Assimilation Of Host Rocks Research Articles

Chemical Characteristics of Ore-Bearing Intrusions and the Origin of PGE–Cu–Ni Mineralization in the Norilsk Area

The composition of the parental magmas of Cu–Ni deposits is crucial for the elucidation of their genesis. In order to estimate the role of magma in ore formation, it is necessary to compare the compositions of silicate rock intrusions with different mineralization patterns, as observed in the Norilsk region. The rock geochemistry of two massifs located in the same Devonian carbonate rocks—the Kharaelakh intrusion, with its world-class platinum-group element (PGE)–Cu–Ni deposit, and the Pyasinsky-Vologochansky intrusion, with its large deposit—was studied. Along with these massifs, the Norilsk 2 massif with noneconomic mineralization intruded in the Ivakinskaya-Nadezhdinskaya basalts was studied as well. Their settings allow the estimation of the parental magma composition, taking into account the possible assimilation of host rocks. Analyses of 39 elements in 97 samples demonstrated the similarity of the intrusions in terms of their major components. The Pyasinsky-Vologochansky intrusion contains the highest trace element contents compared with the Kharaelakh and Norilsk 2 massifs, evidencing its crystallization from evolved parental magma. No influence of host rocks on the silicate rock compositions was found, except for narrow (1–2 m) endo-contact zones. There is no correlation between the mineralization volume and the rock compositions of the studied intrusions. It is assumed that the intrusions were formed from one magma crustal source irregularly rich in sulfur (S). This source inhomogeneity in terms of the sulfur distribution resulted in deposits of varying sizes. The magmas served as a transporting agent for sulfides from deep zones to the surface.

Evolution of Syenite Magmas: Insights from the Geology, Geochemistry and O-Nd Isotopic Characteristics of the Ordovician Saibar Intrusion, Altai-Sayan Area, Russia

In this paper, we provide insight into the evolution of syenite magmas based on geological data and petrographic, geochemical, and O-Nd isotope parameters of rocks of the Saibar intrusion located within the Minusinsk Trough, Altay-Sayan area. The intrusive suite includes predominant syenites, few bodies of melanocratic and leucocratic nepheline syenites (foyaites), and granites. In addition, dykes of granites and mafic rocks are present. The U-Pb zircon age from the melanocratic foyaites was determined to be 457 ± 10 Ma? Examined rocks show fractionated light rare earth element patterns, normalized to chondrite, with (La/Sm)n varying from 4 to 9, and a weakly fractionated distribution of medium and heavy rare elements, with (Dy/Yb)n from 0.35 to 1.23 and (Sm/Yb)n from 0.63 to 2.62. The spidergram normalized to the primitive mantle shows negative Ba, Sr, Nb, Ta, Ti, and Eu anomalies (Eu* = 0.48–0.60) and positive Rb, Th, and U anomalies. The δ18O values vary within 6.3 to 10.2‰, and εNd(t) from +4.1 to +5.0. We observe gradual transitions from syenites to foyaites. Assimilation by syenite magma of the host carbonate rocks was followed to transition from silica-saturated to silica-undersaturated conditions and removal of anorthite from the melt, which then led to nepheline. Granites of the main phase show depleted lithophile incompatible elements in comparison with syenites and foyaites. They originate via interaction of magmas at the marginal part (endocontact zone) of the intrusion, corresponding to north contact of the granites with the host felsic rocks. In comparison, the rock composition of granite dykes is enriched in lithophile incompatible elements, except for Zr, Hf, and Ti. These rocks are formed due to the differentiation of syenite magma without a significant effect of host rock assimilation. Mantle magmas must be used as parent magmas for syenites based on analysis of the formation model of other alkaline intrusions, which are similar in age to the Saibar intrusion. In the line of syenite intrusions of the Altai-Sayan province, the Saibar intrusion is no exception, and its origin is related to the evolution of mafic magmas that arose during the melting of the mantle under the influence of a mantle plume.

