Hedenbergitic Pyroxene Research Articles

Fluctuations of trace elements and Sr isotopes of the multi-stage scheelite in the reduced polymetallic Yupo tungsten skarn deposit, South China: Implications for ore-forming process

The Yupo scheelite (Sch)-bearing deposit is hosted in the external contact zones between the Yupo granodiorite and Cambrian Huangdongkou Formation of South China. The stratiform ore bodies contain hedenbergitic pyroxene (Hd23–69) and andradite-poor garnet (Ad0–22) assemblages, indicating that Yupo is a typical reduced W skarn deposit. The garnet compositions are zoned in a deposit scale inwardly from the periphery of the grossular-rich skarn reaction front to more subcalcic compositions. Scheelite occurs mainly as disseminated grains in the skarn coeval with retrograde alteration (Sch I) and in quartz veins crosscutting skarns (Sch II-1, -2, and -3). Sch II-1 occurs as cores rimmed by Sch II-2, and Sch II-3 as veinlet cutting both of them. The Sch I and Sch II-3 show weak Eu anomalies different from the pronounced positive Eu anomalies of Sch II-1 and -2. All the scheelite contains low Mo contents (<0.1 wt%), together with the subordinate precipitation of molybdenite, indicating a reduced ore-forming fluid system. The Mo content and Eu anomaly jointly reveal that the reducing condition enhanced from Sch I to Sch II-1 and alleviated from Sch II-1 to Sch II-3. The four scheelite generations show gradually increased initial 87Sr/86Sr ratios from 0.7144 to 0.7242, which are higher than that of the magmatic apatite (0.7082–0.7127) and the carbonate rocks (0.7104–0.7109). The Cambrian carbonaceous shale is a high (87Sr/86Sr)i endmember that interacted with the initial ore-forming fluid; continuing interaction was deemed to enhance the reduction of the ore-forming fluid from Sch I to Sch II-1. Weakening of the intensity of fluid-rock interaction with carbonate rocks during vein-type mineralization caused the (87Sr/86Sr)i values of the scheelite to increase gradually. The input of meteoric water decreased the fluid reduction capability during the formation of Sch II-1 to Sch II-3. Fluid-rock interaction dominated the scheelite mineralization of the reduced W skarn deposit.

Skarn mineralogy and in-situ LA–ICP–MS U–Pb geochronology of wolframite for the Caojiaba tungsten deposit, southern China: Implications for a reduced tungsten skarn deposit

The Caojiaba tungsten deposit (19.03 Mt@ 0.37 wt% WO3) is hosted in skarn within clastic and carbonate rocks in the Xiangzhong metallogenic province (XZMP), southern China. The deposit is characterized by an early prograde skarn overprinted by a retrograde assemblage, and then by quartz–scheelite–sulfide veins. The Caojiaba skarn is characterized by grossular garnet (Grs65–85Adr7.1–31) and hedenbergitic pyroxene (Hd63–86Di11–33), similar to the well–studied reduced tungsten skarns worldwide. In this paper, we present in-situ laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) analysis of U–Pb isotopes and trace elements of wolframite grains coexisting with scheelite in the quartz–scheelite–sulfide veins. Uranium is correlated positively with Nb5+, tetravalent (Ti4+, Sn4+, Zr4+, Hf4+), and trivalent (Sc3+, V3+, Y3+, REE3+) cations in all wolframite samples, suggesting that the incorporation of U into wolframite is controlled by the coupled substitution mechanisms. Two wolframite samples (CJB–43 and CJB–44) yielded 206Pb/238U ages of 212.4 ± 1.0 Ma (2σ, n = 50, MSWD = 0.92) and 211.8 ± 1.1 Ma (2σ, n = 43, MSWD = 1.4), overlapping with the late Triassic granitic rocks (225.6–204.5 Ma) in the XZMP. Mineralogical and geochronological evidence collectively indicate that Caojiaba is a typical example of the late Triassic reduced tungsten skarn deposits in the XZMP. This study demonstrates the potential of U–Pb dating of wolframite and highlights its importance for directly dating hydrothermal ore–forming processes.

Magmatic-hydrothermal evolution at the Lyangar redox-intermediate tungsten-molybdenum skarn deposit, western Uzbekistan, Tien Shan: Insights from igneous petrology, hydrothermal alteration, and fluid inclusion study

