Miarolitic Research Articles

Magmatic–hydrothermal evolution of highly fractionated granites: Evidence from the Kuiqi miarolite in Fujian province, SE China

The Kuiqi miarolite, a highly evolved alkaline granite pluton, is situated in Fujian province, southeastern China. Quartz crystals from the Kuiqi miarolite contain abundant inclusions, which have preserved a complete record of the magmatic–hydrothermal evolution. Five inclusion type were identified, i.e., the melt (M-type), melt–fluid (ML-type), daughter mineral-bearing (S-type), vapor-rich (V-type), and aqueous (L-type) inclusions. Integrated petrographic, microthermometric and laser Raman spectroscopy studies demonstrate that the whole magmatic–hydrothermal evolution at Kuiqi comprises three stages, i.e., the magmatic, magmatic–hydrothermal and hydrothermal stages. The M-type inclusions (minimal trapping temperatures: ca. 710–800°C) represent the melts entrapped at the magmatic stage of crystallization, while the occurrence of euhedral prismatic arfvedsonites in these melt inclusions indicates that the granitic magma was probably volatile-rich. At the magmatic–hydrothermal stage, both the ML-type inclusions (Th=ca. 500–710°C) and coexisting M- and V-type inclusions reflect the presence of late magmatic fluid circulation and melt-volatile immiscibility, exhibiting a transition toward the hydrothermal stage. At the hydrothermal stage, the pressure and temperature (700–485°C) drop may have led to oversaturation of the primary saline fluids, resulting in the phase separation a highly saline and a volatile-rich fluid occurred, accompanied by the widespread vapor–brine immiscibility. The L-type inclusions in the miarolitic quartz display bimodal salinity distribution: The Th and salinity of the inclusions at the bottom (L1) miarolitic quartz are ca. 303–427°C and ca. 6.6–17.9wt.% NaCl equiv, respectively, while that of the inclusions at the top (L2) are ca. 165–232°C and ca. 0.2–5.7wt.% NaCl equiv, respectively. This indicates a drop of fluid temperature and salinity during the miarolitic cavity formation. Moreover, the low homogenization temperatures and salinities of the L-type inclusions suggest an influx of meteoric water during the late hydrothermal evolution stage.

Crystal chemistry of brannockite, KLi3Sn2 Si12O30, from a new occurrence in the Golden Horn Batholith, Washington State, USA

A second locality of the milarite-group mineral brannockite (ideally KLi 3 Sn 2 Si 12 O 30 ) has been discovered at Liberty Bell/Washington Pass in the alkaline granite of the Golden Horn Batholith, Okanogan County, Washington, USA as sub-millimeter euhedral bladed clear crystals in a single miarolitic cavity in a biotite granite. The Washington Pass brannockite shows micro-scale compositional zoning involving tetravalent A-site Sn 4+ , Zr 4+ , Ti 4+ , and Hf 4+ substitution and the coupled substitution A (X 4+ )+ B □ ⇔ A (Fe/Al) 3++B (Na, K) + . In contrast, brannockite analyzed from the type locality in the late hydrothermal zone of the Li-Sn-rich pegmatite of the Foote Mine, Kings Mountain, North Carolina, USA exhibits near end-member composition with only minor Na + , Al 3+ , and Ti 4+ . Single-crystal X-ray diffraction of Washington Pass brannockite confirms the space group P 6/ mcc with unit-cell parameters a = 10.014(2)A, c = 14.268(5)A, and V = 1239.16(8)A 3 . Washington Pass brannockite co-exists with its Zr-analogues, both the milarite-group sogdianite and the related tuhualite-group mineral zektzerite. Selected samples of both minerals show Sn as minor element. The sogdianite contains unusually low Na and Ti compared to most other localities and has near endmember composition. The results emphasize the high compatibility of the milarite structure with different heterovalent cations, associated with the cation-size dependent coupled distortion of the octahedral A and tetrahedral T2 sites.

Trace-element compositions and Br/Cl ratios of fluid inclusions in the Tsushima granite, Japan: Significance for formation of granite-derived fluids

