MVT Ore Deposition by Fluid-Mixing and CO₂ Degassing: a Case Study at the San Vicente Zinc-Lead District, Central Peru

Proceedings Chapter

Reference

MVT Ore Deposition by Fluid-Mixing and CO

₂

Degassing: a Case Study at the San Vicente Zinc-Lead District, Central Peru

SPANGENBERG, Jorge Enrique, FONTBOTÉ, Lluís

Abstract

The San Vicente Mississippi Valley-type (MVT) Zn-Pb district is hosted in dolostones of the eastern Upper Triassic to Lower Jurassic Pucará basin, central Peru. The elemental (Ca, Mg, Fe, Mn, Sr, Na, Ba, Zn, Corg, S and rare earth elements) and isotopic (C and O) compositions of the host and gangue carbonates from 20 localities, including the San Vicente main deposit, minor ore occurrences, and barren localities are used to constrain fluid path-ways, ore precipitation mechanisms, and changes in Eh-pH conditions during mineralization.

SPANGENBERG, Jorge Enrique, FONTBOTÉ, Lluís. MVT Ore Deposition by Fluid-Mixing and CO

₂

Degassing: a Case Study at the San Vicente Zinc-Lead District, Central Peru. In: Leach, D.L. & Goldhaber, M.G. International Field Conference on Carbonate-Hosted Lead-Zinc Deposits: extended abstracts . Littleton, CO : Society of Economic Geologists, 1995. p.

295-298

Available at:

http://archive-ouverte.unige.ch/unige:154360

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

1 / 1

r ·~

• . i4 ' MVT Ore Deposition by Fluid-Mixing and C0

2

Degassing: A Case Study at the San Vicente Zinc-Lead District, Central Peru

' ) j

' J

)

' •

'

)

,

t·

Jorge Spangenberg and Lluis Fontbote

Departement de Mineralogie, Uniuersite de Geneue Rue des Maraichers 13, 1211 Geneue 4, Switzerland

Fig. I. Location of the San Vicente belt showing the studied localities (barren and mineralized areas).

ABSTRACT

The San Vicente Mississippi Valley-type (MVT) Zn-Pb district is hosted in dolostones of the eastern Upper Triassic to Lower Jurassic Pucari basin, central Peru. The elemental (Ca, Mg, Fe, Mn, Sr, Na, Ba, Zn, C0,g, S and rare earth elements) and isotopic (C and 0) compo- sitions of the host and gangue carbonates from 20 localities, including the San Vicente main deposit, minor ore occurrences, and barren localities are used to constrain fluid path-ways, ore precipitation mechanisms, and changes in Eh-pH conditions during mineralization.

t/

N

Pucara Group

~

(Upper Triassic - Lower Jurassic)

m

Lower Paleozoic ,."Major tectonic lineament

~ Closed mine

t

MVT ore o cx:urrences

•Other studied occurences

~

/'- ljW•·

I *

^{F'l ~} .... >.

INTRODUCTION

Changes in the concentrations of metals and reduced sulfur, pH, Eh, and temperature due to fluid-mixing, fluid-rock interaction or

co2

degassing can cause the precipitation of Mississippi Valley-type ore (e.g. Anderson, 1976, 1990; Beales, 1975, Barnes, 1983;

Sverjensky, 1981, 1096). This contribution addresses the mechanisms of ore precipitation the San Vicente Mississippi Valley-type (MVT) Zn-Pb district and it is based on geo- chemical data obtained from gangue carbon- ates (Spangenberg, 1995). The San Vicente district is located 300 km east of Lima in cen- tral Peru, in the eastern flank of the Andes cov- ered by tropical rain forest (Fig. 1). The Upper Triassic - Lower Jurassic Pucari basin, host of the San Vicenre deposit (10 % Zn and 0.8 % Pb) and other non-mined MVT occurrences, is a carbonate platform at the western margin of the Brazilian Shield (Fontbote et al., this volume and references therein). In the San Vicente mining area three dolomitic units host the orebodies. The mineralization consists mainly of sphalerite and galena and appears in part as zebra ore and occasionally as cement in breccias or veinlets (Fontbote and Gorzawski, 1990; Spangenberg et al., 1995). A series of normal faults and tectonic lineaments of gen- eral direction NE-SW, NW-SE, and N-S cut the carbonate units in the proximity of the ore- bodies. These fault systems and the basement highs may play an important role in the gene- sis of San Vicente ore as channel pathways of the mineralizing solutions (Fig. 1).

METHODS

The ore-bearing dolomitic units and the Uncush bituminous silty Limestone (UL), that overlies the major ore-bearing dolomitic unit (San Vicente Dolomite), were sampled at dis- trict and mine scale in mineralized and barren areas along a 32 km N-S traverse centered in

CJ{ ';",; ••

--~~ "

(~-,

;~ :, :~

_...

_{~ ­}

.

