Replacement Of Albite Research Articles

Porosity generation via spatially uncoupled dissolution precipitation during plagioclase replacement in quartz undersaturated fluids

The replacement of feldspars is commonly characterized by pseudomorphism and reaction-induced pore generation. However, the effects of compositions of feldspars and fluids on porosity generation during alteration are still poorly understood. In this study, we conducted a series of hydrothermal experiments on plagioclase replacement by 2 M KCl or NaCl aqueous solutions at 600 °C and 150 MPa for 1–8 days, using plagioclase with different compositions (anorthite, An96Ab4; labradorite, An66Ab33Or1; albite, An1Ab99) with or without quartz. Albite replacement by K-feldspar was not affected by the presence of quartz, whereas anorthite was unaltered in the quartz-absent fluid. The replacement of labradorite by KCl(aq) showed different results: in the presence of quartz, labradorite was altered by K-feldspar, whereas in the absence of quartz, alteration proceeded significantly with the generation of large pores hosted by secondary anorthite coupled with euhedral K-feldspar overgrowth. Such textural relationship and oxygen isotope-labeled experiments reveal that silica-deficient fluid enhances the uncoupled dissolution reprecipitation process. The Si and Al ions in the reacted aqueous solution diffused outside the labradorite grains and encountered K+-rich solutions to grow K-feldspar. The experiments with polycrystalline rocks composed of amphibole + labradorite using 2 M KCl aqueous solution indicated the replacement of labradorite grains by anorthite and K-feldspar overgrowth, as found in single-crystal experiments. Our results indicate that the silica concentration in the fluids has different influences on the saturation indices of albite, anorthite, and K-feldspars in saline fluids, which significantly affect the replacement textures and porosity generation in crustal rocks.

Mineral chemistry of hydrothermal alteration assemblage in hanging wall Shahapur granite associated with vein-type Gogi uranium deposit, Bhima Basin, Eastern Dharwar Craton, India: Implications for physico-chemical conditions of ore formation

Eastern Dharwar craton (EDC) is a major uranium province in India hosting number of unconformity related, vein-type and carbonate hosted strata-bound uranium deposits. Gogi uranium deposit (GUD) is a structurally controlled hydrothermal ore deposit (~1.35 Ga), hosted in a reverse fault zone between the foot-wall Mesoproterozoic Bhima Group carbonates and the hanging wall basement granites (~2.5 Ga) of the EDC. The hanging wall granites, locally known as Shahapur granite show extensive chloritisation and epidotisation, and constitute the propylitic outer zone of the main hydrothermal system of the GUD. We conducted systematic mineralogical and geochemical studies on hydrothermally altered assemblages to understand physico-chemical conditions of hydrothermal fluid. Quartz, epidote, chlorite, titanite, fluorite and hematite are the main hydrothermal mineral phases. Chlorites occur in two different forms, as pseudomorphic replacement of hornblende and as vein/cavity fillings. Epidotes occur as coarse prismatic grains in the main veins, or as replacement of albite in the wall rock. Mineralogical, textural data and geochemical mass balance calculation indicate that about 17.9% of total albites (Modal abundance = ~48%) and all the hornblendes (modal abundance = ~7.5%) of the parent rock were broken down and reacted to form quartz, chlorite, epidote, secondary titanite and hematite. The hydrothermal reaction resulted in creation of 13 vol% secondary porosity in the host rock and acidification of the evolved fluid. Thermodynamic calculation on chlorite compositions indicates ~ 250 °C temperature of formation and oxidising nature of ambient fluid, with log fO2 ranging between −37 and −31 (avg. −35). Available fluid inclusion data, in conjunction with chlorite geothermometric results, show fluid entrapment pressure between 625 bar and 792 bar, which is consistent with emplacement of hydrothermal fluid in low-pressure brittle deformation zone. We surmise that U6+ was liberated from basement granite by reaction with oxidised acidic fluid, transported as chloride, carbonate complexes and precipitated in high permeability fault-zone, possibly because of mixing with reduced- basin derived fluids.

The Albitization of K-Feldspar in Organic- and Silt-Rich Fine-Grained Rocks of the Lower Cambrian Qiongzhusi Formation in the Southwestern Upper Yangtze Region, China

The albitization of K-feldspar is a common diagenetic process that has thus far received little attention and is not fully understood in fine-grained sedimentary rocks. To better understand the albitization of K-feldspar, studies in organic- and silt-rich fine-grained rocks of the lower Cambrian Qiongzhusi Formation in the southwestern Upper Yangtze region, China, were carried out via X-ray diffractometry (XRD) and field emission scanning electron microscopy (FE-SEM). The results show that five types of albitized K-feldspar textures have developed: microcrystal albite replacement, irregular blocky replacement along margins, cleavage planes or microcracks of K-feldspars, complete pseudomorphic replacement, albite overgrowth, and albite pore filling. Organic- and silt-rich fine-grained rocks differ from sandstones and mudstones in terms of the rock structure and mineral assemblage, which results in differences in the textures and degree of albitization of K-feldspar. Illitization of clay has an impact on the albitization of K-feldspar. In provenance analyses using feldspar, fine-grained rocks, especially those that underwent mesogenesis, should be treated with caution because detrital feldspars have been destroyed. Theoretically, the albitization of K-feldspar could increase the porosity of reservoirs, although, from our observations, most of the related secondary pores are cancelled out or became isolated pores due to other diagenetic processes (compaction, cementation, etc.) in organic- and silt-rich fine-grained rocks.

Deformation of feldspar at greenschist facies conditions – the record of mylonitic pegmatites from the Pfunderer Mountains, Eastern Alps

