodfellow and Lydon (2007), Large
of the different ore types in the Zhaokalong deposit. Normalized values for chondrite are from
et al (2008), Wilkinson (2010), Radulescu (2010) and Wang et al(2014) discussed the morphological characteristics and/or ore genesisseparately, but few of them explained the genesis of the deposits com-bined with the ore body morphology.
uid inclusions and (d) homogenization temperature and salinity patterns influid inclusionsTaylor and McClennan (1985).
Note: The calculation of density follows the methods in Liu et al. (2000); the calculations of pH and Eh use the methods mentioned in Wang and Wang (2011).
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In this study, we believe that the ore forming fluids of SEDEXdeposits can be considered as a kind of “turbidity currents.” These fluidsoriginate from the exhalative process, passing through syngeneticfaults, then flowing into the sea basin. Transport of these brines followsstratigraphic reservoir pathways toward faults, which isolate the buriedstratigraphy into recognizable sedimentary basins. The brines percolateup the basin-bounding faults and are released into the overlying oceanicwater. The mineralizing fluids originated as seawater or as evaporatedseawater and migrated downward through sedimentary basins intonetworks of interconnected fractures within their basement (Muchezet al., 2005). In regions characterized by pronounced extension andheat production, the ore-bearing fluids were expelled back upwardsalong extensional faults and associated fracture networks during faultrupture. This circulation pattern caused the formation of the pipe faciesof SEDEX-type deposits. The mineralizing fluids were carried upwardswithin sedimentary units toward basin-bounding faults. The fluidsmoved upwards due to thermal ascent and pressure of the underlyingreservoir. Fluid density and sea water overpressure play importantroles in the transportation of fluid. Faults which host the hydrothermalflow can show evidence of this flow due to development of massivesulfide veins, hydrothermal breccias, quartz and carbonate veiningand pervasive ankerite–siderite–chlorite–sericite alteration (Kelleyet al., 1992). During the exhalative process, the fluid density is not sohigh, so fluids can be transported easily. Once the original fluid encoun-ters the seawater, the pH and Eh environment changes tremendously,resulting in a strong reaction between fluid and sea water, and withthe increase of fluid density the fluids change into a kind of turbiditycurrents which contains a lot of mineral particles of different sizes.The turbidity currents migrate under the influence of ocean currents.In this process, the ore-forming temperature decreases gradually;increasing seawater elements and wall rock components add into thecurrents as a whole ore-forming fluid system. Regarding the sources ofthe ore minerals, some researchers expect that the elements could bescavenged from seawater by exhalative particles in a hydrothermallyderived plume that finally accumulated on the seafloor (Canet et al.,2004). This type of accumulation can form the sedimentary fan for theexhalative sulfide deposits. Trap sites are found in lower or depressedareas of the ocean topography where the heavy, hot brines flow andmix with cooler sea water. This causes the dissolved metal and sulfurin the brine to precipitate from solution as a solid metal sulfide ore,deposited as layers of sulfide sediment as a fan shape. Upon mixing ofthe ore fluids with the seawater, the ore constituents and gangue aredispersed across the seafloor and precipitated to form an ore-bodyand mineralization halo which are congruent with the underlyingstratigraphy and are generally fine grained, finely laminated and canbe recognized as chemically deposited from solution, whose density
Table 5Aqueous and gaseous compositions of the quartz-hosted fluid inclusions in the Zhaokalong de
has slightly decreased. Fluids eventually discharge onto the seafloor,forming really extensive, stratiform deposits of chemical precipitates.Sometimes, synthesis of the stratigraphically repeated mineralizedsequences in this kind of deposit suggests that mineralization wasepisodic, accompanied and periodically interrupted by turbiditicsedimentation. Each exhalative episode was preceded by a period ofmixed, both chemical and turbiditic sedimentation (footwall alterationblanket), followed by a peak release of hot metalliferous brines(stratiform ores and associated stringers), and terminated by a waninginflux of hydrothermal fluid accompanying turbiditic deposition(hanging-wall alteration blanket) (Gu et al., 2012).
