Analytical Methods
Samples collection and preparation
Over 50 rock samples were collected in the Star Zinc pit area, most of them in situ
(Table 1; Fig. 5). A few samples originate from the ore piles, some others from the
badly exposed Eastern Orebody (EOB), and most of them from the Western
Orebody (WOB). A few analyses have been carried out on the Star Zinc soils and
on the recent karst infillings (“terra rossa”) around the main ore body (Fig. 5), in
order to evaluate the mineralogy of the eluvial cover.
A small number of samples were collected from several small outcrops around the
Excelsior pit, as the pit itself is presently not accessible. Some willemite
specimens from the collections of the Natural History Museum (London), whose
provenance have been genetically assigned to the prospects of the Lusaka
region, were also petrographically studied.
Pure mineral phases for analyses were obtained by hand picking under a
stereomicroscope. The samples were then cleaned in a sonic bath for 10 minutes,
to remove the impurities deposited on crystal surfaces. 35 polished thin sections
(30μm) were prepared from the most representative samples. Several types of
mineralogical, petrographic and geochemical investigations were then carried out
on the samples.
Cold cathodoluminescence (CL) microscopy
CL microscopy was carried out on all sections, in order to characterize the growth
features and the textural relationship of the willemite with the other mineral
phases. A CITL 8200 Mk3 Cold Cathodoluminescence instrument, coupled with
an optical microscope was used at the Geologisch-Paläontologisches Institut of
the Universität Heidelberg (Germany). The thin sections were placed on a tray
controlled by X-Y manipulators in a vacuum chamber. A beam with a 12-15kV
voltage and a current of 300-350μA was used.
X-ray diffraction analysis (XRPD)
X-ray powder diffractometry (XRPD) was performed on granulometrically
homogeneous powdered (fraction <200μm) whole rock (including ore) samples.
XRPD data were gathered in two different laboratories: (1) Dipartimento di
Scienze Della Terra Università di Napoli “Federico II”, and (2) Mineralogisches
Institut of the Universität Heidelberg (Germany).
The diffraction data at Dipartimento di Scienze della Terra in Naples were
collected with a SEIFERT – GE MZVI automated diffractometer, under the
following conditions: a CuKα radiation at 40kV, and 30mA, 2 and 1 mm divergence
slits, 1 mm receiving slit, 0.1 mm antiscatter slit, step scan 0.05°, counting time 5
sec/step. A zero-background sample holder, consisting of a quartz single crystal
plate cut and polished 6° of the c-axis, was used to obtain long scans performed
over 3° < 2θ < 75° interval. The RayfleX software package (GE Inspection
Technologies) has been used to evaluate the obtained profiles and to permit the
comparison with the ICDD-PDF2 database.
At the Mineralogisches Institut of the Universität Heidelberg, a Bragg-Brentano Xray diffractometer (Siemens D 500) with a Cu tube and graphite monochromator
was used. The diffractometer was also operated at 40 kV and 30 µA and
scanned the samples in the 2θ range from 2.0° to 70.0° with increments of
0.02°[2θ]/sec. The EVA software (part of the DIFFRACPLUS V5 software suite by
the Bruker AXS & Co.) was used to evaluate the profiles and to permit the
comparison with JCPDS-ICDD database. Quartz powder was added to the
powder samples as an internal standard in order to correct the position of the
peaks by reference to the quartz (101) reflection of the internal quartz standard.
Scanning electron microscopy (SEM) and Energy dispersive X-ray
spectroscopy (EDS)
SEM observations, consisting of both qualitative and quantitative analyses, were
carried out using the LEO Electron Microscopy LEO440 scanning electron
microscope, equipped with an Energy Dispersive X-ray spectrometer, at the
Mineralogisches Institut of the Universität Heidelberg (Germany).
The instrument working conditions were: high vacuum, 12mm objective lens to
specimen working distance, 20kV accelerating voltage, 2nA beam current
(stabilized and measured with a Faraday cup). Backscattered electron images
were taken. Quantitative spectra required 50 sec acquisition times. Silicates,
sulphates, sulphides, carbonates, oxides and pure elements were used as
standards. Detection limits are in the order of 0.2wt% oxide. The CO 2 contents in
carbonates and water content in hydrated carbonates and silicates were
evaluated by stoichiometry.
Secondary electron images of representative gold-palladium coated samples
were also obtained. A few observations were also made at the Electron
Microscopy and Mineral Analysis (EMMA) division in the Mineralogy Department
of Natural History Museum in London (United Kingdom) on a Jeol JSM 5310.