The Lyangar W-Mo skarn deposit is situated in a metallogenic belt of W-Mo, Cu-Mo, W-Au, and Au deposits and is related to an Early Permian multiphase (monzodiorite)-granodiorite-granite-leucogranite pluton that was emplaced in a tectonic domain with possible ancient continental crust. The latter, coupled with an inferred ascent of hot asthenospheric material, has defined an enrichment in Mo and fluorine, together with W and Au, to form a distinct metallogenic assemblage in a post-collisional environment. The parental moderately- to high-K, transitional metaluminous to peraluminous, weakly oxidized-weakly reduced, ilmenite-titanite-series granitoids are accompanied by hydrothermal alteration and mineralization that formed in redox-intermediate environment.The deposit incorporates prograde and retrograde pyroxene and garnet skarns composed of hedenbergitic pyroxene and grossular- to almandine+spessartine-rich garnet, respectively. Fluorite is present in the retrograde skarn together with scheelite and sulfides, the latter including most abundant pyrrhotite and molybdenite. Hydrosilicate (amphibole-epidote-chlorite-quartz, locally with albite, sulfides, and scheelite) assemblage has limited development. Phyllic (quartz-sericite-Fe-carbonate, locally with albite, fluorite, scheelite, and sulfides, also Bi and Au minerals) alteration assemblage partially overprints altered skarns and extends over much larger areas. Overall, the deposit represents a superposition of the early W-Mo mineralization related to retrograde skarn and hydrosilicate alteration, and the late Au-W-Cu-Bi mineralization in phyllic alteration assemblages.Prograde and retrograde skarns were formed from dominantly magnesian-sodic-chloride, moderately-saline (11.7–13.9 wt% to 8.6–10.5 wt% NaCl-eq.), high pressure (1500 to 1350 bars), hot aqueous fluids that were followed by moderately-saline (12.3–14.5 wt% NaCl-eq.), Ca-rich, lower pressure (600 bars) fluids during deposition of most scheelite. Hydrosilicate (propylitic) assemblages were formed from boiling fluids at the temperatures of 380–430 °C and pressures of 600±300 bars, whereas phyllic alteration assemblages - from boiling carbonic-aqueous, low-salinity fluids at the temperatures of 300–320 °C and pressures of ~1000 bars. δ18OH2O values range from +7.8 to +8.6‰ for retrograde skarn, and from +6.6 to +7.0‰ for phyllic alteration, whereas δ13CCO2- from −3.3 to −0.5‰ for retrograde skarn, and from −5.5 to −4.5‰ for phyllic alteration. δ34S increases from retrograde skarn (δ34S = +2.0 to +2.3‰) to phyllic alteration (δ34S = +4.3 to +4.6‰) stages.

Mineralogy and titanite geochronology of the Caojiaba W deposit, Xiangzhong metallogenic province, southern China: implications for a distal reduced skarn W formation

The Caojiaba tungsten deposit (19.03 Mt@ 0.37 wt% WO3) is hosted by skarn along the contact between clastic and carbonate rocks in the Xiangzhong Metallogenic Province of southern China. The deposit is characterized by an early prograde skarn containing low andraditic garnet (Ad0.7–21.9) and hedenbergitic pyroxene (Hd52.9–77.3) overprinted by a retrograde biotite–chlorite assemblage and then by quartz–scheelite veins, similar to well-studied reduced tungsten skarns worldwide. Scheelite has low MoO3 (0.01–0.16 wt%), and ore commonly contains up to 1.5 ppm Au and up to 0.33 wt% Sb. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb analyses of hydrothermal titanite coexisting with scheelite in three skarn ore samples provide ages between 206 ± 5 Ma and 196 ± 3 Ma (2σ). Our new ages demonstrate that the tungsten mineralization took place at Caojiaba between 206 and 196 Ma, overlapping the 228–201 Ma emplacement age of granitic rocks in the Xiangzhong Metallogenic Province. Mineralogical and geochronological evidence collectively indicates that Caojiaba is a distal reduced W skarn deposit. The 226–196 Ma granite-related W mineralization recognized throughout the province has a possible link with the widespread Sb–Au mineralization in the region.

Case study of magmatic differentiation trends on the Moon based on lunar meteorite Northwest Africa 773 and comparison with Apollo 15 quartz monzodiorite

