Phanerozoic continental growth and gold metallogeny of Asia

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. 

Chapter 8 Orogenic Gold Deposits of China

. Early Cretaceous granitic intrusions are associated with Au-quartz veins and Cu-Fe skarns in the the Kitakami Mountains, which are underlain by the late Paleozoic of continental margin-type sedimentary rocks and Mesozoic accretionary complexes. The plutonic rocks are divided into potassic, high-Sr/Y calc-alkaline and low-Sr/Y calc-alkaline series. All the metallic mineral deposits are spatially associated with small stocks and plugs; they show no consistent association with the larger plutonic bodies. The plutonic rocks generally belong to the magnetite series but less oxidized in the southwestern part of the Kitakami Mountains where Au-quartz veins occur. The gold deposits are classified into high and low sulfide types.



The high sulfide type contains a high volume of sulfide minerals mostly of chalcopyrite, arsenopyrite and pyrrhotite with low bulk Au/Ag ratios. This type occurs almost exclusively in and surrounding the Orikabe pluton, including two most important gold deposits (Oya and Kohoku) of the Kitakami Mountains. The pluton is composed of potassic gabbroids, potassic granitoids of the shoshonite - high-K calc-alkaline series (Orikabe type), and less potassic Sasamori-type granodiorite. All these rocks belong to a moderately oxidized magnetite series. The Orikabe pluton has one of the lowest initial Sr ratio (0.70392) in the Kitakami Mountains, and the Au-Cu-dominant ore components of the high sulfide type Au deposits are considered magmatic in origin carried by the juvenile magmas from the upper mantle.



The low sulfide type is generally plain quartz vein with a low volume of sulfides and a high bulk Au/Ag ratio. The associated minerals are often scheelite and/or arsenopyrite and pyrrhotite. The ore deposits include historically famed Au-quartz veins at Shishiori and Ogayu. They are widespread in the southwestern Kitakami Mountains and may be later than the high sulfide type in age, and are hosted most commonly in the sedimentary rocks, which surround small weakly oxidized magnetite-series plutons of low to intermediate Sr/Y series. These less differentiated intrusions typically include quartz dior-ite and granodiorite. Some ore components of this type may have derived from the host sedimentary rocks.



Among other mineral deposit types in the region, the largest ore deposit is Kamaishi Cu-Fe skarn (magnetite ores of 58 MT, Fe 50–64 %; Cu 143 KT). It is related to the high-Sr/Y series Ganidake granodiorite stock, which is a strongly oxidized magnetite-series body. In contrast, the second largest deposit in the mountains, Akagane deposit, is a similar-type skarn but associated with an intrusion classified as less oxidized, ilmenite to intermediate series, and that is intermediate in Sr/Y of calc-alkaline series granodiorite. Degree of magmatic differentiation appears to be not critical factor in the formation of Au-quartz vein and Cu-Fe skarn deposits in the region, but is definitely significant for controlling the distribution of the Mo-mineralization to the east.

Granitoid Types Related to Cretaceous Plutonic Au‐Quartz Vein and Cu‐Fe Skarn Deposits, Kitakami Mountains, Japan

Mineral prospectivity mapping (MPM) needs robust predictive techniques so that the target zones of mineral deposits can be accurately delineated at a specific location. Although an individual machine learning algorithm has been successfully applied, it remains a challenge because of the complicated non-linear relations between prospecting factors and deposits. Ensemble learning methods were efficiently applied for their excellent generalization, but their potential has not been fully explored in MPM. In this study, three well-known machine learning models, namely random forest (RF), support vector machine (SVM), and the maximum entropy model (MaxEnt), were fused into ensembles (i.e., RF–SVM, RF–MaxEnt, SVM–MaxEnt, RF–SVM–MaxEnt) to produce a final prediction. The paper aims to investigate the potential application of stacking ensemble learning methods (SELM) for MPM. In this study, 69 hydrothermal gold deposits were split into two parts: 70% for the training model and 30% for testing the model. Then, 11 mineral prospecting factors were selected as a spatial dataset constructed for MPM. Finally, the models’ performance was assessed using the receiver operating characteristic (ROC) curves and five statistical metrics. Compared with other single methods, the SELM framework showed an improved predictive performance in the model evaluation. Therefore, this finding suggests that the SELM framework is promising and should be selected as an alternative technique for MPM.

https://www.mdpi.com/2075-163X/10/12/1126/pdf

A Multi-Model Ensemble Approach for Gold Mineral Prospectivity Mapping: A Case Study on the Beishan Region, Western China