Element mapping and qualitative energy-dispersive (EDS) spectra were obtained
with the INCA microanalysis system (Oxford Instruments).
Wavelength Dispersive X-ray Spectroscopy (WDS)
Chemical analyses of willemite, apatite, franklinite and gahnite were performed on
6 thin sections using both a Cameca SX50 (Mineralogisches Institut of the
Universität Heidelberg) and a Cameca SX100 (Electron Microscopy & Mineral
Analysis -EMMA- division in the Mineralogy Department of Natural History
Museum, London) electron microprobe with a gas proportional WDS. Instrumental
conditions were 15kV, 15nA and 10μm spot size in both cases. Minerals and pure
elements were used as standards. Detection limits are in the order of 0.01 wt% for
most elements.
Laser Ablation-ICPMS
Laser ablation-ICPM analysis was used to measure beryllium in genthelvite. The
analyses were done on an Agilent 7500ce Quadrupole LA-ICPMS, equipped with
a Reaction Cell (run in hydrogen mode), and coupled with a Geolas 193 µm, ArF
excimer Laser, at the Virginia Institute of Technology, Blacksburg VA. The system
is capable of analyzing for major, minor and trace elements simultaneously with
detection limits < 1 ppm for most elements. Spatial resolution is as low as 5 µm.
Loose Fragments of the thin sections were put in the LAICPMS chamber and
analyzed for Li, Be, B, Si and Fe with a fixed 44 µm spot. Details of the analytical
conditions are presented in Table 2.
Microthermometric analyses of fluid inclusions
Five doubly polished thin sections (120μm) were prepared for fluid inclusion
analyses from the willemite and calcite samples of Star Zinc and Excelsior.
Microthermometric measurements were carried out at the Fluid Laboratory of the
Universität Heidelberg, using a Linkam MD 600 stage. The stage was regularly
calibrated using H2O and CO2 inclusions in synthetic quartz (Bodnar and Sterner
1987); the measurements have a precision of ±1 °C in the temperature range of
interest. The degree of fill of the inclusions was recorded at room temperature and
the bubble size was monitored during the heating and freezing cycle. Data from
leaked inclusions or inclusions that stretched or leaked during microthermometric
analyses were discarded. In order to minimize the risk of stretching and/or
leakage, the inclusions were first heated and then cooled with a small gradient (10
to 5° C/min). The gradient was further slowed (1° C/min) in the last phase of each
experiment.
Crush-leach analysis
A preliminary crush-leach analytical study of inclusion fluids was carried out at
the Department of Earth & Atmospheric Sciences, University of Alberta,
Edmonton (Canada). Primarily, this served as a test of the utility of these types
of analyses for nonsulphide Zn deposits. In total 6 samples (four willemite and
two calcite) were analysed. The samples were crushed and hand picked to give a
clean mineral separate. One gram of each sample was crushed and leached
following the method described by Gleeson and Turner (2007). The leachate was
analysed using a DX600 ion chromatograph for fluoride, chloride, bromide,
phosphate and sulphate. Chloride was above the detection limit (0.008 ppm) in
all samples but F- and Br- were detected in only four samples. Sulphate was
detected in two samples at very high levels. These data are unlikely to represent
the sulphate levels in the fluids but are likely the result of small sulphate mineral
inclusions in the calcites. Replicate analyses of standards and unknowns were
within 2% for Cl- and SO42- and 5% for F- and Br-. Sodium, K and Li in the
leachates were analysed by atomic absorption spectroscopy. Lithium was below
detection in 4 samples and will not be discussed further. The precision on these
analyses is 5%.
References
Bodnar RJ, Sterner MS (1987) Synthetic fluid inclusions. In: Hydrothermal
experimental techniques - IAGC Subcommittee on Water-Rock Interactions IV
International Symposium, Misasa, Japan, 1983, Ulmer and Barnes (eds), John
Wiley & Sons, New York, NY, (USA): 423-457
Gleeson SA, Turner WA (2007) Origin of hydrothermal fluids associated with PbZn mineralization at Pine Point and coarse and saddle dolomite formation in
southern Northwest Territories. Geofluids 7: 51-68
The carbonate-hosted willemite prospects of the Zambezi Metamorphic Belt (Zambia)
Boni M*., Terracciano R., Balassone G., Gleeson S.A, Matthews A.
*boni@unina.it
Dipartimento di Scienze della Terra, Università di Napoli “Federico II”, Italy
Mineralium Deposita