Pyroxene and feldspar compositions indicate that most clasts from the Northwest Africa 773 (NWA 773) lunar meteorite breccia crystallized from a common very low-Ti (VLT) mare basalt parental magma on the Moon. An olivine cumulate (OC), with low-Ca and high-Ca pyroxenes and plagioclase feldspar formed during early stages of crystallization, followed by pyroxene gabbro, which is characterized by zoned pyroxene (Fe#=molar Fe/(Fe+Mg)×100 from ∼35 to 90; Ti#=molar Ti/(Ti+Cr)×100 from ∼20 to 99) and feldspar (∼An90–95Ab05–10 to An80–85Ab10–16). Late stage lithologies include alkali-poor symplectite consisting of fayalite, hedenbergitic pyroxene and silica, and alkaline-phase-ferroan clasts characterized by K-rich glass and/or K,Ba-feldspar with fayalite and/or pyroxene. Igneous silica only occurs with the alkaline-phase-ferroan clasts. This sequence of clasts represents stages of magmatic evolution along a ferroan–titanian trend characterized by correlated Fe# and Ti# in pyroxene, and a wide range of increase in Fe# and Ti# prior to crystallization of igneous silica.Clasts of Apollo 15 quartz monzodiorite (QMD) also have pyroxene co-existing with silica, but the QMD pyroxene has more moderate Fe# (∼70). Thus, in AFM components (A=Na2O+K2O, M=MgO, F=FeO), the QMD clasts are similar to the terrestrial calc-alkaline trend (silica-enrichment at moderate Fe#), whereas the ferroan–titanian trend is similar to the terrestrial tholeiitic trend (silica-enrichment only after strong increase in Fe#). However, the variations in SiO2-contents of QMD clasts are due to variable mixing of SiO2-rich and FeO-rich immiscible liquids (i.e., not a progressive increase in SiO2). Immiscibility occurred after fractionation of a KREEP-rich parent liquid.A third trend is based on zoning relations within the NWA 773 OC, where pyroxene Ti# increases at constant Fe# with proximity to intercumulus, incompatible element-rich pockets rich in K,Ba-feldspar and Ca-phosphates. This type of fractionation (increasing refractory trace elements at constant Fe#) in a cumulate parent rock may have been important for generating lunar rocks that combine low Fe# with high incompatible trace element concentrations, such as KREEP basalts and the magnesian suite.MELTS (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998) models of one VLT, one low-Ti and two high-Ti mare basalts and one KREEP basalt all show evolution from low to high Fe# residual liquids during fractional crystallization; however strong enrichments in FeO-concentrations are limited to the VLT and low-Ti liquids. In the high-Ti liquids, crystallization of Fe–Ti-oxides prevents enrichment in FeO, and the increases in Fe# are due to depletion of MgO. Fe–Ti-oxide fractionation results in steady silica-enrichment in the high-Ti mare compositions. Intervals of FeO-enrichment on the VLT and low-Ti mare liquid lines of descent are linked to shifts from olivine to pyroxene crystallization. The onset of plagioclase feldspar crystallization limits the depletion of FeO during crystallization of one high-Ti mare basalt and of the KREEP basalt composition modeled.

Petrology, geochemistry, and evolution of the iron skarns along the northern contact of the Eğrigöz Plutonic Complex, Western Anatolia, Turkey

The Çatak and Küreci skarn districts are located approximately 10 km NW of Emet (Kütahya) in Western Turkey. The skarn and associated ore formations mainly occur at the contact between intrusive rocks of the Eğrigöz Plutonic Complex (EPC) and calcareous pelitic schists with limestone lenses of the Sarıcasu Formation and meta-carbonate rocks of the Arıkaya Formation. The major, trace, and rare earth element analysis of the igneous rocks indicate that they are high level, subalkaline, calc-alkaline, peraluminous to metaluminous I-type intrusions, generated in a continental arc setting. Three distinct skarn-type mineralization, differing in their host rocks and distance from the intrusive body, were chosen to establish the ore-forming conditions in different episodes of skarn formation. The Küreci iron mineralization is hosted in a skarn zone with well-developed zoning from unaltered granodiorite and endoskarn, andradite-diopside exoskarn, to diopside-wollastonite exoskarn towards a marble reaction front. In Sakari, the iron mineralization and associated skarn have formed due to successive fracturing and infiltration processes. From early contact metamorphic rocks to late prograde skarn at the Sakari prospect, the composition of clinopyroxene ranges from (Di_{50-70} Hd_{28-53} Jo_{1-2}) to (Di_{19-73} Hd_{26-77} Jo_{2-6}) and the composition of garnet ranges from (Ad_{95-99} Gr_{1-5}) to (Ad_{40-61} Gr_{36-58}), respectively. The presence of anisotropic grossular garnet with high Fe^{2+}/Fe^{3+} in crosscutting pyrrhotite-pyrite-bearing veinlets coupled with hedenbergitic pyroxene (Mg-poor clinopyroxene with higher Fe^{2+}/Fe^{3+}) is consistent with reducing conditions during the later stage of prograde skarn alteration. The Çatak iron skarn is characteristic, with its high sulphide content due to the presence of pyrrhotite, pyrite, and arsenopyrite, and low proportion of garnet to pyroxene. The sulphur isotope (\delta^{34}S) compositions in the pyrrhotite-dominant skarn zones range between +0.84 to -2.23&permil. We interpret the bulk of the sulphur in the system as of igneous derivation and there has not been any significant sulphur contribution from a crustal source. Fluid inclusion measurements conducted on skarn minerals of the proximal zone and distal zone+vein skarn revealed high homogenization temperatures (371 to >600°C) and varying salinity values (10.5 to >70 wt% NaCl). The fluid inclusion data indicate that there are at least three fluids associated with the genesis of the proximal skarn where the high garnet/pyroxene ratios are found. Fluid inclusions that represent the early stages both in garnet and pyroxene plot in 'Primary Magmatic Fluid' and 'Metamorphic Fluids' fields. A magmatic fluid, presumably located at deeper parts of the system, mixed with a metamorphic fluid during its

Skarn Alteration and Au-Cu Mineralization Associated with Tertiary Granitoids in Northwestern Turkey: Evidence from the Evciler Deposit, Kazdag Massif, Turkey

