Aluminum Phosphate-sulfate Minerals Research Articles

LA-ICP-MS Mapping of Barren Sandstone from the Proterozoic Athabasca Basin (Canada)—Footprint of U- and REE-Rich Basinal Fluids

The Proterozoic Athabasca Basin hosts a large number of high-grade, large-tonnage unconformity-related uranium (U) deposits, many of which are also enriched in rare earth elements (REE). The basin also contains hydrothermal REE mineralization unassociated with U. Previous studies postulated that U and REE were derived from either the basin or the basement; however, the exact source of the metals remains ambiguous. This study provides evidence of U- and REE-rich fluids throughout the Athabasca Basin through laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) mapping of barren sandstone distal to mineralized areas. The results indicate that elevated U and REE concentrations mainly occur in the matrix; there are strong positive correlations between U and REE, Th, P and Sr, and moderate positive correlations between U and Zr, Ba, Fe, Al, K and Ca, but the few spots with the highest U are unrelated to these elements. Quantitative evaluation of the element correlations, together with scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis, suggests that most of the elevated U and REE are hosted in aluminum phosphate sulfate (APS) minerals rather than apatite and monazite. As the APS minerals are of diagenetic-hydrothermal origin, the results testify to the presence of U- and REE-rich fluids within the Athabasca Basin. The elevated Th/U ratio (~10) and REE pattern (strong heavy rare earth element (HREE) depletion) are consistent with a model in which large amounts of U and REE (especially HREE) were leached from the sandstone within the Athabasca Basin and contributed to U and REE mineralization near the unconformity between the sedimentary rocks in the basin and underlying basement rocks. This study demonstrates that LA-ICP-MS mapping can be effectively used to evaluate microscale distribution of elements and their mobility in sedimentary rocks to address mineralization related problems.

Alteration mineralogy and pathfinder element inventory in the footprint of the McArthur River unconformity-related uranium deposit, Canada

ABSTRACTPathfinder elements associated with the exploration footprint of the McArthur River unconformity-related U deposit include U, radiogenic Pb, V, Ni, Co, Cu, Mo, As, Zn, and rare earth elements. In this study, the mineralogical and paragenetic context for their occurrence was established by integrating in situ mineral chemistry and laser ablation mass spectrometry chemical mapping of interstitial assemblages, detrital grains, and cements with whole-rock analyses of drill core samples from the diagenetically altered background and the hydrothermally altered sandstone host rocks. Diagenetically altered background sandstones contain a matrix assemblage of illite and dickite, with trace to minor aluminum-phosphate-sulfate (APS) minerals, apatite, and Fe-Ti oxide minerals. Aluminum-phosphate-sulfate minerals account for the majority of the Sr and light rare earth element concentrations, whereas early diagenetic apatite, monazite, and apatite inclusions in detrital quartz and detrital zircon contribute significant U and heavy rare earth elements to samples analyzed with an aggressive leach (partial digestion) such as aqua regia. Hydrothermally altered sandstone host rocks also contain variable assemblages of Al-Mg chlorite (sudoite), alkali-deficient tourmaline, APS minerals, kaolinite, illite, Fe-oxide, and sulfide minerals. Late pre-mineralization chlorite accounts for a significant portion of the observed Ni concentrations, whereas Co, Cu, Mo, and Zn occur predominantly in cryptic sub-micron sulfide and sulfarsenide inclusions within clay mineral aggregates and in association with Fe-Ti oxides. Elevated concentrations of U were observed in cryptic micro-inclusions associated with sulfides in quartz overgrowths, with Fe-Ti oxide micro-inclusions in kaolinite, and in post-mineralization Fe-oxide veins. The distribution of pathfinder elements throughout the deposit footprint appears to be less related to the primary dispersion of alteration minerals from the hydrothermal system than to the secondary dispersion of elements post-mineralization. Their occurrence throughout pre-, syn-, and post-mineralization assemblages further demonstrates the limitations to defining geochemical footprints from pathfinder element concentrations expressed in lithogeochemical data sets without structural, lithological, and mineralogical context.

Mineralogical Study of the Advanced Argillic Alteration Zone at the Konos Hill Mo–Cu–Re–Au Porphyry Prospect, NE Greece

