Abstract The northern Olympic Cu-Au province, Gawler craton, Australia, includes a series of magnetite-dominated deposits/prospects associated with minor Cu-Au mineralization such as the 8.37 million tonne Cairn Hill deposit. Cairn Hill has long been considered a deep, magnetite end member of the iron oxide copper-gold (IOCG) family that is largely represented in the southern Olympic province by the 1590 Ma hematite-dominated Olympic Dam, Carrapeteena, and Prominent Hill deposits. In contrast to the southern district, the deposits in the northern Olympic Cu-Au province are hosted in rocks that experienced multiple phases of high-temperature metamorphism and deformation. New U-Pb zircon geochronology shows the magnetite-hornblende lodes at Cairn Hill were formed at ca. 1580 Ma at amphibolite facies conditions. The magnetite lodes are crosscut by ca. 1515 Ma granitic dikes. A second high-temperature event is recorded by U-Pb monazite geochronology at ca. 1490 Ma and involved deformation and metamorphism along the Cairn Hill shear zone at conditions of 4.6 to 5.3 kbar and 740° to 770°C. The 1490 Ma event reworked the iron lodes and 1515 Ma granitic dikes. However, Cu mineralization at Cairn Hill occurs in brittle fractures and quartz-biotite veins, overprinting the 1490 Ma deformation and metamorphism. Despite a spatial association between magnetite and Cu, the long thermal history that affected magnetite mineralization and the clear petrographic links between magnetite and high-temperature granulite facies minerals contrast with the late, low-temperature hydrothermal Cu mineralization and indicate the two are not paragenetically related. Therefore, the spatial but not temporal association between magnetite and Cu has effectively overlain two distinct episodes of mineralization to create the Fe-Cu deposit observed today. Although this fits within the broad IOCG deposit family, exploration strategies for Cairn Hill-style composite deposits should be distinct from IOCG deposits with cogenetic Fe and Cu.
Abstract China produces about 450 t Au per year and has government stated in-ground reserves of approximately 12,000 t Au. Orogenic gold, or gold deposits in metamorphic rocks, and associated placer deposits compose about 65 to 75% of this endowment, with lodes existing as structurally hosted vein and/or disseminated orebodies. The abundance of orogenic gold deposits reflects Paleozoic to Triassic closure of Paleo-Tethyan ocean basins between Precambrian blocks derived from Rodinia and Gondwana as well as late Mesozoic-Cenozoic circum-Pacific events and Cenozoic Himalayan orogeny. The deposits range in age from middle Paleozoic to Pleistocene. The Jiaodong Peninsula contains about one-third of China’s overall endowment, and large resources also characterize East Qinling, West Qinling, and the Youjiang basin. Although gold ores in Jiaodong postdate formation and metamorphism of Precambrian host rocks by billions of years, they are nevertheless classified here as orogenic gold ores rather than as a unique Jiaodong-type or decratonic-type of gold deposit. Similarly, although many workers classify the gold lodes in the Youjiang basin and much of West Qinling as Carlin-type gold, they show significant differences from gold ores in Nevada, United States, and are better defined as epizonal orogenic gold deposits. Although there are widespread exposures of Precambrian rocks in China, there are no significant Precambrian gold deposits. If large ancient orogenic gold deposits formed in Archean and Paleoproterozoic rocks, then they have been eroded, because these deep crustal rocks that are now exposed in China’s cratonic blocks have been uplifted from levels too deep for orogenic gold formation. The oldest large gold deposits in China are perhaps those of the Qilian Shan that were formed in association with Silurian tectonism along the present-day southwestern margin of the North China block. Closure of ocean basins in the outer parts of the Central Asian orogenic belt led to late Carboniferous to Middle Triassic orogenic gold formation in the Tian Shan, Altay Shan, Beishan, and northwestern North China block. Deformation associated with amalgamation of the North China block, northern Tibet terranes, South China block, and Indochina, as well as initial Paleo-Pacific subduction, can be related to Late Triassic orogenic gold formation in West Qinling, East Kunlun, Youjiang basin, West Jiangnan (Xuefengshan belt), Hainan Island, and Yunkaidashan gold provinces. In the middle Mesozoic, continued subduction along the Paleo-Pacific margin was associated with gold ores forming in East and Central Jiangnan, whereas early to middle Mesozoic deformation along the northern North China block formed important orogenic lodes in Precambrian basement (e.g., Jiapigou, Zhangjiakou, and Yanshan districts). Continued Yanshanian orogeny in the eastern half of the North China block led to extensive orogenic gold formation during the main period of decratonization and regional extension at ca. 135 to 120 Ma (e.g., Jiaodong, Liaodong, Chifeng-Chaoyang, Zhangbaling, Taihangshan, and East Qinling). At the same time, strike-slip events in central Transbaikal were associated with orogenic gold formation in both Russia and adjacent northeastern China and likely are the source for China’s most productive gold placers in the upper Heilongjiang basin. China’s youngest orogenic gold deposits formed in the Ailaoshan, Lanping basin, Ganzi-Litang belt, Daduhe district, and areas south of the Lhasa terrane in Tibet during the middle Cenozoic, as well as in the northern half of the Central Range of Taiwan during the Pliocene-Pleistocene.
