More rocking geology

Materials World magazine
,
1 Dec 2017

Imperial College students went on a field trip to Morocco to examine local geology and learn more about ore forming processes. Participant George Fry reports.

The modern-day structures of the Atlas Mountains represent the superposition of phases from a complex history of deformation. With multiple contrasting phases of deformation, and different styles of mineralisation associated with each stage, the crust of Morocco is enriched in a diverse range of metals and deposits. Recent legislative developments such as new mining authorisations and licences covering all mineral substances with the exception of phosphates (state prerogative) being valid up to ten years, have paved the way for an expansion of the country’s mining industry. The diverse and abundant basket of minerals make it an exciting location to study.

The 11-day trip included visits to different deposit sites, such as for polymetallic volcanogenic massive sulphide (VMS), hydrothermal vein copper, polymetallic molybdenum containing skarn, porphyry-style copper-gold, manganese karst and the world-class silver deposit of Imiter. In addition, we spent two days in the Jbel Saghro region of the Anti-Atlas Mountains, studying field evidence for magmatic-hydrothermal mineralisation in igneous terranes.

The team consisted of members of the Imperial College London Society of Economic Geologists Student Chapter and students of economic geology, members of staff from the Natural History Museum, London, representatives from the UK metals and mining industry, including two exploration geologists, and Dr Johann Tuduri, an expert in the local geology from the Bureau de Recherches Géologiques et Minières in Orléans, France. 

The field trip began with a short drive and work at the ore processing plant of Hajjar, located roughly half an hour from the city. Hajjar is a polymetallic metamorphosed VMS deposit in the Jebilet massif. It is stratabound, hosted within volcano-sedimentary successions, pyrrhotite rich and associated with chlorite and sericite alteration. This deposit is also pyrrhotite dominated with minor pyrite, containing 8% zinc, 2-3% lead and 0.4-0.6% copper. The afternoon consisted of a tour of the ore processing facilities and a visit to the Managem Research Institute, where scientists conduct research into battery technology, with cobalt being refined at the site.

The Imini manganese deposit

The group then travelled across the High Atlas Mountains to Kalaat M’Gouna. En route, Dr Tuduri organised stops to demonstrate the tectonic evolution of the mountain belt. The High Atlas of Morocco is an intracontinental fold-thrust belt, continuing into Algeria and Tunisia, about 2,000km long and 100km wide, from which the Middle Atlas branches in about 250km in length. Extension in this region occurred through the Mesozoic – first in the Triassic, preserved by red beds and tholeiitic basalts, and second in the Jurassic, marked by deposition of marine carbonates and shales capped by continental red beds. Compressional deformation and uplift occurred through the Oligocene to Pliocene in upper crustal conditions, causing little or no metamorphism. Variations in Mesozoic stratigraphy and thickness across many thrust faults show their origin as syn-sedimentary extensional faults, which later experienced tectonic inversion. The High Atlas and Middle Atlas to the northwest are shown by magnitude and the timing of deformation to be part of the African-Eurasian plate-boundary zone in the western Mediterranean during the Cenozoic. 

The igneous geology

The group drove further into the mountains, to investigate the mineralisation in the Anti-Atlas. The mountains form the northern margin of the West African Craton – a palaeoproterozoic basement established during the Eburnean Orogeny 2-2.2Ga. Unconformably above this granitic basement (exposed as a 20km pluton at Tazenakht), are three major groupings of lithologies – late neoproterozoic igneous, plutonic and volcanic. Plutonic rocks comprise calc-alkaline granites emplaced during the Pan-African orogeny, and volcanic rocks are related to these, with the full spectrum of felsic-contents from basaltic to rhyolitic. Palaeozoic sediments lie non-conformably above the Neoproterozoic units and post-date the Pan-African orogeny. Carbonates and sandstones form beautiful periclines across the region, uplifted and folded by the Variscan Orogeny. Exposed mostly to the west of the inliers of interest, near to Sirwa, these mafic volcanics have produced epithermal (propylitic) alterations of their own. 