The Konos Hill prospect in NE Greece represents a telescoped Mo–Cu–Re–Au porphyry occurrence overprinted by deep-level high-sulfidation mineralization. Porphyry-style mineralization is exposed in the deeper parts of the system and comprises quartz stockwork veins hosted in subvolcanic intrusions of granodioritic composition. Ore minerals include pyrite, molybdenite, chalcopyrite, and rheniite. In the upper part of the system, intense hydrothermal alteration resulted in the formation of a silicified zone and the development of various advanced argillic alteration assemblages, which are spatially related to N–S, NNW–SSE, and E–W trending faults. More distal and downwards, advanced argillic alteration gradually evolves into phyllic assemblages dominated by quartz and sericite. Zunyite, along with various amounts of quartz, alunite, aluminum phosphate–sulfate minerals (APS), diaspore, kaolinite, and minor pyrophyllite, are the main minerals in the advanced argillic alteration. Mineral-chemical analyses reveal significant variance in the SiO2, F, and Cl content of zunyite. Alunite supergroup minerals display a wide compositional range corresponding to members of the alunite, beudantite, and plumbogummite subgroups. Diaspore displays an almost stoichiometric composition. Mineralization in the lithocap consists of pyrite, enargite, tetrahedrite/tennantite, and colusite. Bulk ore analyses of mineralized samples show a relative enrichment in elements such as Se, Mo, and Bi, which supports a genetic link between the studied lithocap and the underlying Konos Hill porphyry-style mineralization. The occurrence of advanced argillic alteration assemblages along the N–S, NNW–SSE, and E–W trending faults suggests that highly acidic hydrothermal fluids were ascending into the lithocap environment. Zunyite, along with diaspore, pyrophyllite, and Sr- and Rare Earth Elements-bearing APS minerals, mark the proximity of the hypogene advanced argillic alteration zone to the porphyry environment.

Geochemistry of advanced argillic altered rocks in the area of Breznik, Western Srednogorie Unit (Bulgaria)

The distribution and behaviour of major and trace elements in the main zones of hydrothermal alteration (propylite, sericite and advanced argillic) in the area of Breznik Town have been studied, coupled with the chemical composition of alunite group minerals and their isotope geochemistry. The content of Sr decreases in propylite and especially in sericite rocks, and increases significantly in advanced argillic altered (AAA) rocks, which is connected with the formation of aluminium phosphate-sulphate (APS) minerals. Rubidium has high content in sericitic rocks, whereas the element is almost depleted in AAA rocks. The Rb/Sr ratio increases in sericitic rocks and strongly decreases in AAA rocks. Zirconium, Ti, Cr, V, Ga, Hf, Nb, Ta, Th, U and partially Sn and Ba have comparatively inert behaviour during alteration. Manganese, Zn, Ni, Co, Cs and Y are mobile in different degree, with a trend toward complete extraction in the most altered rocks. Arsenium concentrates in AAA rocks, while Li is concentrated only in kaolinite AAA rocks. LREE are comparatively inert in AAA rocks, while MREE and HREE are very mobile, especially in alunite rocks. Alunite has Na-K composition, with permanent admixtures of Ca, Sr, Ba and LREE. APS minerals themselves are woodhouseite and svanbergite-woodhouseite solid solutions (s.s.). Stable isotopes of alunite are characteristic for magmatic-hydrothermal alunite, whereas δ34S of jarosite is intermediate between δ34S of alunite and sulphides, which confirms its supergene origin. All these features define a high-sulphidation epithermal environment, and a possible porphyry mineralization at deeper part of the system is suggested.

Characterizing fluids associated with the McArthur River U deposit, Canada, based on tourmaline trace element and stable (B, H) isotope compositions

The origin and nature of fluids responsible for U deposits of the Athabasca Basin have been in debate. This study presents the trace element abundances and stable isotope ratios (D/H and 11B/10B) of well characterized tourmaline samples determined by in situ analytical techniques, in order to evaluate the nature of fluids during the mineralization of the McArthur River U deposit, the world's largest high-grade U deposit. Magnesio-foitite, an alkali-deficient Mg-rich tourmaline, is abundant along the 13-km long P2 fault, a reverse structure that controls the location of the McArthur River deposit. Magnesio-foitite contains variable concentrations of rare earth elements (REEs, up to 46ppm in total) and Y (up to 35ppm). Chondrite normalized patterns indicate low light REEs (LREEs) relative to heavy REEs (HREEs) (CeN<YN, Y as a proxy for HREEs) and slight negative Eu anomalies, similar to those of uraninite from the deposit. The data combined with textural evidence suggest that magnesio-foitite and uraninite co-crystallized with LREE-rich aluminum phosphate sulfate minerals. Relatively high REE contents of magnesio-foitite in the ore zone indicates the incursion of a REE-rich basement fluid. Low values of δD (−98 to −41‰) for magnesio-foitite suggests that the mineralizing fluid originated from groundwater. Variably high values of δ11B (+13.1 to +23.2‰) are explained by the dissolution of B from carbonate or evaporitic rocks and preferential removal of 10B by the crystallization of illite and kaolinite. The results of this study support the ascent of a REE-rich basement fluid during mineralization and extensive modification of basinal fluids through the crystallization of clay minerals.

