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A rare Phanerozoic amphibolite-hosted gold deposit at Danba, Yangtze Craton, China: significance to fluid and metal sources for orogenic gold systems
Introduction
Basically, the formation of orogenic gold deposits is fundamentally associated with either of the two categories: (1) the formation from syn- to post-peak metamorphism or (2) the formation through pre-peak metamorphism with subsequent metamorphic overprint. Apart from the metamorphism of enclosing sedimentary rocks, Goldfarb and Groves (2015) proposed that other mechanisms of formation include: (1) metamorphic devolatilization of subducting oceanic slab and overlying sediment wedge and (2) metasomatized lithosphere.
The research aims to establish the proposition that the formation of the rare Phanerozoic amphibolite-hosted gold deposit at Danba is due to the devolatilization of the metasomatized lithosphere as triggered by the transition from lithospheric transpression to extension in the Jurassic. This is if the orogenic gold deposit at Danba formed at similar P-T conditions to its amphibolite-facies wall rocks.
Geologic Setting
Figure 1a shows the gold deposit which is located on the north-western margin of the Yangtze Craton. Figure 1b shows a detailed geological map of the Songpan-Garzê accretionary prism. In the Late Triassic, convergence between the North China, South China, and Qiangtang blocks, after the closure of the Paleotethyan Ocean, resulted in the filling of a thick sequence of Triassic flyschoid sedimentary rocks in the Songpan-Garzê relict basin and the formation of the accretionary prism. The domal domain comprises a series of extensional metamorphic core complexes formed at ca. 180 to 160 Ma. The Neoproterozoic crystalline basement, which extensively crops out along the domal domain, is a product of Triassic metamorphism of an 860–750 Ma Panxi-Hannan arc assemblage that shows depletion of high field strength elements and enrichment of large ion lithophile elements. In the Danba area (boxed in blue outline in Figure 1b), amphibolite-facies metamorphism peaked during the Late Triassic to Early Jurassic. A sillimanite-grade metapelite in the north-west of the area, very close to the Danba deposit, yielded P-T conditions of 640 25 °C and 4.8 kbar at 193.4 5.2 Ma, representing estimated peak-metamorphic conditions around the deposit. Moreover, the Danba dome and decollement structure is represented by the exposure of the Neoproterozoic basement in a metamorphic core complex to the northwest of Danba. The core of the dome comprises mainly migmatitic granite and granitic gneiss complexes with U-Pb ages of zircon cores of ca. 830 Ma and metamorphic rims of 177 ± 3 Ma.
Deposit Geology
As shown in Figure 2, the amphibolite-facies metasedimentary rocks in the Danba gold deposit form a monocline trending 310–0° and dipping 45–75° southwest to west. Ore bodies of the Danba gold deposit are mainly contained in a series of N-S trending, bedding- and foliation-parallel shear zones (Fig. 2; ESM Fig. 2). The closest large granite intrusion is more than 10 km from the mine, and this is confirmed by the magnetotelluric sounding used during exploration at the mine. There are thin fine-grained granodiorite dykes in the wall rocks of the Danba ore bodies.
Metamorphic petrography
With regards to metamorphic petrography (Figure 3), metamorphic mineral assemblages in the wall rocks of the Danba gold deposit indicate that peak metamorphism reached amphibolite-facies. The biotite wraps around the garnet with a sigmoidal shape, producing pressure shadows that reflect initial growth during shear-related deformation.
Alteration
In Figure 4, the wall rock alteration is shown and this represents the exploration section 14 in Figure 2 which is characterized by sillimanite-garnet-biotite schist. In this figure, it is shown that auriferous quartz veins are surrounded by symmetrical alteration zones of biotite alteration and silicification. Moreover, these small-scale alteration zones are associated with variable amounts of sulfide, carbonate, and K-feldspar minerals. Moreover, quartz-biotite-amphibole-K-feldspar-plagioclase-calcite-scheelite assemblages petrographically characterize alteration zones. Here, quartz and biotite are intergrown with K-feldspar and amphibole. K-feldspar, however, is absent from distal wall rocks.