5.2. Zoning genesis of ore body sedimentary fans
5.2.1. Texture and structureIn the formation process of ore body sedimentary fans, due to the
changes of fluid composition and mineralization environment, eachpart of the fan may be different from the other, having their ownfeatures. According to our sedimentary fan model, the fans can bedivided into at least three facies: pipe, central and marginal facies.Pipe facies occurred near the venting area due to the collapse of therapid deposition and impact of exhalative fluid. It is characterized byblock and brecciated structure of chert, soft sedimentary deformationstructures and slump structure of exhalative sediments. Slightly awayfrom the venting area, temperature decreased and deposition strength-ened, forming the central facies. Sometimes, hydrogen sulfide gas in hotwater is conducive to the growth and reproduction of organisms,resulting in the formation of the spherulitic structure of biological char-acteristics. Away from the vent center, typical sedimentary structuresare the majority because of relatively quiet waters. Further away fromthe venting, it reaches the marginal facies. Because of the low contentof silicon in hot aqueous solution, deposition rate becomes slower; atthe same time, siliceous rocks and sedimentary rocks show transitionalcontact because of terrigenous input. The changes of these facies areparticularly evident in many SEDEX type deposits in China, such as thedeposits in the gold–copper–uranium metallogenic belt in the southsubzone of Western Qinling (Zheng and Liu, 1993). In the Xitieshandeposit, the altered stockwork rocks and unstratified ore bodiesrepresent the pipe facies, whereas stratified ore bodies characterizethe central and marginal facies. Within the ore sedimentary fans, thedecrease in sulfide mineral granularity from the center to the edgecorresponds to the transition of the sedimentary fan facies from thecentral to the marginal.
Based on the different textures and structures in the different faciesof the fans, it becomes much easier to determine the vent and mineral-ization center. For example, in the Dongshengmiao Pb–Zn–S deposit
(Miao and Ran, 1992), cloddy-spotted, brecciated structural ores werefound in the central-northern part of the mine; whereas laminated,nodular and nutty structural ores are located in the eastern, westernand southern areas. It can therefore be speculated that the exhalativevent center lies in the central northern part of the mine, where thestructure of the ores shows the hydrothermal mineralization character-istics. On the other hand, ores from the eastern, southern and westernparts show the deposition characteristics because of their distancefrom the venting center. Subsequent explorations have proven theaccuracy of this speculation (Zhang et al., 1999, 2010).
5.2.2. MineralizationThe ore bodies of SEDEX deposits are usually stratified and zoned.
The formation of zoning in mineralized elements is due to the differentsolubility and migration of compounds which were formed by theore-forming elements (Betts, 2004; Tornos, 2006). In the Xietieshandeposit, high-grade lead–zinc ores are concentrated in the central faciesof the fans, whereas pyrite content increases significantly in themarginalfacies. In the Jinding Pb–Zn deposit, distinct zonation from top (marginalfacies) to bottom (central facies) also exists (Ye et al., 1993). TheZhaokalong deposit also shows significant mineralization zoning in asingle ore-forming system. Laterally, Fe and Cu are deposited in thecentral facies of the fans, whereas Zn, Ag and other relativelylow-temperature minerals are located in the marginal facies (Fig. 9).The ore bodies of the Jingtieshan iron–copper deposit are stratified,with the copper ore body at the bottom and the iron ore body at thetop (Fig. 12a, Xue et al., 1997). The Luchaichong polymetallic deposit inSoutheast Yunnan Province also shows the vertical and horizontalmineralization zoning characteristics (Meng et al., 1998). Vertical zoningfrom bottom to top is Pb–Zn (Ag) → Ba → Cu (Pb, Zn) → Ag (Pb,Zn)→ Ba–Mn; lateral zonation from the east (synsedimentary fault) towest is Pb–Zn (Ag), Ba → Ag → Mn–Fe (Fig. 12b). The ore bodies ofthe Jiande copper deposit (Zhejiang Province) also have a distinctzonation, with Cu rich ores in the bottom yet Pb–Zn rich ores in thetop. Horizontally, the Cu/(Cu + Pb + Zn) ratios in the central fanfacies are much higher than that in the marginal fan facies, whereasZn/(Cu + Pb + Zn) values show the opposite phenomenon (Liu et al.,1996). In the Laerma gold–copper–uranium deposit in West Qinling,Cu mineralization is concentrated at or near exhalative venting areas,whereas most of Au mineralization occurs in chert which is locatedaway from the venting center. Vertically, Cu mineralized in the lowerpart, Au deposited in the medium whereas U gathered at the top (Liuand Zheng, 1992).
