Alteration Rims Research Articles

Laser ablation ICP-MS study of platinum-group elements in sulphides from the Platreef at Turfspruit, northern limb of the Bushveld Complex, South Africa

The Platreef unit of the northern Bushveld Complex comprises a diverse package of pyroxenites, peridotites and mafic lithologies with associated Ni–Cu–platinum-group element (PGE) mineralisation. Base metal sulphides (BMS) are generally more abundant in the Platreef than in other Bushveld PGE deposits, such as the Merensky Reef and the UG2 chromitite, but the Platreef, though thicker, has lower overall PGE grades. Despite a commonly held belief that PGEs are closely associated with sulphide mineralisation, a detailed study by laser ablation ICP-MS (LA-ICP-MS) on a core through the Platreef at Turfspruit suggests that this is not strictly the case. While a significant proportion of the Pd, Os and Ir were found to be hosted by BMS, Pt, irrespective of its whole-rock concentration, was not. Only at the top of the Platreef is Pt directly associated with sulphide minerals where Pt–Pd–(±Sb)–Te–Bi-bearing inclusions were detected in the chalcopyrite portions of large composite sulphides. In contrast, Pd, Os, and Ir occur in solid solution and as discrete inclusions within the BMS throughout the core. For Os and Ir, this is usually in the form of Os–Ir alloys, whereas Pd forms a range of Pd–Te–Bi–(Sb) phases. Scanning electron microscope observations on samples from the top of the core revealed the presence of ≤0.2-mm-long (PtPd)2(Sb,Te,Bi)2 michenerite–maslovite laths within the chalcopyrite portions of large composite sulphides. Additional Pt-bearing minerals, including sperrylite and geversite, and a number of Pd(–Te–Bi–Sb) minerals were observed in, or close to, the alteration rims of these sulphides. This textural association was observed throughout the core. Similar platinum-group minerals (PGMs) were observed within the felsic assemblages composed of quartz, plagioclase, alkali feldspar and clinopyroxene produced by late-stage felsic melts that permeated the Platreef. Many of these PGMs occur a significant distance away from any sulphide minerals. We believe these features can all be linked to the introduction of As, Sb, Te and Bi into the magmatic system through assimilation of sedimentary footwall rocks and xenoliths. Where the degree of contamination was high, all of the Pt and some of the Pd formed As- and Sb-bearing PGM that were expelled to the edges of the sulphide droplets. Many of these were redistributed where they came into contact with late-stage felsic melts. Where no felsic melt interactions occurred, the expelled Pt- and Pd-arsenides and antimonides remained along the margins of the sulphides. At the top of the Platreef, where the effects of contamination were relatively low, some of the Pt remained within the sulphide liquids. On cooling, this formed the micro-inclusions and blade-like laths of Pt–Pd–(Sb)–Bi–Te in the chalcopyrite.

Bimodal Distribution of Gold in Pyrite and Arsenopyrite: Examples from the Archean Boorara and Bardoc Shear Systems, Yilgarn Craton, Western Australia

This study investigates the microstructures, geochemistry, and hydrothermal evolution of gold-bearing pyrite and arsenopyrite from six orogenic gold deposits in the Archean Eastern Goldfields Province, Western Australia. Scanning electron microscope (SEM), electron microprobe (EMP) and laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) analyses show that the gold-bearing minerals possess a number of similar textural features, including the occurrence of invisible gold within initial phases of growth, and later-stage visible gold associated with alteration rims. The alteration rims are characterized by a higher-than-average atomic mass (mainly owing to arsenic enrichment) and are preferentially located along fractures and grain boundaries in the pyrite and arsenopyrite. These observations suggest that visible gold formation is associated with hydrothermal alteration of preexisting pyrite and arsenopyrite. Textural observations and LA-ICP-MS data suggest that some invisible gold was remobilized from early-formed pyrite and arsenopyrite to form visible gold during development of these alteration rims. Gold may also have been added by hydrothermal fluids during a later stage of mineralization. In situ geochemistry and phase relationships of alteration rims are used to further constrain the hydrothermal process responsible for formation of alteration rims and visible gold in fractures. Based on sulfide stability relations, our data indicate that development of arsenopyrite alteration rims associated with late-stage visible gold formation was related to an increase in temperature (maximum increase from 310° to 415°C) and up to of sic orders of magnitude increase in sulfur fugacity, whereas changes in oxygen fugacity were less important. LA-ICP-MS analyses show that the relative and absolute variations in selected trace element (Au, Ag, Sb, Bi, Ba, Te, Pb, Co, and Mo) concentrations can also be used to distinguish between unaltered and altered pyrite and arsenopyrite. In general, trace elements within pyrite and arsenopyrite have a relatively uniform distribution, whereas later-stage alteration rims have more variable trace element distributions. Although the observed textures are typical of prograde metamorphic coronae, we suggest that they are the consequence of variations in fluid conditions and chemistry, and that mineralization occurred in response to syn- and/or postpeak metamorphic fluid infiltration.

