Assimilation Of Country Rocks Research Articles

The Sea Point contact, Cape Town, South Africa: A geological site made famous by Charles Darwin's visit

Abstract The Sea Point intrusive contact on the Atlantic coast, South Africa, is a site of historical significance which continues to be useful for studying the details of granite-wallrock relations, some of which have not been properly evaluated yet. At a time when the origin of granites was still controversial, and the concept of contact metamorphism not yet properly developed, Charles Darwin visited the Sea Point contact to make some geologically important observations. He concluded that the granite originated as a magma deep in the earth, intruded a country rock of sedimentary origin and changed this country rocks' appearance due to the thermal action of the hot granite magma. He also contemplated the long periods of time required to expose the rocks by erosion. Besides its historic value, the Sea Point heritage site exposes unique features giving rare insights into the physical and chemical interaction between intruding granitic magma and a regionally low-grade metamorphosed sedimentary sequence. Questions regarding mechanisms of magmatic emplacement may be answered from this outcrop, including emplacement dynamics and interrelationships of various granitic phases, pre- versus syn-intrusive deformation and metamorphism, assimilation of country rock by a granitic magma, and the origin of feldspar phenocrysts seemingly “stranded” in the country rock.

A case of Te-rich low-sulfidation epithermal Au-Ag deposits in a calc-alkaline magmatic arc, NE China

Tellurium-bearing low-sulfidation epithermal Au-Ag deposits are significant producers of gold, silver, and potentially strategic elements if mineral processing methods are optimized for recovery. Although these deposits are generally related to alkaline magmatism, our study documents an unusual occurrence of Te-rich low-sulfidation epithermal systems in the North Heilongjiang Belt in northeast China that is spatially and temporally associated with calc-alkaline magmatism and tectonic extension in a continental arc‐setting.In the North Heilongjiang Belt, Te-bearing Au-Ag deposits are usually sited in dilatant zones, mostly along extensional NW structures or at their intersections with deep‐seated NE-striking faults such as the Nenjiang-Heihe and Jiayin-Mudanjiang lineaments. Of these deposits, Sandaowanzi is well‐known for its bonanza gold grades. These faults localized andesitic to rhyolitic volcano-plutonic centers that evolved from mantle‐derived mafic melts to intermediate compositions due to differentiation and crustal assimilation as documented by their mineralogy and Sr, Nd, and Pb isotope compositions. During assimilation of country rocks, fluids containing 3He and other volatiles derived from mantle magmas displaced or mixed with external groundwater containing radiogenic 4He derived from country rocks. In this belt, the bulk metal content of Au, Ag and Te was probably introduced by deep mantle‐derived mafic‐intermediate calc‐alkaline intrusions. However, our study does not exclude input of metals leached from underlying metasedimentary rocks and older Te‐bearing Au mineralization. The noble gas, hydrogen, oxygen and lead isotope compositions of fluid inclusions and ore minerals suggest that the deposits formed by mixing between magmatic fluids and convecting meteoric ground waters containing lead leached from surrounding country rocks. At deeper levels, isotopic evidence suggests that Au-Ag-Te (Bi) precipitated with pyrite, quartz, sericite, and carbonate minerals due to mixing with Te-rich fluids. At shallower levels, Au and Ag precipitated contemporaneously with base‐metal sulfides, hydrothermal quartz and sericite during episodic boiling as evidenced by silica morphology and mineral textures.The relatively uniform spacing of epithermal systems along NW- and NE-trending structures in the North Heilongjiang Belt was used to identify unexplored spaces within this mineral belt that are prospective for concealed low-sulfidation epithermal gold deposits. Although some of the Au-Ag deposits in this belt are enriched in Te, there is a lack of coeval alkaline igneous rocks that are commonly associated with Te-bearing gold deposits elsewhere in the world (e.g., Cripple Creek, Emperor, and Lihir Island).

Discovery of antiskarn-hosted strategic metal mineralization in the Upper Cretaceous Twihinate carbonatite intrusion (West African Craton Margin, Moroccan Sahara)

The Upper Cretaceous Twihinate carbonatite in Moroccan Sahara, which is the focus of ongoing exploration for Nb-P-Fe ± LREE ± U-Th resources, consists of a crescent-like intrusion made of a central sövite body encircled by an annular ring of vuggy siliceous breccia. Drill core logging coupled with petrographic observation, whole-rock and mineral chemistry reveal that the metasomatic interaction between the carbonatite melt and the silicate wall-rocks led to development of calc-silicate paragenesis within the sövite itself at depths ranging from ∼60 to >200 m from the surface for which the term antiskarn is attributed. We provide the first petrographic and textural descriptions, and report the whole-rock major- and trace-element geochemistry and chemical compositions of the main antiskarn-forming minerals and related strategic metal-bearing ore. Geochemically, the antiskarn lithotypes are characterized by a compositional range of 7–17 wt% SiO2, 25–72 wt% CaO, 1–2 wt% Al2O3, and 12–26 wt% loss on ignition, and enrichment in large-ion lithophile elements (LILE), particularly Sr (3125–7018 ppm ppm), Ba (393–1300 ppm), U (10–787 ppm) and light REEs (LREEs) (1058–1569 ppm), but not in the heavy REE (HREEs) and high-field strength elements (HFSE) such as Ti, Zr, Ta and Hf. Nb and V show, however, much higher concentrations ranging from 140 ppm to >1000 ppm, and 603 to 1008 ppm, respectively. Mineralogically, the antiskarn lithotypes consist predominantly of diopsidic (Di66-82 Hd18-34) to aegirine-augite clinopyroxene (Di38-55 Ae21-35) and andradite (And86-94 Gro1-9) with subordinate amounts of micas (biotite/phlogopite) and titanite. The paragenetically later mineral assemblages involve amphibole, chlorite, epidotes, baryte, fluorite, kaolinite, carbonates and quartz. The main Nb-P-Fe ± LREE ± U-Th ore minerals consist of Fe oxides (titanomagnetite principally), pyrochlore group minerals, phosphates (apatite, monazite-(Ce)), and REE-fluorcarbonates (bastnäsite-(Ce)) with subordinate sulfides (chalcopyrite, sphalerite, and galena), fluorite and baryte. Development of the antiskarn calc-silicate paragenesis and related strategic metal-bearing mineralization is consistent with derivation from a carbonatite melt through assimilation of silicate country rocks including the Silurian-Devonian gneissic granitoid of the Laknouk suite. From an economic perspective, the widespread occurrence of high-Ti (0.7–16. 9 wt% Ti) and V- (0.2–0.7 wt% V) magnetite opens new opportunities in regard of exploration for V; an emerging strategic commodity whose presence has to be reassessed in the Twihinate prospect. The currently described antiskarn paragenesis may be therefore used as reliable exploration metallotect to target prospective occurrences in the search for critical metals including V as by product.

