Sulfur isotope and trace element systematics of zoned pyrite crystals from the El Indio Au-Cu-Ag deposit, Chile

We present a comparative study between early, massive pyrite preceding (Cu–Ag) sulfosalt mineralization in high-temperature feeder zones (‘early pyrite’) and late pyrite that formed during silicic alteration associated with Au deposition (‘late pyrite’) at the El Indio high-sulfidation Au–Ag–Cu deposit, Chile. We use coupled in situ sulfur isotope and trace element analyses to chronologically assess geochemical variations across growth zones in these pyrite crystals. Early pyrite that formed in high-temperature feeder zones shows intricate oscillatory zonation of Cu, with individual laminae containing up to 1.15 wt% Cu and trace Co, As, Bi, Ni, Zn, Se, Ag, Sb, Te, Au, Pb and Bi. Late pyrite formed after (Cu–Ag) sulfosalt mineralization. It contains up to 1.14 wt% As with trace Cu, Zn, Pb, V, Mn, Co, Ni, Ge, Se, Ag, Sb, Te, Pb and Bi, as well as colloform Cu-rich growth bands containing vugs toward the outer edges of some crystals. Plotting the trace element data in chronological order (i.e., from core to rim) revealed that Co and Ni were the only elements to consistently co-vary across growth zones. Other trace elements were coupled in specific growth zones, but did not consistently co-vary across any individual crystal. The δ34S of early pyrite crystals in high-temperature feeder zones range from −3.19 to 1.88 ‰ (±0.5 ‰), consistent with sublimation directly from a high-temperature magmatic vapor phase. Late pyrite crystals are distinctly more enriched in δ34S than early pyrite (δ34S = 0.05–4.77 ‰, ±0.5 ‰), as a consequence of deposition from a liquid phase at lower temperatures. It is unclear whether the late pyrite was deposited from a small volume of liquid condensate, or a larger volume of hydrothermal fluid. Both types of pyrite exhibit intracrystalline δ34S variation, with a range of up to 3.31 ‰ recorded in an early pyrite crystal and up to 4.48 ‰ in a late pyrite crystal. Variations in δ34Spyrite at El Indio did not correspond with changes in trace element geochemistry. The lack of correlation between trace elements and δ34S, as well as the abundance of microscale mineral inclusions and vugs in El Indio pyrite indicate that the trace element content of pyrite at El Indio is largely controlled by nanoscale, syn-depositional mineral inclusions. Co and Ni were the only elements partitioned within the crystal structure of pyrite. Cu-rich oscillatory zones in early pyrite likely formed by nanoscale inclusions of Cu-rich sulfosalts or chalcopyrite, evidence of deposition from a fluid cyclically saturated in ore metals. This process may be restricted to polymetallic high-sulfidation-like deposits

Silicate-sulfide liquid immiscibility in modern arc basalt (Tolbachik volcano, Kamchatka): Part I. Occurrence and compositions of sulfide melts

Silicate-sulfide liquid immiscibility plays a key role in the formation of magmatic sulfide ore deposits but incipient sulfide melts are rarely preserved in natural rocks. This study presents the distribution and compositions of olivine-hosted sulfide melt globules resulting from silicate-sulfide liquid immiscibility in primitive arc basalts. Abundant sulfide droplets entrapped in olivine from primitive basalts of the 1941 eruption and pre-historic eruptive cone “Mt. 1004” of the Tolbachik volcano, Kurile-Kamchatka arc. Inclusions range from submicron to 250 μm in size, coexist with sulfur-rich glass (≤ 1.1 wt% S), and, in some cases, with magmatic anhydrite. Saturation in sulfide occurred early in the evolution of a water- and sulfur-rich magma, moderately oxidized (QFM + 1 to +1.5), which crystallized high-Mg olivine (Fo₈₆ˍ₉₂), clinopyroxene and Cr-spinel. The process developed dense “clouds” of sulfide in relatively small volumes of magma, with highly variable abundances of chalcophile metals. The low degree of sulfide supersaturation promoted diffusive equilibration of the growing droplets with the melt in Ni and Cu, resulting in high concentrations (≈ 38 mol%) of CuS and NiS in the earliest sulfide liquids. The Tolbachik samples provide a glimpse into deep arc processes not seen elsewhere, and may show how arc magmas, despite their oxidized nature, saturate in sulfide.This study was supported by the Russian Science Foundation grant # 16-17-10145. This is CRPG contribution #253

The competing effects of sulfide saturation versus degassing on the behavior of the chalcophile elements during the differentiation of hydrous melts

