Bou Azzer Ophiolite Research Articles

Ore formation in mantle peridotites before and after emplacement: review and new perspective

Generation of ore deposits in and around mantle peridotite (serpentinite) massifs are reviewed and summarized, and their future studies are discussed as well. Podiform chromitites are essentially formed by magma-peridotite (mainly harzburgite) reaction and subsequent magma mixing at the upper mantle. The UHP chromitite is, however, possibly formed by deep recycling of the low-pressure one formed in the upper mantle. Seawater, which contains Cl, is potentially able to dissolve and transport Cr and precipitate chromian spinel. Chromitite formation in the mantle wedge by slab fluids is possible but has not been well examined. Olivine in peridotites is potentially an important source of some elements (iron and related transition metals), which are liberated upon its breakdown during serpentinization. They are sometimes in-situ precipitated as minute grains of oxides, sulfides or metals in serpentinite, but sometimes transported via hydrothermal solutions responsible for serpentinization and concentrated to form ores. Magnetite veins in serpentinite, possibly as well as Co-Ni arsenides along the serpentinite-diorite boundaries of the Bou-Azzer ophiolite, give us a good example. Precipitation of Mn nodules and other ores from seafloor are possibly related to circulated seawater through peridotite emplaced in the crust. This issue should be examined thoroughly in the future.

Petrogenesis of ultramafic rocks from the Neoproterozoic Bou Azzer Ophiolite (Morocco): new insights into a long-standing geodynamic question

ABSTRACT The Neoproterozoic Bou Azzer Ophiolite, located in the Pan-African orogen of the Anti-Atlas, Morocco, exposes a well-preserved mantle section of the ophiolitic sequence. Although ultramafic rocks are the most common rock types in the sequence, they have received less attention in previous studies. In this work, we investigate the petrogenesis of these ultramafic rocks in detail, including their petrology, mineralogy, and whole rock geochemistry. The mineral assemblages are mainly formed by antigorite with minor lizardite, chlorite, and accessory Cr-spinel. The only preserved primary mantle mineral is Cr-spinel, which shows unaltered cores with Cr# between 0.61 and 0.71 and alteration rims to ferrian chromite and magnetite. Whole rock analyses indicate almost total serpentinization and, locally, strong carbonation (mass loss of ignition between 11.94 and 22.20 wt%). The non-carbonated samples preserve protolith signatures, with decreased MgO/SiO2 suggesting metasomatic processes related to serpentinization. Immobile trace elements compare well with fore-arc peridotites, while melting modelling indicates intense and polyphasic melting events. We propose polyphase melting in a subduction-initiation setting, with a first stage of 14–23% anhydrous melting, forming fore-arc basalts, and a second stage of 15–25% melting with 1 wt% H2O, forming boninites. The Bou Azzer ultramafic rocks represent the fore-arc region of a supra-subduction zone, challenging previous interpretations that suggested a back-arc position.

Insights into the deep structure of the Bou Azzer-El Graara inlier (Central Anti-Atlas, Morocco): Inference from high-resolution magnetic data, and geodynamic implications

The Bou Azzer-El Graara inlier (BAEI) in the Central Anti-Atlas has been extensively studied due to its famous Pan-African ophiolite and its great mineral potential. In 1998, an airborne survey was flown over this inlier, providing high-resolution magnetic data. This study is concerned with reinterpreting these data to help elucidate the BAEI structure. For this purpose, the aeromagnetic data were processed through a series of filters and transformations. They were then subjected to a detailed analysis, taking into account the study area geology. Qualitative analysis of the reduced to pole data indicates that the observed anomalies are broadly consistent with outcrops of Precambrian mafic and metavolcanic rocks in the BAEI. The extension of this type of rocks below the Paleozoic sedimentary cover could explain similar anomalies found outside the inlier. Additional analysis of the processed data helped to identify various lineaments and structures organized into two main trends: ENE-WSW and ESE-WNW. Furthermore, the calculation of anomalous source depths by Euler deconvolution from aeromagnetic data shows that these vary between 0 and 1000 m within the inlier. However, deeper sources occur beneath the detached and slightly folded Paleozoic sedimentary deposits in the northern part of the inlier. Moreover, a quantitative interpretation using the 3D inverse method was applied to high-amplitude anomalies associated with the Bou Azzer ophiolite (BAO) and the Foum Zguid great dyke (FZGD). The results indicate that (i) the BAO consists of three north-northeast dipping magnetic bodies, (ii) the FZGD corresponds to a deeply rooted vertical structure with lateral thickness variations and (iii) the NE-SW FZGD structure represents the eastern limit of the BAO as indicated by geological and magnetic data. This suggests that the FZGD fracture may have played an important role during the Pan-African collages as a key structure controlling the oblique suturing of the ophiolite slices.

