In search of the lost zinc: A lesson from the Jabali (Yemen

Journal of Geochemical Exploration 108 (2011) 209–219
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Journal of Geochemical Exploration
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j g e o ex p
In search of the lost zinc: A lesson from the Jabali (Yemen) nonsulfide zinc deposit
N. Mondillo a, M. Boni a,⁎, G. Balassone a, B. Grist b
a
b
Università di Napoli, Dipartimento Scienze della Terra, Via Mezzocannone 8, 80134 Napoli, Italy
ZincOx Resources plc Knightway House Park Street Bagshot Surrey GU19 5AQ, United Kingdom
a r t i c l e
i n f o
Article history:
Received 13 December 2010
Accepted 24 February 2011
Available online 13 March 2011
Keywords:
Jabali
Zn-dolomite
Mineralogy
Geochemistry
Evaluation
Recovery
a b s t r a c t
The Jabali nonsulfide zinc deposit, located northeast of Sana'a (Yemen) contains a geological resource of
12.6 million tonnes of ore grading 8.9% zinc, 1.2% lead and 68 g/t silver, with a projected recovery of ca. 80%
zinc. The primary sulfide deposit shows features of both Mississippi Valley and Carbonate Replacement types,
and is believed to have been formed by circulating hydrothermal fluids, either associated with Mesozoic
rifting, or generated from Tertiary igneous activity, developed in the area during the Red Sea crustal extension.
An extension of this phenomenon should have also triggered the late uplift, which favored the oxidation of
sulfides. Ore deposition has been accompanied by several dolomitization phases, some of which have been
considered strictly hydrothermal.
A complete quantitative (Rietveld) mineralogical and geochemical study of mineralized full-length core
samples, carried out with the aim of possibly increasing zinc recovery, shows a discrepancy between the zinc
grades recorded in the chemical assays, and those calculated from the sum of the ore minerals occurring in the
same samples. The difference between the assayed and calculated zinc amounts in various parts of the deposit
is due to the presence of Zn-rich dolomite phases (up to 20% Zn in the lattice), as well as of Mg-smithsonite
(up to 12% Mg), both phases replacive of the earlier dolomites in the weathering environment. The
Zn-enriched dolomite phases could be the “missing link” between pure dolomite and smithsonite. Zinc
occurring in dolomite cannot be processed economically with today's methods. Analysis of the total zinc
amount contained in Zn-dolomite, when compared with the zinc occurring in the processable ore minerals
shows that there is a significant proportion of unrecoverable zinc. This explains why at Jabali the projected
metallurgical recovery of around 80% is unlikely to be improved upon, due to the trapped zinc within the
“supergene” dolomite phases. The extensive development of the Zn-dolomite bodies, which occur throughout
the whole mining area, may be highly significant for the evaluation of nonsulfide zinc ores at Jabali and for the
exploration philosophy of the region.
The possible occurrence of Zn-dolomite has to be kept in mind when exploring for supergene Zn-nonsulfides
in other mining districts where the ore is also dolomite-hosted, which may feature a significant nonrecoverable phase.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
“Zinc nonsulfides” is a very general term, which comprises a whole
series of minerals (Boni, 2005; Hitzman et al., 2003; Large, 2001).
However, the only ones considered so far of economic importance for
zinc extraction are: the Zn-carbonates smithsonite and hydrozincite,
the silicates hemimorphite and willemite, and the Zn-smectite
(sauconite). Among silicates, zinc can be hosted also in other layered
phases, as in the Zn-chlorite baileychlore (Blot et al., 1995; Rule and
Radke, 1988), in the serpentines fraipontite and Zn-rich caryopilite or
greenalite (Fransolet and Bourguignon, 1975; Guggenheim and
Bailey, 1990) and in the hendricksite mica (Robert and Gaspérin,
1985). Variable amounts of zinc have been detected in Mn–Fe(hydr)
⁎ Corresponding author. Tel.: + 39 0812535068.
E-mail addresses: [email protected] (N. Mondillo), [email protected] (M. Boni),
[email protected] (G. Balassone), [email protected] (B. Grist).
0375-6742/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.gexplo.2011.02.010
oxides (Boni et al., 2009; Hayes et al., 2000). However, the latter
concentrations (with few exceptions) are relatively uncommon. The
mineralogical association (franklinite, zincite and gahnite), occurring
in the metamorphic high-temperature Franklin-Sterling Hill mine in
North America (Johnson, 2001) is not very common either, being
limited to a small group of deposits.
Generally, nonsulfide zinc ores of both hypogene and supergene
origin have been relatively poorly investigated (Hitzman et al., 2003).
Consequently, the genetic understanding, regional knowledge, and
the ability to explore these mineralogically and geochemically
complex deposit types are still incomplete (Borg, 2010). However,
with the development of solvent-extraction (SX), electrowinning
(EW), and leach to chemical (LTC) processes there has been a
renewed commercial (and scientific) interest for nonsulfide Zn
mineralization throughout the world. Because even small differences
in dissolution rates and in H2SO4 consumption (in the case of an acid
leach) may have strong implications for the production strategies and
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N. Mondillo et al. / Journal of Geochemical Exploration 108 (2011) 209–219
Fig. 1. Geological sketch map of Yemen, with the location of the Zn–Pb–Ag Jabali nonsulfide deposit.
metallurgical requirements, a thorough understanding of the mineralogy, but also of the petrographic associations is a “must” in
exploration targeting and feasibility studies of nonsulfide deposits.
Host rock composition also significantly influences the mineralogy
(and therefore metallurgy) of nonsulfide ores. Those in limestone and
dolomite tend to be dominated by smithsonite and hydrozincite, due
to the interaction of low-pH Zn-rich fluids with host carbonates,
whereas deposits in siliciclastic rocks tend to contain hemimorphiteand sauconite-bearing assemblages (Hitzman et al., 2003). However,
even in the same category of host rocks, the mineralogy tends to be
relatively simple (smithsonite, hemimorphite, and hydrozincite) in
the oxidation products derived from low-temperature sulfide deposits. Most of the historical European Zn–Pb nonsulfide concentrations,
the so-called “Calamine” (Boni and Large, 2003; Large, 2001), are
carbonate-hosted, as well as other deposits of current economic
importance throughout the world: Angouran (Boni et al., 2007), Accha
(Boni et al., 2009) and Jabali (Al Ganad et al., 1994), which is the
subject of this study.
