A rare Phanerozoic amphibolite-hosted gold deposit at Danba, Yangtze Craton, China: significance to fluid and metal sources for orogenic gold systems
- 1.1k Downloads
The Danba gold deposit is located in a poorly-documented gold province on the north-western margin of the Yangtze Craton. It is sited in Devonian sequences in a high-grade metamorphic terrane that includes an extensional metamorphic core complex. Around the deposit, peak metamorphic conditions of 6 ± 0.5 kbar and 650 ± 50 °C at ca. 193 Ma were followed by retrograde sillimanite-grade conditions of 4.5 ± 0.5 kbar and 550 ± 50 °C. The deposit is hosted in a broadly strata-bound ductile-brittle shear zone with high-T proximal alteration assemblages of biotite-amphibole-plagioclase and ore assemblages dominated by pyrrhotite, but with a strong association between gold and bismuth tellurides. Alteration mineral thermobarometers, together with heating/freezing studies of low-salinity H2O–CO2–CH4 fluid inclusions, indicate P-T conditions of early ore deposition of approximately 4–5 kbar and 500–650 °C at around 185 ± 9 Ma indicated by Re-Os geochronology on ore-related molybdenite. In conjunction, all data demonstrate that Danba represents a Lower Jurassic hypozonal orogenic gold deposit that formed during post-peak metamorphic retrogression. The primary high P-T nature of the deposit, combined with its late-metamorphic timing, negate that the ore fluid was sourced via devolatilization of the hosting supracrustal sequences. A deep externally-derived ore-fluid source is required. The most likely source is the K–H2O–CO2 and ore-metal fertilized lithospheric mantle that was metasomatized during Neoproterozoic subduction. It is proposed that transition from lithospheric transpression to extension in the Jurassic triggered the devolatilization of this metasomatized lithosphere to cause the formation of this rare Phanerozoic amphibolite-hosted gold deposit at Danba.
KeywordsHypozonal orogenic deposit Phanerozoic gold Amphibolite-facies metamorphism Metasomatized lithosphere Re-Os age
Hypozonal orogenic lode-gold deposits in amphibolite-facies domains are widely considered to be restricted to Precambrian, largely Archean, terranes (Groves 1993; Knight et al. 1993), perhaps reflecting significantly higher mantle temperatures and resultant-continental thermal gradients (Grove and Parman 2004, and references therein). Phanerozoic hypozonal orogenic gold deposits are generally considered to be essentially absent in the geological record (Goldfarb et al. 2001, 2005; Goldfarb and Groves 2015; Kolb et al. 2015).
A fundamental question related to hypozonal orogenic gold deposits is whether they formed syn- to post-peak metamorphism (Groves et al. 1998; Goldfarb et al. 2005) or formed pre-peak metamorphism with subsequent metamorphic overprint (Phillips and Powell 2009, 2010; Tomkins 2010). The former would imply a deep and external fluid source (Goldfarb and Groves 2015), whereas the latter would support a source of auriferous fluids from greenschist- to amphibolite-facies prograde metamorphism of abundant sedimentary sequences (Tomkins and Grundy 2009) from which gold and associated metals can be extracted by metamorphism at deeper crustal levels (Goldfarb et al. 2005; Pitcairn et al. 2006). In contrast, the Precambrian hypozonal orogenic gold deposits are largely sited in basalt-dominated sequences from which some ubiquitous gold-related elements such as arsenic cannot be extracted (Pitcairn et al. 2015).
Goldfarb and Groves (2015) proposed that, apart from metamorphism of enclosing sedimentary rocks, orogenic gold deposits could form from other fluid sources (Bierlein et al. 2006), including metamorphic devolatilization of subducting oceanic slab and overlying sediment wedge (Goldfarb and Santosh 2014; Groves and Santosh 2016) or from metasomatized lithosphere (Hronsky et al. 2012). Such source regions could explain the anomalous deposits such as those of the Jiaodong Province, China (Goldfarb and Santosh 2014; Deng and Wang 2016), and the Megashear Zone in northern Mexico (Goldfarb et al. 2007) where young deposits occur in terranes that were metamorphosed hundreds to thousands of million years earlier. Such models can also explain the mixed stable and radiogenic isotope signals of the deposits that indicate deep and extensive auriferous fluid pathways (Ridley and Diamond 2000).
In the context of this ongoing debate, the Jurassic Danba gold deposit of the north-western Yangtze Craton, as described below, is potentially critical as it is a very rare Phanerozoic orogenic deposit with typical hypozonal characteristics (Gebre-Mariam et al. 1995). If the deposit formed at similar P-T conditions to its amphibolite-facies wall rocks, it would add support to a model in which ore-forming fluids are sourced from external sub-crustal environments even in a Phanerozoic cooler Earth. The Danba deposit is described and discussed below in order to resolve its genesis and define its significance to the ongoing debate on orogenic gold source models.
The north-western margin of the Yangtze Craton is characterized by a > 1000-km Mesozoic domal domain along the Longmenshan thrust nappe belt (Fig. 1b; Chen and Wilson 1996; Wallis et al. 2003). The domal domain comprises a series of extensional metamorphic core complex formed at ca. 180 to 160 Ma (Zhou et al. 2002, 2008). The Neoproterozoic crystalline basement, which extensively crops out along the domal domain (Fig. 1b), is a product of Triassic metamorphism of a 860–750 Ma Panxi-Hannan arc assemblage (Fig. 1a; Zhou et al. 2002, 2008; Zhao and Zhou 2008) that shows depletion of high field strength elements (HSFE) and enrichment of large ion lithophile elements (LILE) (Zhou et al. 2006a, b). This Neoproterozoic basement is overlain by a thick Silurian-Devonian metasedimentary sequence that itself is overlain by the Triassic flyschoid (Brugier et al. 1997; Roger et al. 2004).
Since the Mesozoic, there have been three widely-recognized tectonic events: 1) Late Triassic compression, in which shortening and thickening of the crust of the Songpan-Garzê accretionary prism induced intense folding, thrust faulting, and Barrovian-type metamorphism (Huang et al. 2003a, b; Weller et al. 2013); 2) Early Jurassic domal extension along the > 1000 km length of the north-western margin of the Yangtze Craton (Fig. 1b); and 3) relatively minor Cenozoic thrusting related to India-Asia collision (Roger et al. 2004).
Lithostratigraphy and metamorphism
Around the Danba area, Neoproterozoic crystalline basement of the Yangtze Craton is unconformably overlain by Paleozoic cover represented by Silurian, Devonian, and Permian schist; quartzite; marble; gneiss; and amphibolite. In turn, Triassic sandstones, carbonates, and turbidites unconformably overlie this thick Paleozoic cover (Harrowfield and Wilson 2005; Zhou et al. 2008). The Danba gold deposit is hosted in Late Devonian strata (Electronic supplementary material: ESM Fig. 1), which comprise a 5300–3200 m sequence of pelites, carbonate rocks, quartzites and mafic rocks metamorphosed at amphibolite to granulite facies conditions (Fan et al. 2013).
In the Danba area, amphibolite-facies metamorphism peaked during the Late Triassic to Early Jurassic (ESM Fig. 1), as identified by Billerot et al. (2017). After re-interpretation by Weller et al. (2013), Huang et al. (2003a, b) indicated that the Barrovian metamorphism evolved progressively from kyanite-grade conditions of 5.3–8 kbar and 570–600 °C at 205–190 Ma, to sillimanite-grade conditions at 4.8–6.3 kbar and 640–680 °C at 197–180 Ma. These metamorphic P-T estimates were based on Thermocalc modeling (version 2.6) and other thermobarometers, mainly on metapelite and amphibolite from the whole area, with timing of these events based on monazite U-Pb geochronology. A sillimanite-grade metapelite (9827-1) in the north-west of the area (ESM Fig. 1), very close to the Danba deposit, yielded P-T conditions of 640 ± 25 °C and 4.8 kbar at 193.4 ± 5.2 Ma (Huang et al. 2003a, b), representing estimated peak-metamorphic conditions around the deposit.
The Danba dome and decollement structure (ESM Fig. 1) is represented by exposure of Neoproterozoic basement in a metamorphic core complex to the north-west of Danba (Jolivet et al. 2015). 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, the latter representing exhumation of the lower plate to form the extensional metamorphic core complex (Zhou et al. 2002). 40Ar/39Ar dating of amphibole associated with foliation provided cooling ages of about 166–159 Ma for the regional doming (Zhou et al. 2008).