The Caixiashan giant carbonate-hosted Zn–Pb deposit (~ 131 [email protected] 3.95% Zn + Pb) formed by replacement of dolomitized marble, with stratiform massive and breccia bodies is located near the base of the Proterozoic Kawabulake Group limestone and marble. It is one of the largest carbonated-hosted massive sulfides Zn–Pb ore deposits in Northwest China to have been discovered in recent years. Abundant pyrite occurs in dolomitized marble, along fractures in dolomitized clasts in the host rocks and filling cracks in the host rock. Locally, colloform or framboidal pyrites are observed in the early period and sometimes replaced by the later sphalerite. The sulfide assemblage of the main ore stage is characterized by massive or disseminated sphalerite and galena, with less pyrite than the earlier stage, and minor pyrrhotite. Galena occurs as small veins cutting the early-formed sphalerite. Dolomite and calcite are the main gangue minerals that co-precipitated with these sulfides. Tremolite and quartz alteration commonly overprints the orebodies. According to the crosscutting relationships and the different mineral associations within the host rocks and ore bodies, three stages are recognized at Caixiashan, i.e., syn-sedimentary pyrite (stage I), pyrite alteration, sphalerite–carbonate and galena–pyrite–carbonate (stage II-1, stage II-2 and stage II-3, respectively) and magmatic/metamorphic reworking (stage III). Calcite and quartz crystals are important host minerals among the three hypogene stages (stages I–III, although quartz mainly occurred in stage III). Stage I contains only aqueous inclusions (W-type), which were homogenized from 110 to 236 °C (main range of 138–198 °C and average at 168 °C; main range = average ± σ) and the salinities are from 0.5 to 16.5 (main range of 5.1–15.1 with average of 10.1) wt.% NaCl eqv. In the pyrite alteration of stage II-1 the W-type fluid inclusions homogenized from 175 to 260 °C (main range of 210–260 with average of 235) and the salinities range from 8.5 to 22.4 (main range of 16.7–20.1 with average of 18.4) wt.% NaCl eqv. In the main Zn–Pb mineralization stage (stage II-2–3), four types of fluid inclusions were identified an aqueous phase (W-type), a pure carbon phase (PC-type), a carbon phase containing (C-type) and mineral bearing inclusions (S-type). The W-type fluid inclusions of stage II-2–3 homogenized at 210 to 370 °C (main range of 253–323 and average at 270) and the salinities range from 5.9 to 23.1 (main range of 13.3–20.3 with average at 16.8) wt.% NaCl eqv.; C-type homogenized at 237 °C to 371 °C and the salinities range from 6.4–19.7 wt.% NaCl eqv.; S-type fluid inclusions homogenized at 211 to 350 °C and daughter minerals melted between 340 and 374 °C during heating, indicating a salinity range of 42 to 44 wt.% NaCl eqv. PC-type fluid inclusions with homogenization temperatures of CO2 phase show large variation from 7.4 °C to 21.2 °C. Laser Raman analyses show that CH4, CO2 and SO42 − coexist in the main mineralization stage fluids. The magmatic/metamorphic reworking stage only contains W-type fluid inclusions which yield homogenized between 220 and 360 °C (main range of 251–325 and average at 288), with salinities ranging from 1.7 to 23.0 (main range of 14.3–20.0 and average at 18.8) wt.% NaCl eqv. The textural features, mineral assemblages and fluid geochemistry suggest that the Zn–Pb ores were formed through hydrothermal convection of hot marine waters along the faults and fractures resulting in metal (Zn, Pb and Fe) enriched stratiform orebodies. Subsequent rapid precipitation of sulfides was triggered by sulfate (SO42 −) thermal reduction with the CH4 preserved in sedimentary rocks and early stage I pyrite bodies. This process occurred at moderate temperatures (ca. 270 °C). Higher-temperature magmatic hydrothermal alteration overprinted the orebodies, but only provided a minor contribution to the mineralization.