There is a lack of consensus regarding the roles of sulfide saturation versus volatile degassing on the partitioning of Cu and Ag during differentiation and eruption of convergent margin magmas. Because of their oxidized character, volatile-rich magmas from the Eastern Manus Back-arc Basin (EMBB) only reach sulfide saturation following magnetite-driven reduction of the melt: the so-called “magnetite crisis.” If sulfide saturation typically precedes volatile saturation, the magnetite crisis will limit the proportion of Cu and Ag that can partition from the melt into an exsolving volatile-rich phase, which may contribute to the sporadic occurrence of magmatic-hydrothermal ore deposits at convergent margins. However, it is unclear whether the magnetite crisis is a common or rare event during differentiation of volatile-rich magmas. We report major and trace element data for submarine volcanic glasses from the Tonga arc-proximal Valu Fa Ridge (VFR; SW Pacific). Cu-Se-Ag systematics of samples erupting at the southern VFR suggest magnetite fractionation-triggered sulfide saturation. The similarity in chalcophile element systematics of the southern VFR and EMBB samples is unlikely to be coincidental, and may indicate that the magnetite crisis is a common event during differentiation of hydrous melts. However, unlike many convergent margin magmas, it is unlikely that the evolving VFR and EMBB were saturated in a S-bearing volatile phase prior to magnetite fractionation. Hence, the metal-depleting magnetite crisis may be restricted to back-arc basin magmas that do not degas volatiles prior to magnetite fractionation and potentially convergent margin magmas fractionating at high pressures in the continental crust

Mineralogy, Paragenesis, and Mineral Zoning of the West Fork Mine, Viburnum Trend, Southeast Missouri

ASARCO\u27s West Fork mine, the newest mine in the Viburnum Trend, began production in 1985 and reached full production in 1988. Although significant mineralogical and paragenetic studies have been performed on the other ore deposits of the Viburnum Trend, no attempt has been made to correlate these studies with mineral zoning patterns. The distinct mineral zoning present at the West Fork mine provides a unique opportunity to relate mineral paragenetic sequence to mineral zoning.The present study concentrated on the main ore horizon (M bed) which is composed of four north-south-trending linear metal zones which together consist of an inner zone of colloform iron and zinc sulfides, bordered outward by zones of disseminated iron and zinc sulfides and fringed by outermost zones of galena that also overprint the inner zone. Principal metallic sulfides in the ores at the West Fork mine are PbS (galena), ZnS (sphalerite and wurtzite), and FeS2 (pyrite and marcasite). Paragenetic sequence and metal zoning show that the ore-forming fluids migrated initially through the inner zone where ZnS was deposited and subsequently traveled outward depositing FeS2 followed by PbS. The presence of rapidly deposited, colloform ZnS and FeS2 and bladed galena in the central Fe-Zn zone indicates that ore fluids which deposited the sulfides in the central zone were more saturated with respect to metal sulfides. Outer layers of the colloform ZnS bodies in the Fe-Zn zone are characterized by an abundant complex intergrowth of sphalerite and wurtzite (schalenblende). X-ray analysis indicates that the wurtzite within the schalenblende inverted to sphalerite, thereby producing pseudomorphous sphalerite after wurtzite. Because wurtzite is the sulfur-deficient form of ZnS, it can be postulated that sulfur fugacities were low at the end of the main stage of zinc sulfide deposition. Subsequent increase in sulfur fugacity during the main stage of iron deposition caused inversion of wurtzite to sphalerite. Interlayered pyrite and marcasite are evidence that the ore fluids during the main stage of deposition were oscillating above and below a pH of 5. Minerals subsequently deposited in the outer zones were precipitated more slowly from fluids that were relatively depleted in metals. The distinct differences in ore character between the metal zones may well be a result of multiple fluids. Separate fluids may have been channelized (each fluid representing a zone), or may have mixed through time. The results of this study are not compatible with ore deposition from a single fluid source

The Effect of FeO on the sulfur content at sulfide saturation (SCSS) and the selenium content at selenide saturation of silicate melts