Fluid Inclusion Evidence for Magmatic-Hydrothermal Fluid Evolution in the Porphyry Copper-Molybdenum Deposit at Butte, Montana

The porphyry Cu-Mo deposit in Butte, Montana, formed where magmatic hydrothermal fluids, introduced with injections of porphyrytic dikes, fractured and permeated the Butte Quartz Monzonite. These fluids formed a stockwork of quartz and quartz-sulfide veinlets with a variety of styles of potassic and sericitic alteration envelopes. The distribution of vein and alteration types and the distribution of fluid inclusions in these veins record the progressive pressure, temperature, and compositional evolution of the hydrothermal fluids that formed this world-class deposit. Deep drilling and 1,300 m of offset along the Continental fault provide a vertical view of almost 3 km through the Butte deposit. Deep veins within and below the highest Mo grades are quartz dominated with thin K-feldspar or, less commonly, biotitic alteration rims. Fluid inclusions in deep veins trapped a single phase aqueous fluid containing 2 to 5 wt percent NaCl equiv and 2 to 8 mol percent CO2 at temperatures between 575° and 650°C and pressures between 200 and 250 MPa, corresponding to depths between 6 and 9 km. Although Cu grades are low in this region, abundant chalcopyrite daughter minerals in fluid inclusions indicate that the fluids were Cu rich. Fluids that formed these veins transported Cu from the magma below, upward into the region of Cu mineralization with only minor Cu precipitation. Over a kilometer above the bulk of deep quartz and quartz-molybdenite veins, the highest Cu grades are in and around chalcopyrite-bearing quartz-sulfide veins with biotitic alteration (early dark micaceous veins), and their upward, equivalent magnetite-chalcopyrite-pyrite-quartz veins with wide K-feldspar, green sericite, and chlorite alteration (pale-green sericitic veins). These veins contain more evidence for brine-vapor unmixing than any other vein type. The upward progression of early dark micaceous veins to pale-green sericitic veins formed where low salinity, CO2-bearing fluids, similar to those trapped in deep quartz veins, ascended, de-pressurized, sometimes unmixed, and cooled from ~650°C at 90 MPa to ~475°C at ~50 MPa. As low salinity, CO2-bearing, aqueous fluids, similar in composition to fluids trapped in deep quartz veins, cooled at shallow depths, they formed late pyrite-quartz veins with sericitic alteration. These veins formed from fluid cooling at temperatures between 370° and 450°C at transiently hydrostatic pressures between 40 and 70 MPa, corresponding to depths of 4 to 7 km. Most pyrite-quartz veins formed at pressures and temperatures above the H2O-NaCl-CO2 solvus, but evidence for brine-vapor unmixing is also present. Pyrite-quartz veins formed at progressively greater depths as the hydrothermal system cooled, overprinting much previous mineralization. Late Cu-Pb-Zn-Ag-As-rich Main stage veins formed from dilute fluids containing <3 wt percent NaCl equiv and <2 mol percent CO2. These fluids were trapped between 230° and 400°C under hydrostatic pressures between 20 and 60 MPa and depths of 2 to 6 km. No evidence of boiling is observed in Main stage veins. Fluid inclusion phase relationships indicate that the Butte porphyry Cu-Mo deposit formed at 5 to 9 km depth, greater than any other porphyry-type deposit. At Butte, the similarity in bulk composition of fluids trapped in early quartz-rich veins with potassic alteration and late pyrite-quartz veins with sericitic alteration implies that an underlying magma continually provided low salinity, CO2-bearing fluids of relatively constant composition during the entire life of the hydrothermal system. We hypothesize that rather than resulting from changes in fluid chemistry due to magma crystallization, the entire suite of vein and alteration types and the ore metal distribution reflect the path of cooling, depressurization, and wall-rock interaction of a parental magmatic-derived fluid of relatively constant initial composition. Fluid inclusions, vein and alteration relations, and ore metal distribution indicate that Cu and Mo were introduced into the hydrothermal system by the same fluids, but that the mechanisms of precipitation of these metals were decoupled. Early dark micaceous and, to a greater extent, pale-green sericitic, veins have wide alteration envelopes and contain more evidence for fluid unmixing than any other vein type, which suggests that chalcopyrite precipitation was driven by a combination of fluid unmixing, fluid-rock reaction, and fluid cooling between 650° and 475°C. Most molybdenite mineralization, however, is in quartz-dominated veins with little or no alteration that are dominated by low salinity inclusions. These veins formed in response to pressure decrease rather than cooling. After chalcopyrite and molybdenite precipitation, low salinity fluids cooled, usually at temperatures and pressures above the H2O-NaCl-CO2 solvus, to produce significant acid and voluminous sericitic alteration accompanied by pyrite-quartz vein formation that overprints much of the deposit and contains anomalous but noneconomic Cu.

Zircon U–Pb dating of water–rock interaction during Neoproterozoic rift magmatism in South China

A combined study of zircon U–Pb dating and mineral O isotope analysis was carried out for Neoproterozoic granite, gabbro and gabbrodiorite from the Beihuaiyang zone that marks a rifted margin between the South China Block and the supercontinent Rodinia. The results provide precise constraints on timing of water–rock interaction resulting in unusual 18O depletion in igneous minerals. CL imaging reveals that zircons mostly show clear oscillatory and/or sector zoning, with or without thin alteration rims. No metamorphic zircon occurs in the igneous intrusives, although they suffered greenschist-facies metamorphism during Triassic continental collision. LA-ICPMS zircon U–Pb dating yields consistent ages of 739 ± 8 to 754 ± 5 Ma, which are interpreted as timing of magma crystallization. Mineral separates have highly variable δ 18O values of − 0.6 to 6.8‰ for zircon, 1.2 to 12.5‰ for quartz, and 0.3 to 8.4‰ for plagioclase. Oxygen isotope heterogeneities exist within single intrusions and between different intrusions. Magmatic zircons with low δ 18O values and a weighted mean U–Pb age of 748 ± 6 Ma crystallized from low δ 18O magmas that were derived from melting of hydrothermally altered low δ 18O rocks. Both low and normal δ 18O intrusives in the Beihuaiyang zone are shown to form at the same time, contemporaneous with the Rodinia breakup. Thus, tectonic evolution from supercontinental rift to breakup is proposed to create conditions for alternating processes of high-T hydrothermal alteration and low δ 18O magmatism. Such processes are a characteristic feature of high-T water–rock interaction in volcanic rift margins associated with localized water-saturated melting. While the occurrence of low δ 18O magmas demonstrates penetration of meteoric water into the rift tectonic zone of crustal melting, the existence of normal δ 18O magmas indicates that their emplacement may serve as a thermal engine to drive the second stage of meteoric water–rock interaction resulting in the unusual 18O depletion in adjacent rocks. The concurrent occurrence of both low and normal δ 18O magmas appears to indicate highly heterogeneous high-T water–rock interaction during rift magmatism.

