Apparent stable isotope heterogeneities in gangue carbonates of the Mississippi Valley-type Zn-Pb deposit of San Vicente, central Peru

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Reference

Apparent stable isotope heterogeneities in gangue carbonates of the Mississippi Valley-type Zn-Pb deposit of San Vicente, central Peru

SPANGENBERG, Jorge Enrique, SHARP, Zachary D., FONTBOTÉ, Lluís

Abstract

The aim of the present communication is to emphasize that some variations of the measured

∂13C and ∂180 values are apparent, and due to analytical interferences caused by the presence of sulfur and organosulfur compounds in the analyzed carbonates. This is particularly relevant for isotopic studies on carbonate-hosted mineral deposits, where the nearly ubiquitous association of the host carbonates with organic matter and sulfides can certainly affect the metallogenetic interpretations. In this work two methods were used to overcome the disturbing effects of sulfides and organic matter: (1) sample pretreatment following the method proposed by Charef and Sheppard (1984), combining the oxidation of organic matter with sodium hypochlorite and trapping of the sulfur species with silver phosphate; and (2) laser-based microprobe extraction. Apparent isotopic variations in sparry dolomite from a single hand sample of zebra ore from the MVT Zn-Pb deposit, San Vicente, central Peru, are as large as 6 per mil ∂13C and 4per mil ∂180. These variations are reduced to several tenths of a per rail when the samples are pretreated. [...]

SPANGENBERG, Jorge Enrique, SHARP, Zachary D., FONTBOTÉ, Lluís. Apparent stable isotope heterogeneities in gangue carbonates of the Mississippi Valley-type Zn-Pb deposit of San Vicente, central Peru. Mineralium Deposita , 1995, vol. 30, p. 67-74

DOI : 10.1007/bf00208878

Available at:

http://archive-ouverte.unige.ch/unige:78927

Disclaimer: layout of this document may differ from the published version.

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Mineral. Deposita 30, 67-74 (1995)

MINERALIUM DEPOSITA

9 Springer-Verlag 1995

Apparent stable isotope heterogeneities in gangue carbonates of the Mississippi Valley-type Zn-Pb deposit of San Vicente, central Peru

J. Spangenberg 1, Z.D. Sharp z, and L. Fontbot~ 1

t D6partement de Min~ralogie, Universit6 de Gen~ve, 13, rue des Maraichers, CH-1211 Gen~ve 4, Switzerland 2 Institut de Min6ralogie, Universit~ de Lausanne, CH-1015 Lausanne, Switzerland

Received: 29 September 1993/Accepted: 8 March 1994

Abstract. The aim of the present communication is to emphasize that some variations of the measured 613C and 6180 values are apparent, and due to analytical interferen- ces caused by the presence of sulfur and organosulfur compounds in the analyzed carbonates. This is parti- cularly relevant for isotopic studies on carbonate-hosted mineral deposits, where the nearly ubiquitous association of the host carbonates with organic matter and sulfides can certainly affect the metallogenetic interpretations. In this work two methods were used to overcome the dis- turbing effects of sulfides and organic matter: (1) sample pretreatment following the method proposed by Charef and Sheppard (1984), combining the oxidation of organic matter with sodium hypochlorite and trapping of the sulfur species with silver phosphate; and (2) laser-based microprobe extraction. Apparent isotopic variations in sparry dolomite from a single hand sample of zebra ore from the MVT Zn-Pb deposit, San Vicente, central Peru, are as large as 6%0 613C and 4%0 6180. These variations are reduced to several tenths of a per rail when the samples are pretreated. A careful examination of the effects of treatment with NaOC1 and/or AgaPO4 in relation to the concentration of sulfide inclusions indicates that the main disturbing effects for 613 C values are the presence of sulfur species and organic matter, whereas the 6180 values are mainly affected by the presence of sulfides. Fine- and medium-grained replacement carbonates from MVT and other sediment-hosted base metal deposits are potentially the most affected during isotope analysis, due to the com- mon presence of organic matter and sulfides. Using in situ laser microprobe techniques, it is possible to determine isotopic variations at a sub-millimeter scale. Our results show that laser extraction analysis allows a more precise sampling of the carbonate minerals, and minimizes con- tamination of the sample with sulfides and to some extent with intergrown organic matter. However, there is an isotopic shift associated with the laser extraction tech- nique, of the order of 0.5-1%o for 613C and 6180 values.

