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Bilihe |
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Inner Mongolia, China |
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Au
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Super Porphyry Cu and Au
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The Bilihe gold deposit is located within the northern margin of the North China Craton, ~10 km south of the craton margin. It is ~400 km northwest of Beijing and 200 km SE of Erenhot in Inner Mongolia, China.
(#Location: 42° 23' 41"N, 113° 33' 43"E).
It has been proposed (e.g., Yang et al., 2016) that the Bilihe gold deposit is a rare example of economically viable gold mineralisation occurring in magmatic unidirectional solidification textures (UST) quartz at the cupola of a granitoid intrusion, in this case one that is associated with a middle Permian granodiorite.
Regional Setting
The transition form the Amurian Superterrane of the Central Asian Orogenic Belt, to the North China Craton is occupied by the Liaoyuan Terrane, which both separates the orogen from, and overlaps onto, the North China Craton. This terrane is divided into two main zones, separated by the major east-west trending Chifeng-Bayan Obo Fault. The southern zone is predominantly composed of Palaeo- to Mesoproterozoic intracratonic rift sequences of metasedimentary and igneous rocks, succeeded locally by Neoproterozoic to Lower Palaeozoic passive margin sedimentary successions developed over the Yinshan Block on northern margin of the North China Craton. The northern zone comprises the east-west trending, Cambrian to Silurian, 520 to 420 Ma Bainaimiao Arc developed on a narrow ribbon-like Proterozoic micro-continent, and the >490 to 450 Ma Ondor Sum subduction-accretion complex immediately to its north. The latter is the remnant of an earlier southward subduction of an ocean to the north (the Solonker Ocean) below the Bainaimiao Arc before subduction flipped and the ocean to the south began to subduct northward below the arc. These two subduction events mean the arc has two components, an early magmatism from the Cambrian until the subduction reversal at ~450 Ma, followed by a younger event that was partially terminated by collision with the passive North China Craton margin. The Ondor Sum Complex is bordered to the north by the Solonker Suture straddling the regional Xar-Moron-Changchun Fault. The Bainaimiao Arc collided with the North China Craton and its passive margin successions over a north dipping subduction zone between ~430 and ~410 Ma in the Silurian to Lower Devonian. The Solonker Suture is the product of collision between the Songnen/Songliao Terrane of the Amurian Superterrane in the north and the Ondor Sum accretionary complex which by then was amalgamated with the North China Craton. This collision was diachronous closing 'scissor-like' from west to east between the Late Permian and ~230 Ma in the Upper Triassic. The collision events led to the formation of the 900 km long Solonker suture zone that is marked by mélanges and remnants of oceanic arcs and ophiolites (Xiao et al., 2009). After closure of the Solonker Ocean, the North China Craton and Central Asian Orogenic Belt were amalgamated into a combined block and a part of the newly forming Asian continent. During and following the collision between the Amurian Superterrane of the Central Asian Orogenic Belt and the amalgamated North China Craton, convergence continued into the Triassic and Jurassic causing post-collisional over-thrusting, imbrication and considerable crustal thickening on the NW margin of the craton (Xiao et al., 2003). Mid to Late Triassic S-type granites were intruded into areas across the Solonker Suture zone with ages of ~230 Ma (whole rock Rb-Sr).
The Bilihe gold deposit is hosted within the Bainaimiao Arc which consists of calc-alkaline volcanic rock sequences of basalt, andesite and felsic lavas, and a plutonic complex of gabbro, diorite, granodiorite dated at 466 ±9 Ma (zircon U-Pb; Tang and Yan, 1993), and 429 ±100 Ma (Sm-Nd wholerock isochron: Nie and Bjørlykke, 1999) and granite. These magmatic rocks mostly represent the early, pre-subduction flip magmatic phase of the arc, and intrude greenschist- to amphibolite-facies metamorphic rocks of the Bainaimiao Group, including two-mica schists and biotite-plagioclase gneisses, dated at 1130 ±16 Ma (U-Pb zircon; Nie et al., 1991) and 1107 ±28 Ma (Sm-Nd wholerock isochron; Nie et al., 1995).