The Jabali zinc-lead-silver deposit is a dolomite-hosted mixed
sulfide- and nonsulfide mineralization, currently under development
by Jabal Salab Company (Yemen) Ltd., a joint venture between ZincOx
Resources plc and a Yemeni company, Ansan Wikfs. The deposit
Fig. 2. Representative results of the LTC — metallurgical test on 35 composite samples.
The amount of zinc recovery vs. zinc grade has been plotted; the circles represent
samples with Zn N5 wt.% and high recovery, rhombs indicate samples with Zn b5 wt.%
and variable recovery (ZincOx Resources plc.).
(Fig. 1) is located 100 km ENE of Sana'a, in a desert terrain, on the east
side of the mountain range that runs along the length of the western
border of the Arabian peninsula, at an altitude of 2000 m above sea
level. In the Middle Ages (7th–9th century AD) this was considered
one of the most important mining areas for silver in the Muslim world.
The nonsulfide concentrations, derived from the supergene alteration
of the primary sulfide deposit, with their 8.7 million tonnes of ore at
an average grade of 9.2% zinc, are currently considered the major zinc
resource of Yemen (Grist, 2006; Mineral Resources of Yemen, 2009;
Watts, Griffis and McOuat Limited, 1993).
After consideration of a number of mineral processing routes, a
good recovery (80%) from the bulk ore was reached in metallurgical
tests carried out on 35 composite nonsulfide samples with the LTC
Fig. 3. Geological map of the Jabali mining site with the location of analyzed drillcores,
and the future open pit area (modified from SRK Consultants, 2005). Description of the
units is in Fig. 4.
N. Mondillo et al. / Journal of Geochemical Exploration 108 (2011) 209–219
211
mining industry, as recoveries are almost never 100%, but can often be
improved by later work on recovery technique. In the case of Jabali
however, the work shows that the current process is close to optimal
and that further improvements to recovery are unlikely. We will
briefly report on the causes of the above phenomenon in the Jabali
deposit, which may be extended also to other dolomite-hosted
nonsulfide concentrations (Boni et al., 2011).
2. Geological setting of the Jabali deposit
Fig. 4. Schematic stratigraphic column of the Jabali area with the units established by
Christmann et al., 1989 (modified). Unit 1 (7–20 m): sandstone and conglomerate,
transgressive over the Late Proterozoic basement (unknown age); Unit 2 (30–35 m):
gypsiferous mudstone overlain by dolomitized calcarenite, marl and nodular limestone
(unknown age); Unit 3 (33–50 m): micritic and biomicritic limestone (Callovian) with
nodular concretions and chert layers; Unit 4 (10–16 m): micritic limestone and finely
bedded lagoonal/lacustrine dolomite (unknown age); Unit 5 (40–45 m): partly dolomitized
bryozoan calcarenite (Late Oxfordian-Early Kimmeridgian), overlain by coral-bearing oolitic
limestone. A local disconformity at the top of Unit 5; Unit 6 (45–100 m): greenish
gypsiferous mudstone grading to micritic, ammonite-bearing limestone (Kimmeridgian)
and marl with sandstone lenses; Unit 7 (60–90 m): massive bioclastic and biomicritic
limestone, locally oolitic with coral bioherms (Kimmeridgian). The unit is dolomitized and
affected by karstic erosion at the top; Unit 8 (0–30 m): black mudstone and argillite with
gypsum crystals and dolomite intercalations, grading laterally to micritic ammonite-bearing
limestone (Late Kimmeridgian–Tithonian); Unit 9 (N 120 m): biomicrite with oncolites and
bio-oocalcarenite (Late Jurassic).
(leach-to-chemical) method using ammonia-based solutions. It has
proven impossible to raise the recovery above this level, especially in
those samples bearing a low metal grade. Fig. 2 shows the most
representative results of the LTC-recovery testwork, where the
amount of zinc recovery is plotted against the zinc grade. The method
selectively recovers the zinc contained in smithsonite and hydrozincite, and, when compared with the classical acid-leach, there is no
interaction between the chemical solutions and the gangue minerals,
which would cause a high acid consumption during metal extraction
under an acid leach. It can be clearly seen that in the samples having a
zinc ore grade higher than 5%, the recovery is very high (80% and
above), whereas in the samples with ore grade below 5% Zn, the
recovery is variable to very low. In order to investigate the possible
cause of the remaining non-recoverable phase, it was necessary to
carry out a wide mineralogical and geochemical study in the
nonsulfide ore zone, to identify how the metal is trapped in this
non-extractable phase.
A key part of the original deposit evaluation by Jabal Salab was to
apply the calculated recoveries when considering the recovered
material that will be extracted from the reserve. This is normal in the
Major extensional basins formed in Yemen during the Late Jurassic/
Early Cretaceous, linked to the separation of India/Madagascar from
Afro-Arabia (Geological Survey and Minerals Exploration Board, 1994).
The location and NW–SE orientation of these basins are controlled by
the Precambrian structural grain of the Arabian shield (Fig. 1).
During Triassic to middle Jurassic times, Yemen was part of the
Afro-Arabian plate of western Gondwanaland. In the Toarcian–Bathonian
western Yemen was a region of subsidence, in which continental
deposits were accumulated. In the Bathonian, part of the marginal area
between western and eastern Gondwanaland was subjected to a
marine transgression leading to the deposition of the Amran Group. In
the late Callovian, the sea transgressed, providing a passage between
the Arabian and African Seas, including Somalia and Ethiopia. The
sediments of the lower part of the Amran Group were deposited during
the earlier part of the Marib-Al-Jawf basin development, prior to any
significant rifting. The downwarping of the basin resulted in a
deepening regime, which caused the deposition of thick carbonate
sequences in a syn- to post-rift environment. Three facies associations
occur in the middle-upper part of the Amran Group: (1) carbonate
platform facies, (2) carbonate-marl facies, and (3) shallow water coral
and stromatoporoid build-up facies (Al Thour, 1997). The Jurassic
sediments are overlain by post-rift Cretaceous sandstone.
The Cenozoic rift basins of Yemen are linked to the Oligocene/
Miocene rifting of the Gulf of Aden and of the Red Sea. They were filled
with thick sedimentary successions displaying continental, evaporite,
and shallow to deep marine facies, locally injected by alkali-basalt rift
volcanic rocks (Geological Survey and Minerals Exploration Board, 1994).
Lead and zinc ore deposits in Yemen occur in sequences consisting
of Jurassic sediments accumulated in the major rift basins as Jabal
Salab (Al Ganad et al., 1994; Christmann et al., 1989), Al Jabal AlAhmar, Dhi Bin etc., as well as in Palaeocene carbonates associated
with the evolution of the same rifts (Fig. 3). Most of the metallic
occurrences appear at the margins of the rifts or in rift-affected blocks
(Mineral Resources of Yemen, 2009), and are mainly dolomite-hosted.