Zonation of Sulfate and Sulfide Minerals and Isotopic Composition in the Far Southeast Porphyry and Lepanto Epithermal Cu–Au Deposits, Philippines

AbstractThe world‐class Far Southeast (FSE) porphyry system, Philippines, includes the FSE Cu–Au porphyry deposit, the Lepanto Cu–Au high‐sulfidation deposit and the Victoria–Teresa Au–Ag intermediate‐sulfidation veins, centered on the intrusive complex of dioritic composition. The Lepanto and FSE deposits are genetically related and both share an evolution characterized by early stage 1 alteration (deep FSE potassic, shallow Lepanto advanced argillic‐silicic, both at ~1.4 Ma), followed by stage 2 phyllic alteration (at ~1.3 Ma); the dominant ore mineral deposition within the FSE porphyry and the Lepanto epithermal deposits occurred during stage 2. We determined the chemical and S isotopic composition of sulfate and sulfide minerals from Lepanto, including stage 1 alunite (12 to 28 permil), aluminum–phosphate–sulfate (APS) minerals (14 to 21 permil) and pyrite (−4 to 2 permil), stage 2 sulfides (mainly enargite–luzonite and some pyrite, −10 to −1 permil), and late stage 2 sulfates (barite and anhydrite, 21 to 27 permil). The minerals from FSE include stage 2 chalcopyrite (1.6 to 2.6 permil), pyrite (1.1 to 3.4 permil) and anhydrite (13 to 25 permil). The whole‐rock S isotopic composition of weakly altered syn‐mineral intrusions is 2.0 permil.Stage 1 quartz–alunite–pyrite of the Lepanto lithocap, above about 650 m elevation, formed from acidic condensates of magmatic vapor at the same time as hypersaline liquid formed potassic alteration (biotite) near sea level. The S isotopic composition of stage 1 alunite–pyrite record temperatures of approximately 300–400°C for the vapor condensate directly over the porphyry deposit; this cooled to &lt;250°C as the acidic condensate flowed to the NW along the Lepanto fault where it cut the unconformity at the top of the basement. Stage 1 alunite at the base of the advanced argillic lithocap over FSE contains cores of APS minerals with Sr, Ba and Ca; based on back‐scattered electron images and ion microprobe data, these APS minerals show a large degree of chemical and S‐isotopic heterogeneity within and between samples. The variation in S isotopic values in these finely banded stage 1 alunite and APS minerals (16 permil range), as well as that of pyrite (6 permil range) was due largely to changes in temperature, and perhaps variation in redox conditions (average ~ 2:1 H2S:SO4). Such fluctuations could have been related to fluid pulses caused by injection of mafic melt into the diorite magma chamber, supported by mafic xenoliths hosted in diorite of an earlier intrusion.The S isotopic values of stage 2 minerals indicate temperatures as high as 400°C near sea level in the porphyry deposit, associated with a relatively reduced fluid (~10:1 H2S:SO4) responsible for deposition of chalcopyrite. Stage 2 fluids were relatively oxidized in the Lepanto lithocap, with an H2S:SO4 ratio of about 4. The oxidation resulted from cooling, which was caused by boiling during ascent and then dilution with steam‐heated meteoric water in the lithocap. This cooling also resulted in the sulfidation state of minerals increasing from chalcopyrite stability in the porphyry deposit to that of enargite in the lithocap‐hosted high‐sulfidation deposit. The temperature at the base of the lithocap during stage 2 was ≥300°C, cooling to &lt;250°C within the main lithocap, and about 200°C towards the limit of the Lepanto orebody, approximately 2 km NW of the porphyry deposit. Approximate 300°C and 200°C isotherms, estimated from S isotopic and fluid inclusion temperatures during stage 1 and stage 2, shifted towards the core of the FSE porphyry deposit with time. This general retreat in isotherms was more than 500 m laterally within Lepanto and 500 m vertically within FSE as the magmatic–hydrothermal system evolved and collapsed over the magmatic center. During this evolution, there is also evidence recorded by large S isotopic variations in individual crystals for sharp pulses of higher temperature, relatively reduced fluid injected into the porphyry deposit.

Composition of Large Zoned Aluminum Phosphate Sulfate Minerals: Implications For Fluid Evolution In the Centennial Uranium Deposit Area, Athabasca Basin, Saskatchewan, Canada

