Robert A. Ayuso

Geochemical and isotopic (Nd-Pb-Sr-O) variations bearing on the genesis of volcanic rocks from Vesuvius, Italy

Abstract Alkaline volcanism produced by Monte Somma–Vesuvius volcano includes explosive plinian and subplinian activity in addition to effusive lava flows. Pumice, scoria, and lava (150 samples) exhibit major- and trace-element gradients as a function of SiO2 (58.9–47.2 wt%) and MgO (0–7.8 wt%); Mg# values are 10 with decreasing age for the Vesuvius system as a whole, yielding similar compositions in the least evolved rocks (low-silica, high-MgO, incompatible element-poor) erupted at the end of each cycle. Internal variations within individual eruptions also systematically changed generally toward a common mafic composition at the end of each cycle, thus reflecting the dominant volume in the magma chamber. At the start of a new eruptive cycle, the rocks are relatively enriched in incompatible elements; younger groups also contain higher abundances than older groups. N-MORB-normalized multielement diagrams exhibit selective enrichments of Sr, K, Rb, Th, and the light rare-earth elements; deep Nb and Ta negative anomalies commonly seen in rocks generated at orogenic margins are absent in our samples. Sr isotopic compositions are known to be variable within some of the units, in agreement with our data ( 87 Sr / 86 Sr ~0.70699 to 0.70803) and with contributions from several isotopic components. Isotopic compositions for δ 18 O (7.3 to 10.2‰), Pb for mineral separates and whole rocks ( 206 Pb / 204 Pb ~18.947 to 19.178, 207 Pb / 204 Pb ~15.617 to 15.769, 208 Pb / 204 Pb ~38.915 to 39.435), and Nd ( 143 Nd / 144 Nd ~0.51228 to 0.51251) also show variability. Oxygen isotope data show that pumices have higher δ 18 O values than cogenetic lavas, and that δ 18 O values and SiO2 are correlated. Radiogenic and stable isotope data plot within range of isotopic compositions for the Roman comagmatic province. Fractional crystallization cannot account for the radiogenic isotopic compositions of the Vesuvius magmas. We favor instead the combined effects of heterogeneous magma sources, together with isotopic exchange near the roof of the magma chamber. We suggest that metasomatized continental mantle lithosphere is the principal source of the magmas. This kind of enriched mantle was melted and reactivated in an area of continental extension (incipient rift setting) without direct reliance on contemporaneous subduction processes but possibly with input from mantle sources that resemble those that produce ocean island basalts.

The 50 Ma granodiorite of the eastern Gulf of Alaska: Melting in an accretionary prism in the forearc