Cambrian rift magmatism recorded in subvolcanic sills of the Ediacaran-Cambrian La Ciénega Formation, NW Mexico

Mafic-ultramafic subvolcanic metabasalt sills intrude Ediacaran and Cambrian strata of the La Ciénega Formation of Sonora, Mexico. Sills are typically 1–7.5 m thick, with minor associated thin (∼10 cm thick) intrusions. Sills are typically bed parallel with upper and/or lower tempered (quenched) borders and porphyritic textures characterized by abundant olivine and pyroxene pseudomorphs that have been transformed to clay and carbonate minerals. Metamorphic halos in adjacent dolostones and quartzites occur at the bottom and top contacts, and metabasalt sills contain carbonate veins typical of hydrothermal alteration and mobilization. Unlike surface flows of the overlying Cerro Rajón Formation, all sills in the La Ciénega Formation lack evidence of crystal settling, yet evidence for assimilation of country rocks is present. Assimilation is more typical of sills rather than flows and is diagnostic of sills in the El Arpa Formation, a stratigraphically lower unit in the succession. Actinolite-chlorite-albite metamorphic mineral paragenesis suggests low-grade greenschist facies metamorphism for these metabasaltic rocks. Although their primary mineralogy is obscured due to alteration, olivine and clinopyroxene pseudomorphs are present and their major and trace element geochemistry suggest they were derived from alkaline basaltic magmas with low (normalized) SiO2N (22.7–43.1 wt%), high MgON (7.25–16.6 wt%), and high TiO2N (4.04–6.27 wt%). Immobile trace elements suggest these sills were originally alkaline rift-related basalts with OIB-type signatures. Tectonic discrimination diagrams suggest continental rift magmatism with low geochemical evolution ratios and relatively low mantle melting rates when compared to contemporaneous rift related magmatism. Sr-Hf-Nd-Pb isotopes suggest the metabasalts originated from an Enriched Mantle I (EMI) reservoir and melting occurred during adiabatic decompression. These geochemical characteristics are remarkably similar to volcanic rocks of the Cerro Rajón Formation. The inferred magmatic composition, mantle melting rates, petrography and field relations of the La Ciénega Formation metabasalt are consistent with a low viscosity feeder system like that envisaged to have sourced volcanic rocks of the overlying Cambrian (Terreneuvian) Cerro Rajón Formation. Together, evidence from these two volcanic units is consistent with a Cambrian continental rift event close to the onset of the Sauk transgression.

Petrographic evidences of open-system magmatic processes in the felsic rocks of the northern part of the Ditrău Alkaline Massif (Eastern Carpathians, Romania)

Abstract Over the almost 190 years-long research of the Ditrău Alkaline Massif (Eastern Carpathians, Romania), felsic rocks have been regarded as homogeneous, uniform units of the igneous complex. Nevertheless, our detailed textural study revealed that the felsic suite (diorite–alkaline feldspar syenite and nepheline-bearing syenite–granite series) cropping out north of the Jolotca Creek valley is more heterogenous at micro-scale than previously thought. This heterogeneity partly derives from abundant mafic mineral-rich clusters; nevertheless, felsic minerals also exhibit various, remarkable textural features. Outcrop to micro-scale traits of felsic crystal settling, mafic mineral aggregates and flow fabrics along with metamorphic country rock xenoliths suggest that the studied rocks crystallized under dynamic magmatic conditions. Cumulate formation, shear flow, convection currents as well as various open-system magmatic processes (e.g., magma recharge, magma mixing and mingling, crystal or mush transfer and recycling, country rock assimilation) played a significant role in the petrogenesis of the examined felsic suite. Based on field observations as well as on the microtextural relationship of the minerals, two major groups of felsic rocks were distinguished: (1) felsic rocks (lacking or containing sparse mafic minerals) spatially associated with mafic rocks and (2) felsic rocks (with mafic minerals and clots) spatially unassociated with mafic rocks. Rocks of the former group are dominated by plagioclase, accompanied by minor alkaline feldspar, biotite and accessory titanite. Distinct structural and textural features suggest the physical accumulation of the rock-forming phases. Such textural properties can also be observed in some rocks of the second group. Isolated mafic minerals are rather scarce in the latter; nevertheless, different types of aggregates made up of either identical or various mafic phases are more common. Clustered minerals are either intact or show different stages of alteration. A detailed petrographic study of the above-mentioned peculiarities has been implemented in order to define their potential origin(s) and petrogenetic significance.