Changes in the stable isotope composition of host and gangue carbonates of Mississippi Valley-type (MVT)

Pb-Zn deposits are mainly explained as an effect of chang- ing fluid composition, temperature, water/rock ratio, or some combination of these effects (e.g. Hall and Friedman 1969; Pinckney and Rye 1972; Sverjensky 1981; Hannah and Stein 1984; Machel 1987; Gregg and Shelton 1989;

Rowan and Leach 1989; Frank and Lohman 1986; Ghaz- ban et al. 1990). In general, it has been observed that with advancing diagenetic stages, the carbon and oxygen iso- tope ratios are lowered (Machel 1987; Banner et al. 1988;

Kaufman et al. 1990, 1991; Fontbot6 and Gorzawski 1990;

Moritz et al. 1993).

In the course of an ongoing investigation on the geo- chemical zonation in gangue carbonates of the Mississippi Valley-type Zn-Pb district San Vicente, central Peru, we attempt to trace potential small carbon and oxygen iso- topic variations (e.g. less than 2%0) at the deposit and the district scale that could reflect basinal brine migration.

For this purpose, defined carbonate generations have been sampled and analyzed from different parts of the district. Preliminary results confirm the general tendency toward lower isotope ratios with advancing diagenetic stages, but reveal variations within single hand specimens of the same order as those observed over the entire deposit (in the range of - 4.1 to 2%0 613C and - 10.3 to - 5.9%0 6taP, see Fontbot~ and Gorzawski 1990). These major isotopic heterogeneities at hand specimen scale may pre- clude the recognition of subtle isotopic variations at ore- body, deposit, or district scale.

The aim of the present communication is to demon- strate that some of the variations of the measured 613C and 6180 may be

apparent,

and due to analytical inter- ferences caused by the presence of sulfides and organic matter in the analyzed carbonates. Sulfides and organic matter, which are abundant in most carbonate-hosted base metal deposits, can strongly affect the carbon and oxygen isotope analyses (e.g. Smith and Croxford 1975;

Weber et al. 1976; Rye and Williams 1981; Charef and

Sheppard 1984), and may account for the large isotopic

variations at a small scale. We want to emphasize the need

for controlling the influence of organic, organosulfur, and

sulfur species of the measured 6t3C and 6180 values

of carbonates. This is particularly relevant for isotopic

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68

studies o n c a r b o n a t e - h o s t e d m i n e r a l deposits, where the n e a r l y u b i q u i t o u s a s s o c i a t i o n o f the h o s t c a r b o n a t e s with o r g a n i c m a t t e r a n d sulfides c a n c e r t a i n l y affect the m e t a l - l o g e n e t i c i n t e r p r e t a t i o n s .

In this w o r k t w o m e t h o d s were used to o v e r c o m e the d i s t u r b i n g effects o f sulfides a n d o r g a n i c m a t t e r : (1) s a m p l e p r e t r e a t m e n t f o l l o w i n g the m e t h o d p r o p o s e d b y C h a r e f a n d S h e p p a r d (1984), c o m b i n i n g the o x i d a t i o n o f o r g a n i c m a t t e r with s o d i u m h y p o c h l o r i t e a n d t r a p p i n g of the sulfur species with silver p h o s p h a t e ; a n d (2) laser- b a s e d m i c r o p r o b e e x t r a c t i o n .

T h e i s o t o p i c v a r i a t i o n s at the c e n t i m e t e r to sub-milli- m e t e r scale were e v a l u a t e d with a c o m b i n a t i o n of a system- atic d e t a i l e d s a m p l i n g using c o n v e n t i o n a l a n d in situ laser m e t h o d s . T r e a t m e n t for o r g a n i c m a t t e r a n d sulfur allows us to r e c o g n i z e slight v a r i a t i o n s of i s o t o p i c r a t i o s (i.e. less t h a n 0.5%o) in a d e f i n e d g e n e r a t i o n o f the h o s t c a r b o n a t e s in San Vicente t h r o u g h the o r e - b o d i e s , ore deposits, o r ore district, which o t h e r w i s e m i g h t be o v e r l o o k e d .

Geologic background

The geology and genetic aspects of the San Vicente Zn-Pb deposit, including Sr, C, and O isotope studies in the carbonate gangue

minerals, are discussed in Fontbot6 and Gorzawski (1990) and Moritz et al. (1993). The ore occurs as lens-shaped bodies, generally concordant with the bedding, and displays a characteristic rhythmic banding, known as zebra rock or diagenetic crystallization rhythmites (Fontbot6 and Gorzawski 1990; Fontbot6 1993). The main ore minerals, sphalerite and galena, are interbanded with white sparry dolomite and grey replacement dolomite. Three distinct dolomite types occur in the host rocks: dark-grey replacement dolomite, ore-stage sparry dolomite, and late-stage void-filling dolomite.

