R. Scheepers

U-Pb SHRIMP and Sm-Nd age constraints on the timing and sources of the Pan-African Cape Granite Suite, South Africa

Geochemical and isotopic UPb SHRIMP and SmNd studies in three granitic plutons from the Pan-African Neoproterozoic/Cambrian Saldania Mobile Belt, southwestern South Africa supports differences in the sources and timing of emplacement of the main plutons related to the Cape Granite Suite as established by previous researchers. The Phase I, early syn-tectonic Darling Batholith Granitoid reflects an overall peraluminous chemical signature compatible with derivation mainly from melts extracted from paraderived crustal sources (Sa1 association). The Phase II, late to post-tectonic Robertson Pluton, reflects affinities to the Australian l-type granites (1a association). Despite the good structural constraints on the syn- and post-tectonic origin, the UPb ages point to a broadly synchronous crystallisation episode at 547±6 Ma and 536±5 Ma, respectively. In addition to UPb, Nd isotopic studies were also carried out for both plutons, as well as for the Riviera Granite, another phase II (1a association) pluton. The initial eNd (550 Ma) based on a depleted mantle model range from −3.5 (Darling), to −3.1 (Robertson) and to −2.6 (Riviera). The Nd mean crustal residence ages are 1559 Ma for Darling, 1626 Ma for Robertson and 1243 Ma for Riviera. Despite the small databank, a dominant Mesoproterozoic (∼ 1600 Ma) crust may be seen as the best candidate to explain the model TDM ages obtained. All the data largely overlap with others recently obtained for other plutons within the Cape Granite Suite and cast doubts on the current correlation between Saldania and the southeastern Brazilian, Dom Feliciano Belts.

Geology, geochemistry and petrogenesis of Late Precambrian S-, I- and A-type granitoids in the Saldania belt, Western Cape Province, South Africa

Late Precambrian granitoids occurring as a series of batholiths and plutons in the southwestern portion of the Saldania belt were subdivided into S-, I- and A-type associations according to geological and petrological criteria. Field relationships, multicationic interpretation, major and trace element geochemistry, rare earth element composition and age were also used to obtain the subdivision. Six major granitic associations in four batholiths were identified, namely two S-type (Sa and Sb), two I-type (Ia and Ib) and two A-type associations (Aa and Ab). The Sa association is further subdivided into the Sal and Sa2 types. P2O5 and Th are used as the principal discriminating elements and Na2O, K2O, Zr, Nb, Y and FREE for further characterization. The S-type granitoids dominate in the western Tygerberg tectonic terrane, the I-type granitoids in the central Swartland, as well as the eastern Boland terranes, and the A-types are intrusive in both the Swartland and Tygerberg ter apes. Plutonism in the southwestern Saldania belt is related to three major phases. Granitoids of phases I and III were precursed by mafic to intermediate magmas of tholeiitic and calc-alkaline affinity, respectively. Mafic and intermediate rocks of phase I are characterized by low K2O and granophile elements. The older (∼600 m.y.) Sa granitoids of this phase have features corresponding to typical S-type granites. Phase I ended with the intrusion of Sb granitoids (540 Ma), which are seen as contaminated metaluminous melts with a high degree of fractionation. Phase II granitoids are typically metaluminous, high-K calc-alkaline quartz monzonites (Ia association) and granites with I-type features. These granitoids are high in radioelements, especially Th. This phase of magmatism ended with the intrusion of the Ib association at ∼517 Ma, typically highly evolved plutons intrusive into slightly older Ia quartz monzonites. Phase III commenced with the intrusion of a K-series of gabbros to quartz syenites at ∼519 Ma. These rocks are high in alkalis as well as Nb, Zr and REE. Phase III culminated with the intrusion of two associations of A-type granites, the Nb, Zr and REE enriched Aa association and the Th, Y and REE enriched Ab association. Plutonism in the southwestern Saldania belt is subduction related. Older mafic and intermediate associations were intrusive in an arc environment, followed by the intrusion of peraluminous late orogenic to anorogenic S-type granitoids. The metaluminous I-type granites are late orogenic and intruded during a long time span (560-517 Ma) in a marginal position with respect to a Proterozoic subduction zone. Granitoids related to the Aa and Ab associations have the typical characteristics of anorogenic alkaline granites.

