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This thesis summarizes the metallogenetic mechanism of the Galinge skarn deposit based on integrated knowledge of tectonics, geochemistry, geochronology, petrology, mineralogy, thermodynamics and hydrothermal fluids. It also discusses the multistage growth characteristics of various skarn minerals in which the varying compositions reflect the evolution of the hydrothermal fluid. The multidisciplinary nature of this research sheds new light on reconstructing metallogenetic processes successfully. It outlines the main aspects of skarn zonation based on the dominant contents of the skarn minerals and the wall rock compositions. In addition, it focuses on volatile-rich minerals including tourmaline and hastingsite, highlighting the importance of the volatile component in the skarn deposit. Lastly, it describes the regional tectonic–magmatic evolutionary history to explain the metallogenic principles, which can be used to guide prospecting in the field.

Inhaltsverzeichnis

Frontmatter

Chapter 1. The Qiman Tagh Orogen as a Window to the Crustal Evolution in Northern Qinghai-Tibet Plateau

Abstract
The Qiman Tagh Orogenic Belt (QTOB), located along the northern part of the Qinghai-Tibet plateau, was constructed through protracted accretion and collision of a collage of terranes during subduction and closure of the Qiman Tagh Ocean, a branch of Paleo-Tethys Ocean from the Neoproterozoic to Early Mesozoic. The orogen is located between the Qaidam Basin and Kumukuri Basin, and cut by the Altun Fault to the west. The early Neoproterozoic (ca. 1000–820 Ma) ages from this orogen suggest a link with the formation of the supercontinent Rodinia. The QTOB is tectonically divided into the North Qiman Tagh Terrane (NQT) and the South Qiman Tagh Terrane (SQT). The NQT developed as an active continental margin, and preserves abundant Early Paleozoic granitoids, which possibly formed through the melting of old basement, and a series of mafic–ultramafic rocks considered as VA (volcanic arc) type ophiolites. In contrast, the SQT witnessed intra-oceanic subduction, where SSZ (supra-subduction zone) type ophiolites are documented together with island arc tholeiite (IAT) and calc-alkaline lavas, in a primary oceanic island arc environment during the Early Paleozoic. With continued subduction, the young island arc was transformed into a mature island arc with thickened crust. This region preserves typical evidence for sedimentation and volcanism in the initial stages of volcanic arc development. The collision between the SQT and NQT occurred probably in the Late Silurian (ca. 422 Ma) and continued until ca. 398 Ma, as evidenced from the ages of the abundant within-plate granitic magmatism in the NQT that formed after 398 Ma. In the SQT, voluminous oceanic island arc granitoids formed during the Early-Middle Devonian (ca. 418–389 Ma), with contrasting geochemical features as to those in the NQT. The SQT is interpreted as an exotic terrane that has been incorporated into the continental margin and contributed significantly to the continental growth in this orogenic belt. A trench jam might explain the large gap (ca. 357–251 Ma) of granitoid magmatism. The final closure of the Paleo Tethyan Qiman Tagh Ocean might have occurred in the Late Permian, and resulted in the accretion of the Kumukuri microcontinent; which formed in response to the orocline formation of western Qiman Tagh Orogen and the rotation of the western South Qiman Tagh Terranes. A series of Y-depleted granitoids formed during Early-Middle Triassic (before 237 Ma), which might be associated to the partial melting of thickened lower crust induced by the oceanic lithosphere delamination. Subsequently, a series of calc-alkaline and alkaline granitoids generated through melting of older crustal material which were emplaced in the SQT, and their formation is interpreted to be linked with the transition from post-collision to within-plate settings. Our model is not only suitable to trace the tectonic evolution of the Qiman Tagh orogen, but also valid for the plate tectonic setting orogens in the modern earth.
Miao Yu

Chapter 2. Skarn Zonation and Mineral Geochemistry of the Galinge Skarn Deposit

Abstract
The Galinge deposit can be roughly divided into three proximal to distal skarn zones Based on the mineral assemblages and their compositions; i.e. the magnesian skarn (Mg-SK), the calcic skarn (Ca-SK) and the manganese-calcium skarn (Mn-Ca-SK) zones. The Mg-SK is developed in the II ore domain, and is featured by the widespread occurrence of Mg-rich minerals, including forsterite, spinel, diopside, tremolite, serpentine, Mg-chlorite, chondrodite and phlogopite. The Ca-SK, occurring in the IV and V ore domains, is associated with the metamorphism of mafic volcanic rocks and marbles, and is characterized by garnet, pyroxene, tourmaline, axinite, magnesian-hastingsite, ferro-actinolite and epidote. The distal Mn-Ca-SK zone of the VI ore domain contains johannsenite, galena and sphalerite.
Miao Yu

