Edward M. Ripley

Copper solubility in a basaltic melt and sulfide liquid/silicate melt partition coefficients of Cu and Fe

The solubility of copper in a sulfur-saturated basaltic melt has been determined at 1245°C as a function of fO2 and fS2. Copper solubilities at log fO2 values between −8 and −11 fall into two distinct populations as a function of fS2. At log fS2 values < −1.65, sulfide liquid that coexists with the basaltic glass quenches to sulfur-poor bornite solid solution. At log fS2 values in excess of −1.65, the sulfide liquid quenches to a complex intergrowth of sulfur-rich bornite and intermediate solid solution. Copper solubilities in the low-fS2 population range from 594 to 1550 ppm, whereas those in the high-fS2 population range from 80 to 768 ppm. Sulfide liquid/silicate liquid partition coefficients (D) for Cu and Fe range from 480 to 1303 and 0.7 to 13.6, respectively. Metal-sulfur complexing in the silicate liquid is shown to be insignificant relative to metal-oxide complexing for Fe but permissible for Cu at high fS2 values. On log DFe (sulfide-silicate) and log DCu (sulfide-silicate) vs. (log fS2 − log fO2) diagrams, both fS2 populations show distinct but parallel trends. The observation of two D values for any fS2/fO2 ratio indicates nonideal mixing of species involved in the exchange reaction. The two distinct trends observed for both Cu and Fe are thought to be due to variations in activity coefficient ratios (e.g., γFeO/γFeS and γCuO0.5/γCuS0.5). Results of the experiments suggest that accurate assessments of fS2/fO2 ratios are required for the successful numerical modeling of processes such as the partial melting of sulfide-bearing mantle and the crystallization of sulfide-bearing magmas, as well as the interpretation of sulfide mineralogical zoning. In addition, the experiments provide evidence for oxide or oxy-sulfide complexing for Cu in silicate magmas and suggest that the introduction of externally derived sulfur to mafic magma may be an important process for the formation of Cu-rich disseminated magmatic sulfide ore deposits.

Sulfur and oxygen isotopic evidence of country rock contamination in the Voisey's Bay Ni–Cu–Co deposit, Labrador, Canada

Abstract The emplacement of basaltic magma into sulfide-bearing country rocks provides a favorable geologic environment for magmatic sulfide ore formation related either directly to assimilation of country rock sulfur or indirectly to a depression of sulfide solubility caused by assimilation-induced changes in magma composition. Pelitic country rocks of the Proterozoic Tasiuyak Gneiss in the area of the Voiseys Bay Ni–Cu–Co deposit contain sulfidic layers that may have provided sulfur to basaltic magmas during emplacement of the Voiseys Bay intrusion. Sulfur isotopic compositions of the Tasiuyak Gneiss range from −0.9 to −17.0‰, values typical for sulfides produced via bacterial sulfate reduction in an open marine environment. Archean gneisses in the area contain low amounts of sulfide and are less likely to have served as a source of externally-derived sulfur. Sulfur isotopic compositions of the sulfide minerals from the Voiseys Bay deposit show consistent variations, both spatially and with rock types. Disseminated and massive sulfides show a decrease in δ 34 S to the west, with values typically between 0 and −2‰ in the Eastern Deeps, Ovoid, and Discovery Hill zone, and between −2 and −4‰ in the Reid Brook zone. δ 34 S values of the Mushua intrusion to the north and the Normal Troctolite in the Eastern Deeps are more positive, ranging between −0.5 and 1.8‰. This range is taken to represent the isotopic composition of primary mantle-derived sulfur in the area because the Mushua intrusion and Normal Troctolite show the least geochemical evidence for contamination by country rocks. Sulfur isotopic data from the Reid Brook zone are consistent with up to a 50% sulfur contribution from the Tasiuyak Gneiss. Correspondingly lower proportions are indicated for the eastern portion of the deposit where country rocks are predominantly low-sulfide enderbitic and quartzofeldspathic gneisses. Oxygen isotopic values of gneiss fragments in the Basal Breccia Sequence and Feeder Breccia suggest that the assimilation process involved a greater proportion of high− 18 O contaminant to the west. δ 18 O values of the Tasiuyak Gneiss (5.9 to 14.0‰), enderbitic gneiss (6.4 to 8.7‰), and Archean quartzofeldspathic gneiss (9.5 to 9.7‰) are consistent with an increased proportion of Tasiuyak Gneiss contaminant to the west. Isotopic data strongly indicate that sulfur from the Tasiuyak Gneiss has been involved in ore deposition at the Voiseys Bay deposit. However, sulfur and oxygen isotopic data also strongly suggest that the addition of externally derived sulfur was not the sole process responsible for mineralization, and that assimilation of both Proterozoic and Archean country rocks played a key role in depressing sulfide solubility prior to sulfide localization via dynamic, physical mechanisms.