Around the Danba area, widespread Late Triassic to Early Jurassic granites (230–170 Ma) intruded into the metamorphic strata and Triassic cover (Fig. 1; ESM Fig. 1). These granites can be subdivided into two age groups (Roger et al. 2004). The first group is represented by late-orogenic, ca. 230–200 Ma granites that have I-type and/or adakitic characteristics (Zhang et al. 2006; Xiao et al. 2007; Yuan et al. 2010). Both have ISr = 0.7050 to 0.7108 and εNd(t) = −1.0 to −9.5, and the adakites also have high Sr/Y = 20–110 and (La/Yb)N = 25–105. The second group comprises late- to post-orogenic A-and S-type granites containing mafic enclaves dated at ca. 210–180 Ma (Zhang et al. 2007; Sigoyer et al. 2014; Jolivet et al. 2015; Chen et al. 2017), consistent with intensive mantle-crustal interaction and lithospheric extension during this period. The A-type granites have ISr = 0.7090–0.7123 and εNd(t) = −2.72 to −4.26 with enrichment of HFSE (Sigoyer et al. 2014).
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). This series of shear zones is cut by a set of later NW-trending reverse faults (Fig. 2). There are thin fine-grained granodiorite dykes in the wall rocks to the Danba ore bodies (Fig. 2), but the closest large granite intrusion is more than 10 km from the mine (ESM Fig. 1). The ore bodies had been mined to a depth of 600 m below the surface and there are no indications of a granite intrusion below the ore bodies. This is confirmed by the results of EH4 magnetotelluric sounding used during exploration at the mine (Fan et al. 2013).
Metamorphic mineral assemblages in the wall rocks of the Danba gold deposit indicate that peak metamorphism reached amphibolite-facies conditions with a typical amphibole-plagioclase assemblage together with coarse-grained high-T, high-Ti red-brown biotite (Thomson 2001; Henry et al. 2005) (ESM Fig. 3a, b). In garnet + biotite + sillimanite + plagioclase + quartz assemblages (ESM Fig. 3c, d), sillimanite is common and ubiquitously associated with retrogressive muscovite that has replaced the biotite along its contacts with garnet grains. The biotite wraps around the garnet with a sigmoidal shape, producing distinctive pressure shadows that reflect initial growth during shear-related deformation (ESM Fig. 3c). Fractured garnet grains, with cracks infilled by minerals from the retrogressive metamorphic assemblage (ESM Fig. 3d), are also common.
The P-T conditions of metamorphism in the immediate vicinity of the gold orebodies are discussed in more detail below.
Petrographically, alteration zones are characterized by quartz-biotite-amphibole-K-feldspar-plagioclase-calcite-scheelite assemblages. Hydrothermal quartz has extensively replaced the amphibole-plagioclase assemblage in the alteration zone (ESM Fig. 4a). Silicification, represented by milky microcrystalline quartz, is intimately associated with biotite and aggregates of sulfide and gold (Fig. 4b–e; ESM Fig. 4b). Adjacent to this silicification, pyrrhotite, which cuts the schistosity, becomes more abundant compared with unaltered wall rocks (ESM Fig. 4c). Plagioclase, K-feldspar and amphibole (ESM Fig. 4d–e) are intergrown with quartz and biotite. K-feldspar is restricted to the gold-related alteration zones and is absent from distal wall rocks.
Ore body characteristics
The deposit is characterized by a thick ore zone known as the Major Lode with a length of about 2 km and dip of 50–90° west (Figs. 2 and 3; ESM Fig. 2), which is mined to a current depth of 300 to 600 m in different sectors of the mine. Exploration data show that the Major Lode has a true thickness range of 0.8–27.3 m (mean 8.0 m) and a gold grade range of 2.6–26.3 g/t (mean 7.0 g/t). The total gold resource, including the Major Lode and other minor ore bodies, is about 50 t (1.6 Moz) Au (Fan et al. 2013). It is thickest closer to the surface and generally thins with depth (ESM Fig. 2). Locally, the thickest gold lode completely occupies the shear zone (Fig. 2; ESM Fig. 2), which developed mainly in planes of weakness in the wall rocks, especially those between relatively incompetent biotite schist and competent quartzite (Figs. 2 and 3; ESM Fig. 2).
Gold grains occur as inclusions in pyrrhotite and quartz (Fig. 5c), in fractures within them, or along grain margins. They are commonly intergrown with bismuth tellurides (Fig. 5h, i). Gold grains vary in diameter from > 5 μm to 1 mm (mainly 25–200 μm). Gold fineness ranges from 778 to 896 (mean 821: Fan et al. 2013), which is lower than the gold fineness of > 900 that typifies most Archean amphibolite-facies gold deposits (Morrison et al. 1991; Knight et al. 1993). The ore minerals at the Danba deposit can be described in terms of two distinct assemblages within a single mineralization event: 1) an early ore-stage with dominant pyrrhotite, scheelite, minor molybdenite, pyrite, chalcopyrite, sphalerite, and trace gold; and 2) a late ore-stage with dominant Bi-tellurides, trace chalcopyrite, sphalerite, galena, pyrite, and major gold.
In the early ore-stage, anhedral pyrrhotite, the dominant ore-related sulfide (Fig. 5a, c-g), occurs within the quartz veins and also along the foliation in wall rocks immediately adjacent to the ore and alteration zones. 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). Sphalerite is normally contained as small inclusions in chalcopyrite and pyrite. Minor euhedral molybdenite is intergrown with pyrrhotite and is sited on the boundaries between sulfides and quartz (Fig. 5f). Some coarse-grained quartz coexists with the sulfides (Fig. 5a), whereas most quartz distal to sulfide aggregates is 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 (Fig. 5g) and commonly intergrown with coarse-grained native gold (Fig. 5h). The most common bismuth telluride mineral is euhedral pilsenite (Bi2 + xTe3 − x) rather than tetradymite (Bi2TeS2) or joseite (Bi4Te2 − xS1 + x) (Fan et al. 2013), suggestive of low fS2 at the late ore-stage. Rare pyrite and galena grains are sited on the margins of pilsenite grains (Fig. 5i). In many places, pilsenite has replaced pyrrhotite, suggesting that the telluride-bearing assemblage formed late in the paragenetic sequence. In a few cases, pilsenite is intergrown with late ore-stage chalcopyrite, galena, and pyrite.
Silver, Bi, Cu, Pb, Te, and Zn are characteristically co-enriched with Au at Danba, with high Bi and Te most strongly associated with high Au (Fan et al. 2013), compatible with the petrography of the ore assemblage. In general, Te and Bi show a strong positive correlation with Au as well as with each other, whereas W appears independent of other metals. Arsenic, with concentrations below 10 ppm, is a noticeable absentee from the ore body.
Twelve core samples of wall rocks and 116 samples of gold ores and wall rocks from underground workings at Danba were collected for petrography, mineralogy, geochronology, fluid-inclusion analysis, and sulfur isotopic studies. Descriptions of all these samples are given in Table 1 of the electronic supplementary material.
Five ore-related molybdenite samples from the deposit were handpicked under a binocular microscope after crushing, cleaning, and sieving to 30–60 mesh. Osmium and rhenium were then separated by distillation and extraction following the procedures described in Du et al. (2004). The Re and Os isotope ratios for each sample were determined using TJA Plasmaquad ExCell inductively-coupled plasma-mass spectrometry (ICP-MS) at the Re-Os Laboratory of the National Research Center of Geoanalysis, Chinese Academy of Geosciences.
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 in order to test analytical reliability (Stein et al. 1997). The 187Re decay constant of 1.666 × 10−11 y−1 was used for calculating molybdenite ages. The uncertainty in each individual 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 (biotite, amphibole, and garnet, in particular), 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. Different synthetic minerals and relevant standard minerals (from SPI Corp., United States) were selected for instrument calibration. In terms of wavelength-dispersive mode, the electron beam had an acceleration voltage of 15 kV and current of 10 nA, with 1 μm diameter and counting time of 30 s on each element. To improve the precision of Ti for Ti-thermometer calculation, the counting time was increased to 60 s. The estimated precision for each element is better than ± 2%.
Fluid inclusion analyses
At Danba, all generations of representative quartz from quartz veins, stockworks, and disseminated ores were examined. 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. Cooling and heating experiments were carried out using a Linkam THMSG 600 heating–freezing stage (−198 °C to 600 °C) attached to a Leitz transmitted-light microscope at the Fluid Inclusion Laboratories, China University of Geosciences, Beijing. Synthetic fluid inclusions of known compositions were used to calibrate the stage. Estimated accuracy from reproducibility of measurements is ± 0.2 °C for clathrate-melting temperatures (Tm.cla), melting temperatures of the carbonic phase (TmCO2) and CO2 homogenization temperatures (ThCO2) with a heating rate of 0.5–1 °C/min. The accuracy is ± 2 °C for homogenization temperatures (Thtot) with a heating rate of 1–2 °C/min. At other times, heating/cooling rates were restricted to be < 10 °C/min. Laser Raman spectroscopic analysis was conducted using a Renishaw-inVia spectrometer at China University of Geosciences, Beijing. The Ar + laser wavelength was 514 nm, laser power 20 mW, diameter of laser beam spot 2 μm, and spectrometer resolution 2 cm−1.