The early stage replacement dolomite, referred to as generation I in Fontbot6 and Gorzawski (1990), is a very fine- to medium- grained, dark-grey dolomite, with abundant inclusions of organic

Table 1. Range of organic carbon and sulfur contents of gangue carbonates in the San Vicente MVT deposit. The number of analyzed samples (by LECO pyrolisis) is given in parentheses

Carbonate Corg (wt. %) S (wt. %)

generation

Dark replacement 0.09-5.2 0.13-2.1

dolomite (5)

White sparry < 0.014).09 0.07-0.08

dolomite (5)

Late filling < 0.01-0.05 < 0.01-0.02

carbonate (3)

Fig. 1. Representative hand sample (FSV-924) of zebra ore showing the sampled sites. Detail of the in situ laser microsampled area. Individual bands (letters) and specific analysis locations (numbers) correspond with sample flames in tables

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matter. In some ore-bearing samples this stage I dolomite is com- pletely replaced by sphalerite and, less commonly, by galena. The total organic carbon and sulfur contents in this dark replacement dolomite are up to 5.2 wt.% and 2.1 wt.%, respectively (Table 1).

The sparry dolomite consists of coarse or very coarse, subhedral, light-grey to white or light-brown dolomite, and corresponds to generation II of Fontbot6 and Gorzawski (1990). This dolomite type occurs as cement in moldic and vuggy porosity, or as bands between the dark replacement dolomite, giving the characteristic rhythmic banding of the zebra rock (Fig. 1). Zonal crystal growth can often be recognized as a simple color difference, due to a low amount of disseminated sulfides (up to 0.08 wt.% total sulfur) and organic matter (up to 0.09 wt.% total organic carbon). The precipitation of sparry dolomite is interpreted as a late diagenetic event, having formed from highly saline basinal brines (Fontbot6 and Gorzawski 1990; Moritz et al. 1993). Sphalerite mineralization is mainly paragenetically associated with this sparry dolomite generation.

Late-stage open-space-filling dolomite is a coarse- to very coarse- grained, milky-white, xenomorphic dolomite, virtually free of organic matter and sulfides (Table 1). It occludes voids in white sparry dolomite, and correlates with the generation III dolomite of Fontebot6 and Gorzawski (1990). Locally, this late-stage diagenetic crystallization generation consist of calcite, galena, or bitumen in- stead of dolomite. In the samples selected for this study the filling carbonate is exclusively dolomite.

Analytical procedures

Three different analytical procedures have been compared: (1) conventional acid CO2-extraction without pretreatment: (2) conventional CO2-extraction with sodium hypochlorite and silver phosphate pretreatment; and (3) in situ laser microprobe CO2-extraction without treatment. Treatment of laser-extracted COz with silver phosphate was attempted, but a successful method was not found.

Conventional method

Individual dolomite types were selectively sampled from roughly polished slabs using a drilling device. The holes of a diameter of about 1-3 mm were aligned along profiles covering the different dolomite generations.

All isotope analyses were carried out at the Laboratory of Iso- topic Geochemistry at the University of Lausanne. Carbon and oxygen isotope analyses were performed following standard carbon dioxide extraction techniques of McCrea (1950). Approximately 10 mg of fine-grained dolomite was dissolved in vacuum at 50 ~ by 100% phosphoric acid. The released carbon dioxide was trapped in liquid nitrogen and cleaned by means of a fractionate sublimation using a dry ice-ethanol mixture. The gas was measured on a Finnigan | MAT-251 mass spectrometer. Data were corrected for fractionation using the carbonate-acid fractionation factor for dolomite of 1.01065 (Rosenbaum and Sheppard 1986) and are expressed conventionally in the 6 notation as the variation in per mil relative to the PDB standard. Analytical reproducibility, reported as standard deviations of replicate analyses of the laboratory working standard Binn Dolomite, is better than +_ 0.05%0 for carbon and

+ 0.1%o for oxygen.