Sulfur isotopic studies of continental flood basalts in the Noril’sk region: implications for the association between lavas and ore-bearing intrusions

Previous studies of both ore and non-ore-bearing intrusives in the Permo-Triassic flood basalts of the Siberian platform in the Noril’sk area have shown that high-grade Ni–Cu–platinum group elements (PGE) mineralization is associated with anomalously high δ34S values of ∼8 to 12‰. In addition, several researchers have proposed that observed depletions in the Cu, Ni, and PGE content of basaltic lavas of the Nadezhdinsky (Nd) Formation are related to diffusional exchange with, and upgrading in metal tenor of, sulfides in the volcanic conduit system. Sulfur isotopic studies of the lavas at Noril’sk were initiated to determine if interaction with crustally derived sulfur in the conduit system was evident, and if the Nd lavas in particular were characterized by an anomalous isotopic signature. δ34S values of the lavas range from −4.5 to 8.7‰ Vienna Canon Diablo Troilite (VCDT), with S concentrations from <40 to 1373 ppm. The majority of δ34S values range from 0 to 4‰, and are similar to those from S-poor intrusions in the Noril’sk area. Although textural data are not supportive of early sulfide saturation and the presence of immiscible sulfide droplets in the lavas, recrystallization may have erased expected mineralogical and textural evidence. Mineralogical data indicate that hydrothermal alteration of the lavas has occurred, but S redistribution has been restricted to localized areas and δ34S values have not been affected. The relatively low S concentrations of the lavas are thought to be due in large part to degassing of the lavas in the shallow conduit system and during eruption. Our calculations are consistent with the premise that degassing of basaltic magmas at temperatures in excess of ∼900°C at QFM leads to only minor 34S-depletion of sulfur remaining in the melt, and decreases in δ34S values of less than 2‰ at 90% degassing. For this reason all lavas with δ34S values in excess of ∼ 2‰ require a contribution of 34S-enriched country rock sulfur. Because of the high S content and δ34S value (∼ 16–20‰) of evaporites in the country rocks at Noril’sk, contamination of less than 0.5% is required to explain the most 34S-enriched lavas. The Nd lavas have an average δ34S of 2.9‰, but show no difference in S isotopic composition relative to the other lavas, suggesting that metal depletion involved only limited S transfer, or that exchange between mantle-derived S and S of crustal origin buffered δ34S values to less than ∼5‰. Anomalously positive δ34S values, similar to those of the ore-bearing intrusives in the Noril’sk region, are not consistently found in low-S rocks, either lavas or intrusives. Although the mechanism for the derivation of sulfide in the ore-bearing intrusions remain speculative, it is clear that the formation of sulfide ores characterized by high metal tenors proceeded only in the presence of sulfur of crustal origin.

Analysis of sulfides in whole rock matrices by elemental analyzer-continuous flow isotope ratio mass spectrometry

Abstract Potential matrix effects in the sulfur isotopic analysis of whole rocks by elemental analyzer–continuous-flow isotope ratio mass spectrometry (EA–CFIRMS) have been investigated by measuring mixtures of a well-calibrated laboratory sulfide reference mineral and common rock-forming minerals. Mixtures of sulfide and quartz, orthoclase, plagioclase, kaolinite, olivine, magnetite, ilmenite, garnet, halite, fluorite, calcite, and graphite show consistent S yields and δ34S values generally within ±0.2‰ of the measured pure sulfide value. However, erratic results are obtained for mixtures of reference sulfide and pyroxene, amphibole, or biotite. These minerals may suppress sample peaks, cause extreme peak tailing, and prohibit isotope ratio measurements. Pure sulfide minerals introduced after the mixtures also showed suppressed sample peaks, suggesting SO2 adsorption on mineral and residual ash surfaces. Removal of the residual ash resulted in normal measurements for reference sulfide minerals. Whole rocks with S concentrations in excess of ∼100 ppm can be successfully analyzed using EA–CFIRMS provided that measurements are carefully monitored for evidence of SO2 retention during and after combustion.