Fluid inclusion assemblages (FIAs), the concept of Goldstein and Reynolds (1994), were used to verify the consistency of the microthermometric data. The mean values of each FIA were used as representative values.
Sulfur isotope analyses
Twenty pyrrhotite samples from the Danba gold deposit were used for sulfur isotope analysis. Pyrrhotite grains were carefully handpicked individually under a binocular microscope after the samples were crushed, cleaned, and sieved to 200 mesh, to attain over 99% purity pyrrhotite separates. The EA-ISOPRIME100 mass spectrometer was used for sulfur isotope measurement, at the analytical laboratory of China University of Geosciences, Beijing. Sulfur isotope analyses were carried out utilizing standard samples GBW04414 and GBW04415, according to the method DZ/T 0184.14-1997. Environmental conditions were 25 °C and 15% humidity. The temperature was 1150 °C in the oxidized column and 850 °C in the reduction furnace. 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.
Metamorphism and wallrock alteration
Contrasts between metamorphic and alteration mineral assemblages
Peak metamorphic P-T conditions
P-T estimates of metamorphism of the wall rocks distal to the ore bodies, together with ore-related wall rock alteration, in the Danba mine were calculated using three methods: 1) amphibole-plagioclase thermometer (Holland and Blundy 1994) and corresponding calibration barometer (Bhadra and Bhattacharya 2007), 2) garnet-biotite-plagioclase-quartz thermobarometer (Berman 1991), and 3) Ti-in-biotite thermometer (Henry et al. 2005).
Calculated metamorphic pressures of wall rock amphibolite are 5.7–6.9 kbar at 690–750 °C.
The Ti-in-biotite thermometer (Henry et al. 2005) 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 (ESM Fig. 3d), the thermometer is ideally suitable for this calculation. The calibration range for this thermometry is XMg = 0.275–1.000, Ti = 0.04–0.60 pfu, P = roughly 4–6 kbar, and temperature = 480–800 °C. Precision of the thermometer is estimated to be ± 12 °C for higher temperatures. The final estimated metamorphic temperature is 640–670 °C (mean 660 °C) for Fe-rich biotite in wall rock amphibolite (ESM Fig. 5c; ESM Table 5). This is slightly lower than the temperatures calculated from the amphibole-plagioclase thermometer but still well within the range of amphibolite-facies conditions.
Post-peak metamorphic P-T conditions
Calculations using the Ti-in-biotite thermometer (Henry et al. 2005) yield 560–640 °C (mean 590 °C) for sillimanite-garnet-biotite schist, with eight of the nine data points concentrating in the range 560–600 °C (ESM Fig. 5c; ESM Table 5), in close agreement with temperatures derived from the garnet thermobarometer.
P-T conditions of wall rock alteration
The calculated temperature for wall rock alteration, based on the amphibole-plagioclase thermometer (Holland and Blundy 1994), is 590–630 °C (mean 610 °C) for the edenite-labradorite assemblage in amphibole-biotite schist from the proximal alteration zone (ESM Figs. 4a and 5a; ESM Table 5). In good agreement is the temperature of 600–650 (mean 620 °C) calculated using the Ti-in-biotite thermometer (Henry et al. 2005) on assemblages from the same sample (ESM Fig. 5c).
Temperatures calculated using the Ti-in-biotite thermometer are 560–670 °C (mean 640 °C) for Mg-rich biotite from the margin of the alteration zone (ESM Figs. 4b–d and 5c; ESM Table 5) and 580–660 (mean 630 °C) for Mg-rich biotite intimately intergrown with ore minerals in the proximal alteration zone (ESM Fig. 4f).
Fluid inclusion data
Fluid inclusion petrography
At room temperature, liquid-dominated inclusions contain both a liquid H2O phase and liquid CO2 phase (10–60 vol% of the inclusion). 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%. They are both discrete inclusions or in coexistence with liquid-dominated inclusions. Vapor-only inclusions contain only CO2. Other inclusions comprise two phases (liquid and vapor H2O). The liquid phase CO2 in all inclusions separates into liquid CO2 and vapor CO2 when temperature decreases to below about 20 °C (Fig. 8d).
Fluid inclusions as clusters in growth zones (Fig. 8a, c) and isolated inclusions are likely primary, while those in short intra-crystal trails are likely pseudo-secondary (Chi et al. 2017). Only inclusions considered to be primary or pseudo-secondary were examined, and obvious secondary fluid inclusions (Fig. 8c), distributed along fractures, are excluded.
Microthermometry of fluid inclusions
Homogenization temperatures of 300–436 °C (mean 355 °C) for the early ore-stage inclusions and 230–350 °C (mean 300 °C) for the late ore-stage inclusions were obtained (Fig. 9a). The highest temperature of 400–436 °C is at the lower end of the temperature range of 400–575 °C at which pyrite converts into pyrrhotite during metamorphism of graphitic schist (Ferry 1981; Yang et al. 2016).
The salinity (equiv. wt% NaCl), bulk composition and density of the two types of inclusions in the early and late ore-stages were estimated using the program FLINCOR of Brown (1989) with internal equations from Brown and Lamb (1989) and Bowers and Helgeson (1983). The relatively consistent results are presented in ESM Table 6. Overall, the aqueous-carbonic fluid inclusions have a total XCO2 of 0.09 and salinity of 2.6 wt% NaCl equiv. For early ore-stage fluid inclusions, the mean XCO2 is 0.10 and salinity is 1.6 wt% NaCl equiv. For late ore-stage inclusions, the mean XCO2 is 0.09 and salinity is 3.4 wt% NaCl equiv.
Volatile compositions of fluid inclusions
The results of laser Raman spectroscopy spot analyses on the CO2 phases in fluid inclusions show that they can contain both CO2 and CH4. In some samples, concentrations of CH4 are high and some exceed those of CO2 (Fig. 8f). This is in accordance with microthermometric results that indicate melting temperatures of the CO2 phase are depressed from the CO2 triple point (−56.6 °C) to −65.6 to −56.8 °C by a species such as CH4. The CH4 content in the CO2 + CH4 phase was estimated from the V-X phase diagram of the CO2–CH4 system (Thiéry et al. 1994). 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%).
Pressure correction for early ore-stage inclusions
In general, the homogenization temperatures of fluid inclusions commonly do not reflect the true temperatures of the ore fluid that formed the deposit. 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 (1989) (Fig. 9b). The pressure range is consistent with contemporarily regional metamorphic conditions of 4.8–6.3 kbar (Huang et al. 2003a, b) and 5 to 6.5 kbar (Billerot et al. 2017).
Correspondingly, the P-T diagrams for a H2O–CO2–NaCl fluid of Brown and Lamb (1989) were used to calculate the pressure correction, as it is applicable for high P-T fluid conditions (P > 350 °C; P = 2–10 kbar). Their representative isochores (ρCO2 = 0.8 and 6 wt% NaCl) were chosen to simulate the fluid conditions of the deposit (ρCO2 = 0.76 and < 5.86 wt% NaCl).
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 (Fig. 9b, blue dashed lines): 4.5 kbar is the trapping pressure at the transition from early to late ore stages, and 5 kbar approximates to the lower limit of the regional metamorphic conditions. Temperature conditions during the early ore stage at Danba are estimated using the maximum CO2 volume percent isochore of the early ore-stage fluid inclusions (50%, S930-10-1 and S680-1-1; ESM Table 6). As salinity has a limited influence on the isochores compared with CO2 density, the widely applied pressure-correction system for H2O–NaCl fluid from Potter (1977) can also be used to verify the above result. In this system, the temperature would be corrected over a broad range from 350 to 400 °C to over 500–550 °C.
The P-T estimates thus fall within amphibolite-facies metamorphic conditions in accordance with more robust P-T conditions calculated from wall rock alteration silicate minerals via thermos-barometers.
Sulfur isotope ratios
Review of critical age data for mineralization
The linearity in the 187Re–187Os plot in Fig. 6 suggests that the mineralized assemblages evolved as an isotopically closed system: the Re-Os system has not been reset (Gannoun et al. 2003). Molybdenites, especially those having high Re concentrations, such as those with up to 105 ng/g at Danba (ESM Table 2), are difficult to disturb (Selby et al. 2002), even after solid-state recrystallization at granulite facies metamorphic P-T conditions and in fluid-present deformation environments (Stein et al. 1998, 2001, 2003; Bingen and Stein 2003). The Re-Os thus represents the timing of gold mineralization under post-peak metamorphic conditions.