Geochemical and Boron Isotopic Evidence that Tourmaline Records Country Rock Assimilation of Leucogranites in the Himalayan Orogen

AbstractTourmaline geochemical and boron (B) isotopic compositions in two‐mica granites (TMG), tourmaline‐bearing leucogranites (Tou‐LG), tourmalites and metapelites from the Gyirong–Malashan areas of the Himalayan orogen provide evidence for country rock assimilation during the intrusion of Himalayan leucogranite. The schorls in Gyirong leucogranitic plutons show low contents of MgO (0.238%–1.160%) and δ11B values (–12.1‰––11.2‰), while dravites gathered in the contact zone between the leucogranitic veins and metapelites show high contents of MgO (4.815%–6.755%) and δ11B values (–10.7‰––9.3‰). This geochemical and isotopic variation of tourmalines can also be identified in the Malashan gneiss dome. As a result, three types of tourmaline were identified in the Himalayan orogen: (1) Tou‐I in the TMG and Tou‐LG, which is the most common tourmaline type of schorl; (2) Tou‐II (dravite and high‐Mg schorl) in the Tou‐LG and tourmalite at the margins of the leucogranite; and (3) Tou‐III (mainly dravite, with minor high‐Mg schorl) in metapelites of the High Himalayan Crystalline Sequence. The lenses and veins of Tou‐LG may have experienced metasomatism and assimilation as a result of interaction with the High Himalayan Crystalline Sequence metasedimentary country rocks, which can be traced by the geochemical and isotopic characteristics of the tourmaline therein.

Metasediment-derived Melts in Subduction-zone Magmas and their Influence on Crustal Evolution

Abstract Subduction is a major process cycling material through Earth’s geochemical reservoirs. Although trends in chemical composition of arc magmas imply assimilation of metasediment, the degree of such assimilation and the loci of that metasediment contamination (whether via subducted sediment or country rock assimilation) are poorly understood. To address these issues, we explore compositional data of oceanic and continental arc systems from circum-Pacific subduction zones. We find that high-silica continental arc rocks of the circum-Pacific are associated with higher aluminium saturation indices interpreted to reflect higher degrees of metasediment assimilation, with Sr/Y suggestive of shallow emplacement levels within the crust. In contrast, high-silica oceanic rocks of the circum-Pacific display lower aluminosity and equilibrated at deeper levels within the crust. Continental arc basalts are often assumed to be the source of high-silica continental arc rocks. However, phase equilibrium modelling of partial melting and crystal fractionation of continental arc basalts yield results that question this assumption. Furthermore, continental arc rock compositions show that the assimilated metasediments have protoliths that are most probably felsic greywacke and pelite rather than mafic greywacke. These findings are consistent with the hypothesis that high-silica rocks in continental arcs are directly influenced by anatexis of metasediment at shallow crustal levels (<20 km). Based upon a new method of discriminating the contribution of metasediment-derived melt, approximately one-third of felsic rocks in continental arcs have a demonstrable and unambiguous metasedimentary component. The degree of metasedimentary reworking in continental arc magmas plays an important role in the evolution of the continental crust and highlights the importance of using sediment-sensitive geochemical proxies and a magma’s petrological history when deconvolving the histories of magmatic arcs. This study also underlines the caveats associated with the calculation of depleted mantle model ages, where traditional techniques may lead to discrepancies of the order of 0·5 billion years.

Cu and Ni Isotope Variations of Country Rock-Hosted Massive Sulfides Located Near Midcontinent Rift Intrusions

Abstract Previous Cu isotope work has documented a clear disparity between δ65Cu values of sheet-style (–0.5 to 0.5‰) and conduit-style (0.5–2.0‰) intrusions associated with the Midcontinent rift system. The application of metal isotopes to the study of magmatic Ni-Cu-platinum group element (PGE) deposits is in an early stage, and very little is known regarding isotope distributions and mechanisms of fractionation at high temperatures. In order to resolve the previously mentioned Cu isotope disparity, to determine metal sources for the intrusions, and to assess sources of high-temperature metal isotope fractionation, we have measured Cu and Ni isotope ratios from a suite of exceptionally well characterized Ni-Cu-PGE massive sulfides that occur in sedimentary country rocks near intrusions within the Midcontinent rift system in Michigan and Minnesota. Previous mineralogic and S, Pb, and Os isotope measurements indicate that the massive sulfides are of magmatic origin and provide a framework for the interpretation of the Cu and Ni isotope data in terms of magmatic processes, including assimilation of Proterozoic country rocks. Copper and Ni isotope ratios were determined for massive sulfides as well as local sedimentary sulfides, and these results were compared with available Cu and Ni isotope results of magmatic and sedimentary sulfides in the Midcontinent Rift and elsewhere. Nickel isotope ratios of the sulfides have been modeled in terms of the effects of variable silicate/sulfide ratios, or R-factors (the mass ratio of silicate magma to sulfide magma), crustal contamination, and olivine fractional crystallization. The near-zero and slightly negative δ60Ni values of country rock-hosted magmatic sulfides (–0.45 to 0.17‰) near Tamarack and Eagle can be explained by minor degrees of crustal contamination of a mantle-derived melt at variable R-factors. Igneous-sourced, Cu-poor sulfides from below the Partridge River intrusion generally have lower δ60Ni values (−0.77 to –0.52‰) that require substantially more contamination from a low δ60Ni source, similar to some of the local sedimentary rocks. Copper isotope ratios of country rock-hosted massive sulfides near Eagle and Tamarack are lower than those reported by previous workers and are mostly about 0‰, similar to those expected for unaltered mantle. Tamarack samples require very little crustal contamination to explain their isotope ratios, whereas Eagle samples require no contamination from local sedimentary rocks to explain their Cu isotope compositions. Copper isotope ratios of samples beneath the Partridge River intrusion are similar to those from previous analyses of igneous-hosted sulfides, supporting their origin from a magmatic sulfide liquid. The low δ65Cu values (–1.14 to 0.25‰) from samples below the Partridge River intrusion cannot be explained by contamination from the Virginia and Thomson Formations, which are characterized by mostly positive δ65Cu values (–0.33 to 3.12‰), unless a lighter reservoir remains to be discovered in the local Proterozoic rocks. We suggest these values may have been produced by a combination of incomplete sulfide melting during partial melt generation and fractionation related to sulfide segregation at variable R-factors.