Oxygen isotope fractionation between chlorite and water from 170 to 350°C: a preliminary assessment based on partial exchange and fluid/rock experiments

Abstract Oxygen isotope fractionations in laboratory systems have been determined between chlorite and water at 170–350°C. In one series of experiments, the Northrop-Clayton partial exchange method was used where three (sometimes four) isotopically different waters were reacted with chlorite [(∑Fe)/∑Fe+Mg = 0.483] for four durations (132–3282 h) at 350°C and 250 b. The percents of exchange determined for the four times from shortest to longest are 4.4, 6.5, 8.0, and 11.9. The fractionations calculated from the Northrop and Clayton (1966) method are in modest agreement for the four run durations: 0.13, 0.26, −0.46, and −0.55 per mil. Errors associated with each of these fractionations are quite large (e.g., ±1.2 per mil for the longest run). The value determined for the longest run of ∼20 weeks is the most reliable of the group and compares very closely with a value of ∼ −0.7 per mil estimated by Wenner and Taylor (1971) based on natural chlorites. Good agreement is also observed with the estimates, −1.2 and −1.3‰ calculated at 350°C for chlorite compositions with [(∑Fe)/∑Fe+Mg] = 0.313 and 0.444, respectively, from equations given by Savin and Lee (1988) based on their empirical bond-type method. Additional fractionation data have been estimated from hydrothermal granite-fluid experiments where chlorite formed from biotite. Detailed thin section, scanning electron microscope (SEM), x-ray diffraction (XRD), and electron microprobe analyses demonstrate that biotite is altered exclusively to chlorite in 13 granite-fluid experiments conducted at the following conditions: T = 170–300°C, P = vapor saturation − 200 b, salinity = H 2 O, 0.1 and 1 m NaCl, fluid/biotite mass ratios = 3−44, run durations = 122−772 h. The amount of chlorite, quantified through point counting and XRD, increased with increasing temperature, salinity, and time. The isotope compositions of chlorite were calculated from mass balance and compared to the final measured δ 18 O of the fluids. The 10 3 ln α values averaged 0.14, 0.8 and 2.9 per mil for 300°, 250°, and 200°C, respectively. A least-squares regression model of the combined data set (all T’s) gives the following expression for fractionation: 1000 ln α chl-w =2.693 (10 9 /T 3 )−6.342 (10 6 /T 2 )+2.969 (10 3 /T) The curve described by this equation is in very good agreement with empirical curves given by Wenner and Taylor (1971), Savin and Lee (1988), and Zheng (1993) .

Re-Os, Sm-Nd, and Pb isotopic constraints on mantle and crustal contributions to magmatic sulfide mineralization in the Duluth Complex

Abstract Previous petrologic and stable isotopic studies of sulfide mineralization in the Duluth Complex have led to the premise that sulfide genesis is strongly linked to the interaction between mantle-derived magmas and sedimentary country rocks in a rift zone environment. In order to more fully evaluate the nature of this interaction, and to gain an insight into the possible importance of externally derived metals in the ore-forming process, Pb, Sm-Nd, and Re-Os isotopic studies of the Babbitt Cu-Ni deposit were initiated. Rock-types examined include low-sulfide troctolite that show very little petrologic evidence for contamination, disseminated sulfide-bearing troctolitic to gabbroic rocks that occur close to the basal contact with country rocks and contain metasedimentary xenoliths, and massive sulfide. Lead isotopic values of whole rocks, plagioclase mineral separates, and massive sulfides show only subtle differences, and are compatible with 3 to 5% contamination of a mantle-derived melt with a Proterozoic crustal contaminant. e Nd (1.1 Ga) values of the troctolite and gabbro samples are chondritic, and only the massive sulfides show strong evidence for contamination based on Sm-Nd isotopic values. Massive sulfides tend to be more sensitive indicators of contamination in the Sm-Nd system because of late-stage incorporation of a light rare earth element-rich fluid into a Ca-PO 4 (apatite) component of the immiscible sulfide liquid. γ Os (1.1 Ga) values are also strongly anomalous, and range from ∼500 to 1200 in disseminated sulfide-bearing troctolites and massive sulfides. These values are also consistent with from 1 to 3% contamination by C-rich and strongly radiogenic Proterozoic sedimentary rocks. Elemental and isotopic mass balance calculations suggest that up to 50% of the Pb and at least 35% of the Os in the sulfide mineralization have been derived from external sources. A corollary is that other metals may also be in part derived from external sources, which would aid in explaining the compositional diversity of the sulfide mineralization. Selective assimilation of country rocks by mantle-derived magmas may occur in crustal staging chambers, during ascent to shallower levels, or in situ as a result of devolatilization reactions and partial melting.

