Controls of lateral and vertical variations in the geochemistry of the Hotazel Fe-Mn Formation at Nchwaning and Gloria mines, Kalahari Manganese Field, South Africa
- Authors: Dorbor Jr., Stephen Baysah
- Date: 2023-10-13
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424621 , vital:72169
- Description: The Paleoproterozoic Kalahari manganese field (KMF) in the Northern Cape Province, South Africa, hosts a large resource of manganese ores that has been of great interest over many decades. The Kalahari Manganese deposit (KMD), which is the largest of five erosional relics of the Hotazel Formation in the KMF, hosts three beds of Mn ores with alternating layers of banded iron formation (BIF) and hematite lutite. These three rock types are all evaluated for their mineralogy and geochemistry in this study, with emphasis on lateral and vertical distributions across the Gloria and Nchwaning Mines in the northernmost KMF, an area of high-grade, hydrothermally altered Mn mineralisation. The Mn ores of the Hotazel formation are traditionally categorised into two types. The carbonate-rich low Mn grade (Mn≤40 wt. %) ores (Mamatwan-type) domninates the largest part of the KMD, while carbonate-free, high Mn grade (Mn≥ 45 wt.%) ore (Wessels-type) occurs in the northernmost KMD. The Wessels-type ores are considered as the hydrothermally altered product of Mamatwan-type ores, and as indicated above, are the focus of this study. Five drill cores containing Wessels-type ores from the Nchwaning and Gloria area of the northern KMD were analysed to help understand the petrographic and particularly the geochemical variations in the Hotazel Fe-Mn Formation, both laterally for a given Mn layer of the three, and vertically across Mn layers as captured in specific drillcores. Petrographic and whole-rock geochemical results obtained from the three rock types of the Hotazel Formation show variations in their mineralogical and geochemical compositions, especially in the high-grade Mn ores themselves. Most of the samples of the BIFs layers are dominated by hematite and chert occurring in banded fashion, which is typical of a normal carbonate-free altered BIF discussed in this thesis. The BIFs can also be locally enriched in hematite (ferruginised), occurring as massive hematite ores usually at the top of the stratigraphic profiles. The presence of aegirine-rich assemblages is also noted occurring in some of the BIF and hematite lutite sections immediately above and below the Mn ore beds. The high-grade Mn ore beds vary greatly in mineralogy and texture of the ores laterally and even within a single drill core. In an extreme case, a single drillcore sampled from the Gloria mine (GL57) contains high-grade Wessels-type ore in the upper Mn bed and low-grade, Mamatwan-type ore in the lower Mn layer. Geochemically, the Mn ore bodies also show substantial geochemical variability, although a net increase in the Mn grade downward is usually characterised by a corresponding depletion in mainly bulk Ca, Si and carbonate. However, the Fe content appears to be consistently higher in the upper ore bodies of the drillcores than the lower ones, and the increase in the concentration of the Fe-oxide expectedly causes a relative decrease in the bulk Mn-oxide concentration, usually expressed as an antithetic relationship between the two elements. In terms of trace element distributions, this appears to be more significant in the Mn ores than the other two rock types affected by the same alteration process, probably due to the presence of Mn phases such as hausmannite and braunite serving as good hosts to several trace elements. Cu, Zn, Pb and to a lesser extent Mo are trace metals that appear to show elevated concentration levels (net enrichments) in high-grade Mn ore by comparison to the presumed Mamatwan-type protolith. Ba is an additional element of clear enrichment, manifested mainly as the mineral barite. The Northern KMD has a complex post-depositional history, which includes the intrusion of NE-SW-trending dykes, formation of the Mapedi/Gamagara erosional unconformity, normal faulting associated with the Wessels event and major thrust faults in the western part of the northern KMD. These structural events all have the potential to have contributed to the alteration and subsequent enrichment of the Mn ores in the Nchwaning and Gloria area. As such, the mineralogical, textural, and geochemical variations observed here can tentatively be attributed to the different structural features in the northern KMD. Classic interpretations suggest that normal N-S-trending fault structures have acted as fluid conduits for hydrothermal fluids, which led to the metasomatic alteration of the Mn ore body laterally. Drill cores proximal to and evidently affected by fault-controlled alteration in the SE and SW-portions of the Nchwaning area, have comparable mineralogical and geochemical characteristics for both ore bodies (upper and lower) with subdued alteration effects from the unconformed contact above. Fluids associated with the Mapedi/Gamagara unconformity, would have percolated down-stratigraphy causing oxidative ferruginisation, which led to the formation of massive hematite ores in the top BIF layers and ferruginised Mn ores in the Mn ore beds. This alteration effect appears more prominent in a drill core from the northern part of the study area where the unconformity contact appears more proximal to the upper Mn bed. Drill cores located in the western part of the Nchwaning area seem to also capture evidence of fluid alteration with enrichment in Na recorded in the local abundance of the mineral aegirine. Finally, the dyke structures appear to have acted as impermeable fluid barriers to both lateral and possibly down-dip fluid-flow. , Thesis (MSc) -- Faculty of Science, Geology, 2023
- Full Text:
- Date Issued: 2023-10-13
- Authors: Dorbor Jr., Stephen Baysah
- Date: 2023-10-13
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424621 , vital:72169
- Description: The Paleoproterozoic Kalahari manganese field (KMF) in the Northern Cape Province, South Africa, hosts a large resource of manganese ores that has been of great interest over many decades. The Kalahari Manganese deposit (KMD), which is the largest of five erosional relics of the Hotazel Formation in the KMF, hosts three beds of Mn ores with alternating layers of banded iron formation (BIF) and hematite lutite. These three rock types are all evaluated for their mineralogy and geochemistry in this study, with emphasis on lateral and vertical distributions across the Gloria and Nchwaning Mines in the northernmost KMF, an area of high-grade, hydrothermally altered Mn mineralisation. The Mn ores of the Hotazel formation are traditionally categorised into two types. The carbonate-rich low Mn grade (Mn≤40 wt. %) ores (Mamatwan-type) domninates the largest part of the KMD, while carbonate-free, high Mn grade (Mn≥ 45 wt.%) ore (Wessels-type) occurs in the northernmost KMD. The Wessels-type ores are considered as the hydrothermally altered product of Mamatwan-type ores, and as indicated above, are the focus of this study. Five drill cores containing Wessels-type ores from the Nchwaning and Gloria area of the northern KMD were analysed to help understand the petrographic and particularly the geochemical variations in the Hotazel Fe-Mn Formation, both laterally for a given Mn layer of the three, and vertically across Mn layers as captured in specific drillcores. Petrographic and whole-rock geochemical results obtained from the three rock types of the Hotazel Formation show variations in their mineralogical and geochemical compositions, especially in the high-grade Mn ores themselves. Most of the samples of the BIFs layers are dominated by hematite and chert occurring in banded fashion, which is typical of a normal carbonate-free altered BIF discussed in this thesis. The BIFs can also be locally enriched in hematite (ferruginised), occurring as massive hematite ores usually at the top of the stratigraphic profiles. The presence of aegirine-rich assemblages is also noted occurring in some of the BIF and hematite lutite sections immediately above and below the Mn ore beds. The high-grade Mn ore beds vary greatly in mineralogy and texture of the ores laterally and even within a single drill core. In an extreme case, a single drillcore sampled from the Gloria mine (GL57) contains high-grade Wessels-type ore in the upper Mn bed and low-grade, Mamatwan-type ore in the lower Mn layer. Geochemically, the Mn ore bodies also show substantial geochemical variability, although a net increase in the Mn grade downward is usually characterised by a corresponding depletion in mainly bulk Ca, Si and carbonate. However, the Fe content appears to be consistently higher in the upper ore bodies of the drillcores than the lower ones, and the increase in the concentration of the Fe-oxide expectedly causes a relative decrease in the bulk Mn-oxide concentration, usually expressed as an antithetic relationship between the two elements. In terms of trace element distributions, this appears to be more significant in the Mn ores than the other two rock types affected by the same alteration process, probably due to the presence of Mn phases such as hausmannite and braunite serving as good hosts to several trace elements. Cu, Zn, Pb and to a lesser extent Mo are trace metals that appear to show elevated concentration levels (net enrichments) in high-grade Mn ore by comparison to the presumed Mamatwan-type protolith. Ba is an additional element of clear enrichment, manifested mainly as the mineral barite. The Northern KMD has a complex post-depositional history, which includes the intrusion of NE-SW-trending dykes, formation of the Mapedi/Gamagara erosional unconformity, normal faulting associated with the Wessels event and major thrust faults in the western part of the northern KMD. These structural events all have the potential to have contributed to the alteration and subsequent enrichment of the Mn ores in the Nchwaning and Gloria area. As such, the mineralogical, textural, and geochemical variations observed here can tentatively be attributed to the different structural features in the northern KMD. Classic interpretations suggest that normal N-S-trending fault structures have acted as fluid conduits for hydrothermal fluids, which led to the metasomatic alteration of the Mn ore body laterally. Drill cores proximal to and evidently affected by fault-controlled alteration in the SE and SW-portions of the Nchwaning area, have comparable mineralogical and geochemical characteristics for both ore bodies (upper and lower) with subdued alteration effects from the unconformed contact above. Fluids associated with the Mapedi/Gamagara unconformity, would have percolated down-stratigraphy causing oxidative ferruginisation, which led to the formation of massive hematite ores in the top BIF layers and ferruginised Mn ores in the Mn ore beds. This alteration effect appears more prominent in a drill core from the northern part of the study area where the unconformity contact appears more proximal to the upper Mn bed. Drill cores located in the western part of the Nchwaning area seem to also capture evidence of fluid alteration with enrichment in Na recorded in the local abundance of the mineral aegirine. Finally, the dyke structures appear to have acted as impermeable fluid barriers to both lateral and possibly down-dip fluid-flow. , Thesis (MSc) -- Faculty of Science, Geology, 2023
- Full Text:
- Date Issued: 2023-10-13
Origin and metallogenic significance of alkali metasomatism in the Paleoproterozoic Mapedi Formation, Kalahari Manganese Field, South Africa
- Authors: Ikwen, Emmanuella Biye
- Date: 2023-10-13
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424632 , vital:72170
- Description: The occurrence of alkali-rich metasomatic assemblages has been widely reported in various regions of the Kalahari Manganese Field (KMF). This alkali metasomatism has been characterized by the secondary introduction of elements such as K, Na, Li, Ba, P, V, Zn, As, amongst others. This study further explores the possibility of widespread alkali metasomatism in the KMF by reporting on and examining the occurrence of sugilite and other alkali-rich minerals at the contact between the Transvaal and Olifantshoek Supergroups in the Hotazel Mine area of the north-eastern KMF. The lithologies observed at the contact show macroscopic (such as cross cutting veins) and microscopic evidence of hydrothermal alteration. Using analytical methods such as X-ray diffraction, X-ray fluorescence, and scanning electron microscopy, results showed that in the north-eastern region of the KMF, the metasomatism observed at the Transvaal-Olifantshoek contact is mainly characterized by enrichment in sodium, and the occurrence of sodium minerals, predominantly in the form of aegirine. The aegirine forms exclusively in the quartzites of the Mapedi Formation along with minerals such as sugilite, baryte, banalsite, amongst others. Albite also occurs within the quartzites, but also within the Mapedi red shales. The secondary nature of these minerals is established by geochemical comparisons with pristine, as well as alkali-metasomatized samples of the same formation which were obtained from other parts of the KMF and Postmasburg. These comparisons showed that the Mapedi quartzites in the north-eastern KMF have undergone extensive oxidation compared to samples of the same formation which were obtained from Postmasburg. The north-eastern quartzites have an average hematite abundance of 17 wt.% compared to Postmasburg quartzite which have an average of 7 wt.% hematite. Furthermore, some quartzite samples contained up to 40 wt.% in hematite content. The comparisons also showed that Mapedi quartzites from the north-eastern KMF are substantially more sodium enriched compared to Mapedi quartzites from the Postmasburg region, which on average have sodium oxide content below detection limits. Geochemical comparisons were made between pristine Hotazel Formation samples from north-western KMF (Gloria Mine) and samples obtained from the north-eastern KMF (Hotazel Mine). Results showed that the samples obtained from the top of the Hotazel Formation (in the Hotazel mine area) are likely altered hematite lutite and not Banded Iron Formation, evident by their substantially high manganese oxide content (over 30 wt.% in some cases). When compared to pristine samples, the lutite also showed evidence of hydrothermal alteration, predominantly in the form of phosphate and barium enrichment, evident by the occurrence of baryte and apatite. The alkali metasomatism occurring at the contact between the Transvaal and Olifantshoek Supergroups was shown to be predominantly characterized by enrichment in Na, K, Li, Al, Ba, Sr, and P. The metasomatism characterized in this study was also proposed to possibly post-date an earlier metasomatic event which was characterized by leaching of silica and extensive oxidation of the rocks observed at the Transvaal-Olifantshoek contact in the north-eastern KMF. The occurrence of the alkali-rich minerals outlined above geochemically parallels other alkali-rich metasomatic assemblages reported in other parts of the KMF, as well as in the Postmasburg Manganese Field. Thus, based on the consistent occurrence of secondary, alkali-rich mineral assemblages across the KMF, characterized by the common occurrence of aegirine along with minerals such as sugilite and albite, there is evidence of a large-scale alkali metasomatism in the KMF. This study also explores the possible role that the Transvaal-Olifantshoek unconformity might have played in acting as a major conduit for fluid propagation because the observed mineral assemblages occur right at the contact between the Hotazel and Mapedi Formations. The occurrence of the alkali-rich minerals predominantly around the unconformity, as well as the relative depletion of phosphates in stratigraphically deeper parts of the Hotazel suggest that the fluid metasomatism was aided by the Olifantshoek-Transvaal unconformity surface. This study concludes that there is evidence for a strong link between the metasomatism occurring at the contact between the Hotazel and Mapedi formations (in the north-eastern KMF) and what is observed in the broader KMF region. , Thesis (MSc) -- Faculty of Science, Geology, 2023
- Full Text:
- Date Issued: 2023-10-13
- Authors: Ikwen, Emmanuella Biye
- Date: 2023-10-13
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424632 , vital:72170
- Description: The occurrence of alkali-rich metasomatic assemblages has been widely reported in various regions of the Kalahari Manganese Field (KMF). This alkali metasomatism has been characterized by the secondary introduction of elements such as K, Na, Li, Ba, P, V, Zn, As, amongst others. This study further explores the possibility of widespread alkali metasomatism in the KMF by reporting on and examining the occurrence of sugilite and other alkali-rich minerals at the contact between the Transvaal and Olifantshoek Supergroups in the Hotazel Mine area of the north-eastern KMF. The lithologies observed at the contact show macroscopic (such as cross cutting veins) and microscopic evidence of hydrothermal alteration. Using analytical methods such as X-ray diffraction, X-ray fluorescence, and scanning electron microscopy, results showed that in the north-eastern region of the KMF, the metasomatism observed at the Transvaal-Olifantshoek contact is mainly characterized by enrichment in sodium, and the occurrence of sodium minerals, predominantly in the form of aegirine. The aegirine forms exclusively in the quartzites of the Mapedi Formation along with minerals such as sugilite, baryte, banalsite, amongst others. Albite also occurs within the quartzites, but also within the Mapedi red shales. The secondary nature of these minerals is established by geochemical comparisons with pristine, as well as alkali-metasomatized samples of the same formation which were obtained from other parts of the KMF and Postmasburg. These comparisons showed that the Mapedi quartzites in the north-eastern KMF have undergone extensive oxidation compared to samples of the same formation which were obtained from Postmasburg. The north-eastern quartzites have an average hematite abundance of 17 wt.% compared to Postmasburg quartzite which have an average of 7 wt.% hematite. Furthermore, some quartzite samples contained up to 40 wt.% in hematite content. The comparisons also showed that Mapedi quartzites from the north-eastern KMF are substantially more sodium enriched compared to Mapedi quartzites from the Postmasburg region, which on average have sodium oxide content below detection limits. Geochemical comparisons were made between pristine Hotazel Formation samples from north-western KMF (Gloria Mine) and samples obtained from the north-eastern KMF (Hotazel Mine). Results showed that the samples obtained from the top of the Hotazel Formation (in the Hotazel mine area) are likely altered hematite lutite and not Banded Iron Formation, evident by their substantially high manganese oxide content (over 30 wt.% in some cases). When compared to pristine samples, the lutite also showed evidence of hydrothermal alteration, predominantly in the form of phosphate and barium enrichment, evident by the occurrence of baryte and apatite. The alkali metasomatism occurring at the contact between the Transvaal and Olifantshoek Supergroups was shown to be predominantly characterized by enrichment in Na, K, Li, Al, Ba, Sr, and P. The metasomatism characterized in this study was also proposed to possibly post-date an earlier metasomatic event which was characterized by leaching of silica and extensive oxidation of the rocks observed at the Transvaal-Olifantshoek contact in the north-eastern KMF. The occurrence of the alkali-rich minerals outlined above geochemically parallels other alkali-rich metasomatic assemblages reported in other parts of the KMF, as well as in the Postmasburg Manganese Field. Thus, based on the consistent occurrence of secondary, alkali-rich mineral assemblages across the KMF, characterized by the common occurrence of aegirine along with minerals such as sugilite and albite, there is evidence of a large-scale alkali metasomatism in the KMF. This study also explores the possible role that the Transvaal-Olifantshoek unconformity might have played in acting as a major conduit for fluid propagation because the observed mineral assemblages occur right at the contact between the Hotazel and Mapedi Formations. The occurrence of the alkali-rich minerals predominantly around the unconformity, as well as the relative depletion of phosphates in stratigraphically deeper parts of the Hotazel suggest that the fluid metasomatism was aided by the Olifantshoek-Transvaal unconformity surface. This study concludes that there is evidence for a strong link between the metasomatism occurring at the contact between the Hotazel and Mapedi formations (in the north-eastern KMF) and what is observed in the broader KMF region. , Thesis (MSc) -- Faculty of Science, Geology, 2023
- Full Text:
- Date Issued: 2023-10-13
Mineral paragenesis of olmiite/poldevaartite and rhodochrosite/shigaite occurrences in the Kalahari Manganese Field, and their relation to the formation of high-grade manganese ore of Nchwaning II mine, Black Rock, South Africa
- Authors: Opperman, Alicia
- Date: 2023-03-29
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/422448 , vital:71943
- Description: Access restricted. Access embargoed until 2025. , Thesis (MSc) -- Faculty of Science, Geology, 2023
- Full Text:
- Date Issued: 2023-03-29
- Authors: Opperman, Alicia
- Date: 2023-03-29
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/422448 , vital:71943
- Description: Access restricted. Access embargoed until 2025. , Thesis (MSc) -- Faculty of Science, Geology, 2023
- Full Text:
- Date Issued: 2023-03-29
The depositional history and evaluation of two late quaternary, diamondiferous pocket beaches, south-western Namibia
- Authors: Milad, Micael George
- Date: 2004-03
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/420934 , vital:71795
- Description: The two Late Quaternary, diamondiferous pocket beach deposits studied here are situated along a 10 km stretch of the storm-dominated, Atlantic coastline of the Sperrgebiet, south-western Namibia. The pocket beaches are approximately 130 km north of the Orange River mouth, which is widely accepted as a long-lived point source for diamonds sourced from the interior of southern Africa. A total of fourteen pocket beach deposits were recently evaluated in this area, but only two of these, namely Site 2 (to the south) and Site 3 (to the north), are considered here. The main diamondbearing horizons are beach gravels, which occur within, and form part of, the pocket beach sequences. The beach gravels are mostly blanketed by sand overburden, meaning that exposures available for study were limited, and much reliance was placed on borehole logging and observations of evaluation sample tailings. The main aims are to unravel the depositional history of the pocket beach sequences, identify the controls on diamond mineralisation in the beach gravels, and critically examine two different methods of estimating average diamond size for the deposits. In pursuit of these aims, sedimentological characteristics of the unconsolidated pocket beach deposits were recorded using small diameter drill holes, hydraulic grab bulk samples, trench exposures and surface outcrops. The surface geology, geomorphology and modern wave patterns were mapped using high-resolution, Airborne Laser Survey imagery coupled with extensive field checking. Three-dimensional geological modeling software was used to gain insight into the subsurface morphology of the deposits. Fossil shell samples were used to aid interpretation of ancient depositional environments and to date parts of the pocket beach sequences. Variations in diamond concentration and the size of diamonds were recorded using bulk samples, some of which were taken from a trench, but most of which were excavated using a hydraulic grab tool called the GB50. Finally, by using diamond size data from Site 3, sample data from diamondiferous beach gravels to the south of the study area and sample campaign simulations, two alternative methods of evaluating average diamond size in marine gravel deposits were appraised.The pocket beach sequences occur within north-south trending valleys of a major deflation basin and are separated from one another by rocky headlands. The ridge-and-valley topography of the deflation basin has resulted from differential erosion of Late Proterozoic basement rock units, alternating layers of which differ greatly in their resistance to the long-lived, local denudationalprocesses. On the basis of the stratigraphic information collected from the unconsolidated pocket beach valley fills, interpreted within the context of global, Late Pleistocene sea level records, the following depositional history is deduced : a) Deposition of sheetflood gravels by ephemeral streams, activated during a regressive phase. b) Transgression, culminating in the deposition of a gravel beach, representing a sea level highstand of +4 metres above mean sea level (mamsl) at between 120 000 and 130 000 BP. c)A regressive phase, resulting in deflation of former valley fills to the bedrock valley floor and accompanied by re-activation of ephemeral stream activity to form sheetflood deposits; this represents a protracted period of subaerial exposure of the +4 m gravel beach deposit. d) Deposition of a great volume of sediment in the valleys during the latter stages of the transgression from the Last Glacial Maximum (LGM). The sequence generated during this phase, which started at ca. 9 000 BP, contains : i) pan/coastal sabkha sediments, ii) shallow, sheltered bay sediments, iii) back-barrier lagoonal sediments, iv) a gravel beach deposit representing a sea level stillstand at -5 mamsl, laid down between 7 600 and 5 600 BP, v) another gravel beach deposit representing the well-known Middle Holocene sea level highstand at +2 to +3 mamsl, laid down at ca. 5 000 BP, and which terminated the transgression from the LGM. e) A minor regression to the current sea level, accompanied by progradation of the shoreline to its current position. This progradational marine unit consists almost entirely of sand and grit, reflecting the lack of gravel supply to this part of the coastline in the most recent past. f) Deposition of modern coastal dunes, which cap the pocket beach sequence and are the youngest sediments in the study area. Using trench and hydraulic grab evaluation sample results, in combination with analysis of wave patterns and field observations, the following local controls on the density distribution (ie. concentration) and size distribution of diamonds in the gravel beach deposits (+4, -5 and +2 to +3 mamsl stands) are recognised: a) Gravel beach depositional processes, which are responsible for clast sorting on the beach, have influenced the density and size distribution of diamonds. The infill zone, or beach toe, favours maximum diamond concentration while diamond size decreases from the imbricate zone (intertidal) to the infill zone (subtidal). b) Wave energy is identified as the dominant local control on diamond size distribution, but has also influenced diamond concentration to a limited degree. Larger diamonds are intimately associated with coarser beach gravels, both of which are a reflection of increased wave energy. Higher concentrations of diamonds are sometimes associated with zones of coarser gravel and therefore greater wave energy. c) The time of deposition of the host gravel beach is seen to be the dominant controlling factor with respect to diamond concentration. This is seen as evidence of significant temporal variation in the availability of diamonds in the littoral evironment. A significant reduction (20%) in average diamond size from Site 2 to Site 3, over a distance of only 6 km, is evident. The following were identified as reasons for this reduction in diamond size : a) Longshore sorting processes, of which the long-lived northerly littoral drift is a key part, are known to have played a role in the diminution of diamond size northwards from the Orange River mouth point source. However, it is believed that this can only partly account for the observed 20% reduction in diamond size. b) Input of sediment and smaller diamonds at Site 3, reworked out of an older, Eocene-aged marine succession in the hinterland, is recognised as a possible additional reason for the large reduction in diamond size from Site 2 to Site 3. It is also speculated that the large size of the pocket beach at Site 3, relative to Site 2, may have resulted in lower average wave energy at Site 3, with consequent reduced average diamond size. Diamond size in the beach gravels of Site 3, as well as in beach gravels elsewhere in the Sperrgebiet, is seen to be lognormally-distributed within geologically homogeneous zones. In theory, lognormal mean estimators represent the best method of estimating average diamond size in such cases, whereas the arithmetic mean estimator has the tendency to overestimate when large outlier values occur. Lognormal mean estimators have the added benefit of providing for the calculation of confidence limits, which are becoming increasingly more important as financial lending institutions insist on better quantification of the risk involved in resource estimates. Sample campaign simulations demonstrate, for the kinds of diamond size-frequency distributions typical of beach gravel deposits at Site 3, that there is no significant improvement in the accuracy of average diamond size estimates when lognormal mean estimators are used instead of the arithmetic mean estimator. This is because the variance (a ) of the diamond populations is low, and large outlier values are extremely unlikely to occur. However, simulation of a diamond population with high variance, drawn from a sample of beach gravels near the Orange River mouth, shows that lognormal estimators produce significantly more accurate results when a is large. Since individual diamond weights were not recorded during evaluation sampling of Site 3, numerical solution of lognormal estimators is not possible, and these would need to be solved using a less accurate graphical method. It is therefore recommended that individual diamond weights are recorded in future sampling campaigns, allowing for the use of lognormal mean estimators, and the calculation of confidence limits for average diamond size estimates. , Thesis (MSc) -- Science, Geology, 2004
- Full Text:
- Date Issued: 2004-03
- Authors: Milad, Micael George
- Date: 2004-03
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/420934 , vital:71795
- Description: The two Late Quaternary, diamondiferous pocket beach deposits studied here are situated along a 10 km stretch of the storm-dominated, Atlantic coastline of the Sperrgebiet, south-western Namibia. The pocket beaches are approximately 130 km north of the Orange River mouth, which is widely accepted as a long-lived point source for diamonds sourced from the interior of southern Africa. A total of fourteen pocket beach deposits were recently evaluated in this area, but only two of these, namely Site 2 (to the south) and Site 3 (to the north), are considered here. The main diamondbearing horizons are beach gravels, which occur within, and form part of, the pocket beach sequences. The beach gravels are mostly blanketed by sand overburden, meaning that exposures available for study were limited, and much reliance was placed on borehole logging and observations of evaluation sample tailings. The main aims are to unravel the depositional history of the pocket beach sequences, identify the controls on diamond mineralisation in the beach gravels, and critically examine two different methods of estimating average diamond size for the deposits. In pursuit of these aims, sedimentological characteristics of the unconsolidated pocket beach deposits were recorded using small diameter drill holes, hydraulic grab bulk samples, trench exposures and surface outcrops. The surface geology, geomorphology and modern wave patterns were mapped using high-resolution, Airborne Laser Survey imagery coupled with extensive field checking. Three-dimensional geological modeling software was used to gain insight into the subsurface morphology of the deposits. Fossil shell samples were used to aid interpretation of ancient depositional environments and to date parts of the pocket beach sequences. Variations in diamond concentration and the size of diamonds were recorded using bulk samples, some of which were taken from a trench, but most of which were excavated using a hydraulic grab tool called the GB50. Finally, by using diamond size data from Site 3, sample data from diamondiferous beach gravels to the south of the study area and sample campaign simulations, two alternative methods of evaluating average diamond size in marine gravel deposits were appraised.The pocket beach sequences occur within north-south trending valleys of a major deflation basin and are separated from one another by rocky headlands. The ridge-and-valley topography of the deflation basin has resulted from differential erosion of Late Proterozoic basement rock units, alternating layers of which differ greatly in their resistance to the long-lived, local denudationalprocesses. On the basis of the stratigraphic information collected from the unconsolidated pocket beach valley fills, interpreted within the context of global, Late Pleistocene sea level records, the following depositional history is deduced : a) Deposition of sheetflood gravels by ephemeral streams, activated during a regressive phase. b) Transgression, culminating in the deposition of a gravel beach, representing a sea level highstand of +4 metres above mean sea level (mamsl) at between 120 000 and 130 000 BP. c)A regressive phase, resulting in deflation of former valley fills to the bedrock valley floor and accompanied by re-activation of ephemeral stream activity to form sheetflood deposits; this represents a protracted period of subaerial exposure of the +4 m gravel beach deposit. d) Deposition of a great volume of sediment in the valleys during the latter stages of the transgression from the Last Glacial Maximum (LGM). The sequence generated during this phase, which started at ca. 9 000 BP, contains : i) pan/coastal sabkha sediments, ii) shallow, sheltered bay sediments, iii) back-barrier lagoonal sediments, iv) a gravel beach deposit representing a sea level stillstand at -5 mamsl, laid down between 7 600 and 5 600 BP, v) another gravel beach deposit representing the well-known Middle Holocene sea level highstand at +2 to +3 mamsl, laid down at ca. 5 000 BP, and which terminated the transgression from the LGM. e) A minor regression to the current sea level, accompanied by progradation of the shoreline to its current position. This progradational marine unit consists almost entirely of sand and grit, reflecting the lack of gravel supply to this part of the coastline in the most recent past. f) Deposition of modern coastal dunes, which cap the pocket beach sequence and are the youngest sediments in the study area. Using trench and hydraulic grab evaluation sample results, in combination with analysis of wave patterns and field observations, the following local controls on the density distribution (ie. concentration) and size distribution of diamonds in the gravel beach deposits (+4, -5 and +2 to +3 mamsl stands) are recognised: a) Gravel beach depositional processes, which are responsible for clast sorting on the beach, have influenced the density and size distribution of diamonds. The infill zone, or beach toe, favours maximum diamond concentration while diamond size decreases from the imbricate zone (intertidal) to the infill zone (subtidal). b) Wave energy is identified as the dominant local control on diamond size distribution, but has also influenced diamond concentration to a limited degree. Larger diamonds are intimately associated with coarser beach gravels, both of which are a reflection of increased wave energy. Higher concentrations of diamonds are sometimes associated with zones of coarser gravel and therefore greater wave energy. c) The time of deposition of the host gravel beach is seen to be the dominant controlling factor with respect to diamond concentration. This is seen as evidence of significant temporal variation in the availability of diamonds in the littoral evironment. A significant reduction (20%) in average diamond size from Site 2 to Site 3, over a distance of only 6 km, is evident. The following were identified as reasons for this reduction in diamond size : a) Longshore sorting processes, of which the long-lived northerly littoral drift is a key part, are known to have played a role in the diminution of diamond size northwards from the Orange River mouth point source. However, it is believed that this can only partly account for the observed 20% reduction in diamond size. b) Input of sediment and smaller diamonds at Site 3, reworked out of an older, Eocene-aged marine succession in the hinterland, is recognised as a possible additional reason for the large reduction in diamond size from Site 2 to Site 3. It is also speculated that the large size of the pocket beach at Site 3, relative to Site 2, may have resulted in lower average wave energy at Site 3, with consequent reduced average diamond size. Diamond size in the beach gravels of Site 3, as well as in beach gravels elsewhere in the Sperrgebiet, is seen to be lognormally-distributed within geologically homogeneous zones. In theory, lognormal mean estimators represent the best method of estimating average diamond size in such cases, whereas the arithmetic mean estimator has the tendency to overestimate when large outlier values occur. Lognormal mean estimators have the added benefit of providing for the calculation of confidence limits, which are becoming increasingly more important as financial lending institutions insist on better quantification of the risk involved in resource estimates. Sample campaign simulations demonstrate, for the kinds of diamond size-frequency distributions typical of beach gravel deposits at Site 3, that there is no significant improvement in the accuracy of average diamond size estimates when lognormal mean estimators are used instead of the arithmetic mean estimator. This is because the variance (a ) of the diamond populations is low, and large outlier values are extremely unlikely to occur. However, simulation of a diamond population with high variance, drawn from a sample of beach gravels near the Orange River mouth, shows that lognormal estimators produce significantly more accurate results when a is large. Since individual diamond weights were not recorded during evaluation sampling of Site 3, numerical solution of lognormal estimators is not possible, and these would need to be solved using a less accurate graphical method. It is therefore recommended that individual diamond weights are recorded in future sampling campaigns, allowing for the use of lognormal mean estimators, and the calculation of confidence limits for average diamond size estimates. , Thesis (MSc) -- Science, Geology, 2004
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- Date Issued: 2004-03
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