Metallogenic implication and geodynamical significance of iron skarn-related deposits in eastern Tianshan

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Skarn-related ore deposits (SROD)

The “skarnization” processes are often associated with economic metal deposition and concentration. In such cases these deposits are grouped into skarn deposits (Einaudi et al., 1981; Meinert et al., 2005). The term “skarn deposits” used by theses authors is ambiguous because mineralization is not necessary genetically and/or temporary linked with the skarnization processes. As “intrusion-related gold deposits” has been defined by Thompson et al. (1999), in the following sections of this manuscript the term “skarn-related ore deposit” (SROD) will be used to characterize an ore deposit associated, at least spatially, with a skarn.
SROD are widespread over the world and constitute important reserves in Fe, W, Pb/Zn, Cu, Au, Mo and Sn. Some skarn are caused by porphyry, which leaded Jebrak and Marcoux (2008) classified part of the so-call porphyry deposits into skarn. Pirajno (2009) also indicated that in the lower Yangtz area of South-East Chine, skarn-type deposits and porphyry deposits were initiallly linked. Tonnage and grade relations in porphyry and non porphyry environment for Au, Cu-Au and Pb-Zn-Au of world class deposits of SROD are listed (Fig. 1-4). Most skarn deposits are associated with intrusions, but distal skarn/skarnoid–related ore deposits are found spatially disconnected from plutons or intrusions. To classify or to establish a typology of SROD is not easy and not unequivocal because too many parameters interact (substance, depth of deposition, telescoping with porphyries etc…). The different metals found in skarn deposits are a product of the different composition of the protoliths, oxidation state, etc…According to Zhao et al. (1990), reserves of 25% iron, 50% W, 30% Pb-Zn, 100% phlogopite, and 100% vermiculite of the world would be contributed by skarn-related deposits. Skarn deposits are also important sources for the non-metallic elements and minerals, such as: tourmaline, phlogopite, diopside, wollastonite, tremolite etc..

Mineralogy: paragenesis, evolution and mineral zonation

Due to the strong temperature gradients from the magmatic intrusions or hot reative fluid toward the sediment wall rock and large fluid circulation cells caused by intrusion of magma intruding, many skarn deposits have zonations (Bowers et al., 1990). Mostly skarn displayed zonation that garnet-pyroxene and vesuvianite-(chlorite) assemblage close and far from the heat source, respectively. This zonation is characterized by mineral assemblage which evolves in different stage during the skarnization. Durand (2006) did a synthesis of the major mineral phases encountered within skarn systems caused by granitic intrusions (Fig. 1-5). As we can see on this figure, skarn is characterized by an extreme mineralogical variety, with a dominance of calc-silicated minerals. And, it seems that from the pluton into the limestone wall rock, zonation generally could be recognized. Garnet-pyroxene anhydrous minerals dominantly occur in the early and close to the pluton. For the garnet, it occurred preferably in the exoskarn side; whereas the K-feldspar alteration occurred in the endoskarn side. The epidote-chlorite-sulphide presents in the distal and later.

Homogenization temperature of fluid inclusions

Fluid inclusion studies are mainly on the minerals which contain numerous of fluid inclusions and are relative transparent, such as quartz, carbonate and fluorite. Most high temperature skarn minerals such as forsterite, garnet, diopside, etc. are unlikely to trap later low temperature fluids without visible evidence of alteration (Meinert, 1992). Thus, fluid inclusions in skarn minerals provide a relatively unambiguous opportunity to measure temperature, pressure, and composition of skarn-forming fluids. Fluid inclusion studies displayed that the homogenization of fluid inclusions of ore skarn solutions have much higher CaCl2 contents and usually very high formation temperatures (>500°C); temperatures generally decrease away from the solution source, both in time and distance; the gradients found at greater distances from the source in distal (far from contact) skarns tend to be less (e.g. 210–350°C) for a particular skarn stage to that in proximal (near contact) skarns (e.g. 400–650°C) (Kwak, 1986); temperatures also tend to decrease with time, which is reflected by the superimposition of various overprinting, retrograde mineral stages.

Homogenization temperature of melt inclusions

In recent 30 years, some researchers devoted to high temperature skarns (Lin and Xu, 1989; Wu and Chang, 1998; Fulignati et al., 2000; Fulignati et al., 2001; Zhao et al., 2003; Zhao et al., 2003; Gaeta et al., 2009). It is strongly supported by melt inclusion research (Fulignati et al., 2001; Zhao et al., 2003; Zhao et al., 2003). Melt inclusions in skarn minerals could hold silicate glass and non-silicate, vapor-bearing globules (Fig. 1-9) (Fulignati et al., 2001; Zhao et al., 2003; Zhao et al., 2003). Melt inclusions were thought to represent silicate melts at the time of skarn formation in magma (Zhang and Ling, 1993; Zhao et al., 2003). Therefore, some researchers believed that the skarn could be form directly by magmatic crystallization (Lin and Xu, 1989; Wu and Chang, 1998). Skarn rocks which are component of tephra ejected by Vesuvius were studied by Fulignati et al. (2000). Fulignati et al. (2000) found that skarn formed at temperatures of about 800~1000°C which leading them to concluded these rocks record in-situ endoskarn genesis at the interface between magma and carbonate rocks.