Tourmaline as a potential mineral for exploring porphyry deposits: a case study of the Bilihe gold deposit in Inner Mongolia, China

Tourmaline is widespread at the Bilihe porphyry gold deposit in Inner Mongolia, China. Five types of tourmaline are recognized based on their occurring locations and microscopic structures: (1) Tur-A in quartz–magnetite ± K-feldspar veins and related potassic-altered rocks, (2) Tur-B in auriferous quartz veinlets, (3) Tur-C in the intermediate argillic shell, (4) Tur-D in late quartz–carbonate–tourmaline–pyrite veins, and (5) Tur-HB in hydrothermal breccia pipes, consisting of three sub-generations (Tur-HB1, -HB2, and -HB3). Almost all these tourmalines have a schorlitic–dravitic composition and belong to the alkali group. Most of the tourmalines contain high Al contents, which may suggest that the fluids are Al-enriched, and that (□Al)(NaFe)-1 and (AlO)(R2+)-1(OH)-1 exchanges have dominated their elemental substitutions. Tur-A and -B yield similar REE patterns with no strong fractionation of LREE and HREE, whereas LREE-enrichment and HREE-depletion characterize Tur-C and Tur-D; Tur-HB1 and -HB2 have flat REE patterns. Tourmalines in hydrothermal veins always show negative Eu anomalies, whereas the ones replacing plagioclase in the altered rocks yield positive Eu anomalies. The mineral precipitation and assimilation of host rocks probably both contributed to the variable compositions of Bilihe tourmaline. Tur-A−D from the main orebody all yield positive δ11B values ranging from +0.7 to +6.3‰, whereas Tur-HB1−HB3 from the hydrothermal breccia pipe show negative δ11B values of −8.8 to −3.5‰. Magmatic hydrothermal fluids enriched in isotopically light boron could have dominated the formation of Tur-HB. In contrast, the heavy boron in Tur-A−D was probably leached from the widespread marine carbonates around the deposit. Systematic contrasts in the trace element compositions of tourmaline from different mineralization types and granitic rocks suggest that (Sn + Li) vs. (Ni + V + Zn), (∑REE + Y + Zr) vs. (Ni + V + Zn) and V vs. Zn are potential elemental groups for distinguishing tourmaline from different environments.

The Ediacaran Rio Doce magmatic arc in the Araçuaí – Ribeira boundary sector, southeast Brazil: Lithochemistry and isotopic (Sm–Nd and Sr) signatures

The Rio Doce magmatic arc, developed from ca. 630 to ca. 580 Ma on an active continental margin, linking the Araçuaí and Ribeira orogens in southeastern Brazil. The arc plutonic portion comprises the G1 supersuite, a calc-alkaline, magnesian, I-type pre-collisional rock-assemblage, mostly composed of tonalite to granodiorite, frequently containing dioritic to mafic enclaves, and their metamorphosed equivalents. We carried out field, petrographic, lithochemical and isotopic (Sm–Nd and Sr) studies on a segment of the Rio Doce arc located in the transition region between the Araçuaí and Ribeira orogens. The studied samples include metamorphozed granitic rocks (referred to by their igneous names in the QAP diagram), consisting of syenogranite, monzogranite, granodiorite, tonalite, quartz diorite, orthopyroxene-bearing tonalite and orthopyroxene-bearing quartz diorite. This rock assemblage defines an I-type, magnesian, metaluminous to slightly peraluminous, medium-to high-K, expanded calc-alkaline series. The numerous mafic to dioritic enclaves and related features indicate magma mixing processes. Isotopic data show moderately to strongly negative εNd(t) values (−2.9 to −13.6) and intermediate to high 86Sr/87Sr ratios (0.7067–0.7165) suggests assimilation of older crustal material (i.e., the Juiz de Fora and Pocrane complexes, enclosing paragneisses), which is also indicated by Nd TDM model ages from 1.19 Ga to 2.13 Ga. Magmatic orthopyroxene and high content of CaO in garnet suggest magma crystallization in the deep crust. Together, our data point out to a combination of partial melting of mantle wedge in the subduction zone, deep crustal anatexis, host rock assimilation, and crystal fractionation for magma genesis in the southeastern Rio Doce arc.

Lower Oligocene Calc-Alkaline Spessartitic Lamprophyres from Central Iran (East of Anarak Area); an Evidence from the Eastern Branch of Neotethys Subduction-Related Mantle Enrichment