Petrogenesis of the Xingluokeng W-bearing granitic stock, western Fujian Province, SE China and its genetic link to W mineralization

The Xingluokeng deposit is the largest tungsten deposit discovered in the Wuyi metallogenic belt, with 30.43 Mt WO3 reserves averaging at 0.23%. The main orebody mainly occurs in an altered granitic stock, comprising veinlet-disseminated and vein-type scheelite and wolframite mineralization. A comprehensive study of field investigations with whole-rock and in-situ geochemical and isotopic compositions was conducted on different intrusive phases of the Xingluokeng ore-bearing granitic stock, aiming to constrain the petrogenesis and its genetic link to W mineralization. The Xingluokeng stock is mainly composed of the porphyritic biotite granite (G1) as the upper part of the stock, intruded by the medium- to fine-grained biotite granite (G2) in the deep. Petrographic observations suggest that the stock experienced intense fractional crystallization as well as assimilation of country rocks close to stock margins. Zircon U − Pb geochronology and trace element analyses indicate that G1 and G2 were emplaced coevally at ~152 Ma at a relatively lower oxygen fugacity condition (Ce4+/Ce3+ = 44 ± 18 for G1, and 104 ± 41 for G2). Ti-in-zircon thermometer and AlT-in-biotite geobarometer reveal average crystallization temperature and depth at 762 ℃−740 ℃ and 7.3–7.0 km, respectively. Geochemically, obvious negative correlation between SiO2 and P2O5 and low A/CNK values (mostly < 1.1) indicate that they belong to the weakly peraluminous I-type granite, rather than S-type granite as proposed previously. Whole-rock geochemical compositions from G1 to G2 display systematic variations, with a general decrease in major oxides (such as Al2O3, TiO2, FeOT, MgO, CaO) and Zr/Hf ratio, but a gradual increase in DI value, Rb/Sr ratio and δEu with increasing SiO2, suggesting fractional crystallization is the major factor that controls the compositional variations between G1 and G2. They have consistent initial 87Sr/86Sr ratios of 0.71170 to 0.71477 and εNd(t) values of −12.5 to −10.8, as well as zircon εHf(t) values of −17.4 to −9.2 with Paleoproterozoic Nd − Hf model ages (1783–2081 Ma), suggesting a mixed crustal source of about 88% Paleoproterozoic meta-basaltic rocks and about 22% Paleoproterozoic gneissic granites, with minor assimilation of country rocks. These results favor that the Xingluokeng granites were formed by co-magmatic evolution via K-feldspar-dominated fractionation. High degrees of fractional crystallization have immensely enriched W in reduced magma, then intensive fluid-rock interaction liberated Ca from the G1 to form the scheelite and some Fe might come from the country rocks (biotite hornfels) to precipitate the wolframite.

Mg–Sr–Nd Isotopic Insights into Petrogenesis of the Xiarihamu Mafic–Ultramafic Intrusion, Northern Tibetan Plateau, China

AbstractTo characterize the integrated magmatic processes for magma plumbing systems from mantle sources to magma chambers, we present Mg–Sr–Nd isotopic data for a representative mafic–ultramafic intrusion in Xiarihamu, northern Tibetan Plateau, China. The intrusion hosts the largest magmatic Ni–Cu sulfide deposit in an orogenic setting in the world and is composed predominantly of harzburgite, orthopyroxenite, websterite, and gabbronorite. The cumulate harzburgites have relatively low δ26Mg of orthopyroxene (–0·49 to –0·34‰) and moderately high (87Sr/86Sr)i and positive εNd(t) values (0·7067–0·7080 and +0·4 to +1·8, respectively). Most of the orthopyroxenites, websterites, and gabbronorites have slightly higher δ26Mg of orthopyroxene (–0·34 to –0·21‰) and higher (87Sr/86Sr)i and lower εNd(t) values (0·7087–0·7118 and –4·0 to –1·4, respectively) than the harzburgites. On the other hand, marginal facies (i.e. a few centimeters to meters from the contact between the intrusion and country rocks of schist, gneiss, and marble) including websterite and gabbronorite are marked by low δ26Mg of orthopyroxene (–0·44 to –0·33‰) and extremely high (87Sr/86Sr)i and negative εNd(t) values (0·7127–0·7172 and –5·4 to –4·8, respectively). The relatively low δ26Mg, moderately high (87Sr/86Sr)i, and positive εNd(t) of the harzburgites are best interpreted by inheritance from the mantle source, which was previously metasomatized by subducted Mg-rich carbonates. Given the fact that the solidus of carbonated mantle is significantly lower than that of volatile-free mantle, mantle metasomatism by carbonates may be critical to generate voluminous mafic magmas and to form the giant Xiarihamu Ni–Cu deposit and other magmatic sulfide deposits in convergent settings worldwide. The relatively high δ26Mg, high (87Sr/86Sr)i, and low εNd(t) of most orthopyroxenites, websterites, and gabbronorites were produced by extensive contamination of isotopically heavy crustal rocks in a deep-seated magma chamber. The slightly low δ26Mg, extremely high (87Sr/86Sr)i and negative εNd(t) of the marginal facies, compared with other websterites and gabbronorites, were governed by assimilation of local low-δ26Mg country rocks (e.g. gneiss and marble) at the Xiarihamu magma chamber during emplacement. These observations indicate that mafic–ultramafic cumulates away from marginal facies can preserve their primitive Mg–Sr–Nd isotope signatures inherited from deep magma chambers, even from mantle sources, with negligible crustal contamination during magma ascent and emplacement. These findings for the first time, to our knowledge, provide observational evidence to characterize the integrated magmatic processes of a single magma plumbing system from deep mantle source to magma chambers, which sheds light on the petrogenesis and mineralization of mafic–ultramafic intrusions.

