View
246
Download
0
Embed Size (px)
Citation preview
7/23/2019 Dolomita Zinciana relacionada a la Alteración Supergena
http://slidepdf.com/reader/full/dolomita-zinciana-relacionada-a-la-alteracion-supergena 1/12
See discussions, stats, and author profiles for this publication at: http://www.researchgate.net/publication/236861821
Zincian dolomite related to supergenealteration in the Iglesias mining district (SWSardinia)
ARTICLE · JANUARY 2013
DOI: 10.1007/s00531-012-0785-0
CITATIONS
2
READS
79
5 AUTHORS, INCLUDING:
Nicola Mondillo
University of Naples Federico II
24 PUBLICATIONS 74 CITATIONS
SEE PROFILE
Giuseppina Balassone
University of Naples Federico II
56 PUBLICATIONS 323 CITATIONS
SEE PROFILE
Michael M. Joachimski
Friedrich-Alexander-University of Erlangen-…
194 PUBLICATIONS 4,219 CITATIONS
SEE PROFILE
Abner Colella
University of Naples Federico II
50 PUBLICATIONS 278 CITATIONS
SEE PROFILE
All in-text references underlined in blue are linked to publications on ResearchGate,
letting you access and read them immediately.
Available from: Michael M. Joachimski
Retrieved on: 02 January 2016
7/23/2019 Dolomita Zinciana relacionada a la Alteración Supergena
http://slidepdf.com/reader/full/dolomita-zinciana-relacionada-a-la-alteracion-supergena 2/12
O RI G I N A L P A P E R
Zincian dolomite related to supergene alteration in the Iglesiasmining district (SW Sardinia)
M. Boni • N. Mondillo • G. Balassone •
M. Joachimski • A. Colella
Received: 13 August 2011/ Accepted: 21 April 2012/ Published online: 29 May 2012
Springer-Verlag 2012
Abstract One of the main effects of supergene alteration
of ore-bearing hydrothermal dolomite in areas surroundingsecondary zinc orebodies (Calamine-type nonsulfides) in
southwestern Sardinia (Italy) is the formation of a broad
halo of Zn dolomite. The characteristics of supergene Zn
dolomite have been investigated using scanning electron
microscopy and qualitative energy-dispersive X-ray spec-
troscopy, thermodifferential analysis, and stable isotope
geochemistry. The supergene Zn dolomite is characterized
by variable amounts of Zn, and low contents of Pb and Cd
in the crystal lattice. It is generally depleted in Fe and Mn
relative to precursor hydrothermal dolomite ( Dolomia
Geodica), which occurs in two phases (stoichiometric
dolomite followed by Fe-Mn-Zn-rich dolomite), well dis-
tinct in geochemistry. Mg-rich smithsonite is commonly
associated to Zn dolomite. Characterization of Zn-bearing
dolomite using differential thermal analysis shows a drop
in temperature of the first endothermic reaction of dolomite
decomposition with increasing Zn contents in dolomite.
The supergene Zn dolomites have higher d18O but lower
d13C values than hydrothermal dolomite. In comparison
with smithsonite-hydrozincite, the supergene Zn dolomites
have higher d18O, but comparable d13C values. Formation
of Zn dolomite from meteoric waters is indicated by low
d13C values, suggesting the influence of soil-gas CO2 in
near-surface environments. The replacement of the dolo-
mite host by supergene Zn dolomite is interpreted as part of
a multistep process, starting with a progressive ‘‘zinciti-
zation’’ of the dolomite crystals, followed by a patchydedolomitization s.s. and potentially concluded by the
complete replacement of dolomite by smithsonite.
Keywords SW Sardinia Zn dolomite Supergene
Nonsulfides
Introduction
In the first decades of the twentieth century, with more than
50 active mines of lead, zinc, and barium, the Iglesiente-
Sulcis (SW Sardinia) was one of the most important mining
districts in Europe (Fig. 1). The metallic ores were hosted
mainly in a Lower Cambrian calcareous formation (Cero-
ide Limestone), which is largely replaced by epigenetic
hydrothermal dolomite, considered to be of late- to post-
Variscan age because of its crosscutting relationships to
both sedimentary and tectonic structures (Boni et al. 2000).
This dolomite forms large-scale bodies, which can be
clearly identified on outcrop due to their yellow–brown
color, caused by supergene oxidation of Fe2? contained in
the dolomite lattice (Fig. 2a, b). The carbonate-hosted Zn-
Pb sulfide ores have also been altered in the oxidation zone,
resulting in the so-called Calamine or nonsulfide ores (Boni
et al. 2003).