Secondary mineralogy and microtextures of weathered sulfides and manganoan carbonates in mine waste-rock dumps, with implications for heavy-metal fixation

The secondary mineralogy and microtextures of weathered waste-rock dumps derived from the mining of galena-sphalerite ore in quartz veins containing manganoan carbonates were examined using back-scattered electron imaging, X-ray diffraction, and chemical analysis. Sphalerite, pyrrhotite, and arsenopyrite were coated or replaced by iron oxyhydroxides in the earliest stage of the weathering, and were then replaced by sulfur. Galena shows a thin alteration rim of anglesite. Oxidation of pyrite has resulted in porous boxworks of Fe oxyhydroxides. The relative resistance to oxidation, from most resistant to least resistant, was observed to be pyrite ≈ galena > arsenopyrite ≈ sphalerite > pyrrhotite. Rhodochrosite dissolved to form hydrohetaerolite pseudomorphs, and manganoan calcite has an outer alteration rim of hydrohetaerolite and an inner zone of smithsonite. Rock and mineral fragments were cemented by fine aggregates of plumbojarosite, Fe oxyhydroxides/sulfates, and manganates. Microchemical analysis and sequential extractions showed a close association of As with Fe oxyhydroxides/sulfates, of Pb, Cu, Zn, and As with plumbojarosite, and of Pb and Zn with manganates. Despite their lower acid-neutralizing capacity, manganoan carbonates played an important role in the fixation of Pb and Zn by the formation of manganates.

Microbial alteration of 0–30-Ma seafloor and sub-seafloor basaltic glasses from the Australian Antarctic Discordance

Scanning electron microscopic observations of alteration rims in basaltic glasses dredged from 0–2.5-Ma seafloor and drilled from 18–28-Ma ocean crust (Ocean Drilling Program (ODP) Leg 187) in the Australian Antarctic Discordance (AAD) document the presence of endolithic microbes in altered basalt glass. In very young AAD lavas, ∼10-μm-thick alteration rims are developed along intersecting fractures and cracks, and the altered glass contains numerous spherical, rod- and star-shaped, partially fossilised microbial cells, similar to those from the Arctic ridges [1,2]. In 2.5-Ma basalt glasses, altered rims are up to 250 μm thick, and zeolite (phillipsite) is present within many fractures. Spherical cells occur in porous outer zones of alteration rims and on zeolite crystal surfaces within fractures, indicating that microbial activity persists in the region for at least 2.5 Ma. Mn-rich microbial cell-encrustations associated with zeolite suggest that Mn is used in an energy yielding metabolic process. Combined with recent results from the Arctic ridges the results from this study demonstrate that endolithic microbial growth is a persistent feature of mid-ocean spreading ridges. In glasses from ODP cores, ∼1-mm-thick alteration rims are developed along the widest fractures lined with Mn(Fe)-oxyhydroxides and/or clay and filled by zeolite and calcite. Most common, however, are <10–200-μm-thick rims developed along zeolite-filled, more narrow fractures and cracks. Zeolite-filled fractures with only minor to no alteration indicate several episodes of fracturing followed by relatively fast sealing. There is no age progression in alteration thickness along fractures or other characteristics, suggesting that alteration is essentially completed between 2.5 and 18 Ma. A comparison of alteration in the 2.5-Ma glass with that in the ODP samples indicates that a significant proportion of the glass alteration in the drilled samples developed prior to burial, although one type of diffuse, highly irregular front that is only observed in the ODP samples most likely developed after burial. These diffuse alteration fronts are caused by dissolution and alteration of the glass into minute globules, 0.05–0.2 μm in diameter, with no associated microbial morphologies. Fossilised, Mn-rich cells do occur within zeolite-filled fractures, possibly indicating that microbial activity continued in the fractures for as long as circulation continued. The apparent non-biological origin of diffuse, irregular alteration fronts in buried AAD glasses indicates that these textural features are not reliable as diagnostic criteria for the existence of a deep biosphere in the volcanic ocean crust.

The spatial distribution and source of arsenic, copper, tin and zinc within the surface sediments of the Fal Estuary, Cornwall, UK