Abstract From integrated textural and compositional studies of auriferous and barren pyrite/marcasite in the epithermal Axi gold deposit, China, we have identified a relationship between multiple gold mineralizing events, mafic magma recharge, and fluid-rock reactions. Three generations of pyrite (Py1–3) and four generations of marcasite (Mar1–4) record episodic gold mineralizing events, followed by silver-copper-lead-zinc-cadmium enrichment. The gold mineralizing events are recorded by high concentrations of subnanometer-sized gold in Py1, Py3, and Mar3 (max. = 147, 129, and 34 ppm, med. = 39, 34, and 12 ppm). Based on previous Re-Os age determinations of pyrite and U-Pb zircon ages of the andesitic wallrock, these gold events slightly postdate pulsed mafic magma recharge and represent the incursion of Au-As-S-rich magmatic volatiles into circulating meteoric water. Silver-Cu-Pb-Zn-Cd enrichment in Py2, Mar2, and Mar4 are consistent with quiescent degassing and gradual Ag-Cu-Pb-Zn-Cd enrichment in an evolved felsic magma. Barren Mar1 records the dominance of meteoric water and a limited magmatic fluid contribution. High-Co-Ni-V-Cr-Ti contents in porous cores of Py1 and Mar2 are attributed to wall rock alteration and dissolution-reprecipitation. The results provide convincing evidence that the metal budget (especially for Au, Ag, Cu, Pb, Zn, Sb) of the hydrothermal fluids and sulfides in epithermal systems are controlled by the influx of magmatic fluids and associated magma, whereas the enrichment of certain fluid-immobile elements, such as Co, Ni, V, Cr, and Ti, is caused in part by fluid-rock interaction.
The Qiman Tagh W–Sn belt lies in the westernmost section of the East Kunlun Orogen, NW China, and is associated with early Paleozoic monzogranites, tourmaline is present throughout this belt. In this paper we report chemical and boron isotopic compositions of tourmaline from wall rocks, monzogranites, and quartz veins within the belt, for studying the evolution of ore-forming fluids. Tourmaline crystals hosted in the monzogranite and wall rocks belong to the alkali group, while those hosted in quartz veins belong to both the alkali and X-site vacancy groups. Tourmaline in the walk rocks lies within the schorl–dravite series and becomes increasingly schorlitic in the monzogranite and quartz veins. Detrital tourmaline in the wall rocks is commonly both optically and chemically zoned, with cores being enriched in Mg compared with the rims. In the Al–Fe–Mg and Ca–Fe–Mg diagrams, tourmaline from the wall rocks plots in the fields of Al-saturated and Ca-poor metapelite, and extends into the field of Li-poor granites, while those from the monzogranite and quartz veins lie within the field of Li-poor granites. Compositional substitution is best represented by the MgFe−1, Al(NaR)−1, and AlO(Fe(OH))−1 exchange vectors. A wider range of δ11B values from −11.1‰ to −7.1‰ is observed in the wall-rock tourmaline crystals, the B isotopic values combining with elemental diagrams indicate a source of metasediments without marine evaporates for the wall rocks in the Qiman Tagh belt. The δ11B values of monzogranite-hosted tourmaline range from −10.7‰ and −9.2‰, corresponding to the continental crust sediments, and indicate a possible connection between the wall rocks and the monzogranite. The overlap in δ11B values between wall rocks and monzogranite implies that a transfer of δ11B values by anataxis with little isotopic fractionation between tourmaline and melts. Tourmaline crystals from quartz veins have δ11B values between −11.0‰ and −9.6‰, combining with the elemental diagrams and geological features, thus indicating a common granite-derived source for the quartz veins and little B isotopic fractionation occurred. Tourmalinite in the wall rocks was formed by metasomatism by a granite-derived hydrothermal fluid, as confirmed by the compositional and geological features. Therefore, we propose a single B-rich sedimentary source in the Qiman Tagh belt, and little boron isotopic fractionation occurred during systematic fluid evolution from the wall rocks, through monzogranite, to quartz veins and tourmalinite.