Abstract Aluminum phosphate sulfate (APS) minerals associated with the Athabasca Basin are compositionally part of the alunite group. Typically 5–20 μm in size, they accompany the regional diagenetic-hydrothermal illite-kaolinite-dickite assemblage in the Athabasca Group sandstones, the illite-sudoite assemblage of the altered paleo-regolith, and alteration zones surrounding uranium deposits. In the vicinity of the Centennial deposit, where illite and sudoite replace coarse-grained aluminosilicate porphyroblasts in the basement phyllitic pelites, cubic APS crystals are as large as 80 μm. Detailed petrography indicates that the APS crystals form broadly coeval to illite. Backscattered electron imaging, elemental mapping, and compositional analysis reveal complex zoning as well as later hydrothermal alteration of the APS crystals. Growth-zoned crystals are a solid solution between crandallite-goyazite-svanbergite [Sr 0.17–0.32 Ca 0.0.18–0.28 LREE 0.0.25–0.46 (Al 2.78–2.92 Fe 0.01–014 )(PO 4 ) 1.82–2.03 (SO 4 ) 0.14–0.35 (OH) 6 ], whereas the later alteration of the APS minerals results in a compositional shift to endmember florencite [Sr 0.15 Ca 0.11 LREE 0.57 (Al 2.86 Fe 0.02 )(PO 4 ) 1.98 (SO 4 ) 0.15 (OH) 6 ]. Fluids responsible for the alteration of APS to florencite are paragenetically linked to late hydrothermal fluids associated with mafic Mackenzie dikes and do not appear to be related to proximity to uranium mineralization. Both zoned and altered portions of the crystals have bulk compositions that overlap with APS minerals in other areas of the basin suggesting a common genetic origin. However, it is critical to link the paragenetic context to observed compositional changes. An increase in MREE from early to late stages of zoned crystal growth correlates with the greatest concentration of REE found in uraninite from unconformity-related uranium deposits. This could be a link between broader APS growth and uraninite precipitation.

Quantifying aluminium phosphate–sulphate minerals as markers of acidic conditions during the Permian–Triassic transition in the Iberian Ranges, E Spain

In this paper, a method based on element mapping of randomly selected areas of thin sections on electron microprobe is proposed to quantify the relative contents of strontium-rich hydrated aluminium phosphate-sulphate (APS) minerals in siliciclastic continental sedimentary rocks. The main problems for these minerals to be quantified are related to their small size, low concentrations, and optical properties.By comparing the element maps obtained for the rocks in the study area of the Iberian Ranges (E Spain) with the results of whole rock analysis and with factors indicating the presence of life (bioturbation, palaeosols, and macro-plant remains), it has been possible to correlate relatively high levels of APS minerals in the first sedimentary record (Cañizar Formation) after the Permian–Triassic boundary, with the lack of living organisms. The APS are related to early diagenetic phases precipitated at low pH conditions and are therefore markers of formation in an acidic environment.Our findings suggest a long period of sustained acidic conditions followed by an environmental change linked with the recovery of life and with lower APS mineral contents. This change is detected at the top of the Cañizar Formation, at the end of the Spathian. The method proposed could be used as a tool to address the environmental changes that took place during the Permian–Triassic transition in continental environments.

Paleoenvironmental significance of aluminum phosphate-sulfate minerals in the upper Cretaceous ooidal ironstones, E-NE Aswan area, southern Egypt

Aluminum phosphate-sulfate (APS) minerals are present as small, disseminated crystals in the upper Cretaceous shallow marine ooidal ironstones, E-NE Aswan area, southern Egypt. Their association with the ironstones is considered as a proxy of subaerial weathering and post-diagenetic meteoric water alteration. The mineralogical composition of the ooidal ironstones was investigated by optical and scanning electron microscopes, X-ray diffraction, Fourier transform infrared and Raman spectroscopy. The ooidal ironstones are composed mainly of ooids and groundmass, both of which consist of a mixture of detrital (quartz) and diagenetic (fluorapatite, chamosite and pyrite) mineral assemblages. These mineral assemblages are destabilized under acidic and oxidizing, continental conditions. These conditions resulted from the oxidation of pyrite and probably organic matter under warm and humid, tropical climate followed the Santonian Sea regression and subaerial exposure. These pedogenic conditions promoted corrosion of quartz, dissolution of chamosite and apatite and hydrolysis of feldspars of the nearby exposed granitoids. The released Si, Al and Sr from quartz, chamosite and feldspars; Fe and S from pyrite and P, Ca and light rare earth elements (LREE) from apatite are reprecipitated as hematite, kaolinite, apatite and APS minerals from the pore fluids or along fractures. The paragenetic sequence and textural relationships of this post-diagenetic mineral assemblage indicate that hematite was formed by replacement of chamosite followed by formation of a secondary generation of pore filling chlorapatite and APS minerals and finally the precipitation of kaolinite in the remaining pore spaces. The formation of APS minerals and chlorapatite is simultaneous, but APS minerals are stable at shallow depths under acidic to neutral pH conditions, whereas chlorapatite is stable under alkaline pH conditions. Alkaline conditions were maintained at greater depths when the infiltrated acidic fluids reacted with chamosite. The APS minerals display a homogeneous chemical composition in all ironstone locations in Aswan area, corresponding to a solid solution between crandallite (CaAl3(PO4)2(OH)5·H2O), goyazite (SrAl3(PO4)2(OH)5·H2O), svanbergite (SrAl3(PO4)(SO4)(OH)6) and woodhouseite (CaAl3(PO4)(SO4)(OH)6) end-members. The variations in the APS mineral chemistry (AB3(XO4)2(OH)6) are essentially due to variable substitutions of Sr and LREE for Ca at the A site and limited S for P at the X site. The spatial distribution of APS minerals and their composition in the ooidal ironstones of Aswan area permitted to consider them as good tracers of physicochemical and paleoenvironmental changes, in particular those associated with subaerial exposure and pedogenesis. The post-diagenetic phosphatization and kaolinization of the Aswan ironstones decrease their economic potentiality; thus, understanding paragenetic sequence and textural relationships is essential for the iron ore beneficiation.