The chromitites of the Neoproterozoic Bou Azzer ophiolite (central Anti-Atlas, Morocco) revisited

• Intermediate-Cr and high-Cr chromitites in the Bou Azzer ophiolite, Morocco. • Chromitites crystallized from fore-arc basalts (FAB) and boninites, respectively. • Fore-arc position of the ophiolite in a subduction-initiation geodynamic setting. • Super-reduced phases (moissanite and native Cu) formed during serpentinization. The Neoproterozoic Bou Azzer ophiolite in the Moroccan Anti-Atlas Panafrican belt hosts numerous chromitite orebodies within the peridotite section of the oceanic mantle. The chromitites are strongly affected by serpentinization and metamorphism, although they still preserve igneous relicts amenable for petrogenetic interpretation. The major, minor and trace element composition of unaltered chromite cores reveal two compositional groups: intermediate-Cr (Cr# = 0.60 – 0.74) and high-Cr (Cr# = 0.79 – 0.84) and estimates of parental melt compositions suggest crystallization from pulses of fore-arc basalts (FAB) and boninitic melts, respectively, that infiltrated the oceanic supra-subduction zone (SSZ) mantle. A platinum group elements (PGE) mineralization dominated by Ir-Ru-Os is recognized in the chromitites, which has its mineralogical expression in abundant inclusions of Os-Ir alloys and coexisting magmatic laurite (RuS 2 ) and their products of metamorphic alteration. Unusual mineral phases in chromite, not previously reported in this ophiolite, include super-reduced and/or nominally ultra-high pressure minerals moissanite (SiC), native Cu and silicates (oriented clinopyroxene lamellae), but “exotic” zircon and diaspore have also been identified. We interpret that clinopyroxene lamellae have a magmatic origin, whereas super-reduced phases originated during serpentinization processes and diaspore is linked to late circulation of low-silica fluids related to rodingitization. Zircon grains, on the other hand, with apatite and serpentine inclusions, could either have formed after the interaction of chromitite with mantle-derived melts or could represent subducted detrital sediments later incorporated into the chromitites. We offer a comparison of the Bou Azzer chromitites with other Precambrian ophiolitic chromitites worldwide, which are rather scarce in the geological record. The studied chromitites are very similar to the Neoproterozoic chromitites reported in the Arabian-Nubian shield, which are also related to the Panafrican orogeny. Thus, we conclude that the Bou Azzer chromitites formed in a subduction-initiation geodynamic setting with two-stages of evolution, with formation of FAB-derived intermediate-Cr chromitites in the early stage and formation of boninite-derived high-Cr chromitites in the late stage.

Hydrothermal alterations of the Bou Azzer East Co-Ni-As-Au deposits (central Anti-Atlas, Morocco)

The polymetallic Co, Ni, Cu, Zn, Au and Ag deposits of Bou Azzer East is linked to Bou Azzer ophiolite located in the Central Anti Atlas. The ultramafic rocks are strongly deformed and altered by serpentinization, carbonatization and silicification. The mineralization is hosted by quartz, quartz-carbonate and/or chlorite veins, intimately associated with the serpentinite-quartz diorite contact. The description of alteration phenomena as mineralizing events, especially in Co-Ni arsenides and precious metals Au-Ag is realized. The petro-mineralogical and geochemical studies of the mineralized structures and their surrounding host rocks reveal several types of alterations that developed a lateral zoning at the serpentinite level, with formations of silica-, talc-, carbonate-, and chlorite-rich serpentinites. Nearly all primary silicate minerals have been replaced by serpentine minerals, leaving only relics of primary olivine and chromian spinel. The presence of antigorite dominated serpentinites indicates that the Bou Azzer ultramafic sequence was hydrated at relatively high temperature between 200 and 350 °C. In agreement with fluid inclusions study and the estimated temperatures from the chemical composition of chlorites (220–320 °C), which showed an evolution from ferrous chlorites (pychnochlorite and ripidolite) to magnesian chlorites (penninite and clinochlore). The altered zone is characterized by high metal contents of As, Co, Mo, Ag and Au in the silica-rich serpentinite and Cr, Cu, Zn and Pb in the carbonate-rich serpentinite. The enrichment in these elements result from the action of brines with a high Si content and fluids rich in Ca, Mg and CO2 on serpentinized peridotite, respectively. The chloritite breccia developed mainly in shear zones also shows elevated levels in As, Co, Mo, Ag and Au, buffered by the interaction between the fluid and the surrounding rocks.

