Controls on Gold Solubility in Arc Magmas: An Experimental Study at 1000°C and 4 kbar

International audienceIn order to (1), explain the worldwide association between epithermal gold-copper-molybdenum deposits and arc magmas and (2), test the hypothesis that adakitic magmas would be Au-specialized, we have determined the solubility of Au at 4 kbar and 1000°C for three intermediate magmas (two adakites and one calc-alkaline composition) from the Philippines. The experiments were performed over a fO2 range corresponding to reducing (not, vert, similarNNO-1), moderately oxidizing (not, vert, similarNNO+1.5) and strongly oxidizing (not, vert, similarNNO+3) conditions as measured by solid Ni-Pd-O sensors. They were carried out in gold containers, the latter serving also as the source of gold, in presence of variable amounts of H2O and, in a few additional experiments, of S. Concentrations of Au in glasses were determined by LA-ICPMS. Gold solubility in melt is very low (30-240 ppb) but increases with fO2 in a way consistent with the dissolution of gold as both Au1+ and Au3+ species. In the S-bearing experiments performed at not, vert, similarNNO-1, gold solubility reaches much higher values, from not, vert, similar1200 to 4300 ppb, and seems to correlate with melt S content. No systematic difference in gold solubility is observed between the adakitic and non-adakitic compositions investigated. Oxygen fugacity and the sulfur concentration in melt are the main parameters controlling the incorporation and concentration of gold in magmas. Certain adakitic and non-adakitic magmas have high fO2 and magmatic S concentrations favorable to the incorporation and transport of gold. Therefore, the cause of a particular association between some arc magmas and Au-Cu-Mo deposits needs to be searched in the origin of those specialized magmas by involvement of Au- and S-rich protoliths. The subducted slab, which contains metal-rich massive sulfides, may constitute a potentially favorable protolith for the genesis of magmas specialized with respect to gold

XANES evidence for sulphur speciation in Mn-, Ni- and W-bearing silicate melts

Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 73 (2009): 6847-6867, doi:10.1016/j.gca.2009.08.013.S K edge XANES and Mn, W and Ni XANES and EXAFS spectra of silicate glasses synthesised at 1400° C and 1 bar with compositions in the CaO-MgO-Al2O3-SiO2-S plus MnO, NiO, or WO3 systems were used to investigate sulphur speciation in silicate glasses. S K-edge spectra comprised a composite peak with an edge between 2470 and 2471.4 eV, which was attributed to S2-, and a peak of variable height with an edge at 2480.2 to 2480.8 eV, which is consistent with the presence of S6+. The latter peak was attributed to sample oxidation during sample storage. W-rich samples produced an additional lower energy peak at 2469.8 eV that is tentatively attributed to the existence of S 3p orbitals hybridised with the W 5d states. Deconvolution of the composite peak reveals that the composite peak for Mn-bearing samples fits well to a model that combines three Lorentzians at 2473.1, 2474.9 and 2476.2 eV with an arctan edge step. The composite peak for W-bearing samples fits well to the same combination plus an additional Lorentzian at 2469.8 eV. The ratio of the proportions of the signal accounted for by peaks at 2473.1eV and 2476.2eV correlates with Mn:Ca molar ratios, but not with W:Ca ratios. Spectra from Ni-bearing samples were qualitatively similar but S levels were too low to allow robust quantification of peak components. Some part of the signal accounted for by the 2473.1 eV peak was therefore taken to record the formation of Mn-S melt species, while the 2469.8 peak is interpreted to record the formation of W-S melt species. The 2474.9 eV and 2476.2 eV peaks were taken to be dominated by Ca-S and Mg-S interactions. However, a 1:1 relationship between peak components and specific energy transitions is not proposed. This interpretation is consistent with known features of the lower parts of the conduction band in monosulphide minerals and indicates a similarity between sulphur species in the melts and the monosulphides. S XANES spectra cannot be reproduced by a combination of the spectra of the component element monosulphides. Mn-, W- and Ni- XANES and EXAFS for synthetic glasses without sulphide exsolution did not show any sensitivity to the presence of sulphur, which is unsurprising as S:O ratios were sufficiently low that metals would be mostly co-ordinated by O. Mn EXAFS spectra were consistent with divalent Mn in 5 co-ordinated Mn-O melt species. W spectra were consistent with tetrahedrally co-ordinated hexavalent W, most likely in scheelite-like melt species, and Ni spectra were consistent with [4] co-ordinated divalent Ni. These results indicate lower coordinations for bothWand Ni than those inferred by some previous workers. Cation coordination may reflect the proportion of non-bridging oxygens, which is lower in the Ca-rich and Al-poor samples investigated here than for previous studies.This work was performed with 814 support from the Australian Synchrotron Research Program (ASRP), which is funded by the 815 Commonwealth of Australia under the Major National Research Facilities Program

Experimental determination of the solubility of gold (Au) in intermediate arc magmas.