In addition to precipitating typical ore carbonates
(smithsonite and hydrozincite) or silicates (hemimorphite),
the supergene alteration has also promoted a widespread
replacement of previously deposited dolomites by new
zincian dolomite phases (Boni et al. 2011). The formation
of a broad halo of Zn dolomite, spottily replacing
the previous hydrothermal dolomite along fractures and
discontinuities, is one of the main effects of supergene
M. Boni (&) N. Mondillo G. Balassone A. Colella
Dipartimento di Scienze della Terra, Universita di Napoli,
Via Mezzocannone 8, 80134 Naples, Italy
e-mail: [email protected]
M. Joachimski
GeoZentrum Nordbayern University of Erlangen-Nuremberg,
Schlossgarten 5, 91054 Erlangen, Germany
1 3
Int J Earth Sci (Geol Rundsch) (2013) 102:61–71
DOI 10.1007/s00531-012-0785-0
7/23/2019 Dolomita Zinciana relacionada a la Alteración Supergena
http://slidepdf.com/reader/full/dolomita-zinciana-relacionada-a-la-alteracion-supergena 3/12
alteration in the areas surrounding the Calamine orebodies.
This replacement process is relatively common in several
mining districts subjected to supergene weathering, even
though the extent of substitution of Zn for Mg (and Pb
for Ca) in the dolomite structure is not easy to quantify.
The supergene ‘‘zincitization’’ of the dolomite has beendescribed at the Jabali (Yemen) and Yanque (Peru) mine-
sites by Boni et al. (2011) and in the Polish ore district by
Zabinski (1959, 1980). This phenomenon is not restricted
to the mentioned localities only, but the precipitation of Zn
dolomite may be characteristic around most dolomite-
hosted sulfide concentrations undergoing supergene alter-
ation. However, the genetic relationships between host
rock, primary sulfide mineralization, and the newly formed
nonsulfide Zn phases (including Zn dolomite) need better
clarification. Therefore, the aim of this study has been the
characterization of the supergene Zn dolomite in southwest
Sardinia, and its relationships with both the primary
hydrothermal dolomite and the Calamine-type Zn nonsul-
fide ores.
Geological setting
The geology of SW Sardinia is largely dominated by
Paleozoic (mainly Cambro-Ordovician) rocks of sedimen-
tary as well as igneous origin, belonging to the so-called
external zones of the Variscan orogen (Carmignani et al.
1994) (Fig. 1). The Lower Cambrian succession is subdi-
vided into the basal Nebida Group and the overlying
IGLESIENTE
S U L C I S
Carbonia
Gonnesa
3
4
5
6
7
8
9
1 0
2
1
Nonsulfide Ore Deposits
& Zn-Dolomite District
10 Km
S a r d i n i a
4
1
2
3
5
Monteponi
San Giovanni
Campo Pisano
Nebida
Buggerru-Malfidano
Mines & Outcrops
Buggerru
Fluminimaggiore
Iglesias
5
31
2
4
Fig. 1 Geological sketch map of southwestern Sardinia with the
location of the main Zn nonsulfide orebodies (1–5), surrounded by
Zn dolomite areas of variable extension. 1 Overthrust; 2 normal
fault; 3 Cenozoic; 4 Mesozoic; 5 Variscan granites; 6 Paleozoic
(allochthonous); 7 Ordovician to Devonian succession; 8 Iglesias Group
(Middle Cambrian-Lower Ordovician); 9 Gonnesa Group (Lower
Cambrian); 10 Nebida Group (Lower Cambrian) (modified from Boni
et al. 2003)
62 Int J Earth Sci (Geol Rundsch) (2013) 102:61–71
1 3
7/23/2019 Dolomita Zinciana relacionada a la Alteración Supergena
http://slidepdf.com/reader/full/dolomita-zinciana-relacionada-a-la-alteracion-supergena 4/12
Gonnesa Group, which consists of siliciclastic sedimentary
rocks with carbonate intercalations toward the top and of
tidal dolomites and limestones, respectively (Bechstadt and
Boni 1994). Middle and Upper Cambrian to Lower Ordo-
vician strata are represented by nodular limestones (Campo
Pisano Formation, Iglesias Group) and slates (Cabitza
Formation, Iglesias Group), respectively. Upper Ordovi-
cian and Silurian lithologies are separated by an angular
unconformity from the underlying series due to partial
erosion of Cambrian and Lower Ordovician sediments.
The pre-Variscan, stratiform, and/or stratabound Zn–
Pb–Ba orebodies are hosted in the Lower Cambrian car-
bonates (Boni 1985). Two groups of genetically distinct ore
types are known: SEDEX and MVT-type ores (Boni et al.
1996). Part of the orebodies are enclosed within an
epigenetic hydrothermal dolomite ( Dolomia Geodica).