Reconstruction of fluid flow involved in skarn

It has been recognized for more than a decade that the mineralogy of many metamorphic rocks is controlled not just by the elevated pressures and temperatures but also by pervasive flow of chemically reactive fluids during metamorphism. One-dimensional models were used to predict the spatial distribution of mineral assemblage developed in siliceous dolomitic limestones (Ferry, 1994; Hanson and Ferry, 1995). Their result emphasized that mineral assemblages in metacarbonate rocks are controlled not only by temperature, pressure, rock composition, and fluid composition but also by the amount and direction of fluid flow. Besides, fluid flow controlled by structure during contact metamorphism was reported (Ferry et al., 1998; Cole et al., 2000). In order to constrict fluid flow during skarn formation, isotopic data were used frequently. The Bungonia limestone was taken as the study target for isotopic analysis, and the result showed that the protolith of limestone did not initially contain a fluid-filled porosity, but hydrofracture play an important role in the fluid circulation (Buick and Cartwright, 2000). It is noteworthy that, for the fluid flow involved in skarn, its geometry was playing an important role during fluid circulation. That is whether fluid flow was horizontal or vertical and up-temperature or down-temperature. There could be many ways to answer this question according for specific aureoles.

Temporal and spatial evolution of skarn

When a pluton intrudes into CaCO3-contained wall rocks, the early isochemical metamorphism and continued metasomatism at relatively high temperature is followed by retrograde alteration as temperature decline (Fig. 1-11). They are characterized by distinguished mineral assemblage. Contact metamorphism is not a typical isochemical recrystallisation (Einaudi et al., 1981; Meinert, 1984; Meinert, 1992). Heat, magmatic components such as Fe, Si, Cu, etc., transfer from the magma to the sediment wall rock, whereas the CaO and CO2 are inverse. This is the process that called bimetasomatism. In addition to the mineral assemblage, some mineral show compositional and colour gradients according to the distance of contact zone. For example, the garnets closed to the pluton may contain relative higher Fe and lower Ca, dark red-brown, towards the wall rock, the Fe and Ca content would systematically decrease and increase respectively (Meinert et al., 2005). Its colour becomes pale green because of appearance of epidote and chlorite. For some skarn systems, these zonation patterns can be « stretched out » over a distance of several kilometers and can provide a significant exploration guide.

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Table of contents :

