The structural and stratigraphic setting of the Rosh Pinah zinc-lead deposit within the Gariep trough

Alchin, Dennis John (1993)

Thesis (M. Sc.) -- University of Stellenbosch, 1993.

Thesis

ENGLISH ABSTRACT: A geological investigation in the Rosh Pinah, Obib and Pickelhaube Peaks and Namuskluft-Dreiqratberg areas established the structural and stratigraphic setting of the Pinah zinc-lead deposits within the Gariep Trough. The sedimentary evolution of the Pan-African Gariep Belt was initiated by widespread rifting and opening of an ocean (the Adamastor Ocean) along a continental margin. Deposition of the basal clastic Stinkfontein Sequence as broad alluvial fans strandline sediments took place in the transgressive sea. The mass flow Kaigas Formation diamictites and volcaniclastics of the Rosh Pinah Formation were subsequently deposited in locally developed grabens. Syn-sedimantary ore beds of hydrothermal origin (includinq Rosh Pinah orebodies) were precipitated on the sea floor while reef carbonates formed along the coast line during the final stages of transgression. These sediments were eventually covered by a marginal marine clastic and carbonate facies with widespread intrusion of basic sills and dykes following uplift of the craton. These rocks are here grouped within the Rosh Pinah Formation. Durinq the sea floor spreading stage of continental evolution the elastic-carbonates of the Hilda Sequence were deposited in a shallow to moderately deep-water environment. The Wallekraal conglomerates were deposited as local submarine fans with minor debris flows and are overlain by the Dabie River Formation carbonates. During regression banded iron formations were deposited in shallow depressions and the glaciogenic Numees Sequence diamictites were deposited unconformably above the Hilda Sequence and iron formations. These in turn are overlain by the deepwater clastics of the Holgat Sequence. The Witputs sequence, which uncontormably overlies the Numees Sequence along the basement in the east is seen as a proximal shallow water equivalent of the distal Holqat Sequence. Gravity tectonics played an important role during the early structural evolution of the Gariep Belt. Mass flow and slump structures featured prominently in half graben structures formed during the rifting staqe. However, the closing of the Pan- African Adamastor Ocean as a result of subduction and continental collision led to a single, very intensive and protracted period of SE directed transpression, which largely obliterated all gravity deformation characteristics. The D1 transpressive phase produced NNW-trendinq basement involved oblique ramps during a major SB-directed tectonic event, which culminated in several deformation pulses. Arcuate nappe structures were formed, e.g. the Marmora, Schakalsberg and possibly the Rosh Pinah Nappes as a result of the oblique emplacement of thrust slices over the cover rocks. Thrusting was accompanied by east-northeast-verging F1 folds with axes rotated sub-parallel to the D1 transport direction, and a penetrative westerly dipping S1 transposition cleavage. The master Schakelsberg Thrust emplaced a thick slice or dense basic rock with ophiolitic affinities onto a foreland situated in the Schakalsberg mountains during the D1 transpressive phase. Gravitational instability was thereby caused in the crust, and an imbricate fan of listric splay faults formed in front of the overlying spreading mass. Several deformation paths for the complicated tectonite containing three partly non-colinear phases of deformation can be envisaged from here which includes oblique ramping, backthrusting and backfoldinq. The Annisfontein anticlinorium may have formed as a D1 basement thrust ramped up section and folded due to continued pressure from the W. During this pulse D1 structures were sub-coaxially refoldled by non-cylindrical WSW-verging F2 shear folds because of E-directed compression across NNW trending ramps. This is displayed in the Namuskluft-Dreigratberg area and is also inferred for the deeper levels of the western areas, and leads to backfolding over most of the area. The semi-allochthonous Rosh Pinah Formation containing the Rosh Pinah orebodies in the type area was refolded during D2 into a hinterland-verging anticlinorium because of its proximity to the major oblique ramps, which are also responsible for the steeply dipping thrusts in the Rosh Pinah-Namuskluft-Dreigratberg area, and which probably represent highly deformed earlier qraben and horst structures. On the eastern limb of the Annisfontein anticlinorium, a hinterland-dipping duplex of imbricate fan sheets developed on the deformed and overthrust sediments (the Pickelhaube Duplex Structure) as a result of ramping and steepening against the basement. Continued upward bulging of the Annisfontein anticlinorium due to either antiformal thrust stacking or multiple ramping along an initial basement irregularity on the footwall resulted in east-dipping parts of the Rosh Pinah Nappe Thrust being inactivated. Late, limited gravitational gliding of the overthrust sediments along the Rosh Pinah Nappe Thrust towards the east may have followed contractional deformation to enhance backfolding of thrust slices of Hilda lithologies i the duplex zone . The oblique ramplng ot litholoqies against the cratonic margin and possible deeper seated ramps finally also accounts for D3 structures. Due to differential movement these F3 folds may be slightly overturned towards the southeast, with slip occurring along cleavage planes to outline minor thrusting oblique to the previous events and trending NE-SW. This folding phase resulted in the present day north-south undulation of the structures and the landscape. The late lateral transtensive phase D4-D5 resulted from the emplacement of the Kuboos-Swartbank plutons and are not well represented in this area. The Gariep Belt has been subject to a protracted period of lower qreenschist facias metamorphism (M1), followed by a second local contact-thermal event (M2) which resulted from the emplacement of the Kuboos and Swartbank plutons in the Richtersveld.