The generation of granitic rocks by melting of flyschoid sediments in an accretionary prism is addressed in this study of 50 Ma silicic igneous rocks in the Gulf of Alaska, near Cordova, Alaska. Plutons of relatively homogeneous biotite and biotite-hornblende granodiorite and minor tonalite and granite are scattered through the Paleocene and Eocene Orca Group. Two masses of cointrusive gabbro and minor dacite dikes also were intruded here. The Orca Group consists of flysch, (quartzofeldspathic graywacke and argillite of turbidite or deep-sea fan origin) and of minor basaltic rocks and pelagic sedimentary rocks. The Orca represents the youngest and structurally lowest part of a late Cretaceous to Eocene composite accretionary prism that extends for 2100 km along the Gulf of Alaska. The plutons are 5–150 km2 in plan and represent less than perhaps 5% of the total volume of this part of the prism. These granitic rocks are unusual in that they were emplaced in a forearc environment during the last stages of deformation of the prism. The three intrusive bodies chosen for study (the McKinley Peak, Rude River, and Sheep Bay plutons) show a range of chemical and initial isotopic compositions (SiO2 = 66.3–71.3%, Na2O = 2.8–3.6%, K2O = 1.8–3.0%, eNd = +2.1 to −3.3, 87Sr/86Sr = 0.7051–0.7067, 206Pb/204Pb = 19.04–19.20, 207Pb/204Pb = 15.60–15.66, and 208Pb/204Pb = 38.59–38.85). Relative to the other two plutons, the McKinley Peak pluton generally shows slightly lower K2O, higher Al2O3, higher eNd, and lower 87Sr/86Sr ratios. All three plutons, however, have similar, well-defined minor and trace element abundances characterized by relative enrichment in light rare earth elements and depletion in high field strength elements. The granodiorites and flysch of the Orca Group show overlapping elemental and isotopic compositions. The only clearly defined chemical differences between the flysch and the granodiorites are weak negative Eu anomalies in the granodiorites and slightly lower Ca and higher Na contents in the flysch. The Nd and Sr isotopic compositions of the Rude River and Sheep Bay plutons completely overlap those of the flysch. The McKinley Peak pluton, however, shows discretely higher eNd slightly lower 87Sr/86Sr values than those of the flysch. The Pb isotopic compositions of the flysch and the Rude River pluton also overlap, but Pb of the other two plutons is slightly less radiogenic. Our chemical data, modeling, and comparison with Conrad et al.s (1988) melting experiments of graywacke indicate that the granodiorite orginated by large fractions (65–90%) of melting of the Orca Group graywacke and argillite. Plagioclase, pyroxene(s), and biotite(?) were residual to melting at about 850°–950°C and at low H2O activities. The distinct chemical and isotopic compositions of the McKinley Peak pluton probably result from variations in the character of the flysch at depth in the prism, rather than from mixing between melts of the flysch and mafic magmas injected into the prism itself. However, basaltic magmas were injected into the accretionary pile, as evidenced by the coeval gabbroic plutons, and these apparently provided the heat necessary for crustal melting. The mafic magmas probably originated from the subjacent oceanic Kula plate. We suggest that the subducting Kula “plate” consisted of several small plates, much as the modern Juan de Fuca and nearby smaller plates lie at the margin of the Pacific plate. Basaltic magmas produced along the boundaries of such small plates were injected for more than 12 m.y., first into the Orca Group deep-sea fans and later into the accretionary prism. Accretionary prisms have been an important, but little discussed, source of granitic magmas since Archean time. Their emplacement as complete sections of lower to upper crust means that any basaltic magma coming up from the mantle will impinge upon and tend to melt them. Furthermore, many prisms, especially those bearing high proportions of quartzofeldspathic graywacke, are fertile in granitic melts. These Alaskan granodiorites do not fit into the alphabetical classification of Australian workers. Being melts of sedimentary rocks, they should have S-type character. Because the source flysch is quartzofeldspathic and of arc origin, however, the granodiorite shows I-type character. Our results also highlight a problem with Pearce et al.s (1984) and Harris et al.s (1986) purportedly tectonic-discriminant plots for granitic rocks. These diagrams classify our granodiorites as “volcanic arc granite” and reflect their source rocks rather than their tectonic environment of origin.

Regional differences in PB isotopic compositions of feldspars in plutonic rocks of the northern Appalachian Mountains, U.S.A., and Canada: A geochemical method of terrane correlation

Feldspar Pb isotopic compositions from Paleozoic tonalitic to leucogranitic Caledonian plutons from the northern Appalachian Mountains of the U.S.A. and Canada range from nonradiogenic to radiogenic relative to the evolution curve of Stacey and Kramers (1975). Pb isotopic values in the feldspars are generally independent of modal and major and trace element contents of the plutons. Plutons with the least radiogenic Pb isotopic signatures (Northern group) have intruded allochthonous rocks closest to the North American craton, and plutons with the most radiogenic Pb (Southern group) have intruded Avalonian basement. Feldspars from plutons in the Central group have intermediate Pb isotopic compositions between the Northern and Southern groups and do not define an isotopically unique group. The predominant source of the felsic magmas was the continental lithosphere as indicated by the general Pb isotopic similarity of the plutonic feldspars with either Grenville or Chain Lakes basement in the case of the Northern group and with Avalonian basement rocks in the case of the Southern group. The source region for plutons in the Central group may have contained a mixture of Grenvillian and Avalonian components. Plutons in the northern Appalachians are as a group more evolved in their Pb isotopic compositions relative to Caledonian plutons in the British Isles.

