Abstract
Big Syncline is the only one of the four Aggeneys-Gamsberg Ore District deposits that remains
unexploited. It is also the only one with no published insights on its process mineralogical
character. This is attributed to its comparatively lower Zn+Pb content of 3.6% and the longterm
low Zn prices. However, the 2011 change in ownership from Anglo American to Vedanta
Resources Plc, with a long-term Zn strategy, coupled with the Broken Hill and Swartberg
nearing their end-of-mine, reignited interest in the Big Syncline. This culminated in the
reinstitution of an intense 47 348m drilling programme of closely spaced boreholes, providing
an opportunity to carry out the first process mineralogical study on the Big Syncline deposit.
In accordance, the current study aims to prrovide a preliminary understanding of the role that
the key properties play in the processing behaviour of Big Syncline ores through an
investigation of the mineralogical properties that influence metallurgical performance, namely,
head grade, ore and gangue mineralogy, particle size distribution, sulphide grain size
distribution, liberation, association and locking
The pelitic sulphide rock and mineralized schist are characterized by coarser-grain size, high
mica and gangue sulphide, pyrite, and pyrrhotite contents, as well as a low manganese
content. The calc-silicate, quartz magnetite, garnet magnetite, and pyrobolite, on the other
hand, are finer-grained, magnetite-rich, Mn-rich with minor gangue sulphide abundances.
Based on these established similarities the pelitic sulphide rock and mineralized schist ores
were grouped as the micaceous ores, and the other ores, as the non-micaceous ores. The
coarser-grained nature of the micaceous ores, coupled with the high mica content is thought
to be responsible for the longer milling times recorded (23 and 31 minutes, respectively to
achieve a target grind of 65% passing 75μm). On the contrary, the finer-grained texture of the
non-micaceous ores is behind the shorter milling times (10 - 20 minutes) observed, due to
their closeness in original grain size to the target grind.
Sphalerite is the main ore mineral, generally occurring in higher abundances than the other
ore minerals, galena, and chalcopyrite. Three sphalerite variations were differentiated based
on colour; opaque and ruby-red varieties prevalent in the micaceous ores, and the honeyyellow
variety, more common in the non-micaceous ores. SEM and MLA examinations
indicated the colour variation to be consistent with increasing Fe content from a relatively
higher content in the opaque to a lower content in the honey-yellow variety. Apart from Fe,
other impurity elements, Mn, Cd, and Cu occurred.
vi
The occurrence of Mn as an impurity element in the opaque and ruby-red sphalerites was
confirmed by laboratory-scale differential flotation tests which showed significant Mn recovery
in the Zn concentrates of the micaceous ores, while the bulk of the Mn in the non-micaceous
ores was rejected to the tailings. The bulk rejection of Mn in the non-micaceous ores informs
its primary host as the non-sulphide gangue such as spessartine, pyroxmangite, pyrophanite,
etc. Cd was also confirmed to occur primarily in sphalerites, regardless of the Fe content, its
recovery patterns mirrored those of sphalerite.
Apart from the penalty element-related issues, flotation tests also showed poor selectivity
against chalcopyrite and galena across the sampled ores, with the bulk of Pb misreporting to
the Cu concentrates. The micaceous ores displayed better selectivity towards sphalerite, with
the bulk of the Zn rightfully recovering to Zn concentrate. Despite this rightful Zn recovery, the
micaceous ores overall reported poor grades and recoveries. The reduced quality in the
micaceous ores concentrates is attributed to the increased dilution associated with the high
abundance of gangue sulphides. The increased occurrence of the impurity elements in
sphalerite also contributed to the lower zinc grade by taking up some of the zinc sites in the
sphalerite lattice thus lowering the overall zinc content of the sphalerite. The non-micaceous
ores reported superior quality concentrates, regardless of the diminished selectivity.
Understanding the nature of the valuable minerals lost to the tailings allowed the assessment
of strategies that could potentially improve performance. Across the studied ores, galena and
sphalerite in the tailings are poorly liberated and in close association with gangue silicates.
This suggests secondary grinding as a possible improvement strategy. Chalcopyrite lost to
the tailings of the non-micaceous ores is also fine-grained and mostly locked in the nonsulphide
gangue, similarly suggesting secondary grinding as a feasible mechanism to improve
Cu recovery.
This study has highlighted key mineralogical and textural variations between the mineralized
horizons at Big Syncline and their influence on the flotation performance, demonstrating that
the valuable metal content of the feed does not always translate into superior processing
performance. Despite the comparatively higher Zn and low Mn contents in the micaceous ore
feed, the high gangue sulphides and mica abundances, and the increased occurrence of Fe
and Mn in the sphalerite, these ores performed poorly. On the contrary, the low abundance of
gangue, and the prevalence of the low Fe and Mn sphalerites in the non-micaceous ores
contributed to better performance, albeit poor selectivity. The study ultimately showed that ore
properties such as mineralogy, grain size, mode of occurrence, and associations of the target
minerals, as well as their chemistry govern their processing efficiency.