The concentration of sulfur in basalt-like silicate melts as S²⁻ is limited to the amount at which the melt becomes saturated with a sulfide phase, such as an immiscible sulfide melt. The limiting solubility is called the 'sulfur content at sulfide saturation' (SCSS). Thermodynamic modelling shows that the SCSS depends on the FeO content of the silicate melt from two terms, one with a negative dependence that comes from the activity of FeO in the silicate melt, and the other with a positive dependence that comes from the strong dependence of the sulfide capacity of the melt on FeO content. The interaction between these two terms should yield a net SCSS that has an asymmetric U-shaped dependence on the FeO content of the melt, if other variables are kept constant. We have tested this thermodynamic model in a series of experiments at 1400°C and 1·5 GPa to determine the sulfur contents at saturation with liquid FeS in melt compositions along the binary join between a haplobasaltic composition and FeO. The SCSS is confirmed to have the asymmetric U-shaped dependence, with a minimum at ∼5 wt % FeO. The effect of FeO on the selenide content at selenide saturation (SeCSeS) was investigated in an analogous fashion. SeCSeS shows a similar, though not identical, U-shaped dependence, implying that the solubility mechanism of selenide in basalt-like silicate melts is similar to that of sulfide. The observation of increasing SCSS with decreasing FeO in hydrous silicic melts was explored by inverse modelling of datasets from pyrrhotite-saturated hydrous silicic liquids, revealing that high SCSS at low FeO can be explained in terms of the low-FeO limb of the 'U', rather than dissolution of sulfur as hydrous species such as H2S or HS–. Recent measurements of the composition of the surface of Mercury prompted examination of the high-SCSS, low-FeO limb of the 'U' as a potential explanation for the sulfur-rich but Fe-poor surface of Mercury.18 page(s

Resolving sub-micron-scale zonation of trace elements in quartz using TOF-SIMS

Quartz is abundant in the Earth’s continental crust and persistent throughout the geological record. Trace element signatures in silica minerals can be used to infer processes operating in magmatic and hydrothermal systems. Conventional analyses of trace elements in silica minerals are limited by either spatial or mass resolution (e.g., wavelength-dispersive X-ray spectroscopy, micro X-ray fluorescence, laser ablation inductively coupled mass spectrometry (LA-ICP-MS),and secondary ion mass spectrometry (SIMS). Time-of-flight SIMS (TOF-SIMS) is a relatively new technique for geological applications and provides both high spatial and mass resolution. This minimally-destructive, in situ technique rapidly acquires a full suite of elements down to tens of nanometers depth. No previous study has utilized TOF-SIMS to analyse quartz or silica. Four samples of silica minerals representing distinct environments in a magmatic-hydrothermal system were characterized with optical microscopy and qualitative cathodoluminescence (CL), quantitatively analysed for trace elements with 157 nm LA-ICP-MS, and qualitatively mapped for trace elements using TOF-SIMS. The novel technique produced maps of trace element distribution in silica minerals to a maximum resolution of 65 nm and consistently resolved light elements (including Li) to 195 nm. That makes this study the highest resolution geochemical characterization of silica minerals, and places it among the highest resolution analyses by TOF-SIMS, or any technique, for that matter. TOF-SIMS isotope maps differentiate trace elements hosted in nano- and micro-inclusions from lattice incorporation in quartz and cryptocrystalline silica – an impossibility for lower resolution techniques, allowing insights into cations substituting for Si4+ in the crystal lattice and their role in activating CL in low-temperature epithermal quartz. Further development of this technique could see TOF-SIMS become a routine tool for measuring diffusion profiles in a range of other geological materials. Quantification of TOF-SIMS would revolutionise mineral characterisation, especially given its temporal efficiency and low sampling volume

The importance of talc and chlorite ‘‘hybrid’’ rocks for volatile recycling through subduction zones; evidence from the high-pressure subduction melange of New Caledonia

The transfer of fluid and trace elements from the slab to the mantle wedge cannot be adequately explained by simple models of slab devolatilization. The eclogite-facies melange belt of northern New Caledonia represents previously subducted oceanic crust and contains a significant proportion of talc and chlorite schists associated with serpentinite. These rocks host large quantities of H2O and CO2 and may transport volatiles to deep levels in subduction zones. The bulk-rock and stable isotope compositions of talc and chlorite schist and serpentinite indicate that the serpentinite was formed by seawater alteration of oceanic lithosphere prior to subduction, whereas the talc and chlorite schists were formed by fluid-induced metasomatism of a melange of mafic, ultramafic and metasedimentary rocks during subduction. In subduction zones, dehydration of talc and chlorite schists should occur at subarc depths and at significantly higher temperatures (* 800C) than other lithologies (400–650C). Fluids released under these conditions could carry high trace element contents and may trigger partial melting of adjacent pelitic and mafic rocks, and hence may be vital for\ud transferring volatile and trace elements to the source\ud regions of arc magmas. In contrast, these hybrid rocks are\ud unlikely to undergo significant decarbonation during subduction and so may be important for recycling carbon into\ud the deep mantle