ABSTRACT Estuarine sediments commonly form major sinks for contaminants released during industrial activity. Many industrial processes lead to the release of metals initially in solution, which can then be adsorbed on to, for example, Fe hydroxides or clay minerals. However, in the mining industry, there are two major contaminant waste streams: (1) metals discharged in solution via mine drainage; and (2) particulate grains of the ore‐forming or related minerals released after ore processing. The release of particulate waste can have a major long‐term impact on environmental geochemistry. In this study, we have mapped the distribution of arsenic, copper, tin and zinc within the surficial sediments of the Fal Estuary, Cornwall, UK, an area that drains a historically important polymetallic mining district. There are clear spatial variations in the contaminants, with the highest levels (&gt; 2800 p.p.m. As, &gt; 5000 p.p.m. Cu, &gt; 3000 p.p.m. Sn and &gt; 6000 p.p.m. Zn) within Restronguet Creek on the western side of the estuary. Mineralogical studies show that small (&lt; 20 µm) grains of detrital arsenopyrite, chalcopyrite, cassiterite and sphalerite are very abundant within the surface sediments. Most of the sulphide grains are fractured, but mineralogically unaltered, although some grains show alteration rims caused by oxidation of the sulphides. The geochemistry and mineralogy are indicative of sediment supply from the discharge of particulate waste into the estuary during historical mining activity. Subsequently, this particulate waste has been largely physically and biologically reworked within the surface sediments. Although considerable effort has been made to minimize contaminants released via mine drainage into the estuary, the potential flux of contaminants present within the intertidal and subtidal sediments has not been addressed. Benthic invertebrates living within the area have adapted to be metal tolerant, and it is likely that the dominant source of bioavailable metals is a result of alteration of the particulate mine waste present within the intertidal and subtidal sediments.

Microfabric analyses applied to the Witwatersrand gold- and uranium-bearing conglomerates: constraints on the provenance and post-depositional modification of rock and ore components

Microfabrics of detrital quartz grains and quartz cement of four gold- and uranium-bearing conglomerates of various goldfields of the Witwatersrand Basin, South Africa, were investigated by optical and cathodoluminescence microscopy. The study revealed that the vast majority of quartz grains (<5 mm) originate from felsic magmatic source rocks. Cherts and polymineralic mineral grains, pointing to greenschist to amphibolite facies metasedimentary rocks, follow in abundance, whereas diagenetic to weakly metamorphosed quartzites are subordinate. Material from hydrothermal vein quartz is almost completely lacking, both in the sand and in the pebble fraction. No obvious relationships are discernible between the varying proportions of detrital siliciclastic components of the different reefs and their ore grades. Assuming a sedimentological control of gold distribution, this finding supports multiple sources for the detrital components, which were thoroughly mixed during transport. The post-depositional history of the sediments is characterized by a complex polyphase succession of deformation, cementation and hydrothermal alteration events. Both fragmentation and pressure solution features within detrital quartz, quartz cement and round grains of pyrite, zircon, chromite and uraninite demonstrate that these materials were present in the conglomerates during diagenesis, and, thus, are true detrital grains with abraded, rounded grain morphologies. By analogy, it is assumed that gold is also a detrital component, although most of the gold grains display characteristics of hydrothermal overprinting. During subsequent metamorphism, micro-shear zones are developed, and brittle–ductile crystal–plastic deformation and limited quartz recrystallization occur. Maximum temperatures of about 350 °C were reached on the prograde metamorphic path. Recrystallization and redistribution of detrital siliciclastic and ore minerals took place, and various hydrothermal/metamorphic minerals including chlorite, sericite, pyrophyllite and chloritoid were formed. These redistribution processes involved existing detrital minerals only and were generally isochemical because little evidence exists for the development of a secondary porosity and permeability that would allow major external inputs into the Witwatersrand conglomerates. Most of the gold grains have hydrothermal characteristics, as evidenced by their authigenic, crystalline shapes and their chemical compositions. However, these features are regarded to result from overprinting. Most likely, the gold grains experienced more drastic modifications relative to other ore components because of the ductile and mobile nature of gold. The retrograde metamorphic path is characterized by percolating radioactive fluids at T<300 °C, recorded by radiation damage indicated by cathodoluminescence alteration rims along quartz grain boundaries and microcracks. The degrees of radiation damage observed are proportional to the uranium contents of the conglomerate ores. The collective evidence of our study supports the modified placer model for the genesis of the Witwatersrand ores. This model explains most of the observations on the detrital mineral assemblage and its post-depositional modification elegantly and in a satisfactory manner.

The Chromite Deposits of the Bacuri Mafic-Ultramafic LayeredComplex, Guyana Shield, Amapa State, Brazil

The chromite deposits of the Bacuri mafic-ultramafic layered complex consist of 8.8 Mt of ore grading 34 percent Cr 2 O 3 and represent the second largest reserves of chromite in Brazil. The complex is a major stratiform complex overprinted by ductile deformation and associated regional amphibolite-facies metamorphism of the Transamazonian cycle (about 2.0 Ga), intrusive into gneiss-migmatite terranes of the Guyana Shield. The Bacuri mafic-ultramafic complex consists of a lower mafic zone (>500 m thick), an ultramafic zone (30–120 m thick) and an upper mafic zone (>300 m thick). The chromitite layers are restricted to the ultramafic zone. This zone consists of interlayered serpentinite (olivine cumulate) and chromitite (chromite cumulate). Most of the chromite is concentrated in a thick chromitite layer, known as the main chromitite, located at the base of the ultramafic zone in direct contact with the underlying lower mafic zone. The thickness of the main chromitite is highly variable owing to deformation and ranges from 3 to 30 m (average of 12 m). Several thinner layers of massive chromitite are located above the main chromitite within the ultramafic zone. Chromitites are generally massive and contain more than 60 vol percent of cumulus chromite. The chromite is mainly euhedral and fine-grained, ranging from 0.1 to 3 mm in diameter. The matrix of massive chromitite consists mainly of metamorphic silicates (serpentine, chlorite, and tremolite), except for the B1 orebody where large igneous orthopyroxene oikocrysts are preserved. Chromite grains in massive chromitite have a homogeneous core and an alteration rim. Microprobe traverses through chromite grains of massive chromitite indicate that the alteration rim is enriched in Cr and Fe 2+ and depleted in Al and Mg. The composition of chromite from massive chromitite changes significantly with stratigraphic height. The consistent stratigraphic variation is well indicated by progressive upward decrease in the Mg/(Mg + Fe 2+ ) ratio as well as the progressive upward increase in the Cr/(Cr + Al) ratio, the Fe 3+ /(Fe 3+ + Al + Cr) ratio, and TiO 2 content. The significant TiO 2 and Fe 3+ enrichments in chromite of the uppermost massive chromitites are typical of stratiform chromite deposits such as the Bushveld Complex. Olivine shows an equivalent trend of compositional variation characterized by an upward decrease in Fo and Ni content. The cryptic variation of chromite and olivine indicates extensive fractionation within the ultramafic zone. Reversals in the Fo content of olivine and in both Mg/(Mg + Fe 2+ ) ratio and TiO 2 content of chromite suggest that successive replenishments of the magma chamber with more primitive magma occurred during the formation of the ultramafic zone.