Goldschmidt Abstracts 2009 – JInternational audienceCu-Au-Mo deposits represent major metal resources known to be spatially and temporally associated with intrusive arc magmatism, and in particular with adakites. Although the bulk of the ore metals seem to originate from the magmas, there is as yet no consensus on processes responsible for their concentration. The fact that primary ore minerals are predominantly sulfides has led to the suggestion that sulfur may play an important role in metal enrichment processes at the magmatic stage. Here, we conducted highpressure/ temperature experiments (0.4 GPa, 1000°C) on 3 natural dacitic magmas from the Philippines (two adakites and one typical calc-alkaline composition) over a wide range of fO2 (~NNO-1 to ~NNO+3), as measured by solid sensors. The experiments were carried out in pure Au capsules, the latter also serving as the source of gold, in the presence of variable amounts of H2O. Both S-free and S-bearing (~1 wt% S added) experiments were performed. Charges consist of dominant silicate melt quenched to glass plus minor silicate phases (mostly Cpx). All S-bearing charges are saturated with either sulfides or sulfates, depending on the fO2. Our data show that Au solubility in S-free charges is very low (30-240 ppb) but increases with fO2. In S-bearing charges Au contents reach much higher values, ranging from ~500 to 5200 ppb depending on the fO2, the most reduced charges being the gold-richest. Also, the behaviour of Au w.r.t. sulfur appears to be radically different according to the fO2. No significant difference of Au solubility is observed between adakitic and non-adakitic melts in both S-free and S-bearing charges. A major consequence of this study is that sulfide crystallization does not scavenge all Au present in the magma, but allows gold-enriched residual magmas to be emplaced in the upper crust. Additional experimental runs are currently being performed at higher pressures (> 1 GPa), that should lead to important supplementary data and results

Experimental Study of the Behaviour of Gold in Calc-alkaline Arc Magmas : Demonstration of the Effect of Sulphur and Geological Implications on the Formation of Gold Deposits

International audienceAB: Copper-gold-molybdenum deposits represent major metal resources known to be spatially and temporally associated with intrusive arc magmatism (Hedenquist and Lowenstern, 1994). Although the bulk of the ore metals seem to originate from the magmas, there is as yet no consensus on processes responsible for their concentration. The fact that primary ore minerals are predominantly sulfides has led to the suggestion that sulfur may play an important role in metal enrichment processes at the magmatic stage (Sillitoe, 1997; Mungall, 2002). Previous experimental studies have shown that sulfides can largely fractionate gold from the melt, and therefore concluded that oxidized magmas are more likely to be Au-enriched. Here, we conducted high-pressure temperature experiments (4 Kbar, 1000°C) on three dacitic and two rhyolitic natural magmas from the Philippines (both adakitic and typical calc-alkaline compositions were used) over a range of fO2 corresponding to reducing (~NNO-1), moderately oxidizing (~NNO+1.5) and strongly oxidizing (~NNO+3) conditions as measured by solid Ni-Pd-O sensors. The experiments were carried out in gold capsules, the latter also serving as the source of gold, in the presence of variable amounts of H2O. The water content of experimental glasses was determined by Karl-Fisher Titration. Both S-free and S-bearing (~1 wt% S added) experiments were performed. Sulfur concentrations in experimental glasses were measured by electron microprobe, and thermodynamic calculations were used to compute the fS2 of the experimental charges. Gold concentrations in glasses were determined by LA-ICP-MS. Charges consist of dominant silicate melt quenched to glass plus minor silicate phases (mostly Cpx, rarely Opx), together with discrete magnetites in the most oxidized charges. All S-bearing charges are saturated with either sulfides or sulfates, together with discrete Au-Fe-S alloys, depending on the fO2. Our data show that gold solubility in S-free charges is low (30-240 ppb) but globally increases with fO2, suggesting the implication of oxidized species in the dissolution of gold. Gold solubility in S-bearing charges appears much higher, ranging from ~500 to 5200 ppb. Gold content is higher in reduced than in moderately oxidizing conditions, and is not affected by melt composition. In very oxidizing conditions, Au solubility is low and nearly constant, and does not seem affected by the presence of S ; it only shows a dependence on fO2 by following the same trend than S-free charges. At ~NNO+1.5, gold content of silicate melt appears to be the result of a complex competition between fO2 and fS2, whereas under more reduced conditions, the effect of fS2 appears the strongest, leading to a direct dependence of gold solubility on sulfur content. This study shows conclusively that Au transport and concentration in silicic melts can be dramatically enhanced by the presence of sulphur. A major consequence is that sulfide crystallization is not likely to scavenge all Au present in the magma, but may allow gold-enriched residual magmas, especially those that are moderately reduced (NNO-1 to NNO+1.5), to be emplaced in the upper crust. Gold enrichment in porphyry- and epithermal- type deposits can be directly linked with sulfur incorporation and abundances in magmas