Fig. 2 Yellow (oxidized) hydrothermal dolomite in the Iglesiente
district. a Scoglio Il Morto, Nebida: ‘‘Yellow’’ dolomite replacing
Cambrian limestone in a carbonate block along the coast; b Hills east
of the Nebida village: unreplaced limestone areas in Dolomia Gialla;
c Cungiaus open pit of the Monteponi mine (Iglesias): the limestone
has been completely replaced by Yellow, locally Zn-rich dolomite;
d Malfidano open pit of the Buggerru mine (Fluminimaggiore): the
limestone is dolomitized and patchily enriched in Zn dolomite;
e Canale San Giuseppe (Nebida): Yellow Zn-rich dolomite hosting
nonsulfide Zinc ores; f San Giovanni Mount (Gonnesa): apophyses of
oxidized hydrothermal dolomite, patchily replacing the Cambrian
limestone
Int J Earth Sci (Geol Rundsch) (2013) 102:61–71 63
1 3
7/23/2019 Dolomita Zinciana relacionada a la Alteración Supergena
http://slidepdf.com/reader/full/dolomita-zinciana-relacionada-a-la-alteracion-supergena 5/12
Contrary to most MVT deposits, this dolomitization phase
clearly postdates both the emplacement of the stratabound
ores and Variscan deformation (Boni et al. 1992). The
epigenetic replacive dolomitization affected the Cambrian
limestones as well as the early diagenetic dolomites in
large areas (more than 500 km2 in outcrop) of the Igle-
siente-Sulcis district. The dolomitization process is espe-
cially pervasive in southern Iglesiente, where only limitedparts of the Ceroide limestone are unaffected and occur as
gray, isolated spots within a sea of weathered, brown-
yellowish dolomite (Fig. 2a, b, f). The large-scale rela-
tionship between dolomite and limestone clearly suggests
a post-deformational origin of the dolomite, since the
dolomite bodies clearly crosscut the vertical foliation and
are apparently controlled by the former as well as later
extensional faults. The epigenetic dolomite is informally
known as Dolomia Geodica (=Geodic Dolomite, due to its
vuggy appearance) and/or Dolomia Gialla (=Yellow
Dolomite; Brusca and Dessau 1968). Boni et al. (2000)
demonstrated that Dolomia Gialla and Dolomia Geodica
are different names for basically the same bodies of Fe-
bearing dolomite. The Dolomia Gialla appears yellow–
brown on outcrop and for several hundred meters under-
ground, due to oxidation of Fe2? contained in the dolo-
mite lattice.
The relative age of the Dolomia Geodica event can be
tentatively inferred from the crosscutting relationship with
the host rocks and tectonic lineaments. This age can be
bracketed between the Late Carboniferous and Middle
Permian, as it has been reported in other European late
Variscan domains as well (Gasparrini et al. 2006).
The nonsulfide ores of the Iglesiente district, derived from
repeated weathering episodes, are generally hosted within the
Dolomia Gialla (hydrothermal Dolomia Geodica weathered
to brownish, rusty colors; Fig. 2c–e). Smithsonite, hydro-
zincite, and hemimorphite are the principal Zn-bearing min-
erals in thenonsulfide zinc (?lead) deposits(Boni et al. 2003).
Cerussite and anglesite also occur, generally associated with
nodules of remnant or supergene galena, iron and manganese
oxy-hydroxides, and clay minerals.
The extent of the oxidized ore zones in the mining
district, which reach deep below the surface, is generally
independent of the present-day water table and highly
variable in different areas of the mining district. These
differences may be related to several distinct phases of
block faulting that displaced mature oxidation profiles
(Boni et al. 2003). The vertical tectonic movements
occurred during both the Tertiary and Quaternary periods.
The base of the oxidation profiles containing nonsulfide Zn
minerals can be both elevated above or submerged below
the recent water table, and the supergene alteration of the
primary ores is considered to be related to fossil, locally
reactivated, oxidation processes (Boni et al. 2003).
However, the Zn nonsulfide ore shoots in most mines are
roughly located within the lower vadose zone of a paleo-
karstic system that is hundreds of meters deep, but above
the water-filled conduits of the phreatic saturated zone. The
mineralization is considered to be the result of in situ
oxidation of the primary sulfide ores by increasingly acidic
meteoric fluids that circulated through the carbonates
(Moore 1972; Boni et al. 2003).The formation of a broad halo of zincian dolomite,
spottily replacing the previous hydrothermal dolomite
along fractures and discontinuities, is another effect of the
supergene alteration in the areas surrounding the supergene
orebodies (Boni et al. 2011).
Based on geological and paleomagnetic arguments, it
was hypothesized that the Middle Eocene to Plio-Pleisto-
cene represents the most probable age interval for the
formation of the supergene nonsulfide Zn-Pb ores, as well
as for the weathering and «reddening» of the hydrothermal
Dolomia Geodica to Dolomia Gialla (Boni et al. 2003,
2005).
Analytical methods
To investigate the mineralogy of the zincian dolomites, we
studied 25 samples from the Iglesiente district (Table 1)
using thin sections, scanning electron microscopy (SEM),
and qualitative energy-dispersive X-ray spectroscopy
(EDS). 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). X-ray
diffraction analyses were performed on all samples using a
Philips PW 3020 automated diffractometer (XRD) at the
University of Heidelberg (CuK a radiation, 40 kV and
30 mA, 10 s/step, and a step scan of 0.022h; data were
collected from 3 to 1102h.
Stable carbon and oxygen isotopes were measured on
five samples of non-oxidized and three samples of oxidized
hydrothermal Dolomia Geodica, two samples of smith-
sonite from the Monteponi and Buggerru mines, and seven
samples containing larger amounts of zincian dolomite
located around the mines of Monteponi (Fig. 2c), Buggerru
(Fig. 2d), Nebida (Fig. 2e), San Giovanni (Fig. 2f), and
Planu Sartu. We were not able to separate the supergene
zincian dolomite from the hydrothermal dolomite, the two
phases being strictly intergrown, but we took care of
choosing those samples in which Zn dolomite was most
abundant, as well as some samples with only traces of Zn
dolomite detected by SEM analysis. All samples were
treated with EDTA solution to eliminate calcite.
