Geological setting and evolution of the Omitiomire Cu deposit in the Southern Zone accretionary prism of the Damara Belt, Namibia

Kitt, Shawn (2017-12)

Thesis (PhD)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: Shear Zone hosted Cu mineralisation in the Deep Level Southern Zone accretionary prism of the Damara Belt is associated with deformation and fluid flow related to the exhumation of the Mesoproterozoic Ekuja basement gneiss dome during the Pan-African convergence of the Congo and Kalahari Cratons. In terms of tectonic setting, metamorphic grade and age structure, the mineralisation has no real equivalent in Namibia and seems to defy current classification schemes of Cu deposits. This study aims to describe the geometry and controls of mineralisation and the sources of fluids and metals in Omitiomire Cu deposit, and constrain the internal dynamics and processes that led to the imbrication of 100 km2 slivers of basement gneisses with the overlying prism metasediments. The Omitiomire Cu deposit (137 Mt at 0.54% Cu) is hosted by a low-angle, late-Damaran (ca. 520 Ma) shear zone system, referred to as the Omitiomire Shear Zone (OSZ), that developed around an older (ca. 1100–1060 Ma), late Mesoproterozoic intrusive breccia between a suite of mafic rocks (originally lava flows) and later tonalitic gneisses. The chalcocite-dominated mineralisation is associated with biotite-epidote assemblages that formed through the progressive hydration of the original mafic rocks to amphibole-biotite gneiss and biotite-epidote schist during deformation and fluid infiltration along the OSZ. Stable isotope (O, H and S) data indicate upper-greenschist-to middle amphibolite-facies conditions, a low fluid-rock ratio and a local redistribution of sulfur during fluid flow and mineralisation. This points to the remobilisation by epigenetic fluid flow of an older Cu mineralisation event. The geometry of the mineralisation is controlled by the spatial coincidence of the OSZ and the inherent heterogeneity of the original intrusive amphibolite–tonalite breccia. The gently undulating, shallowly-east dipping orebody is composed of several mineralised lenses that are contained in the regional S2 fabric. High-grade ore shoots are parallel to a prominent N-S trending L2 stretching lineation and are correlated with the cumulative number and thickness of several thin quartz-biotite-epidote-chalcocite shear zones at the contacts of interleaved schists and felsic gneisses. The kinematics and the association of the Cu mineralisation with retrograde assemblages in the OSZ link the mineralisation with the exhumation of the Ekuja Dome. The timing of exhumation is constrained to between 526.4 ± 3.5 and 521.9 ± 3.6 Ma by 40Ar/39Ar dating of biotite from the OSZ. Peak metamorphic assemblages in amphibolite gneisses from the Ekuja dome record PT conditions of 8.5-9.15 kbar and 635-655 °C, which equates burial to ca. 35 km. In contrast, PT estimates and U-Pb xenotime age determinations in the overlying prism metasediments show that peak metamorphic conditions of 7-9.25 kbar and 640-675 °C were only reached at ca. 515 Ma. This suggest that exhumation of the Ekuja dome started some ~10 Ma before maximum burial and peak metamorphism was reached in the overlying prism sediments and reflects the complexities of subduction-exhumation processes in accretionary complexes. The results of this study highlight the striking similarities with basement dome hosted deposits in the Lufilian Arc of Zambia and suggest the existence of a contiguous convergent margin along the leading edge of the Congo Craton that was active to at least ca. 515 Ma.