Strontium isotope stratigraphy through the Flatreef PGE-Ni-Cu mineralization at Turfspruit, northern limb of the Bushveld Igneous Complex: evidence of correlation with the Merensky Unit of the eastern and western limbs

The origin of PGE-Ni-Cu mineralization in the Platreef, northern limb of the Bushveld Igneous Complex (BIC), and the possible correlation with the Merensky Reef in the eastern and western limbs has been long debated. The Platreef and Merensky Reef share the same stratigraphic position in the uppermost part of the Upper Critical Zone (UCZ), near the transition to the overlaying Main Zone (MZ). However, discrepancies in interpretations have been difficult to resolve due to the effects of intense magma-country rock interaction throughout most of the northern limb succession. To address this problem, we generated a detailed stratigraphic profile of the initial strontium isotopic ratio [Sri = (87Sr/86Sr)i] in plagioclase across a Flatreef interval lacking macroscopic evidence of country rock assimilation. The in situ Sr isotopic ratios in plagioclase were determined using LA-MC-ICP-MS analysis on 37 samples from a drill core (UMT094) at the Turfspruit project. Strontium isotope stratigraphy is useful because of a well-documented shift in Sri near the base of the Merensky Unit in the eastern and western limbs. The results show the existence of a significant shift (from Sri = 0.7060 to Sri = 0.7090) that matches the isotopic shift documented through the Merensky Unit in the eastern and western limbs. Thus, this new Sr isotope data indicates that the main mineralized interval of the Flatreef can be stratigraphically correlated to the Merensky Reef in the remainder of the BIC. In addition, we interpret these results as compelling evidence to suggest that the main mineralization processes in the Flatreef were likely similar to those operating in the eastern and western limbs and that interaction with local country rocks was not a necessary condition.

Ediacaran - Earliest Cambrian arc-tholeiite and adakite associations of the Malcocinado Formation (Ossa-Morena Zone, SW Spain): Juvenile continental crust and deep crustal reworking in northern Gondwana

The Ossa-Morena Zone of the SW Iberian Massif encloses an arc-related assemblage ascribable to subduction processes that encompass the latest Cryogenian-early Cambrian. It started ca. 645 Ma ago and is recorded by coeval N-MORB and E-MORB type magmatic rocks. Mature stages of the arc generated the Malcocinado Formation volcano-sedimentary complex. We study 15 coherent andesite units (usually lava flows) from this unit with a focus on their whole-rock geochemical and isotopic signatures and their geochronology. Lavas consist of sub-alkali andesites and basaltic andesites enriched in Th and LREE, depleted in P, Nb and, to a lesser extent, in Ti, Zr and Yb. Their εNd(t) – Mg# relations disclose three igneous associations: 1) arc-tholeiites derived from fluid-modified garnet lherzolites; 2) silica-poor adakites generated by partial melting of basic/ultrabasic sills; and 3) hybrid rocks generated by mixing of evolved melts from the previous associations plus variable assimilation of volcano-sedimentary country rocks. Formation of youngest lavas is dated ca. 534 Ma. It involved partial melting of enriched garnet lherzolite accompanied by deep crustal reworking of previous, subduction-related igneous rocks. Subduction was either still active at ca. 534 Ma or, likely, stopped some time before. The relationships with regional low-pressure metamorphism and ophiolite emplacement point to interaction between oceanic ridge/transform lithosphere and the trench at a steep subduction scenario. We envisage an active margin involving Gondwana hyper-extended crust North of the West African Craton and the Tuareg Shield.

The Huckleberry Ridge Tuff, Yellowstone: evacuation of multiple magmatic systems in a complex episodic eruption

