Creus, Pieter Koenraad
ENGLISH ABSTRACT: Numerous quartz-vein hosted gold prospects in the Karibib district in central Namibia testify to the presence of a large hydrothermal system during Pan-African times in rocks of the Damara Belt and centred around the Navachab Gold Mine. This study presents the results of the regional and detailed mapping of the Navachab synform, a NE-SW trending, regional-scale structure hosting a number of these gold prospects, locally referred to as the “zoo prospects”, in the direct vicinity of the main Navachab Gold Mine.
The zoo prospects are located in the marble-dominated, up to 800 m thick Karibib Formation, which forms the core of the Navachab synform. Regional mapping of the synform identified six main lithological units comprising massive and banded dolomitic and calcareous marbles, intraformational breccias and interlayered calc-silicate felses. Despite characteristic thickness variations, the six units can be correlated throughout the synform, allowing for a lithostratigraphic correlation of units in the otherwise monotonous marble sequence. All of the prospects are spatially closely associated with and adjacent to unit 5, an up to 100 m thick, competent dolomitic marble unit. This suggests a strong lithological control of the mineralisation.
The first-order Navachab synform formed during the regional D2 phase of deformation. The strongly non-cylindrical, doubly-plunging fold shows open- to close interlimb angles and a pronounced NW vergence in the SW, but is tight- to isoclinal and upright in the NE. Higher fabric intensities and detachment folding are confined to the subvertical limbs of the synform in the north. Here, structures record a subhorizontal, NW-SE directed co-axial shortening strain interpreted to have developed in response to the geometric hardening and layer-normal shortening following the rotation of the fold limbs to subvertical attitudes during progressive D2 shortening. The mineralisation of the zoo prospects is located where D2 high-strain zones intersect unit 5. This suggests an additional structural control of the mineralisation.
The detailed mapping of the zoo prospects identified five distinct quartz-vein sets. The geometry, orientation and progressive deformation of the vein sets indicate that veining occurred during the late stages of the D2 event and during NW-SE directed, subhorizontal shortening. Areas of increased veining and mineralisation occur preferentially in areas of strain incompatibilities, where the combined effects of D2 strains and prominent lithological contacts created zones of localized dilatancy. This includes most prominently dilational jog geometries developed between anastomosing D2 shear zones, where hydrothermal fluid flow has produced pervasive quartz-vein stockworks. This also includes areas of detachment folding, where deformation of the rheological stiffer dolomitic marbles and less competent calcareous marbles has led to detachment surfaces and, locally, dilatancy. Zones of increased permeability are also created where two or more vein sets intersect, which is particularly common within and adjacent to boudin interpartitions of competent dolomite units and along rheologically prominent contacts.
The zoo prospects illustrate the interplay of (1) prominent rheological contrasts between adjacent lithologies, (2) the presence of high-strain zones, and (3) the geometry of host structures for the formation of auriferous quartz veins in the Karibib district. AFRIKAANSE OPSOMMING: Talle goud vooruitsigte, wat kwarts-aar geherberg is, in die Karibib-distrik in Sentraal-Namibië getuig van die teenwoordigheid van 'n groot hidrotermale stelsel tydens die Pan-Afrika-tye in die rotse van die Damara-Belt en is gesentreer rondom die Navachab goudmyn. Hierdie studie stel die resultate van die streeks- en gedetailleerde kartering van die Navachab sinform, 'n NW-SO neiging, streeks-skaal struktuur wat ŉ aantal van hierdie goud prospekteerplek herberg, plaaslik bekend as die "zoo prospects", in die direkte omgewing van die hoof Navachab goudmyn.
Die “zoo prospects” is geleë in die marmer-gedomineerde, tot 800 m dik Karibib formasie, wat deel vorm van die kern van die Navachab sinform. Streeks kartering van die sinform het ses belangrike litologiese eenhede geidentifiseer wat uit massiewe en gestreepte dolomitiese kalk marmer, intraformasie breksie en tussengelaagdheid kalksilikaat felses bestaan. Ten spyte van die kenmerkende dikte variasies, kan die ses eenhede gekorreleer word oor die hele sinform, wat toelaat vir 'n litostratigrafiese korrelasie van eenhede in die andersins eentonige marmer opeenvolging. Al die vooruitsigte is ruimtelik verbind met en aangrensend aan Eenheid 5, 'n tot 100 m dik, bevoegde dolomitiese marmer-eenheid. Dit dui op 'n sterk litologiese beheer van die mineralisasie.
Die eerste-orde Navachab sinform het gevorm tydens die plaaslike D2 fase van deformasie. Die sterk nie-silindriese, dubbelduikende plooi wys oop- tot noue tussenflankhoeke en 'n uitgespreek NW-vergensie in die SW, maar is styf- tot isoclinaal en regop in die NO. Hoër maaksel sterkte en losmaking plooie is beperk tot die subvertikale flank van die sinform in die noorde. Hierdie strukture is 'n aanduiding van 'n subhorisontale, NW-SO gerigte ko-aksiale verkorting wat geïnterpreteer is as vervorming wat ontwikkel het in reaksie op die geometriese verharding en die laag-normaal verkorting as gevolg van die rotasie van die plooi flanke tot subvertikale houdings tydens die progressiewe D2 verkorting. Die mineralisasie van die “zoo prospects” is geleë waar D2 hoë-spanning sones Eenheid 5 sny. Dit dui op 'n addisionele strukturele beheer van die mineralisasie. Die gedetailleerde kartering van die “zoo prospects” het vyf verskillende kwarts-aar stelle geïdentifiseer. Die geometrie, argitektuur en progressiewe vervorming van die aar-stelle dui daarop dat aar-vorming plaasgevind het gedurende die laat stadium van die D2 gebeurtenis en tydens die NW-SO gerugte, subhorisontale verkorting. Gebiede van verhoogde aar-vorming en mineralisasie kom verkieslik voor in die gebiede van vervorming verskille, waar die gekombineerde effek van die D2 vervorming en prominente litologiese kontakte sones van gelokaliseerde dilatansie. Dit sluit die mees prominente uitsettings uitwyking geometrie wat ontwikkel tussen anastomoserend D2 skuifskeursones, waar hidrotermale vloeistof stroming deurdringende kwarts-aar stokwerke geproduseer. Dit sluit ook die gebiede van losmaking plooie, waar die vervorming van die reologiese stywer dolomitiese marmer en minder bevoegde kalk marmer losmaking oppervlaktes gelei het, en plaaslik, dilatansie. Sones van 'n verhoogde deurlaatbaarheid is ook geskep waar twee of meer aar stelle sny, wat is veral algemeen binne en aangrensende boudin tussendeelpartisies van bevoegde dolomiet-eenhede en langs reologiese prominente kontakte.
Die “zoo prospects” illustreer die wisselwerking tussen (1) prominente reologiese kontraste tussen aangrensende litologie, (2) die teenwoordigheid van hoë-spanning sones, en (3) die geometrie van die geherbergte strukture vir die vorming van goudhoudende kwarts are in die Karibib-distrik. Description:
Masters Degrees (Earth Sciences)