The Xiuwenghala gold deposit is located in the Beishan Orogen of the southern Central Asian Orogenic Belt. The vein/lenticular gold orebodies are controlled by Northeast‐trending faults and are hosted mainly in the brecciated/altered tuff and rhyolite porphyry of the Lower Carboniferous Baishan Formation. Metallic minerals include mainly pyrite and minor chalcopyrite, arsenopyrite, galena, and sphalerite, whilst nonmetallic minerals include quartz, chalcedony, sericite, chlorite, and calcite. Hydrothermal alterations consist of silicic, sericite, chlorite, and carbonate. Alteration/mineralization processes comprise three stages: pre‐ore silicic alteration (Stage I), syn‐ore quartz‐chalcedony‐polymetallic sulfide mineralization (Stage II), and post‐ore quartz‐calcite veining (Stage III). Fluid inclusions (FIs) in quartz and calcite are dominated by L‐type with minor V‐type and lack any daughter mineral‐bearing or CO2‐rich/‐bearing inclusions. From Stages I to III, the FIs homogenized at 240–260°C, 220–250°C, and 150–190°C, with corresponding salinities of 2.9–10.9, 3.2–11.1, and 2.9–11.9 wt.% NaCl eqv., respectively. The mineralization depth at Xiuwenghala is estimated to be relatively shallow (<1 km). FI results indicate that the ore‐forming fluids belong to a low to medium‐temperature, low‐salinity, and low‐density NaCl‐H2O system. The δ18OH2O values decrease from Stage I to III (3.7‰, 1.7–2.4‰, and −1.7 to 0.9‰, respectively), and a similar trend is found for their δDH2O values (−104 to −90‰, −126 to −86‰, and −130 to −106‰, respectively). This indicates that the fluid source gradually evolved from magmatic to meteoric. δ34S values of the hydrothermal pyrites (−3.0 to 0.0‰; avg. −1.1‰) resemble those of typical magmatic/mantle‐derived sulfides. Pyrite Pb isotopic compositions (206Pb/204Pb = 18.409–18.767, 207Pb/204Pb = 15.600–15.715, 208Pb/204Pb = 38.173–38.654) are similar to those of the (sub)volcanic ore host, indicating that the origin of ore‐forming material was mainly the upper crustal (sub)volcanic rocks. Integrating evidence from geology, FIs, and H–O–S–Pb isotopes, we suggest that Xiuwenghala is best classified as a low‐sulfidation epithermal gold deposit.

Genesis of the Xiuwenghala Gold Deposit in the Beishan Orogen, Northwest China: Evidence from Geology, Fluid Inclusion, and H–O–S–Pb Isotopes

. The Beishan Mountain spans three provinces - Gansu, Xinjiang and Inner Mongolia, having an area of 120,000 km2 Tectonically, it transverses three different tectonic units, i.e. Siberia, Kazakhstan and Tarim plates, and is composed of nine ter-rains with widely exposed Precambrian and Paleozoic strata, complex structures, intensive magmatic activities and widespread ore deposits. It is not only a main part of Tianshan-Yinshan-Great Hinggan metallogenic belt in China, but also a key to under-stand the evolution of central-Asian orogenic system.



At present, more than 100 gold deposits and prospects have been discovered, explored and mined, among which Nanjinshan, Mazhuangshan, Liushashan, Jinwozi, Zhaobishan and Xiaoxigong are the most important ones. Based on the host rocks and the geological features, all these gold occurrences can be subdivided into three groups (or types): (1) hosted by Carboniferous or Permian volcanic or subvolcanic rocks; (2) hosted by or related to plutonic intrusions; and (3) hosted by Precambrian metamorphic rocks. The first group includes the Mazhuangshan gold deposit, which occurs in Hercynian quartz por-phyry and rhyolite porphyry as gold-bearing quartz veins. The second group is composed of the Liushashan, Nanjinshan Zhaobishan and Jinwozi gold deposits. Gold mineralization at these four deposits occurs within Hercynian granitoids intrusion: or late Paleozoic sedimentary rocks as quartz veins, veinlets and altered rocks. The Xiaoxigong gold deposit belongs to the third group, and is hosted by Precambrian schist, amphibolite and migmatite as quartz veins and altered rocks.



Isotopic age dating data, geological and geochemical evidence suggest that most of the groups 1 and 2 gold deposits were generated during the emplacement of the Hercynian or partly Indosinian intrusions. These intrusions may provide both heat and metals for groups 1 and 2 deposits. In contrast, although the formation is closely related to the Hercynian magma-tism, the ore-forming materials of the group 3 deposits may not only come from the intrusions, but also from the Precambrian metamorphic rocks.

Gold Deposits in Beishan Mountain, Northwestern China

During severe weather, the runup of the devasting surges and waves can cause extensive damages to the coastal facilities and results in significant loss of life. Therefore, it becomes necessary to find reliable measures to reduce the wave runup in coastal regions. As a natural barrier to the shorelines, coastal vegetation has great potential to attenuate wave runup and to prevent the coastal communities from extreme floodings and huge land loss. This study numerically discusses the influences of the emergent vegetation patch on the runup processes of extreme wave groups by applying a nonhydrostatic flow solver. Influences of the major factors, i.e., water depth, peak wave period, significant wave height, and density and length of the vegetation patch, are analyzed in detail. Research findings indicate that the presence of the emergent vegetation patch can substantially dissipate most of the incident wave energy and effectively reduce the maximum wave runup heights of extreme wave groups at the sloped beach. It is hoped that the research results of this study will be helpful to the design of coastal soft protection measures.

/pdf/numerical-analysis-on-influences-of-emergent-vegetation-3c8cweod.pdf

Numerical Analysis on Influences of Emergent Vegetation Patch on Runup Processes of Focused Wave Groups

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Gold Deposits in Beishan Mountain, Northwestern China

Numerical Analysis on Influences of Emergent Vegetation Patch on Runup Processes of Focused Wave Groups