The Moroccan Anti-Atlas ophiolites: Timing and melting processes in an intra-oceanic arc-back-arc environment

The Moroccan Anti-Atlas orogenic belt encloses several Precambrian inliers comprising two major Neoproterozoic ophiolitic complexes: the Sirwa and Bou Azzer ophiolites. These ophiolites expose crustal and mantle units, thrusting over fragments of a long-lived intra-oceanic arc system. We present a detailed geochronological and petro-geochemical study of three mafic/ultramafic units of these two ophiolites: the Khzama sequence (Sirwa ophiolite) and the Northern and Southern Aït Ahmane sequences (Bou Azzer ophiolite). The crystallization of layered metagabbros from the Bou Azzer ophiolite (North Aït Ahmane sequence) has been dated here at 759 ± 2 Ma (U-Pb on zircons). This new age for the Bou Azzer ophiolite is similar to the formation of the Sirwa ophiolite (762 Ma) and suggests that both units formed during the same spreading event. Metabasalts of the three units show tholeiitic signature but with variable subduction-related imprints marked by LILE enrichments, HFSE depletions and variable Ti contents, similar to modern back-arc basin basalts (BABB). Their back-arc origin is also supported by the geochemical signature of ultramafic units showing very low contents in major and trace incompatible elements (Al2O3: 0.12–1.53 wt%, Ti: 3.5–64.2 ppm and Nb: 0.004–0.10 ppm), attesting of a highly refractory protolith. This is in agreement with the high Cr# (0.44–0.81) and low to intermediate Mg# (0.25–0.73) of their constitutive Cr-spinels. Dynamic melting models suggest that these serpentinites experienced intense and polyphased hydrous melting events, strongly influenced by supra-subduction zone SSZ-fluid influx and subduction-related melt percolation. Being particularly affected by these SSZ-melt/rock interactions and closer to arc units to the south, the Sirwa ophiolite and the South Aït Ahmane unit of the Bou Azzer ophiolite likely represent an early stage of the arc-back-arc system, which has been more influenced by the magmatic products of the arc activity compared to the North Aït Ahmane unit of the Bou Azzer ophiolite.

100 myr cycles of oceanic lithosphere generation in peri-Gondwana: Neoproterozoic–Devonian ophiolites from the NW African–Iberian margin of Gondwana and the Variscan Orogen

Abstract The Variscan Orogen in Iberia and the Anti-Atlas Mountains in Morocco contains a set of ophiolites formed between Neoproterozoic and Devonian times, during the complex evolution of the NW African–Iberian margin of Gondwana. During this time interval, the margin evolved from an active margin ( c. 750–500 Ma: the Reguibat–Avalonian–Cadomian arc) to the final collision with Laurussia in Devonian times to form Pangaea. In this context, one of the oldest recognized ophiolites is the Bou Azzer Ophiolite from the Anti-Atlas Mountains, dated at c. 697 Ma and containing two types of mafic rocks, the youngest of which has a boninitic composition. To the north, in the SW Iberian Massif, the Calzadilla Ophiolite contains mafic rocks also of boninitic composition dated at c. 598 Ma. Farther north, in the NW Iberian Massif, the Vila de Cruces Ophiolite is formed by a thick sequence of mafic rocks with an arc tholeiitic composition and minor alternations of tonalitic orthogneisses dated at c. 497 Ma. In the same region, the Bazar Ophiolite has a similar age of c. 495 Ma. Also in NW Iberia, there is a group of ophiolites with varied lithologies and dominant mafic rocks with arc tholeiitic composition (Careón, Purrido and Moeche ophiolites) dated at c. 395 Ma. The composition of all these peri-Gondwanan ophiolites is of supra-subduction zone type, showing no evidence of preserved mid-ocean ridge basalt type oceanic lithosphere. Consequently, these ophiolites were generated in the peri-Gondwanan realm during the opening of forearc or back-arc basins. Forearc oceanic lithosphere was promptly obducted or accreted to the volcanic arc, but the oceanic or transitional lithosphere generated in back-arc settings was preserved until the assembly of Pangaea. Based on the ages of the described ophiolites, the peri-Gondwanan realm has been a domain where the generation of oceanic or transitional lithosphere seems to have occurred at intervals of c. 100 myr. These regularly spaced time intervals may indicate cyclic events of mantle upwelling in the peri-Gondwanan mid-ocean ridges, with associated higher subduction rates at the peri-Gondwanan trenches and concomitant higher rates of partial melting in the mantle wedges involved. The origin of the apparent cyclicity for mantle upwelling in the peri-Gondwanan ocean ridges is unclear, but it could have possibly been related to episodic deep mantle convection. Cycles of more active deep mantle convection can explain episodic mantle upwelling, the transition from low- to fast-spreading type mid-ocean ridges and, finally, the dynamic context for the episodic generation of new supra-subduction zone type oceanic peri-Gondwanan lithosphere.