AbstractThe 2·08 Ma, ∼2500 km3 Huckleberry Ridge Tuff (HRT) eruption, Yellowstone, generated two fall deposits and three ignimbrite members (A, B, C), accompanying a ∼95 x 65 km caldera collapse. Field data imply that the pre-A fall deposits took weeks to be erupted, then breaks of weeks to months occurred between members A and B, and years to decades between B and C. We present compositional and isotopic data from single silicic clasts (pumice or fiamme) in the three ignimbrite members, plus new data from co-eruptive mafic components to reconstruct the nature and evacuation history of the HRT crustal magmatic complex. Geochemical data, building on field characteristics, are used to group nine silicic clast types into seven compositional suites (A1-A3; B1; C1-C3) within their respective members A, B and C. Isotopic data are then added to define four magmatic systems that were tapped simultaneously and/or sequentially during the eruption. Systems 1 and 2 fed the initial fall deposits and then vented throughout member A, accompanied by trace amounts of mafic magma. In member A, volumetrically dominant system 1 is represented by a rhyolite suite (A1: 73·0–77·7 wt % SiO2, 450–1680 ppm Ba) plus a distinct low-silica rhyolite suite (A2: 69·2–71·6 wt % SiO2, &gt;2500 ppm Ba). System 2 yielded only a low-Ba, high-silica rhyolite suite (A3: 76·7–77·4 wt % SiO2, ≤250 ppm Ba). Glass compositions in pumices from systems 1 and 2 show clustering, indicative of the same multiple melt-dominant bodies identified in the initial fall deposits and earliest ignimbrite. Member B samples define suite B1 (70·7–77·4 wt % SiO2, 540–3040 ppm Ba) derived from magmatic system 1 (but not 2) that had undergone mixing and reorganisation during the A: B time break, accompanying mafic magma inputs. Mafic scoriae erupted in upper member B cover similar compositions to the member A clasts, but extend over a much broader compositional range. Member C clast compositions reflect major changes during the B: C time break, including rejuvenation of magmatic system 2 (last seen in member A) as suite C3 (75·3–77·2 wt % SiO2, 100–410 ppm Ba), plus the appearance of two new suites with strong crustal signatures. Suite C2 is another rhyolite (74·7–77·6 wt % SiO2, with Ba decreasing with silica from 2840 to 470 ppm) that defines magmatic system 3. Suite C2 also shows clustered glass compositions, suggesting that multiple melt-dominant bodies were a repetitive feature of the HRT magmatic complex. Suite C1, in contrast, is dacite to rhyolite (65·6–75·0 wt % SiO2, with Ba increasing with silica from 750 to 1710 ppm) that defines magmatic system 4. Compositions from magmatic systems 1 and 2 dominantly reflect fractional crystallization, but include partial melting of cumulates related to earlier intrusions of the same mafic magmas as those syn-eruptively vented. Country rock assimilation was limited to minor amounts of a more radiogenic (with respect to Sr) evolved contaminant. In contrast, systems 3 and 4 show similar strongly crustal isotopic compositions (despite their differences in elemental composition) consistent with assimilation of Archean rocks via partial melts derived from cumulates associated with contrasting mafic lineages. System 3 links to the same HRT mafic compositions co-erupted in members A and B. In contrast, system 4 links to olivine tholeiite compositions erupted in the Yellowstone area before, sparsely during, and following the HRT itself. All four magmatic systems were housed beneath the HRT caldera area. Systems 1 and 2 were hosted in Archean crust that had been modified by Cretaceous/Eocene magmatism, whereas systems 3 and 4 were hosted within crust that retained Archean isotopic characteristics. The extreme compositional diversity in the HRT highlights the spatial and temporal complexities that can be associated with large-volume silicic magmatism.

U–Pb zircon dating of Paleozoic volcanic rocks from the Rheno-Hercynian Zone: new age constraints for the Steinkopf formation, Lahn-Dill area, Germany

Small and volumetrically subordinate outcrops of alkali-rhyolitic to trachytic metavolcanic rocks are widespread in the Lahn-Dill area of the Rheinisches Schiefergebirge, Germany. These occurrences are either related to late Early Devonian or to Middle Devonian episodes of volcanic activity. An unambiguous age assignment for the time of eruption could not be established so far. Most volcanic rocks of the study area are not suitable for application of U–Pb geochronology. A notable exception is the Steinkopf rock suite in the SW Lahn-Dill area, but a straightforward interpretation of the LA-ICP-MS U–Pb data is hampered by complex zircon populations recording various processes including crystallization from newly formed melt, inheritance, assimilation of country rocks, variable degrees of Pb-loss and fluid-assisted Zr-mobility. A plausible evaluation of U–Pb zircon data of two metatrachytes and a metavolcaniclastic rock document a coherent age group at ca. 390–384 Ma and a probable eruption age at ca. 385 Ma, suggesting a relationship to Middle Devonian (Givetian) igneous processes. Zircon-bearing veinlets in a metatrachyte indicate post-eruptive infiltration of external hydrothermal fluids and/or internal Zr-redistribution in Late Devonian to Early Carboniferous time (364 ± 10 Ma), but this process is of rather limited importance.

Magmatic evolution of the Cerro Maricunga gold porphyry-epithermal system, Maricunga belt, N-Chile

The Cerro Maricunga gold project is located in the Maricunga gold belt in northern Chile, within the remnant edifice of the Tertiary Ojo de Maricunga volcano. The gold mineralization is hosted within a 23‒15 Ma old complex system of interwoven porphyry intrusions, flows, dikes and diverse breccia types, all forming part of a large diatreme system. In particular, phreatomagmatic and phreatic processes contributed to the formation of the synmineral diatreme complex. The rocks are calc-alkaline (daci)andesites and very rare dacites of the medium-K series with a high-K tendency and a conspicuous “adakitic” signature. Rare-earth element patterns reflect formation during a period of distinctly thickened continental crust with retention of heavy rare-earth elements by a garnet-bearing residue at a depth of ≥45 km. U-Pb zircon ages show a wide spectrum from Proterozoic, over Permian and Triassic ages to Paleocene, Oligocene, early and mid-Miocene ages. The Phanerozoic zircon ages indicate the assimilation of country rocks, whereas the Proterozoic age reflects the presence of a lower crustal Neoproterozoic basement that is also suggested by εNd values of −3.9 to −3.1 for bulk rock, i.e. mantle depletion model ages around 1 Ga. The last magmatic activity at the Ojo de Maricunga volcano was at ∼14.9 Ma. Stable isotope data on hydrothermal barite and supergene acanthite and gypsum (δ34Sacanthite: −0.6 to −0.4; δ34Ssulfate: 0.2 to 4.6; δ18Osulfate: 9.2 to 13.8) indicate an igneous origin of sulfur and arid conditions during mineral formation. The emplacement of the diatreme complex took place during the adjustment of the Chilean flat slab setting in a time of increased crustal thickness. Although the rocks at Ojo de Maricunga show a pronounced adakite-like signature, they formed in a common MASH zone or a deep hot zone within the lower crust. Gold mineralization took place at ∼17.9 Ma (Re-Os on molybdenite), syngenetic with diatreme formation and emplacement of hydrous magma. The thickening of continental crust was more advanced in the northern part of the Maricunga belt in the early to mid-Miocene than previously assumed, and then shifted south where several younger gold porphyries formed.