Fluid inclusions in quartz samples from a miarolitic cavity, two quartz veins, and a hydrothermal ore vein in the Tsushima granite, Japan, were analyzed by particle-induced X-ray emission to examine the chemistry and process of formation of hydrothermal fluids in an island-arc granite. Most of the inclusions were polyphase or vapor, and there were smaller numbers of two-phase aqueous inclusions. The inclusions contained Cl, K, Ca, Ti, Mn, Fe, Cu, Zn, Ge, Br, Rb, Sr, Ba, and Pb. For each inclusion, there was a strong positive correlation between Cl content and contents of other elements identified. Concentration ranges for most elements (other than Rb and Ge) in polyphase inclusions from the miarolitic cavity were comparable to those from cavities in alkaline granites; those from the ore vein were comparable to large-scale continental hydrothermal ore deposits. The lower Rb and higher Ge contents in the polyphase inclusions of the Tsushima granite may be characteristic of hydrothermal fluids from calc-alkaline granites in an island-arc setting. Br/Cl ratios (by weight) for the vapor and two-phase inclusions were 0.0013–0.0030 and differed among the three geological settings. Br/Cl ratios of polyphase inclusions increased with increasing Cl content in single-crystal and polycrystalline quartz, and high values of more than 0.0100 were found. The high Br/Cl ratios and the differences among the geological settings sampled may be due to pressure dependences of partitioning of Cl and Br between fluid and magma during fluid segregation and between liquid and vapor during boiling. Using a simple model based on these dependences, we calculated Br/Cl ratios greater than 0.01 in brine generated at pressures <0.89kbar. Differences in Br/Cl ratios in polyphase and vapor inclusions from each geological setting were attributed to mixing between two end-member fluids: a high Br/Cl fluid generated at low pressure and a low Br/Cl fluid generated at high pressure. Br/Cl ratios of polyphase inclusions in quartz may be a key for understanding the conditions under which boiling fluids are generated in simple granite systems.

Carbonate inclusions in Lower Cretaceous picrites from the Hončova hůrka Hill (Czech Republic, Outer Western Carpathians): Evidence for primary magmatic carbonates?

Porphyritic picrites from the Honcova hůrka site in the Silesian Unit (Western Carpathians) are composed mostly of olivine phenocrysts enclosed in a black fine-grained groundmass, which consists of clinopyroxene, biotite, magnetite, chlorite, feldspars, and zeolites. The rocks are variably affected by hydrothermal alteration. The freshest samples contain potentially primary igneous calcite and aragonite, which occur as globular inclusions hosted by olivine phenocrysts, or as fillings of the miarolitic cavities in the picrite groundmass. In this paper, we try to clarify the nature of investigated carbonates using the combination of several petrological methods. Based on the texture, mineral composition, and relationship to the alteration patterns of the host mineral, we distinguished three basic types of inclusions: carbonate inclusions, silicate inclusions, and a combined type consisting of both carbonate and silicate domains. Only the fresh olivine-hosted round carbonate globules can contain the primary igneous calcite. These globules cannot represent immiscible carbonatite melt since they lack Si, alkalis, and other essential components (e.g. P, F, Cl, and S) present in natural carbonate melts. Instead, they can be interpreted as product of equilibrium crystallization of calcite from carbonated silicate melt (i.e. crystal cumulates). In contrast, the calcite–aragonite assemblage in inclusions hosted by altered olivine and in miaroles most probably originated during recrystallization of primary calcite during late-magmatic or post-magmatic stages or is related to the superimposed hydrothermal alteration.

Textures and melt-crystal-gas interactions in granites

Felsic intrusions present ubiquitous structures. They result from the differential interactions between the magma components (crystal, melt, gas phase) while it flows or when the flow is perturbed by a new magma injection. The most obvious structure consists in fabrics caused by the interactions of rotating grains in a flowing viscous melt. New magma inputs through dikes affect the buk massif flow, considered as global within each mineral facies. A review of the deformation and flow types developing in a magma chamber identifis the patterns that could be expected. It determines their controlling parameters and summarizes the tools for their quantification. Similarly, a brief review of the rheology of a complex multi-phase magma identifies and suggests interactions between the different components. The specific responses each component presents lead to instability development. In particular, the change in vorticity orientation, associated with the switch between monoclinic to triclinic flow is a cause of many instabilities. Those are preferentially local. Illustrations include fabric development, shear zones and flow banding. They depend of the underlying rheology of interacting magmas. Dikes, enclaves, schlieren and ladder dikes result from the interactions between the magma components and changing boundary conditions. Orbicules, pegmatites, unidirectional solidification textures and miarolitic cavities result from the interaction of the melt with a gaseous phase. The illustrations examine what is relevant to the bulk flow, local structures or boundary conditions. In each case a field observation illustrates the instability. The discussion reformulates instability observations, suggesting new trails for ther description and interpretation in terms of local departure to a bulk flow. A brief look at larger structures and at their evolution tries to relate these instabilities on a broader scale. The helical structures of the Říčany pluton, Czech Republic and by the multiple granitic intrusions of Dolbel, Niger illustrate such events.