Fluid inclusion evidence for magmatic silicate/saline/CO2 immiscibility and geochemistry of alkaline xenoliths from Ventotene Island, Italy

Abstract Fluid and melt inclusions and geochemical features of alkali syenite, mafic, and ultramafic cumulate xenoliths in the last ignimbritic event (volcanism up to 300 ky.b.P.) at the island of Ventotene in the Pontine archipelago (Gaeta Gulf) were investigated to establish the genesis and evolution of the fluids trapped in the inclusions. Ranges in lead isotopic compositions of the xenoliths as a group are narrow: 206Pb/204Pb:18.778–18.864; 207Pb/204Pb:15.641–15.701; 208Pb/204Pb:38.858–39.090; the values overlap among the groups, implying that the xenoliths are closely related. The xenoliths straddle the best-fit line describing the regional NW-SE variation of values for uranogenic Pb in volcanic rocks from the Roman alkaline province. The similarity between the xenoliths and volcanic rocks permits the interpretation that the xenoliths are representative of the source region of the volcanic rocks, residues after partial melting of the source region or fractional crystallization of the volcanic rocks, or even that the xenoliths represent assimilants obtained during the evolution of the magmas that produced the volcanic rocks. Xenoliths belonging to the ultramafic-mafic cumulate group contain only silicate melt inclusions ± vapor bubble ± droplets of an opaque phase and rarely some CO2 trapped in silicate melt inclusions. Xenoliths in the alkali syenite group have three types of fluid inclusions: (1) single phase vapor and silicate melt inclusions; (2) two-phase silicate melt + salt, silicate melt + CO2 (V), aqueous (L + V), and silicate melt + vapor inclusions, and (3) three-phase and multiphase inclusions: CO2 (L) + CO2 (V) + H2O; silicate melt + saline melt + H2O ± birefringent or opaque trapped minerals; H2O + salt + silicate glass ± birefringent trapped minerals. During heating experiments, melting of salt occurs at temperatures from 565 to 815°C, depending on the water content of the inclusions. Homogenization of the vapor bubble occurs from 850–1160°C, and complete melting of the silicate glass at about 950°C. The highly variable proportions of the individual phases in the silicate melt + salt + H2O inclusions and the coexistence of silicate melt + CO2 inclusions indicates immiscibility during the crystallization of the magma. Primary and secondary CO2 inclusions in the alkali syenite suite indicate lower densities (from 0.10 to 0.22 g/cm3) than those resulting from primary CO2 inclusions in the gabbroic suite (from 0.34 to 0.42 g/cm3). The P-T trajectory of the probable fluid evolution path shows that the crystallization of gabbro occurred between ∼1 and 1.4 kbars, whereas alkali syenite crystallized between ∼200 and 400 bars. The secondary H2O inclusions in alkali syenite were trapped in the later stages of the hydrothermal process and at much lower temperatures (130–290°C), but at pressures relatively close to those of alkali syenite crystallization. The almost isobaric conditions during the final stage of the fluid evolution path are explained by the very shallow emplacement of the alkali syenite intrusive body.

Geochemistry and argon thermochronology of the Variscan Sila Batholith, southern Italy: source rocks and magma evolution