The Lower Oligocene Kal-e-kafi (East of Anarak, Central Iran) lamprophyres occur as stocks and dikes, which cross-cut the Eocene volcanic and Cretaceous sedimentary rocks. The predominant minerals of these lamprophyres are hornblende (magnesiohastingsite) and clinopyroxene (diopside) phenocrysts set in a fine- to medium-grained matrix of the same minerals plus plagioclase (labradorite to bytownite), sanidine, apatite, and magnetite. Secondary minerals are chlorite, magnetite, calcite, and epidote. Petrography, mineral chemistry, and whole rock compositions classify these rocks as calc-alkaline lamprophyre, in general, and spessartite in particular. These samples have intermediate compositions (SiO2 ~ 58 wt %). The chondrite-normalized REE patterns and primitive mantle-normalized multi-element spider diagram of Kal-e-kafi lamprophyres are remarkably parallel and suggest that these dikes and stocks were derived from the same parental magma and underwent similar melt extraction. These rocks are enriched in alkalies, large-ion lithophile elements (e.g., Rb, Ba, K), and light rare-earth elements (e.g., La, Ce), and exhibit moderate to high fractionation in LREE patterns, with an average La/Lu ratio of 112. The large amount of hydrous fluids coming from the subducted slab rather than sediments caused to the enrichment and metasomatism of subcontinental lithospheric mantle source. Crustal contamination and assimilation of host rocks also played role in the genesis of these lamprophyres. Geochemical characteristics of the studied rocks suggest that parental magma have been derived from partial melting of a metasomatized amphibole-bearing spinel lherzolite of lithospheric mantle, which was previously modified by dehydration of a subducting slab. Subduction of oceanic crust around the Central-East Iranian Microcontinent (CEIM) is the most reasonable mechanism to explain enrichment in volatiles of the mantle, and the lamprophyric magmatism of the Kal-e-kafi area in Lower Oligocene times. Several tectonomagmatic discrimination diagrams indicate that the Kal-e-kafi lamprophyres occurred during postcollisional period of lithospheric extension.

Source contamination, crustal assimilation, and magmatic recycling during three flare-up events in the Cretaceous Peruvian Coastal Batholith: An example from the Ica-Pisco plutons

Source contamination, crustal assimilation and magmatic recycling of older and coeval magmatic materials can potentially have dramatic effects on magma petrology, emplacement and thermal histories, as well as magma addition rates and crustal growth. To establish sources and magnitudes of assimilation and recycling processes in the Ica-Pisco area of the Peruvian Coastal Batholith (PCB), we use data at different scales collected from 125 field localities, 124 thin sections, 10 new U-Pb zircon ages, and 117 elemental and 49 isotopic whole rock analyses.Our new U-Pb zircon ages for the northern Arequipa segment of the PCB document three magmatic flare-up events over 80 m.y. (ca. 136-56 Ma), younging toward the northeast: pre-PCB gabbros at ~136 Ma; the Quilmana volcanics and the Linga, Pampahuasi, and Tiabaya plutons as part of a flare-up at ~120-80 Ma; and a final Incahuasi flare-up event at ~70-56 Ma. Zircon geochronology of xenocrysts and antecrysts provides a record of recycling from isotopically evolved materials, as well as from earlier magmatic events.Field observations reveal widespread evidence for mass transfer processes of older volcanic and plutonic units into younger plutons over a range of scales: (1) host block subsidence and disaggregation at the kilometer-scale during cauldron subsidence; (2) widespread stoping at the meter-scale; and (3) mechanical and chemical disaggregation of xenoliths at the centimeter-scale. Observations at these scales indicate that generally 1–10% of host rock assimilation and 1–5% of magmatic recycling occurred. The high alkalinity in the Linga, Pampahuasi, and Incahuasi plutons is associated with contamination from high-K materials at the magma source and minor crustal contamination at emplacement levels. The Sr/Y ratio and REE whole rock geochemistry indicate that the youngest inboard plutons have a deeper magma source, suggesting a thicker lower crust with greater possibility for assimilation during magma ascent. Sr, Nd, and Pb isotopes indicate a significant contribution from mantle melting, a moderate contribution from melting of subducted Pacific Ocean sediments, and minor assimilation from lower crustal Precambrian and Paleozoic gneisses during magma ascent and emplacement. Assimilation and fractional crystallization (AFC) calculations with Sri and Sr suggest that the Ica-Pisco plutons are composed of at most 25% recycled Pacific Ocean sediments. In summary, the composition of the Ica-Pisco plutons appears to be derived from about 65 to 70% mantle melts and 30–35% isotopically evolved material that is composed of Pacific Ocean sediments, arc and basement crust. Geochemical variations from older to younger plutons show an initial decrease in the mantle component, a general increase in the isotopically evolved component, and an initial increase and later decrease in the volcanic and plutonic host rock components.