At Jabali (Jabal Salab) the primary and secondary Zn–Pb ores are
located on the southwestern flank of the NW-trending Sab'atayn
Jurassic rift basin (Al Ganad et al., 1994), where the sediments of the
Amran Group are locally condensed to some 300 m (Fig. 3). The
majority of the Amran lithofacies comprise lagoonal, lacustrine, reef
and biohermal carbonates. After Christmann et al. (1989), and Al
Ganad (1991) the Jurassic succession in the mining area consists from
bottom to top of nine units, whose characteristics are described in the
legend of Fig. 4.
2.1. Sulfides and nonsulfides mineralization
The mineralization is structurally and lithologically controlled, and
this control is reflected in the morphology of the orebodies, which are
variously tabular and parallel to stratigraphy, vertical along fractures
and fissures and along the intersection of structures. The mineralized
bodies may also occur as “keel” like features. The Jabali mineralization
is particularly developed along structural planes (feeder zones?),
which dip between 60° and 80°, and at their intersection. The same
directions seem to have controlled the distribution of the epigenetic
(hydrothermal) dolomitization phases. The stratiform bodies occur in
two zones: a laterally extensive “Upper” zone, directly underlying the
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N. Mondillo et al. / Journal of Geochemical Exploration 108 (2011) 209–219
Fig. 5. a) General view of the Jabali minesite and future plant, looking northeast; typical ore facies in drillcore: b) J125-7: the oxidation proceeds from the fractures of the dolomite
host rock (gray) causing the formation of Zn-dolomite, c) J125-15: typical oxidized ore (red-brown), consisting of replaced smithsonite and Zn-dolomite. White smithsonite in the
cavities, d) J125-31: typical oxidized ore, with remnants of the original dolomite, replaced by finely intergrown Zn-dolomite and smithsonite.
base of the Unit 7/Unit 8 contact and the more sporadic “Lower” and
“Middle” zones, that occur as limbs off the “keel” type mineralization.
These bodies are generally flat; however the dip increases to angles
greater than 30° from the base of Jabal Salab to the Salab Fault
(SRK Consultants, 2005) (Fig. 5a). The black argillites of Unit 8 appear
to have acted as an impermeable barrier to the migration of fluids
(Al Ganad et al., 1994). To the stratiform mineralization should be
added a series of crosscutting bodies (the so-called “chimneys”),
which are related to faults. The sulfide association consists of two
generations of sphalerite (predominant), galena, pyrite/marcasite and
other minor sulfide phases. Silver, cadmium, copper, germanium and
mercury are generally contained in sphalerite.
The primary sulfide deposit, which shows features of both
Mississippi Valley type and Carbonate Replacement models, is believed
to have formed by a combination of processes. After Al Ganad et al.
(1994) the mineralization was deposited by fluids circulating in a karstic
network related to the emersion surface at the top of Unit 7, and was
possibly emplaced only slightly later than sedimentation of Unit
8 (Late Jurassic–Cretaceous), still in association with Mesozoic rifting.
Ore deposition has been accompanied by several dolomitization phases,
only some of which have been considered strictly hydrothermal. Sparry
dolomite crystals (the baroque dolomite of Al Ganad et al., 1994, which is
equivalent to the saddle dolomite of Radke and Mathis, 1980) occur
within cavities, or fill fractures crosscutting the previous dolomite
generations. Saddle dolomite is typically deposited by hydrothermal
fluids and is generally a precursor phase for MVT or CRD-type
carbonate-hosted Zn–Pb mineralization (Diehl et al., 2010).
A different genetic concept has been presented in some unpublished
reports. After this model the primary sulfides were deposited by
circulating hydrothermal fluids, ascending along basinal faults, boosted
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N. Mondillo et al. / Journal of Geochemical Exploration 108 (2011) 209–219
Table 1
Mineral abundances in the Jabali cores samples, as deduced from X-ray quantitative phase analysis (QPA).
Sample
no.
Drillcore interval
J109-1
J109-2
J109-3
J109-4
J109-5
J125-1
J125-2
J125-3
J125-4
J125-5
J125-6
J125-7
J125-8
J125-9
J125-10
J125-11
J125-12
J125-13
J125-14
J125-15
J125-19
J125-20
J125-21
J125-22
J125-23
J125-24
J125-29
J125-30
J125-31
J125-32
J125-33
J125-34
J125-35
J138-4
J138-5
J138-6
J138-7
J138-8
J138-9
J138-10
57.30–58.30
58.30–59.30
59.30–60.30
60.30–61.65
61.65–62.70
50.78–51.78
51.78–53.10
53.10–54.73
54.73–55.73
55.73–56.73
57.92–59.45
59.45–60.97
60.97–62.00
62.00–64.00
64.00–65.00
65.00–66.00
66.00–67.00
67.00–68.00
68.00–69.00
69.00–70.00
74.50–75.50
75.50–76.50
76.50–77.50
77.50–78.50
78.50–79.50
79.50–80.50
84.50–85.50
85.50–86.50
86.50–87.50
87.50–88.50
88.50–89.50
89.50–90.50
90.50–91.50
68.00–69.00
69.00–70.00
70.00–71.00
71.00–72.00
72.00–73.00
73.00–74.00
74.00–75.00
Dol
Cal
Sm
Cer
Gp
Ang
Sp
Gn
Cha
Hem
Gth
Kln
Sau
Ilt
Qz
(wt.%)a
(meters)
82.3
85.1
76.9
86.5
78.6
94.0
83.6
2.7
92.9
77.9
19.5
46.0
2.6
61.1
94.0
95.1
91.3
93.7
95.7
53.2
56.4
15.6
30.6
20.8
47.9
76.3
55.0
85.6
16.9
59.4
93.9
97.5
86.9
95.9
82.8
76.9
89.4
83.8
62.1
8.0
9.8
15.3
4.0
1.3
0.1
0.2
0.4
0.1
41.4
32.9
38.1
76.9
50.5
16.6
0.1
0.1
8.9
3.4
29.7
1.5
3.5
5.2
7.8
17.7
0.1
3.3
38.0
0.9
5.9
63.3
36.8
82.5
82.8
34.5
2.4
1.7
2.2
1.9
2.9
0.1
0.7
0.8
0.3
4.0
0.9
2.6
7.8
52.5
3.2
7.7
8.7
11.1
8.9
5.1
1.7
1.1
0.3
3.7
2.6
0.6
2.6
3.1
2.8
4.7
0.1
2.1
7.6
0.7
2.0
0.2
0.2
0.1
0.3
0.3
0.5
0.4
0.6
1.1
1.0
3.2
1.3
0.1
0.2
0.4
9.8
16.1
6.4
10.1
2.0
0.2
0.3
0.4
0.6
0.1
0.2
0.7
0.2
1.3
0.5
0.1
1.8
0.7
0.6
0.4
0.4
5.8
45.4
10.4
4.1
39.0
10.5
78.1
35.6
0.5
4.7
0.1
0.2
0.3
0.4
0.4
0.2
0.1
0.2
0.1
1.6
3.2
1.6
1.2
1.2
2.1
0.4
0.3
3.0
0.1
1.4
0.6
0.1
2.3
0.9
1.6
3.2
1.2
3.8
2.4
1.7
1.1
1.6
2.1
1.8
1.2
3.6
4.4
7.7
5.5
2.0
1.7
2.8
2.1
3.8
4.6
4.3
4.9
2.5
3.3
2.5
4.2
2.4
2.3
2.1
3.9
1.6
1.7
2.2
0.5
1.1
0.1
0.5
30.8
15.0
0.4
0.3
0.5
0.3
0.4
3.3
1.6
0.5
Notes: mineral abbreviations mostly after Whitney and Evans (2010). Dol; dolomite; Cal, calcite; Sm, smithsonite; Cer, cerussite; Gp, gypsum; Ang, anglesite; Sp, sphalerite;
Gn, galena; Cha, chalcophanite; Hem, hematite; Gth, goethite; Kln, kaolinite; Sau, sauconite; Ilt, illite; Qz, quartz.