Abstract. Deformation microstructures of albitic plagioclase and K-feldspar were investigated in mylonitic pegmatites from the Austroalpine basement south of the western Tauern Window by polarized light microscopy, electron microscopy and electron backscatter diffraction to evaluate feldspar deformation mechanisms at greenschist facies conditions. The main mylonitic characteristics are alternating almost monophase quartz and albite layers, surrounding porphyroclasts of deformed feldspar and tourmaline. The dominant deformation microstructures of K-feldspar porphyroclasts are intragranular fractures at a high angle to the stretching lineation. The fractures are healed or sealed by polyphase aggregates of albite, K-feldspar, quartz and mica, which also occur along intragranular fractures of tourmaline and strain shadows around other porphyroclasts. These polyphase aggregates indicate dissolution–precipitation creep. K-feldspar porphyroclasts are partly replaced by albite characterized by a cuspate interface. This replacement is interpreted to take place by interface-coupled dissolution–precipitation driven by a solubility difference between K-feldspar and albite. Albite porphyroclasts are replaced at boundaries parallel to the foliation by fine-grained monophase albite aggregates of small strain-free new grains mixed with deformed fragments. Dislocation glide is indicated by bent and twinned albite porphyroclasts with internal misorientation. An indication of effective dislocation climb with dynamic recovery, for example, by the presence of subgrains, is systematically missing. We interpret the grain size reduction of albite to be the result of coupled dislocation glide and fracturing (low-temperature plasticity). Subsequent growth is by a combination of strain-induced grain boundary migration and formation of growth rims, resulting in an aspect ratio of albite with the long axis within the foliation. This strain-induced replacement by nucleation (associated dislocation glide and microfracturing) and subsequent growth is suggested to result in the observed monophase albite layers, probably together with granular flow. The associated quartz layers show characteristics of dislocation creep by the presence of subgrains, undulatory extinction and sutured grain boundaries. We identified two endmember matrix microstructures: (i) alternating layers of a few hundred micrometres' width, with isometric, fine-grained feldspar (on average 15 µm in diameter) and coarse-grained quartz (a few hundred micrometres in diameter), representing lower strain compared to (ii) alternating thin layers of some tens of micrometres' width composed of fine-grained quartz (<20 µm in diameter) and coarse elongated albite grains (long axis of a few tens of micrometres) defining the foliation, respectively. Our observations indicate that grain size reduction by strain-induced replacement of albite (associated dislocation glide and microfracturing) followed by growth and granular flow simultaneous with dislocation creep of quartz are playing the dominating role in formation of the mylonitic microstructure.

Two metasomatic events recorded in apatite from the ore-hosting dolomite marble and implications for genesis of the giant Bayan Obo REE deposit, Inner Mongolia, Northern China

In the Bayan Obo REE deposit in Inner Mongolia, Northern China, three major orebodies are hosted in dolomite marble of the Bayan Obo Group. There are carbonatite dikes in the ore district. Apatite is a common accessary mineral in the ore-hosting dolomite marble (DM apatite) and in carbonatite dikes (IC apatite). These two types of apatite are both fluorapatite, and have low SiO2, uniform P2O5, and variable CaO contents. Total REY (REEs + Y) contents are correlated with Na2O contents, indicating that REY of both types of apatite enter lattice via the substitution reaction: Na+ + (REY)3+ = 2Ca2+. These features, combined with high REY (6230–18,906 ppm) and Sr (9653–17,200 ppm) contents of DM apatite, indicate that DM apatite likely had a carbonatite origin. Some DM apatite grains are partially replaced by albite and quartz. Fluid inclusions crosscutting both apatite and albite or quartz indicate that they formed later than quartz and albite replacement. The back-scattered electron images show that DM apatite grains contain many micro-pores (fluid inclusions), and monazite inclusions formed from the fluid inclusions. However, no monazite inclusions are observed within quartz and albite, excluding the possibility that the monazite inclusions were precipitated directly from the fluids. The monazite inclusions were therefore formed during fluid-induced dissolution–reprecipitation processes, where DM apatite served as the source of LREEs. This also explains the depletion of some LREEs in DM apatite. The formation of monazite inclusions in apatite requires fluids with relatively low Na and Si concentrations, different from the fluids responsible for quartz and albite replacement. DM apatite was affected by two stages of fluid activities: the first stage of metasomatism by alkaline fluids that were likely derived from carbonatite magmas when the deposit first formed (represented by quartz and albite replacement), followed by a second stage of modification that caused LREEs depletion and the formation of new REE minerals. Thus, the Bayan Obo REE ore deposit was modified by a significant thermal event after the formation, which provided negligible or only small amounts of REEs.

Metasomatic Replacement of Albite in Nature and Experiments

Replacement of albite by sodium-rich, secondary phases is a common phenomenon, observed in different geological settings and commonly attributed to alkaline metasomatism. We investigated growth of nepheline and sodalite on albite in time series experiments between two and 14 days. A total of 42 hydrothermal experiments were performed in cold-seal hydrothermal vessels at a constant pressure of 4 kbar and 200–800 °C in the system SiO2–Al2O3–NaCl–H2O. To allow for fluid flow and material transport, a double-capsule technique was used; hereby, a perforated inner Pt capsule was filled with cleavage fragments of natural albite, whereas the shut outer Au capsule was filled with γ-Al2O3 and the NaCl–H2O solution. Complete overgrowth of albite by sodalite and nepheline occurred after just two days of experiments. At high salinity (≥17 wt % NaCl) sodalite is the stable reaction product over the whole temperature range whereas nepheline occurs at a lower relative bulk salinity than sodalite and is restricted to a high temperature of ≥700 °C. The transformation of albite starts along its grain margins, cracks or twin lamellae. Along the reaction front sodalite crystallizes as small euhedral and highly porous grains forming polycrystalline aggregates. Coarse sodalite dominates in the outermost domains of the reaction zones, suggesting recrystallization. Sodalite may contain fluid inclusions with trapped NaCl-rich brine, demonstrating that the interconnected microporosity provides excellent pathways for fluid-assisted material transport. Highly porous nepheline forms large, euhedral crystals with rectangular outline. Sodalite and nepheline in natural rock samples display only minor porosity but fluid and secondary mineral inclusions, pointing to coarsening of a previously present microporosity. The reaction interface between sodalite and albite in natural rock samples is marked by open channels in transmission electron microscopy. In many of the experiments, a zone of Si–H-rich, amorphous material is developed at the reaction front, which occurs at a temperature of up to of 750 °C as nanometer to 350 µm wide reaction zone around albite. This change in composition corresponds with the abrupt termination of the crystalline feldspar structure. The presence of sodalite as micro- to nanometer-sized, euhedral crystals within the amorphous zone demonstrates, that both the sodalite reaction rim and the amorphous material allow for fluid-assisted material transport between the crystalline albite (release of Si, Al) and the bulk fluid (H2O, Na, Cl). This texture, moreover, suggests that the amorphous phase represents a metastable interstage reaction product, which is progressively replaced by sodalite and nepheline. Remarkably, product sodalite, nepheline, and the amorphous material largely inherit the trace element budget of the respective ancestor albite, indicating that at least part of the trace elements remained fixed during the reaction process. The observed reaction textures in both natural and experimental samples indicate an interfacial dissolution–reprecipitation mechanism. Results of our study bear important implications with respect to mineral replacement in the presence of a fluid phase, especially regarding the interpretation of trace element patterns of the product phases.