AFRIKAANSE OPSOMMING: 'n Geologiese ondersoek in die Rosh Pinah, Obib. Pickelhaube en Namuskluft-Dreiqratberg omqewinqs het tile strukturele en stratigraflese plasing van die Rosh Pinah lood-sinkatsetting in nie Gariep Troq aangedui. Die sedimentere evolusie van die Pan-Afrika Gariepgordel het begin met wydverspreide vorminq van slenkdale, en die oopmaak van 'n oseaan (die Adamastoroseaan) met die skepping van 'n kontinentale rand. Afsetting van die basale klastiese Stinktonteinopeenvolqinq as breë alluviale waaiers en kuslynsedimente het plaasgevind in 'n transgressiewe see. Daarna is die Kaigas diamiktiete en vulkanoklastiese gesteentes van die Formasie Rosh Pinah as massavloei-afsettings gevorm in lokaal ontwikkelde grabens. Gedurende die finale fases van transgressie is sinsedimentere ertslae in stiller waters neerqelê terwyl rifkarbonate langs die kuslyn gevorm het. Hierdie sedimente is uiteindelik bedek deur mariene klastiese en karbonaat fasies wat afgeset is in 'n marginale see. Na die opheffing van die kraton het wydverspreide basiese gange en plate al hierdie sedimente geïntrudeer. Hulle word hier gegroepeer in die Formasie Rosh Pinah. Gedurende die seevloeruitbreidingsfase van kontinentale evolusie is die klastiese en karbonatiese gesteentes van die Hildaopeenvolging afgeset in water van vlak tot intermediêre diepte. Die basale Wallekraalkonglomerate is as lokale waaiers met ondergeskikte puinstortings neergelê en word onderlê deur die karbonale van die Formasie Dabierivier. Hierna vind regressie plaas en is gestreepte ysterklip formasies afgeset in vlak depressies terwyl die glasiogene Numeesopeenvolging se diamiktiete onkonform oor die Hilda en die ysterklip neerqelê is. Hulle word opgevolg deur die klastiese diepseegesteentes van die Holgatopeenvolging. Die Witputsopeenvolging oorlê die Numeesopeenvolging onkonform lanqs die kontak met vloerqesteentes in die ooste, en word gesien as 'n afqeleë vlakwater ekwivalent van die Holgatopeenvolging. Gravitasietektoniek het 'n belangrike rol gespeel in die aanvanklike strukturele ontwikkeling van die Gariepgordel. Massavloeie en versakkingsstrukture vertoon prominent in half-grabenstrukture gedurende die slenkdalvormingsfase. Die sluiting van die Pan-Afrika Adamastoroseaan as gevolg van kontinentale botsing het egter gelei tot 'n periode van intense en uitgerekte SE-gerigte transpressie wat meeste gravitasiestrukture uitgewis het. Die D1 transpressiewe fase het NNW-strekkende vloeroorskuiwings gedurende 'n SE-gerigte tektoniese hoof-gebeurtenis veroorsaak. Dit het uiting gevind in 'n aantal tektoniese pulse om boogvormige nappe-strukture te vorm (bv. die Marmora-, Schakalsberg- en moontlik ook die Rosh Pinah-nappe) wat as skubbe oormekaar op die dekgesteentes ingeplaas is. Oorskuiwings het gepaard gegaan met oos-noordoos vergerende F1 plooie met asse wat subparallel aan die D1 vervoerrigting asook aan die strekkinq van 'n deurdringende en weshellende S1 transposisie kliewing gerig is. Gedurende die D1 -transpressiewe fase het die Schakalsberg oorskuiwing wat nou in die Schakalsberge dagsoom, 'n dik laag van digte basiese gesteentes met ofiolitiese eienskappe op 'n voorland