5.2.3. Trace elementsIn sedimentary exhalative deposits, trace elements show different
enrichments in different fan facies. Pipe facies are enriched in elementsthat have a close relationshipwith deep sourced hydrothermal activities(Canet et al., 2004), whereas central and marginal facies have moreconcentrated elements from sea water.
Fig. 12. (a) Synsedimentary section diagrams of (a) the Jingtieshan iron–copper deposit (after
In the Zhaokalong deposit, As, Sb, Hg, Mn, Co, Ni and other indicativeelements are generally abundant in the ores. Part of this enrichmentcould be derived from a direct precipitation from seawater, favored bythe euxinic conditions of the basin. These elements could be scavengedfrom seawater by exhalative particles in a hydrothermally derivedplume that finally accumulated on the seafloor (Canet et al, 2004).Profiles of redox-sensitive trace elements show great W, Ni, Co, Moand Bi enrichments in the magnetite ores, and high values of theseelements could be of deep hydrothermal origin. Additionally, magnetiteore has lower Hg and As content, higher Fe3+/Fe2+ ratios, with Co/Nivalues greater than 1 (Fig. 11b), indicating that oxide-type ore has acloser genetic relationshipwith deeply sourced fluid, formed in relativelyoxidizing environments. It is further suggested that the iron formations,which are often, but not always genetically connected with volcanism,have their iron source from volcanic exhalations. In this study's orebody sedimentary fan model, magnetite ores and sulfide ores belong tothe pipe (or central) facies and marginal facies, respectively.
There is another example from the Woxi SEDEX type polymetallicdeposit in Hunan Province (Gu et al., 2012). Geochemical data of thisdeposit show that, with increasing distance above and below themineralized horizons, contents of W, Au, As, and Sb systematicallydecrease, whereas concentrations of other trace elements such as Hf,Sc, Th, Ta, Y, Zr, Nb and REEs gradually increase. This may indicate adecreasing hydrothermal input in the sediments and an increasingdominance of detrital and seawater-derived components. The highconcentrations of W, Au, As and Sb are commonly interpreted to be ofhydrothermal origin, whereas the low contents of the other traceelements are attributed to rapid accumulation rates and rapid burial ofhydrothermal precipitates in the sediment pile (Lottermoser, 1991).
Rare earth element geochemistry has been used to constrain thegenesis of exhalative deposits, especially for the exhalites that arespatially associated with them (Parr, 1992; Song et al., 1997; Steineret al., 2001). In the Zhaokalong deposit, different REE patterns betweenthe magnetite ores and sulfide ores suggest the different ore-formingenvironments and the transition frompipe (or central) facies tomargin-al facies in the ore body sedimentary fans. Negative Ce of ore-forminghydrothermal fluid has a close relationship with the added water fromthe sea (Ding et al., 2003; Yan et al., 2005). As the samples exhibitminor negative Ce anomalies, it may suggest that a submarineore-forming process existed. The magnetite ores have similar REE pat-terns with the andesite (Li et al., 2011), suggesting the iron-containingfluid originated from deep sources. On the other hand, sulfide oreshave positive Eu and negative Ce anomalies with similar REE patternsto the liquids and sediments of modern submarine hydrothermalsedimentary systems, indicating a similar formation process as subma-rine hydrothermal sedimentary system deposits (Canet et al., 2004;Douville et al., 1999; Gu et al., 2007; Li et al., 2007; Li et al., 2013).
Some ratios of trace elements (e.g., Co/Ni, S/Se) can be used todetermine the source of ore forming fluid of SEDEX deposits. In theZhaokalong deposit, the Co/Ni values of the sulfide ores are generallyless than 1. In the Jiande copper deposit, the pyrite of Fe–Cu ores from
Xue et al., 1997) and (b) the Luchaichong polymetallic deposit (after Meng et al., 1998).