The Evciler Au-Cu district is located in the Kazdag region of northwestern Anatolia, Turkey. Skarn alteration and Au-Cu mineralization assemblages are related to an elliptical I-type, magnetite series, metaluminous, calc-alkaline body, which has intruded the Kazdag Massif. The skarns have an oxidized mineralogy dominated by garnet (Ad85Gr15-Ad50Gr50), hedenbergitic pyroxene (Di70Hd30-Di25Hd75), epidote, amphibole, and chlorite and contain up to 80 percent sulfides (pyrrhotite >> pyrite + chalcopyrite). The massive pyrrhotite-bearing mineralization replaced prograde skarn and marble. At least two paragenetic stages of skarn formation and ore deposition have been recognized: stage I hedenbergitic pyroxene ± garnet ± scapolite ± quartz ± magnetite, and stage II amphibole ± epidote ± chlorite ± quartz ± calcite and pyrrothite + chalcopyrite ± pyrite. Early skarn formation (stage I) was dominated by anhydrous minerals, hedenbergitic pyroxene, and garnet. The hydrous skarn assemblage (stage II) replaced early-formed skarn assemblages. Clinopyroxene is hedenbergite rich, whereas garnet is andraditic. The mineralogy and character of the Evciler skarn is consistent with deposition under oxidized conditions, formation at shallow crustal levels and similar to those of other Cu-Au, Fe-bearing skarn systems. Stable isotope data indicate that magmatic water was involved in the early stage of skarn formation (δ18OH2O= 6.3–10.9‰) and a mixed magmatic-meteoric water was responsible for the subsequent mineralization (δ18OH2O = −4.6 to +1.0‰, δDH2O= −30.9 to −17.7‰).

Symplectites derived from metastable phases in martian basaltic meteorites

The martian basalts Los Angeles, QUE 94201, Shergotty, and Zagami contain several late-stage mineral types including oxides, sulfides, phosphates, and associated silicate assemblages. Symplectites consisting of two- and three-phase assemblages are present in Los Angeles, QUE 94201, and Shergotty. The three-phase symplectite is composed of hedenbergitic pyroxene, fayalitic olivine, and an SiO 2 polymorph, and the two-phase symplectite consists of fayalitic olivine and an SiO 2 polymorph. These symplectites are commonly found in close association with the Ca-phosphate merrillite [general formula Ca 9 (Mg,Fe 2 + )(PO 4 ) 7 ]. Scanning Electron Microscopy was used to examine the distribution and occurrence of the symplectites, and point-count and electron-microprobe data were used to recast symplectites in terms of pyroxene stoichiometry. The reconstructed three-phase symplectite compositions plot close to lunar (metastable) pyroxferroite on the pyroxene quadrilateral diagram and indicate that pyroxene compositions ranged into the forbidden region and eventually crystallized pyroxferroite, which subsequently broke down to the three-phase symplectite upon cooling. Two-phase symplectites, which occur directly adjacent to merrillite, may be the result of breakdown of metastable ferrosilite. The crystal chemistry of merrillite and its close association with the symplectites indicates that during crystallization of pyroxene, co-crystallizing merrillite contributes to depletion of Ca and Mg and the formation of metastable pyroxferroite and ferrosilite.

The Geochemistry of Three Tin-Bearing Skarns and TheirRelated Plutonic Rocks, Atlin, Northern British Columbia

The Late Cretaceous Surprise Lake batholith in the Atlin district of northern British Columbia is a highly differentiated, fluoritic, and peraluminous subalkaline body of adamellite-granite composition. The presence of miarolitic cavities and chilled, fine-grained margins suggests it was emplaced at a high structural level, and its trace element signature indicates it has some within-plate granitoid characteristics. The batholith shows many chemical similarities to the I-type igneous rocks associated with W skarns in British Columbia, although it averages >2,700 ppm F, whereas plutons related to W skarns average <400 ppm F. It also shares similar characteristics to the Seagull batholith in south-central Yukon which, like the Surprise Lake suite, is associated with several skarns, including the largest Sn skarn in the Canadian Cordillera, the 1.25 Mt JC deposit. The Surprise Lake batholith and a nearby satellite body, the Mount Leonard stock, intrude an oceanic package of sedimentary, mafic volcanic, and ultramafic rocks belonging to the allochthonous Cache Creek terrane. Hornfels adjacent to the intrusive bodies host three Sn-bearing skarns, the Silver Diamond, Atlin Magnetite, and Daybreak occurrences. Despite the close proximity (<500 m) of all three skarns to the Surprise Lake intrusive bodies, they exhibit differences in their mineral compositions and assemblages, skarn textures, and presumed oxidation states. The presence of andraditic garnets (Ad 76 –99 ) and the overall higher pyrite and Cu content at the Atlin Magnetite skarn suggests it formed in relatively more oxidizing conditions. By contrast, the abundance of pyrrhotite and hedenbergitic pyroxene (Hd90–99) at the Silver Diamond occurrence points to a more reduced and sulfur-rich chemical state. The Daybreak occurrence is notable for having an extensive distal skarn with garnet-vesuvianite-pyroxene veins and a much smaller proximal ribbon or wrigglite skarn with thin rhythmic layers (0.25 mm to 1 cm) dominated by either magnetite, garnet, fluorite, or vesuvianite. Some of its skarn silicates contain elevated amounts of Cr, F, Mg, or Sn (up to 1.19 wt % Cr 2 O 3 , 2.07 wt % F, and 1.68 wt % MgO in the garnets and 1.77 wt % F and 0.15 wt % SnO 2 in the vesuvianites). There are two chemically distinct garnet populations in the distal veined skarn, one of which contains higher quantities of F, Mg ,and Ti, whereas the other population is characterized by more Cr, Fe, and Mn. Although many Sn and W skarns worldwide contain F-bearing garnets, it is unusual for such systems to also contain Cr- and Mg-bearing garnets. The Cr and Mg enrichment in some Daybreak garnets is probably due to contamination from ultramafic and dolomitic units in the host rocks. This in turn reflects a tectonic history that enabled mineralizing fluids from the Surprise Lake batholith to interact with ultramafic and oceanic rocks of the Cache Creek terrane.