Alteration at the Olympic Dam IOCG–U deposit: insights into distal to proximal feldspar and phyllosilicate chemistry from infrared reflectance spectroscopy

ABSTRACTTwenty thousand metres of diamond drill core representing a 14 km cross-section from weakly to intensely altered Roxby Downs Granite through the Olympic Dam Breccia Complex, host to the Olympic Dam iron-oxide–copper–gold–uranium deposit in South Australia, was analysed using the HyLogger-3 spectral scanner. Thermal and shortwave infrared spectroscopy results from 30 drill holes provide insight into the spatial relationships between quartz, orthoclase–microcline, albite–oligoclase and progressively changing sericite and chlorite compositions. The relative proportions of quartz, feldspars and phyllosilicates were mapped with thermal infrared spectroscopy. Variations in the chemistry of sericite and chlorite were extracted by proxy from their shortwave infrared spectral response, together with their relative spatial distribution. HyLogger scanning has revealed four deposit-scale mineralogical trends, progressing from least-altered Roxby Downs Granite into mineralisation where most of the feldspar has been replaced by sericite + hematite + quartz: (1) a progressive Al–OH wavelength shift of 2205 nm to 2210 nm for sericite, followed by a spatially rapid reversal corresponding to lower phengite/muscovite abundance ratios; (2) progressive Mg/Fe–OH wavelength shift of 2248 nm to 2252 nm reflecting an increase in the Fe:Mg ratio of chlorite; (3) increasing ratio of microcline to orthoclase followed by a rapid decrease; and (4) slightly decreasing ratio of albite to oligoclase followed by plagioclase destruction prior to albite replacement by sericite. The HyLogger feldspar results support recent petrographic evidence for hydrothermal albite and K-feldspar at the Olympic Dam deposit, not previously reported. The spectral results from continuous HyLogger scans also show that the microscopic observations and proposed feldspar replacement reactions are not locally isolated phenomena, but are applicable at the deposit and regional-scale. A modified quartz–K-feldspar–plagioclase ternary diagram utilising mineralogy interpreted from HyLogger thermal infrared spectra (QAPTIR) diagram along with supporting data on the abundance ratios of orthoclase/microcline and albite/plagioclase, and the wavelength shifts in characteristic absorption features for sericite and chlorite, can be used as empirical vectors towards mineralisation within the Olympic Dam mineral system, with potential application to other IOCG ore-forming systems. Intrusion of Gairdner Dyke Swarm dolerite dykes into sericite ± hematite altered Roxby Downs Granite results in retrograde albite–chlorite–magnetite alteration envelopes (up to tens of metres thick) overprinting the original sericite ± hematite alteration zone and needs to be carefully evaluated to ensure that such areas are not falsely downgraded during exploration.

Signatures of Multiple Mineralization Processes in the Archean Orogenic Gold Deposit of the Pampalo Mine, Hattu Schist Belt, Eastern Finland

The Hattu schist belt is an emerging gold-producing ore district in the western part of the Archean Karelian Province of the Fennoscandian Shield. The belt consists of 2.76 to 2.70 Ga tonalite, granodiorite, and leucogranite intruded into a mafic-felsic epiclastic-volcanic sequence of an only slightly older age. Complex and successive folding, shearing, and hydrothermal processes affected these rocks prior to the lower amphibolites facies peak metamorphism (550° ± 50°C; 3–5 kbar) at ca. 2.70 Ga. Orogenic gold deposits are hosted by the highly strained zones that developed during the Archean deformation of the belt. However, previous K-Ar and Rb-Sr geochronological studies indicated that a second tectonothermal overprint affected the Hattu schist belt between 1.7 and 1.8 Ga during the Svecofennian orogeny. Results of our field, mineralogical, textural, and Pb isotope studies suggest that ore deposition at Pampalo was initiated by hydrothermal processes at the time of the emplacement of feldspar porphyry and tonalite intrusions and dikes, at around 2.72 Ga. Albitization and quartz-tourmaline-biotite-muscovite veining characterize this hydrothermal activity. The major stage of gold ore deposition can be confined to the subsequent development of high-strain zones in an intermediate-felsic tuffaceous unit (mafic schist) characterized by biotite-carbonate-pyrite alteration. Ar-Ar studies revealed the complete resetting of the Ar-Ar system in muscovite and biotite due to the Svecofennian orogeny, with closure of the isotopic system at 1.81 Ga. Results of Pb isotope studies of hydrothermal K-feldspar, galena, and altaite by laser ablation-inductively coupled plasma-mass spectrometry indicate that K-feldspar alteration and remobilization of metals also took place during the Svecofennian reactivation of high-strain zones. The petrographic manifestation of this process is the replacement of albite by hydrothermal K-feldspar. Fluid inclusion data from hydrothermal K-feldspar suggest that carbonic-aqueous, low-salinity fluids interacted with the Archean ore at 350° to 400°C and 1.8 to 2.4 kbar during the Svecofennian overprint. Results of mass transfer calculations indicate that potassium gain correlates with increase of gold concentrations in the mafic schist and feldspar-porphyry units. Therefore, the Svecofennian overprint locally also enhanced the grade of gold mineralization, mostly along competency differences of the mafic schist and feldspar porphyry blocks/dikes. Later percolation of relatively low temperature (<300°C) saline basin fluids in some fractures of the crystalline basement also left their Pb isotope and fluid inclusion signatures on the mineralization while further modifying the composition of the ore. Occurrences of hydrothermal proto-ores, as well as fluid flow events along structures reactivated by overprinting orogenic processes, are not exceptional features in Archean orogenic gold provinces. This study shows that the combination of in situ Pb isotope studies of U-poor alteration minerals and Pb-rich ore minerals, together with evaluation of relationships between gold enrichment and hydrothermal alteration and fluid inclusion studies, can be very useful in determining the significance and conditions of overprinting processes, as well as their potential implications for genetic and exploration models.

