Mineralised pegmatites of the Damara Belt, Namibia: fluid inclusion and geochemical characteristics with implications for post-collisional mineralisation
Date
2014-07-30
Authors
Ashworth, Luisa
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Abstract
Namibia is renowned for its abundant mineral resources, a large proportion of which are
hosted in the metasedimentary lithologies of the Damara Belt, the northeast-trending
inland branch of the Neoproterozoic Pan-African Damara Orogen. Deposit types include
late- to post-tectonic (~ 523 – 506 Ma) LCT (Li-Be, Sn-, and miarolitic gem-tourmalinebearing)
pegmatites, and uraniferous pegmatitic sheeted leucogranites (SLGs), which have
an NYF affinity.
Fluid inclusion studies reveal that although mineralization differs between the different
types of pegmatites located at different geographic locations, and by extension, different
stratigraphic levels, the fluid inclusion assemblages present in these pegmatites are
similar; thus different types of pegmatites are indistinguishable from each other based on
their fluid inclusion assemblages. Thorough fluid inclusion petrography indicated that
although fluid inclusions are abundant in the pegmatites, no primary fluid inclusions could
be identified, and rather those studied are pseudosecondary and secondary. Fluid
inclusions are aqueo-carbonic (± NaCl), carbonic, and aqueous. It is proposed that all of
the pegmatites studied share a similar late-stage evolution, with fluids becoming less
carbonic and less saline with the progression of crystallisation.
Oxygen isotope ratios allow the discrimination of different pegmatites into two groups,
Group A (Sn-, Li-Sn-, and gem-tourmaline-bearing LCT pegmatites), and Group B (Li-Bebearing
LCT, and U-bearing NYF pegmatites). Group A pegmatites have O-isotope ratios
ranging from 11 to 13 ‰ suggesting that they have an I-type affinity. These values are,
however, elevated above those of typical I-type granites (7 - 9 ‰), indicating either a postemplacement
low-temperature exchange with meteoric fluid, high-temperature
hydrothermal exchange with δ18O country rocks during emplacement, or the derivation of
these pegmatites from a non-pelitic/S-type metaigneous source. Group B pegmatites
have higher δ18O ratios (δ18O = 15 - 16 ‰), indicative of their S-type affinity, and their
derivation from metapelitic source rocks. δD values of all the pegmatites range from -40
‰ to -90 ‰ indicating that the pegmatitic fluids are primary magmatic with a
metamorphic fluid component.
Trends in the trace element concentrations of both Group A and Group B pegmatites are
very similar to each other, making the two groups indistinguishable from each other on
this basis. The Damaran pegmatites also share similar geochemical trends with their
country rocks. There is, however, no direct field evidence to suggest that the pegmatites
were derived from the in situ anatexis of the country rocks. It is more likely that anatexis
occurred some distance away from where the pegmatites were ultimately emplaced, and
that the melts migrated and were finally emplaced in pre-existing structures, possibly
formed during Damaran deformation.
O-isotope and Ti-in-quartz geothermometry indicate that Damaran pegmatites can be
subdivided into two groups based on their crystallisation temperatures. LCT pegmatites
crystallised at temperatures ranging from ~ 450 - 550 ºC, while the NYF pegmatites
crystallised at higher temperatures, ranging from 630 - 670 ºC. It is important to note that
the subdivision of pegmatites in Groups A and B based on their O-isotope systematics
does not correspond with their subdivision into the LCT and NYF pegmatite families
according to their crystallisation temperatures.
In addition to clarifying aspects of the emplacement and evolution of the Damaran
pegmatites, this study points out that there are several discrepancies in the current
classification schemes of pegmatites. It shows that in addition to the problems
encountered when trying to distinguish between LCT and NYF pegmatites based on their
mineralogy, they also cannot truly be distinguished from each other using their
geochemical and isotopic characteristics, or their tectonic settings. It is tentatively
proposed that crystallisation temperature be considered as an alternative or additional
characteristic in the classification of pegmatites, and that it be considered on a regional
scale rather than only in the evaluation of the highly evolved end-members of a pegmatite
swarm.
Description
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, in fulfilment of the requirements for the degree of Doctor of Philosophy, Johannesburg 2014