Ore minerals
The petrography of ore and mineral assemblages is shown in Figure 5. Here, quartz veins, quartz stockworks, and sulfides dominantly form the mineralization types. The gold-bearing sulfides are distributed along the border of quartz veins with wall rocks and within the quartz. Carbonate veins or veinlets are virtually absent in quartz veins. In places, small milky quartz veins cut early ore-stage, fine to coarse-grained translucent quartz veins. Gold grains occur as inclusions in pyrrhotite and quartz, in fractures within them, or along grain margins (Fig. 5c). They are commonly intergrown with bismuth tellurides (Fig. 5h, i). Gold grains vary in diameter from > 5 μm to 1 mm. Pyrrhotite contains some native gold. Fine- to coarse-grained scheelite aggregates occur both in quartz veins and within massive polymetallic sulfide aggregates (Fig. 5b). Chalcopyrite and fine-grained pyrite are everywhere distributed along fractures in other minerals, commonly associated with pyrrhotite (Fig. 5d). Some coarse-grained quartz coexists with the sulfides, (Fig. 5a), whereas most quartz distal to sulfide aggregates are fine to medium-grained and translucent with a greasy luster (Fig. 5e). In the late ore stage, bismuth tellurides, absent in early-stage ore, are sited in fractures in milky white medium-grained quartz and commonly intergrown with coarse-grained native gold. Rare pyrite and galena grains are sited on the margins of pilsenite grains (Fig. 5g,h).
Ore geochemistry
Arsenic, with concentrations below 10 ppm, is a noticeable absentee from the ore body. On the other hand, Au at Danba is coenriched with Ag, Bi, Cu, Te, and Zn. `
Analytical Methods
For the analytical methods, the samples were subjected to petrography, mineralogy, geochronology, fluid-inclusion analysis, and sulfur isotopic studies. At Danba, 12 core samples of wall rocks and 116 samples of gold ores and wall rocks from underground workings were collected.
Re-Os Geochronology – repeated analyses of molybdenite standard HLP from a carbonate vein-type Mo-Pb deposit in the Jinduicheng-Huanglongpu area of Shaanxi province, China, were performed to test analytical reliability. The 187Re decay constant of 1.666 10−11 y−1 was used for calculating molybdenite ages. The uncertainty in each age determination is about 1.4%, comprising the uncertainty of the decay constant of 187Re, uncertainty in isotope measurement, spike calibration for 185Re and 190Os, and individual weighing and analytical random errors.
Mineral Chemistry (EPMA) – compositions of metamorphic and alteration minerals, bismuth tellurides, and gold in 16 samples were analyzed using a JEOLJXA-8230 electron probe micro-analyzer combined with an INCAX-ACT energy spectrometer at Wuhan University of Technology. The counting time was increased to 60 s to improve the precision of the Ti calculation.
Fluid Inclusion Analyses – fluid inclusions were described and analyzed from samples representative of both the early and late-ore stages. Twenty-four polished sections about 100 μm thick were prepared for fluid inclusion analyses. Synthetic fluid inclusions of known compositions were used to calibrate the stage. The Ar+ laser wavelength was 514 nm, the laser power was 20 mW, the diameter of the laser beam spot was 2 μm, and the spectrometer resolution was 2 cm−1.
Sulfur Isotope Analyses – twenty pyrrhotite samples from the Danba gold deposit were used for sulfur isotope analysis. The sulfur isotope values, with analytical precision of about ± 0.2‰, are reported using the δ notation in per mil, relative to the Cañón Diablo Troilite (CDT) standard.
Figure 6 shows the molybdenite Re-Os isochron for the Danba gold deposit. Analyses of five molybdenite samples from the Danba gold deposit yield a 187Re-187Os isochron age of 185.3 ± 9.4 Ma (MSWD = 1.4) at the 95% confidence level. As the isochron age is more reliable than model ages, it is chosen to represent the formation age of the early ore stage of the deposit.
Metamorphism and Wall Rock Alteration
As shown in Figure 7, Although some minerals, such as biotite, amphibole, and plagioclase, occur in both the gold-related alteration zones and the more distal metamorphosed wall rocks, there are clear compositional differences between them. In contrast, amphibole intergrown with quartz from amphibole-biotite schist within the alteration zone is mainly edenite. Iron-rich biotite dominates in unaltered garnet-biotite schist and amphibolite wall rocks, whereas Mg-rich biotite is abundant in alteration zones, especially where layers of coarse-grained euhedral hydrothermal biotite occur at quartz vein margins or as aggregates within those veins.
Peak Metamorphic P-T conditions – Metamorphic temperature estimates for peak sillimanite-grade metamorphism in unaltered amphibolite were obtained from the amphibole-plagioclase thermometer which has good precision in the range 400–1000 °C and 1–15 kbar over a broad range of bulk compositions. The Ti-in-biotite thermometer was used to check the temperature calculated above. As the biotite schist in the deposit is either a metapelite or altered metapelite that contains peraluminous garnet and Ti saturation minerals, such as ilmenite in biotite, the thermometer is ideally suitable for this calculation. XMg = 0.275–1.000, Ti = 0.04–0.60 pfu, P = roughly 4–6 kbar, and temperature = 480–800 °C. The precision of the thermometer is estimated to be 12 °C for higher temperatures.