Apatite at Olympic Dam, South Australia: A petrogenetic tool

The >10,000milliontonne Olympic Dam Cu–Au–U–Ag deposit, (eastern Gawler Craton, South Australia) is one of the largest orebodies in the World. The deposit is hosted within the Olympic Dam Breccia Complex, placed at the centre of, and resulting from multiple brecciation and Fe-metasomatism of the Roxby Downs Granite (RDG). The latter is part of a larger batholith emplaced at ~1.6Ga. Apatite petrography and chemistry were studied in non-mineralised RDG and coeval granitoids and dolerites, as well as in mineralised RDG from deep (>2km) and distal (2.7km to NE) locations. In both latter cases, although the mineralisation corresponds to the same, early chalcopyrite–pyrite–magnetite±hematite stage identified in the outer and deeper zones of the deposit itself, the character of granite alteration differs: sericite–chlorite alteration with all feldspar replaced in the deep location; and red-stained K-feldspar on top of prevailing albitization in the distal location. Close-to end-member fluorapatite is a key accessory mineral in all igneous rocks and a common product of early hydrothermal alteration within mineralised granite. Variations in habit, morphology and textures correlate with chemical trends expressed as evolving Cl/F ratios, and concentrations of REE+Y (hereafter REY), Sr, Mn, S, Si and Na. Magmatic apatite is unzoned in the dolerite but features core to REY-enriched rim zonation in the granitoids. Increases in Cl- and Sr-contents correlate with rock basicity. Calculation of Cl in the vapour phase relative to melt at the apatite saturation temperature for zoned apatite in the RDG shows higher values for grains with inclusion-rich cores associated with mafic enclaves, concordant with assimilation of exotic material during magma crystallisation. Hydrothermal alteration of magmatic apatite is most varied in the dolerite where interaction with fluids is expressed as subtle changes in Cl- versus F- and REY-enrichment, and most importantly, S-enrichment in grains adjacent to crosscutting, carbonate–sulphide-bearing veins. Highly acidic fluids, which not only corroded apatite but also quartz, led to recycling of REY via subsequent crystallisation of discrete REY-bearing species, notably monazite. Hydrothermal apatite often occurs as interstitial aggregates and displays a wider range of textures indicating variability in local growth conditions and/or sustained interaction with evolving hydrothermal fluids. Apatite from the deep mineralisation is richer in Cl whereas increases in ΣREY and S correlate with higher Si in both deep and distal cases. The co-correlation between Na and S in apatite from distal mineralisation ties apatite crystallisation to replacement of albite by sericite+chlorite. The textural and chemical variation of apatite relative to magmatic to hydrothermal transition suggests that the mineral represents a petrogenetic tool to gain insights into mineralising processes at Olympic Dam. Apatite evolution clearly played a key contributing role in the elevated REY-concentrations seen at Olympic Dam. The marked differences in apatite chemistry between mineralised and non-mineralised lithologies show promise for potential application as a tool in mineral exploration, in which apatite chemistry represents a means for outlining spatial position within an alteration envelope, or identifying if a particular alteration system is barren or fertile with respect to Cu–Au mineralisation.

REPLACEMENT PROCESSES INVOLVING HIGH FIELD STRENGTH ELEMENTS IN THE T ZONE, THOR LAKE RARE-METAL DEPOSIT

The Thor Lake rare-element (Y-REE-Nb-Ta-Zr-Be) deposit, Northwest Territories, Canada, is a world-class deposit hosted by alkaline syenite and granite and has two main mineralized zones, namely the Nechalacho and T Zones deposit. The T Zone is shown to have originally been a pegmatite, but has experienced a strong hydrothermal overprint. Coarse-grained to megacrystic, elongate crystals defining the pegmatitic texture are completely pseudomorphed and are interpreted to have originally been nepheline. The T Zone underwent three main alteration stages, which are (from earliest to latest): (I) silicification of the principal phases of the pegmatite and the associated replacement of igneous aegirine and arfvedsonite by quartz + magnetite + high field strength element (HFSE)-bearing minerals; (II) polylithionite [KLi 2 AlSi 4 O 10 (F,OH) 2 ] alteration (Li metasomatism) that caused pervasive replacement of albite by polylithionite; and (III) phenakite (Be 2 SiO 4 ) alteration (Be metasomatism). Replacement of the primary mineralogy resulted in the formation of abundant pseudomorphs that are characterized by various HFSE minerals [typically, bastnasite-(Ce), zircon, rutile, xenotime-(Y), and columbite] and phenakite (the predominant Be mineral). The habit and mineralogy of the pseudomorphs, as well as textural relationships in partially altered crystals, make aegirine and mica-group minerals the most likely precursors of the HFSE-rich and Be-rich pseudomorphs, respectively. Although aegirine and arfvedsonite from alkaline rocks can contain HFSE, LA-ICP-MS analysis of aegirine and arfvedsonite from the T Zone indicates that the HFSE concentrations in these two minerals are insufficient to account for the bulk HFSE concentrations of the HFSE-rich pseudomorphs. Therefore, HFSE must have been added to the pseudomorphs during fluid-mineral interaction, requiring hydrothermal mobility of HFSE. An open-space filling process may have been involved in the precipitation of HFSE minerals, including zircon, rutile, anatase, and REE fluorcarbonates. Initial replacement of aegirine by quartz + magnetite increased porosity in some of the T Zone rocks, which may have facilitated HFSE precipitation. Our study also demonstrates that most hydrothermal zircon shows no association with fluorite and that the fluorite in REE-rich pseudomorphs commonly postdates the REE fluorcarbonate minerals [ e.g. , bastnasite-(Ce)]. These relationships indicate that HFSE precipitation in the T Zone requires a model that does not rely on the mixing of HFSE-, F-bearing fluids with Ca-rich fluids, as has been proposed for other HFSE deposits hosted by alkaline igneous rocks. As the precipitation of HFSE appears to be restricted to pseudomorphs where aegirine and micas have been replaced, the degree of hydrothermal enrichment of HFSE in the T Zone, and possibly in other deposits containing such minerals, may be dictated by the primary abundance of these minerals.

The mechanism of cation and oxygen isotope exchange in alkali feldspars under hydrothermal conditions

The mechanism of re-equilibration of albite in a hydrothermal fluid has been investigated experimentally using natural albite crystals in an aqueous KCl solution enriched in 18O at 600°C and 2 kbars pressure. The reaction is pseudomorphic and produces a rim of K-feldspar with a sharp interface on a nanoscale which moves into the parent albite with increasing reaction time. Transmission electron microscopy (TEM) diffraction contrast and X-ray powder diffraction (XRD) show that the K-feldspar has a very high defect concentration and a disordered Al, Si distribution, compared to the parent albite. Raman spectroscopy shows a frequency shift of the Si-O-Si bending vibration from ~476 cm−1 in K-feldspar formed in normal 16O aqueous solution to ~457 cm−1 in the K-feldspar formed in 18O-enriched solution, reflecting a mass-related frequency shift due to a high enrichment of 18O in the K-feldspar silicate framework. Raman mapping of the spatial distribution of the frequency shift, and hence 18O content, compared with major element distribution maps, show a 1:1 correspondence between the reaction rim formed by the replacement of albite by K-feldspar, and the oxygen isotope re-equilibration. The textural and chemical characteristics as well as the kinetics of the replacement of albite by K-feldspar are consistent with an interface-coupled dissolution-reprecipitation mechanism.