Sodium hypoehlorite and silver phosphate pretreatment Earlier publications report the oxidation of organic matter by sodium hypochlorite (e.g. Weber et al. 1976; Land et al. 1977) and the removal of acid-volatile sulfur compounds by a silver phosphate pretreatment (e.g. Smith and Croxford 1975; Rye and Williams 1981). We have used the pretreatment procedure proposed by Charef and Sheppard (1984) to minimize the effects of organosulfur

Table

2. Average and standard deviation of conventional carbon and oxygen isotope analyses of laboratory standard Binn Dolomite.

Pretreatment (n) 813C%oPDB 3 1 8 0 % o PDB

None (21) 1.55 (0.04) --9.41 (0.10)

NaOCI + Ag3PO4 (8) 1.61 (0.03) -- 9.46 (0.06) Ag3PO4 (8) 1.61 (0.05) - 9 . 4 8 (0.10)

species. It consists of the combined oxidation of organic matter by sodium hypochlorite (NaOCI) followed by silver phosphate (Ag3PO4) treatment of the phosphoric acid evolved gas.

The powdered samples were reacted with 5% fresh NaOC1 solution for 15 h at room temperature, filtered, washed several times with distilled water, and dried at 80 ~ for 4 h. The gas released by the phosphoric acid was exposed to coarsely powdered Ag3PO4 at room temperature for 10 rain to eliminate the evolved H2S. The Ag3PO4 was dried at 150 ~ for 30 rain and left in the vacuum line for at least another 30 min prior to extraction.

No significant differences of the isotope ratios of the laboratory dolomite standard were observed by using the Ag3PO4- or the combined NaOC1 + Ag3PO4 pretreatment, within the analytical precision of +_ 0.05%0 for 613C and +_ 0.1%o for 8~80 (Table 2). As stated by Charef and Sheppard (1984), the sodium hypochlorite treatment does not necessarily remove all types of organic carbon in the carbonate samples, but it certainly destabilizes all 'active' organosulfur compounds, sensitive to oxidation by the 5% NaOC1 solution, leaving only stable organic carbon species.

In situ laser microprobe determination

In situ laser measurements, following the procedure of Sharp (1992) were made at the University of Lausanne to study the isotopic zonation at sub-millimeter scale across a single 1-cm-thick band of white sparry dolomite. The CO2-extractions were carried out at 100% laser power (20 W). Twenty laser shots, each of 30 ms dura- tion, performed in two closely neighboring sites, produced 150-200 lain-diameter holes (Fig. 1). The evolved COa was trapped with liquid N2, cryogenically purified, and then transferred on-line to the mass spectrometer for isotopic analysis. The precision of the oxygen and carbon isotope ratios reported for homogeneous sam- pies is better than +_ 0.1%o and _+ 0.2%0, respectively (Sharp 1992).

The procedure for the in situ measurements with Ag3PO4 treatment were identical to the conventional in situ method except that a cold finger with Ag3PO4 was included in the extraction line. The Ag3PO4 treatment was similar to that described for conventional analyses. Various methods of heating and degassing of the Ag3PO4 were attempted in order to remove the isotopic shift caused by the phosphate interaction; none was found to alleviate the problem.

Results and discussion

T w o r e p r e s e n t a t i v e o r e - b e a r i n g h a n d specimens with m e g a s c o p i c a l l y r e c o g n i z a b l e different d o l o m i t e crystalli- z a t i o n g e n e r a t i o n s from the S a n Vicente m i n e have been a n a l y z e d in detail. T h e isotopic analyses of a z e b r a ore s a m p l e with typical r h y t h m i c b a n d i n g are given in Fig.

1 a n d T a b l e s 3 a n d 4. T h r e e sets of analyses are c o m p a r e d : c o n v e n t i o n a l acid e x t r a c t i o n m e t h o d w i t h o u t p r e t r e a t - m e n t , with p r e t r e a t m e n t , a n d laser extraction.

A c o m p a r i s o n b e t w e e n the results of the c o n v e n t i o n a l isotopic analyses with a n d w i t h o u t p r e t r e a t m e n t (Fig.