Neoproterozoic to Cambrian (Namibian) rocks of South Africa: a geochronological and geotectonic review

Abstract The Namibian Erathem in South Africa is defined by lower and upper age limits of 900 Ma and 570 Ma and therefore includes most of the Neoproterozoic Erathem. This time bracket incorporates the Pan-African Gariep and Saldania mobile belts as well as some preceding intrusions in the Richtersveld. The upper age limit is surpassed by continuous post-orogenic granite intrusion up to about 500 Ma and deposition in the Nama foreland basin up to about 530 Ma. These rocks are therefore also discussed. An analysis of all existing age data on these rocks reflect polyorogenic and polymagmatic histories related to collisional events for both the Gariep and Saldania Belts that are comparable to that of the Damara Belt in Namibia and other Pan-African belts in Africa and South America. The Pan-African tectonic cycle in South Africa (±650 Ma – 500 Ma) also led to partial resetting of older basement rocks along the so-called West Coast Belt.

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation

Abstract Much research has been conducted on these banded iron formations (BIF) over the last 15–20 years. This contribution seeks to provide an overview of old and new facts and critical discussion on the latest ideas regarding the origin of these sediments in the early Proterozoic. The recently suggested stratified ocean water model and a new stratified lake water model are compared using new evidence of a stratigraphic, major and trace element, stable isotope and REE nature. It appears that any hypothesis on the genesis of these Transvaal Supergroup rocks will have to satisfactorily account for at least the following: 1. i) A constant supply of enough Fe and Si over at least several hundred thousand years. 2. ii) A macro- and a micro-cyclicity. The latter is the most basic building stone and any hypothesis that can not explain these phenomena must be considered unrealistic. 3. iii) The stratigraphic and isotopic evidence for heterogeneity of the waterbody. 4. iv) The REE and trace element evidence for contributions from different sources. 5. v) The distribution of organic carbon in rock facies and minerals. 6. vi) The difference between Proterozoic and present day atmospheres and surface waters. 7. vii) The fossil record of the early Proterozoic, and coupled to this the role that chelation, complex formation, stable colloids and co-precipitation played in weathering, transportation and deposition of Fe. 8. viii) Factors influencing or controlling cyclicity. 9. ix) The role that atmospheric and crust-mantle evolution plays in producing most large BIF-deposits over a time span of about 500 Ma from the late Archaean into the early Proterozoic. 10. x) The concomitant evidence provided by early Proterozoic paleosols. 11. xi) The fact that several large Proterozoic BIF deposits are immediately preceded by platform carbonates. Finally, the two important ore districts, Sishen in the Northern Cape Province and Thabazimbi in the Central Transvaal, are dealt with. The general geology, mineralogy and genesis of these very high-grade major deposits are presented. New information on ore morphology and new evidence on multiple epigenetic enrichment of BIF-protore are presented and discussed. The modern and specialised ore-mining, -processing and blending techniques at Sishen are explained.

Rhyolitic ignimbrites of the Cape Granite Suite, southwestern Cape Province, South Africa.

Abstract Based on textural and geochemical evidence, the existence of Late Precambrian rhyolitic volcanism related to the Cape Granite Suite is illustrated. Recrystallised rhyolitic ignimbrites represent the volcanic phase of a subvolcanic to volcanic sequence composed of rhyodacite to rhyolitic magma. Textural features including faimme, pumice fragments and glass shards provide unquivocal evidence for the volcanic origin of these rocks.