Mineralogy, geochemistry and emplacement of the Conakry Igneous Complex, Guinea: implications for the Ni–Cu–PGE mineralization

ABSTRACTThe Conakry Igneous Complex is a mafic-ultramafic intrusion emplaced contemporaneously with the opening of the Atlantic, forming a complex, 55 km x 5 km dyke-like body within which three main episodes of injection have been recognized, characterized by a lack of mineral layering. Unit 1 consists of dunite and related facies, Unit 2 of wehrlite and pyroxene peridotite and Unit 3 corresponds to various gabbro facies. Units 1 and 2 constitute the Kaloum Peninsula; Unit 3 is its NW extension, forming the 1010 m high Mount Kakoulima. Unit 3 intrudes the two previous units and corresponds to a tholeiitic liquid that crystallized in an almost closed system, and thus exhibits a strong differentiation trend, in contrast to Units 1 and 2. Mineral compositions suggest the existence of a deeper magma chamber where a first stage of differentiation occurred.Disseminated base-metal sulfides (BMS) are present in all units of the complex and earlier descriptions have mentioned a “massive sulfide layer” with 2 to 4 g/t PGE. Platinum-group minerals (PGM) are almost everywhere included in or attached to composite Ni–Fe–Cu sulfides. Most PGM grains form complex associations resulting either from exsolution or alteration. It is characteristic of the Conakry Igneous Complex PGM, described here for the first time, to be dominated by (Pd,Pt)(Te,Bi) minerals with rare Pd,Sn and Pd,Pb compositions and an absence of Pt,Pd sulfides and Pt,Pd antimonides.The constant association of the PGM with the BMS shows that the magmatic sulfide liquid acts as an efficient collector of PGE. In such a dynamic environment, the process leading to the formation of massive sulfides must be sought in the accumulation of sulfides in the conduit following host-rock assimilation. Accordingly, considering the multiple injection processes that characterize the whole intrusion, the potential for discovering additional Ni–Cu–PGE mineralization in the Conakry Igneous Complex remains high.

Multiple sulfur isotope and mineralogical constraints on the genesis of Ni-Cu-PGE magmatic sulfide mineralization of the Monchegorsk Igneous Complex, Kola Peninsula, Russia

We present the results of a pilot investigation of multiple sulfur isotopes for the Ni-Cu-PGE sulfide mineralization of the ∼2.5 Ga Monchegorsk Igneous Complex (MIC). Base Metal Sulfide (BMS) compositions, Platinum Group Element (PGE) distributions, and Platinum Group Mineral (PGM) assemblages were also studied for different types of Ni-Cu-PGE mineralization. The uniformly low S content of the country rocks for the MIC as well as variable Sm-Nd isotope systematics and low-sulfide, PGE-rich mineralization of the MIC suggest that S saturation was reached via assimilation of silicates rather than assimilation of sulfur-rich lithologies. R-factor modeling suggests that the mixing ratio for silicate-to-sulfide melt was very high, well above 15,000 for the majority of our mineralized samples, as might be expected for the low-sulfide, PGE-rich mineralization of the MIC. Small, negative Δ33S values (from −0.23 to −0.04 ‰) for sulfides in strongly metamorphosed MIC-host rocks indicate that their sulfur underwent mass-independent sulfur isotope fractionation (MIF) in the oxygen-poor Archean atmosphere before it was incorporated into the protoliths of the host paragneisses and homogenized during metamorphism. Ore minerals from the MIC have similar Δ33S values (from −0.21 to −0.06 ‰) consistent with country rock assimilation contributing to sulfide saturation, but, also importantly, our dataset suggests that Δ33S values decrease from the center to the margin of the MIC as well as from early to late magmatic phases, potentially indicating that both local assimilation of host rocks and S homogenization in the central part of the large intrusion took place.

Geochemical aspects of the assimilation of host rocks by basaltic magmas during the formation of Noril’sk Cu-Ni ores

In models for the genesis of the Noril’sk Pt-Cu-Ni ore deposits, much importance is attached to the processes of assimilation of host rocks by basaltic melts. This idea is based on unusual relations between the silicate and sulfide constituents of this type of ore deposits and also on the heavy sulfur isotopic composition of the sulfide ores. The reason for this unusual composition is thought to be the assimilation of anhydrite from the host rocks. However, no other factors able to influence this process have ever been analyzed in the literature. We were the first to thoroughly analyze the inner structure of contact aureoles of the intrusions hosted in various rocks: the Maslovsky intrusion in Early Triassic basalts of the Ivakinsky and Nadezhdinsky formations and the Talnakh intrusion in Devonian anhydrite-bearing carbonate-terrigenous rocks. The distributions of trace elements, the 87Sr/86Sr isotopic ratio, and Sm and Nd isotopes indicate that host rocks were either not assimilated at all, or their effect is perceptible only within a very narrow (1 m) zone in the eastern apophyse in the southern portion of the Maslovsky intrusion. The Sr, Nd, and particularly, Pb isotopic composition indicate that the anhydrite could not be the source of isotopically heavy sulfur for sulfides at Noril’sk deposits. The ores of the Maslovsky and Talnakh intrusions have similar sulfur isotopic composition of their sulfides (the maximum δ34S values of these sulfides reach +10.8 and +14.2‰, respectively), in spite of the significant differences in the rocks hosting these intrusions. Our newly obtained data indicate that assimilation was insignificant and could not affect the origin of the ores.