a
Statistical indicator ranges: Rp 5.11–6.40%, wRp 6.65–8.85%, χ2 1.199–1.825, Dwd 0.398–1.711.
by the heat generated during Tertiary igneous activity, developed in the
area during the Red Sea crustal extension (~22 Ma). An extension of this
phenomenon would have also triggered the late uplift, favoring the
oxidation of sulfides (Allen, 2000).
The supergene nonsulfide ore is massive, semi-massive and
disseminated, and is characterized by vuggy to highly porous, buff,
brown, and orange to white zinc nonsulfide minerals with variable
densities, averaging 2.6 g/cm3 (SRK Consultants, 2005). A porous cellular
boxwork structure accompanied by numerous cavities coated with zinc
minerals, dolomite and calcite is quite common. Gypsum can be very
abundant through the entire mineralized area. Kernels of partially
oxidized galena are common in high-grade sections. Most of the host
rock dolomites at Jabali are dedolomitized and patchily replaced by
calcite. Manganese and iron, previously contained in the dolomite lattice,
can be seen as newly deposited hydr(oxides) in the interstices of the
crystals and in small vugs and fissures. Iron staining is common
throughout the mining area, resulting again in variable concentrations
of goethite, hematite, and Mn(hydr)oxides.
The most common secondary zinc mineral is smithsonite, intimately
intergrown with dolomite. Fine to granular amorphous aggregates of
hydrozincite have been observed in outcrop, but are very uncommon at
depth and in drill cores. Hemimorphite and relict sphalerite occur in
minor amounts. Lead is present both as relict galena and cerussite. Silver
is contained in argentite and galena, as well as native metal (Al Ganad
et al., 1994).
3. Methods of study
To investigate the mineralogy linked to the recovery problem, we
have studied 40 samples from the Jabali cores J109, J125 and J138
(each sample consisting of a quarter core 1 m long) (Fig. 5b, c, d). We
have carried out first a petrographical study in thin section of several
core fragments (65), followed by scanning electron microscopy (SEM)
observation and qualitative energy dispersive X-ray spectroscopy
(EDS) analyses. SEM examination was carried out using a Jeol JSM
5310 instrument at the University of Napoli (CISAG). Element
mapping and EDS spectra were obtained by the INCA microanalysis
system (Oxford Instruments).
The core samples have been crushed to 1 mm and fully
homogenized. We have carried out X-ray diffraction on all samples
(40), with the aim of identifying the occurring mineral phases. We
have used a Philips PW 3020 automated diffractometer (XRD) at the
University of Heidelberg, with CuKα radiation, 40 kV and 30 mA, 10 s/
step and a step scan of 0.02° 2θ. The data were collected from 3 to 110°
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N. Mondillo et al. / Journal of Geochemical Exploration 108 (2011) 209–219
Table 2
Major and minor element concentrations derived from the chemical assays (CA).
Sample
no.