Classification of Danba gold deposit
These relationships between P-T conditions of regional metamorphism and gold mineralization are broadly compatible with available age constraints that indicate that gold was introduced at ca. 185 ± 9 Ma after regional peak amphibolite-facies metamorphism at ca. 193 ± 5 Ma (Huang et al. 2003a, b), with all events overlapping within geochronological constraints and probably occurring within a 10-My time frame (Fig. 11). This period of regional metamorphism and gold mineralization also overlaps the age range of the late- to post-orogenic A- and S-type granites dated at about 200–180 Ma (ESM Fig. 6).
The high temperature derived from alteration assemblages and fluid inclusions, combined with the presence of gold-related bismuth tellurides, raises the possibility that Danba is an intrusion-related gold deposit (IRGD formed from H2O-CO2 ± CH4 fluids: Lang et al. 2000; Baker 2002). However, there are a number of lines of evidence against this hypothesis. 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. In support, zircons from the small dykes within the deposit (Fig. 2) are hydrothermally altered (Wang QF, unpublished data), indicating that the dykes were emplaced prior to the deposit. In addition, the minimum pressure estimate of about 4 kbar is outside the depth range of documented IRGDs. The sulfur isotope data (Fig. 10; ESM Table 8) are also inconsistent with a magmatic sulfur source, the δ34S of which is normally considered to be around 0‰ (Gemmell and Large 1992).
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 (2015). A number of lines of evidence again argue against this model. First, the regional peak metamorphism is indistinguishable in age from that of the deposit. Second, hydrothermal quartz extensively replaced peak amphibole-plagioclase assemblage in alteration zones (ESM Fig. 4a). As elegantly demonstrated by Stanton (1972), 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. The hydrothermal gold-related pyrrhotite crosscuts silicate minerals in the alteration zone (ESM Fig. 4c), whereas pyrrhotite in wall rock is interstitial to silicate minerals along the schistosity (ESM Fig. 3a, b). Third, deposits formed under greenschist-facies conditions in fault zones universally have distal alteration zones that are far more extensive than proximal alteration zones (Eilu et al. 1999; Eilu and Groves 2001) and this geometry should be preserved during subsequent metamorphism. In contrast, Danba has essentially no discernable distal alteration (Fig. 7; ESM Fig. 2), implying that ore fluid was channeled along the hosting ductile-brittle shear zone. Finally, the presence of a strong Au-Bi-Te association is also consistent with formation of the Danba deposit under high-T conditions by analogy to their association in intrusion-related deposits, as discussed above. 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 (Groves et al. 1998; Golfarb et al. 2005). 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. (1995): ~ 475 °C at 3 kbar to ~ 700 °C at 6 kbar (Smith 1996). Such hypozonal orogenic gold deposits are relatively common in well-documented Archean greenstone belts from Western Australia (Bloem et al. 1994; Napier et al. 1998; Knight et al. 2000; Vielreicher et al. 2002), where they form an integral part of a crustal continuum (Groves 1993). Archean hypozonal orogenic gold deposits elsewhere, such as the Renco deposit in Zimbabwe (Kolb and Meyer 2002), Hutti deposit in India (Kolb et al. 2005), and New Consort deposit in South Africa (Dziggel et al. 2010) also formed under post-peak amphibolite-facies metamorphic conditions on a retrograde P-T path.
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 (Goldfarb et al. 2005; Goldfarb and Groves 2015).
Source of ore fluid
As discussed by Goldfarb and Groves (2015) and Groves and Santosh (2016), orogenic gold deposits that formed in terranes that were metamorphosed millions of years before the gold mineralization event, or were deposited under amphibolite-facies P-T conditions, cannot have a fluid source derived from regional metamorphism of enclosing continental rock sequences: the most commonly proposed ore-fluid source (Kerrich and Fyfe 1981; Phillips and Groves 1983; Colvine et al. 1984; Cox et al. 1991; Powell et al. 1991; Bierlein and Crowe 2000; Goldfarb et al. 2001, 2005; Dubé and Gosselin 2007; Phillips and Powell 2009, 2010; Tomkins 2010).
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, generation of the fluids responsible for gold mineralization must have been external to the hosting continental rock sequences (Otto et al. 2007; Kolb et al. 2015), and must be deeply derived (Ridley and Diamond 2000).
The major possibilities for such a deep fluid source appear to be devolatilization of the sediment wedge above a subduction zone (Goldfarb and Santosh 2014; Groves and Santosh 2016) or lithosphere metasomatized during subduction-related fluid release (Hronsky et al. 2012; Goldfarb and Santosh 2014).
In order to determine if either of these is a viable ore-fluid source for the Danba gold deposit, it is necessary to examine the tectonic history of the hosting terrane.
As discussed above, the Danba gold deposit formed in a Mesozoic post-collisional transpressional to extensional regime after the Late Triassic closure of Paleotethys. The regional granites (ESM Fig. 6) in the Songpan-Garzê accretionary prism were generally emplaced later than the magmatic rocks in the Yindun arc that were related to subduction of the Paleotethyan Ocean (Reid et al. 2007). The regional hybrid granites evolved from late-orogenic adakites and I-type granites (230–200 Ma) to late- to post-orogenic A- and S-type granites (210–180 Ma) that overlap the period of regional amphibolite-facies metamorphism and gold mineralization (ESM Fig. 6). Importantly, the occurrence of mafic enclaves in the granite indicates a mixing between mantle-derived melts and the granitic magma (Sigoyer et al. 2014; Chen et al. 2017). The marked diversity of granite types in the late- to post-collisional environment, especially the occurrence of A-type granite, has been interpreted to be related to delamination of lower lithospheric mantle or other similar processes (Zhang et al. 2007; Yuan et al. 2010; Sigoyer et al. 2014; Chen et al. 2017), all involving asthenosphere upwelling.
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 (Fig. 12). The mechanisms for migration into the crust by fluids created through devolatilization of metasomatized lithospheric mantle have been rarely discussed (Kennedy et al. 1997; Burnard and Polya 2004; Finlay et al. 2010; Klemperer et al. 2013), with seismic pumping along crustal-scale faults one logical mechanism (Cox 2016). Rapid passage of such deeply-sourced ore fluid through faults with zones of local water saturation may prevent the ore fluids from being consumed through partial melting (Schrauder and Navon 1994; Bureau and Keppler 1999; Klein-BenDavid et al. 2011; Rospabé et al. 2017).
The δ34S range of + 3.1 to + 9.9 ‰ falls within the normal range between about 0 and + 10‰ for orogenic gold deposits (Kerrich 1987, 1989; Golding et al. 1990; Nesbitt 1991; Partington and Williams 2000). 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 (Goldfarb and Groves 2015). Importantly, the nearby Yanzigou gold deposit, that is also contained in Devonian metasedimentary rocks, has δ34S values of + 7.47 to + 9.35‰ for five pyrite samples in ore (Hou 2010), 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 (Fig. 10) 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 (5.3–3.2 km) Devonian hosting sequence are also possibilities.
The consistent 187Re and 187Os values that plots on a linear array in Fig. 6 suggest a common age and an isotopically homogeneous source (Gannoun et al. 2003). The high initial 187Os/188Os values 11 ± 14 also indicate a probable source from metasomatized lithospheric mantle (McInnes et al. 2008). The subducted crustal components incorporated into the lithosphere have provided large amount of radioactive Os and have increased 187Os/188Os ratios (Gannoun et al. 2003). Both the high Re concentrations of molybdenites (up to 105 ng/g; ESM Table 2) and 187Re/187Os ratios (up to ~ 300) support this explanation (Stein et al. 2001; Gannoun et al. 2003; Çelik et al. 2018).
The Re-Os data, although equivocal, are therefore at least consistent with a fluid and metal source derived from metasomatized lithospheric mantle.
Potential analogs for the Danba gold deposit
The Danba gold deposit is located on the edge of a metamorphic core complex during doming of the enclosing belt (Fig. 1; ESM Fig. 1), related to post-orogenic transpression to extension and related Jurassic asthenosphere upwelling. Some other Phanerozoic orogenic gold deposits in high-grade metamorphic rocks are also related to decollement structures during metamorphic core complex formation related to regional extension caused by asthenosphere upwelling. Most of these are mesozonal orogenic gold deposits, deposited under P-T conditions similar to those deposits in Phanerozoic greenschist-facies domains, but they formed in Precambrian rocks that were metamorphosed many hundreds to thousands of million years before the gold mineralization event, as summarized by Goldfarb et al. (2001, 2007). These include the well-documented Jiaodong gold province of China (Goldfarb and Santosh 2014; Deng et al. 2015; Deng and Wang 2016) and the less-well documented deposits of the Megashear Zone of northern Mexico (summarized in Goldfarb et al. 2007).