The role of external fluid in the Shanggusi dynamic granitic magma system, East Qinling, China: Quantitative integration of textural and chemical data

It is well recognized that various degrees of mantle-derived materials are incorporated in the formation of granite, and mantle-derived mafic melts are generally considered to mix with crustal felsic melt. Here, however, we provide an example of the Shanggusi leucocratic granite where external mantle-derived hydrous fluid, rather than mafic melt, might be incorporated into a nearly pure crustal granitic melt system. Field observations suggest that the Shanggusi granite consists of granite porphyry, granite dyke and granitic pegmatite and they have consistent zircon U–Pb ages and molybdenite Re–Os ages. The marginal pegmatite, interconnected miarolitic cavities, heterogeneous molybdenite mineralization and significant variation of micro-texture of the Shanggusi granite physically indicate that strong fluid activities occurred in the granitic system. Accumulation of quartz and K-feldspar and bulk-rock major element data imply that fractional crystallization played an important role in the evolution of the granitic system which, however, cannot reasonably explain the significant trace elements fractionation, non-CHARAC trace elements behavior and simultaneous concave and convex REE tetrad effect of the Shanggusi granite, but which can be best explained by the influence of external fluorine-rich hydrous fluid. Importantly, the chemical fractionation, including bulk-rock trace elements and isotopes, is closely correlated with quantitative textural parameter L max (the average length of the four largest quartz crystals in each sample), indicating that the vast majority of physical and chemical characteristics of the granitic system were most likely controlled by the wholesale fluid flow. The Shanggusi granite is highly siliceous (SiO 2 = 74.91–79.50 wt.%, except granitic pegmatite with SiO 2 = 67.41 wt.%), extremely poor in mafic minerals, and with relative homogeneous bulk-rock major element chemistry and mineralogy, which approximate experimentally pure crustal melt that generated by dehydration melting of sedimentary rocks. The mantle-derived Nd–Pb–O isotopic features, however, require a great deal of contributions from mantle materials. Therefore, it is proposed that external mantle-derived fluid was infiltrated into the Shanggusi granitic system at least before emplacement, and fluid–melt mixing may be an important but not easily recognized part in granite origin. • Quantitative integration of textural and chemical data in igneous petrology • External mantle-derived fluid was involved in the Shanggusi granitic system. • Fluid-induced chemical and textural diversity

Anatomy of the Cretaceous Hobenzan pluton, SW Japan: Internal structure of a small zoned pluton, and its genesis

Abstract Field, petrographic, geochemical, and K–Ar and U–Pb age data were used to elucidate the internal structure of the Cretaceous Hobenzan pluton, SW Japan, and the processes which generated that structure. The pluton is elongated E–W with dimensions of about 6.5 × 2.0 km (13 km2), and was emplaced at ~ 95 Ma as a pluton in accretionary complexes. The pluton contains an early tonalite, but most of the body consists of later granitoids that show a continuous differentiation series from biotite–hornblende granodiorite (GD) to hornblende–biotite granite (HBG) and biotite granite (BG). The contacts between the GD and HBG are gradational. The pluton provides an exceptional cross-sectional view of a simple cooling magma body. The GD shows no vertical variations in modal and chemical compositions, whereas the HBG displays differentiation from the lowermost exposure to the top of the pluton. Initial Sr isotope ratios (SrI) in the HBG increase from the lower part to the top of the pluton. The granitoids show continuous compositional variations from 65 to 79 wt.% SiO2 (anhydrous basis), and magmatic differentiation was dominantly controlled by crystal fractionation of hornblende and plagioclase. Field, elemental and Sr–Nd isotope data are consistent with limited operation of assimilation with pelitic rocks and fractional crystallization (AFC), in which assimilation increased with higher degrees of differentiation. The Hobenzan pluton retains a history of granitoid magma evolution in a subvolcanic magma reservoir. The GD formed as a rigid sponge, and melt fraction increases inwards from the walls, forming the HBG mush by fractional crystallization, coupled with small degrees of assimilation of adjacent schists. A more evolved and enriched low-density melt segregated from the mushy cumulate of the HBG by incomplete crystal–melt separation, and moved upwards with the assistance of gas-driven filter pressing, as indicated by the presence of miarolitic cavities, thus forming the BG at the roof of the pluton.