Evidence for solar nebula signatures in the matrix of the Allende meteorite

Carbonaceous chondrites of type 3 (e.g. Allende) are among the most primitive meteorites. They contain components that formed prior to the accretion of a parent asteroid and thus record conditions of the ambient nebular gas, the source of the material from which the solid bodies of our solar system formed. Identification of nebular signatures is often difficult as thermal metamorphism and/or aqueous alteration on the meteorite parent body may have erased mineralogical evidence of nebular processes. The major fraction of Ca in the Allende matrix is contained in relatively large, 20–50 μm, Ca,Fe-rich aggregates (CFA) commonly assumed to have formed by parent body processes. To better constrain their origin a transmission electron microscopic study of these CFA was performed. They consist mainly of hedenbergitic pyroxenes with minor andradite and sulfide. We found that pyroxenes with low Mg content belong to the space group P2/n, whereas the expected C2/c structure is restricted to pyroxenes with higher Mg content. A hedenbergitic pyroxene with space group P2/n has never been reported in the literature and is considered metastable. The relationship between composition and space group can be explained best assuming ferrobustamite (with wollastonite structure) as a precursor phase for the P2/n pyroxenes. Above 970°C a two phase field exists between ferrobustamite and augite. The miscibility gap widens towards higher temperatures. In one case intergrown P2/n with C2/c pyroxenes were found. Their compositions fit well into the ferrobustamite–augite two phase field above 1050°C. Very fast cooling (>10°C/h) controls the incomplete transformation from ferrobustamite to hedenbergite resulting in the observed P2/n space group of Mg-poor pyroxenes. Thus, the Ca,Fe-pyroxenes provide strong evidence for a high temperature origin (>1050°C) followed by rapid cooling (>10°/h), implying that the CFA in the Allende matrix formed in the solar nebula as the Allende parent asteroid has never seen such temperatures and possible cooling rates on a kilometer sized body are orders of magnitude lower. The conditions required for the formation of the CFA suggest either transport from a high temperature to a low temperature environment or very localized heating events in the solar nebula. In addition strongly oxidized conditions (log fO 2 (bar)=−15 to −10) are required to stabilize andradite against hedenbergite.

Crystallization processes in an artificial magma: variations in crystal shape, growth rate and composition with melt cooling history

A large (4.8 m3, 1.3x107 g) artificial mafic melt with a bulk composition similar to that of a basalt (but with a high CaO content of 17 wt%) was generated during a demonstration of in situ vitrification and was allowed to cool naturally. During the melting process, convection was vigorous, resulting in a chemically and thermally homogeneous melt body. Once heating was complete, the cooling rate was rapid with the temperature dropping from 1500°C to 500°C in ∼6 days within the interior of the 3 m diameter, 1.5 m thick body. A ∼20 h period of constant temperature (1140°C) observed during colling was the result of latent heat released by widespread crystallization. The final crystalline assemblage consists of diopsidic to hedenbergitic pyroxene and anorthitic feldspar, with a subordinate amount of potassic feldspar, plus a small amount of evolved glass. The compositions and proportions of phases agree well with those predicted by the MELTS thermodynamic model. Thermal and textural evidence suggest that convection within the melt ceased coincident with formation of the first crystals. Textural investigation of core samples reveals large (up to 1 cm in length) acicular diopsidic pyroxenes in a matrix of smaller feldspar and zoned pyroxene crystals (∼500 μm in length). Crystal shape and pyroxene composition vary as a function of position within the solidified body, as a function of cooling rate. Both crystal size and degree of crystallinity are highest in the central, most slowly-cooled parts of the rock. Crystal shape is characterized by tabular, equilibrium-growth forms in the slowly-cooled areas, grading to highly skeletal, dendritic forms at the rapidly-cooled edges of the body. The pyroxene crystals are dominantly homogeneous diopside, but crystals are characterized by thin Fe-rich hedenbergitic rims. These rims were deposited when Mg in the melt was depleted by diopside growth, and melt temperature had cooled sufficiently to allow Fe-rich pyroxene growth. Crystal growth rates can be calculated based on thermal behavior of the melt, reinforced by thermodynamic modelling, and are determined to be between 10-7 and 10-8 cm/s in the central part of the melt. These estimates agree well with growth rates in natural systems with similar cooling rates. Pyroxene crystals that formed at a higher cooling rates are characterized by higher Al and lower Mg contents relative to tabular equilibrium crystalline forms, presumably as a result of disequilibrium melt compositions at the crystal-melt interface.