However, the late Carboniferous to early Permian (315–285 Ma) gold deposits of the Variscan belt in the Massif Central of France, defined as orogenic gold deposits by Bouchot et al. (2005), are an exception. They include deposits with significant gold production in the Saligne (117 t = 4 Moz gold) and St-Yreix (37 t = 1.3 Moz gold) districts. Here, the time gap between early subduction and compressional orogenesis and gold mineralization related to post-orogenic transpressional to extensional doming and exhumation (Olivier et al. 2004) was less than 200 My, with gold mineralized domes parallel to the orogen (Whitney et al. 2004). The deposits described as “deep-seated gold deposits” by Bouchot et al. (2005) fit the classification of hypozonal orogenic gold deposits and show strong similarities to Danba. As summarized by Bouchot et al. (2005) they have an early ore-stage dominated by pyrrhotite and arsenopyrite with some loellingite, common in Archean hypozonal orogenic gold deposits from Western Australia (Neumayr et al. 1993), followed by a late ore-stage with gold, base metals and Bi-bearing minerals. Homogenization temperatures of low-salinity H2O–CO2–CH4 fluid inclusions range from 260 to 450 °C, with interpreted P-T depositional conditions of up to 4.0–5.5 kbar and 450–500 °C, slightly lower than those for Danba, and commensurate with the presence of some pyrite and carbonate minerals in the ore bodies.
Other exceptions include some ~ 370 Ma lode gold deposits hosted in turbidite sequences in the Paleozoic Meguma Group, Nova Scotia, Canada (Kontak et al. 1990; Ryan and Smith 1998). 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 by Kontak et al. (1990) to be associated with retrogression during regional doming. They have ore mineral assemblages of pyrrhotite-scheelite-molybdenite-Bi-Te minerals, fluid temperatures approaching 450–500 °C, and sulfide δ34S values of 9 ± 1‰. On the basis of these parameters, Kontak et al. (1990) suggest derivation of ore components from a sub-crustal source.
The hypozonal orogenic gold deposits of the French Massif Central appear to be the best analogs of Danba in terms of tectonic evolution, structural association, nature of economic ore bodies, P-T conditions of gold deposition, and interpreted fluid and metal sources, but several of the Meguma deposits are also probable analogs.
The anomalous Danba gold deposit is located in a poorly-documented gold province on the north-western margin of the Yangtze Craton. The province is characterized by a > 1000-km 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. It most likely formed during post-peak sillimanite-grade retrogression (4.5 ± 0.5 kbar and 550 ± 50 °C) subsequent to peak sillimanite-grade metamorphism (6 ± 0.5 kbar and 650 ± 50 °C). A combination of textural relationships between ore-related sulfides and silicate alteration minerals, the Au-Bi-Te association, and the virtual restriction of wall rock alteration to the proximal zone strongly indicate that the Danba deposit formed during the regional metamorphic event and was not metamorphosed after formation.
The primary high P-T nature of the deposit, combined with its late-metamorphic timing, make it highly unlikely that the ore fluid was sourced via devolatilization of the hosting rock sequences. A deep externally-derived fluid source is required. The timing of tectonic events in the region negates a direct source of ore fluid via devolatilization of the sediment wedge above a down-going subduction slab, as has been suggested for anomalous orogenic gold deposits such as those of the Jiaodong Province. The most likely source that meets both the constraints of the tectonic evolution of the area and the geological and isotopic constraints for the deposit is 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. This asthenosphere upwelling was responsible for granite intrusion, regional metamorphism, doming, including the generation of metamorphic core complexes, and devolatilization of fertilized lithosphere to provide the Danba ore fluid. This is interpreted to have been advected into the crust, probably via the crustal-scale Xianshuihe Fault, and focused into the shear zone at Danba by seismic pumping.
Danba appears to be part of only the third well-documented and well-endowed Phanerozoic hypozonal orogenic gold district in an amphibolite-facies metamorphic terrane. The best analog is represented by the Variscan gold districts of the French Massif Central, but several deposits in the Meguma gold districts of Nova Scotia also have strong similarities. Danba is similar in many respects to the majority of Archean hypozonal orogenic deposits which have been attributed by some authors (Kolb et al. 2015) 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. At 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.
As for the Archean examples of hypozonal deposits, a deep external source is required. For the Archean deposits, devolatilization of the sedimentary wedge above a subducting slab meets tectonic and crustal-scale architectural constraints. For Danba, a more indirect origin via devolatilization of lithosphere that was metasomatized in an earlier subduction-related event is the only reasonable model that meets all geological and geochronological constraints.
The key question, as was asked for the Jiaodong gold deposits (Groves and Santosh 2016), is whether Danba and its Phanerozoic analog deposits are simply exceptions in terms of Phanerozoic auriferous-fluid source or whether it has a more fundamental importance and questions acceptance of the widely-accepted Phanerozoic model of metamorphic devolatilization of crustal rock sequences, just as the Archean analogs question that model for the genesis of Precambrian orogenic gold deposits?
David Groves sincerely thanks Professor Jun Deng and Professor Liqiang Yang for their invitation, travel support, and accommodation at CUGB which allowed interaction with the co-authors of this paper.
This research was jointly supported by the National Key Research and Development Project of China (2016YFC0600307) and the National Key Basic Research Development Program (973 Program; 2015CB452606).
- Abdel-Rahman AFM (1994) Nature of biotites from alkaline, calc-alkaline, and peraluminous magmas. J Petrol 35:525–541Google Scholar
- Baker T (2002) Emplacement depth and CO2-rich fluid inclusions in intrusion-related gold deposits. Econ Geol 97:1109–1115Google Scholar
- Berman RG (1991) Thermobarometry using multi-equilibrium calculations: a new technique with petrological application. Can Mineral 29:833–855Google Scholar
- Bhadra S, Bhattacharya A (2007) The barometer tremolite + tschermakite + 2 albite = 2 pargasite + 8 quartz: constraints from experimental data at unit silica activity, with application to garnet-free natural assemblages. Am Mineral 92:491–502Google Scholar
- Bierlein FP, Crowe DE (2000) Phanerozoic orogenic lode gold deposits. Rev Econ Geol 13:103–139Google Scholar
- Bierlein FP, Groves DI, Goldfarb RJ, Dubé B (2006) Lithospheric controls on the formation of provinces hosting giant orogenic gold deposits. Mineral Deposita 40:874–886Google Scholar
- Billerot A, Duchene S, Vanderhaeghe O, Sigoyer JD (2017) Gneiss domes of the Danba metamorphic complex, Songpan Ganze, eastern Tibet. J Asian Earth Sci 140:48–74Google Scholar
- Bingen B, Stein H (2003) Molybdenite Re-Os dating of biotite dehydration melting in the Rogaland high-temperature granulites, S Norway. Earth Planet Sci Lett 208:181–195Google Scholar
- Bloem EJM, Dalstra HJ, Groves DI, Ridley JR (1994) Metamorphic and structural setting of amphibolite-hosted gold deposits near Southern Cross, Southern Cross Province, Yilgarn Block, Western-Australia. Ore Geol Rev 9:183–208Google Scholar
- Bouchot V, Ledru P, Lerouge C, Lescuyer JL, Milesi JP (2005) Late Variscan mineralizing systems related to orogenic processes: the French Massif Central. Ore Geol Rev 27:169–197Google Scholar