Zn
J109-1
J109-2
J109-3
J109-4
J109-5
J125-1
J125-2
J125-3
J125-4
J125-5
J125-6
J125-7
J125-8
J125-9
J125-10
J125-11
J125-12
J125-13
J125-14
J125-15
J125-19
J125-20
J125-21
J125-22
J125-23
J125-24
J125-29
J125-30
J125-31
J125-32
J125-33
J125-34
J125-35
J138-4
J138-5
J138-6
J138-7
J138-8
J138-9
J138-10
4.55
5.31
5.98
6.29
14.16
1.30
11.49
21.78
3.15
12.16
24.44
19.41
37.62
38.02
25.43
2.30
2.76
3.55
3.09
5.93
1.06
12.43
29.10
15.53
1.13
2.27
8.05
24.93
14.42
37.14
29.32
5.03
4.32
3.77
4.66
6.32
9.45
9.96
11.53
5.42
a
b
Fe
Mg
Pb
Ca
Mn
S
Ag
Cd
(wt.%)a
3.86
2.74
1.99
2.30
2.32
5.05
2.39
2.99
3.57
1.92
2.14
1.91
5.52
2.21
2.74
2.44
1.87
3.56
3.40
2.06
2.15
3.61
7.36
4.57
1.99
1.97
3.32
4.73
3.44
3.93
3.54
5.78
2.60
3.60
3.42
4.05
3.18
3.59
2.99
3.18
8.94
9.26
8.73
10.08
8.36
8.84
8.71
0.69
10.58
8.88
4.02
5.73
0.75
0.54
6.13
12.27
12.17
11.37
11.32
10.71
6.85
5.29
1.83
2.36
2.10
7.38
8.56
3.93
7.45
2.72
4.67
9.48
10.26
8.46
10.14
7.51
8.66
8.63
7.22
6.35
1.05
0.13
5.55
0.12
0.06
0.02
1.37
9.33
1.18
2.75
3.65
3.36
4.11
13.20
1.58
0.18
0.16
0.39
0.07
0.10
0.06
0.43
0.68
1.10
0.25
0.45
0.91
3.63
0.95
0.83
1.91
0.97
0.64
0.68
0.75
1.31
2.38
1.41
10.12
4.60
Cu
Ni
P
30
5
5
5
5
10
5
5
10
5
20
5
20
10
20
5
5
10
10
5
5
20
60
40
5
5
20
20
20
20
20
10
5
5
10
30
20
20
20
20
300
50
50
50
50
500
100
100
200
200
300
100
300
300
200
200
200
100
100
200
300
400
600
500
300
300
400
500
500
300
200
300
200
100
100
600
200
400
200
200
(ppm)b
19.89
21.06
20.13
20.61
15.57
19.68
17.39
9.37
19.90
16.68
7.91
12.23
1.46
0.54
8.78
20.34
20.62
19.46
19.75
19.69
28.97
19.29
2.70
16.18
35.32
27.18
17.16
7.44
15.62
3.70
7.59
18.62
20.25
23.24
19.73
15.84
18.03
17.07
14.61
23.51
0.48
0.58
0.55
0.65
0.60
0.66
0.65
0.13
0.59
0.50
0.80
0.48
0.41
0.39
0.58
0.71
0.55
0.58
0.60
0.61
0.53
0.52
0.48
1.33
0.53
0.57
0.42
0.32
0.55
0.36
0.48
0.59
0.65
0.98
0.65
0.58
0.78
0.66
0.68
0.86
0.03
0.19
0.58
0.15
0.03
2.71
5.61
7.75
4.44
5.00
3.12
4.62
1.59
1.37
0.25
0.06
0.03
0.03
0.03
0.13
0.26
0.03
0.03
0.03
0.03
0.03
0.03
0.24
0.03
0.03
0.07
0.03
0.03
0.03
0.03
0.03
0.12
0.08
0.90
0.43
3
3
11
22
31
5
56
567
45
50
210
118
231
230
289
21
24
44
24
14
5
3
3
3
5
9
102
433
81
264
353
15
18
10
11
44
116
143
93
112
670
760
750
550
1330
90
1120
1380
130
760
1330
1250
1830
2550
1470
280
270
420
280
640
70
1290
1430
850
80
320
780
2060
1320
2120
1160
330
380
260
330
440
560
740
990
440
25
25
25
25
70
25
90
240
25
25
50
25
190
210
70
25
25
25
25
25
25
25
25
25
25
25
25
100
25
90
25
25
25
25
25
25
50
50
70
25
Detection limits (wt.%): Zn 0.0001, Fe 0.01, Mg 0.01, Pb 0.01, Ca 0.01, Mn 0.0001, S 0.05.
Detection limits (ppm): Ag 0.5, Cd 0.5, Cu 0.5, Ni 0.5, P 5.
2θ. Quantitative phase analysis (QPA) was performed on the XRD
traces using the Rietveld method (Bish and Howard, 1988; Bish and
Post, 1993; Hill, 1991; Rietveld, 1969). X-ray powder diffraction data
were analyzed using the GSAS package (General Structure Analysis
System, Larson and Von Dreele, 2000) and its graphical interface
EXPGUI (Toby, 2001).
Whole rock chemical analyses (CA) of major and minor elements
for the same core samples were committed by Jabal Salab Company
(JSC) to OMAC Laboratories Ltd (Co Galway, Ireland). Diamond drill
cores have been split and the entire half-core samples have been
homogenized and pulverized to obtain 30 g of pulps for chemical
analysis. After aqua regia digestion, the samples have been analyzed by
multi-element inductively-coupled plasma mass spectrometry (ICP-MS).
Samples holding N9% Zn have been also analyzed by atomic absorption
spectrometry (AAS), with an excellent agreement between the two data
sets (SRK Consultants, 2005). For our study, we have performed a quality
check on the previous chemical analyses, testing 20 random samples
from our cores at the ACME Laboratories (Vancouver) as well.
The zinc values calculated by QPA through X-ray analyses
(Rietveld method) have been compared with the chemical assay
data of the same cores.
Differential Thermal Analysis (DTA, TG) has been performed only
on two dolomite samples, which were chosen with the aim of testing
this technique for a future exploration perspective of nonsulfide
deposits. The samples were analyzed on a Netsch Instrument model
STA 409, under air atmosphere at the CISAG Laboratory of the
University of Napoli. A sample mass of 100 mg was heated from room
temperature to 1100 °C, at the rate of 10 °C min−1. Two pure dolomite
samples from the Norian of Southern Apennines (Italy) have been
analyzed for comparison.
4. Results
Table 1 records the results from quantitative phase analysis (QPA)
of the Jabali cores samples set, while Table 2 shows the data from the
ICP-MS chemical assays (CA). Table 3 shows the zinc percentages
calculated from the minerals indicated by the QPA Rietveld analyses of
the core samples, the comparison with the amounts quoted in the
chemical assays, and the difference between the two sets of data to
indicate the excess or defect in metal contents.
The most abundant zinc mineral in the Jabali cores (Fig. 5b, c, d) is
the Zn-carbonate smithsonite (ZnCO3), which is generally averaging a
few % to 20 wt.%, with a maximum abundance of about 80 wt.% in the
J-125 drill core. Hemimorphite [Zn4Si2O7(OH)2H2O] was not detected
in XRD, being probably very scarce in the Jabali deposit, as well as
hydrozincite [Zn5(CO3)2(OH)6]. However, the latter is very abundant
N. Mondillo et al. / Journal of Geochemical Exploration 108 (2011) 209–219
Table 3
Zinc amount (Zn%) calculated from whole rock chemical assays (CA), compared with
the metal percentages derived from the Zn-bearing minerals measured by X-ray
quantitative method (QPA). The columns CA-ΣQPA correspond to defect or excess of Zn
percentages calculated with CA respect to QPA method.
Sample
no.