ALTERATION RELATED TO URANIUM DEPOSITS IN THE KIGGAVIK-ANDREW LAKE STRUCTURAL TREND, NUNAVUT, CANADA: NEW INSIGHTS FROM PETROGRAPHY AND CLAY MINERALOGY

The Kiggavik project, located 70 km west of Baker Lake (Nunavut), is a major uranium exploration project in the Canadian arctic, with three significant basement-hosted uranium deposits (Kiggavik, End, and Andrew) which spread along a NE SW trend a few kilometers from the south-eastern border of the Thelon Basin. These deposits are closely associated with alteration zones in which clay minerals are abundant. At the scale of the whole structural trend, the alteration paragenesis is composed of illite +/- sudoite +/- hematite +/- aluminum phosphate sulfate minerals (APS). Alteration petrography and mineral paragenesis are similar to those identified in basement-hosted uranium deposits related to Paleoproterozoic unconfornaities in the Athabasca Basin (Canada) or Alligator River (Australia). The alteration haloes are characterized by two similar types of phyllosilicate assemblages (dioctahedral micas or illite and chlorites) corresponding to a regional retrograde metamorphic stage that was overprinted by hydrothermal alteration during the mineralization event. These two assemblages can be distinguished on the basis of crystallographic and chemical properties and mapping of structural parameters, such as the variation of crystallinity along the c-axis or the polytypes of phyllosilicates, which can be used as a vector to mineralization. The crystal chemistry of the hydrothermal phyllosilicates replacing the previous metamorphic minerals indicates a release of iron. This last point is fundamental to the occurrence of hematite in alteration zones and points out the potential effects of the iron redox state in the control of uranium precipitation during the hydrothermal event

Origen y naturaleza de los minerales fosfato-sulfato alumínicos (APS) asociados a la mineralización de uranio en los red-beds triásicos (Cordillera Ibérica, España)

Este trabajo se centra en el estudio de los minerales fosfato-sulfato alumínicos (APS) que se producenen una secuencia clástica del Triásico (Buntsandstein) de la Cordillera Ibérica. El depósito está constituido por areniscas, lutitas y conglomerados con matriz arenosa, que fueron depositados en ambientes de fluviala marino somero. En la secuencia clástica además de los APS están presentes los siguientes minerales dia-genéticos: cuarzo,feldespato potásico, minerales del grupo de la caolinita, illita, óxidos-hidróxidos de Fe,cementos-reemplazamientos de carbonatos y sulfatos y minerales secundarios uraníferos. Los mineralesAPS fueron identificados y caracterizados por microscopía óptica, difracción de rayos X, microscopía elec-trónica de barrido, y microsonda electrónica. Los microcristales de APS reemplazan a minerales uraníferos,asociados con caolinita-dickita, mica y rellenando la porosidad de arcosas-subarcosas fluviales a marealesdistales. Dado su contenido en Ca, Sr y Ba, los minerales APS se pueden definir como una solución sólida de crandallita-goyacita-gorceixita (0,53 Ca, 0,46 Sr y 0,01 Ba). La baja concentración en LREE y elcontenido de Sr&gt;S, sugieren que los minerales APS se originaron durante la alteración supergénica de lasareniscas del Buntsandstein, debido a la presencia de fluidos mineralizantes que provocaron la formaciónde areniscas uraníferas en una zona de alteración distal. Además, la aparición de dickita asociada a losminerales APS, indica que éstos precipitaron a temperatura superior a 80ºC, relacionada con el levanta-miento que tuvo lugar durante la etapa post-rift del Cretácico Superior.