Layered intrusions of the Duluth Complex, Minnesota, USA

Abstract The Duluth Complex and associated subvolcanic intrusions comprise a large (5,000 km 2 ) intrusive complex in northeastern Minnesota that was emplaced into comagmatic volcanics during the development of the 1.1 Ga Midcontinent rift in North America. In addition to anorthositic and felsic intrusions, the Duluth Complex is composed of many individual mafic layered intrusions of tholeiitic affinity. The cumulate stratigraphies and cryptic variations of six of the better exposed and better studied intrusions are described here to demonstrate the variability in their cumulus mineral paragenesis. Although the general paragenetic sequence is: O1(±Pl) → Pl+Ol → Pl+Cpx+Ox(±Ol±Opx) → Pl+Cpx+Ox+Ap(±Ol) considerable differences exist among the six intrusions in the relative order and timing of cumulus arrivals, most notably with regard to augite (Cpx) and Fe-Ti oxide (Ox). The various cumulate stratigraphies and cryptic variations represented by the six intrusions described here largely reflect differences in the degree of open-system behaviour to recharge, eruption, and country-rock assimilation, but also may have been influenced by differences in parent magma composition, in the efficiency of fractional crystallization, and in the conditions of crystallization ( e.g. P total , f O 2 P H 2 O ). The Sonju Lake intrusion, wherein cumulus augite arrived before ilmenite, formed by essentially closed-system fractional crystallization. However, its compositional evolution may have been affected to an uncertain degree by assimilation of a granitic hanging-wall. The monotonous, thick troctolitic cumulate sequences of the lower parts of the South Kawishiwi and Partridge River Intrusions appear to represent the effects of frequent magma recharge coupled with in situ (or boundary layer) crystallization though other interpretations have been proposed. Extensive Cu-Ni sulphide mineralization at the base of these intrusions is attributed to country-rock contamination of the earliest intruded magmas. The cyclical progression of cumulates in the Layered Series at Duluth, wherein cumulus augite and oxide arrived nearly simultaneously, formed in a moderately open system characterized by periodic eruption and recharge. The Wilder Lake Intrusion is different from other intrusions in that cumulus ilmenite appears before augite and that olivine and augite composition define an inverted cryptic variation. The latter phenomenon may be related to a strong upward gradation toward lesser amounts of trapped liquid in the cumulates. Finally, the incomplete cumulate stratigraphy of the Bald Eagle Intrusion, which is composed of approximately equal thicknesses of troctolite (PO) and gabbro (PAO) adcumulates, is unique in that Fe-Ti oxide did not arrive as a cumulus phase despite prolonged crystallization of cumulus augite. These different cumulus parageneses probably produced a variety of derivative magmas, which may have contributed in part to the compositional diversity of the Midcontinent rift volcanics.

Olivine and sulfur isotopic compositions of the Uitkomst Ni–Cu sulfide ore-bearing complex, South Africa: evidence for sulfur contamination and multiple magma emplacements

