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Zincian dolomite related to supergenealteration in the Iglesias mining district (SWSardinia)

 ARTICLE · JANUARY 2013

DOI: 10.1007/s00531-012-0785-0

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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: boni@unina.it

M. Joachimski

GeoZentrum Nordbayern University of Erlangen-Nuremberg,

Schlossgarten 5, 91054 Erlangen, Germany

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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)

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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

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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 %

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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

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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

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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

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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

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(‘‘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.

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