Chapter 3. Thermodynamic Model for the Galinge Fe Skarn Deposit in Qinghai

Abstract
The Galinge iron deposit in the Qiman Tagh orogen, western part of the East Kunlun, Qinghai province, occurs as lens-shaped magnesian skarn, with magnetite and base-metal sulfide orebodies, and is hosted in dolomitic limestone. It experienced a complete skarn and retrograde stage under varying fluid compositions resulting in thermodynamically controlled formation of magnesian skarn and mineralogical zonation. A series of Mg- and Ca-rich solid solutions were generated in the skarn stage, including forsterite-fayalite, spinel-hercynite-gahnite and diopside-hedenbergite solid solutions. A thermodynamic model setting pressure of 0.6 kbar and X(CO2)=0.3 was set up to trace the skarn evolution in the skarn stage. Magnetite is stabilized at fluid conditions of ca. 460 – 520°C and Δlog fO2 (HM) = -5 – -11 in the skarn stage, and co-precipitates with diopside and forsterite. Magnetite precipitation always shows strong relations with diopside and forsterite rather than fayalite and hedenbergite, which deplete iron from the fluid. The retrograde alteration stage is characterized by the formation of tremolite, chondrodite, phlogopite, clinochlorite, epidote, prehnite, serpentine, magnesiomagnetite and ludwigite. In the thermodynamic model of the retrograde alteration evolution, setting P= 0.6 kbar and X(CO2)=0.01. Most of the tremolite + diopside + magnetite and clinohumite + diopside + magnetite assemblages are stable at 360 – 460°C and Δlog fO2 (HM) = -16 – -5. The phlogopite is formed at a temperature range of ca. 360 – 420°C and Δlog fO2 (HM) = -11 – -6, and serpentine are stabilized below 460°C in the late retrograde stage. Their stability in the system are intensively effected by the Al2O3 activity of the fluid. The paragenetic sequence of retrograde minerals is most likely a result of internally buffered increasing oxidation state as the precipitation of magnetite. This suggests that oxidizing process is most important for understanding the major causes of skarn iron deposit formation in other areas.
Miao Yu

Chapter 4. Multistage Skarn-Related Tourmalines from the Galinge Deposit: A Significant Indicator for Varying Fluid Composition

Abstract
The Galinge skarn deposit, the largest iron polymetallic skarn deposit in the Qiman Tagh metallogenic belt (western China), was formed via multi-stage fluid-rock interactions. It is divided into six ore domains from east to the west. Skarn-related tourmaline is ubiquitous in the V ore domain of the Galinge deposit, occurring both in the altered basaltic andesite (Tour-I) and in the sandstone (Tour-II). The tourmaline composition in both rock types is within the dravite–uvite solid solution. Some Tour-I crystals show compositional growth zoning in which the early stage uvite cores (Gen-1) are overgrown by second-stage dravite rims (Gen-2). Some Tour-I crystals also show overgrowth rims and fracture-infilled textures (Gen-3). Some other Tour-1 tourmalines without clear growth zoning (Others) show an intermediate composition between Gen-2 and Gen-3.The varying composition of the zoned tourmalines records important information about the evolving hydrothermal fluids and host rocks. Gen-1 and Gen-2, displaying a narrow and high range of Fe2+/(Fe2+ + Mg) ratios, are much more equilibrated with mafic host rocks. The alkaline (K + Na) content of tourmalines is associated with the salinities of the ore-forming fluids. The lowest Na t K content of Gen-3 indicates that it may have been equilibrated with a low-salinity fluid environment in which the concentration of metal-chlorite complexes decreased. The Gen-3 stage is considered to be the main ore-forming event. Tour-II have similar Ca/(K + Na + Ca) ratios with Gen-1 and Gen-2 ratios, which indicates that they are contemporarily formed by the same fluid as Tour-I. Through compositional comparison of the tourmalines with those from other hydrothermal deposit types, the Galinge skarn-related tourmalines are overwhelmingly controlled by the MgFe-1 substitution mechanism. This is different from the compositions of tourmalines in porphyry, VMS, and vein-greisen type deposits, which are, respectively, controlled by the Fe3+Al-1, (Ca Mg)(Na Al)–1 and (Na Mg)(□Al)-1, and (Fe2+Fe3+)(MgAl)-1 substitution mechanisms. Different tourmaline compositions and substitution mechanisms could be used as guides for mineral exploration.
Miao Yu