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the bottom of the ore bodies has average Co/Ni ratios that are greaterthan 1. The S/Se ratio is less than 1.15 million, indicating that Fe–Cumineralization is closely linked to volcanic hydrothermal activity. Incontrast, the pyrite of the Pb–Zn enriched ore from the upper andflanking sides of the ore bodies has opposite Co/Ni and S/Se ratios,suggesting a hydrothermal sedimentary origin for the Pb–Zn ore (Liuet al., 1996). Therefore, it can be concluded that the Fe–Cu ores fromthe bottom of the ore bodies were formed during the initial stage ofsubmarine exhalation, whereas the Pb–Zn mineralization was formedby later deposition when the exhalative fluid vented into the sea basinand mixed the seawater. Similar phenomenon also occurred in theDongshengmiao Pb–Zn–S deposit, Inner Mongolia. The pyrite from thenorthern part of the mine has a higher ratio of Co/Ni, which is greaterthan 1; the pyrite from the eastern, western and southern areas showsopposite characteristics, with the ratios of Co/Ni smaller than 1. Theseindicate that the exhalative venting center of the Dongshengmiaodeposit lies in the northern part of this mine. The lower ratio of Co/Nican be explained by constant addition of seawater, which contains alow ratio of Co/Ni, when endogenetic hydrothermal fluid vents intothe sea basin and migrates to the seafloor. Furthermore, these gradienttrends of Co/Ni and S/Se ratios indicate that sedimentationwas graduallyenhanced from the central facies tomarginal facies, and all of this provedthe reliability of the sedimentary fan model.
5.2.4. IsotopesIn the Xitieshan deposit, the pipe facies and stratified ore bodies
(central facies and marginal facies) of the fans have different δ13CCO2values (ranging from −5 ± 2‰ to 0‰), indicating that deep-sourcedfluid played an important role in the pipe facies, whereas shallow fluid(containing seawater) might be essential in the formation of the centraland marginal facies.
Meng et al (1998) carried out C and O isotope analysis for the 18carbonate samples that are related to different types of ores in theLuchaichong SEDEX-type polymetallic deposit, Yunnan Province. Theresults showed that the δ13C and δ18O values at the bottom of eachsedimentary cycle (central facies) are higher than that at the mediumor top parts (marginal facies). The general range of isotopes frombottom to medium (or top) are δ13C: −0.6‰ to −2‰, δ18O: −6‰ to−8‰ → δ13C: −2.0‰ to 3.5‰, δ18O: −10‰ to −15‰. Thermaldiffusion and water desalination can explain the isotopic zoning (Liuet al., 2012; Meng et al., 1998). In each exhalative-sedimentary cycle,ore-forming thermal fluid vented into the sedimentary basins andmixed seawater, causing the temperature of the fluids to decreasewith increasing distance of transportation. In this process, heavyisotopes (e.g. 13C, 18O) were likely precipitated due to the highertemperatures, whereas the content of light isotopes (e.g. 12C, 16O)tends to increase at the medium or top of the cycles. Another possiblefactor is the desalination of seawater.With the intensification of deposi-tion, surfacewater that containedmore light isotopes (e.g. 12C, 16O)mayhave affected the deposition, resulting in the decrease of δ13C and δ18Ovalues in the marginal facies of the ore body sedimentary fans.
5.2.5. Temperatures and salinitiesThe fluid inclusions in each facies of the ore body sedimentary fans
show different characteristics. The temperatures and salinities in theinclusions associated with the pipe facies are generally high, followedby the central facies and the marginal facies. In the Jiande copperdeposit, the homogenization temperature of the massive Fe–Cu sulfideores from the central facies is around 300 °C, whereas the bandedPb–Zn enriched ores from the marginal facies have a temperature of150 °C (Liu et al, 1996).
Based on the fluid compositional analysis in the Xitieshan and theZhaokalong deposit, it can be conceded that the pipe and central faciesusually contain higher Ar, N2, H2, CO2, F–, Na+ and K+. On the contrary,the marginal facies generally has higher contents of SO4
2–, Mg2+, Ca2+
and CH4. This suggests that from the pipe and central facies to the
5.2.6. Redox environmentThe redox environment of the pipe facies can be reduced (e.g., the
Xitieshandeposit) or oxidized (e.g., the Zhaokalongdeposit), dependingon the conditions of the deep-sourced fluids and exhalative processes.In these deposits, the massive ores from the central facies have higherpH but lower Eh values, compared to banded ores from the marginalfacies. It may suggest that the environment at the marginal faciescould be more oxidized than that at the central facies (e.g., Liu et al.,1996; Meng et al., 1998). Additionally, the hematite ores are foundlocated at the edge of the ore body fans in the Zhaokalong deposit(Fig. 9), indicating that the marginal facies was formed in a relativelyoxidizing environment.