Hydrothermal alteration and magnetic properties of rocks in the Carolina de Michilla stratabound copper district, northern Chile

In the Carolina de Michilla district, northern Chile, stratabound copper mineralization is hosted by Jurassic volcanic rocks along the trace of the Atacama fault system. In this study, we present the overall effects of hydrothermal alteration on the magnetic properties of rocks in this district. Two types of metasomatic alteration associations occur, one of regional extent and the other of local hydrothermal alteration associated with copper mineralization (e.g., Lince–Estefania–Susana). Regional alteration is interpreted as a low-grade “propylitic association” characterized by an epidote–chlorite–smectite–titanite–albite–quartz–calcite association. The local hydrothermal alteration is characterized broadly by a quartz–albite–epidote–chlorite–calcite mineral assemblage. The most pervasive alteration mineral is albite, followed by epidote and, locally, actinolite. These minerals contrast sharply against host rock minerals such as chlorite, calcite, zeolite, prehnite, and pumpellyite, but alteration is constrained to mineralized bodies as narrow and low contrast alteration halos that go outwards from actinolite–albite to epidote–albite, to epidote–chlorite, and finally to chlorite. Hydrothermal alteration minerals, compared to regional alteration minerals, show iron-rich epidotes, a lower chlorite content of the chlorite–smectite series, and a nearly total albite replacement of plagioclase in the mineralized zones. Opaque minerals associated with regional alteration are magnetite and maghemite, and those associated to hydrothermal alteration are magnetite, hematite, and copper sulphides. We present paleomagnetic results from nine sites in the Michilla district and from drill cores from two mines. Local effects of hydrothermal alteration on the original magnetic mineralogy indicate similar characteristics and mineralogy, except for an increase of hematite that is spatially associated with the Cu–sulphide breccias with low magnetic susceptibilities. Results indicate that it is impossible to magnetically differentiate mineralized bodies from unmineralized lavas, except for pyrite-rich hydrothermal breccias. In conclusion, for stratabound copper deposits of the Michilla type, the overall effect of hydrothermal alteration on the paleomagnetic properties of rocks is of low contrast, not clearly discernable even at a small scale. From an exploration point of view, magnetic exploration surveys should not discern mineralized bodies of Cu–sulphide breccias except in detailed ground surveys due to the small size of contrasting bodies. Unoriented drill cores with primary ore mineralization record a characteristic remanent magnetization of reverse polarity. Taking into account the azimuth and dip of the drill cores, we were able to compare the magnetization of the mineralized bodies with the characteristic directions from sites drilled in situ from Late Jurassic–Early Cretaceous intrusives mostly. The characteristic direction recorded by the Pluton Viera is similar to the magnetization of the ore bodies of the Estefania mine. If copper mineralization mostly postdates the tilt of the volcanic flows, the low paleomagnetic inclinations suggest an age for the mineralization near 145 Ma, the time of the lowest paleolatitude for the South American plate during the Mesozoic.

Copper-gold endoskarns and high-Mg monzodiorite–tonalite intrusions at Mt. Shea, Kalgoorlie, Australia: implications for the origin of gold–pyrite–tennantite mineralization in the Golden Mile

Five Cu–Au epidote skarns are associated with the Mt. Shea intrusive complex, located in the 2.7–2.6 Ga Eastern Goldfields Province of the Archean Yilgarn craton, in greenstones bounded by the Boulder Lefroy and Golden Mile strike-slip faults, which control the Golden Mile (1,435 t Au) at Kalgoorlie and smaller “orogenic” gold deposits at Kambalda. The Cu–Au deposits studied are oxidized endoskarns replacing faulted and fractured quartz monzodiorite–granodiorite. The orebodies are up to 140 m long and 40 m thick. Typical grades are 0.5% Cu and 0.3 g/t Au although parts are richer in gold (1.5–4.5 g/t). At the Hannan South mine, the skarns consist of epidote, calcite, chlorite, magnetite (5–15%), and minor quartz, muscovite, and microcline. Gangue and magnetite are in equilibrium contact with pyrite and chalcopyrite. The As–Co–Ni-bearing pyrite contains inclusions of hematite, gold, and electrum and is intergrown with cobaltite and Cu–Pb–Bi sulfides. At the Shea prospect, massive, net-textured, and breccia skarns are composed of multistage epidote, actinolite, albite, magnetite (5%), and minor biotite, calcite, and quartz. Gangue and magnetite are in equilibrium with Co–Ni pyrite and chalcopyrite. Mineral-pair thermometry, mass-balance calculations, and stable-isotope data (pyrite δ34SCDT = 2.5‰, calcite δ13CPDB = −5.3‰, and δ18OSMOW = 12.9‰) indicate that the Cu–Au skarns formed at 500 ± 50°C by intense Ca–Fe–CO2–S metasomatism from fluids marked by an igneous isotope signature. The Mt. Shea stock–dike–sill complex postdates the regional D1 folding and metamorphism and the main phase of D2 strike-slip faulting. The suite is calc-akaline and comprises hornblende–plagioclase monzodiorite, quartz monzodiorite, granodiorite, and quartz–plagioclase tonalite porphyry. The intrusions display a wide range in silica content (53–73 wt% SiO2), in $$ {{\text{Mg}}} \mathord{\left/ {\vphantom {{{\text{Mg}}} {{\left( {{\text{Mg}} + {\text{Fe}}_{{{\text{TOTAL}}}} } \right)}}}} \right. \kern-\nulldelimiterspace} {{\left( {{\text{Mg}} + {\text{Fe}}_{{{\text{TOTAL}}}} } \right)}} $$ ratio (0.37–0.89), and in $$ {\text{K}} \mathord{\left/ {\vphantom {{\text{K}} {{\left( {{\text{K}} + {\text{Na}}} \right)}}}} \right. \kern-\nulldelimiterspace} {{\left( {{\text{K}} + {\text{Na}}} \right)}} $$ ratio (0.02–0.31). Chromium (62–345 ppm), Ni (23–158), Sr (311–1361 ppm), and Ba (250–2,581 ppm) contents are high, Sr/Y ratios are high (24–278, mostly >50), and the rare earth element patterns are fractionated $$ {\left( {{{\text{Ce}}_{N} } \mathord{\left/ {\vphantom {{{\text{Ce}}_{N} } {{\text{Yb}}_{N} = 17 - 41}}} \right. \kern-\nulldelimiterspace} {{\text{Yb}}_{N} = 17 - 41}} \right)} $$ . These features and a negative niobium anomaly relative to the normal mid-ocean ridge basalt indicate that the suite formed by hornblende fractionation from a subduction-related monzodiorite magma sourced from metasomatized peridotite in the upper mantle. The magnesian composition of many intrusions was enhanced due to hornblende crystallization under oxidizing hydrous conditions and during the subsequent destruction of igneous magnetite by subsolidus actinolite–albite alteration. At the Shea prospect, main-stage Cu–Au epidote skarn is cut by biotite–albite–dolomite schist and by red biotite–albite replacement bands. Post-skarn alteration includes 20-m-thick zones of sericite–chlorite–ankerite schist confined to two D3 reverse faults. The schists are mineralized with magnetite + pyrite + chalcopyrite (up to 0.62% Cu, 1.6 g/t Au) and are linked to skarn formation by shared Ca–Fe–CO2 metasomatism. Red sericitic alteration, marked by magnetite + hematite + pyrite, occurs in fractured porphyry. The biotite/sericite alteration and oxidized ore assemblages at the Shea prospect are mineralogically identical to magnetite–hematite-bearing gold lodes at Kambalda and in the Golden Mile. Published fluid inclusion data suggest that a “high-pressure”, oxidized magmatic fluid (2–9 wt% NaCl equivalent, $$ X_{{{\text{CO}}_{2} }} = 0.1 - 0.2 $$ , 200–400 MPa) was responsible for gold mineralization in structural sites of the Boulder Lefroy and Golden Mile faults. The sericite–alkerite lodes in the Golden Mile share the assemblages pyrite + tennantite + chalcopyrite and bornite + pyrite, and accessory high-sulfidation enargite with late-stage sericitic alteration zones developed above porphyry copper deposits.