Chapter 5. Multistage Amphiboles from the Galinge Skarn Deposit: Evidence of Igneous Rocks Replacement

Abstract
Amphiboles from the Galinge skarn deposit, the largest iron (Fe) polymetallic skarn deposit in the Qiman Tagh metallogenic belt (western China), were formed by multistage fluid-rock interactions. Mineral analysis of the various amphiboles suggest that they were formed by the replacement of mafic to intermediate igneous rocks. The two alteration phases have formed three generations of compositionally distinct amphiboles: Amp-I: Ferro-edenitic hornblende (FE); Amp-II: Deep bluish-green magnesian-hastingsite (MH); Amp-III: Light greenish-beige ferro-actinolite (FA). The Amp-I preserves the primary igneous amphibole composition, and was subsequently replaced by Amp-II. The amphibole Cl content markedly increases from the FE (0.176 – 0.582 wt.%)to the MH (0.894 – 3.161 wt.%), and abruptly drops in the FA(0.017 – 0.039 wt.%). The Cl-rich MH contains the lowest concentration of Si [5.64 – 6.28 atoms per formula unit (apfu)], and the highest (K + Na) values (0.72 – 1.06 apfu) in the amphibole A-site with a high K/(K + Na) of 0.491 to 0.429. Both Mg and Fe contents of the MH and FA vary widely, possibly due to the interactions of magma-derived hydrothermal fluids with the basaltic / andesitic host rocks. Formation of the Cl-rich MH may have been associated with the early high-temperature and high-saline hydrothermal fluids, meanwhile the Cl-poor FA may have formed from later low-temperature and low-saline hydrothermal fluids. The MH plays an important role for consuming Cl carried by hydro-thermal fluids. The Cl-rich fluids may have mobilized some elements, such as Fe, Al, Mg, Ca and Ti from the host rocks.
Miao Yu

Chapter 6. Formation and Breakdown of Ilvaite in the Large Galinge Skarn Fe Deposit, Western China: A Record of Multistage Retrograde Alteration

Abstract
The Galinge deposit, the largest Fe skarn deposit in the Qiman Tagh porphyry-skarn metallogenic belt (western China), is noteworthy for its well-developed Ca-rich retrograde alteration. The ilvaite-bearing skarn associations were studied to determine their physicochemical formation conditions. Petrographic evidence for replacement of garnet and magnetite by ilvaite in the early retrograde stage (Stage I) combined with thermodynamic modeling suggests that the alteration may have occurred at ca. 400°C – 470°C under moderately high oxygen fugacity (ΔlogfO2(HM): ca. -4 − -4.2). The model is based on a maximum pressure of 0.5 kbar calculated from magmatic amphibole geobarometer. The continuous breakdown of ilvaite with quartz to form ferro-actinolite and magnetite occur in the late retrograde stage (Stage II). The reactions occurred at about 400°C – 440°C under moderate fO2 (ΔlogfO2(HM): ca. -4 − -4.4). In Stage III, the breakdown of ilvaite to form calcite, pyrite and ferroactinolite depends on X(CO2) which is unknown but can be estimated to be a range of 0.005 to 0.05.Under these conditions the breakdown occurs at ca. 270–350°C and low fO2 (up to -5.2 log units below HM), but the reaction would occur at higher temperatures with increasing X(CO2). The thermodynamic model for continuous evolution from Stage I to Stage III completely records the conditions of the retrograde alteration. Although Mn is absent, the presence of substantial Fe and Mg strongly affects the stability field of ilvaite in the skarn system. Therefore, the petrography and phase relations of ilvaite are useful indicators of reaction conditions in various skarn deposit types.
Miao Yu

Chapter 7. Fluid Evolution and Stable Isotope Characters

Abstract
Fluid inclusions are particularly useful in documenting the temporal and spatial evolution of skarn-forming fluids, and provide evidence for the temperature and salinity shifts between skarn- and retrograde alteration stage. In the skarn stage of the Galinge deposit, ascending SiO2-bearing NaCl saturated magmatic fluids separated to a hypersaline liquid and a vapor phase at 500~550°C and 300~400bar 1~1.5km paleodepth, at higher lithostatic pressure compared to hydrostatic pressure. Brine and vapor continued to ascend, and replaced the wall rock to form unmineralised skarns such as garnet, pyroxene etc with minor ore minerals. As they cooled to ~400°C in the retrograde stage, due to great consumption of SiO2, the fluid predominantly consisted of saturated salty liquid. The residual fluid reacted with the earlier skarn to form epidote, amphibole etc with abundant ore minerals. This was followed by a decline in temperature to between 250~380°C at a hydrostatic pressure of 150~50 bar, corresponding a paleodepth of 0.5~1.5km as result of the mixing of magmatic water with lower-salinity meteoric waters. The overall hydrothermal cooling event took place during the geological background reversing, like mentioned above, the post-orogenic stage consists of a transition of compressional environment to extensional environment, while the magmatic fluids spread upward and outward on a larger scale.
Miao Yu