The Bou Skour copper deposit

Day five started with a lecture on the geological setting of the Bou Skour deposit and on the mineralisation style given by Dr Tuduri and the chief geologist on site. After, the group visited the main above-ground workings of the mine, including two open pits, to take samples from the ore heaps beside the main pit, yielding some impressive samples of copper oxides and sulphides. A visit to the second pit, an historical working, allowed a glimpse at some rare minerals, including agardite, for which Bou Skour is the type locality. The Bou Skour copper deposit is located in the northeast-southwest trending Saghro massif, an elongate mountain chain that contains multiple magmatic inliers disposed to mineralisation. The magmatic rocks of the Bou Skour deposit exhibit hydrothermal alteration, which is directly associated with mineralising processes. 

The Imiter silver mine

A strong candidate for the highlight of the trip, day eight comprised a visit to the famous silver deposit at Imiter. The world-class, 8.5Mt (700 g/t) Imiter silver mine is located 125km east of Ouarzazate at an altitude of 1,500m on the northern side of the Precambrian Jbel Saghro inlier, north of the West African Craton. Archaelogical evidence suggests mining occurred here as early as the 8th century AD, and more than 1,000 tonnes of silver were extracted before present day work began. 

The steeply dipping, east-west trending regional faults system mainly consists of a complex paragensis of silver-amalgam, silver-sulphides, sulphosalts and other base metal sulphides, which formed during four distinct mineralising events, characterised by different gangue mineral assemblages and hydrothermal fluids. Deposit formation is genetically associated with a felsic volcanic event, dated at 550Ma, and believed to result from a regional extensional tectonic regime.

The deposit is situated in the northern part of the Precambrian Saghro massif, which is part of the 680–580Ma Pan African Anti-Atlas orogenic belt, on the northern margin of the West African Craton. The rocks of the Saghro group are subdivided into two major units, lower and upper complex, and were deposited on the Neoproterozoic basement. The lower complex consists of a strongly folded Cryogenian succession of greenschist-facies greywacke and organic-rich black shale, locally intruded by 570-580Ma granodioritic to dioritic intrusions. The black shales, which host the majority of Imiter silver, were deposited at the outer part of a passive continental margin that developed in an extensional back-arc environment. Silver mineralisation predominantly occurs within the Cryogenian black shale and to a lesser extent in the Ediacaran volcanic rocks. It is localised along the late Neoproterozoic, transcrustal Imiter fault zone, at the boundary between the lower and upper complexes. The Imiter fault zone consists of a complex series of N90°E and N60-70°E faults that exhibit sinistral pull-apart structures. 

The Azegour skarn deposit

On the final, we got the chance to see an intriguing polymetallic deposit in the foothills of the High-Atlas Mountains, southeast of Marrakesh. The skarn deposits of Azegour were first mined in 1921, then primarily for copper, and subsequently for molybdenum from 1930 to 1946, during which time a peak production of 200 tonnes of 85% molybdenum disulfide (MoS2) concentrate was achieved per year. Exploitation continued variably until 1972, after which activity ceased, although recent rises in metal prices have spurred a flurry of interest and new research in the site. Estimated reserves were 1.5Mt 0.2%-0.7% MoS2, 2Mt of 0.4% tungsten trioxide, 1Mt of 1.4%-2.8% copper before exploitation began. The site included an abandoned processing plant, which the group explored extensively.

The deposits occur as irregularly shaped clusters and pods, or thin lenticular bands following compositional banding. Mineralisation is predominantly molybdenite and chalcopyrite with minor associated scheelite, sphalerite, galena and pyrite. Trace amounts of uranium, beryllium and bismuth minerals are also observed. It has been suggested, based on mineralogical and textural observations, that mineralisation at Azegour occurred in two stages – an initial sulphide/arsenosulphide phase, similar to other hydrothermal sulphide deposits found throughout Morocco, and a second skarnification phase that provided molybdenum and tungsten as well as additional metallic sulphide phases.

Apply for Institute’s research funds and Imperial trip

Students were able to participate in the trip at reduced cost due to the generosity of the society’s sponsor organisations such as IOM3. To enable more trips like these, the Institute is inviting applications for its 2018 scholarship and bursary awards. Full details can be downloaded from www.iom3.org/content/scholarships-bursaries. Additionally, the Imperial College student chapter is planning a trip in summer 2018 to Namibia and is looking for industry collaborators to sponsor and attend the field course. More info can be obtained by emailing the student chapter at seg.chapter@imperial.ac.uk