Carbonate powders for stable isotope analyses were
collected with a dental drill and reacted with 103 %
64 Int J Earth Sci (Geol Rundsch) (2013) 102:61–71
1 3
7/23/2019 Dolomita Zinciana relacionada a la Alteración Supergena
http://slidepdf.com/reader/full/dolomita-zinciana-relacionada-a-la-alteracion-supergena 6/12
phosphoric acid at 70 C using a Gasbench II connected to
a Thermo Finnigan Five Plus mass spectrometer (Univer-
sity of Erlangen-Nuremberg). All values are reported inper mil relative to V-PDB by assigning a d13C value of
?1.95 % and a d18O value of -2.20 % to NBS19.
Reproducibility was checked by replicate analysis of lab-
oratory standards and was better than ±0.07 % (1r) for
both carbon and oxygen isotope analyses. Oxygen isotope
values of dolomite and smithsonite were corrected using
the phosphoric acid fractionation factors given by Kim
et al. (2007), Rosenbaum and Sheppard (1986), and Gilg
et al. (2008).
Differential thermal analysis (DTA) is a quick method to
provide additional information on carbonate minerals
assemblages (Zabinski 1959; Mondillo et al. 2011), since it
allows distinguishing between pure dolomite and Zn
dolomite on the basis of the temperature of the first
endothermic reaction. This reaction concerns MgCO3 de-
carbonatization occurring in the dolomite crystal structure,
and its temperature is subjected to variations induced by
substitution of metals, in particular Zn, in place of Mg in
the lattice (Zabinski 1959, 1980; Mondillo et al. 2011).
Thermal analysis was performed on few samples that were
analyzed for stable isotopes as well. The analyses were
conducted at the CISAG Laboratory of the University of
Napoli using a multiple thermoanalyzer Netzsch STA 409
(sample mass of 100 mg, air atmosphere, continuousheating from room temperature to 1,100 C at 10 C
min-1). Elemental composition of the whole rock was
obtained, analyzing powder pellets at the CISAG Labora-
tory with a Philips PW1400 X-ray fluorescence spectrom-
eter, following the methods described by Melluso et al.
(2005). LOI (weight loss on ignition) was measured
gravimetrically igniting the samples at 1,100 C.
Results
X-ray analyses of most samples show the occurrence of
dolomite with its usual peak at about 312h. No shifting or
doubling of this peak could be observed in the samples
containing important amounts of Zn dolomite (detected by
SEM). Goethite and haematite occur as well. Small
amounts of quartz and barite have been detected in few
dolomite samples from the San Giovanni mining area.
Variable amounts of smithsonite were identified in the
Cungiaus (Monteponi mine) and Buggerru (Malfidano
mine) samples.
Table 1 Dolomite, Zn
dolomite, and smithsonite
samples from several localities
of southwestern Sardinia
Sample Location Mineral species
Bugr Buggerru Smithsonite
CP 2 Campo Pisano-Iglesias ‘‘Saddle’’ dolomite
Cung Monteponi-Cungiaus Smithsonite
Cung 2 Monteponi-Cungiaus Smithsonite, Zn dolomite, dolomite
DG 5B San Giovanni-Gonnesa ‘‘Saddle’’ dolomite
GT25-B San Giovanni-Gonnesa White ‘‘Saddle’’ dolomite
GT25-GR San Giovanni-Gonnesa Gray dolomite
GT26-B San Giovanni-Gonnesa White ‘‘Saddle’’ dolomite
GT26-GR San Giovanni-Gonnesa Gray dolomite
MP-TC Monteponi-Iglesias ‘‘Saddle’’ dolomite
M Poni 2 Monteponi-Iglesias Dolomite[Zn dolomite
Malf Buggerru-Malfidano Dolomite[Zn dolomite
Malf 5 Buggerru-Malfidano Dolomite[Zn dolomite
NEB1 Nebida Zn dolomite[dolomite
NEB6 Nebida Dolomite[Zn dolomite
NEB7 Nebida Zn dolomite[dolomite
PS ? 55 Buggerru-Planu Sartu Zn dolomite[dolomite
PSV1 Buggerru-Planu Sartu Zn dolomite[dolomite
PSV2 Buggerru-Planu Sartu Zn dolomite[dolomite
PSV3 Buggerru-Planu Sartu Zn dolomite[dolomite
PSV4 Buggerru-Planu Sartu Zn dolomite[dolomite
SG-GON-DG1 San Giovanni-Gonnesa ‘‘Saddle’’ dolomite, Fe-hydroxides
SG-GON-DG2 San Giovanni-Gonnesa ‘‘Saddle’’ dolomite, Fe-hydroxides
S MAR Santa Margherita-Nebida Dolomite, Zn dolomite
S MAR 2 Santa Margherita-Nebida Dolomite, Zn dolomite
Int J Earth Sci (Geol Rundsch) (2013) 102:61–71 65
1 3
7/23/2019 Dolomita Zinciana relacionada a la Alteración Supergena