Reappraisal of uranium-series isotope data in Kamchatka lavas: implications for continental arc magma genesis

Abstract Uranium-series isotopes can be used to determine constraints on the timescale of slab dehydration and melt production at subduction zones. However, interpretations of U–Th–Ra data suggest very different timescales of slab dehydration. Here, we present new U–Th–Ra data from Kamchatka along with a number of alternative models for production of radioactive disequilibrium. Variations in ( 226 Ra/ 230 Th) and ( 231 Pa/ 235 U) activity ratios are best explained by crystal fractionation with host rock assimilation for a duration of less than c. 6000 years. The association of the largest 226 Ra excesses with high Sr/Th in the most primitive lavas suggests that Ra–Th fractionation is controlled by slab dehydration less than 10 ka ago. We show that U–Th data can be explained by dynamic melting of a recently (<10 ka) metasomatized mantle wedge. Dynamic melting of an oxidized source metasomatized several hundreds of thousands of years ago cannot produce significant 231 Pa excess. Because 238 U– 230 Th disequilibrium is inferred to be controlled by partial melting, there is no requirement for multi-stage slab dehydration commencing ∼150 ka. We suggest that Ra–Th disequilibria constrain the timing of slab dehydration, whilst U–Th fractionation is dominated by partial melting, at least at the Kamchatka arc.

Vertical zonation of the Barcroft granodiorite, White Mountains, California: Implications for magmatic processes

Petrological and geochemical variations within plutons reflect their magmatic and emplacement histories. Here we present new magnetic susceptibility ( K m ) data on the ~163 Ma Barcroft granodiorite pluton in eastern California, which is exceptionally well exposed, especially in the vertical dimension. The Barcroft pluton offers exposures over a total of 2560 m of elevation and is an appropriate target to investigate variations of magnetic susceptibility. In ferromagnetic plutonic rocks, K m reflects mainly the abundance of magnetite, whereas in paramagnetic plutonic rocks it reflects primarily the abundance of mafic silicates. Magnetic susceptibility is also determined by magmatic processes such as crystal fractionation and by intensive parameters such as oxygen fugacity. Other magmatic processes, including magma replenishment, hybridization, and host-rock assimilation, may also influence K m variations. A first data set is based on 622 core samples that were measured in the laboratory. Our second data set comes from 1960 field measurements collected at 196 stations between ~1600 and 4000 m elevation. Detailed surveys were performed at the outcrop scale to evaluate the impact of the ~100 Ma McAfee Creek intrusion on the Barcroft background magnetic susceptibility. The combined data sets display a broad positive correlation between K m and elevation. Pluton mineralogy also appears to vary with elevation but is more difficult to quantify. At the outcrop scale, small dikes of the McAfee Creek granite transect the pluton and are responsible for a decrease in K m of the host granodioritic rocks toward the dikes due to late-stage magmatic or hydrothermal alteration. A contour map of K m shows a high degree of correlation with local topographic features such as deep canyons. Magnetic susceptibility of the Barcroft mafic rocks varies at the outcrop scale as a result of presence of petrological heterogeneities. However, these small-scale variations are embedded in a broader magnetic susceptibility trend due primarily to elevation, which reflects petrologic stratification of the pluton. The late-magmatic and hydrothermal alterations described in previous studies do not affect the spatial distribution of magnetic susceptibility. We propose that vertical increase of K m was primarily caused by crystal fractionation or another magmatic differentiation mechanism rather than by an externally driven increase in oxygen fugacity toward the roof of the intrusion.