One of the most controversial aspects of the genesis of sedimenthydrothermal mineralization is the source of fluids and metals and thephysical and chemical conditions of material transport. Solubilityconsiderations suggest that metals and reduced sulfur cannot beefficiently transported together in sufficiently high concentrations in amildly acid to neutral fluid at temperatures below about 350 °C(Tornos and Heinrich, 2008). Unrealistically high volumes of fluid andlong-lived convection systems would be required to form a significantdeposit (Solomon and Heinrich, 1992; Spirakis and Heyl, 1995). Thus,it seems a prerequisite that effective metal-transporting fluids must bepoor in reduced sulfur (e.g., Anderson et al., 1998; Kyle and Saunders,1996; Plumlee et al., 1994). Two main alternatives have been proposedby Tornos and Heinrich (2008), including (a) the transport of metalsand sulfur by different solutions that later mix, or (b) the joint transportof metals with sulfate, which is later reduced in the depositionalenvironment (e.g., Cooke et al., 2000; Hinman, 1996; Plumlee et al.,1994; Spirakis and Heyl, 1995). Therefore, it seems that during theformation of the deposited fans, the redox environment can be changedfrom reduction to oxidation, corresponding to the changes of faciesfrom central to marginal.
5.2.7. Fluid densityIn the Xitieshan and the Zhaokalong deposit, the central facies have
the highest fluid densities, followed by the marginal facies and the pipefacies. The different density values between the same facies of these twodeposits can be explained by the different overpressure of the seawater,and the ore bodies from the Zhaokalong deposit are estimated to bedeposited in relatively shallowwaters. The lowdensity of the pipe faciesfluid might be related to the buoyant effect of the exhalative process,whereas the high density of the central facies fluid could be generatedby the mixing of deep-sourced fluid and seawater, resulting in thedeposition of coarse ore minerals. In pace with the migration of the“turbidity currents,” the fluid densities slightly decreased, resulting inthe relatively longer transportation and finer mineral deposition.
5.2.8. SummaryBase on the Xitieshan, Zhaokalong and other typical sedimentary-
exhalative deposits in China, the facies characteristics for the sedimentaryfan model are summarized below (Table 6; Fig. 13).
6. The application of the model: Metallogenic target prediction
Using the “sedimentary fan”model, the overall ore-body shapes canbe easily discerned. According to the zoning characteristics of thesedeposits, it is essential to determine the location of the vent centerand the extending trend of ore belts. This study attempted to restorethe whole form of the fans in the Xitieshan and the Zhaokalong deposit,and it would have great significance for metallogenic study andmineralization forecast.
Table 6Facies characteristics of the sedimentary ore-body fans from the typical sedimentary-exhalative deposits in China.