AFRIKAANSE OPSOMMING: Cu-mineralisasie in `n skuifskeer sone in die Deep Level Southern Zone (DLSZ) van die aanwas prisma van die Damara gordel word geassosieer met deformasie en vloeistof vloei wat verband hou met die herblootstelling van die Mesoprotersoïese Ekuja vloergneis koepel tydens die Pan-Afrika botsing tussen die Kongo en Kalahari kratone. Die mineralisasie het geen werklike ekwivalent in Namibië in terme van tektoniese omgewing, metamorfe graad en ouderdom, en pas ook nie werklik in die huidige klassifikasieskemas van Cu afsettings nie. Die doel van hierdie studie is om die geometry en beheer van die mineralisasie en die bronne van vloeistowwe en metale in Omitiomire Cu afsetting te beskryf, en om die interne dinamika en prosesse te ontsyfer wat daartoe gelei het dat 100 km2 plate van vloergesteentes met die oorliggende prisma afsettings geimbrikeer word. Die Omitiomire Cu afsetting (137 Mt by 0.54% Cu) kom voor in `n lae-hoek, laat-Damaran (ongeveer 520 Ma) skuifskeer sone, bekend as die Omitiomire Shear Zone (OSZ), wat ontwikkel is rondom `n ouer (ongeveer 1100-1060 Ma), laat Mesoprotersoïese intrusiewe breksie tussen `n reeks mafiese gesteentes (oorspronklike lavavloei) en later tonalitiese intrusiewe gesteentes. Die chalcosiet gedomineerde mineralisasie word geassosieer met biotite en epidoot samestellings wat gevorm het deur die progressiewe hidrasie van die oorspronklike mafiese gesteentes na amfibool-biotiet gneis en biotite-epidoot skis gesteentes tydens deformasie en infiltrasie van vloeistof langs die OSZ. Stabiele isotoop (O, H en S) data dui hoë-graadse metamorfe toestande aan, `n lae vloeistof-tot-rots verhouding en `n plaaslike migrasie van swael tydens mineralisasie. Dit dui op die hermobiliseering deur epigenetiese vloeisof vloei vanaf `n ouer singenetiese Cu mineralisasie. Die geometry van die mineralisasie word beheer deur die ruimetelik toeval van die OSZ en die inherente heterogeniteit van die oorspronklike intrusiewe amfiboliet-tonaliet breksie. Die sag golwende, oos hellende ertsliggaam bestaan uit verskeie gemineralisserde lense wat in die plaaslike S2-maaksel voorkom. Hoë graadse ertsstroke is parallel aan `n prominente noord-suid neigende L2 strekkings lineasie en word gekorrelleer met die kumulatiewe aantal en dikte van verskeie dun kwarts-biotiet-epidote-chalcocite-skuifskeer sones langs die kontak tussen mafiese biotite skis gesteentes en tonalitiese gneis. Die kinematika en die assosiasie van die Cu mineralisasie met retrogressiewe samestellings in die OSZ verbind die mineralisasie met die herblootstelling van die Ekuja koepel. Die tydsberekening van herblootstelling is beperk tussen 526.4 ± 3.5 en 521.9 ± 3.6 Ma met 40Ar/39Ar dateering van biotiet vanaf die OSZ. Piek metamorfiese samestellings in amfiboliet gneis van die Ekuja koepel dui aan op druk en temperatuur (PT) toestande van 8.5-9.15 kbar en 635-655 °C, wat aandui dat die Ekuja vloergsteentes begrawe was tot ongeveer 35 km. In teenstelling hiermee toon PT beramings en U-Pb xenotime dateering aan dat die oorliggende prisma metasedimente piek druk en temperatuur toestande van 7-9.25 kbar en 640-675 °C bereik het eers teen 515 Ma. Dit wys dat die herblootselling van die Ekuja koepel begin het ongeveer ~ 10 Ma voordat piek metamorfiese toestande bereik is in die oorliggende prisma metasedimente en weerspieël die kompleksiteite van subduksie-herblootstelling prosesse in aanwas komplekse. Die resultate van hierdie studie beklemtoon die opvallende ooreenkoms met Cu afsettings in vloergesteente koepels in die Lufilian Arc van Zambia en stel voor dat daar 'n aaneenlopende konvergente plaatgrens aan die voorpunt van die Kongo kraton is wat aktief was tot ten minste 515 Ma.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/102724
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