2 a n d T a b l e 4) shows a d r a m a t i c difference. T h e use of the c o m b i n e d N a O C 1 + A g 3 P O 4 p r e t r e a t m e n t for the

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Table 3. Variation of 613C and 6180 values of gangue dolomites of a zebra ore sample a with method of pretreatment

Sample name ~13C~oo P D B 61sO~oo PDB

Pretreatment Pretreatment

N o n e NaOC1 + Ag3PO4 Ag3PO4 None NaOC1 + Ag3PO4 Ag3PO~

Dark replacement dolomite

G1 7.9 0.0 1.1 - 7.0 - 8.7

I1 1.5 0.8 1.2 - 7.9 - 8.1

K1 2.0 0.7 - 8.4 - 8.9

G3 8.4 0.3 1.3 - 6.3 - 8.7

13 1.7 1.l - 7.2 - 7.7

14 1.4 0.7 - 8.4 - 8.5

G2 0.4 - 8.9

I2 0.4 - 8.0

K2 1.0 - 8.6

Average 3.8 0.6 1.2 - 7.5 - 8.4

(lo) (3.4) (0.4) (0.1) (0.8) (0.4)

Light-grey sparry dolomite

C1 2.0 0.8 - 7.1

D1 1.3 0.7 0.7 - 8.0 - 8.0

E1 18.7 b 0.5 0.7 - 4.1 - 8.0

F1 1.5 1.1 1.3 - 8.2 - 8.1

H1 1.3 1.0 1.3 - 8.2 - 8.3

J1 1.4 1.0 1.3 - 7.6 - 8.1

L2 1.5 1.3 - 7.8

C3 1.3 0.7 1.2 - 7.9 - 9.4

D2 5.4 0.7 1.2 - 6.5 - 8.1

E2 3.1 0.8 1.0 - 5.2 - 8.0

F2 1.3 0.9 1.0 - 8.7 - 9.1

H2 0.8

J2 1.2

A1 1.2

B1 1.2

C4 1.1

D3 0.7

F3 1.5

H3 1.3

H4 1.2

H5 1.0

L4 1.3

Average 2.0 0.8 1.1 - 7.2 - 8.3

(la) (1.3) (0.2) (0.3) (1.4) (0.5)

White late-stage void-filling dolomite

L1 1.1 1.0 - 11.5

C2 1.3 1.1 - 10.8

J3 1.7 1.1 - 10.0

J4 1.1

Average 1.4 1.1 - 10.8

(1~) (0.3) (0.1) (0.7)

- - 8 . 3 - - 8 . 5

- 8.6

--8.5 (0.2)

- - 7.8

- 9 . 1

- 8.0

--8.3

- 8.2

- 7.7

- 8 . 1 - 8 . 1

- 7.7

- 9 . 3

- - 9.0

- 8 . 4

- 8 . 2

- 8.8

- 8 . l --8.5

- - 8.4

- 8 . 4

- 7 . 8

- 8 . 3

- - 8.2

--8.3 (0.4)

- 1 1 . 6

- 1 1 . 3

- 1 1 . 2

- - 1 1 . 1 - 11.3

(0.2) a Sample FSV-924 from San Vicente N mine, level 1570, local coordinates 20602 N and 19797 E

b Value excluded from average and from Fig. 2

r e p l a c e m e n t d o l o m i t e a n d o f t h e A g 3 P O 4 t r a p f o r t h e s p a r r y d o l o m i t e a n d l a t e - s t a g e v o i d - f i l l i n g d o l o m i t e e l i m - i n a t e s m o s t o f t h e h e t e r o g e n e i t i e s f o u n d i n t h e n o n - t r e a t e d s a m p l e s . W i t h o u t p r e t r e a t m e n t , t h e d a r k r e p l a c e m e n t d o l - o m i t e d i s p l a y s a s t r o n g v a r i a t i o n o f fi 13 C ( b e t w e e n 1.4 a n d 8 . 4 ~ o ) a n d 6 a S o ( b e t w e e n - 8.4 a n d - 6.3%~ T h e t w o m o s t e r r a t i c 6 1 3 C v a l u e s (7.9%~ a n d 8.4Too) a r e p r o b a b l y m a i n l y d u e t o t h e p r e s e n c e o f s u l f u r s p e c i e s a s s h o w n b y t h e s t r o n g d e c r e a s e o f t h e v a l u e s a c h i e v e d w i t h t h e A g 3 P O g - o n l y t r e a t m e n t . I n t h e s e s a m e t w o s a m p l e s t h e o x y g e n i s o t o p e r a t i o s a r e c l e a r l y l o w e r e d w i t h t h e

A g 3 P O 4 p r e t r e a t m e n t . T h e u s e o f t h e c o m b i n e d p r e t r e a t - m e n t d e c r e a s e s s l i g h t l y b u t c o n s i s t e n t l y t h e 6 1 3 C v a l u e s a n d h a s n o c l e a r i n f l u e n c e o n t h e 6 l s o v a l u e s , s u g g e s t i n g t h a t t h e m a i n d i s t u r b i n g effects f o r t h e c a r b o n i s o t o p i c r a t i o s a r e d u e t o t h e p r e s e n c e o f o r g a n i c m a t t e r a n d s u l f u r w h e r e a s t h e o x y g e n i s o t o p e r a t i o s a r e d i s t u r b e d m a i n l y b y t h e p r e s e n c e o f sulfides.