The Sila batholith is the largest granitic massif in the Calabria-Peloritan Arc of southern Italy, consisting of syn to post-tectonic, calc-alkaline and metaluminous tonalite to granodiorite, and post-tectonic, peraluminous and strongly peraluminous, two-mica±cordierite±Al silicate granodiorite to leucomonzogranite. Mineral 40Ar/39Ar thermochronologic analyses document Variscan emplacement and cooling of the intrusives (293–289 Ma). SiO2 content in the granitic rocks ranges from ∼57 to 77 wt%; cumulate gabbro enclaves have SiO2 as low as 42%. Variations in absolute abundances and ratios involving Hf, Ta, Th, Rb, and the REE, among others, identify genetically linked groups of granitic rocks in the batholith: (1) syn-tectonic biotite±amphibole-bearing tonalites to granodiorites, (2) post-tectonic two-mica±Al-silicate-bearing granodiorites to leucomonzogranites, and (3) post-tectonic biotite±hornblende tonalites to granodiorites. Chondrite-normalized REE patterns display variable values of Ce/Yb (up to ∼300) and generally small negative Eu anomalies. Degree of REE fractionation depends on whether the intrusives are syn- or post-tectonic, and on their mineralogy. High and variable values of Rb/Y (0.40–4.5), Th/Sm (0.1–3.6), Th/Ta (0–70), Ba/Nb (1–150), and Ba/Ta (∼50–2100), as well as low values of Nb/U (∼2–28) and La/Th (∼1–10) are consistent with a predominant and heterogeneous crustal contribution to the batholith. Whole rock δ18O ranges from ∼+8.2 to +11.7‰; the mafic cumulate enclaves have the lowest δ18O values and the two-mica granites have the highest values. δ18O values for biotite±honblende tonalitic and granodioritic rocks (9.1 to 10.8‰) overlap the values of the mafic enclaves and two-mica granodiorites and leucogranites (10.7 to 11.7‰). The initial Pb isotopic range of the granitic rocks (206Pb/204Pb ∼18.17–18.45, 207Pb/204Pb ∼15.58–15.77, 208Pb/204Pb ∼38.20–38.76) also indicates the predominance of a crustal source. Although the granitic groups cannot be uniquely distinguished on the basis of their Pb isotope compositions most of the post-tectonic tonalites to granodiorites as well as two-mica granites are somewhat less radiogenic than the syn-tetonic tonalites and granodiorites. Only a few of the mafic enclaves overlap the Pb isotope field of the granitic rocks and are consistent with a cogenetic origin. The Sila batholith was generated by mixing of material derived from at least two sources, mantle-derived and crustal, during the closing stages of plate collision and post-collision. The batholith ultimately owes its origin to the evolution of earlier, more mafic parental magmas, and to complex intractions of the fractionating mafic magmas with the crust. Hybrid rocks produced by mixing evolved primarily by crystal fractionation although a simple fractionation model cannot link all the granitic rocks, or explain the entire spectrum of compositions within each group of granites. Petrographic and geochemical features characterizing the Sila batholith have direct counterparts in all other granitic massifs in the Calabrian-Peloritan Arc. This implies that magmatic events in the Calabrian-Peloritan Arc produced a similar spectrum of granitic compositions and resulted in a distinctive type of granite magmatism consisting of coeval, mixed, strongly peraluminous and metaluminous granitic magmas.

Anthropogenic vs. natural pollution: an environmental study of an industrial site under remediation (Naples, Italy)

Heavy metal concentrations and Pb isotopic composition were determined in the soils, slags, scums and landfill materials from a shut down industrial (brownfield) site. This was the second largest integrated steelworks in Italy, and is now under remediation by a Government project. It is located in the outskirts of Napoli on the Bagnoli–Fuorigrotta plain (BFP), which is part of the Campi Flegrei (CF) volcanic caldera, where many spas and geothermal springs occur. The purpose of this work is to distinguish the natural (geogenic) component, originated by hydrothermal activity, from anthropogenic contamination owing to industrial activity. ‘In-situ sediments’ (soils), slags, scums and landfill materials from 20 drill-cores were selected from a network of 197 drills carried out on a 100 × 100 m grid, covering the entire brownfield site. In general, heavy metal enrichments in the upper 3 m of the cores strongly suggest mixing between natural (geogenic) and anthropogenic components. Pb isotopic data are suggestive of three potential end members, and confirm the existence of a strong natural component in addition to contamination from anthropogenic activities. The slags, scums and landfill materials have been proved, through mineralogy and leachate experiments, to be geochemically stable; this shows that metal pollutants are not bio-available and, hence, do not pose a risk to future developments on this site. The natural contribution of hydrothermal fluids to soil pollution, in addition to the non-bio-availability of metal pollutants from industrial materials, indicate that heavy metal remediation of soils in this area would be of little use. Continuous discharge from mineralized hydrothermal solutions would cancel out any remediation effort.

Lead-isotopic evidence for distinct sources of granite and for distinct basements in the northern Appalachians, Maine

Lead-isotopic compositions of feldspars in plutons of the coastal lithotectonic block (Avalonian basement), central Maine (Merrimack) synclinorium, and Connecticut Valley-Gaspe synclinorium in northern Maine (Granville basement) indicate the presence of three fundamentally different sources of Devonian granitic rocks in Maine. Plutons in the coastal lithotectonic block have the most radiogenic values ( 206 Pb/ 204 Pb: 18.25–19.25; 207 Pb/ 204 Pb: 15.59–15.67; 208 Pb/ 204 Pb: 38.00–38.60); plutons in northern Maine are the least radiogenic ( 206 Pb/ 204 Pb: 18.00–18.50; 207 Pb/ 204 Pb: 15.51–15.55; and 208 Pb/ 204 Pb: 37.80–38.38). Intermediate lead-isotope values characterize the plutons in central Maine ( 206 Pb/ 204 Pb: 18.20–18.40; 207 Pb/ 204 Pb: 15.56–15.60; and 208 Pb/ 204 Pb: 38.00–38.30). All plutons show relatively radiogenic lead values for their ages and suggest the imprint of continental crustal sources especially in the coastal block and in central Maine. Plutons from the Connecticut Valley–Gaspe synclinorium allow for a more significant isotopic input from the subcontinental mantle. The three distinct lead-isotope groups probably identify plutons formed in different crustal fragments in a continental collisional environment that were juxtaposed after emplacement of the granites.