Occurrence and late re-equilibration of pollucite from the Koktokay no. 3 pegmatite, Altai, northwestern China

The Koktokay no. 3 pegmatite, Altai, NW China, is a strongly zoned spodumene-subtype pegmatite. Pollucite is the unique ore mineral exploited for Cs in this pegmatite. It occurs in internal textural zones of the pegmatite in different abundances. Primary pollucite is largely homogeneous, but it displays a broad range of composition between different textural zones with CRK [=100(Cs + Rb + K)/(Na + K + Rb + Cs + Mg + Ca)] = 74–86 and Si/Al = 2.20–2.51. Four principal types of pollucite were distinguished. (1) First is pollucite with blebby mosaic texture consisting of Na-enriched and Cs-enriched phases, which clearly resulted from local exsolution of primary pollucite in the sub-solidus state. (2) The second type is nearly end-member pollucite (CRK > 90) that occurs as an aureole of primary pollucite in contact with lepidolite or feldspar clusters. This is formed by dissolution/re-precipitation of primary pollucite. (3) Symplectic pollucite is associated with quartz, observed in sub-parallel veinlets penetrating surrounding albite crystals; compositionally, this pollucite attains a CRK ratio of up to 96 and resulted from replacement of albite by Cs-rich fluids. (4) Oscillatory-zoned pollucite is the fourth type, typically restricted to the contact of pollucite with small miarolitic cavities. The zonation compositionally oscillates about the Cs/Na variations, and is simply related to locally changing fluid composition.

Hydrothermal alteration in metasedimentary rock-hosted orogenic gold deposits, Reefton goldfield, South Island, New Zealand

Orogenic or mesothermal quartz lodes in lower Palaeozoic Greenland Group metasedimentary rocks of the Reefton area have produced 67 tonnes (t) of gold prior to 1951, and recent exploration has identified new gold resources in several deposits, including the largest past producers, Blackwater and Globe-Progress. The metasedimentary rocks consist of alternating sandstone and mudstone beds that were metamorphosed to lower greenschist facies prior to being hydrothermally altered adjacent to the quartz lodes. The sandstones are feldspathic litharenites averaging Q65–F10–R25, with detrital grains of quartz, rock fragments, muscovite, and plagioclase and biotite that were altered to albite and chlorite, respectively, during metamorphism. Accessory minerals are graphite, apatite, zircon, tourmaline and titanite. Hydrothermal alteration of the sandstones has developed a mineral assemblage of K-mica, carbonate (dolomite, ankerite, ferroan magnesite and magnesian siderite), chlorite, pyrite and arsenopyrite. The abundance of hydrothermal chlorite is greater at Blackwater than at the other prospects studied. Hydrothermal alteration associated with the quartz lodes is marked by bleaching, magnesian siderite spots, disseminated arsenopyrite and pyrite and thin carbonate, quartz and sulphide veins. These trends are accompanied by increasing concentrations of S, As and Sb and decreasing Na, and a decrease of Fe and Mg in K-mica. The alkali alteration indices 3K/Al (representing K-mica) and Na/Al (representing albite) generally show antipathetic trends, with 3K/Al increasing near the lodes and Na/Al decreasing. These trends reflect the replacement of albite by K-mica. Carbonate alteration indices CO2/(Ca + Mg +Fe) and CO2/[Ca + Mg + Fe –0.5(S + As)] quantify the abundance of hydrothermal carbonates, but they show variable correlation with the lodes. They increase the width of the alteration halo in the hanging wall of the lodes at the Globe-Progress and General Gordon prospects, but the peak values are as far as 150 m from the lodes. By contrast, peak values of the carbonate alteration indices are within 10 and 2 m of the lodes, respectively, at the Merrijigs and Blackwater deposits. Data show that for deposits with wide hydrothermal alteration halos, such as at the Globe-Progress and General Gordon prospects, the use of a suite of geochemical indicators can assist exploration by indicating trends in hydrothermal alteration that provide vectors to mineralisation. They also increase the size of the exploration target. By contrast, the alteration halo of the Blackwater deposit is restricted to within less than 5 m of the quartz lode and, therefore, the geochemical indicators are of more limited assistance to exploration.