Interpreting magmatic processes from accessory phases: titanite—a small-scale recorder of large-scale processes

In this study we examined variations in ore and other trace-metal concentrations in titanite, a ubiquitous product of magmatic (and subsequent sub-solidus) crystallisation in oxidised silicic magmas. Accessory titanite occurs in the Tuolumne Intrusive Suite (TIS), Sierra Nevada Batholith, as euhedral to anhedral, poikilitic, or interstitial grains. Zoned crystals of titanite were analysed by electron microprobe and synchrotron X-ray fluorescence for major and trace elements. Backscatter electron images reveal zoning, with bright areas correlating positively with total REE concentrations. REE concentrations generally decrease toward the edge of titanite crystals; however, some crystals are reversely zoned, and others exhibit oscillatory or patchy zoning; some grains contain discrete anhedral cores. Most elements in magmatic titanite decrease in concentration towards crystal rims, independent of host rock composition.At least one major reduction event in the magma chamber(s) transiently stabilised ilmenite, now present only as inclusions in titanite, and resulted in a reduction in the REE concentration in titanite. We suggest the hypothesis that the reduction in the REE concentration in these zones is due to the diminished activity of the (REE)Fe3+Ca−1Ti−1exchange component; however, the scatter in the data, together with the operation of other exchange vectors for Fe and Al, did not allow us to test this hypothesis herein. Secondary (i.e. sub solidus, hydrothermal) titanite can be recognised on the basis of its chemistry, sometimes by its anhedral form, and by its position as an alteration rim around primary magmatic phases; however, secondary titanite growth on primary titanite crystals may be harder to discern. Secondary titanite rims on magnetite contain higher Cr, Zr and Mo, and lower REE, relative to magmatic titanite. U/Th ratios increase toward the rim of most titanite grains; however, Th decreases in concentration from core to rim. This is due, most likely, to complications resulting from the coupled substitutions necessary for replacement of Ca by tetravalent Th; factors of this sort are commonly overlooked in trace element analysis.The analysed titanites are from rocks of the normally zoned TIS which ranges in87Sr/86Sri, from 0·7059 (tonalite and quartz-diorite) to 0·7066 (granite). Many element ratios in the titanites exhibit little to no functional dependence on87Sr/86Sri. However, log Mo/W increases with increasing87Sr/86Sri, of the host unit from the equigranular quartz-diorite and tonalite, to the interior granodiorites, possibly reflecting the greater crustal contribution to the interior, more felsic units. Neither Mo nor W increase significantly from core to rim in titanite. If these trends are indicative of the general behaviour of these elements duringin-situfractionation, then these data suggest that Mo and W are not strongly incompatible, and indeed may behave compatibly, in some titaniteand magnetite-bearing granodioritic magmas.

Mobilization and attenuation of heavy metals within a nickel mine tailings impoundment near Sudbury, Ontario, Canada

The oxidation and the subsequent dissolution of sulfide minerals within the Copper Cliff tailings area have led to the release of heavy metals such as Fe, Ni, and Co to the tailings pore water. Dissolved concentrations in excess of 10 g/l Fe and 2.2 g/l Ni have been detected within the shallow pore water of the tailings, with increasing depth these concentrations decrease to or near analytical detection limits. Geochemical modelling of the pore-water chemistry suggests that pH-buffering reactions are occurring within the shallow oxidized zones, and that secondary phases are precipitating at or near the underlying hardpan and transition zones. Mineralogical study of the tailings confirmed the presence of goethite, jarosite, gypsum, native sulfur, and a vermiculite-type clay mineral. Goethite, jarosite, and native sulfur form alteration rims and pseudo-morphs of the sulfide minerals. Interstitial cements, composed of goethite, jarosite, and gypsum, locally bind the tailings particles, forming hardpan layers. Microprobe analyses of the goethite indicate that it contains up to 0.6 weight % Ni, suggesting that the goethite is a repository for Ni. Other sinks detected for heavy metals include jarosite and a vemiculite-type clay mineral which locally contains up to 1.6 weight % Ni. To estimate the mass and distribution of heavy metals associated with the secondary phases within the shallow tailings, a series of chemical extractions was completed. The experimental design permitted four fractions of the tailings to be evaluated independently. These four fractions consisted of a water-soluble, an acid-leachable, and a reducible fraction, as well as the whole-rock total. Twenty-five percent of the total mass of heavy metals was removed in the acid-leaching experiments, and 100% of the same components were removed in the reduction experiments. The data suggest that precipitation/coprecipitation reactions are providing an effective sink for most of the heavy metals released by sulfide mineral oxidation. In light of these results, potential decommissioning strategies should be evaluated with the recognition that changing the geochemical conditions may alter the stability of the secondary phases within the shallow tailings.