Mineralogical and seismic properties of serpentinite of Ait Ahmane fault zone of Bou Azzer ophiolite, central Anti-Atlas of Morocco

This paper presents the first seismic measurements of serpentinite of Bou Azzer ophiolite, central Anti-Atlas of Morocco, including seismic velocities and anisotropy. Two serpentinite samples collected from the Ait Ahmane fault zone were analyzed in order to define the mineralogical and seismic features of the natural serpentinite of the Bou Azzer ophiolite. The mineralogical features were investigated using microscopic observation and Raman spectroscopy, while the seismic features were performed using an Electron Backscatter Diffraction (EBSD) instrument. Microscopy and spectroscopy analyses confirmed that the investigated serpentinite suffers from a variable degree of serpentinization, and the antigorite is the dominant variety of serpentine minerals in the study area. The crystal preferred orientation (CPO) results show that the axis [001] of the antigorite deformation is aligned subnormal to the foliation, while the axis [010] is mostly aligned subparallel to the lineation. The seismic anisotropy results are depending on serpentine amount in the rock samples. The sample with a low serpentine amount (30%) shows lowest P- and S-wave anisotropy (Vp = 7.2% and AVs = 6.55%), while the sample with a high amount of serpentine (85%) presents highest P-wave and S-wave anisotropy (Vp = 8.6% and AVs = 11.06%). Consequently, the results indicate that seismic anisotropy increases when increasing the antigorite amount.

Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater

The evolution of the seawater oxygen isotopic composition (δ18O) through geological time remains controversial. Yet, the past δ18Oseawater is key to assess past seawater temperatures, providing insights into past climate change and life evolution. Here we provide a new and unprecedentedly precise δ18O value of −1.33 ± 0.98‰ for the Neoproterozoic bottom seawater supporting a constant oxygen isotope composition through time. We demonstrate that the Aït Ahmane ultramafic unit of the ca. 760 Ma Bou Azzer ophiolite (Morocco) host a fossil black smoker-type hydrothermal system. In this system we analyzed an untapped archive for the ocean oxygen isotopic composition consisting in pure magnetite veins directly precipitated from a Neoproterozoic seawater-derived fluid. Our results suggest that, while δ18Oseawater and submarine hydrothermal processes were likely similar to present day, Neoproterozoic oceans were 15–30 °C warmer on the eve of the Sturtian glaciation and the major life diversification that followed.

Intra-oceanic arc growth driven by magmatic and tectonic processes recorded in the Neoproterozoic Bougmane arc complex (Anti-Atlas, Morocco)

The Bougmane complex forms a Neoproterozoic tectonic unit located south of the Bou Azzer ophiolite in the Moroccan Anti-Atlas orogen. We show here that it corresponds to the lower crustal section of a Neoproterozoic intra-oceanic arc, which recorded 100–120 Ma of arc activity. The Bougmane complex is composed of two units: The 750–730 Ma gneissic unit is made of granodioritic gneiss-amphibolite which recorded a first magmatic pulse in the oceanic arc (IGN1). They are intruded during IGN2 event (710–690 Ma) by undeformed hydrous rocks consisting of hornblende gabbro (dated here at 706 ± 9 Ma, U-Pb protolithic zircons), hornblendite and minor tonalites. The high ƐNd values (+3.6 and +6.7) for all Bougmane rocks and their geochemical similarities ([La/Lu]N > 1.61; high LILE/HFSE ratio) with rocks in modern (Mesozoic to active) oceanic arcs, attest that the parental basic magmas were formed in a mantle wedge in an ocean-ocean subduction zone. The IGN2 event led to partial ‘garnetisation’ of the hornblende-gabbro along contacts with hornblendite bodies. Garnet formed by dehydration (sub-) and dehydration-melting (supra-solidus) of the hornblende-gabbro under HP conditions (>8 kbar). Field and geochemical modelling of Bougmane tonalites, showing low HREE ([La/Yb]N > 11) and high Sr/Y ratio (>1150), suggest that they formed by garnet-present disequilibrium melting of the hornblende-gabbro. U-Pb dating on rutiles in a hornblendite yielded an age of 686 ± 6 Ma, consistent with a post-IGN2 isobaric cooling. Rutiles from a garnet-hornblende gabbro yields a younger age of 658 ± 7 Ma suggesting that a high temperature event locally affected the Bougmane complex, likely related to a third igneous event (IGN3: 660–640 Ma) marked by intrusive diorites in the northern sector of the paleosuture. The building of the Anti-Atlas oceanic arc complexes occurred via three magmatic flare-ups (IGN1-2-3, centred at 750, 700 and 650 Ma respectively) interspersed with an early major tectonic episode (D1: 730–700 Ma). This suggests that the arc thickening in oceanic settings can be driven by magma intrusions at several levels of the crust but it can also be enhanced by intra-oceanic crustal shortening (hence, back-arc on arc stacking) – a still poorly documented geodynamic context in modern oceanic arc settings.

Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)

If magnetite is a common serpentinization product, centimetric, massive and pure magnetite veins are rarely observed in serpentinites. Unique example, in the Aït Ahmane ultramafic unit (Bou Azzer Neoproterozoic ophiolite, Anti-Atlas, Morocco) allows to assess the hydrothermal processes that prevailed at the ending Precambrian. In this study, rock magnetism, petrography, mineral and bulk chemistry are combined to assess iron behavior in these meta-ultramafics, in order to constrain the serpentinites alteration and magnetite veins formation processes.Very high Cr#, low Mg#, high Fe3+# and low Ti content of Cr-spinels cores reflect a supra-subduction zone origin for the Aït Ahmane serpentinites precursor. Typical lizardite/chrysotile pseudomorphic texture in fresh serpentinites reveals an initial oceanic-like serpentinization, involving temperature <350°C while the abundance of magnetite (up to 10.14wt%) in these unaltered serpentinites attests of a relatively high serpentinization temperature >200°C. Magnetic measurements reveal a lower magnetite content in hydrothermalized serpentinites hosting the magnetite veins, with lowest values (down to 0.58wt%) for bleached serpentinites constituting the wall rock of the veins. These magnetic data are consistent with bulk rock chemistry showing a lower total iron content in hydrothermalized serpentinites. Hysteresis parameters and thermomagnetic measurements denote a magnetic grains size that increases with the alteration, associated with the emergence of a new magnetic phase (Cr-magnetite) produced by Cr-spinels alteration. A new proxy, based on thermomagnetic measurements, the CrM/M ratio, provides a quantification of its contribution to the magnetic susceptibility. Mineral chemistry allowed to identify the Cr-spinels alteration sequence and reveals an important chlorine enrichment in serpentine phases from hydrothermalized serpentinites.These results suggest that a Cl-rich acidic hydrothermal event involving temperatures below 350°C produced an intense magnetite leaching in the host serpentinite and an advanced Cr-spinels alteration to ferritchromite and Cr-magnetite. Iron provided by this leaching have led to the formation of unique magnetite veins in the Aït Ahmane ultramafic unit. Two different settings are proposed for the hydrothermal event: (1) a continental hydrothermal system as advanced for the Co-Ni-As ores in the Bou Azzer inlier or (2) an oceanic black smoker type hydrothermal vent field on the Neoproterozoic seafloor.

Structural and geochronological constraints on the evolution of the Bou Azzer Neoproterozoic ophiolite (Anti-Atlas, Morocco)

The Bou Azzer ophiolite in the Moroccan Anti-Atlas represents the suture of the Neoproterozoic Pan-African orogen. The Bou Azzer inlier includes, from NE to SW, three main geological units: a volcanic-arc, an ophiolite and a continental platform. Several subduction-related plutons intrude the ophiolite and the volcanic-arc, and syn-to-late orogenic deposits unconformably cover the three terranes. The Eburnean (≈2 Ga) continental basement observed in other inliers of the Anti-Atlas appears not to crop out at Bou Azzer, where rocks traditionally attributed to that old basement are in fact Neoproterozoic rocks, either metagranitoids of the continental margin or ophiolitic metagabbros. The age of a metagabbro of the ophiolite is reported here for the first time, as 697 ± 8 Ma (SHRIMP U–Pb on zircons). U–Pb geochronological data are also presented for two syn-kinematic subduction-related plutons which, together with previous data, establish a time-span of ca. 655–635 Ma for these arc-type rocks. Moreover, the Oumlill granite, previously attributed to the Eburnean basement, has been dated as 741 ± 9 Ma. The earliest tectonometamorphic event affecting the ophiolite is recorded as a relic foliation and a garnet–rutile metamorphic assemblage, which might attest to an early peak-pressure exceeding the 5–6 kbar medium pressure reported so far. Thus, we suggest a first stage of ophiolite underthrusting below a volcanic-arc, followed by ophiolite exhumation and the concomitant development of the main low-grade planar–linear tectonic fabric. The ages of the calc-alkaline, syn-kinematic subduction-related plutons (655–635 Ma) might be taken as the end of subduction and the beginning of the ophiolite exhumation-obduction. The author's work in the Bou Azzer region supports the ophiolite-continental platform interpretation of Leblanc (1975) and Leblanc and Moussine-Pouchkine (1994), rather than the tectonic mélange interpretation proposed by Saquaque et al. (1989b) and others. A recumbent anticlinal and the basal thrust of the ophiolitic terrane are the main macrostructures formed during the obduction stage, characterized by top-to-the-W/SW kinematics. This deformational event is also recorded in the rocks of the underlying continental platform. A syn-orogenic unit, namely the Tiddiline Formation, unconformably overlies the arc volcanics, the ophiolite and the continental platform. Further collisional deformation affected the Tiddiline sediments, giving way to ESE-WSW striking upright folds and oblique reverse faults. The latest structures in this Neoproterozoic suture were left-lateral strike-slip faults, pointing to an increase in the left-lateral component of the continental collision.