Genesis and composition of endogenous borates in the skarns of the Eastern and Central Pyrenees

This paper addresses the genesis and composition of endogenous borates and other minerals from the magnesian-skarn aureoles at the contacts between dolomites and Hercynian granitoid intrusions of the Eastern and Central Pyrenees (Querigut and Costabonne peak massifs in France, and the Monchi deposit in Spain). It was shown that these occurrences and other magnesian skarns in the Sierra Morena Range, Spain, genetically belong to the periclase depth facies: zoned metasomatic aureoles of dolomites of primitive structure complicated by the development of periclase marble zones. The near-contact zones of the intrusions are represented by the granitoids of increasing basicity and alkalinity, which indicates the assimilation of host rocks by overheated granitic magmas. The postmagmatic stage was marked by the formation of magnesium and magnesium-iron borates of diverse composition in calciphyres and marbles, replacement of forsterite by humites in the calciphyres, and the development of silicates of decreasing Mg mole fraction after pyroxene skarns; the latter is accompanied by magnesium migration into the outer zones of the aureole. It was determined that the studied deposits of France contain boron minerals (kotoite, suanite, and pertsevite) previously unknown for this region in association with fluoborite and late szaibelyite. In skarns of Spain, the high-Fe borates are represented by monomineral aggregates of fine-prismatic parallel oriented crystals of vonsenite or its coarse-grained masses. It is conceivable that sulfide-bearing magnesian skarns and calciphyres of the studied deposits contain magnesium hydroxysulfides: tochilinite (after pyrrhotite) and valleriite (after chalcopyrite).

Differentiation and emplacement of the Worthington Offset Dike of the Sudbury impact structure, Ontario

Abstract— The Offset Dikes of the 1.85 Ga Sudbury Igneous Complex (SIC) constitute a key topic in understanding the chemical evolution of the impact melt, its mineralization, and the interplay between melt migration and impact‐induced deformation. The origin of the melt rocks in Offset Dikes as well as mode and timing of their emplacement are still a matter of debate. Like many other offset dikes, the Worthington is composed of an early emplaced texturally rather homogeneous quartz‐diorite (QD) phase at the dike margin, and an inclusion‐ and sulfide‐rich quartz‐diorite (IQD) phase emplaced later and mostly in the centre of the dike. The chemical heterogeneity within and between QD and IQD is mainly attributed to variable assimilation of host rocks at the base of the SIC, prior to emplacement of the melt into the dike. Petrological data suggest that the parental magma of the Worthington Dike mainly developed during the pre‐liquidus temperature interval of the thermal evolution of the impact melt sheet (>1200 °C). Based on thermal models of the cooling history of the SIC, the two‐stage emplacement of the Worthington Dike occurred likely thousands to about ten thousand years after impact. Structural analysis indicates that an alignment of minerals and host rock fragments within the Worthington Dike was caused by ductile deformation under greenschist‐facies metamorphic conditions rather than flow during melt emplacement. It is concluded that the Worthington Offset Dike resulted from crater floor fracturing, possibly driven by late‐stage isostatic readjustment of crust underlying the impact structure.

Crustal Contamination of Picritic Magmas During Transport Through Dikes: the Expo Intrusive Suite, Cape Smith Fold Belt, New Quebec

The Proterozoic Expo Intrusive Suite comprises a series of mafic to ultramafic intrusions crosscutting the Povungnituk Group of the Cape Smith Fold Belt in New Quebec. The intrusions are mainly in the form of blade-shaped dikes that penetrate a sediment-rich horizon in the middle of the Beauparlant Formation and terminate downward against massive basalts of the lower Beauparlant Formation. Significant accumulations of magmatic sulfide occur at the basal terminations of the dikes. At stratigraphic levels above the Beauparlant Formation the intrusions appear as broad dikes or sills within the Nuvilik Formation, below the mineralized lava flows and subvolcanic intrusions of the Raglan Formation.The Expo Intrusive Suite and the mineralized bodies of the Raglan Formation are probably coeval and comagmatic with the overlying Chukotat Group. Post-emplacement folding has exposed the Expo Intrusive Suite over about 5 km of structural relief, revealing the basal sulfide concentrations where dike segments terminate on the flanks of anticlines. The parent magma as preserved in chilled margins and narrow dikes was a picrite containing � 17 wt % MgO (i.e. komatiitic basalt) and slightly depleted in Th, U and Nb relative to middle and heavy rare earth elements. The compositions of ultramafic cumulate rocks within the intrusions are strongly enriched inTh, U and Nb relative to heavy rare earth elements, reflecting assimilation of the enclosing basalts and metasediments. Modeling of the assimilation process suggests that the picritic magma was capable of assimilating masses of basalt or sediment up to 50% of the original mass of magma. Assimilation of � 10% of a mixture of basalt and sediment caused the magma to become sulfide-saturated, and was accompanied by the crystallization of masses of ultramafic cumulates approximately equal to the mass of rock assimilated. The presence of dikes whose chilled margins resemble uncontaminated primary magmas but that contain abundant cumulates recording wholesale assimilation of host-rocks indicates that the process of assimilation and fractional crystallization required to produce continental tholeiites from picritic parent magmas may not require the presence of long-lived magma chambers, but can occur during transport along dikes and reaction with wall-rocks.