Contaminant source and mobilization in industrial wastes in Dalälven river drainage area, Sweden

From a summary of mineralogical and geochemical studies, this paper presents patterns of contaminant source, mobilization, and retention in three types of industrial wastes deposited in the Dalälven river drainage area in central Sweden. These industrial wastes contain significant amounts of sulfides and have been subjected to atmospheric oxidation for several tens to hundreds of years. They are the major sources of metals and non-metal contaminants discharged into the Dalälven river. The collected data are of considerable value toward the remedial management of this kind of industrial wastes. The mill-tailings initially contain carbonate minerals which neutralize the acidity and maintain the leachate pH at near neutral values at the bottom of the oxidation zone (about 40 cm deep beneath the surface). The acid neutralization results in a great deal of precipitation of ferric iron hydroxides, leading to the formation of Fe-precipitated layers - a potential barrier to metal movement and O2 diffusion downward through the tailings. The mine waste dump is composed of coarse-grained rock fragments and has been covered by crushed limestone since 1985 in an attempt to neutralize the acid leachate. However, limestone cover has much less buffering effect than expected. As a result, the underlying soil and ground water contains high levels of SO4, Fe, and other metals, and have low pH values. The sulfuric acid industry wastes consist of 70-80% iron oxides which are porous in texture. The porous texture provides a large surface area for the adsorption of SO4. During roasting, sphalerite acquired alteration rim which is composed of Zn-oxides. The mineralogical properties of the SAI wastes play an important role in the formation of hydrated sulfates of Fe and Zn. Due to the occurrence of these highly soluble sulfates, in situ flushing is a practical remedial approach for the SAI wastes, and a cleansing project is carrying out in Falun.

Mineralogical and chemical characterization of wastes from the sulfuric acid industry in Falun, Sweden

Wastes from the sulfuric acid industry are an environmental concern, because of the emission of acids, heavy metals, and sulfate to the environment. The wastes in Falun consist of 70–80% iron oxides, 10–20% silicates, less than 10% residual sulfides, and small amounts of secondary precipitates (iron hydroxides and Fe-, Zn- and Cu-sulfates). Due to the different behavior of sulfides during the roasting process, pyrrhotite and sphalerite are the major sulfide residues associated with lesser amounts of pyrite, chalcopyrite, and galena. The leachates are low-pH and enriched in Zn, Fe, and SO4. The acid ferric Fe-rich solution promotes the dissolution of sphalerite and favors the formation of Pb-sulfate coatings on galena, providing an armoring effect which slows down the further oxidation of the galena. The residual sulfides are the potential source for acid generation and metal release. During the roasting process, iron oxides retain small amounts of sulfur and sphalerite forms alteration rims containing Zn-oxides. The iron oxides and Zn-oxides are important contributors to SO4 and Zn in the leachates. The conditions in the waste deposit are favorable for the precipitation of Zn-, Cu-Fe-sulfates (e.g. gunningite, chalcanthite, Zn-copiapite). The highly soluble sulfates play important roles in controlling the concentrations of Cd, Cu, Fe, Zn, and SO4 in the leachates. The mineralogical and geochemical data help to develop the reclamation strategies of this type of industrial wastes.

Organic matter and clay anomalies associated with base-metal sulfide deposits

Organic matter and clay mineral anomalies surround the Polaris, Gays River and Gaspé Mines base-metal sulfide deposits. The Polaris mine, in the Canadian Arctic, is an MVT deposit formed by low temperature solutions (homogenization temperatures, T h: 105–120°C) and is characterized by a zone of pure illite above a kaolinite envelope surrounding the deposit. These clay mineral assemblages are laterally replaced by kaolinite-illite and chlorite-corrensite zones, while the background assemblage in the host rocks is illite-interstratified illite / smectite-chlorite. The organic matter reflectance ( R o) shows two trends depending on the permeability and chemical reactivity of the host rock: (1) an increase with depth above the ore zones in the overlying impermeable shaly cap rock. R o reaches a maximum value of 1.3% above the ore, compared to 0.8% in equivalent country rocks; (2) an anomalous decrease with depth in the mineralized carbonate horizon that hosts the deposit. The Gays River mine, Nova Scotia, is a medium temperature ( T h: 250°C) carbonate-hosted deposit associated with the replacement of well-crystallized detrital illite and chlorite by smectite forming a halo around the main ore zone and by kaolinite and chlorite-corrensite within the ore zone. Reflectance and its absolute variability increase in the host rocks toward the ore zone adjacent to the deposit (0.8 to 1.88%) and shows a relatively low R o in the ore. The economic mineralization at Gaspé Mines, Québec, is a typical high-temperature skarn and porphyry-copper deposit with the concomitant clay mineral alteration assemblages: chlorite-corrensite-illite, smectite-chlorite-illite and almost pure illite with traces of chlorite. The first assemblage is observed within the ore zone and extends slightly outside the procellanite halo. Smectite is also found near the mineralization, but its limit of occurrence is wider than the chlorite assemblage. The illite assemblage is found outside the chlorite zone and is present as irregular tongues. R o increases (1.2 to 15%) approaching ore over a lateral distance of 60 km. In the bleached zone, the organic matter exhibits mosaic and degasification textures and isotropic alteration rims with very low R o. These three deposits, as well as others studied by the authors, are centrally located within thermal aureoles whose size and intensity correlate directly with increasing temperature of formation. The decreasing values of R o in the ore zones of these deposits and the associated authigenic clay minerals reflect changes in chemical conditions due to precipitation of sulfides and to reaction of the ore and post-ore fluids with host rocks.