The geochemistry and isotopic composition of the mafic and intermediate igneous components of the Cape Granite Suite, South Africa

Abstract A number of small outcrops of igneous rocks, gabbroic to granodioritic in composition, occur scattered throughout the western Cape in South Africa. These outcrops, which are mostly intrusive into the Malmesbury metasedimentary sequence, define a small number of medium-sized plutons. A characteristic feature of the larger plutons is their composite nature. Each pluton consists of multiple intrusive phases; a few show extensive hydrothermal alteration. Plutons from Yzerfontein and Mud River, which occur in the Tygerberg terrane, are high-K calcalkaline (K-trans-alkaline) while all plutons from Malmesbury, which occurs in the Swartland terrane, are tholeiitic. The high-K suite correspond to other I-type Cape granites, whereas the tholeiitic Malmesbury plutons may be a precursor of A-type Cape granites in both the Tygerberg and Swartland terranes. Monzonite from the Yzerfontein pluton yielded a UPb age of 519±7 Ma on zircon mineral separates. Rb-Sr isotope data did not yield reliable ages, possibly due to alteration, but yielded distinct initial isotope ratios for the high-K calc-alkaline and the tholeiitic plutons. The δ34S isotope ratios of pyrite range from 1 to 2.7% for Yzerfontein and from 3.6 to 4.6% for Mud River, suggesting a magmatic source for the pyrite. PbPb data on whole rock and pyrite samples imply crustal input into all the original magmas.

Redistribution and fractionation of U, Th and rare-earth elements during weathering of subalkaline granites in SW Cape Province, South Africa

Abstract Major and trace element behaviour during initial and advanced weathering stages in a weathering profile and saprolith developed on two subalkaline granites respectively indicated that Ca, Na and Sr are preferentially removed. Fe2+, K and Y are also lost, followed by P, Mg, Rb and Si. Zr and Nb remained constant relative to Ti. Mn, Fe3+, Al, Ga and Th initially increase eventually decreasing during advanced stages of weathering. U, Th and the REE behaviour could be related to the primary and secondary mineralogy of the granites. The HREE are removed in solution, while the concentrations of LREE are significantly increased relative to HREE during early weathering stages. Ce is initially slightly depleted relative to other LREE, but positive Ce animalies develop during advanced stages of weathering in the profile. Previous hydrothermal alteration of the granite is largely responsible for the behaviour of REE, U and Th during weathering. U and Th values increase with increasing oxidation. U correlates with Fe3+ adn Th with Mn and Fe3+.

Magmatic and related mineral deposits of the Pan-African Saldania belt in the Western Cape Province, South Africa

Abstract Mineral deposits and prospects of the Pan-African Saldania orogenic belt in the Western Cape Province, South Africa, are reviewed. The polyphase, deformed, low-grade metamorphosed, volcano-sedimentary Malmesbury Group constitutes a complex, poorly understood supracrustal sequence that has been loosely subdivided into the Tygerberg, Swartland and Boland tectono-stratigraphic terranes on the basis of NW-trending fault zones. Syn- and post-tectonic granitoids of the Cape Granite Suite selectively intruded these terranes. Early S-types preferred the Tygerberg terrane, whereas the later I-types dominate the remaining areas. Anorogenic A-type granites, however, occur in all three terranes. Despite the absence of operating base or precious metal mines in the area, this study has established at least four metal associations directly or indirectly related to the intrusions: 1. i ) Cassiterite-wolframite (±Au, Cu, Mo, Zn, As, Fe-sulphides) in quartz and quartz/aplite veins hosted by tour-malinized and locally greissenized S-type granite. Similar exo-granitic veins occur in proximal metamorphites; 2. ii ) Juxtaposed, disseminated, stockwork breccia and vein style CuMoFe(Au)-sulphide mineralization hosted by mafic- to intermediate-intrusions of high-K calc-alkaline, I-type affinity; 3. iii ) CuMoAu-sulphides hosted by hydraulic breccia pipes, stocks and veins occurring in anorogenic A-type alkali feldspar granites and amphibole quartz syenites; 4. iv ) Scheelite with minor CuMoAu-sulphides associated with endo- and exo-skams spatially related to I-type monzogranite, granite and alkali feldspar granite. The first three associations occur along the Yzerfontein-Helderberg-zone, a 180 km lineament in the Tygerberg terrane, exploited by syn-, late- and post-tectonic intrusions and their related mineralization. The fourth association is typical of the Boland terrane. The spatial and temporal relationships among the various metal associations are interpreted as the result of sequential intrusion of the granitoid suite. Relatively early, high level S-type, followed by intermediate level I-type and finally anorogenic A-type granitoids intruded during the evolution of a contracting continental margin arc and its inner arc. The spatial configuration of the various terranes at the present erosional levels is the result of rifting and significant differential vertical displacement following the cessation of subduction. This terminal tectonic event was synchronous with the intrusion of the A-type granitoids.