Petrologia dos granitóides sintectônicos à Zona de Cisalhamento Transcorrente Dorsal de Canguçu, Encruzilhada do Sul, RS

The Quiteria, Arroio Francisquinho and Cordilheira metagranites are syntectonic intrusions in the Dorsal of Cangucu Transcurrent Shear Zone. The Quiteria metagranite shows an I-type signature taking into account its mineralogy, type of enclaves and chemistry, although the initial isotope ratios ( 87 Sr/ 86 Sr = 0.716), negative e Nd . values, lower Zr and V contents suggest a mixed origin involving crustal and mantle derived materials in its generation. Field data, petrograpmc, geochemical and numerical modelling evidence indicate the AFC process as the dominant mechanism during the evolution of this granitoid. The parental magma is represented by microdioritic enclaves, but crustal assimilation of host rocks of calc-alkaline orthogneisses and subordinately paragneisses contributed to the hybrid genesis for these rocks. The emplacement of leucogranites of the Cordilheira Granitic Suite represents the late magmatism syntectonic to this shear zone. These rocks have a similar S-type signature as demonstrated by their mineralogical and chemical compositions, with high SiO, normative corindon (> 2 %) and high initial 87 Sr/ 86 Sr isotope ratios and strong e Nd negative values. This indicates crustal melt as the dominant mechanism in the evolution of this suite. On the other hand, field relations, enclave typology, petrologic and chemical characteristics of these rocks do not indicate an S-type source for these peraluminous rocks. Dominant melting of crustal orthogneisses with xenoliths of metasedimentary rocks may explain their origin.

Geology of the Two Duck Lake Intrusion and the Marathon Cu-PGE Deposit, Coldwell Complex, Northern Ontario

Detailed mapping of the Eastern Gabbro, Coldwell Complex, Ontario, revealed that the copper and platinum-group-element deposit on the Marathon property is confined to a coarse-grained to pegmatitic gabbro-diorite intrusion, the Two Duck Lake intrusion (TDLI). Exposed along the eastern boundary of the Coldwell Complex, the TDLI was emplaced as a sill whose orientation is slightly oblique to the general structure of the Eastern Gabbro. The TDLI has been subdivided into lower , middle and upper sub-units on the bases of lithological, textural, mineralogical, and geochemical criteria. Both border sub-units are sulfide-, base- and precious metal-enriched, and exhibit abundant evidence of host-rock assimilation. The primary mineral assemblage in the TDLI is plagioclase, olivine, clinopyroxene, Fe-Ti oxides, with accessory orthopyroxene, biotite, apatite; this primary mineral assemblage is essentially preserved in the core of the intrusion (i.e., the middle subunit). The Cu-PGE Marathon deposit, delineated mainly in the upper sub-unit of the intrusion, consists of disseminated Fe-Cu-Ni sulfides, and platinum-group-minerals derived by hydrothermal remobilization from primary magmatic sulfides. Enrichment in Cu and PGE resulted from partial replacement of original Fe-Ti oxides and pyrrhotite in the primary mineral assemblage by reaction with (a) residual magmatic fluid, and (b) fluids derived from the breakdown of host rock xenoliths and hydrothermal alteration along the lower contact of the TDLI.

Formation of chromitites by assimilation of crustal pyroxenites and gabbros into peridotitic intrusions: North Arm Mountain massif, Bay of Islands ophiolite, Newfoundland, Canada