Petrology of the polymict eucrite Petersburg

The polymict eucrite Petersburg contains eucritic and diogenitic materials which span a broad range of compositions. The present study focuses on four eucritic clasts for which bulk compositions and mineralogical data were acquired; the broad compositional range of these clasts documents the diversity of the parent regolith. Clast A is REE-rich (~26 × chondrite) and magnesian (Mg# = 48.7); composition of this clast can best be explained by partial melting of a source region distinctly different from that of other noncumulate eucrites. Composition of eucritic clast B is similar to compositions of cumulate eucrites in REE and Sr abundances, but it has a much lower Mg# than Moore County. Clasts C and D are similar in texture, mineralogy, and composition to main group eucrites. The presence of anomalous clast A in the same polymict breccia as typical eucrite clasts C and D emphasizes the variability of source materials from which eucritic melts were derived on the HED parent body(ies). After final accretion, this breccia apparently was exposed to a minor episode of thermal metamorphism causing ~30 μm wide alteration rims around larger pyroxene fragments and causing alteration of pyroxenes at the edges of eucritic clasts. This metamorphism completely altered the Mg# of smaller matrix pyroxenes, less than 40–60 μm in size, to values closer to the bulk Mg# of the meteorite.

Predicting the mobility of Zn, Fe, Cu, Pb, Cd from roasted sulfide (pyrite) residues—A case study of wastes from the sulfuric acid industry in Sweden

Leachates from roasted sulfide residues, which are the wastes from the production of sulfuric acid at Falun, Sweden, have low pH and high concentrations of Zn, Fe, and SO 4. The minerals are mainly hematite and maghemite and, because the various sulfides in the feed behave differently during the roasting process, the residual sulfides minerals are pyrrhotite and sphalerite. Oxidation of the residual sulfides contributes acidity, Zn, Fe, Cu, Cd, and sulfate to the effluents from the waste deposits. The dissolution of sphalerite is most likely accelerated in acid solution rich in Fe (III). The formation of Pb-sulfate coatings on galena may provides an armoring effect which slows the oxidation of the galena. Residual sulfides are source phases controlling long-term contaminant release. Other source minerals for Zn, Fe, Pb, Cu, Cd and SO 4 in the effluents are iron oxides which retained percentage quantities of SO 4, roast-derived alteration rims of Zn oxides on sphalerite, alterated silicates formed during the roasting process, and secondary minerals (e.g. Zn, Fe, Cu sulfates, iron hydroxides) that were precipitated by in-site oxidation in the waste dumps. The Zn, Fe, and Cu sulfates most likely control short-term changes in the chemistry of the leachate, while Pb concentration in the leachates may be controlled predominantly by Pb-release from the altered silicates. The mineralogical and geochemical data provide fundamental information essential for the remedial management of this type of industial waste.

Remagnetization of the folded Belden Formation, northwest Colorado

Paleomagnetic and geochemical results are used to evaluate the origin of an apparent synfolding magnetization in Belden Formation (Pennsylvanian) limestones on an asymmetric fold in Colorado. The natural remanent magnetizations contain three components: a modern viscous magnetization removed at low temperatures, a reversed Tertiary magnetization (declination (D) = 186.7°, inclination (I) = −54.6°) of thermoviscous origin removed at intermediate temperatures, and an apparently synfolding Late Cretaceous/early Tertiary characteristic magnetization (D = 338.7°E, I = 62.5°, precision parameter k = 54.9, radius of 95% probability cone ∝95 = 5.9°, 65% unfolding) removed by 580°C. Hysteresis properties suggest that the characteristic magnetization resides in single‐domain magnetite. Low burial temperatures and the presence of authigenic magnetite are consistent with a chemical origin for this magnetization. The 87Sr/86Sr values (0.7088–0.7100) show the limestones to be radiogenic, with 87Sr enriched halos around calcite‐cemented tectonic fractures. There is no control on the distribution and intensity of the magnetization by the veins or structural position (e.g., highly mineralized versus less mineralized rock). Lithology, rather than fluid alteration, controls the magnetization. Fecal pellets contain pyrite, with alteration rims identified as magnetite by scanning transmission electron microscope X ray diffraction patterns. Selective demagnetization has established that the fecal pellets carry the characteristic magnetization. It is not possible to unequivocally establish whether the characteristic magnetization is synfolding or prefolding. A synfolding magnetization caused by the syndeformational mineralizing fluids, however, can be ruled out. Structural modification of a prefolding component and a synfolding piezoremanent magnetization cannot be ruled out but are not considered likely mechanisms of remanence acquisition. Overprinting of a prefolding chemical magnetization by a postfolding viscous magnetization residing in multidomain grains is a likely explanation for the synfolding test result. The replacement of pyrite by magnetite is interpreted as prefolding in origin, perhaps triggered by an oxidizing fluid or by diagenesis of hydrocarbons.