Platinum-group elements distribution and spinel composition in podiform chromitites and associated rocks from the upper mantle section of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco

The distribution of platinum-group elements (PGEs), together with spinel composition, of podiform chromitites and serpentinized peridotites were examined to elucidate the nature of the upper mantle of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco. The mantle section is dominated by harzburgite with less abundant dunite. Chromitite pods are also found as small lenses not exceeding a few meters in size. Almost all primary silicates have been altered, and chromian spinel is the only primary mineral that survived alteration. Chromian spinel of chromitites is less affected by hydrothermal alteration than that of mantle peridotites. All chromitite samples of the Bou Azzer ophiolite display a steep negative slope of PGE spidergrams, being enriched in Os, Ir and Ru, and extremely depleted in Pt and Pd. Harzburgites and dunites usually have intermediate to low PGE contents showing more or less unfractionated PGE patterns with conspicuous positive anomalies of Ru and Rh. Two types of magnetite veins in serpentinized peridotite, type I (fibrous) and type II (octahedral), have relatively low PGE contents, displaying a generally positive slope from Os to Pd in the former type, and positive slope from Os to Rh then negative from Rh to Pd in the latter type. These magnetite patterns demonstrate their early and late hydrothermal origin, respectively. Chromian spinel composition of chromitites, dunites and harzburgites reflects their highly depleted nature with little variations; the Cr# is, on average, 0.71, 0.68 and 0.71, respectively. The TiO 2 content is extremely low in chromian spinels, <0.10, of all rock types. The strong PGE fractionation of podiform chromitites and the high-Cr, low-Ti character of spinel of all rock types imply that the chromitites of the Bou Azzer ophiolite were formed either from a high-degree partial melting of primitive mantle, or from melting of already depleted mantle peridotites. This kind of melting is most easily accomplished in the supra-subduction zone environment, indicating a genetic link with supra-subduction zone magma, such as high-Mg andesite or arc tholeiite. This is a general feature in the Neoproterozoic upper mantle.

Gold mineralization in the Proterozoic Bleida ophiolite, Anti-Atlas, Morocco

Abstract The newly discovered (1998) West Bleida gold mineralization (3 tonnes metal Au) lies west of the main Moroccan Bleida copper deposit (1981–1991) in the central Anti-Atlas (southern Morocco). It is hosted by metamorphosed and deformed mafic to intermediate volcanic rocks that are part of the Neoproterozoic tholeiitic volcanosedimentary series forming the stratigraphically upper part of the Bou Azzer ophiolite sequence. Strong sericitization and local silicification are associated with mineralization. These altered rocks represent a proximal hydrothermal alteration halo around the West Bleida ore zones. Normative chlorite characterizes the metamorphic assemblage away from the ore zones. Gold mineralization primarily occurs as deformed gold-bearing quartz veins and disseminations in Cu-rich chert zones (chalcopyrite–malachite), Fe-rich lithofacies and breccia zones. Gold is accompanied by small amounts of copper sulphides (&lt;1% modal chalcopyrite). Scanning electron microscope–energy dispersive spectrometry analyses of gold grains from veins and disseminations reveal the presence of palladium as inclusions of Pd–As–Sb, Pd–Bi–Se and Pd–Te mineral phases. An electron microprobe study confirms the presence of two types of gold. The first is an alloy of Au–Ag–Pd, typically bordered by small grains of Pd and Bi (Te,Sb) phases and associated with a metamorphic assemblage. Isomertieite, Pd 11 (Sb 2 ,As 2 ), was identified as one of the phases. The second type of gold is electrum (10% Ag, 90% Au), which is always associated with fractures and occurs with hematite and white mica. Based on its form and habits, West Bleida gold reflects two distinct generations of fluid activity. The primary event precipitated Au–Ag–Pd alloys from Au–Pd-bearing hydrothermal fluids and produced auriferous quartz veins and disseminations within mafic rocks of the Bleida ophiolitic accretionary complex. It was structurally and lithologically controlled. This early event is preserved in the deeper (and thus fresher) zones more than 80 m below the surface. Intense tectonic overprinting obscures the genetic relationship between vein and disseminated styles of mineralization, both of which contain Pd-rich gold, but some of the auriferous quartz veins are observed to crosscut disseminated mineralization. Two possible hypotheses are considered: the pre-tectonic root of a volcanogenic massive sulphide system, or a late tectonic orogenic (mesothermal) deposit. The presence of Pd minerals and anomalous cobalt concentrations suggest a source in ultramafic rocks. The second event, characterized by inclusion-free electrum, occurred much later and represents the alteration and weathering of the primary Pd-rich gold assemblage by oxidizing surface fluids. It affected all mineralized units and structures to a depth of 80 m. This post-tectonic surficial alteration also caused leaching of Cu-sulphides, which may explain their low abundances in the upper parts of the ore zones.