GEOCHEMISTRY, MINERALOGY, AND EVOLUTION OF Li-Al MICAS AND FELDSPARS FROM THE MOUNT MICA PEGMATITE, MAINE, USA

Mount Mica is a poorly zoned, sodic LCT-type pegmatite consisting dominantly of quartz, albite, and muscovite in the outer portion of the pegmatite. Micas are the principal K minerals throughout the pegmatite. Potassium feldspar and rare-element minerals such as lepidolite, elbaite, Cs-rich beryl, fluorapatite, and pollucite are restricted to the core zone of the pegmatite. The dearth of rare element (Li, F, Cs, and Rb) substitution in minerals of the outer portion of the pegmatite and the relatively low K/Rb ratio of K-feldspar in the core zone indicate that the overall degree of fractionation of the pegmatite is moderate. Aluminum-micas belong to the muscovite–polylithionite chemical trend, with a compositional gap between muscovite and trilithionite. Lithium is incorporated into the micas mainly via the Li 3 Al −1 □ −2 substitution mechanism. Lithium-Al micas in the wall zone are chemically homogeneous, but abruptly evolve into higher Cs + Rb bearing Li-rich muscovite and lepidolite in the core zone. An abrupt change in grain size, texture, and chemical composition is evident in the micas from the core zone. Pods of fine- to coarse-grained lepidolite occur sporadically throughout the core zone. Muscovite crystals located adjacent to lepidolite pods or adjacent to or within miarolitic cavities may be overgrown by a coarse-grained mosaic of lepidolite crystals. Some of the overgrown muscovite crystals display interlayering on a microscopic μm scale. The interlayering suggests late-stage rapid crystallization along a diffusion-controlled crystal-liquid interface. The abrupt increase of the Cs, Rb, and F in K-feldspar, muscovite, and lepidolite and the occurrence of such highly evolved species as F-rich lepidolite, pollucite pods, elbaite tourmaline, Cs-rich beryl, and spodumene in miarolitic cavities in the core zone suggest that incompatible elements were retained in the residual fluid until their concentration was high enough to initiate crystallization of incompatible-rich mineral phases. The relatively low abundance of incompatible elements in the hanging-wall zone (HWZ) and footwall zone (FWZ) suggest that the fractionation process was very efficient in sweeping the incompatibles into the core zone of the pegmatite, producing proportionally small volumes inside the pegmatite with very high enrichment in incompatible elements.

TERTIARY PEGMATITE DIKES OF THE CENTRAL ALPS

The largest field of Alpine Oligocene pegmatite dikes is in the Central Alps within the Southern Steep Belt (SSB) of the Alpine nappes; it extends for about 100 km in an E–W direction and 15 km in a N–S direction north of the Periadriatic Fault, from the Bergell pluton (to the east) to the Ossola valley (to the west). The pegmatite field geographically overlaps (1) the highest temperature domain of the Lepontine Barrovian metamorphic dome and (2) the zone of Alpine migmatization. We have studied pegmatites in two areas: (1) the Codera area on the western border of the Bergell pluton and (2) the Bodengo area between the Mera and the Mesolcina valleys. Most pegmatites show a simple mineral assemblage consisting of K-feldspar, quartz, and muscovite ± biotite, and only a minor percentage of the dikes (< 5%) contains Sn-Nb-Ta-Y-REE-U oxide, Y-REE phosphate, Mn-Fe-phosphate, Ti-Zr-silicate, Be-Y-REE-U-silicate and oxide minerals (beryl, chrysoberyl, bertrandite, bavenite, and milarite), garnet (almandine-spessartine), tourmaline (schorl to rare elbaite), bismuthinite, magnetite, and rarely dumortierite and helvite. The mineral assemblages, geological context, and chemical compositions allow the distinction between LCT (lithium, cesium, tantalum) and mixed LCT-NYF (niobium, yttrium, fluorine) pegmatites (with only one exception of an NYF dike in the Bodengo area). The LCT pegmatites of the Central Alps did not reach a high degree of geochemical evolution. The most fractionated pegmatites are found in the Codera area and contain Mn-rich elbaite, triplite, pink-beryl, and Cs-Rb-rich feldspar. In the Bodengo area pegmatites locally contain miarolitic cavities and the most evolved pegmatites correspond to the beryl-columbite-phosphate type. From a structural point of view two main types of pegmatites can be distinguished: (1) pegmatites that were involved in ductile deformation and (2) pegmatites that postdated the main ductile deformation of the SSB. Many pegmatites of the Codera valley belong to the first structural type: they were emplaced at relatively high ambient temperature (ca. 500 °C) and locally show a pervasive recrystallization of quartz and a mylonitic structure. The Codera dikes trend about 70° and are steeply dipping. In the Bodengo area the main set of pegmatites (trending approximately N–S to NNE–SSW) crosscuts the ductile deformation structures of the SSB, but the area also includes an earlier generation of boudinaged and folded pegmatite dikes. The undeformed pegmatites from this area may contain miarolitic pockets. There is no systematic difference in the mineral assemblage between the two structural types of pegmatites. However, the chemistry of pegmatite minerals, especially of garnet, in addition to field data suggests that the dikes of the Codera and Bodengo areas represent two distinct generations of pegmatites. Structural data and the few existing radiometric ages suggest that pegmatites were emplaced over a time span between 29 and 25 Ma (and possibly as young as 20 Ma), with the youngest dikes postdating the ductile deformations of the Alpine nappes. The present work presents a first comprehensive field description and geochemical – mineralogical characterization of the Alpine pegmatite field of the Central Alps.