In this paper we use published isotopic ages for gold deposits and related rocks in the Jiaodong peninsula ( East Shandong Province) to investigate the time and tectonic setting of the large scale gold metallogeny in the region, which contains world-class lode gold deposits. According to this database, metallogenic processes in this area occurred in the Mesozoic, with peak activities between 110 Ma and 130 Ma. In the Jiaodong gold province the mineralising events are coeval with or postdates Mesozoic granitoid intrusions. Both the Rb-Sr isochron ages and zircon SHRIMP dating results suggest that Mesozoic granitoids were emplaced during several thermal events. The identification of inherited zircons coupled with ISr ratios ( 0. 709) indicate that these granitoids were mainly sourced from the continental crust by remelting or partial melting. The ISr values obtained from ores and fluid inclusions are generally higher than 0. 709, and slightly higher than those for Mesozoic granitoids. This also indicates that both ore fluids and metals were mainly sourced from the crust. A synthesis of the available data suggests that collision between the South and North China continents was probably the dominant factor responsible for the gold metallogeny in the Jiaodong gold province. Granitoid emplacement and large-scale gold metallogenesis can be related to three important stages in the geodynamic evolution of a collisional orogen ( compression-crustal thickening-uplift, lithospheric delamination and transition to extension and a final extension phase). The most important metallogenic phase occurred at the transition from collisional compression to extension tectonics. A previously developed model for collisional orogeny, metallogeny and fluid flow (CMF) can be used to interpret the key characteristics of ore deposits and igneous rocks in the Jiaodong gold province.
Orogenic gold is one of the most important gold resources in the world, however, the origin of ore-forming fluid has been controversial for a long-term history. The Dunbasitao is a newly identified orogenic gold deposit, which is the largest among a few gold deposits reported along the Armantai suture zone, East Junggar, China. The ore bodies are hosted in volcano-sedimentary clastic rocks of the Lower Carboniferous Jiangbasitao Formation and porphyritic quartz diorite. Based on the cross-cutting relationships of veins and mineral assemblage characteristics, the hydrothermal ore-forming process can be divided into the early (quartz–pyrite), middle (quartz–polymetallic sulfide), and late (quartz–carbonate) stages and the gold mineralization occurred in the middle stage. Four types of fluid inclusions (FIs) were observed at Dunbasitao, including aqueous FIs (W-type), CO2–H2O FIs (C-type), pure–CO2 FIs (PC-type), and daughter crystal-bearing aqueous FIs (S-type). Both the early- and middle-stage quartz contain C-, PC- and W-type FIs, but only the W-type FIs are present in the late-stage minerals. The S-type FIs were only found in middle-stage quartz. Laser Raman spectrometry analyses, microthermometry, and isotope analyses show that the initial ore-forming fluids in the early stage were a moderate-high temperature (284−372°C), low-salinity (1.0−10.2 wt.% NaClequiv.), H2O−CO2−NaCl ± CH4 ± H2S ± N2 metamorphic fluid system with their δ18OH2O ranging from 7.6 to 7.8 ‰, δ18DH2O ranging from −100.3 to −96.2 ‰, and δ13Cfluid ranging from −3.8 to −2.5 ‰. In the middle stage, the fluids evolved into moderate-temperature (225−326°C), immiscible fluid system, including a low-salinity (0.4−5.0 wt.% NaClequiv.) H2O−CO2−NaCl system and a high-salinity (3.0−38.6 wt.% NaClequiv.) H2O−NaCl system with their δ18OH2O ranging from 5.5 to 8.4 ‰, δ18DH2O ranging from −108.6 to −104.2 ‰, and δ13Cfluid ranging from −3.4 to −2.6 ‰. And finally, the ore-forming fluids became a low temperature (124−237°C), low-salinity (0.2−7.8 wt.% NaClequiv.) fluid system in the late stage due to the continuous addition of meteoric water with their δ18OH2O ranging from −0.2 to 3.4 ‰, δ18DH2O ranging from −119.5 to −114.9 ‰, and δ13Cfluid ranging from −7.1 to −4.0 ‰. Fluid immiscibility and mixing were inferred to be key mechanisms for gold precipitation. The Dunbasitao gold deposit was formed in collisional orogeny between the Altai and East Junggar, during the Early to Middle Permian. The initial ore-forming fluids were probably derived from the metamorphic devolatilization of the overlying sedimentary cover of the subducted southern margin of Altai orogen. The calculated mineralization pressures of the early and middle stages were 69−261 MPa and 45−188 MPa, corresponding to the depths of 6.6−9.7 km and 4.4−7.0 km, respectively. The depth of the middle stage is significantly above the brittle-ductile transition zone at ~10 km, which shows that there is still a considerable orogenic gold potential in the deep. The homogenization temperatures of middle-stage FIs gradually increase downward from 1082.5 m to 1014.8 m above sea level (ASL), but suddenly decrease at 988.5 m ASL, which implies there is possibly a new orebody below the NO. 14 drill hole in NO. 0 exploration line.
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