Generation of rhyolitic melt in an artificial magma: Implications for fractional crystallization processes in natural magmas

Abstract A large (1.3 × 107 g) artificial mafic melt with a bulk composition similar to an unusually calcic basalt (17 wt.% CaO) was produced by in-situ heating of soil, and subsequently cooled and crystallized. The final crystalline assemblage consisted dominantly of acicular diopsidic to hedenbergitic pyroxene and anorthitic feldspar, with a subordinate amount of potassic feldspar. Electron microprobe analyses reveal the presence of a small amount (∼10 vol.%) of rhyolitic glass (71% SiO2) within the crystalline network. This glass is the residual material left after crystallization of pyroxene and feldspar, analogous to rhyolitic melt which may be generated from fractional crystallization of a basaltic magma. Ion microprobe imaging indicates that the rhyolitic glass is generally present in isolated triangular- to rectangular-shaped interstices left by crystallization of acicular and tabular phases, and that the glass is enriched in trace elements (such as Zr and Cs) which are incompatible within the crystalline phases. No evidence of coalescence or migration of the rhyolitic melt is suggested by glass morphology. Although these observations were made in an artificial magmatic system, analogies to a natural system may be drawn. The isolated nature and low abundance of the rhyolitic glass highlights the difficulty of extracting and segregating evolved melts produced by high degrees of crystallization of a primitive parent, particularly one characterized by elongated, rather than equant, crystals. The distribution of the rhyolitic glass supports the suggestion that extraction of significant amounts of evolved melts from rocks may require repeated partial melting of crystallized material in order to allow the evolved liquid to physically separate from the parent, and also suggests that the geometry of crystals may be an important factor in melt segregation.

Volcanic stratigraphy and ore deposits of the Moctezuma District, Sonora, Mexico

An ash-flow tuff sequence characterizes the rocks of the Moctezuma district, Sonora. Within the sequence, a 50-m-thick unit, the Arenillas Formation, consists of volcaniclastic sedimentary rocks and limestone lenses. Tensional deformation of the region has produced basin-and-range structure. A strong north-trending normal fault forms the western boundary of the principal range of the district, normal faults on the east side of the range define a horst, and the sequence is tilted 10 degrees to 30 degrees east. At the Oposura mine, a Zn-Pb skarn deposit in the limestone of the Arenillas Formation contains galena, sphalerite, magnetite, bustamite, Mn-bearing andradite, hedenbergitic pyroxene, and strong late chloritic alteration. Elsewhere in the district, steep quartz veins contain three characteristic associations: (1) Au with Te, Ag, and barite, (2) Ag with galena, sphalerite, tetrahedrite, and high values of Sb, As, and Mn, and (3) galena, sphalerite, chalcopyrite, and ankerite with low Ag and Au values. Structural reconstructions suggest that these vein deposits developed at different depths, with the first association shallowest, the third association deepest. The skarn may have formed deeper than all the others.

