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Badu |
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Guangxi, China |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The carbonate and dolerite hosted Badu gold deposit gold deposits are located in Tianlin County, in northern Guangxi Province, ~10 km SW of the major Jinfeng deposit and ~250 Km SSW if Guiyang (Guizhou Province). It lies within the Golden Triangle of southern China.
The Golden Triangle is located at the southwestern margin of the Archaean to Proterozoic Yangtze Craton within the Phanerozoic Youjiang (or Nanpanjiang) basin. The Youjiang basin was initiated by Cambrian extension of the Yangtze Precambrian basement, and contains sedimentary rocks that range from Cambrian to Triassic in age, although the bulk of outcrop is of Permian and Triassic age.
For background on the geological setting and distribution of mineralisation see the Golden Triangle record.
The Badu gold deposit is located 40 km SW of the regional the Youjiang fault in the Youjiang Basin. The Youjiang fault divides the basin into northern and southern parts. Badu together with the Gaolong and the Nabi deposit constitute the Gaolong-Badu gold belt. The Badu deposit is located on the western flank of the Badu anticline, which was the product multiple of tectonic events that led to the formation of a complex dome-like anticline (Yang et al., 2018). From west to east, the principal axis of the structure rotates from NNE to east-west to NW trending, to form form an arcuate superposed fold that has northward arcuate axis. The anticline is, in turn, crosscut by numerous faults that parallel the fold axis, suggesting the fold and faulting are contemporaneous. The structural style and deformation sequence is consistent with the D1 deformation characteristics of the Youjiang fold-and-thrust belt (Qiu et al., 2016). The core of the structure is occupied by Devonian rocks whilst Permian to Middle Triassic sequences are found on the limbs of the anticline, taken to suggest the fold and related faults formed after the Middle Triassic. Three undated generations of dolerites dykes occupy faults in both the core and limbs of the anticline.
Lower Devonian Yujiang Formation rocks in the core of the structure are predominantly composed of thick layered mudstone and siltstone, intercalated with limestone and dolomite containing disseminated and nodular pyrite. The overlying Upper Devonian Liujiang Formation comprises chert intercalated with tuff and lenses of banded limestone. The Carboniferous succession is primarily composed of mudstone and chert, whilst the Lower-middle Permian Sidazhai Formation comprises limestone interbedded with chert and is locally converted to marble and dolomite. The Upper Permian Linghao Formation consists of mudstone, argillaceous sandstone and tuff, intercalated with siliceous mudstone. The Lower and Middle Triassic Shipao and Baifeng formations are dominated by sandstone, siltstone and mudstone.
Dolerite is principally emplaced into cross-cutting NEE- and NW-trending faults into Devonian, Carboniferous and Permian sedimentary sequences, to as late as the Middle Triassic Baifeng Formation. They have weakly metamorphosed intrusive contacts, and sometimes contain xenoliths of folded country-rock. However, these dolerites were disrupted by subsequent faulting that localised gold mineralisation.
Badu comprises seven major mineralised zones that occur as tabular bodies or veins. Most trend NNE, with a few NW striking, and are controlled by secondary faults. Each mineralised zone is composed of sets of parallel veins, with individual veins having widths of from 2 to 70 m, strike lengths of between 10 and 500 m that persist for ~200 m below the surface. The No. III to VII mineralised zones constitute 70% of the gold resources and are predominantly hosted in altered dolerites that are cut by fault zones or occur along faulted contacts separating dolerites and sedimentary rocks. The No. I and II mineralised zones make up <30% of the total gold resources, and are principally hosted by mudstone, siltstone and sandstone of the Lower Devonian Yujiang and upper Permian Linghao Formations. Average grades vary from zone to zone and average from 1 to 6 g/t Au. The richest is the No. VI zone with local grades of up to 20 g/t Au.
Faults cutting dolerite and faulted contacts between dolerites and sedimentary rocks are typically filled by graphitic calcite veins, with gold being associated with lenses of pyrite within the veins, although grades are commonly <1 g/t Au. The two sides of these dolerite hosting faults side of the faults, dolerites are fractured and altered, although the density of fracturing and intensity of alteration progressively decreases away from the faults. Some mineralised dolerite contains abundant, up to 20 cm thick, un-mineralised milky quartz veins which envelop cm-sized fragments of altered dolerite that has been sheared and brecciated. The least altered dolerite is green and has a porphyritic texture with phenocrysts of pyroxene and plagioclase. Mineralised dolerite, in contrast, is bleached and intensely altered, characterised by the major gold-bearing ore minerals that are disseminated pyrite and arsenopyrite, although Fe sulphides account for <5 to 20 vol.% of the mineralised dolerites. The relative amounts of pyrite and arsenopyrite are variably developed in different mineralised zones. Samples with more arsenopyrite have lower gold grades. The richest No. VI mineralised zone has grades up to 20 g/t Au, which is dominated by pyrite, whilst the No. VII orebody is dominated by arsenopyrite and with grades that are generally <2 g/t Au. Although, there are no apparent correlations between the quantity and size of pyrite with gold grades, samples with higher gold grades are usually associated with very fine grained pyrite ('dust pyrite'), which is nearly invisible in hand specimens. Whilst hydrothermal quartz grains are rare in strongly altered and mineralised dolerite, samples from faults intersecting dolerite exhibit obvious silica and carbon alteration. Silicification is primarily expressed as quartz or quartz-ankerite veinlets crosscutting dolerite. Pyrite grains adjacent to the veinlets are coarse, up to 1 cm, and occur as aggregates. Fe sulphides are mainly disseminated in altered host rocks but are not found in quartz veins or veinlets. Plagioclase is altered to sericite in mineralised dolerite, whilst clusters of hydrothermal sericite are locally found in quartz-ankerite veinlets that are cut by quartz veinlets. Another distinct characteristic of mineralised dolerite is that coarse-grained pyrite and arsenopyrite are crosscut by graphitic carbon veinlets. These veinlets may be hydrothermal in origin or bitumen derived from the intruded sedimentary strata.
Some 70% of the gold in the Badu deposit is hosted by veining within altered and sulphidised, fault controlled, dolerite that intersects Lower Devonian and upper Permian calcareous sedimentary rocks which host the remainder, as described above.
The ore stage mineralisation and alteration within the dolerites can be divided into three stages: i). an early ankerite + sericite + rutile alteration stage; ii). the main pyrite + arsenopyrite + quartz + ankerite + sericite + apatite + monazite alteration stage; and iii). a late quartz + carbonate + (sulphide) + (barite) stage. Ankerite alteration is marked by pervasive replacement of primary pyroxene, plagioclase and titanite by ankerite. An overprinted pre-ore stage in the least altered dolerite is mainly composed of plagioclase (~50 vol.%), clinopyroxene (~45 vol.%), minor titanite and ilmenite (~5 vol.%), trace apatite, rare sulphides, and quartz, and is characterised by typical doleritic or poikilophitic texture with laths of plagioclase intergrown with clinopyroxene. The post-ore stage involves multiple generations of late veinlets of quartz, ankerite and calcite, with remaining open space filled by trace amounts of stibnite, realgar, cinnabar and barite. The As-Sb-Hg sulfides and barite generally do not coexist. Barite is locally found as ~1 mm thick veinlets that crosscut earlier stages (Dong, 2017). Multiple stages of precipitation of quartz and ankerite are indicated by mutually crosscutting relationships.
Within the sedimentary sequence, samples studied by Gao et al. (2021) with low-grade (0.75 g/t Au) altered mineralisation contain disseminated pyrite and arsenopyrite. Ankerite is still common but is partially replaced by quartz with abundant tiny residual inclusions of ankerite. The ankerite is patchy due to replacement and has obvious zoning with darker rims surrounding lighter cores, a result of the lower Fe content of the rims. The darker rim is in contact with hydrothermal quartz, because Fe in pre-ore-stage ankerite was released during replacement. In higher grade (4.40g/t Au) mineralised samples studied by the same authors, most of the ankerite is replaced by hydrothermal quartz with a jasperoid texture. Illite is found as inclusions in quartz and penetrates or crosscuts quartz, suggesting they formed simultaneously. Minor amounts of Fe-free dolomite are also present. Fe-free dolomite occurs in contact with, and contains tiny inclusions of, ankerite, interpreted to suggest it formed by replacement of ankerite. Hydrothermal pyrite, the principal gold-bearing sulphide, is significantly more abundant compared to the low-grade sample. EPMA and LA-ICP-MS data show the pyrite is arsenian and contains gold in its lattice (W. Gao, unpub. data, 2020). Arsenian pyrite is mainly distributed along the boundary of quartz grains. Small amounts of arsenian pyrite are also enclosed within quartz and non-ferroan dolomite grains, indicating that arsenian pyrites were trapped during quartz crystal growth. The consistent association of gold-bearing arsenian pyrite and hydrothermal quartz suggests that they are contemporaneous and formed from a single stage. As in the dolerite-hosted ore, late sulphosalt and base metal sulphides, including stibnite, tetrahedrite, tennantite, chalcopyrite and sphalerite, fill microfractures in gold-bearing arsenian pyrite (quote from Gao et al., 2021). Overprinted minimally altered pre-ore sedimentary rocks have weak to no gold and are composed of abundant ankerite that is distributed in a clastic matrix of extremely fine grained detrital quartz and illite. The ankerite is compositional zoned and contains numerous mineral inclusions. Locally, it occurs as veins or veinlets that crosscut mudstone. Framboidal pyrite is commonly disseminated in the matrix. Post or late ore alteration and mineralisation includes coarse-grained stibnite, and intergrowths of realgar and orpiment which occur within fractures on the periphery of gold-rich zones or locally in vugs and open space with euhedral quartz crystals. Quartz that is intergrown with stibnite and realgar contains abundant small fluid inclusions. The late ore-stage euhedral quartz formed before, during, and probably after realgar and cinnabar precipitation.
The Badu deposit has reserves of 17.5 Mt @ 2 g/t for 35 t of contained gold (Jinfeng Mining Company, 2018 internal exploration report quoted by Gao et al., 2021).
This summary is drawn from Gao et al., 2021.
The most recent source geological information used to prepare this decription was dated: 2021.
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.
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Gao, W., Hu, R., Hofstra, A.H., Li, Q., Zhu, J., Peng, K., Mu, L., Huang, Y., Ma, J. and Zhao, Q., 2021 - U-Pb Dating on Hydrothermal Rutile and Monazite from the Badu Gold Deposit Supports an Early Cretaceous Age for Carlin-Type Gold Mineralization in the Youjiang Basin, Southwestern China: in Econ. Geol. v.116, pp. 1355-1386.
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