Petrography and mineral chemistry of the composite Deboullie pluton, northern Maine, U.S.A.: Implications for the genesis of CuMo mineralization

Abstract Biotite and apatite mineral chemistry, particularly halogen abundances and ratios, are used to investigate the relation of the two contrasting parts of the Deboullie composite pluton (syenite-granodiorite) located in northern Maine. Biotite mineral chemistry helps to classify the weakly developed porphyry-style mineralization (CuMo) associated with syenitic rocks of the Deboullie pluton. Unmineralized syenite consists of K-feldspar+plagioclase+quartz+biotite+hornblende±clinopyroxene and minor amounts of apatite, titanite, magnetite, zircon and allanite. Biotite and apatite occur within the matrix of the rocks and within small multiphase inclusions hosted by clinopyroxene. The inclusions are interpreted to be crystallized melt inclusions rather than solid inclusions, that were trapped by clinopyroxene during growth. The multiphase inclusions consist of K-feldspar+quartz+biotite+apatite+magnetite. The F contents of biotite display a strong positive correlation with the phlogopite component, or Xphl [Xphl=Mg/(sum octahedral cations)]. Biotites from the multiphase inclusions have higher F contents and higher values of F ( F + Cl + OH ) and Xphl than biotites in the matrix that presumably formed later during crystallization and/or re-equilibrated with late-stage fluids. Contents of TiO2 largely overlap for the inclusion and matrix biotite. Apatite compositions plot within the fluorapatite field [ F ( F + Cl + OH ) = 0.57−0.99] . Apatites in the multiphase inclusions have higher Cl contents than apatites in the matrix or those enclosed in biotite. Biotite-apatite geothermometry for the multiphase inclusions yields magmatic or near-magmatic temperatures (725–850°C); biotite-apatite pairs outside the inclusions yield temperatures that range from near-magmatic (600–775°C) to much lower ( ( X F X Cl ) and Xphl was used to infer HF/HCl fugacity ratios in associated hydrothermal fluids, and indicates that biotite from the Deboullie pluton is strikingly similar to biotite from the porphyry-Cu deposit at Santa Rita, New Mexico, but distinctly different from biotite from the porphyry-Mo deposit at Henderson, Colorado. The assemblage quartz+magnetite+titanite implies an oxidation state for the Deboullie pluton well above the quartz-magnetite-fayalite buffer, which is also consistent with magmas associated with porphyry-Cu deposits. On a regional scale, biotite compositions from granitic plutons in Maine do not vary in a systematic manner. This implies that biotite compositions no longer directly reflect source compositions. Finally, biotite and apatite compositions of the syenite and granodiorite are different; compositional ranges in the granodiorite are more restricted than those in the syenite. This provides evidence that the two parts of this composite pluton had different origins and are not related by crystal fractionation.

A Coast Mountains provenance for the Valdez and Orca groups, southern Alaska, based on Nd, Sr, and Pb isotopic evidence