Mesozoic Tectonomagmatic Evolution and Gold Metallogeny in Jiaodong Area, East China

There were twice major collision orogenic events in Jiaodong area in Mesozoic period. It showed as three times of magmatic activities and stretching in Jiaodong area. In this paper, based on collecting age datas, referring to the previous classification scheme, a chronological frame pattern of Yanshanian granites had been put forward: Linglong-Kunyushan granite emplacement was in in 160~150Ma; the formation of Guojialing granodiorite was in 130~126Ma; Weideshan granodiorite-granite emplacement was in 120~110Ma; Laoshan A-type miarolitic cavity parlkaline alkali feldspar granite emplacement was in 110~100Ma and represented the end of Yanshan movement. Gold mineralization in three periods in this area had coupled relation with Linglong-Kunyushan granite, Guojialing granodiorite and Weideshan granodiorite-granite. Jiaodong tectonic-magmatic events and gold mineralization were controlled by the interactions among Tethyan tectonic domain, Paleo-ocean tectonic domain and the Pacific tectonic domain.

Copper–zinc albite porphyry in the Hersai porphyry copper deposit, East Junggar, China: A transition between late magmatic and hydrothermal porphyry copper deposit

A 418Ma albite porphyry dike emplaced in a 429Ma granite pluton in the Hersai porphyry copper deposit (Nom, East Junggar, China) consists of euhedral and pale-orange luminescing albite phenocrysts and groundmass, zircons, blue-violet luminescing apatites and chalcopyrite–sphalerite-rich miarolitic cavities. This dike is characterized by two-domain textures presented by albites and disseminated intergranular miarolitic cavities, and by geochemical features of continental island arc and adakitic intrusions.The miarolitic cavities consist of various minerals that crystallized in the following sequence: epidote, rutile and sphalerite–chalcopyrite solid solution, titanite and K-feldspar; chalcopyrite with pyrite inclusions; and chlorite, zoisite and calcite. The miarolitic cavities exhibit magmatic to hydrothermal internal textures, such as aplitic textures presented by intergrowths of albite, epidote, rutile and sphalerite–chalcopyrite solid solution, zoning and intersertal texture, poikilitic textures, and layered chlorites. The sphalerite contains a high concentration of exsolved chalcopyrite (up to 10 vol.%). The rutiles intergrown with epidotes contain 137 to 528ppm Zr corresponding to Zr-in-rutile temperatures of 507 to 625°C, and the rutiles intergrown with sphalerite–chalcopyrite solid solutions have 381 to 420ppm Zr and Zr-in-rutile temperatures of 585 to 605°C. Mineralization temperatures of sphalerite–chalcopyrite solid solutions are possible approximately 600°C. The formation of the chalcopyrite–sphalerite-rich miarolitic cavities is related to interstitial residual sulfide-rich fluids. The copper–zinc albite porphyry may represent a copper–zinc (Cu–Zn) subtype of porphyry copper deposit that formed as a transition between late magmatic and classical hydrothermal porphyry copper deposits.

Hillesheimite, (K,Ca,□)2(Mg,Fe,Ca,□)2[(Si,Al)13O23(OH)6](OH) · 8H2O, a new phyllosilicate mineral of the Günterblassite group

A new mineral, hillesheimite, has been found in the Graulai basaltic quarry, near the town of Hillesheim, the Eifel Mountains, Rhineland-Palatinate (Rheinland-Pfalz), Germany. It occurs in the late assemblage comprising nepheline, augite, fluorapatite, magnetite, perovskite, priderite, gotzenite, lamprophyllite-group minerals, and akermanite. Colorless flattened crystals of hillesheimite reaching 0.2 × 1 × 1.5 mm in size and aggregates of the crystals occur in miarolitic cavities in alkali basalt. The mineral is brittle, with Mohs’ hard-ness 4. Cleavage is perfect parallel to (010) and distinct on (100) and (001). D calc = 2.174 g/cm3, D meas = 2.16(1) g/cm3. IR spectrum is given. Hillesheimite is biaxial (−), α = 1.496(2), β = 1.498(2), γ = 1.499(2), 2V meas = 80°. The chemical composition (electron microprobe, mean of 4 point analyses, H2O determined from structural data, wt %) is as follows: 0.24 Na2O, 4.15 K2O, 2.14 MgO, 2.90 CaO, 2.20 BaO, 2.41 FeO, 15.54 Al2O3, 52.94 SiO2, 19.14 H2O, total is 101.65. The empirical formula is: K0.96Na0.08Ba0.16Ca0.56Mg0.58Fe 0.37 2+ [Si9.62Al3.32O23(OH)6][(OH)0.82(H2O)0.18] · 8H2O. The crystal structure has been determined from X-ray single-crystal diffraction data, R = 0.1735. Hillesheimite is orthorhombic, space group Pmmn, the unit-cell dimensions are: a = 6.979(11), b = 37.1815(18), c = 6.5296(15) A; V=1694(3) A3, Z = 2. The crystal structure is based on the block [(Si,Al)13O25(OH)4] consisting of three single tetrahedral layers linked via common vertices and is topologically identical to the triple layers in gunterblassite and umbrianite. The strong reflections [d A (I %)] in the X-ray powder diffraction pattern are: 6.857(58), 6.545(100), 6.284(53), 4.787(96), 4.499(59), 3.065(86), 2.958(62), 2.767(62). The mineral was named after its type locality. Type specimens are deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow, registration number 4174/1.