Mélanges and ophiolites during the Pan-African orogeny: the case of the Bou-Azzer ophiolite suite (Morocco)

Abstract Since the discovery of ophiolite sequences, the Bou-Azzer inlier has been considered a key area for understanding the evolution of the northern margin of the West African craton during the Pan-African orogeny. For about 20 years, it had been commonly accepted that the Bou-Azzer inlier represents an accretionary mélange accreted onto the West African craton under blueschist metamorphic conditions, similar to the Franciscan Complex and the Sanbagawa facies series. This would imply that a low geothermal gradient was prevalent during the subduction of the Pan-African oceanic plate, and that the ocean was subducted with a high convergence rate. A reinvestigation of the metamorphic conditions by a thermodynamic approach shows that the ophiolite sequence of Bou-Azzer underwent HT greenschist metamorphic conditions instead of blueschist metamorphic conditions. We propose that the ophiolites of Bou-Azzer are not similar to the Sanbagawa facies series or to the Franciscan Complex, but bear similarities to the Albanian or Cyprus ophiolites, which represent dismembered ophiolite sequences overprinted by greenschist conditions.

Platinum group minerals in podiform chromitites of the Bou Azzer ophiolite, Anti Atlas, Central Morocco

The Neoproterozoic Bou Azzer ophiolite complex hosts numerous, small lenticular bodies of massive and disseminated chromite. Metallurgical-grade high-Mg and high-Cr spinels (cores with 48–62 wt% Cr2O3) reveal complex alteration patterns of successive Cr and Mn enrichment and loss of Al towards the rims, while the Mg# ratios [(Mg/(Mg + Fe2+)] remain almost constant. Concentration patterns of platinum-group elements are typical for ophiolitic chromitite poor in sulfides, with predominance of the IPGE, variable Rh, and low Pt and Pd. The most abundant platinum-group mineral is Rh-bearing laurite that occurs either included in spinel or in silicate matrix, whereas Os-Ir-Ru alloy is always included in spinel. Laurite inclusions reveal complex intergrowth textures with Rh-Ru-Pt rich alloy, and with Rh-rich sulfide. Most laurites display trends to sulfur-poor compositions leading to local formation of very fine-grained Ru-Os-Ir alloy phases. Ni-Co-Fe sulfides, arsenides and sulfarsenides devoid of PGE are associated with the alteration of chromite. Textural position and chemical composition of the base metal inclusions, as well as comparison of alteration features between chromite and accessory chromian spinel in the Co-Ni-As ores of the Bou Azzer ophiolite indicate a close connection. It is suggested that hydrothermal fluids percolated through the marginal zones of the ophiolite belt during greenschist facies metamorphism and deposited Ni-Co-Fe arsenides, sulfarsenides and minor sulfides as accessories within altered chromitites, and also in structurally favourable zones as Ni-Co-As ores.

Genesis of peculiarly zoned Co, Zn and Mn-rich chromian spinel in serpentinite of Bou-Azzer ophiolite, Anti-Atlas, Morocco

The peculiar Co, Zn and Mn-rich chromian spinels are hosted by magnetite veins, serpentinites and chromitites of the mantle section of the Proterozoic Bou-Azzer ophiolite, Morocco. The spinel is complexly zoned either optically or chemically, and exhibits anomalously high MnO, ZnO and CoO contents (up to 22, 7.5 and 2 wt%, respectively). It has four distinct optical zones particularly in the magnetite veins and less typically in serpentinites. The highest level of these elements, probably divalent, is recorded within the ferritchromite zone and/or within the core zone if the ferritchromite zone is absent. These elements as well as Fe exhibit enrichment along grain boundaries and fractures of the altered spinels. Fe, Mn, Zn and Co were most probably supplied from olivine upon severe serpentinization during and after obduction of the ophiolite. The enrichment of the Bou-Azzer chromian spinel in Mn, Zn and Co was governed mainly by the fluid/mineral (spinel) ratio and partition coefficient between spinel and the relevant fluid among many factors. The Co-, Zn- and Mn-rich chromian spinels can be used as an exploration guide for Co-Ni-Zn-Cu sulfide mineralization associated with serpentinized peridotites.