http://slidepdf.com/reader/full/dolomita-zinciana-relacionada-a-la-alteracion-supergena 7/12
7/23/2019 Dolomita Zinciana relacionada a la Alteración Supergena
http://slidepdf.com/reader/full/dolomita-zinciana-relacionada-a-la-alteracion-supergena 8/12
Table 2 Selected chemical analyses of three dolomite phases from southwestern Sardinia mining district
(a) (b) (c)
CaO 31.59 31.82 30.33 28.55 27.70 27.71 29.80 29.19 29.07 27.77
MgO 20.83 20.38 20.78 13.63 12.86 10.97 16.14 16.38 15.01 12.17
MnO N.D. N.D. 0.42 1.39 0.09 N.D. N.D. 0.24 0.19 0.34
FeO N.D. N.D. 3.35 12.18 N.D. 0.38 0.40 0.22 2.44 1.17
ZnO N.D. N.D. N.D. N.D. 14.95 17.33 4.42 6.95 8.18 12.79
CdO N.D. N.D. N.D. N.D. N.D. N.D. 0.30 0.04 N.D. 0.20
PbO N.D. N.D. N.D. N.D. N.D. N.D. 1.12 0.15 0.42 0.31
CO2* 47.48 47.92 45.05 44.27 44.37 43.59 47.78 46.80 44.70 45.34
Total 99.90 100.12 99.92 100.01 99.97 99.98 99.96 99.97 100.00 100.10
All the analyses were performed by energy-dispersive spectroscopy and all the chemical compositions are in oxide weight percentage.
N.D. = not determined
* Calculated from stoichiometry
(a) Stoichiometric Phase 1 dolomite (hydrothermal)
(b) Fe- to Zn-rich Phase 2 dolomites (hydrothermal)
(c) Phase 3 Zn dolomites related to supergene processes
Low Fe-Mndolomite
30 µm 20 µm
30 µm
40 µm 15 µm
30 µm
a b
c d
e f
stoichiometricdolomite
Zn-rich
dolomite
Zn-Fe-richdolomite
Low Fe-Mndolomite
Fe-hydroxides
calcite/ dolomite
remnantdolomite
calcite
supergeneZn dolomite
remnantdolomite
supergeneZn dolomite
smithsonite
Mg smithsonite
Fig. 3 a Monteponi mine.
Stoichiometric hydrothermal
Ca–Mg dolomite (hypogene,
phase 1), with a border of
(hypogene?) zincian dolomite
(phase 2). b San Giovanni mine.
Stoichiometric Ca–Mg dolomite
(hypogene, phase 1), with a
border of hydrothermal ferroan
dolomite (phase 2). c Malfidano
mine. Stoichiometric Ca–Mg
and low ferroan dolomite(hypogene), patchily
dedolomitized. d Malfidano
mine. Stoichiometric Ca–Mg
and low ferroan dolomite
(hypogene), patchily replaced
by calcite and Fe–
Mn(hydr)oxides, and locally, by
supergene Zn dolomite (phase
3). e Nebida mine. Hypogene
dolomite (phase 1), diffusely
replaced by supergene Zn
dolomite (phase 3); the white
spots are Fe(hydr)oxides.
f Nebida mine. Mg-rich
smithsonite concretions(supergene), associated with
phase 3 Zn dolomite
Int J Earth Sci (Geol Rundsch) (2013) 102:61–71 67
1 3
7/23/2019 Dolomita Zinciana relacionada a la Alteración Supergena
http://slidepdf.com/reader/full/dolomita-zinciana-relacionada-a-la-alteracion-supergena 9/12
Pb and Cd in the crystal lattice. They are also generally
depleted in Fe and Mn relative to precursor phases 1 and 2 of
hydrothermal dolomite ( Dolomia Geodica).
The carbon and oxygen isotope ratios of unweathered
(or slightly weathered) hydrothermal dolomite vary
between -1.5 and ?1.0 % VPDB and from -7.0 to
-10.0 % VPDB, respectively, and confirm earlier published
values (Boni et al. 2000). The oxygen isotopes values aredepleted in 18O with respect to d
18O values of Cambrian
early diagenetic intertidal dolomites and limestone (Boni
et al. 1988; 2000), but the d13C values are never lower than
-2 % VPDB. The oxygen isotope ratios, together with the
rather uniform cathodoluminescence pattern, indicate a
water-dominated fluid-flow system (Boni et al. 2000). The
smithsonite d18O values are within the range of the pub-
lished d18O (average value: -4 % VPDB) and d13C values
(-10.4 to -0.6 % VPDB; Boni et al. 2003), with the low
d13C values being characteristic for supergene smithsonites
(Gilg et al. 2008). The d13C and d18O values of supergene
phase 3 Zn dolomite plot between the d13C/ d18O field of hydrothermal dolomite and that of smithsonite-hydrozinc-
ite with the d13C values of zincian dolomite being com-
parable to those of smithsonite and hydrozincite. The large
variation in the carbon isotope values of zincian dolomite
(similar to those of smithsonite) combined with the
restricted range of oxygen isotope values suggests that only
meteoric waters were involved in the oxidation.