Geology and Mineralisation
The Bilihe and the Hadamiao gold deposit (12 km to the SE) constitute the Bilihe-Hadamiao gold district. Four main stratigraphic units are found in the area (Yang et al., 2016):
• Late Carboniferous to Early Permian, which comprise marine sedimentary rocks, mainly sandstone, siltstone, mudstone and interbedded bioclastic limestone, which together account for 50% of the surface outcrop of the district. These represents cover rocks to the early Palaeozoic segment of the arc;
• Early Permian volcano-sedimentary sequences, composed of andesitic to rhyolitic tuff and breccia, with interbedded sandstone and limestone, the host to the Bilihe gold deposit;
• Middle Permian granodiorite, quartz diorite and diorite porphyry, as well as small bodies of quartz diorite and granite porphyry; and
• massive Late Permian syenogranite that cuts the previous three older geologic units.
The last three units are part of the Permian to earliest Triassic magmatism within the ancient arc as imbrication continued while the Bainaimiao Arc was thrust further over the North China Craton to the south by compression related to the closure on the Solonker Suture to the north. Silurian granodioritic rocks (429 ±3Ma; Z.-M. Yang, unpublished data quoted by Yang et al., 2016) represent the second, post-flip subduction event of the Bainaimiao arc.
Two sets of NE-striking faults are recognised in the district. The first set are normal structures, the latter are reverse faults. Two east-west trending strike-slip faults have also been identified (IMBGMR, Inner Mongolian Bureau of Geology and Mineral Resources, 1990). Most intrusions in the district have a strong NE elongation, suggesting their emplacement was controlled by faults. The strike-slip faults cut the normal faults and syenogranite stocks, and thus are later than magmatism in the district (Yang et al., 2016).
Two principal orebodies have been outlined in the Bilihe gold deposit. The Centre I orebody contained a small resources of 3.1 t Au @ an average grade of 6.2 g/t Au (i.e., 0.5 Mt of ore) and has been exhausted. It is associated with a small NW-trending quartz diorite dyke with a ~2 × 20 m outcrop (Ge et al., 2009) known as the B1 intrusion. The Centre II orebody contains 21.6 t Au @ an average grade of 2.7 g/t Au (i.e., 8 Mt of ore), spatially associated with the small buried and elongated B2 intrusion that strikes NW-SE and dips to the NE. This intrusion has numerous narrow branches that are more or less parallel to the bedding of the sedimentary country rocks. Drilling indicate both the B1 and the B2 intrusions are rooted in a 3 x 2 km quartz diorite to diorite pluton that is 200 to 500 m below the surface (Yang et al., 2016).
The B2 intrusive rocks are strongly fractionated, and range from quartz-diorite to granite in composition. Most of the gold, particularly in the high-grade >3 g/t Au ores, is hosted in a section of the intrusion containing dendritic quartz phenocrysts that range from <2 mm to several cm in diameter and form unidirectional solidification textures (UST), typically with interstices filled by red K feldspar-rich aplite. The presence of abundant melt inclusions in the dendritic quartz and high homogenisation temperatures (>950°C) of the melt inclusions, suggest the dendritic quartz is igneous (Yang et al., 2016). More than 70% of the gold is hosted in the dendritic quartz, occurring as trails of gold grains. These grains are mostly 4 to 7 µm across and typically polygonal (with 8 to 10 sides) or sub-spherical, (Yang et al., 2016). Most gold grains are native metal, with no detectable Ag in electron microprobe, and with only one strongly anomalous Cu value of 0.98 wt % Cu. No primary fluid inclusions have been encountered in the dendritic quartz, although liquid-vapor secondary fluid inclusions are locally found along healed fractures. Detailed cathodoluminescence and petrographic studies (Yang et al., 2016) show most of the gold grains and trails occur along crystallographic elements of the dendritic quartz, indicating simultaneous precipitation of both gold and quartz, interpreted to represent a magmatic origin for the gold (Yang et al., 2016). The B2 intrusion is cut by <1 cm thick quartz-K feldspar dykes and minor younger hydrothermal dolomite veins, and has minor tourmaline-quartz alteration and extensive illite-calcite ±chlorite alteration, with trace amounts of pyrite, chalcopyrite, tetrahedrite, tennantite and scheelite. Tourmaline alteration is generally found in and around the B2 intrusion, and locally extends for tens of metres into the surrounding andesitic and rhyolitic tuff, and sandstone wallrocks. The close relationship between the B2 intrusion and tourmaline alteration has been interpreted to indicate the magma was boron-rich during its late evolutionary stage (Yang et al., 2016). Illite-calcite ±chlorite alteration is widespread in the district, covering an area of at least 16 km2 and is not directly related to deposition of the majority of the gold, i.e., the euhedral gold grains occurring as trails in dendritic quartz. The illite has been dated at 249.8 ±2.8 Ma (Ar–Ar; n=1; Yang et al., 2016), and hence if the gold is magmatic, it should be older. However, minor gold occurs in fractures in the B2 intrusion and illite-calcite±chlorite altered wallrocks, particularly where gold grades are >3 g/t. These gold grains typically have an irregular shape and are accompanied by chalcopyrite, galena and/or tennantite-tetrahedrite, and are mainly found close to the primary gold zone. These fracture-hosted gold grains are interpreted to have been remobilised from early primary gold ore during the later illite-carbonate ±chlorite alteration event (Yang et al., 2016), most likely by a magmatic-hydrothermal fluid. No high-temperature K-silicate alteration has been observed in the deposit area, that would be typical of porphyry-type systems (Lowell and Guilbert, 1970). Syenogranite porphyry occurs extensively in the Bilihe area, mainly in the southeastern and northern parts of the district, where it locally cuts across the B1 intrusion as dykes. The structures that are known below the widespread Quaternary sedimentary cover, suggest the buried B2 intrusion, syenogranite dykes, and some of the Au-bearing quartz-potassic feldspar dykes and dolomite veins all have a similar NW strike direction, suggesting structural control by an unexposed NW striking fault system (e.g., Ge et al., 2009; Qing et al., 2011).