Mineralogy of Sn–W–As–Pb–Zn–Cu-bearing alteration zones in intracontinental rare metal granites (Central Mongolia)

The metalliferous intrusive complexes in the Khentii Uplift evolved post-collisionally in an intracontinental environment. Their felsic magmatic rocks are surrounded by a small marginal facies of more granodioritic to monzonitic composition and were attributed to the A2 type granites, based on discrimination diagrams using K, Na, Si, Nb, Y and Rb. These rare metal granites have elevated contents of fluorite, topaz and tourmaline and are characterised by the light rare earth element enriched minerals allanite-(Ce) and monazite (Ce) prevailing over the xenotime, which is the only host with heavy rare earth element enrichment in these Mongolian granites. Together with zircon, these heavy minerals also show up in the clastic apron around these granites under study, providing a clue to the temperature of formation when the intrusive bodies were emplaced, and moreover provide a tool for exploration in pegmatitic and granitic terrains. Albite alteration is associated with a moderate U–Th–Ti mineralisation and greisen bodies were enriched in Sn and W of subeconomic grade. Only cassiterite re-appears as a heavy mineral in alluvial–fluvial placer-type deposits around the granite stocks, whereas only W occurs in a wide range of limonite and leucoxene like minerals. Sn-, W-, Fe, Mn- and Ti-bearing chemical residues can be used as a short-range marker to pinpoint the loci of enrichment of Sn and W, whereas the aforementioned heavy minerals are applicable as long-range tracers for metalliferous A-type granites. The Pb–Cu–Zn mineralisation is rather homogeneous and of little value as an exploration tool. Three different stages characterised by REE-carbonates and kaolinite, metalliferous duricrust abundant in Sn, W and Ti and aluminium–phosphate–sulphate minerals developed under oxidising conditions. The way how fluids moved though the granitic rocks, either as per descensum or per ascensum fluids, cannot be determined precisely. Therefore epithermal processes cannot be ruled out and have been held responsible for this near-surface alteration zone superimposed on a high-temperature Sn–W mineralisation in the Late Triassic to Early Jurassic A-type granites. These intrusive rocks are related to an extensional regime of the Triassic Indosinian orogeny in southern China.

Coupled SEM-microRaman system: A powerful tool to characterize a micrometric aluminum-phosphate–sulfate (APS)

A micrometric aluminum-phosphate–sulfate (APS) with an intermediate composition between the minerals woodhouseite (CaAl3(PO4)(SO4)(OH)6) and crandallite (Ca(PO4)2(OH)5⋅H2O) has been identified in a natural sample and characterized using a coupled SEM-microRaman system. The Raman spectrum obtained was characterized by various bands assigned to vibrational modes of sulfate and phosphate units. A comparison with other APS minerals, with different PO4/SO4 ratios, enables some differences to be highlighted, which concern particularly the intensities of some sulfate and phosphate bands. It is established that the intensity of the sulfate bands located at around 653 and 1026cm−1 decrease as a function of the mole fraction of phosphate in the APS minerals. This correlation reflects well the effect of the substitution of sulfate anions by phosphate anions. In addition, this study has also shown that the use of a coupling system between a SEM and a microRaman spectrometer is a powerful tool to identify micro-minerals of the alunite supergroup occurring in a complex geological sample.

Palaeoenvironmental implications of aluminium phosphate-sulphate minerals in Early–Middle Triassic continental sediments, SE Iberian Range (Spain)

The presence of Sr-rich aluminium-phosphate sulphate (APS) minerals in continental sedimentary rocks from the Cañizar and Eslida Formations along the eastern part of the Iberian Range (Spain) is considered as evidence of acidic and oxidising conditions during Early–Middle Triassic times. The formation of APS minerals occurred shortly after sedimentation, in early diagenetic stages, prior to the compaction of the sediments and most probably was related to the circulation of acidic meteoric waters. Such conditions might result from a sustained, damaged environment or from multiple environmental crises, but would have delayed the recovery of life after the end-Palaeozoic mass extinction.APS minerals occur as small disseminated and idiomorphic pseudo-cubic crystals (0.5 to 6μm long) or as massive and polycrystalline aggregates replacing fragments of fine-grained metamorphic rocks (mainly metapelites). Textural data indicate that the formation of the APS minerals predated the quartz and illite cements, and that they resulted from the destabilisation of pre-existing minerals, as evidenced by the replacement of slate fragments by APS minerals and hematite and by the close association of the disseminated APS crystals and kaolinite with altered detrital mica plates. Electron microprobe analyses and X-ray diffraction study of the APS minerals indicate a rather homogeneous composition in different parts of the basin, corresponding to solid solutions among woodhouseite, svanbergite, crandallite and goyazite. The sources of strontium in the APS minerals remain unclear. Phosphorous was primarily supplied by dissolution of detrital phosphates under acidic conditions, and sulphur derives from the weathering of pyrite.