Differential thermal analysis confirms the results of
Zabinski (1980) and Mondillo et al. (2011). The dissocia-
tion of a stoichiometric dolomite is characterized by two
endothermic reactions representing the decomposition of
the MgCO3 component at around 800 C and the CaCO3
component at around 900 C (Fig. 5; Webb and Kruger
1970; Smykatz-Kloss 1974; Gunasekaran and Anbalagan
2007). However, the substitution of Mg2? by Zn2? in the
dolomite structure causes a decrease of the first endother-
mic reaction by about one hundred degrees in comparison
with pure dolomite (Hurlbut 1957). The DTA traces of
samples MALF and MP-TC (where Zn dolomite is scarce)
show the dissociation reaction of the MgCO3 at a tem-
perature of about 780 C, slightly less than the dehydration
value of stoichiometric dolomite (Gunasekaran and An-
balagan 2007). In the NEB1 sample, where the Zn value is
higher (Table 4; Fig. 6), this reaction occurs at about
710–720 C (Fig. 5). The difference of temperature of the
first endothermic reaction between the MALF and NEB1
samples should not only be due to the different Zn amount
registered by the chemical analysis (Table 4), but also to
the higher ‘‘Zn grade’’ of the NEB1 Zn dolomite, relative
to the MALF Zn dolomite (Fig. 6).
On the base of textural evidence, the zincian dolomite
phases from the supergene zone of sulfide zinc deposits in
the Iglesiente mining district are interpreted as a possible
smithsonite
Santa Barbara Fm.
Cambrian tidal dolomite
Dolomia Geodica
-12.0
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
2.0
4.0
-14.0 -12.0 -10.0 -8.0 -6.0 -4.0 -2.0 0.0
GT25-B, GT25-GR, GT26-B , GT26-GR, MP-TC: hydrothermal dolomiteM Poni 2, Malf, Malf 5, Bugr old1, NEB6 a, NEB6 b: hydrothermal dolomite > Zn dolomite
Bugr old2, NEB1 a, NEB1 b, NEB7, PS+55, PSV2: Zn dolomite > hydrothermal dolomite
Bugr, Cung: smithsonite
δ18O PDB
δ 1 3 C P DB
Fig. 4 Plot of d13
C versus d18
O for dolomites, smithsonites, and Zn
dolomites from southwestern Sardinia. Zincian dolomite is cogenetic
with smithsonite. The Tidal Dolomite Field comprises the values
published in Boni et al. (1988), the Dolomia Geodica Field comprises
the values published in Boni et al. (2000), and the smithsonite Field
comprises the values published in Boni et al. (2003). The Buggerru
dolomite values have been published in Boni et al. (2003). Symbols as
in Table 1
Table 3 Carbon and oxygen isotope ratios of hydrothermal Dolomia
Geodica, Zn dolomite, and smithsonite from southwestern Sardinia
Sample d13
C (% V-PDB) d18
O (% V-PDB)
Bugr old1 (Boni et al. 2003) -0.73 -8.80
Bugr old2 (Boni et al. 2003) -3.73 -6.98
Bugr -3.56 -2.90
Cung -6.46 -2.37GT25-B -0.02 -9.91
GT25-GR 0.33 -9.86
GT26-B 0.65 -10.60
GT26-GR 0.63 -10.67
MP-TC 1.06 -9.16
M Poni 2 0.66 -8.40
Malf 1.71 -8.11
Malf 5 0.96 -8.52
NEB1 a -5.93 -6.39
NEB1 b -6.41 -7.30
NEB6 a -1.96 -9.26
NEB6 b -1.44 -10.03
NEB7 -2.04 -5.04
PS ? 55 -1.53 -6.36
PSV2 -5.34 -5.73
68 Int J Earth Sci (Geol Rundsch) (2013) 102:61–71
1 3
7/23/2019 Dolomita Zinciana relacionada a la Alteración Supergena
http://slidepdf.com/reader/full/dolomita-zinciana-relacionada-a-la-alteracion-supergena 10/12
missing link between dolomite and smithsonite. This
interpretation is supported by the carbon isotope ratios of
the samples rich in zincian dolomite, which are very sim-
ilar to those of the Iglesiente smithsonites (Boni et al.
2003). The observation that samples poor in zincian
dolomite plot close to the Dolomia Geodica d18O field,
whereas samples rich in zincian dolomite are generally
enriched in 18O and plot closer to the smithsonite d18O fieldsuggests that the variation in zincian dolomite d18O can be
explained by different proportions of precursor dolomite
relative to newly formed zincian dolomite. It must be taken
into account that it was impossible to separate completely
the different dolomite phases before isotope analyses.