However, Huang et al. (2020) note there is a key dispute about whether gold precipitated from i). low-diffusion silicate melt (as discussed above) or ii). a high-activity fluid phase. At Bilihe, the unidirectional solidification textures (UST) quartz represents the most fertile unit. However, statistical results show quartz-K feldspar and grey banded quartz veins, both cutting through the UST layers, host ~22 to 27% of the total gold resources. These non-UST veins are similar to those associated with porphyry-type deposits, including limited potassic alteration, pervasive and intensive yellow-green intermediate argillic alteration, local pale-green phyllic alteration, with gold mineralisation which mainly occurred before and partly during intermediate argillic alteration. These represent the fracture hosted mineralisation described by Yang et al. (2016) as late remobilisation. Huang et al. (2020) note a decreasing trend in Au/Ag, and Au/S, but elevated Au/Cu ratios of gold compositions indicate a relatively continuous fluid evolution in the Bilihe system. Petrographic and spectroscopic studies show that the Bilihe UST quartz can be divided into two different domains, i). gold-rich brownish and ii). barren colourless zones. The former can contain more water, occurring as tiny fluid inclusion or as structural OH-, whilst the latter usually contain melt inclusions. Huang et al. (2020) concluded that the Bilihe UST quartz grew in variable growth mediums, namely fluctuating alternation between fluid-dominant pocket producing the gold rich veins and silicate melt phase near the melt-fluid interface at the apex of the magma body for the barren colourless veins. Huang et al. (2020) suggest early mineralisation from a relatively H2O-poor parent magma, where water and gold saturation might be reached in isolated pockets to form earlier exsolving low-salinity fluids at low pressure (as low as 0.5 kbar) and high temperature (~660 to 700°C) conditions. Subsequently, voluminous fluids are released from the source batholith, accompanied by hydrofracturing, and a late overprint by a hydrothermal phase as the system evolved with pervasive intermediate argillic alteration, which intensely overprinted earlier potassic alteration.
The most recent source geological information used to prepare this decription was dated: 2020.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Bilihe
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Selected References:
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Huang, K., Zhu, M., Zhang, L., Bai, Y and Cai, Y., 2020 - Geological and mineralogical constraints on the genesis of the Bilihe gold deposit in Inner Mongolia, China: in Ore Geology Reviews v.124, doi.org/10.1016/j.oregeorev.2020.103607.
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Wang, Y., Zeng, Q., Guo, L. and Guo, Y., 2018 - Magmatic and tectonic setting of the Permian Au mineralization in the Xing-Meng Orogenic Belt: constraints from the U-Pb ages, Hf-O isotopes and geochemistry of granitic intrusions in the Bilihe and Hadamiao gold deposits: in Mineralogy & Petrology v.113, p. 99-118.
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Yang, Z., Chang, Z., Hou, Z. and Meffre, S., 2016 - Age, igneous petrogenesis, and tectonic setting of the Bilihe gold deposit, China, and implications for regional metallogeny: in Precambrian Research v.34, pp. 296-314.
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Yang, Z., Chang, Z., Paquette, J., White, N.C., Hou, Z. and Ge, L., 2015 - Magmatic Au mineralisation at the Bilihe Au deposit, China: in Econ. Geol. v.110, pp. 1661-1668.
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