Geology of the Chuquicamata Mine: A Progress Report

Chuquicamata, in northern Chile, is the world’s greatest copper orebody. It was controlled, from the initial intrusions (probably at 36–33 Ma) through mineralization (last major hydrothermal event at 31 Ma) to postmineral brecciation and offset, by the West fault system. East porphyry, West porphyry, Banco porphyry, and Fine Texture porphyry make up the Chuqui Porphyry Complex. East porphyry, the dominant host rock, has a coarse, hypidiomorphic-granular texture. Intrusive contacts between most porphyries have not been found, but early ductile deformation, subsequent pervasive cataclastic deformation, and faulting affects all of the rocks and makes recognition of intrusive contacts very difficult. Potassic alteration affects all porphyries, comprises partial K feldspar and albite replacement of plagioclase, and more widespread biotite replacement of hornblende, with igneous texture largely preserved. It is accompanied by granular quartz and quartz-K feldspar veinlets, which contain only trace disseminated chalcopyrite ± bornite remains from this early stage. Fine-grained quartz-K feldspar alteration, with destruction of biotite and apparently following albitization of plagioclase, accompanies strongest cataclastic deformation and destruction of igneous texture. A band of quartz-K feldspar alteration, up to 200 m wide and 1,500 m long, lies along the southward extension of Banco porphyry dikes and is the locus of the bornite-digenite center of the sulfide zoning pattern. This passes east through chalcopyrite-bornite to chalcopyrite-pyrite as sulfide abundance fades out. Sulfides in quartz-K feldspar alteration are abundant only where there is intense crackle brecciation. Propylitic alteration is superimposed on biotitic alteration at the eastern edge of the deposit, but there is no pyritic fringe. Westward, this zoning is interrupted by the superposition of pyritic main-stage veins with pervasive quartz-sericite. Veins of quartz-molybdenite, up to 5 m wide and cutting all porphyries, were emplaced between the early and the main stages. These veins and early-stage quartz veins are commonly segmented and sheared, with fine recrystallization of quartz that eliminates all original fluid inclusions. It is not clear whether quartz-K feldspar alteration was formed later or earlier than quartz-molybdenite veins. Main-stage veins were focused along a structural zone adjacent to the West fault. This stage is distinctly younger than early-stage mineralization, although it occupies many of the same structures and may involve massive remobilization of earlier mineralization. It may represent a more brittle and much shallower environment, which followed significant erosion of the upper parts of the early mineralization system. Main-stage veins with quartz, pyrite, chalcopyrite, and bornite were formed during dextral shear of the West fault system. The last mineralization of the main stage was enargite, digenite, covellite, pyrite, and minor coarse sphalerite, along with sericite, and locally alunite but only local traces of pyrophyllite and dickite. Some northwest enargite veins were apparently opened after the sense of shear on the West fault system changed to sinistral. Vein and veinlet filling faults and fault-related shatter zones contain the overwhelming proportion of copper at Chuquicamata in all alteration zones and assemblages, including pyrite-free early-stage assemblages. Practically all of these fractures have been opened and mineralized more than once. A still poorly understood late stage formed digenite with relatively coarse grained covellite from deep in the sericitic zone and flaring upward and outward under what became the supergene chalcocite enrichment blanket. The presence of associated anhydrite, typical also of earlier stages but largely leached or hydrated to gypsum by later supergene action, proves this is not supergene covellite, but it is otherwise very difficult to distinguish from supergene covellite. Rims of sphalerite on primary sulfides, almost invariably with inner rims of coarse-grained covellite and/or digenite, occur below the chalcocite blanket from which Zn has been leached. The sphalerite rims are interpreted by most as supergene, but the close association with apparently late hydrothermal covellite-digenite and their absence in all other porphyry copper deposits suggests they too may be hydrothermal. A partially preserved leached capping and oxide copper ore, replacing an upper chalcocite blanket, overlie a high-grade supergene chalcocite body that extends up to 800 m in depth in the zone of fault brecciation and pervasive main-stage sericitic alteration. Some leached copper moved laterally to form exotic copper oxides and silicate in adjacent gravels. Continued movement on the West fault produced a wide zone of brecciation and major displacements of mineralized rock. Net sinistral displacement of about 35 km is indicated by regional mapping, but the details of how much of each stage of mineralization was displaced and how far on which splits of the fault are not well understood. The uniqueness of Chuquicamata is due to its intimate and complex relationship with active regional faulting and to superposition of at least two distinct periods of mineralization.

Modeling fluid–rock interaction during greisenization at the East Kemptville tin deposit: implications for mineralization

Interaction between a tin-bearing greisenizing fluid and a leucogranitic rock was modeled in order to evaluate the possible role of alteration induced changes in fluid chemistry in cassiterite deposition. The interaction was modeled using simple titration and multiple-pass fluid flow models with the program CHILLER, modified to permit calculations at 450°C and 4.0 kb. The compositions of the mineralizing fluid and the unaltered leucogranite were based, respectively, on analyses of fluid inclusions and whole-rock chemistry, of samples from the East Kemptville greisen-hosted tin deposit, Nova Scotia. The starting fluid contained 4.85 M Cl, 3.7 M Na, 0.53 M Fe, and minor K, Si, F and Sn, and was at a pH of 3.0, while the leucogranite contained 35 wt.% quartz, 29 wt.% albite, 19 wt.% muscovite and 17 wt.% K-feldspar. In both models, the calculations show that with increasing alteration, K-feldspar was the first mineral consumed, and that this was followed by replacement of albite by muscovite. With further alteration, muscovite was replaced by topaz and pyrrhotite, and cassiterite precipitated in quartz–topaz rocks. In the most altered rocks, only topaz and quartz are present. These changes accurately reproduce those determined for natural samples at East Kemptville. However, only the multiple-pass fluid flow model predicts the changes in fluid inclusion composition. The multiple-pass fluid flow model quantified changes in fluid composition as a function of its interaction with the wall rock and thus enabled us to assess better the factors that controlled cassiterite precipitation. Important conclusions of the model are that the fluid and the rock equilibrated at each stage of the alteration and the water/rock ratio remained constant across the alteration halo. We further concluded that phases precipitated close to the vein in an earlier stage of the alteration were re-dissolved with evolution of the mineralizing system. In particular, re-dissolution of cassiterite caused an increase in the concentration of SnCl + in the fluid. As a result, cassiterite saturated at a lower pH, and the chemical potential of SnCl + increased away from the vein. This could, in turn, explain the low tin grades at East Kemptville.