However, since the oxygen as well as carbon isotope ratios
of zincian dolomite-rich samples and smithsonite are
comparable, the formation of zincian dolomite is suggested
to have occurred under conditions that are comparable with
those of the formation of smithsonite. Formation from
meteoric waters is supported by low d13C values indicative
of the influence of soil-gas CO2 in near-surface environ-ments (Gilg et al. 2008). Temperatures during smithsonite
precipitation are estimated to have been between 11 and
23 C, assuming an oxygen isotopic composition of pre-
cipitation fluid in southwestern Sardinia of -6.5 %
VSMOW (Gilg et al. 2008).
Conclusions
The occurrence of zincian dolomites in the oxidation zone
of base metal sulfide deposits in SW Sardinia confirms thesupergene origin of these carbonates. There is strong evi-
dence that the oxidation profiles and related nonsulfide
mineral deposits evolved throughout late Tertiary and were
later displaced and rejuvenated by younger block tectonics.
The precipitation temperature of the Zn dolomite is inter-
preted to correspond to the temperature of the meteoric
fluids during the main weathering periods, when the sulfide
deposits were oxidized. We interpret the replacement of the
dolomite host as a multistep process, starting with a progres-
sive ‘‘zincitization’’ of the dolomite crystals, followed by a
patchy dedolomitization (resulting in the formation of calcite
and Fe-Mn-hydroxides), potentially concluded by the com-plete replacement of dolomite by smithsonite (Fig. 7).
This progressive ‘‘zincitization’’ phenomenon has been
described also in other dolomite-hosted zinc deposits, as
Jabali (Yemen) and Yanque (Peru) (Boni et al. 2011;
Mondillo et al. 2011). As it is the case in the above-men-
tioned mining districts, the extent of the replacement
bodies of Zn dolomite may be highly significant for the
exploration of nonsulfide Zn ores (Boni et al. 2011). In fact,
the amount of the total Zn contained in Zn dolomite
Table 4 Whole rock chemical analysis of the MP-TC, MALF, and
NEB1 samples
MP-TC MALF NEB1
SiO2 0.55 0.56 0.68
TiO2 0.01 0.06 0.01
Al2O3 0.00 0.19 0.00
FeO 5.40 5.52 9.68
MnO 0.22 0.44 1.16
MgO 13.85 9.72 7.53
CaO 32.34 34.63 27.37
Na2O 0.10 0.10 0.10
K 2O 0.01 0.12 0.06
P2O5 0.01 0.21 0.01
ZnO 0.98 3.50 11.25
LOI 45.52 44.93 42.16
Total 100.00 100.00 100.00
Compositions are in oxide weight percentage. LOI loss of ignition
Fig. 5 Differential thermal analysis curves of selected dolomite
samples from the Iglesiente mining district. In the box at the top of the
figure are highlighted two DTA reference traces: * Triassic dolomite
from Southern Italy, ** Zn dolomite from Tsumeb (Hurlbut 1957).
MALF Malfidano mine, MP-TC Monteponi mine, NEB1 Nebida mine
Int J Earth Sci (Geol Rundsch) (2013) 102:61–71 69
1 3
7/23/2019 Dolomita Zinciana relacionada a la Alteración Supergena
http://slidepdf.com/reader/full/dolomita-zinciana-relacionada-a-la-alteracion-supergena 11/12
(‘‘concealed’’ Zn), together with Zn measured from the
processable ore minerals (i.e., smithsonite and hydrozinc-
ite), might lead to a strong overestimation of the metallic
resources calculated from the assay data only (Mondillo
et al. 2011). This fact has to be kept in mind whenexploring for Zn nonsulfide ores in dolomite host rocks.
Acknowledgments This study has been carried out partly with a
PhD bursary of the University of Napoli to Nicola Mondillo. The
authors would like to thank R. de’ Gennaro (CISAG Napoli) for his
support during SEM–EDS analysis, L. Franciosi and L. Melluso for
their help with XRF analyses. A special thank is reserved to an
anonymous reviewer and to the Editor of the Journal, whose criticism
greatly improved the quality of the paper.
References
Bechstadt T, Boni M (eds) (1994) Sedimentological, stratigraphical and
oredeposits fieldguideof theautochthonous Cambro-Ordovician of
Southwestern Sardinia. Servizio Geologico d’Italia Memorie
Descrittive Carta Geologica d’Italia, v. XLVIII, 434 pp
Boni M (1985) Les gisements de type Mississippi Valley du Sud
Ouest de la Sardaigne (Italie): une synthese. Chronique recher-
ches minieres BRGM 489:7–34
Boni M, Iannace A, Pierre C (1988) Stable isotopes in the lowerCambrian ore deposits and their host rocks in SW Sardinia.