The Uitkomst Complex in northern South Africa is host to a Ni–Cu deposit containing a minerable reserve of 2.9 Mt massive ore with grades of 2% Ni, 1% Cu and 6 ppm Pt+Pd and an indicated plus inferred resource of 98 Mt disseminated ore with grades of 0.6% Ni, 0.2% Cu and 1 ppm Pt+Pd. Based on similar age and composition to that of the Bushveld Complex, the Uitkomst Complex is believed by some investigators to be a satellite body of the Bushveld Complex. It has a tubular shape with an exposed surface area of about 0.8×8 km and a thickness of up to 1000 m, apparently exploiting the bedding planes of gently dipping footwall sediments that include shale, quartzite and dolomite. The Uikomst Complex comprised of stratiform mafic and ultramafic rock units that from base to top include gabbro, sulfide-mineralized harzburgite, unmineralized harzburgite, pyroxenite, gabbronorite and gabbro. The compositions of olivine from different rock units are significantly different. Olivines from the gabbronorite unit are highly fractionated, containing <30 mol% Fo (forsterite, Mg2SiO4) and <300 ppm Ni. Olivines from the underlying pyroxenite and harzburgite units are much more primitive, containing 86–91 mol% Fo. Abrupt changes in the contents of Ni in olivine occur within the pyroxenite unit, and between the mineralized and unmineralized harzburgite units. The contents of Ni in olivine from the unmineralized harzburgite unit are between 2900 and 3600 ppm. The contents of Ni in olivine from the underlying mineralized harzburgite units are ∼1500 ppm lower. Olivines from the lower part of the pyroxenite unit are similar to olivines from the underlying harzburgite unit, whereas olivines from the upper part of the pyroxenite unit contain much lower Ni (<600 ppm). Numerical modelling suggests that the strikingly different compositions of olivine are related to different parental magmas with different MgO/FeO ratios and/or Ni contents. The δ34S values of the basal gabbro and unmineralized rock units range from −0.9‰ to 2.6‰, which are similar to typical mantle values (∼0±3‰). In contrast, the δ34S values of the sulfide ores in the harzburgite units are significantly lower, varying between −2.6‰ and −7.1‰. The low δ34S values of the sulfide ores indicate addition of 34S-depleted crustal sulfur. The obvious sources of such crustal sulfur are the Malmani dolomite and the Timebal Hill shale that contain pyrite with negative δ34S values up to −11‰ and −18‰, respectively. Sulfide saturation in the magma of the mineralized harzburgite units is thought to have resulted from addition of the crustal sulfur. The resulting immiscible sulfide liquid droplets were then concentrated in the base of the magma channel. Some of the sulfide liquid was expelled into the basal gabbro that separated the active magma channel from the footwall quartzite. New magma then entered the channel, displacing most of the resident magma to form the unmineralized harzburgite. The overlying pyroxenite, gabbronorite and gabbro units formed either by in situ differentiation of the same magma giving rise to the underlying unmineralized harzburgite or by subsequent emplacement of a more evolved magma.

Petrogenesis of the Pt–Pd mineralized Jinbaoshan ultramafic intrusion in the Permian Emeishan Large Igneous Province, SW China

The Jinbaoshan ultramafic intrusion is a sheet-like body with a thick wehrlite unit in the center and thin pyroxenite units at the margins. PGE are enriched in several disseminated sulfide zones in the intrusion. Olivine from the intrusion has low Fo and depleted Ni contents compared to olivine from coeval Emeishan picrites. Whole rock major and trace element concentrations suggest that the Jinbaoshan wehrlites originally contained <30% trapped liquid. The total amount of sulfide in the rocks exceeds that which could have been dissolved in the trapped liquid. The Jinbaoshan wehrlites are interpreted to represent residual assemblages formed by dissolution of plagioclase by passing magma. No clear evidence of crustal contamination is indicated by S, Nd and Os isotopes. We envision that sulfide saturation occurred at depth due to olivine and chromite crystallization. Immiscible sulfide droplets were transported to the Jinbaoshan conduit where they accumulated and reacted with magma successively passing through the conduit to achieve high PGE concentrations.

Redox front propagation and banding modalities

Abstract Oxygenated waters flowing through a reduced sandstone cause propagating redox fronts. Mathematical reaction-transport models of these fronts studied here show a number of nonlinear phenomena including one-parameter families of constant-velocity fronts, decelerating fronts and two types of front instabilities leading to pattern formation. These redox front phenomena are examples of nonlinear wave propagation and self-organization. Redox fronts in nature are economically important because they can trap accumulations of metallic ores. Furthermore, they are but one example of a wider class of water-rock interaction systems rich in nonlinear reaction-transport phenomena. Analytical results are presented on conversation law and free boundary methods to study the velocity and profiles of the waves. A new model of the Ostwald supersaturation-depletion cycle is presented that incorporates features of Liesegang banding not predicted by other formulations: these include finite band widths and continuous undulatory as well as discrete banding.