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Morila

Mali

Main commodities: Au
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The Morila deposit is located in southern Mali, some 180 km south of the capital Bamako, and 600 km south-east of Sadiola. Construction commenced in late 1999 and the first gold was poured in late 2000.

The Morila deposit lies within the West Africa Craton, hosted by Palaeoproterozoic Birimian volcano-sedimentary rocks of the the Massigui region of Mali, between the Kalana-Yanfolila and Syama volcano-sedimentary belts.   These Birimian rocks represent juvenile volcanic terranes developed adjacent to and over the Archaean Man (Liberian) craton. They were emplaced over a prolonged (2098 to 2065 Ma) period of magmatism during the late stages of the Eburnean orogeny.

The Massigui region is characterised by Birrimian meta-volcanic and meta-sediment sequences intruded by a large Eburnian granitoid batholith. The batholith comprises i). a homogeneous potassic, pink leucogranite (Massigui type) to the east of the Morila deposit and ii). an older phase comprising a group of biotite granites (Doubalokoro suite) to the west and south of the deposit. The principal regional structure is the 50 km wide, NNE-SSW trending Banifin shear zone which strikes for some 200 km through southern Mali and northern Cote d'Ivoire. This shear zone largely follows the contact between the two grantoid suites described above and contains deformed volcano-sedimentary formations which have been intruded by the granitoid plutons.

The Morila mineralisation is hosted by a large enclave of deformed sediments within a granodiorite batholith, close to the northwest oriented contact of the batholith. Exposure is extremely poor with only 3 outcrops in the mine area. The metasediments are metagreywacke and metavolcaniclastic rocks with minor metapelitic layers that have been metamorphosed to lower amphibolite facies and probably represent original sandstones and siltstones with a volcanic component. Much of the gold in the deposit is localised in a discontinuous, folded zone of hornfelsed biotite schist that is more strongly foliated than the adjacent unmineralized rocks. The southern limit of the orebody is terminated against a granodiorite stock, while the mineralised package is intruded by pegmatites and numerous sills and dykes of diorite and granodiorite.

In regionally equivalent rocks surrounding the deposit, well-preserved way-up indicators such as fining-upward sequences, turbiditic scours, and rare crossbedding are locally well preserved and provide evidence for a complexly folded stratigraphic sequence that includes a series of upright to overturned folds dissected by steep brittle faults.

The shallowly NE plunging orebody, which is over 850 m long, has been subjected to polyphase deformation, including folding, shearing, thrusting and normal faulting, with NW, NE and east-west trends. The pelites show a subtle alignment of biotite, while the psammites are massive without a foliation.

Visible gold at Morila is associated with variably deformed polymineralic veins, occurring as a quartz-sulphide stockwork and disseminated arsenopyrite hosted in an intrusive contact zone. Up to 60% of the gold is free and can be recovered by gravity. The ore zone is enveloped by silica-feldspar alteration, which includes an array of foliation parallel and folded silica-feldspar veins. The ore assemblage includes gold associated with predominantly arsenopyrite / loellingite and pyrrhotite. Smaller quantities (<5% of opaques) comprise chalcopyrite, pyrite and ilmenite, while minor galena, molybdenite and fahlore (tetrahedrite-tennantite) are sporadically evident. Gold is either free and interstitial to the main silicates of biotite, plagioclase (andesine), silica and muscovite, or is attached to the sulphides. Less commonly minor amounts are within arsenpoyrite grains and occassionally within pyrite and pyrrhotite.

Early formed mineralisation is contained within a zone of hornblende hornfels contact metamorphism and is spatially associated with syn- to post-D2 emplacement of 2098 to 2091 Ma quartz-diorite, granodiorite and leucogranite magmas. Immiscible Au-Sb-Bi-Te blebs within sills or dykes associated with gold mineralisation implies a linkage with granitic magmatism. This early intrusion-related phase was overprinted by a post-D2 stage of hydrothermal alteration reflected by sulphidation along a NNE-trending zone of disseminated idioblastic arsenopyrite porphyroblasts that contain polygonal gold blebs. Silicate alteration (U-Pb dated at 2074±14 Ma) accompanying this stage includes albitisation of plagioclase and the growth of randomly distributed biotite and titanite, the latter typically surrounding ilmenite.

The total resource in 2001 was estimated to be just over 185 t (5.6 Moz) of contained gold, comprising 1 Mt @ 6.7 g/t Au as free digging oxide ore, followed by sulphide ore averaging 4.35 g/t Au, together comprising a resource of 33.5 Mt @ 5.5 g/t Au.

Remaining resources and reserves at 31 December, 2010, were (Randgold Resources website, 2012):
    Both Probable reserves and Measured + Indicated resources - 12.55 Mt @ 1.39 /t Au
    Inferred resources - 1.95 Mt @ 0.79 /t Au.

By 2010 mining had ceased and the operation was only involved in stockpile treatment. The deposit is quoted on the Randgold Resources database as having contained 233 t (7.5 Moz) of gold.

For details consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2001.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


Morila

  References & Additional Information
   Selected References:
Anonymous  2000 - Joint Announcement by Anglogold and Randgold Resources: in    Press Release, 5 April 2000    5p
Anonymous  1999 - Randgold Resources Mali: in    Mining Magazine, May 1999    pp RSA12-15
Anonymous  1999 - Morila: in    Register of African Gold 1999/2000    p128
Bristow DM, Reading DJ  2001 - Morila - a company maker product of foucused strategy and applied commercial science: in Yates K (Ed.), 2001 NewGenGold 2001, Conference Proceedings AMF, Adelaide    pp 57-76
Foster R P  1999 - Destination Africa: New Frontiers, New Mineral Exploration Opportunities: in   African Mining: Exploration and Investment Opportunities Conference, Perth, Australia: 16-17 November, 1999 AJM, Sydney    pp 1-11
McFarlane C R M, Mavrogenes J, Lentz D, King K, Allibone A and Holcombe R,  2011 - Geology and Intrusion-Related Affinity of the Morila Gold Mine, Southeast Mali : in    Econ. Geol.   v.106 pp. 727-750


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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