The solubility of copper in hydrous rhyolitic melts: The effects of oxygen, chlorine and sulphur

Goldschmidt ConferenceInternational audiencePorphyry-Cu deposits are unquestionably linked to subduction zone (arc) magmas. Yet the source magmas for these deposits remain unclear, despite recognition that arc-magmas are water-bearing and relatively oxidised. This study focuses on understanding some of the chemical parameters that may affect the solubility of Cu in hydrous melts using experimental petrology. In particular, the effect of oxygen fugacity and Cl and S content of the melt was investigated.The experiments were conducted in an internally heated pressure vessel where the oxygen fugacity is controlled by the addition of H2 to the Ar pressure medium. The sample charges consisted of powdered synthetic glass of rhyolitic composition containing water (7 wt%) and various amounts of NaCl or a S-bearing phase (FeS or CaSO4 depending on fO2). The samples were sealed in welded AuCu capsules and run at 4 kbars and 800 °C and 900 °C for up to 10 days. Twelve samples can be run simultaneously in the same vessel enabling the P, T and fO2 conditions to be identical for each sample. The fO2 was checked using solid Ni–Pd and Co–Pd sensors. The fO2 ranged from NNO−1 to approx. NNO+3; the Cl contents varied from O to 9000 ppm at 900 °C, and O to 6500 ppm at 800 °C; the S content in the melt was very low for all runs, typically <80 ppm. Results show that although there is a very slight positive correlation between Cl and Cu, Cl does not appear to play a dominant role in the solubility of Cu. The main control on Cu solubility is oxygen fugacity, which correlate linearly, Cu increasing with fO2. The oxidation state of Cu in these experiments can be determined using the reaction Log aCuOglassn = n/2 x log ∞O2 + C, where 2n is the valence of dissolved Cu. A slope of 0.25–0.30 suggests dissolution as a Cu2O-like compound where Cu is present as Cu+. This explains the strong correlation between oxidised magmas and porphyry-Cu deposits. The solubility of S was too low to be varied significantly within detection limits and excess sulfur was found to form a discrete sulfide phase. At lower oxygen fugacities (approx. NNO−1) it consists of a Cu–Fe–S phase, whereas at higher oxygen fugacities (up to NNO+3) a Cu–S phase precipitates. This is contrary to the expectation that at high oxygen fugactities sulfide is no longer stable and suggests that Cu greatly expands the stability field of sulphide liquids

The connection between high K melts and Au deposits: Evidence from natural and experimental systems

Goldschmidt Conference Abstracts 2006International audienceIt has long been suspected that K-enriched melts are efficient carriers of Au. This assumption is the result of clear spatial/temporal associations between Au deposits and High-K (or shoshonitic) intrusives in the deposits of Lihir Island (Muller et al., 2001), Bajo de la Alumbrera, Argentina (Muler and Forrestal, 1998), Quesnilla, BC, Canada (Barrie, 1993) and in the Lower Yangtze region of China (Zhou et al., 1996) and others. However, no direct link has been made between the high K nature of these melts and their Au endowment. As a test of this hypothesis, we analysed Au by LA-ICPMS in clinopyroxene-hosted melt inclusions from an ore-associated dyke at the Masbate Gold Deposit in the Phillipines. Pyroxenes were re-equilibrated at 1100 C for 24 h in a controlled gas mixing furnace, quenched in water and polished to expose glassy, homogenised ultrapotassic (8–11wt% K2O) inclusions. All inclusions analysed contain high levels of dissolved gold (up to 20 ppm Au). To independently test this empirical link, we equilibrated hydrous rhyolitic glasses of variable K contents (plus either FeS or FeS and NaCl) with Au capsules in internally-heated pressure vessels. At constant water content, temperature, pressure, fO2 (controlled by H–Ar mixtures) the addition of K2O increases Au solubility by nearly an order of magnitude. In runs with FeS only, Au content increased from 0.08 ppm Au (@1.6% K2O) to 0.66 ppm Au (@ 6.8%K2O), while those with FeS and NaCl contained 0.45 ppm @ 1.6% K2O and 2.2 ppm Au @ 6.1%K2O. These analyses and experiments do not explain the mechanism for increased Au solubility, but they do strengthen the link. Furthermore, in terranes where pervasive alterration obscures primary igneous compositions, melt inclusions may provide the best information, and may provide a valuable exploration tool. References Barrie, C.T., 1993. J. Geochem. Expl. 48, 225–258. Muller, D., Franz, L., Herzig, P.M., Hunt, S., 2001. Lithos 57, 163–186. Muler, D., Forrestal, P., 1998. Min. Pet. 64, 47–64. Zhou, J.C., Zhou, J.P., Liu, J., Zhao, T.P., Chen, W., 1996. J. Geochem. Expl. 57, 273–283. doi:10.1016/j.gca.2006.06.815 A404 Goldschmidt Conference Abstracts 200