Skarn and porphyry copper mineralization at Mines Gaspe, Murdochville, Quebec

Mines Gaspe extracts copper and molybdenum from skarn and porphyry copper orebodies at Murdochville in the Gaspe Peninsula, Quebec. Both types of orebodies occur in the zoned Copper Brook aureole, which formed in calcareous Lower Devonian strata around the Copper Mountain plug, one of a group of small rhyodacite porphyry sills, dikes, and plugs of Upper Devonian to Mississippian age. Progressive metamorphism is marked by the alteration of diagenetic pyrite to pyrrhotite and the successive appearance of phlogopite, tremolite, diopside, and grossularitic garnet toward the plug. Diopside formation was accompanied by destruction of calcite, pyrrhotite, and carbonaceous material to form a bleached inner aureole. Similar metamorphic zones surround the nearby Porphyry Mountain plug and occur (without garnet) in the Porphyry Brook aureole. The three aureoles appear to coalesce some 1,200 to 1,500 m below surface.Crackle veinlets in the strata domed by eraplacement of the Copper Mountain plug were healed by quartz, feldspars, diopside, and grossularitic garnet, the same minerals that were formed in the metamorphosed wall rocks. Wollastonite partially replaced diopside in a ring around the plug. Iron metasomatism near the plug then replaced diopside with hedenbergitic pyroxene and replaced pale grossularitic garnet and wollastonite with darker andraditic garnet in veinlets and concretions. Massive skarn formed in some metamorphosed limestone units. The Porphyry Mountain plug also domed the surrounding sedimentary rocks. No crackle veinlets or wollastonite formed but andraditic garnet and hedenbergitic pyroxene were deposited in adjacent joints and bedding fractures. Metamorphism and metasomatism were followed, in some parts of the Copper Brook aureole, by formation of scapolite and a second generation of tremolite. Two major periods of mineralization followed.Early mineralization is characterized by pyrrhotite, minor chalcopyrite, and traces of sphalerite throughout the three aureoles. It is associated with alteration of Ca-Mg-Fe silicates to tremolite or actinolite, chlorite, and epidote or clinozoisite and with deposition of albite, calcite, and quartz. More intense mineralization formed replacement orebodies at Needle Mountain and Needle East, some distance from the Copper Mountain plug. These orebodies were cut by late 1 quartz veins containing chalcopyrite, pyrrhotite, fluorite, and traces of scheelite.Porphyry copper mineralization occurs as disseminations and in four sets of veins centered on the Copper Mountain plug. Late 2 veins, which contain quartz + anhydrite + K-feldspar with traces of chalcopyrite and magnetite, may be coeval with biotization and K-feldspar alteration in the plug. Late 3 veins, which account for most of the copper and molybdenum mineralization, contain quartz + or - anhydrite with chalcopyrite, pyrite, and molybdenite. They are variously associated with kaolinization, sericitization, and minor anhydrite alteration in the plug and adjacent intrusions and with formation of tremolite, epidote, chlorite, and minor anhydrite in the metasedimentary rocks. Late 4 veins contain calcite or dolomite, quartz + or - anhydrite, and pyrite, with traces of K-feldspar, chalcopyrite, sphalerite, and galena. They are associated with calcite + sericite + pyrite alteration of the plug and calcite, dolomite, and phlogopite alteration of the metasedimentary rocks. Late 3 and late 4 veins often occupy almost vertical fractures parallel to the local north-northwest joint trend. Late 5 veins contain calcite, quartz, laumontite, and apophyllite, with traces of K-feldspar, chlorite, molybdenite, chalcopyrite, and pyrite or pyrrhotite. Traces of scheelite occur in late 1, late 3, and late 5 veins and in quartz + garnet veins which predate all sulfide mineralization.A model is proposed whereby emplacement of a pluton generated a convective flow of water which metamorphosed the overlying strata to form the aureoles. The Copper Mountain and Porphyry Mountain plugs, apophyses of the pluton, intruded the metasedimentary rocks and superimposed further metamorphic and metasomatic effects on them. Early mineralizaotion, like the prograde metamorphism, is widespread throughout the aureoles (although very weak in most areas). Late mineralization, by contrast, is closely associated with the Copper Mountain plug. The succession of widespread and spatially restricted events suggests that the postulated pluton and the visible plugs alternately controlled the sequence of metamorphism, iron metasomatism, and early and late mineralization. The difference in zoning patterns between Mines Gaspe and other porphyry copper-skarn complexes was probably due to the fact that the Copper Mountain orebody formed in previously decarbonated wall rocks.

An experimental study of sulfidation reactions involving andradite and hedenbergite

Three sulfur-buffering, sulfide-silicate reactions, which limit the stabilities of the skarn silicates andradite garnet and hedenbergite pyroxene under sulfidizing conditions, were studied hydrothermally at a pressure of 2 kb over the temperature range 700 degrees to 350 degrees C:Reaction (1): andradite + S 2 = wollastonite + anhydrite + quartz + pyrite or pyrrhotite,Reaction (2): andradite + magnetite + S 2 = anhydrite + quartz + pyrite or pyrrhotite, andReaction (3): hedenbergite + S 2 = andradite + quartz + pyrrhotite.The equilibrium value of the sulfur fugacity (f (sub S 2 ) ) for each reaction assemblage at any given temperature was calculated from the measured composition of a coexisting sulfur fugacity indicator (pyrrhotite or sphalerite, as appropriate). The log f (sub S 2 ) ) data were adequate for the determination of the log f (sub S 2 ) )-1/T relations for each of the andradite sulfidation reactions. Owing to excessive scatter in the data for the hedenbergite sulfidation reaction, however, the log f (sub S 2 ) -1/T relation for this reaction, derived from the log f (sub O 2 ) data of Burton et al. (1982), is preferred to the one developed by Gamble (1976, 1978). The log f (sub S 2 ) -1/T relations for the three sulfidation reactions were combined with equilibrium data available in the literature for other reactions in the model system Ca-Si-Fe-O-S and used to construct quantitative, isothermal-isobaric, log f (sub S 2 ) -log f (sub O 2 ) phase diagrams applicable to the formation of Ca-Si-Fe skarns. The quantitatively derived phase relations were found to be consistent in all major respects with the schematic phase relations deduced by Burt (1972a and b).The influence of diopside component on the stability of hedenbergitic pyroxene was also studied. Diopside was found to enlarge the stability field of hedenbergitic pyroxene, increasing the temperature at which pyroxene becomes stable with pyrite from 288 degrees C for end-member hedenbergite (Burton et al., 1982) to 395 degrees C for 50 mole percent diopside. Diopside was found to be less effective than the johannsenite component in stabilizing hedenbergitic pyroxene.