Facies Ore body occurrences Mineralization Enriched trace elements Fluiddensity
Fluidtemperature
Fluidsalinity
Redoxenvironment
Pipe Unstratified ore bodies; Block, brecciated and stockworkstructures
Fe, Cu (Au) W, Mo, Bi Low High Medium Reducing–oxidizing
Central Stratified ore bodies; massive and banded ore structures withlarge mineral particles; Bouma sequences
Fe, Cu, Pb, Zn W, Mo, Bi, Ni, As,Hg Co/Ni N 1
High Medium High Reducing
Marginal Stratified ore bodies; bedding structure withsmall mineral particles and sedimentary rhythm
In the two-fan model of the Xitieshan deposit, an asymmetric shapeand structure have been ascertained. The asymmetry of the sedimentaryfans can be ascribed to the roles of relay slope and directional currents(Fig. 4). The deep extensions of ore bodies are controlled by the directionof ore fluid migration, whereas marine slopes can also change the basicforms of the sedimentary fans as an external factor (Philip and John2005; Stow and Mayall 2000). The upper parts of typical ore fans alwayshave emanative characteristics, in a two-way form plunging downtowards the deep. In theXitieshandeposit, the ore body is generally tiltedeastward. This is because the relay slope controlled the direction of theexhalative fluid migration. Hence, the shape of the ore body is formedby the superposition of the standard fan and relay slope. These twofactors have the same direction to strengthen the east plungingoccurrence in the eastern part of the deposit. On the other hand, thesetwo factors have the opposite direction in the western portion. The orebodies still plunge to the west in the western portion of the upper partof the deposit. At approximately 3000 m level, the boundary becamevertical and then changed to an eastern plunge at depth. These morpho-logical characteristics correspond to the existing drilling data. Thewestern boundary of this deposit is forecasted to lie around Line 75 toLine 80. The ore bodies are unlikely to continue to extend westwards.In the east, the ore bodies extend at depth eastward. The boundarieshave a large plunge with a dip of about 70°. Notably, there is a limitedexpansion at the 2600 m level. We suppose that the main ore bodyboundary can be extended to Line 05. The deposit has been exploredwith a width of 2 km (from Line 75 to Line 0). So far, the vertical miningrange is 500 m (from surface to 2942 m level). The overall depth of thewestern fan is estimated to be at least 1000 m according to this study'sfan model. Therefore, there is still a 500 m extension from the 2942level to the deeper portions. Based on the observations of the recentdeep drilling cores, the ore bodies in the nearby levels are characterizedby large thickness, coarse crystal size and high-grade mineralization,still showing a central facies feature of the deposit. This indicates that itstill has a great extension space at depth.
6.2. The Zhaokalong deposit
Because of later fold transformations and fault cuttings (particularlythe large fault F1 in the northern part of themine), the sedimentary fans
Fig. 13. Sketch model of “sedimentary fans” to sedimentary-exhalative deposits.
of the Zhaokalong deposit are incomplete at present time; therefore, theore bodies that have been found in this deposit should only be part of thesedimentary fans. As mentioned earlier, we divided the ore-formingsystem into two parts, which correspond to the two sedimentary fansrespectively. Mineralization Zone I, II and Zone VI, VII belong to a samesedimentary-exhalative system, showing the transitional relations fromcentral facies to marginal facies. On the other hand, Mineralization ZoneIII and V have Pb–Zn–Ag mineralization, showing the characteristics ofmarginal facies of the other fan (Fig. 8b). So where is the central faciesto this fan? It indicates that the mineralization center of this system stilllies to the east, which is not completely exposed. We believe that thereis a considerable extension for the ore bodies, and it is possible to findthe new vent center outside of this mine in the east.
7. Conclusions
1. The ore-bodies from the Xitieshan and the Zhaokalong SEDEX-typedeposit are fan-shaped. Based on the zoning characteristics ofminer-alization, trace elements, fluid inclusions and isotopes, the single fanhas been classified into three facies: pipe, central, and marginalfacies.
2. The pipe facies is located close to synsedimentary fault channelways,characterized by unstratified altered stockwork rocks or ores, deeplysourced fluids and Fe–Cumineralizationwith high temperatures andsalinity. Central and marginal facies are formed by stratiform orebodies, showing typical characteristics of sedimentary origin, withPb, Zn, Ag and other relatively low-temperature mineralization.
3. Ore-forming fluids of SEDEX-type deposits can be considered as“turbidity currents.” These fluids come from the exhalative ventingcenter, passing through the syngenetic fault, flowing into the seabasin, and forming the ore bodies as a fan shape. The sedimentaryfan model can be used for metallogenic prognosis to obtain goodexploration results.
Acknowledgements
This work was supported by the China Scholarship Council (CSC)and the Global COE Program inNovel Carbon Resource Sciences, KyushuUniversity. Field work was supported by the Xitieshan Mine and theZhaokalong Mine, Qinghai, China. We thank Dr. Jillian Aira S. Gabo andDr. Thomas D. Tindell for their comments on the early draft. Twoanonymous reviewers are appreciated for the detailed and valuablesuggestions that helped us to improve the article. Special thanks aredue to Dr. Franco Pirajno (Editor-in-Chief) for his help in polishing thefinal article.
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