Chapter 8. Geochronological and Geochemical Constraints on the Galinge Skarn Deposit

Abstract
Galinge, the largest iron (Fe) polymetallic skarn deposit in the Qinghai province (NW China), is located in the Qiman Tagh metallogenic belt. At Galinge, post-collisional calc-alkaline metaluminous intrusions, including granodiorite, diorite and diorite porphyry dikes were emplaced into the Cambrian to Ordovician Qiman Tagh strata. Zircon U-Pb dating for granitic rocks yielded ca. 229 ‒ 217 Ma. Phlogopite coexisting with disseminated magnetite was dated to be 234.2 ± 3.5 Ma by Ar-Ar technique, indicating a close temporal magmatic-metallogenic relationship. Geochemically, the Qiman Tagh granodiorite is enriched in light earth element (LREE) with moderately negative Eu anomalies. Such geochemical data and zircon Hf isotopic data suggest that the studied granodiorite might be generated by low-degree partial melting of the amphibolite-facies metamorphosed rocks, whereas the Galinge diorite and diorite porphyry dykes were probably generated by higher degree of partial melting of the residual materials after granodioritic magma. We propose that the Galinge granitic magmatism and skarn Fe mineralization were formed under Late Triassic post-collisional extension after the closure of the Paleo-Tethys.
Miao Yu

Chapter 9. Genesis of Post-collisional Calc-Alkaline and Alkaline Granitoids in Qiman Tagh

Abstract
The post-collisional magmatism of Qiman Tagh is characterized by the intrusion of voluminous intermediate to felsic granitoids, including syenogranite, monzogranite, granodiorite, tonalite and diorite. The granitoids can be divided into two magmatic suites: Calc-alkaline (CA) and alkaline (Alk), which were emplaced from ~ 236 Ma to ~ 204 Ma. The CA suite contains metaluminous granodiorites and monzogranites. Typical Qiman Tagh CA granodiorites show moderately fractionated REE patterns ((La/Yb)N= 4.35–25.11) with significant negative Eu anomalies (Eu/Eu* = 0.54–1.34), and the primitive mantle-normalized spidergrams show strong depletion of Nb and Sr. The Qiman Tagh CA monzogranites show similar fractionated REE patterns ((La/Yb)N= 2.70–13.5) with less prominent negative Eu anomalies, and the chondrite-normalized spidergrams show strongly depleted Ba, Nb and Sr. The Alk suite, including syenogranite, is highly potassic (K2O/Na2O = 1.09–3.56) and peraluminous (A/CNK = 0.91–1.06). Compared to typical Qiman Tagh CA granodiorites, the Qiman Tagh Alk granitoids can be distinguished by their higher Rb, Nb, Ga/Al, FeO*/MgO, Y/Sr and Rb/Sr, as well as their lower Mg#, MgO, CaO, Al2O3, Sr, Co, V, Eu/Eu*, Ba/Nb, La/Nb, Ba/La and Ce/Nb. The Qiman Tagh CA rocks were most likely to be derived from the partial melting of garnet-amphibolite-facies rocks in the lower crust, leaving behind anhydrous granulite-facies rocks with plagioclase and garnet in the residue. The Alk rocks may have formed by the continued partial melting of granulite-facies rocks at elevated temperatures (> 830 °C).
Miao Yu

Chapter 10. Metallogenic Mechanism of the Galinge Polymetallic Iron Skarn Deposit, Qiman Tagh Mountains, Qinghai Province

Abstract
In the Qiman Tagh metallogenic belt, Fe, Zn, Pb, Cu and Au skarns and epithermal Cu and Mo deposits are spatially and temporally associated with Triassic granitoid rocks including granodiorite, monzogranite and syenogranite which are only occurred in the South Qiman Tagh Terrane as a result of completely different geotectonic setting between the SQT and NQT. The plutonic associations related to mineralization have various ages between middle-Triassic (237 Ma–226 Ma) and late-Triassic (226 Ma–204 Ma). The abundant volatile components evolved from magma are responsible for the significant transportation of metals. The initial oxygen fugacity of magma will affect the different mineralization types.
Miao Yu
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