T h e l i g h t - g r e y s p a r r y d o l o m i t e is v i r t u a l l y f r e e o f o r - g a n i c m a t t e r b u t b e a r s v a r i a b l e a m o u n t s o f sulfides. A c - c o r d i n g l y , i m p o r t a n t d i f f e r e n c e s a r e a l r e a d y a c h i e v e d w i t h t h e A g 3 P O 4 - o n l y t r e a t m e n t , w i t h t h e h e l p o f w h i c h , f o r

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Table 4. In situ stable isotope-analyses of the zebra ore sample using a laser extraction technique

Analysis number a 613 C %0 PDB 61 s o ~o PDB

1 LC43-15 1.5 - 7.4

2 LC43-3 1.3 - 7.3

3 LC43-4 1.4 (0.1) - 7.4

4 LC43-5 1.1 - 6.3

5 LC43-6 0.9 - 7.6

6 LC43-9 0.8 - 7.9

7 LC43-8 0.7 - 7.2

8 LC43-10 0.8 - 7.9

10 LC43-12 b 1.5 - 7.9

13 LC43-2 1.3 - 7.9

(0.1)

average(la) 1.0 (0.3) - 7 . 5 (0.6)

9 LC43-11 r 0.5 - 10.6

11 LC43-14 0.3 - 11.1

12 LC43-13 ~ 0.6 - 10.3

average (la) 0.5 (0.2) - 10.7 (0.4)

a Location of the analysed microsamples are shown in Fig. 1 b Sample site rich in sphalerite

Possibly contaminated by sparry dolomite

i n s t a n c e , i n s t e a d of the e r r a t i c a l l y h e a v y 613C v a l u e o f 18.7%o a r a t i o of 0.7Too is o b t a i n e d . T h e results s h o w t h a t the t r e a t m e n t with N a O C 1 is n o t n e c e s s a r y for this d o l - o m i t e g e n e r a t i o n .

T h e use of the a b o v e d e s c r i b e d p r e t r e a t m e n t p r o d u c e s less i m p o r t a n t c h a n g e s in the i s o t o p i c v a l u e s of the v e r y c l e a n void-filling d o l o m i t e , b u t e l i m i n a t e s t h e still signifi- c a n t d i s t u r b a n c e s (see T a b l e 3 a n d Fig. 1).

V a r i a t i o n s in the

t~13C

a n d 6 1 8 0 v a l u e s at a sub- m i l l i m e t e r scale were e v a l u a t e d w i t h the in s i t u laser ex- t r a c t i o n technique. T h e r a n g e in the 6 1 8 0 v a l u e s for the d a r k r e p l a c e m e n t d o l o m i t e a n d l i g h t - g r e y s p a r r y d o l o m i t e are s m a l l ( - 7.9 t o - 7.2%o, e x c e p t for o n e a n o m a l o u s l y h i g h v a l u e at the c o n t a c t b e t w e e n these t w o c a r b o n a t e g e n e r a t i o n s , T a b l e 4). As e x p e c t e d , t h e 6 1 8 0 v a l u e s for the l a t e - s t a g e d o l o m i t e a r e 3%~ l i g h t e r t h a n the e a r l i e r d o l - o m i t e g e n e r a t i o n s . T h e 6~3C v a l u e s for t h e s p a r r y d o l - o m i t e a r e in g o o d a g r e e m e n t w i t h the c o n v e n t i o n a l pre- t r e a t e d analyses, while the 613C v a l u e s for the d a r k r e p l a c e m e n t d o l o m i t e are closer to t h e u n p r e t r e a t e d values. T h e l a t e - s t a g e v o i d - f i l l i n g d o l o m i t e is d e p l e t e d in

13C r e l a t i v e to the c o n v e n t i o n a l analyses.