Post-Peak Metamorphic P-T Conditions – P-T estimates reflect the retrograde sillimanite-grade metamorphism of the wall rocks of the Danba deposit with a pressure of 4–5 kbar and temperature of 540–580 °C.
P-T Conditions of Wall Rock Alteration – the calculated temperature for wall rock alteration, based on the amphibole-plagioclase thermometer, is 590–630 °C (mean 610 °C) for the edenite-labradorite assemblage in amphibole-biotite schist from the proximal alteration zone.
Fluid Inclusion Data
As presented in Figure 8, fluid inclusions were studied in both early-stage and late-stage quartz. Both the medium to coarse-grained subhedral and euhedral early ore-stage quartz and turbid milky fine to medium-grained late ore-stage quartz contain major liquid-dominated and minor vapor-dominated fluid inclusions. At room temperature, liquid-dominated inclusions contain both a liquid H2O phase and a liquid CO2 phase. These inclusions are 5–30 μm in diameter, appearing both as clusters and discrete inclusions. Vapor-dominated inclusions, similar to liquid-dominated inclusions, also contain two phases, but the volume of liquid CO2 is > 60%. A total of 61 fluid inclusions assemblages and isolated inclusions were studied for micro thermometry. Liquid-dominated inclusions homogenized to the liquid phase through the disappearance of the vapor phase. The salinity, bulk composition, and density of the two types of inclusions in the early and late ore stages were estimated. Overall, the aqueous carbonic fluid inclusions have a total XCO2 of 0.09 and salinity of 2.6 wt% NaCl equiv. In inclusions from the early ore stage, the carbonic phase has higher CH4 contents (range 2–18 mol%, mean 5 mol%) than inclusions from the late ore stage (range 1–7 mol%, mean 4 mol%).
In general, the homogenization temperatures of fluid inclusions commonly do not reflect the true temperatures of the ore fluid that formed the deposit (Figure 9). The temperatures of the early ore-stage fluid require estimation using an isochore P-T diagram with a known trapping pressure derived from the program FLINCOR of Brown. Employing this correction, average trapping P-T conditions in the early ore stage at Danba are approximately 500–550 °C at 4.5–5 kbar: 4.5 kbar is the trapping pressure at the transition from early to late ore stages, and 5 kbar approximates the lower limit of the regional metamorphic conditions. CO2 volume percent isochore of the early ore-stage fluid inclusions. In this system, the temperature would be corrected over a broad range from 350 to 400 °C to over 500–550 °C. On the other hand, Figure 10 presents the δ34S data for pyrrhotite from the Danba gold deposit. Twenty analyses range from 3.1 to 9.9‰ with a mean of 7.8‰ and median of 8.3‰. The first and third quartiles are 7.2‰ and 8.9‰, respectively.
Discussion
Molybdenites, especially those having high Re concentrations, such as those with up to 105 ng/g at Danba, are difficult to disturb, even after solid-state recrystallization at granulite facies metamorphic PT conditions and in fluid-present deformation environments. Re-Os thus represent the timing of gold mineralization under post-peak metamorphic conditions (Figure 11). The Danba gold deposit is situated adjacent to a ductile-brittle shear zone within an amphibolite-facies metamorphic terrane. The Danba gold deposit formed broadly within the P-T range from peak to post-peak metamorphism. These P-T conditions are consistent with the dominance of pyrrhotite rather than pyrite and the absence of gold-related carbonates in the ore assemblage. Importantly, the nearest exposed granite intrusion is over 10 km from Danba and there is no evidence of a significantly proximal intrusion within the deposit environment from either geological or geophysical evidence.