Chemistry and mineralogy of some Plio-Pleistocene tuffs from the Shungura Formation, southwest Ethiopia

The Shungura Formation of southwestern Ethiopia has yielded many tens of thousands of vertebrate fossils including hominids and microvertebrates, and in addition has also yielded fossil wood, pollen, and invertebrates. Widespread tuffs have made subdivision and detailed mapping of the formation possible, have provided material for potassium-argon dating, and have allowed direct lithostratigraphic correlation with the Koobi Fora Formation in northern Kenya. The basis for correlation between the two formations is the distinctive chemistry of the tuffs, but systematic chemical variation within some tuffs invalidates some statistical correlation techniques. Here chemical analysis of glass separates and minerals from tuffs of the Shungura and Usno Formations are presented which may allow further ties to be established when data become available on other tuffs of the Koobi Fora Formation. The tuffs consist primarily of glass, but also contain phenocrysts of anorthoclase, hedenbergitic pyroxene, sodic amphibole, ilmenite, titanomagnetite, chevkinite, quartz, zircon, and rarely orthopyroxene and plagioclase. The glasses show evidence of alkali loss during hydration, and are not now peralkaline, although it is likely that they were initially. The source volcanoes were most likely situated within the Ethiopian rift valley, or on its margins.

Garnets in some metasomatic syenitic rocks of southwestern Japan

In Southwest Japan, ugrandite garnests of various compositions are frequently found in the metasomatic syenitic rocks occurring in granites. Some of these garnests were separated, and studied chemically and optically. Their chemical compositions, and physical and optical properties are given in Table 2 and 3, respecitvely. The equilibrium relations between the garnests and some other coexisthing minerals such as hedenbergitic pyroxene, aegirine-augite, and plagioclase, are discussed in detail.

Synthesis and stability relations of the hornblende ferropargasite

Ferropargasite has been synthesized and its stability relations investigated. From most to least oxidized, assemblages produced on decomposition are magnetite, plagioclase, andraditic garnet plus fluid; magnetite, plagioclase, garnet, hedenbergitic pyroxene, nepheline plus fluid; and hercynite, plagioclase, fayalite, hedenbergitic pyroxene, nepheline plus fluid. Principal effects caused by substituting Fe for Mg are lowering the high temperature stability limit at 1 kb fluid pressure from 1040 degrees C to 835 degrees C, and strong dependence of the ferropargasite stability field boundary on oxygen fugacity. Addition of silica lowers the upper thermal stability limit at low relative oxygen fugacities by 250 degrees C or more for fluid pressures of 2 kb. At high relative oxygen fugacities, addition of silica results in an amphibole whose upper stability limit is raised nearly 75 degrees C.

Synthesis and stability relations of ferrotremolite

Phase relations for the bulk composition 2CaOX5FeO x X8SiO 2 +H 2 O at moderate temperatures and fluid pressures, and low oxygen fugacity (fO 2 ) values, indicate a field of stability for the amphibole ferrotremolite, Ca 2 Fe 5 Si 8 O 22 (OH) 2 . With fO 2 defined by the magnetite-iron buffer, ferrotremolite is stable up to 437 degrees C at 500 bars, 543 degrees C at 3,000 bars; the high-temperature assemblage of equivalent composition consists of fayalite+quartz+hedenbergitic pyroxene+fluid. With fO 2 specified by the magnetite+quartz-fayalite buffer, the amphibole high-temperature stability limit is reduced about 4 degrees C, and magnetite accompanies the other high-temperature phases. At still higher fO 2 values, represented by the Ni-NiO 2 buffer, ferrotremolite is not stable at any temperature. Reasons for restriction of ferrotremolite in nature are examined.

Iron enrichment and pyroxene fractionation in tholeiites

AbstractDifferentiation in tholeiite magma, particularly iron enrichment and silica variation, as traced in lava successions and dolerite‐granophyre suites, is discussed in the light of analytical data on phenocryst and groundmass pyroxenes and their fractionation. Broadly two trends may be distinguished: (1) an intratelluric equilibrium trend ending in a hedenbergitic pyroxene phase fayalite, such as is exhibited by the Tasmanian Red Hill dolerite‐granophyre suite, and (2) a quench trend characteristic of the subcalcic augite groundmass sequence of the volcanic suites as exemplified in the Hawaiian tholeiite series (e.g. Kilauea). This trend may in more siliceous differentiates be extended to close with assemblages carrying a ferropigeonite phase.