9

OG O0

a

-3,

-4 -5, -6, -7, -8, -9, -10, -11, -12,

8,

6 84

4.

t~ 2

0

b -2

0

W i t h o u t p r e t r e a t m e n t

[ ]

I 9 replacement dolomite [] sparry dolomite ]..O late-sta~e dolomite

9 [] []

I I

~ []

0 9

[] 9

O n [] []

[]

0

[]

0

[]

[] 9 nno~

10 20 30 40 5b 60 7() 8() 90

Distance (mm)

Fig, 2. Variations of 613C (a) and 6180 (b) values in replacement dolomite, sparry dolomite and late-stage void-filling dolomite of a zebra ore hand sample (FSV-924) using conventional acid extrac-

W i t h c o m b i n e d p r e t r e a t m e n t

[ ]

o

[]

^{[ ]}

I

-3 -4 -5 -6 -7

-8

-9 .10 --11 --12

8 6 4 2 0 -2

0 10 20 3() 4 50 60 70 80 90 100

Distance (ram)

tion with and without pretreatment. Distances are referred to the lower edge of the sample, as illustrated in Fig. 1

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72

Table 5. Summary of the average 613C and 6180 values and l~r of the gangue dolomite analyzed with different procedure

Dolomite generation (Sample FSV-924) Analytical procedure 613C~oPDB ~5180%oPDB

Conventional 3.8 (3.4) - 7 . 5 (0.8)

NaOC1 + Ag3PO4 0.6 (0.4) - 8.4 (0.4)

Ag3PO4 1.2 (0.3) - 8.5 (0.2)

In situ 1.4 (0.1) - 7.4 (0.1)

Conventional 2.0 (1.3) - 7 . 2 (1.4)

NaOC1 + Ag3PO4 0.8 (0.2) - 8.3 (0.5)

Ag3PO4 1.1 (0.3) - 8 . 3 (0.4)

In situ 1.0 (0.3) - 7.5 (0.6)

Conventional 1.4 (0.3) - 1 0 . 8 (0.7)

NaOC1 + Ag3PO4

Ag3PO 4 1.1 (0.1) - 1 1 . 3 (0.2)

In situ 0.5 (0.2) - 10.7 (0.4)

Table 6. Isotopic composition of the different stage dolomites of an ore sampl& using the combined pretreatment

Sample name 613C%o PDB 6180~o PDB

Dark replacement dolomite b

A1 1.4 - 8.3

A2 1.3 - 8.2

B1 1.0 ^- 9.2

B2 1.2 - 8.8

average (la) 1.2 (0.2) - 8.6 (0.5)

Light-grey sparry dolomite c

C1 0.7 - 10.8

C2 0.6 - 10.7

D1 0.4 - 10.6

D2 0.5 - 10.8

D3 1.0 - 10.2

D4 0.7 - 11.1

D5 0.5 - 11.8

D6 0.9 - 11.2

D7 0.8 - 11.1

D8 1.0 - 10.4

D9 1.0 - 10.4

average (la) 0.7 (0.2) - 1 0 . 8 (0.5) White late-stage void-filling dolomite ~

E1 0.0 - 10.9

E2 - 0.2 - 12.5

E4 0.1 - 12.4

E3 0.1 - 12.2

average (la) 0.0 (0.1) -- 12.0 (0.7)

"Sample FSV-919 from San Vicente N mine, level 1570, local coordinates 20561.5 N and 19795 E

b NaOC1 + Ag3PO4 - pretreatment c Only Ag3PO4 - pretreatment

T h e following c o n c l u s i o n s c a n be d r a w n r e g a r d i n g the in situ laser a n a l y s e s of sulfides- a n d o r g a n i c m a t t e r - r i c h c a r b o n a t e s ( T a b l e 5): (1) in s a m p l e s rich in o r g a n i c m a t t e r (e.g. d a r k r e p l a c e m e n t d o l o m i t e ) the 6 ~ 3C a n d 618 O values are e q u i v a l e n t to c o n v e n t i o n a l analyses, b u t different from samples p r e t r e a t e d to r e m o v e the d i s t u r b i n g effects of sulfur species a n d o r g a n i c c o n t a m i n a n t s ; (2) in samples rich o n l y in sulfides (e.g. light-grey s p a r r y dolomite) the 6~3C values are correct, b u t the 6 1 8 0 values are elevated b y ~ 1%o to those o b t a i n e d with p r e t r e a t m e n t ; (3) the lower cSa 3C values for the late-stage d o l o m i t e m a y reflect

Fig. 3. Hydraulic breccia (sample FSV-919) with grey-white sparry dolomite (C and D) surrounding the dark replacement dolomite (A and B), and milky-white dolomite (E) filling the open spaces

kinetic f r a c t i o n a t i o n effects c o m m o n to all laser analyses (Sharp 1992) or m a y be explained b y a m o r e precise m i c r o - s a m p l i n g t h a n c a n be o b t a i n e d with c o n v e n t i o n a l drilling; (4) simple Ag3PO4 t r e a t m e n t does n o t alleviate the p r o b l e m of sulfur c o n t a m i n a t i o n .