- Bowers TS, Helgeson HC (1983) Calculation of the thermodynamic and geochemical consequences of non-ideal mixing in the system H2O-CO2-NaCl on phase relations in geologic systems: equation of state for H2O-CO2-NaCl fluids at high pressures and temperatures. Geochim Cosmochim Acta 47:1247–1275Google Scholar
- Brown PE (1989) FLINCOR: a microcomputer program for the reduction and investigation of fluid-inclusion data. Am Mineral 74:1390–1393Google Scholar
- Brown PE, Lamb WM (1989) P-V-T properties of fluids in the system H2O ± CO2 ± NaCl: new graphical presentations and implications for fluid inclusion studies. Geochim Cosmochim Acta 53:1209–1221Google Scholar
- Brugier O, Lancelot JR, Malavieille J (1997) U-Pb dating on single zircon grains from the Triassic Songpan-Garzê flysch (Central China): provenance and tectonic correlations. Earth Planet Sci Lett 152:217–231Google Scholar
- Bureau H, Keppler H (1999) Complete miscibility between silicate melts and hydrous fluids in the upper mantle: experimental evidence and geochemical implications. Earth Planet Sci Lett 165:187–196Google Scholar
- Burnard PG, Polya DA (2004) Importance of mantle derived fluids during granite associated hydrothermal circulation: He and Ar isotopes of ore minerals from Panasqueira. Geochim Cosmochim Acta 68:1607–1615Google Scholar
- Çelik ÖF, Marzoli A, Marschik R, Chiaradia M, Mathur R (2018) Geochemical, mineralogical and Re-Os isotopic constraints on the origin of Tethyan oceanic mantle and crustal rocks from the Central Pontides, northern Turkey. Mineral Petrol 112:25–44Google Scholar
- Chang ZS, Large RR, Maslennikov V (2008) Sulfur isotopes in sediment-hosted orogenic gold deposits: evidence for an early timing and a seawater sulfur source. Geology 36:971–974Google Scholar
- Chen Q, Sun M, Zhao GC, Yang FL, Long XP, Li JH, Wang J, Yu Y (2017) Origin of the mafic microgranular enclaves (MMEs) and their host granitoids from the Tagong pluton in Songpan-Ganze terrane: an igneous response to the closure of the Paleo-Tethys ocean. Lithos 290-291:1–17Google Scholar
- Chen SF, Wilson CJL (1996) Emplacement of the Longmenshan thrust-nappe belt along the eastern margin of the Tibetan Plateau. J Struct Geol 18:413–430Google Scholar
- Chi GX, Haid T, Quirt D, Fayek M, Blamey N, Chu HX (2017) Petrography, fluid inclusion analysis, and geochronology of the End uranium deposit, Kiggavik, Nunavut, Canada. Mineral Deposita 52:211–232Google Scholar
- Colvine AC, Andrews AJ, Cherry ME, Durocher ME, Fyon JA, Lavigne MJ, Macdonald AJ, Marmont S, Poulsen KH, Springer JS, Troop DG (1984) An integrated model for the origin of Archean lode-gold deposits. Ontario Geological Survey open-file report no 5524, 98 ppGoogle Scholar
- Cox SF (2016) Injection-driven swarm seismicity and permeability enhancement: implications for the dynamics of hydrothermal ore systems in high fluid-flux, over-pressured faulting regimes-an invited paper. Econ Geol 111:559–588Google Scholar
- Cox SF, Etheridge MA, Cas RAF, Clifford BA (1991) Deformational style of the Castlemaine area, Bendigo-Ballarat Zone--implications for evolution of the crustal structure across Southeast Australia. Aust J Earth Sci 38:151–170Google Scholar
- Deng J, Wang C, Bagas L, Carranza EJM, Lu Y (2015) Cretaceous–Cenozoic tectonic history of the Jiaojia Fault and gold mineralization in the Jiaodong Peninsula, China: constraints from zircon U–Pb, illite K–Ar, and apatite fission track thermochronometry. Mineral Deposita 50:987–1006Google Scholar
- Deng J, Wang QF (2016) Gold mineralization in China: metallogenic provinces, deposit types and tectonic framework. Gondwana Res 36:219–274Google Scholar
- Deng J, Wang QF, Li GJ (2017) Tectonic evolution, superimposed orogeny, and composite metallogenic system in China. Gondwana Res 50:216–266Google Scholar
- Deng J, Wang QF, Li GJ, Santosh M (2014) Cenozoic tectono-magmatic and metallogenic processes in the Sanjiang region, southwestern China. Earth-Sci Rev 138:268–299Google Scholar
- Du AD, Wu SQ, Sun DZ, Wang SX, Qü WJ, Markey R, Stein H, Morgan JW, Malinovskiy D (2004) Preparation and certification of Re-Os dating reference materials: molybdenite HLP and JDC. Geostand Geoanal Res 28:41–52Google Scholar
- Dubé B, Gosselin P (2007) Greenstone-hosted quartze-carbonate vein deposits. Geological Association of Canada, Mineral Deposits Division, Special Publication 5, pp 49–73Google Scholar
- Dziggel A, Poujol M, Otto A, Kisters AFM, Trieloff M, Schwarz WH, Meyer FM (2010) New U–Pb and 40Ar/39Ar ages from the northern margin of the Barberton greenstone belt, South Africa: implications for the formation of Mesoarchaean gold deposits. Precambrian Res 179:206–220Google Scholar
- Eilu PK, Groves DI (2001) Primary alteration and geochemical dispersion haloes of Archaean orogenic gold deposits in the Yilgarn Craton: the pre-weathering scenario. Explor Environ Anal 1:183–200Google Scholar
- Eilu PK, Mathison CI, Groves DI, Allardyce W (1999) Atlas of alteration assemblages, styles and zoning in orogenic lode-gold deposits in a variety of host rock and metamorphic settings. Geology Department & University Extension, University Western Australia Publication 30, 50 ppGoogle Scholar
- Fan T, Ying YY, Wang Y, Zhao HS (2013) Exploration of the Meihe gold mine in Danba County. Geology Bureau of Sichuan Province, Mineral Deposit Exploration report no 12, 219 pp (in Chinese)Google Scholar
- Ferry JM (1981) Petrology of graphitic sulfide-rich schists from south-central Maine: an example of desulfidation during prograde regional metamorphism. Am Mineral 66:908–930Google Scholar
- Finlay AJ, Selby D, Osborne MJ, Finucane D (2010) Fault-charged mantle-fluid contamination of United Kingdom North Sea oils: insights from Re-Os isotopes. Geology 38:979–982Google Scholar
- Gannoun A, Tessalina S, Bourdon B, Orgeval JJ, Birck JL, Allègre CJ (2003) Re–Os isotopic constraints on the genesis and evolution of the Dergamish and Ivanovka Cu (Co, Au) massive sulphide deposits, south Urals, Russia. Chem Geol 196:193–207Google Scholar
- Gebre-Mariam M, Hagemann SG, Groves DI (1995) A classification scheme for epigenetic Archaean lode-gold deposits. Mineral Deposita 30:408–410Google Scholar
- Gemmell JB, Large RR (1992) Stringer system and alteration zones underlying the Hellyer volcanogenic massive sulfide deposit, Tasmania, Australia. Econ Geol 87:620–649Google Scholar
- Goldfarb RJ, Baker T, Dubé B, Groves DI, Hart CJR (2005) Distribution, character, and genesis of gold deposits in metamorphic terranes. Economic Geology 100th Anniversary Volume, pp 407–450Google Scholar
- Goldfarb RJ, Groves DI (2015) Orogenic gold: common or evolving fluid and metal sources through time. Lithos 233:2–26Google Scholar
- Goldfarb RJ, Groves DI, Gardoll S (2001) Orogenic gold and geologic time: a global synthesis. Ore Geol Rev 18:1–75Google Scholar
- Goldfarb RJ, Hart CJR, Davis G, Groves DI (2007) East Asian gold: deciphering the anomaly of Phanerozoic gold in Precambrian cratons. Econ Geol 102:341–346Google Scholar
- Goldfarb RJ, Miller LD, Leach DL, Snee LW (1997) Gold deposits in metamorphic rocks of Alaska. In: Goldfarb RJ, Miller LD (eds) Mineral deposits of Alaska. Economic Geology Monograph, pp 151–190Google Scholar
- Goldfarb RJ, Santosh M (2014) The dilemma of the Jiaodong gold deposits: are they unique? Geosci Front 5:139–153Google Scholar
- Golding SD, Groves DI, McNaughton NJ, Mikucki EJ, Sang JH (1990) Source of ore fluid and ore components—sulphur isotope studies. Geology Department and University Extension 20, University of Western Australia Publication, pp 259–262Google Scholar
- Goldstein RH, Reynolds TJ (1994) Systematics of fluid inclusions in diagenetic minerals. Soc Sediment Geol Short Course 31:1–199Google Scholar
- Grove TL, Parman SW (2004) Thermal evolution of the Earth as recorded by komatiites. Earth Planet Sci Lett 219:173–187Google Scholar
- Groves DI (1993) The crustal continuum model for late-Archaean lode-gold deposits of the Yilgarn Block, Western Australia. Mineral Deposita 28:366–374Google Scholar