The lowermost crust of the North Arm Mountain massif, Bay of Islands ophiolite, is dominated by peridotitic intrusions that assimilated and reacted with host gabbros, pyroxenites and peridotites. Cr‐spinel is a common accessory and locally forms chromitite layers, schlieren, or rims. Thermobarometric calculations and fractionation models suggest that the Cr‐spinels formed at oxidation states between QFM and QFM+1.5. Postcumulus and subsolidus diffusional exchanges with silicate minerals increased the Fe/Mg ratio of Cr‐spinels at North Arm but cannot explain the diversity of Cr‐spinel Cr/Al. Nor can closed or open‐system fractional crystallization of a single parental magma in cotectic proportions generate the required range of Cr/Al in Cr‐spinel. Either a variety of primary magmas was present or the Cr/Al ratio of the melt was perturbed by assimilation of host rocks, with assimilation inducing crystallization of spinel in greater than cotectic proportions. Outcrop‐scale structures and phase relationships suggest that high‐Cr/Al chromitites that are associated with contacts between intrusive peridotites and host pyroxenites are by‐products of the incongruent dissolution of pyroxenes into a Cr‐spinel‐saturated magma. Aluminous chromitites occur higher in the section, in association with hybrid feldspathic peridotites and gabbros that formed by disaggregation of feldspathic cumulates into peridotitic intrusions. These rocks commonly have disseminated anhedral Cr‐spinels and may also contain thin chromitite schlieren, both with low Cr/Al spinels. The disseminated Cr‐spinels fill dissolution pits in feldspar xenocrysts, implying that they formed by incongruent dissolution of feldspar. The H2O needed to stabilize the amphibole included in the low‐Cr/Al Cr‐spinels was probably derived from the reactant gabbros, many of which were previously affected by subseafloor alteration. Locally, chromitites and disseminated Cr‐spinels have higher concentrations of Ti, V, Fe2+, and Fe3+, which we attribute to assimilation of gabbros rich in Fe‐Ti oxides. The composition of Cr‐spinels in chromitites at North Arm depends more on the nature of the minerals being assimilated than on the composition of the magma.

Mineralogical and geochemical study of granular xenoliths from the Alban Hills volcano, Central Italy: bearing on evolutionary processes in potassic magma chambers

Granular xenoliths (ejecta) from pyroclastic deposits emplaced during the latest stages of activity of the Alban Hills volcano range from ultramafic to salic. Ultramafic types consist of various proportions of olivine, spinel, clinopyroxene and phlogopite. They show low SiO2, alkalies and incompatible element abundances and very high MgO. However, Cr, Co and Sc are anomalously low, at a few ppm level. Olivine is highly magnesian (up to Fo%=96) and has rather high CaO (1% Ca) and very low Ni (around a few tens ppm) contents. These characteristics indicate a genesis of ultramafic ejecta by thermal metamorphism of a siliceous dolomitic limestone, probably with input of chemical components from potassic magma. The other xenoliths have textures and compositional characteristics which indicate that they represent either intrusive equivalents of lavas or cumulates crystallized from variably evolved ultrapotassic magmas. One sample of the former group has major element composition resembling ultrapotassic rocks with kamafugitic affinity. Some cumulitic rocks have exceedingly high abundances of Th (81–84 ppm) and light rare-earth elements (LREE) (La+Ce=421–498 ppm) and extreme REE fractionation (La/Yb=288–1393), not justified by their modal mineralogy which is dominated by sanidine, leucite and nepheline. Finegrained phases are dispersed through the fractures and within the interstices of the main minerals. Semiquantitative EDS analyses show that Th and LREE occur at concentration levels of several tens of percent in these phases, indicating that their presence is responsible for the high concentration of incompatible trace elements in the whole rocks. The interstitial position of these phases and their association with fluorite support a secondary origin by deposition from fluorine-rich fluids separated from a highly evolved potassic liquid. The Nd isotopic ratios of the cjecta range from 0.51182 to 0.51217. 87Sr/86Sr ratios range from 0.70900 to 0.71036. With the exception of one sample, these values are lower than those of the outcropping lavas, which cluster around 0.7105±3. This indicates either the occurrence of several isotopically distinct potassic magmas or a variable interaction between magmas and wall rocks. However, this latter hypothesis requires selective assimilation of host rocks in order to explain isotopic and geochemical characteristics of lavas and xenoliths. The new data indicate that the evolutionary processes in the potassic magmas of the Alban Hills were much more complex than envisaged by previous studies. Interaction of magmas with wall rocks may be an important process during magmatic evolution. Element migration by gaseous transfer, often invoked but rarely constrained by sound data, is shown to have occurred during the latest stages of magmatic evolution. Such a process was able to produce selective enrichment of Th, U, LREE and, to a minor degree, Ta and Hf in the wall rocks of potassic magma chamber. Finally, the occurrence of xenoliths with kamafugitic composition points to the existence of this type of ultrapotassic magma at the Alban Hills.