Setting and Evolution of the Archean Synvolcanic Mooshla Intrusive Complex, Doyon-Bousquet-LaRonde Mining Camp, Abitibi Greenstone Belt: Emplacement History, Petrogenesis, and Implications for Au Metallogenesis

The synvolcanic Mooshla Intrusive Complex intrudes coeval ~2699 to 2696 Ma volcanic rocks of the Blake River Group within the southern margin of the Archean Abitibi greenstone belt. The upper Blake River Group is host to the Doyon-Bousquet-LaRonde mining camp that contains Au-rich volcanogenic massive sulfide (VMS) deposits, possible subsea-floor epithermal-style deposits, and orogenic Au deposits. In total, the camp contains to date in excess of 28 million ounces (Moz) Au, making it a world-class example of Au-rich paleosea-floor environments. The Mooshla Intrusive Complex is spatially, temporally, and most probably genetically associated with all of the above types of mineralization. It is host to parts of the Doyon (5.5 Moz Au), Mouska (0.8 Moz Au), and Mic Mac (0.11 Moz Au) Au deposits and host to the smaller Mooshla A and B Au occurrences. Host volcanic units to the Mooshla Intrusive Complex are intensely deformed, metamorphosed, altered, and mineralized, as is the intrusion itself. The Mooshla Intrusive Complex was formed by nine distinctive phases of subvolcanic dikes, sills, and stocks. These were emplaced in two stages to form a shallow, multiphase synvolcanic intrusion along the contact between the Hebecourt and Bousquet volcanic formations. The Mouska stage is represented by a preliminary swarm of thin diabase sills, intruded by a well-layered gabbroic sill, a more crudely layered quartz diorite, and tonalite. A period of devolatilization accompanied crystallization of the xenolith-rich top of the tonalite magma chamber, as evidenced by the presence of an aplite dike swarm and associated extensive alteration zones and miarolitic cavities. The younger Doyon stage comprises a series of fine-grained aphyric to porphyritic, tonalite and trondhjemite dikes and sills, which also contain evidence of in situ devolatilization. The geochemical signatures of the Mooshla Intrusive Complex indicate emplacement during formation of an evolved, extensional oceanic island arc-style succession. Primitive mantle-normalized spider plots suggest a common origin for this island-arc intrusive suite that is similar to that of the volcanic succession of the upper member of the Bousquet Formation. Various element ratio plots used to further define magma origin and emplacement history suggest that whereas the Mouska-stage magmatic phases have a relatively straightforward, coexisting fractionation history, the Doyon-stage tonalite-trondhjemite has a more complex interplay of assimilation-fractionation-contamination, suggesting midcrustal partitioning and interaction with both earlier formed, partially hydrated ~2720 Ma oceanic crust and upper Blake River host strata (~2699-2696 Ma). The protracted and mulitphased magmatic evolution of the Mooshla Intrusive Complex led to the generation of volatile-rich phases that contributed to the development of a submarine magmatic-hydrothermal system that is thought to be responsible for the formation of the Doyon Au-Cu deposit. Geologic and timing relationships suggest that this magmatic-hydrothermal system might also have contributed to the generation of Au-rich VMS deposits higher in the host volcanic succession as part of a large Archean magmatic and hydrothermal center.

Crystal-Filled Cavities in Granitic Pegmatites: Bursting the Bubble

In November 1991, Louis B. Spaulding Jr. (deceased 2013) invited me to his Little Three pegmatite mine near Ramona, California, to view the last of a string of crystal-filled miarolitic cavities th...

The Metal Content of Silicate Melts and Aqueous Fluids in Subeconomically Mo Mineralized Granites: Implications for Porphyry Mo Genesis