- Groves DI, Goldfarb RJ, Gebre-Mariam M, Hagemann SG, Robert F (1998) Orogenic gold deposits: a proposed classification in the context of their crustal distribution and relationship to other gold deposit types. Ore Geol Rev 13:7–27Google Scholar
- Groves DI, Santosh M (2016) The giant Jiaodong gold province: the key to a unified model for orogenic gold deposits? Geosci Front 7:409–417Google Scholar
- Harrowfield MJ, Wilson CJL (2005) Indosinian deformation of the Songpan-Garzê fold belt, northeast Tibetan Plateau. J Struct Geol 27:101–117Google Scholar
- Henry DJ, Guidotti CV, Thomson JA (2005) The Ti-saturation surface for low-to-medium pressure metapelitic biotites: implications for geothermometry and Ti-substitution mechanisms. Am Mineral 90:316–328Google Scholar
- Holdaway MJ, Mukhopadhyay B (1993) A re-evaluation of the stability relations of andalusite: thermochemical data and phase-diagram for the aluminum silicates. Am Mineral 78:298–315Google Scholar
- Holland T, Blundy J (1994) Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry. Contrib Mineral Petrol 116:433–447Google Scholar
- Hou L (2010) Initial study on geological and geochemical characteristics of the Yanzigou gold deposit, Danba. M.Sc. Thesis, Chengdu University of Technology, 52 pp (in Chinese with English abstract)Google Scholar
- Hronsky JMA, Groves DI, Loucks RR, Begg GC (2012) A unified model for gold mineralisation in accretionary orogens and implications for regional-scale exploration targeting methods. Mineral Deposita 47:339–358Google Scholar
- Huang M, Buick IS, Hou LW (2003a) Tectono-metamorphic evolution of the eastern Tibet Plateau: evidence from the Central Songpan-Garze Orogenic Belt, western China. J Petrol 44:255–278Google Scholar
- Huang M, Maas R, Buick IS, Williams IS (2003b) Crustal response to continental collisions between the Tibet, Indian, South China and North China blocks: geochronological constraints from the Songpan-Garzê orogenic belt, western China. J Metamorph Geol 21:223–240Google Scholar
- Jolivet M, Roger F, Xu ZQ, Paquette JL, Cao H (2015) Mesozoic-Cenozoic evolution of the Danba dome (Songpan Garzê, East Tibet) as inferred from LA-ICP-MS U-Pb and fission-track data. J Asian Earth Sci 102:180–204Google Scholar
- Kennedy BM, Kharaka YK, Evans WC, Ellwood A, DePaolo DJ, Thordsen J, Ambats G, Mariner RH (1997) Mantle fluids in the San Andreas fault system, California. Science 278:1278–1281Google Scholar
- Kerrich R (1987) Stable isotope geochemistry of Au-Ag vein deposits in metamorphic rocks. In: Kyser TK (ed) Stable isotope geochemistry of low temperature fluids. Mineralogical Association of Canada short course 13:287–336Google Scholar
- Kerrich R (1989) Geochemical evidence on the sources of fluids and solutes for shear zone hosted mesothermal Au deposits. In: Bursnall JT (ed) Mineralization and shear zones. Geological Association of Canada short course 6:129–197Google Scholar
- Kerrich R, Fyfe WS (1981) The gold-carbonate association: source of CO2, and CO2 fixation reactions in Archaean lode deposits. Chem Geol 33:265–294Google Scholar
- Klein-BenDavid O, Pettke T, Kessel R (2011) Chromium mobility in hydrous fluids at upper mantle conditions. Lithos 125:122–130Google Scholar
- Klemperer SL, Kennedy BM, Sastry SR, Makovsky Y, Harinarayana T, Leech ML (2013) Mantle fluids in the Karakoram fault: helium isotope evidence. Earth Planet Sci Lett 366:59–70Google Scholar
- Knight JT, Groves DI, Ridley JR (1993) The Coolgardie goldfield, Western Australia: district-scale controls on an Archaean gold camp in an amphibolite facies terrane. Mineral Deposita 28:436–456Google Scholar
- Knight JT, Ridley JR, Groves DI (2000) The Archean amphibolite facies Coolgardie Goldfield, Yilgarn Craton, Western Australia: nature, controls, and gold field-scale patterns of hydrothermal wall-rock alteration. Econ Geol 95:49–84Google Scholar
- Kolb J, Dziggel A, Bagas L (2015) Hypozonal lode gold deposits: a genetic concept based on a review of the New Consort, Renco, Hutti, Hira Buddini, Navachab, Nevoria and the Granites deposits. Precambrian Res 262:20–44Google Scholar
- Kolb J, Meyer MF (2002) Fluid inclusion record of the hypozonal orogenic Renco gold deposit (Zimbabwe) during the retrograde P–T evolution. Contrib Mineral Petrol 143:495–509Google Scholar
- Kolb J, Rogers A, Meyer FM (2005) Relative timing of deformation and two-stage gold mineralization at Hutti mine, Dharwar Craton, India. Mineral Deposita 40:156–174Google Scholar
- Kontak DJ, Smith PK, Kerrich R, Williams PF (1990) Integrated model for Meguma Group lode gold deposits, Nova Scotia, Canada. Geology 18:238–242Google Scholar
- Lang JR, Baker T, Hart CJR, Mortensen JK (2000) An exploration model for intrusion-related gold systems. Soc Econ Geol Newslett 40:6–15Google Scholar
- Leake BE, Woolley AR, Arps CES (1997) Nomenclature of amphiboles: report of the subcommittee on amphiboles of the International Mineralogical Association, Commission on New Minerals and Mineral Names. Can Mineral 35:219–246Google Scholar
- McInnes BIA, Keays RR, Lambert DD, Hellstrom J, Allwood JS (2008) Re–Os geochronology and isotope systematics of the Tanami, Tennant Creek and Olympic Dam Cu–Au deposits. Aust J Earth Sci 55:967–981Google Scholar
- Morrison GW, Rose WJ, Jaireth S (1991) Geological and geochemical controls on the silver content (fineness) of gold-silver deposits. Ore Geol Rev 6:333–364Google Scholar
- Napier RW, Guise PG, Rex DC (1998) 40Ar/39Ar constraints on the timing and history of amphibolite facies gold mineralisation in the Southern Cross area, Western Australia. Aust J Earth Sci 45:285–296Google Scholar
- Nesbitt BE (1991) Phanerozoic gold deposits in tectonically active continental margins. In: Foster RP (ed) Gold metallogeny and exploration. Blackie and Sons Ltd, Glasgow, pp 104–132Google Scholar
- Neumayr P, Cabri LJ, Groves DI, Mikucki EJ, Jackman JA (1993) The mineralogical distribution of gold and relative timing of gold mineralization in two Archean settings of high metamorphic grade in Australia. Can Mineral 31:711–725Google Scholar
- Olivier P, Gleizes G, Paquette JL (2004) Gneiss domes and granite emplacement in an obliquely convergent regime: new interpretation of the Variscan Agly Massif (Eastern Pyrenees, France). Geol Sco Am Special Paper 380:229–242Google Scholar
- Otto A, Dziggel A, Kisters AFM, Meyer FM (2007) The New Consort gold mine, Barberton greenstone belt, South Africa: orogenic gold mineralization in a condensed metamorphic profile. Mineral Deposita 42:715–735Google Scholar
- Partington GA, Williams PJ (2000) Proterozoic lode gold and (iron)-copper-gold deposits—a comparison of Australian and global examples. Rev Econ Geol 13:69–101Google Scholar
- Phillips GN, Groves DI (1983) The nature of Archaean gold-bearing fluids as deduced from gold deposits of Western Australia. J Geol Soc Aust 30:25–39Google Scholar
- Phillips GN, Powell R (2009) Formation of gold deposits: review and evaluation of the continuum model. Earth-Sci Rev 94:1–21Google Scholar
- Phillips GN, Powell R (2010) Formation of gold deposits: a metamorphic devolatilization model. J Metamorph Geol 28:689–718Google Scholar
- Pitcairn IK, Craw D, Teagle DAH (2015) Metabasalts as sources of metals in orogenic gold deposits. Mineral Deposita 50:373–390Google Scholar
- Pitcairn IK, Teagle DAH, Craw D, Olivo GR, Kerrich R, Brewer TS (2006) Sources of metals and fluids in orogenic gold deposits: insights from the Otago and Alpine schists, New Zealand. Econ Geol 101:1525–1546Google Scholar
- Potter RW (1977) Pressure corrections for fluid-inclusion homogenization temperatures based on the volumetric properties of the system NaCl-H2O. J Res US Geol Surv 5:603–607Google Scholar
- Powell R, Will T, Phillips G (1991) Metamorphism in Archaean greenstone belts: calculated fluid compositions and implications for gold mineralization. J Metamorph Geol 9:141–150Google Scholar
- Reid A, Wilson CJL, Shun L, Pearson N, Belousova E (2007) Mesozoic plutons of the Yidun arc, SW China: U/Pb geochronology and Hf isotopic signature. Ore Geol Rev 31:88–106Google Scholar