Microbiological, chemical, and mineralogical characterization of the kidd creek mine tailings impoundment, Timmins area, Ontario

Bacterial enumeration and geochemical characterization were undertaken at three sites on the sulfide‐rich tailings impoundment at the Kidd Creek metallurgical site, Timmins, Ontario, Canada. The three sites were selected to represent varying degrees of sulfide oxidation to assess the changes in water chemistry, in the mineralogical composition of the tailings, and in bacterial populations as the sulfide oxidation process proceeds under natural field conditions. The first site was characterized as having negligible oxidation‐derived alteration, the pH of the porewater varied from 6.5 to 7.5, and the concentrations of dissolved constituents were similar to those observed in the deeper, unaltered tailings. Mineralogical examination of the tailings grains indicated that the sulfide surfaces were sharp and unreplaced. At this site, the predominant sulfur‐oxidizing bacteria were Thiobacillus thioparus and related species. The second site showed evidence of the onset of acidification, the pH of the near‐surface porewater had decreased to 5.5, and the concentrations of some sulfide oxidation products, principally SO4 and Zn, had increased beyond those observed in the unaltered tailings. At the second site, T. thioparus and related species predominated. At the third site the acidification process was well established. The pH of the shallow porewater had decreased to <4.0, and high concentrations of several sulfide oxidation products were observed. Mineralogical examination of the tailings indicated the presence of alteration rims of ferric oxyhydroxide minerals that surrounded the oxidizing sulfide grains. At this site the predominant bacterial groups were T. ferrooxidans and T. thiooxidans types. At the third site the bacterial abundance reached a maximum at 25–45 cm below the tailings surface. The occurrence of the peak abundances coincided with the interface between the zones of the unaltered tailings and the overlaying altered sulfides. This coincidence suggests that the iron oxyhydroxide coatings, which surround the partly oxidized sulfide grains, may shield the partly altered grains, thereby inhibiting bacterial growth. The data are consistent with an initial chemical oxidation of the metal sulfides to produce elemental sulfur and polysulfides, which are subsequently oxidized by neutrophilic thiobacilli to produce sulfuric acid. After the pH has decreased to the 3.5–4.0 range, the acidophilic thiobacilli, such as T. ferrooxidans, can directly oxidize the metal sulfides.

Fragipan formation in argillic brown earths (Fragiudalfs) of the Milfield Plain, north‐east England. III. Micromorphological, SEM and EDXRA studies of fragipan degradation and the development of glossic features

SUMMARYThe micromorphology of eluvial and glossic areas with weak soi1, strength from the upper fragipan, and the upper parts of grey polygonal zones, in some argillic brown earths (Glossic Fragiudalfs) from north‐east England shows unstable void walls stripped of clay with remnants of ferri‐argillans, pale grey grainy clay coatings and darker grainy coatings. These occur alongside loose surface residues of skeleton grains and thin grey fine silt coatings of quartz and muscovite. SEM and EDAX studies of coating surfaces, and optical microscopy, SEM and EDXRA of very thin polished sections, show that some grainy clay coatings form as alteration rims of ferri‐argillans through localized waterlogging, iron oxide loss and micro‐erosion leading to micropitting and disoriented fabrics. The inclusion of coarse clay to fine silt‐sized quartz, feldspar and muscovite in other grainy coatings suggests either alteration of impure ferri‐argillans or accumulation of degradational products derived from elsewhere on void walls. This is more certainly the case for dark grainy coatings in layered compound illuviation coatings adjacent to glossic areas. These fragipans are degrading from the top downwards by processes which are partly a consequence of the effects that the fragipan has on water percolation and root penetration. The destabilization of void walls, the degradation of ferri‐argillans and the remobilization of clay to form glossic features did not begin until seasonal waterlogging in parts of the upper pan was sufficient to mobilize iron.

Maturation thermique de la matière organique dans un bassin du Paléozoïque inférieur, basses-terres du Saint-Laurent, Québec, Canada

The organic matter (OM) sampled in 15 oil exploration wells and 102 outcrops of Cambrian-Ordovician rocks in the St. Lawrence Lowlands consists of zooclasts (chitinozoans, graptolites, and scolecodonts) and solid bitumen (mainly pyrobitumen). The reflectance of pyrobitumen, transformed in vitrinite-equivalent (Ro-Std-B), indicates that the upper part of the platform sequence is mature (condensate zone) in the Québec area, but overmature in the Montréal area.The platform is divided into three domains based on optical texture of OM and types of bitumen: domain 1, south of Montréal, contains a highly reflecting and coked pyrobitumen showing alteration rims; domain 3, east of Trois-Rivières, contains low-reflecting, late solid bitumen commonly associated with oil impregnations; domain 2, located midway, contains a pyrobitumen with moderate reflectance and, locally, fine mosaic texture. The thermal maturation in the autochthonous sequences of the St. Lawrence Lowlands and in the Appalachian allochthons increases from the north-east toward the south-west and in the direction of the Appalachian belt. In wells, the gradient of Ro-Std-B with depth decrease from the autochthonous zone toward the Appalachian belt, and is inversely related with thickness of the sequences. Isoreflectance values parallel the outline of the Chambly–Fortierville Syncline in the central and eastern parts of the basin. Consequently, thermal maturation predates folding. Reflectance jumps observed between the Lowlands and the first Appalachian overthrusts, and observed when crossing Logan's Line, demonstrate that the maximum burial of Appalachian sequences predates the tectonic transport. The Ro-Std-B in allochthonous zones shows higher values in the St-Francis River cross-section than in structural equivalent units of the Québec area. Therefore the increase of thermal maturation observed from the northeast toward the southwest, in the St. Lawrence Lowlands, is also developed in the Appalachian allochthonous units. A post-Taconic regional thermal event explains this similarity in both autochthonous and allochthonous sequences, with the sequences of the Connecticut Valley – Gaspé Synclinorium being the most thermally mature.Zones of highest thermal maturation, locally observed in the Montréal area, are explained by (i) hydrothermal activity (Ro-Std = 3–4%), accounted for by sulfate and sulfide mineralization (Ba, Zn, Pb) and by (ii) contact metamorphism, related to alkaline intrusions (Ro-Std-B = 13%). The contact metamorphism is restricted to aureoles less than 5 km wide around the Monteregian alkaline intrusions, but the hydrothermal alteration, apparently not related to contact metamorphism, covers an area of 10 km around mineralized domains.