Conditions of formation of the Mavrokoryfi high-sulfidation epithermal Cu–Ag–Au–Te mineralization (Petrota Graben, NE Greece)

The Mavrokoryfi Cu–Ag–Au–Te prospect, northeastern Greece, consists of atypical, high-sulfidation mineralization where precious metals were introduced contemporaneously with advanced argillic alteration from magmatic vapors. It occurs as veins of massive sulfides in zones of silicic and advanced argillic alteration spatially associated with an andesitic lava dome and hyaloclastites. Mineralogical data demonstrate an unusual ore and gangue mineralogy that is compatible with formation under very oxidizing conditions (logfO2 values of >−31.8) at temperatures of 200°C to 250°C. Oxidizing conditions favored the formation of hypogene lead sulfates (anglesite and barian celestite) instead of galena. Selenian acanthite, cadmian freibergite, and argentian goldfieldite are the main carriers of silver in the deposit and are reported in Greece for the first time. They were deposited at logfS2 of −9 to −7 and logfTe2 values of −9 to −12.5 (250°C). Ag-poor goldfieldite at Mavrokoryfi has up to 3.7 apfu Te and is the most Te-rich goldfieldite yet reported. The mineralization is accompanied by aluminum–phosphate–sulfate minerals of magmatic-hydrothermal origin and an unusual Pb-enrichment. Ore-forming components were likely derived from andesite porphyries.

Aluminium phosphate and phosphate-sulphate minerals in kyanite schists of the Ichetuyskoye area, West Transbaikalia, Russia: crystal chemistry and evolution

Aluminium phosphate and aluminium phosphate-sulphate (APS) minerals occur as disseminated crystals and fine-grained aggregates in kyanite schists near Ichetuyskoye, in the Dzhida basin (West Transbaikal region, Russia). Petrographical, mineralogical and geochemical data suggest a metamorphic evolution through prograde and retrograde stages. Lazulite-scorzalite and trolleite in association with kyanite, quartz, muscovite, paragonite, topaz, rutile, magnetite and hematite formed during the prograde stage. More than 25 minerals identified in schists are genetically related to the retrograde stage. Some phosphates and all of the APS minerals belong to the alunite, beudantite and plumbogummite groups. Electron-microprobe data reveal the presence of wide compositional variations and complex solid-solution series among the members. The main crystal-chemical variations of the APS solid-solution series in the alunite supergroup concern the relative proportions of svanbergite, woodhouseite, goyazite, crandallite, florencite-(Ce) and natroalunite. Chemical analyses of APS minerals indicate extremely high amounts of LREE, Sr, Ba, Ca and Na. In some cases, APS minerals have compositions of Ba and Ca,Ba phosphates– sulphates.

A hydrothermal clay mineral assemblage at the Late Proterozoic unconformity in the Buenos Aires Complex – La Tinta Formation, Barker area, Tandilia Ranges (Argentina)

AbstractA general alteration pattern of two transitional clay mineral assemblages was determined by petrography and X-ray diffractometry studies at the Tandilia Late Proterozoic unconformity zone, around the Barker locality: (1) K-white mica + chlorite + calcite ± anatase-rutile ± secondary quartz (farther from the unconformity) and (2) pyrophyllite + K-white mica + Ti-rich hematite ± aluminium phosphate-sulphate minerals ± tourmaline ± anatase-rutile (closer to the unconformity). The local occurrence of Na in K-white micas and K-Na in pyrophyllite is described for the first time. Possible interlayering with intermediate K-Na mica and paragonite is indicated by detailed X-ray diffraction analyses. A negative Eu anomaly (Eu/Eu* = 0.24), strongly positive Eu anomaly (Eu/Eu* =11.7) and positive Ce anomaly characterized rapid changes from an unaltered basement to the most altered basement and sedimentary rocks at the unconformity. Aluminium phosphate-sulphate minerals dominate patterns of LREE. A model of hydrothermal alteration comparable to that of unconformity-related uranium deposits is presented.

Mineral Zoning and Geochemistry of Epithermal Polymetallic Zn-Pb-Ag-Cu-Bi Mineralization at Cerro de Pasco, Peru

Mineral Zoning and Geochemistry of Epithermal Polymetallic Zn-Pb-Ag-Cu-Bi Mineralization at Cerro de Pasco, Peru

Spectroscopic Mapping of the White Horse Alunite Deposit, Marysvale Volcanic Field, Utah: Evidence of a Magmatic Component

