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

[摘要] ENGLISH ABSTRACT: A geological investigation in the Rosh Pinah, Obiband Pickelhaube Peaks and Namuskluft-Dreiqratberg areasestablished the structural and stratigraphic setting of thePinah zinc-lead deposits within the Gariep Trough.The sedimentary evolution of the Pan-African Gariep Belt wasinitiated by widespread rifting and opening of an ocean (the Adamastor Ocean) along a continental margin. Deposition of thebasal clastic Stinkfontein Sequence as broad alluvial fansstrandline sediments took place in the transgressive sea.The mass flow Kaigas Formation diamictites and volcaniclasticsof the Rosh Pinah Formation were subsequently deposited inlocally developed grabens. Syn-sedimantary ore beds ofhydrothermal origin (includinq Rosh Pinah orebodies) wereprecipitated on the sea floor while reef carbonates formed alongthe coast line during the final stages of transgression. Thesesediments were eventually covered by a marginal marine clasticand carbonate facies with widespread intrusion of basic sills anddykes following uplift of the craton. These rocks are heregrouped within the Rosh Pinah Formation.Durinq the sea floor spreading stage of continental evolution theelastic-carbonates of the Hilda Sequence were deposited in ashallow to moderately deep-water environment. The Wallekraalconglomerates were deposited as local submarine fans with minordebris flows and are overlain by the Dabie River Formationcarbonates.During regression banded iron formations were deposited inshallow depressions and the glaciogenic Numees Sequencediamictites were deposited unconformably above the Hilda Sequenceand iron formations. These in turn are overlain by the deepwaterclastics of the Holgat Sequence. The Witputs sequence,which uncontormably overlies the Numees Sequence along thebasement in the east is seen as a proximal shallow waterequivalent of the distal Holqat Sequence. Gravity tectonics played an important role during the earlystructural evolution of the Gariep Belt. Mass flow and slumpstructures featured prominently in half graben structures formedduring the rifting staqe. However, the closing of the Pan-African Adamastor Ocean as a result of subduction and continentalcollision led to a single, very intensive and protracted periodof SE directed transpression, which largely obliterated allgravity deformation characteristics.The D1 transpressive phase produced NNW-trendinq basement involvedoblique ramps during a major SB-directed tectonic event,which culminated in several deformation pulses. Arcuate nappestructures were formed, e.g. the Marmora, Schakalsberg andpossibly the Rosh Pinah Nappes as a result of the obliqueemplacement of thrust slices over the cover rocks. Thrusting wasaccompanied by east-northeast-verging F1 folds with axes rotatedsub-parallel to the D1 transport direction, and a penetrativewesterly dipping S1 transposition cleavage.The master Schakelsberg Thrust emplaced a thick slice or densebasic rock with ophiolitic affinities onto a foreland situatedin the Schakalsberg mountains during the D1 transpressive phase.Gravitational instability was thereby caused in the crust, andan imbricate fan of listric splay faults formed in front of theoverlying spreading mass.Several deformation paths for the complicated tectonitecontaining three partly non-colinear phases of deformation canbe envisaged from here which includes oblique ramping,backthrusting and backfoldinq. The Annisfontein anticlinoriummay have formed as a D1 basement thrust ramped up section andfolded due to continued pressure from the W. During this pulseD1 structures were sub-coaxially refoldled by non-cylindrical WSW-vergingF2 shear folds because of E-directed compression acrossNNW trending ramps. This is displayed in the Namuskluft-Dreigratbergarea and is also inferred for the deeper levels ofthe western areas, and leads to backfolding over most of thearea.The semi-allochthonous Rosh Pinah Formation containing the RoshPinah orebodies in the type area was refolded during D2 into ahinterland-verging anticlinorium because of its proximity to the major oblique ramps, which are also responsible for the steeplydipping thrusts in the Rosh Pinah-Namuskluft-Dreigratberg area,and which probably represent highly deformed earlier qraben andhorst structures.On the eastern limb of the Annisfontein anticlinorium, ahinterland-dipping duplex of imbricate fan sheets developed onthe deformed and overthrust sediments (the Pickelhaube DuplexStructure) as a result of ramping and steepening against thebasement. Continued upward bulging of the Annisfonteinanticlinorium due to either antiformal thrust stacking ormultiple ramping along an initial basement irregularity on thefootwall resulted in east-dipping parts of the Rosh Pinah NappeThrust being inactivated.Late, limited gravitational gliding of the overthrust sedimentsalong the Rosh Pinah Nappe Thrust towards the east may havefollowed contractional deformation to enhance backfolding ofthrust slices of Hilda lithologies i the duplex zone . Theoblique ramplng ot litholoqies against the cratonic margin andpossible deeper seated ramps finally also accounts for D3structures.Due to differential movement these F3 folds may be slightlyoverturned towards the southeast, with slip occurring alongcleavage planes to outline minor thrusting oblique to theprevious events and trending NE-SW. This folding phase resultedin the present day north-south undulation of the structures andthe landscape.The late lateral transtensive phase D4-D5 resulted from theemplacement of the Kuboos-Swartbank plutons and are not wellrepresented in this area.The Gariep Belt has been subject to a protracted period of lowerqreenschist facias metamorphism (M1), followed by a second localcontact-thermal event (M2) which resulted from the emplacement ofthe Kuboos and Swartbank plutons in the Richtersveld.

[发布机构] Stellenbosch University