Iron oxide-rich melt separation from mafic magma: the case study from Cihai skarn-related magnetite deposit, Eastern Tianshan, NW China

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9 Skarn genetic models

Several models have been proposed to the skarnization process. The following section mainly discusses the two prevailing models.

Metasomatic model

In its broadest sense, it is a process of mass and chemical transport and reaction between adjacent lithologies. It seems that all skarn forming processes involved fluids. Fluids enriched in Si, Fe, Al and Mg were mainly derived from the magma. These fluids would dissolve the carbonated and convert them into skarn. This process is known as part of metasomatism. According to experimental result, as well as field observation, the metasomatic process could effectively concentrate economic elements (Liang, 2000). For a long time, skarn are also called contact metasomatic rock because they develop in/near the contact zone between acid-basic- intrusions and carbonate rocks (Fig. 1-13). High temperature magma will produce a temperature gradient toward the relatively cooler sediment wall rock; exsolved fluids from the magma during its emplacement could cause extensive reactions and results in the deposition of metals.
Skarn can be subdivided into exoskarn and endoskarn depending on whether the metasomatic assemblage is internal or external to the intruding pluton. Exoskarns occur at and outside the granite which produced them, and are alterations of wall rocks. Endoskarns, including greisens, form within the pluton, usually late in the intrusive emplacement and consist of cross-cutting stockworks, cooling joints and around the margins and uppermost sections of the granite itself.
The diversity of different metals found in skarns is controlled by different compositions, fO2, and even different tectonic context of the igneous intrusions. Another criterion, skarns could be classified into magnesian and calcic, to describe the dominant composition of the protolith and resulting skarn minerals. These terms can be used combined. Most of large, economically viable skarn deposits are associated with calcic exoskarns (Robb, 2005). Even though there are different metal associations in skarn deposits, the processes by which these deposits form are similar: First of all, magma and/or hot fluid emplacement, and then the extensive metasomatic reactions between the intrusions and the wall rocks.

Magmatic hydrothermal model

An alternative is the magmatic hydrothermal model. This model is not new but not much mentioned. In 1953, in order to explain vein-like skarn, a skarn-type liquid/melt had been proposed (« #$%&&’ « &()* X. M.(translated by Xu, 1953). The author believed that the skarn-type liquid/melt was different from the metasomatic-type. In recent years, this point was reviewed and confirmed, mostly basing on melt inclusion and petrologic observation (Lin and Xu, 1989; Wu and Chang, 1998; Fulignati et al., 2001; Zhao et al., 2003; Zhao et al., 2003). The mainly difference between the metasomatic and magmatic hydrothermal model is that the magmatic hydrothermal model has extremely high temperature mineral assemblage, such as garnet-pyroxene formed at about 900-1000°C. This temperature range is not long belonging to the scope of metasomatism. In this temperature range, all has become melt or liquid state (Lin and Xu, 1989). When the temperature decline, some Ca-silicate melt could crystallize into the skarn mineral assemblage. Fulignati et al. (2001) indicated that the high temperature skarns represent the magma chamber–carbonate wall-rock interface. The skarnization process results from the crystallization of a Ca silicate melt or from the consolidation of a cryptoexplosion breccia (Lin and Xu, 1989). Wu and Chang (1998) divided high temperature magmatic skarn forming process into two stages:
1) Stage of crystallization of Ca-silicate minerals. In this stage, mainly euhedral/subhedral crystals form with obvious accumulation features and often holes. Clinopyroxene is generally earlier than garnet. Early clinopyroxene is diopside, accompanied by magnetite and andradite; later clinopyroxene are mainly salite with small amount of aegirine composition. Early garnet is homogeneous andradite; the later are mainly strong anisotropic grandite, such as Tongling skarn Fe deposit (Fig. 1-14).
2) Stage of alkali aluminum silicate and carbonate minerals crystallization. Later alkali aluminum silicate minerals, potassium feldspar and/or sodium oligoclase (An =11% ~ 13%) and calcite and anhydrite, etc. would crystallize from the melt. They infill gap or crack. This mineral assemblage do not overprint on the primary stage, these minerals do not altered diopside and grandite, and especially, the gross alkali feldspar and calcite containing diopside/garnet/sphene/apatite inclusion indicate that it is rather a later crystallization.
The mechanism of the magmatic skarn forming process is not clear, however, the sediment assimilation of magma may provide some clues. Skarn xenoliths found in the large (70km*10km) picrite pluton leaded the previous researcher to conclude that the skarn are the products of dolomitic xenoliths to suffer high temperature contact-thermal metasomatism (Yu, 1985). After the study of skarns and cumulates formed at the contact between a magma chamber and its wall rocks, Gaeta et al. (2009) concluded that a ‘skarn environment’ can act as a source of CaO-rich silicate melts. In certain conditions, this CaO-rich silicate melts could crystallize into special mineral assemblage, such as garnet and pyroxene (Wu and Chang, 1998). This CaO-rich silicate melts could be come from the assimilation of sediment wall rocks. In addition, experimental study of limestone assimilation by hydrated basaltic magmas under the physical conditions of 1050–1150°C in temperature and 0.1–500 MPa in pressure shows that desilicated and alkali-rich magmas could be generated by assimilation of sedimentary carbonates, and the desilicated trends are negatively correlated to the increase of the limestone assimilation (Iacono Marziano et al., 2008).
a. Peritectic massive sulphide ore aggregates andradite and grandite growth boundary: 1-andradite (homogeneous) aggregates; 2-strong un-homogeneous andradite grow boundary; 3-sulfide.
b. Garnet adcumulate:1-Automorphic andradite; 2-Non-homogeneous grandite growth boundary; 3-K-feldspar; 4-sulfide.

Skarn-related ore deposits (SROD)

Intrusion composition and tectonic setting of SROD

As a general rule: (1) Fe and Au SROD tend to be associated with intrusions of mafic to intermediate compositions (low silica, iron-rich, relatively primitive magmas); (2) Cu, Pb, Zn and W SROD are linked to calc-alkaline, magnetite-bearing, oxidized (I-type) granitic intrusions; (3) and Mo and Sn SROD are related to more differentiated granites that might be reduced (S-type). The igneous rocks associated with skarn deposits range in composition from gabbro to high evolved granite. It seems that all the deposits subclasses are associated with subalkaline to alkalic calca-calklaline magmatism; and they show systematic geochemical variation from Au, Fe Cu, Zn-Pb, W to Sn deposits (Fig. 1-15) (Ray and Webster, 1991).
Fig. 1-15: AFM plot illustrating the calcalkaline affinities of the skarn-related intrusions to the left and principal oxide plot illustrating variable compositions of the skarn-related intrusions to the right (Ray and Webster, 1991).
Tectonic setting and magma petrogenesis maybe intimately linked, at first approximation, types of SROD might be favored by certain tectonic setting: Mo and Sn SROD would be favored by highly evolved crust in a post-subduction/collision context, whereas large iron SROD would be most related to volcanic arc, close to the subduction zone (Ray and Webster, 1991; Kuscu et al., 2002). Towards the inner stable continental, protolith plutons are more SiO2 in chemical composition and associated skarns are most likely rich in Mo or W-Mo with lesser Zn, Bi, Cu, and F. Many skarn are polymetallic, and with locally important Au and As occasionally. This transition from subduction zone to the inner part of the continental probably reflects the thickness, contamination, heat flux, composition of the crust. Because magmatism associated with shallow subduction angles may have more crustal interaction (Corbett and Leach, 1998). However, it is noteworthy that the presence of skarn does not necessarily indicate a particular geological setting or a particular protolith (Meinert, 1992). An idealized tectonic model for skarn formation is illustrated in Fig. 1-16. However, occurrence of SROD does not indicate certain geological back ground. It is noteworthy that there is very few Precambrian skarn and SROD which mostly formed during Phanerozoic times where plate tectonics and carbonate sedimentation occurred.
Fig. 1-16: Idealized tectonic setting models for SROD formation (after Meinert et al. 2005). (A) Oceanic subduction and back-arc basin environment. (B) Continental subduction environment with accreted oceanic terrane. (C) Transitional low-angle subduction environment. (D) Post-subduction, continental rifting or continental plume environments.


Calc-silicate/metal paragenesis

The mineralogical composition, such as refractory garnet and pyroxene, are informative to indicate skarn-related mineralization types. Garnet and pyroxene almost present in all skarn types and show marked compositional variability. Meinert (1992) indicated that the assemblage of manganiferous pyroxene and johannsenite is important criteria to identity zinc skarns. That the valence of iron could vary leads it to act as redox indicator. Systematic compositional plots reveal significant difference between different skarn related deposits (Fig. 1-17). These triangular plots showed that Fe skarns are preferable for Al-Fe3+ garnet and Mg-Fe2+ pyroxene; Cu, Zn, Au, and Mo skarns are similar with Fe skarn for the garnets, while the Sn and W skarn are trend to be with (Mn+Fe2+) garnets. Cu, Mo, Sn, Au and Fe skarns are low in Mn for the pyroxene composition.

Giant Fe and Cu-Zn SROD

This section is to briefly introduce two famous types giant skarn deposits: Antamina porphyry skarn Cu-Zn deposit and Fe skarn deposit in the Andes Cordillera, Argentina and in Yangtze area, China. However, other skarn types deposit would share more or less common features. The aim is to provide a common outline of features of these types of deposits. These deposits are limited but representative.

Fe skarn deposits in Yangtze and the Andes Cordillera area

Except the massive sulfide deposits, the Fe skarn deposits are conspicuous in Yangtze area, China. According to Zhao (1989) who study hundreds of Fe skarn deposit in China, the occurrence of ore bodies of Fe SROD have some common features, such as (1) associated with small intrusion or along cupola/apex at the top of larger plutons; (2) the economic ore bodies mainly develop close to side of the sediment wall rocks, typically for iron skarn deposits; (3) the ore bodies are poor continuity. Besides, the top of the fold are favourable for ore bodies (Zhao et al., 1986).
The features of Fe skarn deposits in Andes Cordillera, Argentina have been summarized in other angle (Franchini et al., 2007). They concluded that the main features of the Fe skarns include: (1) association with mantle-derived middle Miocene (~15–11 Ma) diorite stocks and sills; (2) widespread alteration including epidote ± amphibole ± magnetite endoskarns, and zoned garnet (Grs:0–66; Adr:32.5–100) ± magnetite ± pyroxene (Di:24–50; Jo:2–9; Hd:74–41) exoskarns formed from oxidized, saline, high-temperature brines (530º–660ºC; 60–70wt.% NaCl equiv.); (3) and magnetite-hematite orebodies associated with quartz ± epidote
± calcite ± actinolite formed at lower temperatures (290º–436ºC) from saline fluids (32–50 wt.% NaCl equiv.) of magmatic origin.

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
Figures and tables in the article of section 3.6:
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
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


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