Thesis (MSc)--University of Stellenbosch, 2007. / ENGLISH ABSTRACT: The aim of the study is to ascertain whether geochemical exploration techniques can be
used in the search for tanzanite deposits in the Merelani area, NE Tanzania. Previous
studies have successfully demonstrated a partial extraction method (in situ soil leaching) in
identifying prospective ultramafic bodies at the Rockland ruby mine in the Mangare area,
Kenya, thereby demonstrating the usefulness of geochemical methods in gemstone
exploration. In this study, a partial extraction as well as a whole-rock geochemical method
was used to determine the applicability of these methods in prospecting for tanzanite
mineralisation using different sampling media, such as soil, stream sediment and calcrete.
It is possible that this geochemical approach may not be as effective as physical methods
such as the separation and examination of heavy mineral suites. However, its viability
needs to be evaluated due to the potential efficiency and relative logistic ease of the
method. In essence the scientific method employed is to compare overburden (soils,
stream sediments and calcrete) chemistry with known underlying geology, the latter having
been established via diamond core drilling. A positive correlation would allow the
prediction of overburden covered tanzanite mineralisation.
Soil samples were collected from a trench dug perpendicular to regional lithological strike
over both barren and tanzanite-bearing horizons. XRF trace element data for the soils
was compared to the chemistry of the underlying lithologies. ICP-AE data derived from 1
molar HCL soil leachate (12 hour leach) and soil XRF data, from the same samples, was
compared, using a mass balance index, to discern any hydromorphic dispersion of
selected trace elements and to evaluate the leachate as a viable alternative to XRF
analysis. In general, a good correlation exists between the soil and rock trace element
data profiles over the length of the section. However, Ti- and Zr-normalised mass balance
calculations show some down-hill drift, but this does not disrupt the overall pattern. The
ICP-AE acid leach data show that hydromorphic dispersion is low, that the trace elements
of interest (V, Cr, Ni and Cu) are hosted within non-soluble phases. Consequently, the
leach technique is not a viable alternative to XRF analysis of the soils. XRF analysis of the
soils was shown to be potentially useful in identifying new areas of mineralisation as the
soils overlying a graphitic calc-silicate schist, that always occurs adjacent to the tanzanite
mineralisation in the Merelani area, was found to be easily identifiable based on
anomalous concentrations of V. An exploration concession was chosen for stream sediment sampling on the basis of the
presence of large streams, of a few tsavorite mines indicating high prospectivity for
tanzanite, and because of a variation in geology on the property. Tanzanite and tsavorite
are cogenetic in the known tanzanite deposits. In this case the aim was to investigate the
possible occurrence of tanzanite-like geochemical anomolies (i.e. the anomalous V
observed in the soil chemistry investigation) could be detected in the vicinity of the
tsavorite mines. Tsavorite, the gem variety of grossular garnet, also contains high
concentrations of V. The samples were analysed by XRF whole-rock methods for trace
element content. The data shows a number of clear positive V anomalies in the study
area. The data also shows that each of the existing or abandoned mines in the area is
marked by a positive V anomaly. This section of the study also demonstrated a relatively
low degree of stream sediment dispersion of the trace elements of interest – most likely a
function of the semi-arid climate. The fine fraction (<90μm), however was shown to be
mobilised to a relatively larger degree than the coarse (180μm – 300μm) and medium
(90μm - 180μm) fractions. As is predictable from the leachate analysis, factor analysis of
the data shows that the trace elements are dominated by heavy mineral geochemistry and
that a study in heavy mineral exploration might provide a cheaper and more viable option
to those explored in this study.
Calcrete samples were taken from an abandoned, 10m deep mine shaft, which was sunk
through the calcrete to reach the tanzanite deposit. The shaft was sampled from the
bottom, closest to the tanzanite mineralisation, to the surface to investigate the association
between trace element geochemistry and proximity to the deposit. There was no vertical
association between the trace element geochemistry of the calcrete and proximity to the
tanzanite deposit. There was also no clear indication in the geochemistry of the calcrete of
the existence of the tanzanite deposit beneath it. This further indicates the immobility of
the elements of interest in this environment.
This study has demonstrated that properly constrained soil and stream sediment
geochemical studies may be of use in tanzanite exploration. However, it must be stressed
that this is only the case if the geochemical signature of the lithological package
associated with the mineralisation is unique and well known. / AFRIKAANSE OPSOMMING: Die doel van hierdie studie is om te bepaal of geochemiese eksplorasie tegnieke vir die
soek na tanzaniet afsettings in die Merelani area, noord-oos Tanzanië, gebruik kan word.
Voorige studies het gewys dat ‘n gedeeltelike ekstraksie metode (in situ grond looging)
gebruik kon word om prospektiewe ultramafiese liggame by the Rockland rubyn myn in die
Mangare area, Kenia te identifiseer. Hierby is gedemonstreer dat geochemiese
eksplorasie metodes suksesvol in edelsteen eksplorasie toegepas kan word. In hierdie
studie is ‘n gedeeltelike ekstrasksie en heel-rots geochemiese metodes gebruik om die
toepaslikheid van hierdie metodes op tanzaniet eksplorasie te toets. Verskillende
geologiese materiale is gemonster, naamlik grond, stroom sedimente en kalkreet. Dit is
moontlik dat hierdie geochemiese benadering nie so effektief soos fisiese metodes soos
swaar mineraal skeidings mag wees nie. Dit is nogtans belangrik om die toepaslikheid
van hierdie metodes op tanzanite eksplorasie te toests, as gevolg van die potensiële
effektiwiteit en relatiewe logistiese gemak van die metodes. Die essensie van die
wetenskaplike metodiek wat in hierdie studie gebruik is, is om die geochemie van die
grond, stroom sedimente en kalkreet te vergelyk met die geochemie van die
onderliggende geologie wat deur middel van diamant boorwerk vasgestel is. ‘n Positiewe
korrelasie sou dan dui op ‘n bedekte tanzaniet afsetting.