Previous studies have demonstrated that the replacement alunite deposits just north of the town of Marysvale, Utah, USA, were formed primarily by low-temperature (100°–170° C), steam-heated processes near the early Miocene paleoground surface, immediately above convecting hydrothermal plumes. Pyrite-bearing propylitically altered rocks occur mainly beneath the steam-heated alunite and represent the sulfidized feeder zone of the H 2 S-dominated hydrothermal fluids, the oxidation of which at higher levels led to the formation of the alunite. Maps of surface mineralogy at the White Horse deposit generated from Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data were used in conjunction with X-ray diffraction studies of field samples to test the accuracy and precision of AVIRIS-based mineral mapping of altered rocks and demonstrate the utility of spectroscopic mapping for ore deposit characterization. The mineral maps identified multiple core zones of alunite that grade laterally outward to kaolinite. Surrounding the core zones are dominantly propylitically altered rocks containing illite, montmorillonite, and chlorite, with minor pyrite, kaolinite, gypsum, and remnant potassium feldspar from the parent rhyodacitic ash-flow tuff. The AVIRIS mapping also identified fracture zones expressed by ridge-forming selvages of quartz + dickite + kaolinite that form a crude ring around the advanced argillic core zones. Laboratory analyses identified the aluminum phosphate-sulfate (APS) minerals woodhouseite and svanbergite in one sample from these dickite-bearing argillic selvages. Reflectance spectroscopy determined that the outer edges of the selvages contain more dickite than do the medial regions. The quartz + dickite ± kaolinite ± APS-mineral selvages demonstrate that fracture control of replacement processes is more prevalent away from the advanced argillic core zones. Although not exposed at the White Horse deposit, pyrophyllite ± ordered illite was identified using AVIRIS in localized, superimposed conduits within propylitically altered rocks in nearby alteration systems of similar age and genesis that have been eroded to deeper levels. The fracture zones bearing pyrophyllite, illite, dickite, natroalunite, and/or APS minerals indicate a magmatic component in the dominantly steam-heated system.

Mineralogic and Stable Isotope Zonation at the Surface overthe El Salvador Porphyry Copper Deposit, Chile

We examined in detail the mineralogic and stable isotope characteristics of alteration minerals exposed at the surface of the El Salvador porphyry copper deposit. A total of 276 samples was collected from 203 localities over an area of >5 km 2 at elevations between 2,900 and 3,300 m. The alteration assemblages can be separated into two groups, broadly corresponding to the stages previously identified from underground studies, although the early stage K silicate alteration identified underground is not evident at the surface. A transitional to late stage is characterized at the surface by assemblages of (1) muscovite-andalusite with trace diaspore (including some areas rich in andalusite) and (2) muscovite with trace diaspore. There is a sharp contact outward with a marginal propylitic assemblage (chlorite-calcite-epidote). Pyrophyllite occurs as a retrograde overprint of muscovite in both of the transitional to late-stage assemblages. These assemblages are concentrically zoned and centered on a core of least-altered granodiorite porphyry, a late intramineral intrusion. The muscovite-andalusite assemblage occurs up to 500 m away from the granodiorite porphyry, and andalusite-rich zones are located near the brecciated margin of the porphyry. Further outward is the muscovite zone, from 500 to 1,500 m wide. The pyrophyllite overprint overlaps the contact of the muscovite-andalusite and muscovite zones, and occurs mainly above 3,000 m elevation, such that its present distribution is controlled largely by topography. Very late alteration minerals occur in radial pebble dikes of hydrothermal breccia and their extensions, and they are dominated by an advanced argillic assemblage of alunite-(± aluminum phosphate-sulfate minerals)-diaspore-zunyite-pyrophyllite-dickite, with residual muscovite. In one case a breccia extends upward and appears to be the root of a 300 × 500 m outcrop, 80 m thick, of massive quartz-alunite replacement of rhyolite. Pure mineral separates were prepared from these samples for O, H, and S isotope analysis, including 48 samples of silicate minerals (muscovite, pyrophyllite, dickite, and supergene kaolinite), and 20 samples of alunite, some supergene in origin. Information on mineral assemblage was used to estimate temperatures of formation, which then allowed calculation of the isotopic composition of associated hydrothermal fluids. The fluid responsible for forming the muscovite had a relatively narrow range of isotopic composition, 6.5 ± 1.5 per mil δ 18 O and –40 ± 10 per mil δ D. The heaviest δ 18 O and δ D values, about 8 and –35 per mil, respectively, were obtained from the samples with andalusite-muscovite-diaspore assemblage. This composition is close to that expected for a magmatic fluid, with <10 to 20 percent meteoric-water component responsible for forming the other muscovite samples. The fluid compositions calculated for the very late stage alunite range from 5 and –30 per mil to –1 and –40 per mil δ 18 O and δ D, respectively. This range in composition agrees with the trend predicted for magmatic vapor absorbed by local groundwater, the latter similar in composition to that responsible for forming the supergene kaolinite. There is no systematic spatial zonation of the alunite samples in terms of isotopic composition or paleotemperature. The temperature estimates from S isotope geothermometry are higher than those based on mineral assemblage, possibly caused by partial supergene replacement of some hypogene alunites. There is no strong isotopic zonation of hydrothermal minerals from surface alteration assemblages at El Salvador that can be applied to generate clear exploration guidelines. By contrast, mapping of alteration mineralogy in the field, backed up by laboratory checking of mineralogy and examination of relict minerals and replacement textures in thin section, clearly indicates the center of the late intramineral intrusion.