Tungsten isotopes as tracers of core-mantle interactions: The influence of subducted sediments

The postulated difference in W isotopic composition of the Earth’s core of ~2 εW units, compared to the bulk silicate earth (BSE) has previously been used to search for evidence of core–mantle interaction (CMI) in ocean island basalts (OIB). The absence of W isotope anomalies has thus been taken as evidence that CMI does not occur. However, the addition of subducted sediment with high W to the sources of OIB could obscure a core signature. This possibility brings into question the utility of W isotopes as tracers for CMI. To accurately consider the effects of sediment addition to mantle sources of OIB with respect to W requires improved constraints on the abundances of W in subducting sediment. Here, we present high-precision W abundance data (and other HFSE) for a suite of sediments from the Banda subduction regime in East Indonesia. Subducting East Indonesian sediments have trace element concentrations that resemble those of average upper continental crust (UCC), making these sediments valuable to consider as typical of subducted sediments. Average W abundances of 2.1 ppm, corrected for carbon content coupled with current models of 0.5% core addition and 1% sediment addition to EM1 or HIMU plume, suggest that a model hybrid source should exhibit values of εW = −0.24 with ~25 ppb W. Prior studies have not reported such low W isotopic compositions or high estimated W concentrations present in the sources of either Hawaiian or French Polynesian lavas, so such large additions of core material to these plume sources seems unlikely. Given these constraints, core contributions to these source, if present, can be no more than ~0.1%

Petrogenesis of contact-style PGE mineralization in the northern lobe of the Bushveld Complex: comparison of data from the farms Rooipoort, Townlands, Drenthe and Nonnenwerth

In the present study, we document the nature of contact-style platinum-group element (PGE) mineralization along >100 km of strike in the northern lobe of the Bushveld Complex. New data from the farm Rooipoort are compared to existing data from the farms Townlands, Drenthe, and Nonnenwerth. The data indicate that the nature of the contact-style mineralization shows considerable variation along strike. In the southernmost portion of the northern Bushveld, on Rooipoort and adjoining farms, the mineralized sequence reaches a thickness of 700 m. Varied-textured gabbronorites are the most common rock type. Anorthosites and pyroxenites are less common. Chromitite stringers and xenoliths of calcsilicate and shale are largely confined to the lower part of the sequence. Layering is locally prominent and shows considerable lateral continuity. Disseminated sulfides may reach ca. 3 modal % and tend to be concentrated in chromitites and melanorites. Geochemistry indicates that the rocks can be correlated with the Upper Critical Zone. This model is supported by the fact that, in a down-dip direction, the mineralized rocks transform into the UG2-Merensky Reef interval. Between Townlands and Drenthe, the contact-mineralized sequence is thinner (up to ca. 400 m) than in the South. Chromitite stringers occur only sporadically, but ultramafic rocks (pyroxenites, serpentinites, and peridotites) are common. Xenoliths of calcsilicate, shale, and iron formation are abundant indicating significant assimilation of the floor rocks. Sulfides may locally form decimeter- to meter-sized massive lenses. PGE grades tend to be higher than elsewhere in the northern Bushveld. The compositions of the rocks show both Upper Critical Zone and Main Zone characteristics. At Nonnenwerth, the mineralized interval is up to ca. 400 m thick. It consists largely of varied-textured gabbronorites, with minor amounts of igneous ultramafic rocks and locally abundant and large xenoliths of calcsilicate. Layering is mostly weakly defined and discontinuous. Disseminated sulfides (<ca. 3 modal %) occur throughout much of the sequence. Geochemistry indicates that the rocks crystallized mainly from tholeiitic magma and thus have a Main Zone signature. The implication of our findings is that contact-style PGE mineralization in the northern lobe of the Bushveld Complex cannot be correlated with specific stratigraphic units or magma types, but that it formed in response to several different processes. At all localities, the magmas were contaminated with the floor rocks. Contamination with shale led to the addition of external sulfur to the magma, whereas contamination with dolomite may have oxidized the magma and lowered its sulfur solubility. In addition to contamination, some of the magmas, notably those of Upper Critical Zone lineage present at the south-central localities, contained entrained sulfides, which precipitated during cooling and crystallization