- Ridley JR, Diamond LW (2000) Fluid chemistry of orogenic lode gold deposits and implications for genetic models. Rev Econ Geol 13:141–162Google Scholar
- Roger F, Jolivet M, Malavieille J (2010) The tectonic evolution of the Songpan Garzê (North Tibet) and adjacent areas from Proterozoic to present: a synthesis. J Asian Earth Sci 39:254–269Google Scholar
- Roger F, Malavieille J, Leloup PH, Calassou S, Xu ZQ (2004) Timing of granite emplacement and cooling in the Songpan Garzê fold belt (eastern Tibetan Plateau) with tectonic implications. J Asian Earth Sci 22:465–481Google Scholar
- Rospabé M, Ceuleneer G, Benoit M, Abily B, Pinet P (2017) Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: the interplay between percolating magmas and high-temperature hydrous fluids. Geology 45:471–474Google Scholar
- Ryan RJ, Smith PK (1998) A review of the mesothermal gold districts of the Meguma Group, Nova Scotia Canada. Ore Geol Rev 13:153–183Google Scholar
- Schrauder M, Navon O (1994) Hydrous and carbonatitic mantle fluids in fibrous diamonds from Jwaneng, Botswana. Geochim Cosmochim Acta 58:761–771Google Scholar
- Selby D, Creaser RA, Hart CJR, Rombach CS, Thompson JFH, Smith MT, Bakke AA, Goldfarb RJ (2002) Absolute timing of sulfide and gold mineralization: a comparison of Re-Os molybdenite and Ar-Ar mica methods from the Tintina Gold Belt, Alaska. Geology 30:791–794Google Scholar
- Sengör AMC, Natalin BA (1996) Paleotectonics of Asia: fragment of a synthesis. In: Yin A, Harrison TM (eds) The tectonics of Asia. Cambridge University Press, New York, pp 486–640Google Scholar
- Sigoyer JD, Vanderhaeghe O, Duchene S, Billerot A (2014) Generation and emplacement of Triassic granitoids within the Songpan Ganze accretionary-orogenic wedge in a context of slab retreat accommodated by tear faulting, eastern Tibetan Plateau, China. J Asian Earth Sci 88:192–216Google Scholar
- Smith DS (1996) Hydrothermal alteration at the Miner Hill mine, Jardine, Montana: a lower amphibolite facies Archean lode gold deposit of probable syn-metamorphic origin. Econ Geol 91:723–750Google Scholar
- Stanton RL (1972) Ore petrology. McGraw-Hill, New York 711 ppGoogle Scholar
- Stein HJ, Markey RJ, Morgan MJ, Du A, Sun Y (1997) Highly precise and accurate Re-Os ages for molybdenite from the East Qinling molybdenum belt, Shanxi Province, China. Econ Geol 92:827–835Google Scholar
- Stein HJ, Markey RJ, Morgan JW, Hannah JL, Schersten A (2001) The remarkable Re–Os chronometer in molybdenite: how and why it works. Terra Nova 13:479–486Google Scholar
- Stein HJ, Scherstén A, Hannah JL, Markey RJ (2003) Subgrain-scale decoupling of Re and 187Os and assessment of laser ablation ICP-MS spot dating in molybdenite. Geochim Cosmochim Acta 67:3673–3686Google Scholar
- Stein HJ, Sundblad K, Markey RJ, Markey RJ, Motuza G (1998) Re-Os ages for Archean molybdenite and pyrite, Kuittila-Kivisuo, Finland and Proterozoic molybdenite, Kabeliai, Lithuania: testing the chronometer in a metamorphic and metasomatic setting. Mineral Deposita 33:329–345Google Scholar
- Thiéry R, Vidal J, Dubess J (1994) Phase equilibria modelling applied to fluid inclusions: liquid-vapour equilibria and calculation of the molar volume in the CO2-CH4-N2 system. Geochim Cosmochim Acta 58:1073–1082Google Scholar
- Thomson JA (2001) A counter-clockwise P-T path for anatectic pelites, south-central Massachusetts. Contrib Mineral Petrol 141:623–641Google Scholar
- Tomkins AG (2010) Windows of metamorphic sulfur liberation in the crust: implications for gold deposit genesis. Geochim Cosmochim Acta 74:3246–3259Google Scholar
- Tomkins AG, Grundy C (2009) Upper temperature limits of orogenic gold deposit formation: constraints from the granulite-hosted Griffin’s Find deposit, Yilgarn Craton. Econ Geol 104:669–685Google Scholar
- Vielreicher NM, Ridley JR, Groves DI (2002) Marymia: an Archean, amphibolite facies-hosted, orogenic lode-gold deposit overprinted by Palaeoproterozoic orogenesis and base metal mineralisation, Western Australia. Mineral Deposita 37:737–764Google Scholar
- Wallis S, Tsujimori T, Aoya M, Kawakami T, Terada K, Suzuki K, Hyodo H (2003) Cenozoic and Mesozoic metamorphism in the Longmenshan orogen: implications for geodynamic models of eastern Tibet. Geology 31:745–748Google Scholar
- Weislogel AL (2008) Tectonostratigraphic and geochronologic constraints on evolution of the northeast Paleotethys from the Songpan-Garzê complex, Central China. Tectonophysics 451:331–345Google Scholar
- Weislogel AL, Graham SA, Chang EZ, Wooden JL, Gehrels GE, Yang HS (2006) Detrital zircon provenance of the Late Triassic Songpan-Garzê complex: sedimentary record of collision of the North and South China blocks. Geology 34:97–100Google Scholar
- Weller OM, St-Onge MR, Waters DJ, Rayner N, Searle MP, Chung SL, Palin RM, Lee YH, Xu X (2013) Quantifying Barrovian metamorphism in the Danba structural culmination of eastern Tibet. J Metamorph Geol 31:909–935Google Scholar
- Whitney DL, Teyssier C, Fayon AK (2004) Isothermal decompression, partial melting and exhumation of deep continental crust. Geol Soc Lond, Spec Publ 227:313–326Google Scholar
- Xiao L, Zhang HF, Clemens JD, Wang QW, Kan ZZ, Wang KM, Ni PZ, Liu XM (2007) Late Triassic granitoids of the eastern margin of the Tibetan Plateau: geochronology, petrogenesis and implications for tectonic evolution. Lithos 96:436–452Google Scholar
- Xu ZQ, Hou LW, Wang ZX, Fu XF, Huang MH (1992) Orogenic process of Songpan-Ganzi belt in China (in Chinese with English abstract). Geological Publication House, Beijing 190 ppGoogle Scholar
- Yang LQ, Deng J, Wang ZL, Guo LN, Li RH, Groves DI, Danyushevsky LV, Zhang C, Zhao H (2016) Relationships between gold and pyrite at the Xincheng gold deposit, Jiaodong Peninsula, China: implications for gold source and deposition in a brittle epizonal environment. Econ Geol 111:105–126Google Scholar
- Yuan C, Zhou MF, Sun M, Zhao M, Wilde S, Long X, Yan D (2010) Triassic granitoids in the eastern Songpan Garzê fold belt, SW China: magmatic response to geodynamics of the deep lithosphere. Earth Planet Sci Lett 290:481–492Google Scholar
- Zhang HF, Parrish P, Zhang L, Xu WC, Yuan HL, Gao S, Crowley QG (2007) A-type granite and adakitic magmatism association in Songpan-Garzê fold belt, eastern Tibetan Plateau: implication for lithospheric delamination. Lithos 97:323–335Google Scholar
- Zhang HF, Zhang L, Harris N, Jin LL, Yuan HL (2006) U–Pb zircon ages, geochemical and isotopic compositions of granitoids in Songpan–Garze fold belt, eastern Tibetan Plateau: constraints on petrogenesis and tectonic evolution of the basement. Contrib Mineral Petrol 152:75–88Google Scholar
- Zhao JH, Zhou MF (2008) Neoproterozoic adakitic plutons in the northern margin of the Yangtze block, China: partial melting of a thickened lower crust and implications for secular crustal evolution. Lithos 104:231–248Google Scholar
- Zhou MF, Ma Y, Yan DP, Xia X, Zhao JH, Sun M (2006a) The Yanbian terrane (Southern Sichuan Province, SW China): a Neoproterozoic arc assemblage in the western margin of the Yangtze block. Precambrian Res 144:19–38Google Scholar
- Zhou MF, Yan DP, Kennedy AK, Li YQ, Ding J (2002) SHRIMP zircon geochronological and geochemical evidence for Neoproterozoic arc-related magmatism along the western margin of the Yangtze Block, South China. Earth Planet Sci Lett 196:51–67Google Scholar
- Zhou MF, Yan DP, Vasconcelos PM, Li JW, Hu RZ (2008) Structural and geochronological constraints on the tectono-thermal evolution of the Danba domal terrane, eastern margin of the Tibetan Plateau. J Asian Earth Sci 33:414–427Google Scholar
- Zhou MF, Yan DP, Wang CL, Qi L, Kennedy A (2006b) Subduction-related origin of the 750 Ma Xuelongbao adakitic complex (Sichuan Province, China): implications for the tectonic setting of the giant Neoproterozoic magmatic event in South China. Earth Planet Sci Lett 248:286–300Google Scholar
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.