oorgeskuif. Gravitasionele onstabilitiet het gevolglik in die kors ontstaan en 'n waaier van listriese takverskuiwinqs is gevorm aan die voorlandkant van die oorliggende en uitbreidende massa. Soos die komplekse tektoniet vervolgens verder ontwikkel kon dit verskeie vervormingspaaie gevolg het. Drie gedeeltelik nie-kolineêre fases van vervorming sluit in: skuins opritte, terugskuiwing en terugplooiing. Die Annisfontein antiklinorium kon gevorm het as 'n D1 vloer oorskuiwing wat oor-opritte geplooi is as gevolg van voortgesette druk vanuit die weste. Gedurende hierdie puls is ander D1 strukture ook sub-ko-aksiaal herplooi deur nie-sillindriese WSW vergerende F2 skuifskeurplooie as gevolg van oos-gerigte druk oor NNW strekkende opritte. Sulke strukture is blootgestel as teruqplooie in die Namuskluft-Dreigratberg omgewing en word ook.afgelei vir die dieper vlakke van die westelike gebied. Die semi-allochtone Formasie Rosh Pinah met die Rosh Pinah ertsliggame in die tipe gebied is herplooi gedurende D2 in 'n hinterland vergerende antiklinorium as gevolg van sy nabyheid aan die groot skuinsopritte. Dit was ook verantwoordelik vir die steil opskuiwings in die Rosh Pinah-Namuskluft-Dreigratberg gebied, wat waarskynlik omgekeerde, hoogs vervormde graben en horst strukture verteenwoordig. Op die oostelike flank van die Annisfontein antiklinorium het 'n hinterland-hellende dupleks uit opeenqestapelde strukturele waaiers ontstaan bo-op die vervormde en oorgeskuifde sedimente (die Pickelhaube duplekstruktuur). Dit is die gevolg van die vasdruk en versteiling van opritte teen die vloergesteentes. Voortdurende opwaartsboggeling van die Annisfontein anticlinorium, of as gevolg van opstapeling van oorskuiwing, of as gevolg van veelvuldige opritte gevorm langs 'n oorspronklike vloerongelykheid, veroorsaak dat die oos-hellende dele van die Rosh Pinah-nappe gede-aktiveer is. In die ooste mag beperkte laat (post-kompressie) qravitasionele gly van die oorgeskuifde sedimente in die dak van die Rosh Pinah-nappe die terugplooiing van oorgeskuifde Hildalitologieë aangehelp het. Die feit dat litologieë skuins opgery het teen die kratoniese rand en dalk ook teen dieper gesetelde opritte, verklaar heel moontlik die D3 strukture. As gevolg van differensieële beweging is hierdie F3 plooie soms effens oorgeplooi na die suid-ooste. Sodoende het gly plaasgevind langs kliewingsvlakke om kleinskaalse oorskuiwing met 'n NE-SW strekking skuins tot die ouer strukture te vorm. Die interferensie van hierdie plooifase met die ouer strukture is verantwoordelik vir die noord-suid golwing van alle sigbare strukture en daarmee die landskap as sulks, soos gesien vanuit die ooste. Die laat, laterale transtensiewe fases D4-D5 het gevolg op die intrusie van die Kuboos-Swartbank plutone en is nie goed verteenwoodig in hierdie studie area nie. Die Gariepgordel was onderhewig aan 'n verlengde periode van laer qroenskisfasies metamorfose (M1), gevolg deur 'n tweede en lokale kontakmetamorfe episode (M2) wat veroorsaak is deur die intrusie van die Kuboos en Swartbank plutone in die Richtersveld.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/58205
This item appears in the following collections: