Thesis (PhD)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: The Gamsberg Zn-Pb deposit is a metamorphosed and multiply deformed sediment-hosted base metal
deposit in the central Namaqua Province of South Africa. The deposit is hosted by the Bushmanland
Group, a late Palaeoproterozoic (2000-1600 Ma) supracrustal succession of quartzite, metapelitic schist
and interbedded metavolcanic rocks. Mineralisation occurs within the central part of the Gams Formation,
a heterogeneous sequence of metamorphosed metalliferous sediments and fine-grained organic-rich
shales. The ore horizon is subdivided into a lower unit of metapelite-hosted ore, an intermediate layer
of phosphorite-hosted ore, and an upper unit of banded garnet-apatite ore. The ore body is enveloped by
unmineralised silicate-, carbonate- and oxide-facies metalliferous rocks, which originally represented
mixtures of Fe-Mn-rich hydrothermal precipitates, authigenic carbonate, and variable concentrations of
detrital material. Based on mineralogical and geochemical characteristics, the metalliferous host rocks are
subdivided into iron formations, coticules, Fe-Mn silicates, impure marbles and barite/Ba-rich quartzite.
Minerals of the Gams Formation mostly represent solid solution between the Fe and Mn end-members
of garnet, pyroxene, pyroxenoid, amphibole, olivine, spinel and ilmenite. Calcium-rich rock types are a
typical feature and characterized by the occurrence of manganoan calcite, clinopyroxene, andradite-rich
garnet and titanite. A successive increase in the (Mn+Ca):Fe value of rocks and minerals is evident with
increasing distance from the ore horizon. Amphibole is restricted to Fe-rich ore-bearing assemblages,
whereas orthopyroxene, clinopyroxene, Fe-rich pyroxenoid and olivine are present in intermediate
assemblages, and Mn-rich rhodonite and pyroxmangite in the most manganiferous assemblages. These
variations are mimicked by an increase in the Mn:Fe value of coexisting garnet and ilmenite group
minerals with increasing distance from ore. LA-ICP-MS analyses have been used to constrain the REE
patterns of garnet and apatite. In the ore-body, these minerals display a positive Eu anomaly, which is
interpreted to reflect a distinct hydrothermal signature. In contrast, garnet and apatite in unmineralised
metalliferous rocks display nil or a negative Eu anomaly.
Primary features of the Gams Formation, such as REE patterns, the banded nature of garnet-apatite ore,
the presence of diagenetic apatite nodules, and the distribution of the redox-sensitive elements Ba and
Mn have been used to constrain palaeo-environmental conditions. The results indicate that metapelitehosted
ore has been deposited in a stratified ocean that was characterised by anoxic bottom waters and
precipitation of Fe and Zn sulphides into organic matter-rich shales. These rocks were superceded by
phosphorite-hosted ore, garnet-apatite ore and metalliferous host rocks that developed in a suboxic to
oxic environment. The large size of the deposit, the internal lamination of the ores and the predominance
of sphalerite and barite are consistent with a vent-distal setting and precipitation of the ore-forming
constituents from dense and reduced hydrothermal fluids, which originated due to reactivation of dormant
growth faults. Collectively, the geological evidence indicates that Gamsberg is bridging the gap betweenthe SEDEX and BHT classifications. The relationships demonstrate that differences between these two
classes of sediment-hosted Zn-Pb deposits are predominantly related to environmental conditions within
localised third order basins and not to fundamental differences in ore-forming processes. / AFRIKAANSE OPSOMMING: Die Gamsberg Zn-Pb afsetting is ‘n meerfasig vervormde en gemetamorfiseerde sedimentgesetelde onedel
metaal afsetting in die sentrale Namakwa Provinsie van Suid Afrika. Die afsetting word geherberg deur
die Boesmanland Groep, ‘n laat Paleoproterosoïse (2000 – 1600 Ma) bokors-opeenvolging van kwartsiet,
metapelitiese skis en tussengelaagde metavulkaniese gesteente. Mineralisasie word gevind in the sentrale
deel van die Gams Formasie. Die Gams Formasie is ‘n heterogene opeenvolging van gemetamorfiseerde
metaalhoudende sediment en fynkorrelrige organiese skalie. Die erts horison word onderverdeel in ‘n
onderste laag van metapeliet-gesetelde erts, n sentrale laag van fosforiet-gesetelde erts, en ‘n boonste
laag van gebande granaat-apatiet erts. Die erts-liggaam word omhuls deur ongemineraliseerde silikaat-,
karbonaat- en oksied-fasies metal-ryke rotse. Hierdie gesteentes word geinterpreteer as oorspronklike
mengsels van Fe-Mn-ryke hidrotermale partikels, outigeniese karbonaat, en verskeie hoeveelhede
detritale materiaal. Gebaseer op mineralogiese en geochemiese kenmerke word hierdie rotse onderverdeel
in ysterformasies, „coticules“, Fe-Mn silikate, onsuiwer marmer en barite/Ba-ryke kwartsiet.
Minerale van die Gams Formasie form meestal soliede oplossingsreekse tussen die Fe en Mn endlede
van granaat, pirokseen, piroksenoid, amfibool, olivien, spinel en ilmeniet. Kalsium-ryke rots tipes is ‘n
tipiese kenmerk van die Gams Formasie en word gekenmerk deur mangaan-ryke kalsiet, klinopirokseen,
andradiet-ryke granaat en sfeen. Daar word ‘n stapsgewyse vergroting van die (Mn+Ca):Fe verhouding
in gesteentes en minerale gevind met toeneemende afstand van die erts horison. Amfibool is beperk
tot Fe-ryke ertsdraende gesteentes, ortopirokseen, klinopirokseen, Fe-ryke piroksenoid en olivien tot
intermediêre gesteentes, en Mn-ryke rodoniet en piroksmangiet tot Mn-ryke gesteentes. Hierdie variasies
gaan gepaard met vergroting van die Mn:Fe verhouding in granaat en ilmeniet-groep minerale met
toeneemende afstand van die erts. LA-ICP-MS analises was gebruik om die skaars-aarde element patrone
van granaat en apatiet te bepaal. In die erts-liggaam wys hierdie minerale ‘n positiewe Eu anomalie, wat
geinterpreteerd word as ‘n hidrotermale kenmerk. In ongemineraliseerde gasheer gesteentes wys granaat
en apatiet geen of ‘n negatiewe Eu anomalie.
Primêre kenmerke van die Gams Formasie, soos skaars-aarde patrone, the gebande voorkoms van
granaat-apatiet erts, die teenwoordigheid van diagenetiese apatiet knolle, en die verspreiding van die
redox-sensitiewe elemente Ba en Mn, was gebruik om afleidings oor die paleo-omgewing te maak.
Die resultate het gewys dat metapeliet-gesetelde erts afgeset was onder anoksiese bodem water deur
presipitasie van Fe en Zn sulfiedes in organiese skalie. Hierdie erts gaan oor in fosforiet-gesetelde erts,
granaat-apatiet erts en metaal-ryke gasheer gesteente wat in ‘n suboksiese tot oksiese omgewing ontstaan
het. Die grootte van die afsetting, die interne gelaagdheid van die erts, asook die teenwoordigheid van
sfaleriet en bariet dui op ‘n distale omgewing relatief tot die hidrotermale bron en presipitasie van die ertsuit digte en gereduseerde hidrotermale vloeistowwe, wat ontstaan het deur die heraktiveering van rustende
groeiverskuiwings. Gesaamentlik bewys die geologiese kenmerke van Gamsberg dat gemetamorfiseerde
SEDEX en Broken Hill-tipe mineralisasie binne die perke van ‘n enkele afsetting kan voorkom. Die
geologiese verhoudings dui aan dat verskille tussen hierdie twee tipes van sedimentgesetelde afsettings
meestal veroorsaak word deur omgewings-toestande binne in gelokaliseerde derde orde komme en nie
deur fundamentele verskille in ertsvormende prosesse nie.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/16072 |
| Date | 12 1900 |
| Creators | Stalder, Marcel |
| Contributors | Rozendaal, A., University of Stellenbosch. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | xxii, 347 leaves : ill. |
| Rights | University of Stellenbosch |
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