Zn%
CA
QPA
Sm
J109-1
J109-2
J109-3
J109-4
J109-5
J125-1
J125-2
J125-3
J125-4
J125-5
J125-6
J125-7
J125-8
J125-9
J125-10
J125-11
J125-12
J125-13
J125-14
J125-15
J125-19
J125-20
J125-21
J125-22
J125-23
J125-24
J125-29
J125-30
J125-31
J125-32
J125-33
J125-34
J125-35
J138-4
J138-5
J138-6
J138-7
J138-8
J138-9
J138-10
4.55
5.31
5.98
6.29
14.16
1.30
11.49
21.78
3.15
12.16
24.44
19.41
37.62
38.02
25.43
2.30
2.76
3.55
3.09
5.93
1.06
12.43
29.10
15.53
1.13
2.27
8.05
24.93
14.42
37.14
29.32
5.03
4.32
3.77
4.66
6.32
9.45
9.96
11.53
5.42
0.78
1.83
2.71
4.07
9.23
0.05
1.72
19.82
0.47
3.08
33.01
19.19
43.02
43.18
17.99
1.25
0.89
1.15
0.99
1.51
Sp
CA-ΣQPA
Cha
ΣQPAa
3.02
23.68
5.42
0.78
1.84
2.71
4.29
9.26
1.85
4.73
19.88
1.81
7.95
33.46
20.47
43.02
43.18
18.03
1.30
0.95
1.22
1.03
1.53
0.03
3.02
23.68
5.42
2.14
20.34
5.48
40.73
18.57
0.26
2.14
20.34
5.48
40.73
18.57
0.26
0.31
5.11
8.40
3.34
5.27
1.04
0.02
0.19
0.03
0.03
1.79
3.01
0.06
1.35
4.87
0.45
1.28
0.03
0.05
0.07
0.07
0.03
0.02
0.03
0.13
0.38
0.02
0.46
5.11
8.40
3.34
5.27
1.43
3.77
3.47
3.27
1.99
4.90
−0.55
6.76
1.90
1.34
4.21
−9.02
−1.06
−5.40
−5.16
7.40
1.00
1.81
2.33
2.06
4.40
1.03
9.41
5.42
10.11
1.13
2.27
5.91
4.59
8.94
−3.59
10.75
4.77
4.32
3.31
4.66
1.21
1.05
6.62
6.26
3.99
Notes: mineral abbreviations as in Table 1.
a
ΣQPA is the sum of Zn% coming from smithsonite (Sm), sphalerite (Sp) and
chalcophanite (Cha).
in the mineralized outcrops. Sauconite (Zn-smectite) and other clay
minerals are of restricted distribution, and can locally reach values of a
few %. Cerussite (PbCO3) is not always present but it may have values
from 0.1 to 4 wt.%. Anglesite (PbSO4) (up to 4 wt.%) has been found
only in five samples. Sphalerite (ZnS) (up to 8 wt.%) occurs in several
samples. Galena (PbS) is ubiquitous but is normally present at levels
below 3 wt.%.
All the analyzed core samples contain dolomite, generally over 50 wt.%
(Fig. 6a, b), seldom below 20%, together with variable amounts of
calcite and Fe–Mn(hydr)oxides. A few samples from the J-125 core
may contain up to 52 wt.% gypsum. The amount of Fe(hydr)oxides
(goethiteNNhematite) is always below 5 wt.%. Zn–Mn(hydr)oxides
and Pb-Mn(hydr)oxides as chalcophanite [(Zn,Fe2+,Mn2+)Mn4+3O7·3
(H2O)], and possibly less crystalline compounds occur in many samples
(below 1 wt.%).
Under microscopic observation, smithsonite can be seen in different
forms, both replacing the dolomite host rock (Fig. 5d), and as zoned,
colloidal-like cements growing in vugs (Figs. 5c and 6c). In a few samples,
215
smithsonite occurs as perfect rhombohedral crystals, mimicking dolomite.
A sharp and/or gradational transition between the host dolomite and the
(iron-stained) smithsonite replacing dolomite has been commonly
observed. In most cases, however, it is very difficult to distinguish
optically the difference between a “dirty”, oxidized dolomite and a “true”
euhedral smithsonite. As in many other nonsulfide deposits of the world
(Boni et al., 2003; Boni et al., 2007; Boni et al., 2009; Coppola et al., 2009),
smithsonite can be also roughly intergrown with Fe(hydr)oxides in
reddish agglomerates, as well as with Mn(hydr)oxides and thin layers of
clays (Fig. 6d). Some of the concretionary smithsonites contain also
variable proportions of MgO (up to 15 wt.% MgO) (Fig. 6e, f).
Fe(hydr)oxides at Jabali are never pure goethite. EDS measurements have demonstrated the constant occurrence of ZnO (up to
12%), PbO (up to 7%) and SiO2 (up to 6%) together with Fe. Mn(hydr)
oxides can also consist of small amounts of amorphous phases,
containing Mn–Pb–Fe in variable proportions.
SEM-EDS analyses of many dolomites sampled in nonsulfide ore
zone have shown that this carbonate seems to be fairly pure, with only
local impregnations of Fe- and Mn(hydr)oxides. However, most
dolomites have a “spotty” texture (Fig. 6b). Chemical compositions of
dolomite fabrics are variable. The most abundant phase is the typical
hydrothermal “saddle” dolomite, which is likely to have replaced the
host rock at the time or shortly before precipitation of primary
sulfides. Its composition is stoichiometric, with only trace amounts of
Mn (up to 0.6 wt.%) (Fig. 6a). Instead, the “spotty” dolomites can be
very metal-enriched: their ZnO content ranges from 7–8 wt.% up to
17–22 wt.%, and CdO is around 1.5 wt.% (Fig. 6b and e). Cadmium-free,
Zn-enriched dolomites have also been detected throughout the cores.
These different phases are generally mixed in various proportions.
Proper minrecordite (a dolomite where Mg is almost totally replaced
by 29% Zn, Garavelli et al., 1982) has not been recorded in the Jabali
samples.
Thermal analysis is a quick method to provide additional
information on specific minerals and/or mineral assemblages and
can be useful in the exploration of nonsulfide ores in carbonate rocks
(Zabinski, 1959), in comparison with other investigation methods.
Therefore, thermodifferential/thermogravimetric analysis was carried
out on two samples showing different zinc grade, i.e. samples J125-31
(14.4% Zn), and J125-34 (5.1% Zn). In J125-31, the DTA peaks were
recorded at 400 °C, 500 °C, 685 °C, 750 °C and 870 °C (Fig. 7). The
peaks at 400 °C and 500 °C are in the range of the dehydration values
for smithsonite (Garcia-Guinea et al., 2009), while the 750 °C and
870 °C peaks are in the range of the dolomite dehydration signatures.
In fact, the decomposition of dolomite takes place through two
different steps: a first around 800 °C, which can be associated with the
decomposition of the MgCO3 layers in the structure, and a second around
900 °C, bound to the decomposition of the CaCO3 layers. The 685 °C peak
does not belong to any of the above-mentioned minerals. The Norian
dolomite, analyzed as a standard compound, has two dehydration peaks
at 810 °C and 890 °C. Both temperatures are perfectly comparable with
existing literature data for pure dolomite (Gunasekaran and Anbalagan,
2007; Rowland and Beck, 1952 and references therein).