Mg–Sr–Nd isotopic constraints on the genesis of the giant Jinchuan Ni–Cu–(PGE) sulfide deposit, NW China

Magmatic sulfide deposits are principal resources of nickel and platinum-group elements (PGE) in the world. These deposits form as the result of the concentration of immiscible sulfide-liquid droplets segregated from mafic or ultramafic magma. Sulfide saturation in the magmas is commonly linked to contamination by the continental crust. The Jinchuan Ni–Cu–(PGE) deposit is the largest single magmatic sulfide deposit in the world. It is hosted by a small mafic–ultramafic intrusion in the North China Craton. The Jinchuan mafic–ultramafic intrusion intruded Paleoproterozoic marbles, gneisses, and granitic migmatites. This paper reports the magnesium, strontium and neodymium isotopic compositions of the intrusion and the country rocks. Olivine, pyroxene and whole rocks of the lherzolites from the Jinchuan intrusion have δ26Mg values ranging from −0.40‰ to −0.26‰, with a weighted mean value of −0.31 ± 0.06‰ (2SD), which is slightly lighter than expected in a mantle-derived magma. The marble country rocks have relatively light δ26Mg values varying from −2.00‰ to −1.17‰, with a weighted mean value of −1.53 ± 0.02‰ (2SD). The variations are attributed to the interaction between basaltic magma and marble country rocks as the principal mechanism which generated the low Mg isotopic signatures in the Jinchuan deposit. The (87Sr/86Sr)i and εNd values of the lherzolites vary from 0.706615 to 0.714612 and from −8.29 to −6.13, respectively. The marble country rocks have relatively high (87Sr/86Sr)i ratios (0.713662 to 0.740069) and low εNd values (−25.43 to −12.41). The negative Nb anomalies in all samples from the intrusion indicate that the parental magma underwent crustal contamination. Mg–Sr–Nd isotopic mixing calculations indicate that the Jinchuan intrusion underwent 10–20% bulk contamination involving the lower continental crust, followed by <10% bulk contamination by marbles. Our petrological and geochemical investigation of the Jinchuan intrusion provides evidence that a combination of the two processes led to the formation of the ore deposit. The assimilation of lower crustal rocks reduced the sulfur solubility; the assimilation of marble country rocks made the magma more oxidized and hence decreased the sulfur solubility further. The results from this study show that Mg isotopes are useful in the study of carbonate assimilation by mafic magma, and marble assimilation by basaltic magma is an important mechanism to produce sulfide saturation in the genesis of Ni–Cu–PGE sulfide deposits.

The Guangshigou uranium deposit, northern Qinling Orogen, China: A product of assimilation-fractional crystallization of pegmatitic magma

Uranium mineralization associated with biotite pegmatites occurs in the Shangdan triangular domain of the northern Qinling Orogen, constituting a significant uraniferous province. The Guangshigou deposit is the most productive uranium deposit in this area. Several Caledonian intrusions were emplaced within the ore district including the Huichizi granite (HG, 441.1 ± 1.3 Ma), Damaogou granite (DG, 420.9 ± 1.3 Ma), biotite pegmatite (BP, 415.6 ± 1.5 Ma), and muscovite-biotite pegmatite (MBP, 412.1 ± 1.4 Ma). Based on uranium concentration, each BP displays two different zones, i.e., an uranium-bearing zone (UBP hosted at the contact zone) and a non-mineralized zone (NBP hosted in the non-assimilation zone). Two magmatic pulses are distinguished. The Early Silurian HG was derived from a low degree of partial melting of thickened lower basaltic crust interfused mantle-derived materials following the subduction of the Shangdan Ocean. The Late Silurian–Early Devonian intrusions exhibited the same source region but formed during the weakening of convergent action. Fractional crystallization of high-K granitic magma (DG) produced a set of fractionated pegmatites, whereas UBP deviates from a normal magma evolution trend due to country rock contamination in the hybrid zone.Most of the U in UBP is hosted in primary uraninite. The biotite gneiss (BG) is the most favorable country rock for uranium mineralization. Bulk assimilation of country rock is driven by reactions during xenolith melting and pegmatitic magma crystallization. The mineralization occurs at the assimilation/hybrid zone, e.g., the contact zone between BP and BG, as well as the partially assimilated xenoliths.The enrichment of contact-hosted peritectic biotite and increase of FeOT, MgO, TiO2, MnO, LILE (i.e., Rb, Ba, K), HFSE (i.e., Nb, Ta) contents and initial 87Sr/86Sr ratios as well as decrease of εNd(t) values in BP arose from chemical interaction (assimilation/hybridization) with the country rock. The reactive bulk assimilation modified the normal crystallization process of pegmatitic magma. The uranium ores are the products of the assimilation-fractional crystallization of the pegmatitic magma. U-F complexes were destabilized by reducing fluorine activities during the formation of peritectic biotite in the assimilation/hybrid zone, triggering decrease of U solubility and saturation.