T h e results of a second r e p r e s e n t a t i v e h a n d specimen are presented in T a b l e 6 a n d Fig. 3. T h e c o m b i n e d NaOC1 a n d A g 3 P O 4 p r e t r e a t m e n t was used for the analyses of the d a r k r e p l a c e m e n t dolomite. T h e s p a r r y d o l o m i t e a n d the late-stage filling d o l o m i t e were o n l y subjected to the pre- t r e a t m e n t with Ag3PO4. C o n s i s t e n t clear t r e n d s w i t h o u t erratic values are illustrated, for example, in profiles D - E a n d E ' - A across the three d o l o m i t e g e n e r a t i o n s (Fig. 4).

T h e white s p a r r y d o l o m i t e exhibits significantly lighter

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Distance (mm)

Fig. 4. Cross section

(D-E

and

E'-A,

see Fig. 3) of carbon and oxygen isotope ratios, covering dark replacement dolomite (NaOC1 + Ag3PO4 pretreatment), grey-white sparry dolomite and late-stage void-filling dolomite (both Ag3PO4 pretreatment)

613C and filsO values than the early stage replacement dolomite, and heavier values than the paragenetically later open-space-filling dolomite. The decreasing isotopic ratios with increasing distance relative to the replacement dolomite m a y be explained by changes in the water to rock ratio, having precipitated the late-stage dolomite from an isotopic lighter fluid with a smaller contribution from the original replacement dolomite.

Conclusions

1. O u r results show that small-scale isotopic variations in the M V T gangue carbonates of San Vicente deposit are mainly due to the presence of sulfides in the carbonate sample, and that the associated organic matter plays a mi- nor role.

2. The combined sodium hypochlorite and silver phos- phate pretreatment, proposed by Charef and Sheppard (1984) is necessary for the accurate isotopic analysis of samples containing organic matter and sulfides, i.e. m o s t fine- and medium-grained carbonate samples from M V T and other sediment-hosted base metal deposits.

3. F o r the isotopic analyses of white sparry and open- space-filling dolomite, which are virtually free of organic matter, the treatment with silver phosphate is sufficient.

4. Using this m e t h o d o l o g y the total variation of the 61aC and 6180 values of a defined carbonate generation in

73 a hand specimen can be lowered to the global analytical and sampling error, i.e. not larger than +_ 0.1 to 0.4%~

Without this pretreatment an additional error in the range of + 1 to + 2%0, and occasionally up to + 10%o m a y be introduced.

5. There is an isotopic shift associated with the

in situ

laser techniques. The 613C and 6180 values are elevated relative to conventional pretreated samples by 0.5 to 1%o.

The

in situ

m e t h o d yields reproducible results. These data indicate that there is no appreciable isotopic variations at the sub-millimeter scale within any of the three carbonate generations.

6. In the samples studied in the present work, the vari- ations of the stable isotope compositions within and be- tween bands of a defined carbonate generation are very small ( ___ 0.1 to 0.4%o 613C and 0.2 to 0.7%o 6180), sugges- ting uniform chemical and physicochemical conditions during precipitation of a given carbonate generation, at least at a centimeter scale.

7. The pretreatment m e t h o d o l o g y allows us to recognize subtle isotopic variations in the gangue carbonates that can be relevant for tracing basinal fluid pathways, and that would otherwise go unnoticed without pretreatment.

Acknowledoements.

We acknowledge the financial support of the Swiss National Science Foundation (Grant n ~ 20-36397.92). This study is a contribution to IGC Project n ~ 342

Age and Isotopes of South American Ores.

Thanks are due to the San Ignacio de Mo- rococha S.A. Mining Company, and in particular the staffs of the Geology and Exploration Departments for their help in the field work. The manuscript benefited from critical comments by a anony- mous reviewer of

Mineralium Deposita.

We also gratefully acknow- ledge J. Hunziker (University of Lausanne) and R. Moritz (Univer- sity of Geneva) for their suggestions during this study, and M.

Doppler and J. Metzger (University of Geneva) for their help in the preparation of the illustrations.

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Editorial handling: K. Shelton