Chapter 1 Skarn and iron oxides ore deposits: the state of art and problems
1.1 General definitions
1.1.1 Metamorphic rocks
1.1.2 Metasomatic rocks
1.1.3 Skarn
1.1.4 Skarn-related ore deposits (SROD)
1.2 Skarns: from objects to processes
1.2.1 Wall rocks
1.2.2 Mineralogy: paragenesis, evolution and mineral zonation Mineral paragenetic evolution Mineral zonation
1.2.3 Mass transfer
1.2.4 Skarn forming conditions Temperature Homogenization temperature of fluid inclusions Homogenization temperature of melt inclusions Pressure Chemical conditions Volatiles Salinity Redox and PH value
1.2.5 Isotopic geochemistry
1.2.6 Reconstruction of fluid flow involved in skarn
1.2.7 Temporal and spatial evolution of skarn
1.2.8 Experimental studies of “skarnization”
1.2.9 Skarn genetic models Metasomatic model Magmatic hydrothermal model
1.3 Skarn-related ore deposits (SROD)
1.3.1 Intrusion composition and tectonic setting of SROD
1.3.2 Calc-silicate/metal paragenesis
1.3.3 Giant Fe and Cu-Zn SROD Fe skarn deposits in Yangtze and the Andes Cordillera area Antamina Cu-Zn porphyry skarn deposits
1.3.4 A focus on skarn-related iron deposit Features of the largest skarn-related iron deposits Alkaline alteration Origin of iron Association with mafic magmatism Differences and similarities with Iron-oxides Copper Gold deposits
(IOCG) and other magnetite deposits
1.4 Problems: Major scientific questions on skarns and skarn-related iron deposits.
Chapter 2 Geodynamics and Metallogeny of eastern Tianshan orogenic belt
2.1 Tectonic units
2.1.1 The Northernmost belt (Dananhu island arc belt)
2.1.2 Northern belt (Aqishan-Yamansu belt)
2.1.3 Middle Tianshan terrane (MTT)
2.1.4 Southern belt (Beishan orogen)
2.1.5 Northern shearing Zone (Kangguer shearing zone)
2.2 Strata
2.2.1 Northernmost belt (Dananhu island arc belt)
2.2.2 Northern belt (Aqishan-Yamansu belt)
2.2.3 Middle Tianshan terrane (MTT)
2.2.4 Southern belt (Beishan orogen)
2.2.5 Summary of the strata
2.3 Magmatism
2.3.1 Granitic plutons
2.3.2 Mafic-ultramafic plutons
2.3.3 Volcanite
2.3.4 Chronology of magmatism
2.4 Tectonic evolution of eastern Tianshan
2.4.1 Welding of the Central Asia Orogenic Belt (CAOB)
2.4.2 Welding of the eastern Tianshan
2.5 Mineralization in the eastern Tianshan
2.5.1 Geochronological synthesis of mineralization
2.5.2 Mafic-ultramafic intrusion related Cu-Ni-(V)-(Ti) deposits
2.5.3 Gold mineralization Northern gold belt (Kangguer belt) Epithermal gold Quartz vein gold Orogenic gold Southern gold belt (Beishan belt) Geochronology of major gold deposits
2.5.4 Porphyry Cu deposits
2.5.5 Skarns in eastern Tianshan Iron skarn deposits Cu-Ag-Pb-Zn skarn deposits
Chapter 3 Yamansu magnetite deposit
3.1 Introduction
3.1.1 Previous studies on YMD Strata Geophysical characteristics Isotopic data Metallogeny Ages
3.1.2 Questions
3.2 Research methods
3.3 Geology
3.3.1 Outcrop morphology, faults and kinematic analysis
3.3.2 Arkose Geology Zircon dating
3.3.3 Dykes
3.4 Skarn
3.4.1 Wall rocks Basalt Limestone
3.4.2 Skarn
3.4.3 Ore shoots
3.4.3 Skarn transitions Transition from iron-rich-fluid to skarn Transition from basalt to skarn Later altercation on skarn
3.4.4 Skarn mineralogy Prograde stage minerals Garnet Pyroxene Magnetite Retrograde stage minerals Stilpnomelane K-feldspar Tourmaline Axinite Epidote Chlorite Sulphides
3.4.5 Dating on the K-feldspar-related stage
3.5 Geochemistry
3.5.1 Major and trace elements Basalt Limestone Skarn
3.5.3 Mass balance Equations Mass balance result
3.6 Rock Magnetism study
1. Introduction
2. Geological setting
3. Yamansu magnetite deposit (YMD)
4. Paleomagnetic and magnetic fabric study
5. Discussion
3.7 Interpretation and discussion
3.7.1 Contribution for the massive magnetite ore shoots
3.7.2 Felsic fluid contributions
3.7.3 Magnetite Silicon in the magnetite Phosphorus content Intercalated magnetite and garnet
3.7.4 Constriction on the skarn forming time
3.7.5 Skarn forming model
3.8 Conclusion
Chapter 4 Iron oxide-rich melt separation from mafic magma: the case study from Cihai skarn-related magnetite deposit, Eastern Tianshan, NW China
The Cihai magnetite deposit (CMD): geological background and previous studies
Lithology and petrology
(1) Pyroxene-rich skarn
(2) Garnet-pyroxene skarn
(3) Late alteration
The metapelites
The mafic dyke swarm
Ore shoots
The “black phase” in CMD
Mineral texture and chemistry
Albite within black phase
Hydrothermal VS magmatic magnetite
Chemical corrosion of the pyroxene in black phase
Melt immiscibility
Table C-2: EPMA analysis of pyroxene (wt%) and structural formula
Table C-3: EPMA analysis of garnet (wt%) and structural formula
Table C-4: Representive EPMA analysis result of magnetite (wt%) and structural formula
Table C-5: EPMA analysis of albite (wt%) and structural formula
Table C-6: EPMA analysis of chlorite (wt%) and structural formula
Table C-7: Chemical analyses of major elements (wt%) and trace elements (ppm) for Cihai deposit
table C-7 continued:
Chapter 5 Metallogenic implication and geodynamical significance of iron skarn-related deposits in eastern Tianshan
5.1 Short reviews on Yamansu and Cihai magnetite deposits
5.2 Short review on ore deposits of eastern Tianshan
5.2.1 Iron deposits
5.2.2 Cu-Ni-V-Ti-Au deposits
5.3 Geochronology and ore forming in eastern Tianshan
5.4 Discussion
5.4.1 Source of the magma
5.4.2 Plume VS. post-collisional stage
5.4.3 Skarn and mafic/ultramafic magmatism
5.4.4 Geodynamic evolution and mineralization
Conclusions and perspectives
APPENDIX A: Method of rock magmatism
1: Magnetic anisotropy
2:The Earth’s magnetic field
3: Geocentric axial dipole model
4: Demagnetization methods
5: IRM acquisition curves
6: Magnetic minerals
APPENDIX B: U-Pb dating result
APPENDIX C: Mineral chemistry of Yamansu magnetite deposit
APPENDIX D: Whole rock geochemistry of Yamansu deposit
APPENDIX E: Isotope composition


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