Chem Geol (Isotope Geoscience) 72:267–282
Boni M, Iannace A, Koppel V, Hansmann W, Fruh-Green G (1992)
Late- to post Hercynian hydrothermal activity and mineralization
in SW Sardinia. Econ Geol 87:2113–2137
Boni M, Iannace A, Balassone G (1996) Base metal ores in the lower
Palaeozoic of South-Western Sardinia. Econ Geol 75th Anniv
Vol, Spec Publ 4:18–28
Boni M, Parente G, Bechstadt T, De Vivo B, Iannace A (2000)
Hydrothermal dolomites in SW Sardinia (Italy): evidence for
a widespread late-Variscan fluid flow event. Sed Geol 131:181–200
Boni M, Gilg HA, Aversa G, Balassone G (2003) The ‘‘calamine’’ of
SW Sardinia: geology, mineralogy and stable isotope geochem-
istry of a supergene Zn mineralization. Econ Geol 98:731–748
Boni M, Dinares-Turell J, Sagnotti L (2005) A first attempt topaleomagnetic dating of non-sulfide Zn ores in SW Sardinia
(Italy). Ann Geophys 48(2):1–12
Boni M, Mondillo N, Balassone G (2011) Zincian dolomite: a
peculiar dedolomitization case? Geology 39:183–186
Brusca P, Dessau G (1968) I giacimenti piombo-zinciferi di S.Giovanni
(Iglesias) nel quadro della geologiadel Cambrico sardo. L’Industria
Mineraria 19(9):470–494, (10):533–556, (11):597–609
Carmignani L, Carosi R, Di Pisa A, Gattiglio M, Musumeci G,
Oggiano G, Pertusati PC (1994) The Hercynian chain in
Sardinia. Geodin Acta 7:31–47
CaZn(CO3)2
CaMg(CO3)2 Ca(Fe,Mn)(CO3)2
Ca(Mg,Zn)(CO3)2
CaZn(CO3)2
CaMg(CO3)2 Ca(Fe,Mn)(CO3)2
Malfidano - MALF
M. Poni - MP-TC
Nebida - NEB1 -Boni et al. 2011
Fig. 6 Composition of dolomites of the MP-TC, MALF, and NEB1 samples, in the system CaMg(CO3)2–Ca(Fe,Mn)(CO3)2–CaZn(CO3)2(SEM–EDS analyses)
Primary Sulfides
Dolomite ph1
Dolomite ph2
Zn dolomite ph3
Calcite
Hydroxides
Smithsonite
Pre-Variscan
V a r i s c a
n c o m p r e s s i v e t e c t o n i c s
Permian(?) Tertiary Recent
Mg smithsonite
Fig. 7 Paragenesis of the main mineralogical phases observed in the
SW Sardinia nonsulfide Zn district
70 Int J Earth Sci (Geol Rundsch) (2013) 102:61–71
1 3
7/23/2019 Dolomita Zinciana relacionada a la Alteración Supergena
http://slidepdf.com/reader/full/dolomita-zinciana-relacionada-a-la-alteracion-supergena 12/12
Garavelli CG, Vurro F, Fioravanti GC (1982) Minrecordite, a new
mineral from Tsumeb. Mineralog Rec 13:131–136
Gasparrini M, Bechstadt T, Boni M (2006) Massive hydrothermal
dolomites in the southwestern Cantabrian Zone (Spain) and their
relation to theLate Variscan evolution. MarPetrol Geol 23:543–568
Gilg HA, Boni M, Hochleitner R, Struck U (2008) Stable isotope
geochemistry of carbonate minerals in supergene oxidation
zones of Zn-Pb deposits. Ore Geol Rev 33:117–133
Gunasekaran S, Anbalagan G (2007) Thermal decomposition of
natural dolomite. Bull Mater Sci 30:339–344
Hurlbut CS Jr (1957) Zincian and plumbian dolomite from Tsumeb,
South-West Africa. Am Mineral 42:798–803
Kim ST, Mucci A, Taylor BE (2007) Phosphoric acid fractionation
factors for calcite and aragonite between 25 and 75 C: revisited.
Chem Geol 246:135–146
Melluso L, Morra V, Brotzu P, Tommasini S, Renna MR, Duncan RA,
Franciosi L, D’Amelio F (2005) Geochronology and petrogenesis
of the Cretaceous Antampombato-Ambatovy complex and asso-
ciated dyke swarm, Madagascar. J Petrol 46:1963–1996
Mondillo N, Boni M, Balassone G, Grist B (2011) In search of the lost
zinc: a lesson from the Jabali (Yemen) nonsulfide zinc deposit.
J Geoch Expl 108:209–219
Moore JMcM (1972) Supergene mineral deposits and physiographic
development in southwest Sardinia, Italy. Trans Inst Min and
Metallurgy (Sect B: Appl Earth Sci) B81:59–66
Rosenbaum J, Sheppard SM (1986) An isotopic study of siderites,
dolomites and ankerites at high temperatures. Geochim Cosmo-
chim Acta 50:1147–1150
Smykatz-Kloss W (1974) Differential thermal analysis: application
and results in mineralogy. Springer, NY
Webb TL, Kruger JE (1970) Carbonates. In: Mackenzie RC (ed)
Differentialthermalanalysis. Academic Press,London, pp 303–341
Zabinski W (1959) Zincian dolomite from the Warynski mine, Upper
Silesia. Bulletin de l’Academie Polonaise des Sciences, S Sci
Chim, Geol et Geogr 7:355–358
Zabinski W (1980) Zincian dolomite: the present state of knowledge.
Mineralog Pol 11:19–31
Int J Earth Sci (Geol Rundsch) (2013) 102:61–71 71
1 3