Origin of magnetite veins in serpentinite from the Late Proterozoic Bou-Azzer ophiolite, Anti-Atlas, Morocco: An implication for mobility of iron during serpentinization

Vein-like deposits of low-TiO 2 (<0.03 wt%) magnetite are present in the mantle section of Bou Azzer ophiolite, Anti-Atlas, Morocco. Two types of magnetite veins, I and II, can be recognized: magnetite is fibrous in Type I, but is stout or idiomorphic in Type II. Mode of occurrence and petrography indicate they had formed filling the open space of cracks. Magnetite is Ni-bearing in Type I, and is Ni-free and sometimes includes Mn-rich chromian spinel in Type II. Magnetite is commonly accompanied by serpentine mainly and magnesite in Type I, and by chlorite (clinochlore), serpentine, talc and lesser amount of garnet (andradite) in Type II. In situ trace-element analysis shows depletion with Al, Si, V, Cr and Zn in the vein magnetite relative to the disseminated one in wall serpentinite. Positive slope of PGE distribution pattern and Au enrichment were the result of hydrothermal activity. The formation of magnetite veins was apparently associated with enrichment of magnetite component in chromian spinel. The iron was supplied from olivine upon serpentinization which accompanied the obduction of the Bou-Azzer complex. The iron mobility may have been enhanced by high water/rock ratio of the serpentinization.

Spinel composition as a petrogenetic indicator of the mantle section in the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco

The Neoproterozic Bou Azzer ophiolite, Anti-Atlas, Morocco, is one of the best preserved ophiolites of Neoproterozic age. However, due to severe serpentinization, almost all primary minerals have been converted to secondary minerals except chromian spinel which has survived alteration in ultramafic rocks and is used here as the only reliable petrogenetic indicator. Relics of the primary textures indicate that the mantle section at Bou Azzer is composed mainly of harzburgite with dunite lenses and veins and small scale chromitite pods. Wehrlite is also found as late-stage intrusions into the mantle section and still has preserved the primary silicates, olivine and clinopyroxene. The chromian spinel chemistry in both peridotite rocks and chromitite pods exhibits a very limited compositional range. The Cr# [= Cr/(Cr + Al) atomic ratio] of spinel is about 0.8 in all serpentinized peridotites and ranges from 0.65 to 0.85 in chromitites, while it is slightly lower, about 0.6, in wehrlite. This suggests the upper mantle from which the Bou Azzer peridotite had been derived was highly refractory. The frequent occurrence of the highly refractory mantle in Precambrian ophiolites may indicate that the high-degree partial melting was much more common in the Precambrian era.

Precise U–Pb zircon dates from the Avalon Terrane in Newfoundland

The following new U–Pb dates are provided for zircons from volcanic and intrusive rocks of the Avalon Terrane of Newfoundland: Burin Group ophiolite (Wandsworth pegmatitic gabbro), [Formula: see text]; Marystown Group ash-flow tuff, [Formula: see text]; Rock Harbour Group rhyolite clast in conglomerate, [Formula: see text]; Harbour Main Group ash-flow tuff, [Formula: see text]; Harbour Main Group flow-banded rhyolite, [Formula: see text]; porphyritic rhyolite plug intruding Harbour Main Group, 632 ± 2 Ma; Holyrood Granite, [Formula: see text]; rhyolite dyke, Harbour Main Group, [Formula: see text]; and welded ash-flow tuff, "Grand Beach porphyry," [Formula: see text]. All of the dated zircons are simple, single-age populations with no trace of inheritance. These dates confirm that the ophiolitic Burin Group represents an older and separate event that correlates precisely with the Bou Azzer ophiolite of Morocco and that the Rock Harbour Group does not represent continental volcanism that preceded that event. The Rock Harbour, Marystown, and Harbour Main groups and the Holyrood Granite (632–608 Ma) are all part of the younger 650–550 Ma pulse of Pan-African orogenesis that affected a broad area extending through Africa, Asia, western Europe, southern England and Wales, and eastern North America. However, a shortage of high-precision dates throughout these terranes precludes very exact correlations and thus very exact interpretations in terms of specific tectonic or magmatic events. Although these dates require some revisions in the stratigraphy of the Newfoundland Avalon Terrane, they do not support suggestions that the Avalon Terrane comprises a "collage of suspect terranes." The age of the Grand Beach porphyry (394 Ma) places it squarely with the Acadian granites of the Appalachians and removes it from consideration as part of the Marystown Group.