Abstract To better understand the factors leading to porphyry Mo mineralization, we studied melt and fluid inclusions in three subeconomically Mo mineralized granites in well-known Mo provinces: the Treasure Mountain dome in the Colorado mineral belt (USA), and the Drammen and Glitrevann granites in the Oslo rift (Norway). Melt and fluid inclusions were investigated in samples ranging from coarsely crystallized whole rocks to euhedral quartz crystals within miarolitic cavities. The major and trace element chemistry of individual inclusions was determined by laser ablation-inductively coupled plasma-mass spectrometry. Melt inclusions are rhyolitic in composition and record a clear trend of increasing Mo concentrations with increasing degree of melt differentiation as monitored by Cs, extending from ~5 to 10 ppm Mo at 5 ppm Cs to ~17 to 40 ppm Mo at 100 ppm Cs. Coexisting magmatic fluids were single phase, had a salinity of 4 to 6 wt % NaCl equiv and a density of 0.6 to 0.7 g/cm3, and contained ~0.5 wt % S and up to 6 mol % CO2. Molybdenum concentrations in these fluids ranged from ~20 to ~200 ppm Mo, except for some highly evolved fluids that had lower Mo contents. Comparison of our data with published fluid and melt inclusion data from porphyry Mo deposits, porphyry Cu (Mo, Au) deposits, and barren intrusions reveals that most subduction-related magmas have lower Mo/Cs ratios than within-plate magmas, but that within these two groups there are no systematic differences between barren and productive intrusions. This suggests that the mineralization potential was not primarily controlled by the metal content of the melts and fluids, but rather by other factors such as size of the magma chamber and the efficiency of residual melt and fluid extraction from the magma chamber and their focusing into a small apophysis at its top. Based on our data, it can be calculated that at least several tens of km3 of magma were necessary to form intermediate-sized Mo deposits, and at least several hundred km3 to form giant (≥1 Mt Mo) deposits. All three granites investigated in this study would have been large enough to produce at least an intermediate-sized Mo deposit, but they nevertheless are only subeconomically mineralized. Their low productivity thus appears to be the result of poor fluid focusing. Factors promoting a high degree of fluid focusing include (1) accumulation of major volumes of fractionated, crystal-poor melts at the top of the magma chamber, (2) formation of an apophysis, and (3) development of convection cells, leading to an efficient circulation of these fractionated melts through the apophysis.

SEM–EDX, Raman and infrared spectroscopic characterization of the phosphate mineral frondelite (Mn2+)(Fe3+)4(PO4)3(OH)5

We have analyzed a frondelite mineral sample from the Cigana mine, located in the municipality of Conselheiro Pena, a well-known pegmatite in Brazil. In the Cigana pegmatite, secondary phosphates, namely eosphorite, fairfieldite, fluorapatite, frondelite, gormanite, hureaulite, lithiophilite, reddingite and vivianite are common minerals in miarolitic cavities and in massive blocks after triphylite. The chemical formula was determined as (Mn0.68, Fe0.32)(Fe3+)3,72(PO4)3.17(OH)4.99.The structure of the mineral was assessed using vibrational spectroscopy. Bands attributed to the stretching and bending modes of PO43- and HOPO33- units were identified. The observation of multiple bands supports the concept of symmetry reduction of the phosphate anion in the frondelite structure. Sharp Raman and infrared bands at 3581cm−1 is assigned to the OH stretching vibration. Broad Raman bands at 3063, 3529 and 3365cm−1 are attributed to water stretching vibrational modes.

Strathbogie batholith: field-based subdivision of a large granitic intrusion in central Victoria, Australia

The Late Devonian Strathbogie batholith is a 1500 km2, semi-contiguous mass of undeformed, peraluminous monzogranite and syenogranite in SE Australia. The rocks are S-type and contain abundant igneous cordierite. Internal variations within the batholith have been investigated here by systematic fieldwork guided by modern concepts of granite batholith structure, combined with geophysical and geomorphological data. Important field parameters include the outcrop character and abundance of tors and pavements, feldspar phenocryst size and abundance, groundmass grain size, the textures, grain sizes and abundances of quartz, cordierite, garnet and biotite, and the presence or absence of tourmaline, vugs and enclaves. Mapping has been supplemented with aeromagnetic data to define the external boundaries of this essentially non-magnetic batholith, but these data cannot be used to delineate internal structures. The K-Th-U distribution, from airborne radiometric surveys, was found to assist in sub-division of the batholith, and the digital elevation model formed the basis of an interpretation of batholith structure, including lineaments. Some parts of the batholith have quite distinctive field characteristics and are interpreted as discrete plutons (e.g. the Kerrisdale and Lightning Ridge plutons). Broad trends in feldspar phenocryst abundance and the presence of tourmaline and vugs (miarolitic cavities) indicate an upper contact (roof) of the batholith in the northwest. Cordierite is ubiquitous, most rocks contain a few percent of biotite, there is widespread accessory garnet, and tourmaline is present within the inferred near-roof zones. The field-based approach described here offers a method for mapping large batholiths without resorting to grid sampling and the accumulation of vast sets of whole-rock analyses. This field-based method is especially applicable in cases where there are no obvious pluton boundaries, such as internal wall-rock screens or contacts exposed on glacial pavements. The field approach records outcrop- and district-scale variations, and many textural and mineralogical features that would be omitted in any map based only on geochemistry.