The DTA trace of sample J125-34 exhibits some differences,
showing the goethite peak (290 °C), and three peaks at 685 °C,
745 °C and 880 °C (Fig. 7). As in the previous sample, the peaks at
745 °C and 880 °C should correspond to the dolomite dehydration,
whereas the 685 °C peak could again not be attributed to any of the
minerals determined by XRD in the chosen samples (see Section 5).
5. Discussion
Discrepancies have been commonly detected between the zinc
contents recorded in the chemical assays and those stoichiometrically
calculated by Rietveld analyses from the total amounts arising from the
zinc minerals (smithsonite and locally also sphalerite and chalcophanite) detected in the Jabali core samples (Table 3). In fact, the
216
N. Mondillo et al. / Journal of Geochemical Exploration 108 (2011) 209–219
Fig. 6. a) J125-33: stoichiometric Ca–Mg (hydrothermal) dolomite replaced along the border by Zn-rich (ZnO 13 to 19%) dolomite; b) J138-5: stoichiometric Ca–Mg dolomite patchily
replaced by Zn-rich (ZnO 15–17%) dolomite; c) J109-5: concretions of zoned, locally Mg-enriched smithsonite around nuclei of Zn-rich dolomite; d) J109-5: zoned crystals of hydrothermal
dolomite, internally replaced by Zn-rich dolomite. White spots are smithsonite intergrown with clays; e) J125-11: zoned crystals of hydrothermal dolomite, internally replaced by Zn-rich
dolomite (13–18% ZnO) and then by smithsonite. The external border has also been patchily replaced along fractures, the cement between crystals consist of Mg-smithsonite. A crystal of
weathered sphalerite (sphalerite ox.) also occurs; f) J125-11: enlargement of the quadrangle in e): zoned, variably enriched (4–19% MgO) Mg-smithsonite cement.
quantitative mineralogy of the analyzed samples is not always in accord
with the metal grade derived from the assays: in some samples the zinc
values in the assay data are higher than the corresponding zinc content
calculated from the mineralogy. This may be due either to a high
proportion of zinc concealed in an unknown phase, or to sampling
issues. In fact, the samples collected for the current work have been
taken from the remaining quarter core available, half core having been
used for assay, and another quarter being fragmented due to previous
analyses. However, the latter issue cannot justify the wide discrepancy
found in most of the considered samples, also because this discrepancy
occurs also in the core fragments – analyzed by the ACME Laboratories –
from which the thin sections have been made. We suggest the difference
between the assayed and calculated zinc amounts in the cores is likely to
be because of the presence of several dolomite phases with variable Zn
content, as well as to the local occurrence of Mg-smithsonite, both of
which have been detected with the SEM-EDS analyses. In fact, also the
samples where smithsonite, or another zinc mineral was not detected
by XRD have shown some zinc percentage in the assays (~5% Zn),
and, from the EDS analyses we know they contain variable amounts of
Zn-dolomite.
In Fig. 8 we have depicted a diagram where the zinc values
detected from the chemical assays of several drillcores are plotted
versus those calculated from the amounts of Zn-minerals determined
with XRD-QPA (data in Table 3). In a hypothetical case of a perfect
stoichiometry of all the minerals occurring in the rocks, the zinc
calculated by XRD-QPA minerals and that measured directly in the
N. Mondillo et al. / Journal of Geochemical Exploration 108 (2011) 209–219
Fig. 7. DTA-TG-DTG traces of the samples J125-31 and J125-34.
217
Fig. 9. Comparison between DTA traces from various sites: Jabali dolomites 1) J125-31,
and 2) J125-34; 3) Tsumeb Zn-dolomite (Hurlbut, 1957); 4) Warynski Mine “Carbonate
Zn-ore” (Zabinski, 1959); 5) Triassic dolomite from Southern Italy. The first
endothermic peaks of the Zn-dolomite in the carbonate zinc ore from the Warynski
mine (Poland) and the Tsumeb dolomite (Namibia) are located around 700 °C, about
100 °C below the first peak of the dolomite standard (Triassic of Southern Italy, 800 °C).
The other endothermic peaks are compatible with the second peak of the dolomite
standard (900 °C). The samples J125-31 and J125-34 have two “first” endothermic
peaks: one is below 700 °C, and is related to the Zn-rich dolomite phases, the other is
above 700 °C, and comprehends both Zn-poor and stoichiometric dolomites. The other
peaks, which are around 900 °C, can be considered compatible with those of the
standard dolomite.
samples should be the same (excluding possible measurement errors)
and, in this diagram, all the points should be in a straight line with a
unitary coefficient. In contrast, many sample points lie away from this
direct line. The samples located below the line contain more zinc than
calculated from the stoichiometry of the Zn-minerals, detected by
XRD (smithsonite, sphalerite and chalcophanite). The samples located
above the line contain less zinc than calculated from the standard
mineral chemistry of the above-mentioned minerals.
The mentioned non-linear trend seems to be due to the fact that at
least part of the zinc minerals are non-stoichiometric and different
from the theoretical mineralogical phases (from XRD-QPA) used to
calculate the amount of the zinc in the samples. The samples
positioned below the line, which contain more zinc than the
calculated values, point to the presence of zincian dolomite. Testified
by SEM-EDS, this dolomite contains variable zinc amounts in the
lattice. Minerals positioned above the bisector line have less zinc than
calculated: this group may include Zn-minerals where zinc has been
partially replaced by other elements, as indicated by the occurrence of
Mg-smithsonites.
Fig. 8. Zinc contents, calculated from the amounts of smithsonite, sphalerite and
chalcophanite determined by QPA, vs. zinc contents measured in the assays. The dotted
line is the bisector of the diagram and indicates the theoretical unitary correlation.
Fig. 10. Compositions of the zincian carbonates (mol%) from Jabali drillcore samples in
the system CaCO3-(Mg, Fe, Mn)CO3-ZnCO3. Circles: zincian dolomites; triangles:
smithsonites.
218
N. Mondillo et al. / Journal of Geochemical Exploration 108 (2011) 209–219
The main problem encountered in the X-ray QPA is that the
Zn-dolomite peaks could not be distinguished from those of the other
dolomite phases in the analyzed samples.