BLADE-SHAPED DIKES AND NICKEL SULFIDE DEPOSITS: A MODEL FOR THE EMPLACEMENT OF ORE-BEARING SMALL INTRUSIONS

Most of the world's major deposits of nickel and copper were formed as accumulations of magmatic sulfide liquid within small mafic-ultramafic intrusions. Ore-hosting intrusions exhibit a variety of forms but typically show greater horizontal than vertical extents, occurring as tube-shaped chonoliths, narrow elongate sills, boatshaped bodies, or sword blade-shaped dikes with originally horizontal principal axes. Sulfide accumulation at the downward termination of blade-shaped dikes is noted in a number of deposits worldwide. Based on evidence from continental dike swarms, Icelandic fissure eruptions, and active shield volcanoes, we suggest that these blade-shaped bodies formed by magma migration within vertical fractures that propagated laterally rather than vertically. Some examples, notably the bladed dikes of the South Raglan trend, have contact relationships indicating processes of active erosion, assimilation, and replacement of country rock. Where such dikes are injected into mixed volcanic-sedimentary sequences, thermal modeling of interaction with nonrefractory country rock predicts repeated formation and collapse of transient chilled margins against narrow zones of partially molten wall rock. Where the wall rock is sulfide-rich sediment, this generates a slurry of sulfide liquid, country-rock xenoliths, and chilled margin fragments that flows down the wall to accumulate at the bottom edge of the dike. This type of mixture is commonly seen as sulfide-matrix ore breccias in intrusion-hosted deposits. Widening of the conduit along zones of easily eroded country rock may result in a transition from a dike-like to a tubular chonolith geometry. Multiple magma pulses within the same magma conduit may give rise to complex superimpositions of ore zones, further complicated by formation of sulfide-silicate melting-infiltration fronts. Hence we argue that the bladed-dike geometry is an end-member type that commonly serves as a precursor form to a range of eventual geometric forms; recognition of this basic geometry provides general insights into the origins of other important deposits.

Time scales and length scales in magma flow pathways and the origin of magmatic Ni–Cu–PGE ore deposits

Ore forming processes involve the redistribution of heat, mass and momentum by a wide range of processes operating at different time and length scales. The fastest process at any given length scale tends to be the dominant control. Applying this principle to the array of physical processes that operate within magma flow pathways leads to some key insights into the origins of magmatic Ni–Cu–PGE sulfide ore deposits. A high proportion of mineralised systems, including those in the super-giant Noril'sk-Talnakh camp, are formed in small conduit intrusions where assimilation of country rock has played a major role. Evidence of this process is reflected in the common association of sulfides with vari-textured contaminated host rocks containing xenoliths in varying stages of assimilation. Direct incorporation of S-bearing country rock xenoliths is likely to be the dominant mechanism for generating sulfide liquids in this setting. However, the processes of melting or dissolving these xenoliths is relatively slow compared with magma flow rates and, depending on xenolith lithology and the composition of the carrier magma, slow compared with settling and accumulation rates. Chemical equilibration between sulfide droplets and silicate magma is slower still, as is the process of dissolving sulfide liquid into initially undersaturated silicate magmas. Much of the transport and deposition of sulfide in the carrier magmas may occur while sulfide is still incorporated in the xenoliths, accounting for the common association of magmatic sulfide-matrix ore breccias and contaminated “taxitic” host rocks. Effective upgrading of so-formed sulfide liquids would require repetitive recycling by processes such as re-entrainment, back flow or gravity flow operating over the lifetime of the magma transport system as a whole. In contrast to mafic-hosted systems, komatiite-hosted ores only rarely show an association with externally-derived xenoliths, an observation which is partially due to the predominant formation of ores in lava flows rather than deep-seated intrusions, but also to the much shorter timescales of key component systems in hotter, less viscous magmas. Nonetheless, multiple cycles of deposition and entrainment are necessary to account for the metal contents of komatiite-hosted sulfides. More generally, the time and length scale approach introduced here may be of value in understanding other igneous processes as well as non-magmatic mineral systems.

Magmatic controls on the genesis of porphyry Cu–Mo–Au deposits: The Bingham Canyon example

Bingham Canyon is one of the world's largest porphyry Cu–Mo–Au deposits and was previously used as an example to emphasize the role of magma mixing and magmatic sulphide saturation in the enhancement of ore fertility of magmatic systems. We analyzed whole rocks, minerals, and silicate melt inclusions (SMI) from the co-genetic, ore-contemporaneous volcanic package (∼38 Ma). As opposed to previous propositions, whole-rock trace element signatures preclude shoshonite–latite genesis via mixing of melanephelinite and trachyte or rhyolite, whereas core to rim compositional profiles of large clinopyroxene phenocrysts suggests the amalgamation of the ore-related magma reservoir by episodic recharge of shoshonitic to latitic magmas with various degrees of differentiation. Major and trace element and Sr and Nd isotopic signatures indicate that the ore-related shoshonite–latite series were generated by low-degree partial melting of an ancient metasomatized mantle source yielding volatile and ore metal rich magmas. Latite and SMI compositions can be reproduced by MELTS modeling assuming 2-step lower and upper crustal fractionation of a primary shoshonite with minimal country rock assimilation. High oxygen fugacities (≈NNO+1) are prevalent as evidenced by olivine-spinel oxybarometry, high SO3 in apatite, and anhydrite saturation. The magma could therefore carry significantly more S than would have been possible at more reducing conditions, and the extent of ore metal sequestration by magmatic sulphide saturation was minimal. The SMI data show that the latites were Cu rich, with Cu concentrations in the silicate melt reaching up to 300–400 ppm at about 60 wt% SiO2. The Au and Ag concentrations are also high (1.5–4 and 50–200 ppb, respectively), but show less variation with SiO2. A sudden drop in Cu and S concentrations in the silicate melt at around 65 wt% SiO2 in the presence of high Cl, Mo, Ag, and Au shows that the onset of effective metal extraction by fluid exsolution occurred at a relatively late stage of magma evolution. Overall, our results show that fluid exsolution during simple magmatic differentiation of oxidized alkaline magmas is capable of producing giant porphyry Cu deposits.