Grond monsters is van ‘n sloot geneem wat loodreg op die strekking van die tanzaniet
gemineraliseerde en ongemineraliseerde horisonne gegrawe is. XRF spoor element data
van die gronde is vergelyk met die chemie van die onderliggende gesteentes. IGP-AE
data wat bekom is deur die monsters met 1 molaar HCl te loog (12 uur loging) is vergelyk
met XRF data van dieselfde monsters deur middel van ‘n massa balans indeks om te
bepaal of daar enige hidromorfiese dispersie van sekere spoor elemente is en om die
toepaslikheid van loging as ‘n alternatief tot die heel-rots metode te bepaal. In die
algemeen is daar ‘n goeie korrelasie tussen die grond en rots spoor element data profiele
oor die lengte van die seksie. Alhoewel, Ti- en Zr-genormaliseerde massa balans data
profiele wys dat daar ‘n mate van afwaartse beweging van grond na die voet van die
heuwel is, maar dat hierdie ‘n breuk in die algemene patroon vorm nie. Die IGP-AE data
dui daarop dat die hidromorfiese verspreiding van spoor elemente laag is en dat die spoor
elemente wat van belang is (V, Cr, Ni en Cu) in nie-oplosbare fases gesetel is. Gevolglik
is die logings metode nie ‘n toepaslike alternatief tot die heel-rots XRF metode op gronde
nie. XRF analises op die gronde het gewys dat die XRF metode moontlik nuttig kan wees om nuwe areas van tanzanite mineralisasie aan te dui, omdat die gronde wat ‘n grafietiese
kalk-silikaat skis oorlê, wat altyd langs die tanzaniet draende horisonne voorkom, is op
grond van anomale konsentrasies van V geïdentifiseer.
‘n Eksplorasie konsessie is op die basis van die teenwoordigheid van groot strome, ‘n paar
tsavoriet myne wat aanduidend is van hoë prospektiwiteit vir tanzaniet is en as gevolg van
‘n variasie in geologie in die area vir stroom sediment monstering gekies. Tanzaniet en
tsavoriet is kogeneties in bekende tanzaniet afsettings. In hierdie geval was die doel om
te ondersoek of tanzanietagtige anomalieë (nl. die anomale konsentrasies van V wat in die
ondersoek van die grond chemie opgemerk is) in die omgewing van die tsavoriet myne
geïdentifiseer kan word. Tsavoriet, die edelsteen variëteit van grossulaar granaat, bevat
hoë konsentrasies V. Die monsters is deur middel van die XRF heel-rots metode vir spoor
elemente geanaliseer. Die data dui op ‘n paar monsters met hoë V konsentrasies in die
ondersoek area. Hierdie studie het ook gedui op ‘n lae stroom sediment verspreiding van
die spoor elemente van belang, heel waarskynlik is dit ‘n funksie van die semi-ariede
klimaat. Die fyn fraksie (< 90μm) blyk tot ‘n groter mate as die growwer (90μm tot 180μm
en 180μm - 300μm) fraksies gemobiliseer te word. Soos voorspel kan word deur die
loogings analise het faktor analise gewys dat die spoor elemente deur swaar mineraal
geochemie gedomineer word en dat ‘n studie op swaar minerale moontlik ‘n goedkoper en
meer toepaslike eksploraise metode is as die wat in hierdie studie ondersoek is.
Kalkreet monsters is van ‘n ongebruikte, 10m diep myn skag wat deur die kalkreet gesink
is om by die tanzaniet gemineraliseerde horison uit te kom geneem. Monsters is van die
bodem van die skag, naaste aan die tanzaniet mineralisasie, tot die oppervlak geneem om
die assosiasie tussen die spoor element geochemie en afstand van die tanzaniet
mineralisasie te ondersoek. Geen vertikale assosiasie tussen spoor element geochemie
en die nabyheid tot die tanzaniet afsetting kon vasgestel word nie. Daar was geen
duidelike aanduiding in die geochemie van die kalkreet op die onderliggende tanzanite
afsetting nie. Hierdie is ‘n verdere annduiding op die nie-mobiele toestand van spoor
elemente in hierdie omgewing.
Hierdie studie het suksesvol gedemonstreer dat goed gedefinieerde grond en stroom
sediment geochemiese studies moontlik in geochemiese eksplorasie vir tanzaniet
bruikbaar kan wees. Dit is belangrik om in gedagte te hou dat dit slegs die geval is as die geochemie van die litologiese paket wat met die mineralisasie geassosieer is uniek en
goed bekend is.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/21455 |
| Date | 12 1900 |
| Creators | Hansen, Robert N. (Robert Neill) |
| Contributors | Scheepers, R., Stevens, G., Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 77 leaves (some folded) : ill., maps |
| Rights | Stellenbosch University |
Page generated in 0.0027 seconds