P22. Standards for backscattered electron (BSE) imaging of skeletal and dental tissues

The > 10,000 million tonne Olympic Dam Cu–Au–U–Ag deposit, (eastern Gawler Craton, South Australia) is one of the largest orebodies in the World. The deposit is hosted within the Olympic Dam Breccia Complex, placed at the centre of, and resulting from multiple brecciation and Fe-metasomatism of the Roxby Downs Granite (RDG). The latter is part of a larger batholith emplaced at ~ 1.6 Ga. Apatite petrography and chemistry were studied in non-mineralised RDG and coeval granitoids and dolerites, as well as in mineralised RDG from deep (> 2 km) and distal (2.7 km to NE) locations. In both latter cases, although the mineralisation corresponds to the same, early chalcopyrite–pyrite–magnetite ± hematite stage identified in the outer and deeper zones of the deposit itself, the character of granite alteration differs: sericite–chlorite alteration with all feldspar replaced in the deep location; and red-stained K-feldspar on top of prevailing albitization in the distal location. Close-to end-member fluorapatite is a key accessory mineral in all igneous rocks and a common product of early hydrothermal alteration within mineralised granite. Variations in habit, morphology and textures correlate with chemical trends expressed as evolving Cl/F ratios, and concentrations of REE + Y (hereafter REY), Sr, Mn, S, Si and Na. Magmatic apatite is unzoned in the dolerite but features core to REY-enriched rim zonation in the granitoids. Increases in Cl- and Sr-contents correlate with rock basicity. Calculation of Cl in the vapour phase relative to melt at the apatite saturation temperature for zoned apatite in the RDG shows higher values for grains with inclusion-rich cores associated with mafic enclaves, concordant with assimilation of exotic material during magma crystallisation. Hydrothermal alteration of magmatic apatite is most varied in the dolerite where interaction with fluids is expressed as subtle changes in Cl- versus F- and REY-enrichment, and most importantly, S-enrichment in grains adjacent to crosscutting, carbonate–sulphide-bearing veins. Highly acidic fluids, which not only corroded apatite but also quartz, led to recycling of REY via subsequent crystallisation of discrete REY-bearing species, notably monazite. Hydrothermal apatite often occurs as interstitial aggregates and displays a wider range of textures indicating variability in local growth conditions and/or sustained interaction with evolving hydrothermal fluids. Apatite from the deep mineralisation is richer in Cl whereas increases in ΣREY and S correlate with higher Si in both deep and distal cases. The co-correlation between Na and S in apatite from distal mineralisation ties apatite crystallisation to replacement of albite by sericite + chlorite. The textural and chemical variation of apatite relative to magmatic to hydrothermal transition suggests that the mineral represents a petrogenetic tool to gain insights into mineralising processes at Olympic Dam. Apatite evolution clearly played a key contributing role in the elevated REY-concentrations seen at Olympic Dam. The marked differences in apatite chemistry between mineralised and non-mineralised lithologies show promise for potential application as a tool in mineral exploration, in which apatite chemistry represents a means for outlining spatial position within an alteration envelope, or identifying if a particular alteration system is barren or fertile with respect to Cu–Au mineralisation.

Differential stress control on the growth and orientation of flame perthite: A palaeostress-direction indicator

Albite flames result from the replacement of K-feldspar by albite through alkali exchange. In flame perthites from deformed granites within the Grenville Front Tectonic Zone, replacement is driven by retrograde breakdown of plagioclase and hydration of K-feldspar. Within the strain fabric of the host rock, albite flames preferentially form subparallel to the inferred maximum compression direction and also develop at high-stress points such as grain-to-grain contacts. Flame tips are generally parallel to the ‘normal’ perthite crystallographic direction i.e. the Murchison plane, which is the orientation of the plane of minimum crystallographic lattice misfit between the albite and K-feldspar. We propose that the morphology of an albite flame is controlled by the coherent propagation of its tip into the K-feldspar crystal lattice, and that the orientation of the flames with respect to the rock fabric is controlled by the differential stress imposed during metamorphism. This type of replacement requires dry conditions in which there would be inefficient dissolution-reprecipitation along completely incoherent interfaces (normal replacement), and increased strength of feldspar. Under these conditions coherency is maintained to avoid a very large kinetic barrier. The differences in lattice misfit along the Murchison plane between K-feldspar and albite requires either high elastic strain energy (if the fit is coherent), or lattice dislocation energy (if the fit is semi-coherent). Such energy might be expected to inhibit the replacement growth of albite lamellae in a host feldspar. However, compression parallel to the Murchison plane reduces the lattice misfit along the compression direction. This results (1) in an increase in the Helmholtz free energy of the K-feldspar, and (2) in a reduction in the Helmholtz energy of the albite lamellae or of the semi-coherent interface. Thus, albite replacement allows a decrease in net free energy without the destruction of the alumino-silicate framework and is therefore favoured under conditions of high differential stress. Since flame growth is controlled by the imposed differential stress and not by strain in the host rock, flame perthite has potential as a palaeostress-direction indicator.

The role of greisenization in cassiterite precipitation at the East Kemptville tin deposit, Nova Scotia

The role of fluid-rock (leucogranite) interaction in forming the greisen-hosted tin (Cu-Zn-Ag) deposit at East-Kemptville, Nova Scotia was investigated by evaluating compositional and mineralogical changes in a representative zoned greisen, and by using this information to estimate physicochemical changes in the fluid-rock system at each step of the alteration. A large sample comprising one side of a symmetrically zoned greisen was cut into 1.5-cm-wide slabs and each slab was analyzed for its major and trace element content. The results of these analyses were used to calculate the modal mineralogy of each slab which, in conjunction with petrographic observations, revealed two main alteration zones. First, or distal to the vein, the leucogranite was converted to a quartz-sericite greisen through the replacement of albite by muscovite. This alteration, specifically the dissolution of albite, created some porosity. Closer to the vein, the neoformed muscovite was converted to topaz and quartz, creating a quartz-topaz greisen; pyrite, sphalerite and cassiterite precipitated in the earlier created pore space.Mass balance calculations show that during alteration almost all major elements, including aluminum, were mobile. Only silicon was essentially immobile because the fluid was saturated with quartz due to earlier interaction with the leucogranite. The alteration of the leucogranite resulted in a quantitative removal of the alkalies, Mg and Mn, and an addition of Fe, S, F, Zn, and Sn. The difference in the modal mineralogy of each pair of adjacent slabs was used to quantify mineralogical changes in the two main alteration reactions responsible for the observed zonation. Variations in the molar abundances of species in solution were evaluated by balancing the alteration reactions and by estimating the water/rock ratios at each step of the alteration using oxygen isotope data. The study indicates that there was an increase in the pH of the fluid and a decrease in f (sub O 2 ) during alteration, and that these changes were most important close to the vein. Cassiterite deposition occurred contemporaneously with the alteration of muscovite to topaz and quartz and is interpreted to have been caused by the sharp increase in pH that accompanied this reaction. Changes in f (sub O 2 ) and Cl (super -) acted against cassiterite precipitation, but were apparently insufficient to prevent its saturation.