Phosphorus as a typological and mineralization potential indicator: the Cape Granite Suite of the Saldania belt as a case study

Abstract Granitoids of the Cape Granite Suite can be classified as S-, I- and A-types according to their P 2 O 5 contents. The S-type granitoids dominate in the western Tygerberg terrane, the I-type granitoids in the central Swartland as well as the eastern Boland terranes and the A-types are intrusive in both the Swartland and Tygerberg terranes. The subdivision of the three major groups into six granitic associations, namely two Stype ( Sa and Sb ), two I-type ( Ia and Ib ) and two A-type associations ( Aa and Ab ) was performed by a combination of mineralogical and petrochemical parameters utilizing P 2 O 5 , Th, Nb, Zr, Na 2 O, K 2 O and the rare earth elements. The S-type granitoids are enriched in P 2 O 5 above values for theoretical apatite saturation curves. A distinction is made between the Sa1 association with P enrichment regardless of the degree of differentiation and the Sa2 association with considerable enrichment during advanced stages of differentiation. Granitoids of the Ia association follow high temperature apatite saturation curves. Fractionated Ib association granitoids adhere to these curves up to advanced stages of differentiation, where P 2 O 5 is slightly enriched. A-type granitoids of both associations are depleted in P 2 O 5 . The mineralization potential of granitoids of the Suite is specific [WMo(Cu) or WSn(Au, As) or MoCu(Au, As)] and related to granitoid association as well as geochemical parameters in each association. High P 2 O 5 granitoids (S-types) have the potential for WSn(Au, As) mineralization when late differentiates ( Sa2 association) are particularly enriched in P 2 O 5 following a trend perpendicular to theoretical apatite saturation curves. Intermediate P 2 O 5 granitoids (1-types) are potentially mineralized [WMo(Cu)] when they follow high temperature apatite saturation curves and have high Ca/P values, as well as a wide range in Ca/P ratios. A-type granitoids with MoCu(Au) mineralization are low in P 2 O 5 and high in Nb, Th, Zr and CO 2 . Barren A-type granitoids are low in P 2 O 5 and CO 2 and high in Y and Th. High P 2 O 5 peraluminous Stype granitoids are late orogenic to post-orogenic. The intermediate P 2 O 5 metaluminous I-type granites are anorogenic and intruded during a long time span (560–517 Ma) in a marginal position with respect to a Proterozoic subduction zone. P 2 O 5 depleted granitoids of the Aa and Ab associations have the typical characteristics of anorogenic alkali granites.

Fractionation of the rare earths in a suite of highly evolved metaluminous granitoids and felsic dykes from the Western Cape Province, South Africa

Abstract The rare earth element (REE) distribution in a comagmatic, metaluminous, igneous suite, consisting of a monzogranite, differentiating to an even-grained granite and eventually a sequence of dyke rocks, was investigated. During the early stages of the differentiation process, the REE behaved compatibly, the LREE being preferentially removed by fractionation of early allanite and monazite. With increasing fractionation, higher silica content and increasing alkalinity in the dyke rocks, evidence for the incompatible behaviour of the REE were obtained. LREE, Eu and, to a lesser extent, Gd were fractionated from the HREE and concentrated in the dyke centres. Partitioning of F and F-REE complexes towards the highly fractionated melt phase contributed towards this process. An increase in oxidation conditions during the very late evolutionary phases of the dykes again stabilized allanite as an accessory phase. LREE were sufficiently enriched at this stage to be incorporated in the mineral, leading to a subsequent lowering of the La Sm ratio in some highly evolved dyke centres. Late crystallizing fluorite in dyke centres accommodated some of the enriched LREE by a process of coupled substitution. Evidence for these fractionation processes in evolved metaluminous felsic rocks have implications for the use of REE in modelling the generation of such rocks.