The possibility that Danba represents a pre-existing gold deposit that was metamorphosed during the regional metamorphism, discussed above in terms of its Re-Os isotopic age, must be considered further in view of controversies related to high T-P deposits in high metamorphic environments, as summarized by Goldfarb and Groves. Second, hydrothermal quartz extensively replaced peak amphibole-plagioclase assemblage in alteration zones. As elegantly demonstrated by Stanton, the textures of mineral grains during solid-state recrystallization are controlled by their interfacial free energy, such that pyrrhotite, for example, should everywhere be interstitial to the silicate alteration minerals, but this is clearly not the case. These are compared with variations in the sulfur isotopic compositions of sulfides in global sediment-hosted orogenic gold deposits through geologic time that relate to the time-dependent marine sulfate curve. Such an association is very rare in mesozonal orogenic gold deposits, the probable precursor deposit type in any metamorphic overprint model. The ore-element associations, potassic wall rock alteration, and low-salinity H2O–CO2–CH4 ore fluid, combined with the strong structural control and late orogenic timing of gold mineralization, are all consistent with the classification of Danba as an orogenic gold deposit. The P-T conditions fall within the range of those of slightly post-peak metamorphic, hypozonal orogenic gold deposits in amphibolite-facies metamorphic terranes elsewhere, as overviewed by Gebre-Mariam et al. Danba appears to be an exceptionally rare, Phanerozoic example of a hypozonal orogenic gold deposit with substantial gold production, with previously recorded deposits almost universally representing mesozonal to epizonal orogenic deposits in sub-greenschist to greenschist-facies environments. At Danba, the previously devolatilized amphibolite-facies wall rocks of the gold deposit already appear to have been on a retrograde path during gold mineralization. Hence, the generation of the fluids responsible for gold mineralization must have been external to the hosting continental rock sequences and must be deeply derived.
This metasomatism was most likely related to the subduction of oceanic crust from ca. 950 to ca. 750 Ma in the north-western Yangtze Craton, which resulted in the partial melting of the mantle wedge by the addition of fluids and melts from the subducted oceanic plate (Figure 12). Neoproterozoic basement, while underplating the oceanic crust and overlying sediment, created a metasomatized lithospheric mantle wedge. This metasomatized Neoproterozoic lithosphere appears to be the only viable source of ore fluid for the Danba orogenic gold deposit. The deeply derived fluid could be advected into the crust, probably via the strike-slip Xianshuihe Fault, and focused into the shear zone at Danba. The 34S range of + 3.1 to + 9.9 ‰ falls within the normal range between about 0 and + 10‰ for orogenic gold deposits. The limited concentration range of + 7.2 to + 8.9 ‰ for most samples is also indicative of a single source as changes in redox and other chemical parameters at the site of gold deposition can only shift sulfur isotope compositions by a few per mil. Importantly, the nearby Yanzigou gold deposit, which is also contained in Devonian metasedimentary rocks, has 34S values of + 7.47 to + 9.35‰ for five pyrite samples in ore, implicating a similar sulfur source to Danba. Although inconclusive, a comparison between the sulfur isotope data for Danba and sea-water sulfate plus global sediment-hosted orogenic gold deposits through time shows that the sulfur in the ore sulfides of the Danba deposit could be derived from Neoproterozoic age sulfur in a subduction-related sediment wedge, whose devolatilization metamorphosed adjacent lithosphere, although Neoproterozoic sulfur in the basement or in the thick Devonian hosting sequence are also possibilities. Although most gold deposits are sited in greenschist-facies domains, several, including Beaver Dam and Cochrane Hill, are located in amphibolite-facies domains and are interpreted to be associated with retrogression during regional doming.
Conclusion
The anomalous Danba gold deposit is located in a poorly documented gold province on the north-western margin of the Mesozoic domal domain along the Longmenshan thrust nappe belt. The conjunction of a variety of research data from structural geology, metamorphic petrology, ore and alteration petrology, geochronology, and fluid inclusion and sulfur isotope studies demonstrate that Danba represents a Lower Jurassic hypozonal orogenic gold deposit formed within a P-T range of 4–5 kbar and 500–650 °C. The primary high P-T nature of the deposit, combined with its late-metamorphic timing, makes it highly unlikely that the ore fluid was sourced via the devolatilization of the hosting rock sequences. The most likely source that meets both the constraint of the tectonic evolution of the area and the geological and isotopic constraints for the deposit is a lithospheric mantle that was metasomatized during a Neoproterozoic subduction-related event. The lithospheric mantle was heated and reactivated by asthenosphere upwelling during a major Early Jurassic event involving a transition from transpressional to extensional tectonics. Danba is similar in many respects to the majority of Archean hypozonal orogenic deposits which have been attributed by some authors to higher thermal gradients of 40–60 °C/km and locally up to 80 °C/km when the thermal regime of the early Earth was greater, due to significantly higher mantle temperatures. Danba, as for the deposits of the Massif Central, such a thermal regime was probably derived more locally by widespread asthenosphere upwelling during a tectonic transition to extension.
Reference:
Zhao, H., Wang, Q., Groves, D. I., & Deng, J. (2018). A rare Phanerozoic amphibolite-hosted
gold deposit at Danba, Yangtze Craton, China: significance to fluid and metal sources for
orogenic gold systems. Minerallium Deposita.
https://doi.org/10.1007/s00126-018-0845-x