Abstract Nd, Sr, and Pb isotopic data were obtained for fourteen fine- to coarse-grained samples of accreted flysch of the Late Cretaceous and early Tertiary Valdez and Orca Groups in southern Alaska to determine the flysch provenance. Argillites and greywackes from the Orca Group, as well as compositionally similar but higher metamorphic grade rocks from the Valdez Group, show a restricted range of correlated eNd ( −0.6 to −3.8) and 87Sr/86Sr (0.7060–0.7080) at the time of sediment deposition ( ∼ 50 Ma). Pb isotopic compositions also vary over a narrow range ( 206Pb/204Pb= 19.138–19.395, 207Pb/204Pb= 15.593–15.703, 208Pb/204Pb= 38.677–39.209), and in the Orca Group the samples generally become more radiogenic with decreasing eNd and increasing 87Sr/86Sr. All samples have similar trace element compositions characterized by moderate light rare earth element enrichments, and low ratios of high field strength elements to large ion lithophile elements. Based on petrographic, geochemical, and isotopic data the sedimentary rocks are interpreted to have been derived largely from a Phanerozoic continental margin arc complex characterized by igneous rocks with eNd values between 0 and −5. The latter conclusion is supported by the eNd values of a tonalite clast and a rhyodacite clast in the Orca Group (eNd = −4.9and−0.9, respectively). However, trondjemitic clasts in the Orca Group have significantly lower eNd ( ∼ −10) and require a derivation of a portion of the flysch from Precambrian crustal sources. The Nd, Sr, and Pb isotopic compositions of both the Valdez and Orca Groups overlap the values determined for intrusive igneous rocks exposed within the northern portion of the Late Cretaceous to early Tertiary Coast Mountains Plutonic Complex in western British Columbia and equivalent rocks in southeastern Alaska. The isotopic data support previous conclusions based on geologic studies which suggest that the flysch was shed from this portion of the batholith, and from overlying continental margin arc-related volcanic rocks, following its rapid uplift in the Late Cretaceous and early Tertiary. The Precambrian crustal material present in the flysch may have been derived from Late Proterozoic or older metasedimentary and metaigneous rocks now exposed along the western margin of the Coast Mountains Plutonic Complex.

Mineral sources and transport pathways for arsenic release in a coastal watershed, USA

Metasedimentary bedrock of coastal Maine contains a diverse suite of As-bearing minerals that act as significant sources of elements found in ground and surface waters in the region. Arsenic sources in the Penobscot Formation include, in order of decreasing As content by weight: löllingite and realgar (c. 70%), arsenopyrite, cobaltite, glaucodot, and gersdorffite (in the range of 34–45%), arsenian pyrite (<4%), and pyrrhotite (<0.15%). In the Penobscot Formation, the relative stability of primary As-bearing minerals follows a pattern where the most commonly observed highly altered minerals are pyrrhotite, realgar, niccolite, löllingite > glaucodot, arsenopyrite-cobaltian > arsenopyrite, cobaltite, gersdorffite, fine-grained pyrite, Ni-pyrite > coarse-grained pyrite. Reactions illustrate that oxidation of Fe-As disulphide group and As-sulphide minerals is the primary release process for As. Liberation of As by carbonation of realgar and orpiment in contact with high-pH groundwaters may contribute locally to elevated contents of As in groundwater, especially where As is decoupled from Fe. Released metals are sequestered in secondary minerals by sorption or by incorporation in crystal structures. Secondary minerals acting as intermediate As reservoirs include claudetite (c. 75%), orpiment (61%), scorodite (c. 45%), secondary arsenopyrite (c. 46%), goethite (<4490 ppm), natrojarosite (<42 ppm), rosenite, melanterite, ferrihydrite, and Mn-hydroxide coatings. Some soils also contain Fe-Co-Ni-arsenate, Ca-arsenate, and carbonate minerals. Reductive dissolution of Fe-oxide minerals may govern the ultimate release of iron and arsenic – especially As(V) – to groundwater; however, dissolution of claudetite (arsenic trioxide) may directly contribute As(III). Processes thought to explain the release of As from minerals in bedrock include oxidation of arsenian pyrite or arsenopyrite, or carbonation of As-sulphides, and most models based on these generally rely on discrete minerals or on a fairly limited series of minerals. In contrast, in the Penobscot Formation and other metasedimentary rocks of coastal Maine, oxidation of As-bearing Fe-cobalt-nickel-sulphide minerals, dissolution (by reduction) of As-bearing secondary As and Fe hydroxide and sulphate minerals, carbonation and/or oxidation of As-sulphide minerals, and desorption of As from Fe-hydroxide mineral surfaces are all thought to be involved. All of these processes contribute to the occurrence of As in groundwaters in coastal Maine, as a result of variability in composition and in stability of the As source minerals. Arsenic contents of soils and groundwater thus reflect the predominant influence and integration of a spectrum of primary mineral reservoirs (instead of single or unique mineral reservoirs). Cycling of As through metasedimentary bedrock aquifers may therefore depend on consecutive stages of carbonation, oxidation and reductive dissolution of primary and secondary As host minerals.