Ring schlieren: Description and interpretation of field relations in the Halifax Pluton, South Mountain Batholith, Nova Scotia

A ∼35 km coastal transect of the Halifax Pluton in South Mountain Batholith (SMB) reveals 96 m-scale ring schlieren, clustered in four locations. Ring schlieren are alternating melanocratic gradational to leucocratic bands in granites forming open to closed, nested, circular to elliptical, concentric to eccentric structures with centres aligned along the major axis or minor axis of the outer ellipse, with a cylindrical form in three dimensions, and in which cross-cutting relations suggest a younging direction towards the centre. We use detailed geometric measurements of these ring schlieren to interpret the process(es) of their formation, and to indicate the relative timing of emplacement of the SMB. The dimensions and orientations of the rings are highly variable: major-axis lengths (1.39 ± 1.43 m), minor-axis lengths (1.01 ± 0.79 m), ellipticities (1.34 ± 0.25), and major-axis orientations (008°±54°). Ring schlieren appear to be the youngest structures in this part of the batholith. Modal-abundance and grain-size variation in the rings suggests that they result from shear flow. In three dimensions, the rings are nested vertical cylinders, and probably represent vertical fossil pathways, either of descending xenoliths or of ascending supercritical vapour bubbles. Such processes can produce, and preserve, a track in a crystal-melt mush having a degree of crystallinity between 55 and 75%. Two miarolitic cavities within one multi-ring structure suggest that a rising bubble train may have produced the rings.

Fluor-elbaite, Na(Li1.5Al1.5)Al6(Si6O18)(BO3)3(OH)3F, a new mineral species of the tourmaline supergroup

Fluor-elbaite, Na(Li1.5Al1.5)Al6(Si6O18)(BO3)3(OH)3F, is a new mineral of the tourmaline supergroup. It is found in miarolitic cavities in association with quartz, pink muscovite, lepidolite, spodumene, spessartine, and pink beryl in the Cruzeiro and Urubu mines (Minas Gerais, Brazil), and apparently formed from late-stage hydrothermal solutions related to the granitic pegmatite. Crystals are blue-green with a vitreous luster, sub-conchoidal fracture and white streak. Fluor-elbaite has a Mohs hardness of approximately 7.5, and has a calculated density of about 3.1 g/cm3. In plane-polarized light, fluor-elbaite is pleochroic (O = green/bluish green, E = pale green), uniaxial negative. Fluor-elbaite is rhombohedral, space group R 3 m, a = 15.8933(2), c = 7.1222(1) A, V = 1558.02(4) A3, Z = 3 (for the Cruzeiro material). The strongest eight X-ray-diffraction lines in the powder pattern [ d in A( I )( hkl )] are: 2.568(100)(051), 2.939(92)(122), 3.447(67)(012), 3.974(58)(220), 2.031(57)(152), 4.200(49)(211), 1.444(32)(642), and 1.650(31)(063). Analysis by a combination of electron microprobe, secondary ion mass spectrometry, and Mossbauer spectroscopy gives SiO2 = 37.48, Al2O3 = 37.81, FeO = 3.39, MnO = 2.09, ZnO = 0.27, CaO = 0.34, Na2O = 2.51, K2O = 0.06, F = 1.49, B2O3 = 10.83, Li2O = 1.58, H2O = 3.03, sum 100.25 wt%. The unit formula is: X(Na0.78□0.15Ca0.06K0.01)Y(Al1.15Li1.02Fe2+0.46Mn2+0.28Zn0.03) ZAl6T(Si6.02O18)B(BO3)3V(OH)3W(F0.76OH024). The crystal structure of fluor-elbaite was refined to statistical indices R 1 for all reflections less then 2% using Mo K α X-ray intensity data. Fluor-elbaite shows relations with elbaite and tsilaisite through the substitutions WF ↔ WOH and Y(Al + Li) + WF ↔ 2YMn2+ + WOH, respectively.

MINERALOGICAL AND MINERAL-CHEMICAL HETEROGENEITY INDICATED BY THE DISTRIBUTION OF ACCESSORY MINERALS IN PEGMATITES OF THE NINE MILE PLUTON OF THE WAUSAU SYENITE COMPLEX, MARATHON COUNTY, WISCONSIN

The Proterozoic Wausau syenite complex, exposed in central Marathon County, Wisconsin, is an A1-type within-plate granitoid consisting of four intrusive centers. The Nine Mile pluton is the youngest and most siliceous intrusion; the next oldest is the Rib Mountain pluton, then the Wausau pluton, and the Stettin complex is the oldest and most strongly alkaline of the four intrusive bodies. Pegmatites abound at all four intrusive centers and are generally rich in REE minerals. Locally, Be, Nb–Ta, and Zr minerals abound. The Nine Mile pluton is the best exposed of the four intrusives and has been studied in detail. Heterogeneity with respect to its mineralogy exists. The northern portion is rich in Be minerals and carbonates, but poor in fluorite. Carbonate likely formed a weak complex with Be, which sequestered it in the melt until it was released at the miarolitic cavity stage, forming phenakite and other Be minerals. Fluorite is much more abundant in the western and southern portions of the Nine Mile pluton, where it is associated with late-stage Ta and Mn enrichment in columbite–tantalite species and Hf enrichment in zircon. The high F content is likely responsible for the enrichment owing to the formation of coordination complexes, which then released higher Ta, Mn and Hf late in the crystallization history. The REE minerals are fairly constant in their abundance in all four intrusive centers.