A better way to detect the possible zinc content in dolomite may
arise from the use of thermodifferential/thermogravimetric (DTA/TG)
analysis, as firstly mentioned by Hurlbut (1957) and Zabinski (1959)
and confirmed by Zabinski (1980) and our preliminary data. However,
the quantitative application of this method to nonsulfide exploration is
still in the experimentation phase by our research group. As mentioned,
the DTA of a pure dolomite sample is mirrored by a spectrum with two
distinct dehydration peaks, which are related to the two cations Ca and
Mg in the crystal structure. The position and dimensions of the peaks
depend on the dolomite composition. There have been many attempts
to register how the variations in composition, induced by the
substitution of metals in the dolomite structure could modify the DTA
trace, and if it was possible to clearly distinguish the related peaks.
Several studies have been carried out to check the effects due to the
substitution of certain metals (e.g. Pb) in the Ca positions (A sites,
second endothermic reaction), and of others (e.g. Zn, Fe, Mn, Co) in the
Mg positions (B sites, first endothermic reaction). The DTA analyses
carried out by Hurlbut (1957) on Zn–Pb dolomites from Tsumeb
(Namibia) showed a progressive decrease in temperature of the first
endothermic reaction, with an increasing percent of ZnO in the dolomite
(Fig. 9). A Mg:Zn substitution of 10:1 should reduce the temperature of
the related peak from 815 °C to 740 °C and Mg:Zn of 3.3:1 to 725 °C.
Fig. 9 also shows the DTA curve of a mixed nonsulfide zinc ore from the
Warynski Mine in Upper Silesia (Poland) measured by Zabinski (1959),
with the following endothermic peaks for the carbonate minerals: a
peak at 400 °C caused by the dissociation of smithsonite, followed by
two endothermic peaks at 680 °C and at 900 °C related to dolomite
dissociation. Considering the marked drop in temperature of the first
dissociation effect of the dolomite, in respect to the standard values
(Rowland and Beck, 1952), a partial substitution of Mg by Zn in the
structure of the Warynski dolomite can be assumed. In Fig. 9, our DTA
traces are compared with those obtained by Hurlbut (1957) and
Zabinski (1959), for the dolomites of Tsumeb and of the Warynski zinc
mine, respectively. In the Jabali samples considered for this study, the
above-mentioned unknown peak centered at 685 °C could be finally
attributed to the first endothermic peak of a Zn-rich dolomite phase.
The “zincification” of the Jabali dolomite is genetically related to
supergene oxidation. This host rock alteration can take two different
aspects. The first one (and the more common after the dedicated
literature, Boni et al., 2011 and references therein) may consist of a
strong alteration of the dolomite lattice by Ca-rich meteoric waters,
followed by precipitation of calcite, and is called “dedolomitization”
(Fairbridge, 1978). This is a common phenomenon at Jabali (Al Ganad
et al., 1994), due to the abundance of gypsum in the sedimentary
succession, which is easily water-soluble and can produce a Caenrichment of the groundwater. In the process of dedolomitization of
hydrothermal, metal-bearing dolomites, the dolomite crystals are
replaced by newly formed calcite, while Mn and Fe usually contained
in the hydrothermal dolomite lattice precipitated as a network of hydr
(oxides), giving the carbonate rocks of many mining districts their typical
brownish, “rusty” appearance. However, at Jabali, as in other dolomitehosted Zn–Pb nonsulfide districts in the world, as Southwestern Sardinia
(Italy) (Boni et al., 2011), the dedolomitization can take a very peculiar
form, because the circulating supergene fluids are extremely Zn-rich. In
this case the process starts with a partial replacement of Mg by Zn in the
dolomite lattice, followed by the formation of new Zn-rich (up to 70 mol%
Zn) dolomite phases (Fig. 10). The last step of this peculiar
dedolomitization may be the total destruction of the dolomite lattice
and the formation of the Zn-carbonate smithsonite. In our opinion, the
supergene zincian dolomite can be considered the “missing link”
between dolomite and smithsonite.
This phenomenon had been already described, even if not fully
understood, in the Upper Silesia (Poland) Zn–Pb mining district, by
Zabinski (1980, 1986), Rosenberg and Champness (1989) and
Coppola et al. (2009). In the latter case, however, some doubts have
been raised if all the zincian dolomites are really supergene, or partly
associated with the hydrothermal fluids, which have precipitated the
Lower Silesian sulfide concentrations (Coppola et al., 2009).
6. Importance of mineralogy for detection of the “lost zinc” at
Jabali
In the Jabali nonsulfide zinc deposit the dolomite host rock has
been partly modified through circulating zinc-rich fluids, during
supergene weathering processes. One of the main changes consists of
the substitution of part of the Mg component by Zn in the dolomite
lattice. The mineralogical study that has been carried out at Jabali
shows how widespread this process may be, and the importance of
understanding the extent of its occurrence in the exploration of
dolomite-hosted nonsulfide ores. This “concealed” zinc at Jabali reaches
discrete amounts, up to several percents. A similar pattern may also be
critical in making an economic evaluation of other supergene zinc
deposits.
As in many other nonsulfide zinc deposits (Boni et al., 2011), at Jabali
it is extremely difficult to define with quantitative diffractometric
methods alone the zinc contained in phases like Zn-dolomites (or Zngoethites), instead as that contained in the better-known nonsulfide Znminerals, like smithsonite, hemimorphite or hydrozincite. The DTA
method, on the contrary, can show immediately if the chosen samples
contain significant amounts of Zn-dolomite, mixed to stoichiometric
dolomite. Further developments of this method could provide a
quantitative evaluation of the Zn-dolomite (and hence of the so far
non-extractable metal) contained in the ore.
Considering that the LTC-metallurgical process chosen for the
Jabali deposit is mineralogically specific, and recovers selectively only
some zinc-nonsulfide minerals (predominantly smithsonite and
hydrozincite) discarding the others, as dolomite, the zinc contained
in the latter is currently a non-recoverable phase. For this reason, as
discussed in Boni et al. (2011), we think that the potential for other
nonsulfide Zn deposits to show the same phenomenon of anomalous
zinc enrichment in the dolomite host rock during supergene processes
would need careful investigation.
Acknowledgements
This study has been carried out partly with ZincOx funds, and
partly with a PhD bursary of the University of Napoli to Nicola
Mondillo. The authors would like to thank ZincOx for permission to
publish, R. de'Gennaro of the CISAG Napoli for his support during SEM
analyses and A. Colella from the Dipartimento di Scienze della Terra of
the University of Napoli, for helping with the DTA-TG. Thanks are also
due to R. Herrington and D. Large for commenting on a first version of
this manuscript.
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