I

295

"' - ~

_(O~

'9' ^{~ ~~·}

, , , .. /

">P'~

"'"1

...,

296

4 2

0

l

-2

,...., I

i:Q

~ ^-4

-J

.._,

"" ~

u

-6

-8

·10 -12 _·18

L

the main deposit (Fig. I). The San Vicente main mining area was submitted to an exten- sive multiple sampling, which includes samples from different mamos from che chree ore-bear- ing dolomitic units and textural carbonate gen- erations sampied at outcrop and hand speci- men scale. Five different generations of carbonates can be distinguished in the ore- bearing dolostones: dark replacement dolomite (ORD, I), white sparry dolomite (WSO, II), lace filling dolomite (LFO, Ill) or calcite (LFC, Ill), and dolomite or calcite replacing evaporic- ic sulfate (EPd, EPc). The ORD presents dif- ferent degree of hydrothermal alceration which permits itsclassificacion inro the following three groups: "Ivf" very fine-grained, "If" fine- grained, and "Im" medium-grained. Most of the sphalerite occurs in the same parageneric situation of cbe white hydrothermal dolomite:

(1) as fine-crystalline anhedral sphalerite inter- grown with recrystallized dolomite I; (2) as coarse open-space filling subhedral sphalerite predating and/or incergrown with the white

0

()()

-16

0

'

__..,_:a. •

•

[111111111 !ill

~

UL

0 0

• Uncush limestone (UL) 0 1 + Dark replacement do!. (IvO

• Dark replacement dol. (If) C Dark replacement do!. (Im)

o White sparry dolomite (In

1111 Late filling dolomite (Illd)

O Lale filling calcite (IIIc)

~ Evaporite pseudomorph (dol

<> Evaoorlre oscuuomoroh (cal

-14 ·12 ·10 -8 -6

al

Bo

^{(%0 PDB)}

Fig. 2. D¹⁸0 versus fJ¹³C of the carbonates of the San Vicente dis- trict. The symbols are for samples of the main deposit. The outlined fields represent the carbonates from rest of the San Vicente belt. The

two agree except for the Uncush Limestone.

sparry dolomite II on the pervasive altered dar~~

dolomite I; and (3) as very coarse euhedral crystals within the lace filling white dolomite III in cement of dissolution-breccias.

The different carbonate generations were separated using a dental diamond-drill device and analyzed for Ca, Mg, Mn, Fe, Sr, Na, Zn and Ba by ICP-AES, for [he rare earth ele- ments (REE) by ICP-MS, and for the C and 0 isotopes. The C and 0 isotopic covariation and the REE patterns of the diverse carbonate gen- erations have proven to be the most sensitive geochemical tools for constraining the process- es of ore deposition.

RESULTS

Carbon and oxygen isotopes

The different generations of gangue carbon- ates of the San Vicente deposit display a gener- al tendency toward lighter isotopic composi- tion in the following sequence: dark

replacement dolomite -> white sparry dolomite -> late filling dolomite-> late filling calcite->

carbonates replacing evaporicic sulfaces (Fig. 2).

The wide isotopic ranges of the hydrothermal carbonates suggest that mulciple fluids were involved in their precipitation. Mixing berween an incoming hot, saline, slightly acidic

(H₂C0₃-dominant) fluid and the native incra- formational alkaline (HC0₃·-dominant) reduc- ing waters explains rhe overall isotopic varia- tion (-11.5 to 2.5%o 0¹³C and -12.5 ^to -6.4%0 PDB C'i¹⁸0) in the different generations of hydrothermal carbonates. Tht [)JJC and ()¹

so

values of the ore-stage altered host DRD and the WSD (0.2 to 1.7 %0 ()uC, -11.4 to 7.3 o/oo

() 180), and the carbon isotope composition of

the associated organic matter (-23.3 to -27.5

%0 C'i¹³C), show similar patterns throughout the studied area, suggesting the existence of a com- mon regional mineralizing hydrothermal sys- tem with interconnected plumbing. The iso- topic covariacions of the ore-stage dolomites (Fig. 3) is explained by interaction of the min- eralizing fluid (F) and the host dolomite (R

=

Ivf). The trend R-E of ORD towards lighter isotopic composicion reflects the pro- gressive alteration of the host dolomite by the hydrothermal ore fluid. The coupled composi- cional evolution of the ore fluid is recorded by the C and 0 isotopic enrichment of the WSO textural subtypes (trend F-E in fig. 2), where cl

~ c=

-

^~

_~

a e:

c: t:

c:

~ c:

t:

~

C:I

t - ^..

^~

2

0

~

~ -1

f;;J '-'

to

-2·

-3

-4 -13

<>

-12

to t6 are different WSD fabrics: tl=spot, t2=fine veinlets, t3=ordered bands, t4=bands in zebra rock, t5=broad crosscutting veins, and t6=breccia cement. The isotopic composition of both the fluid and the host dolomites were progressively modified until they attain isotopic equilibrium in the fluid-rock system (point E in Fig. 3).

Rare-earth elements

The REE spectra normalized to the average host dolostone (Ivf) show significant trends (Fig. 4): 1) all the hydrothermal carbonates are markedly depleted in LREE compared to HREE; 2) the ore-stage dolomite samples (WSD, LFD) show pronounced negative Ce anomalies, suggesting that the incoming ore fluid was depleted in LREE, and particularly in Ce. This is consistent with its early (pre-ore) migration along an oxidizing main aquifer, such as the red sandstones and other elastic units at the base of the basin, in which Ce was preferentialy retained compared to the neigh- boring La and Pr; 3) late filling calcite inter- grown with late sphalerite cementing dissolu- tion breccias (IllAc) is depleted in LREE but has a significant Eu anomaly (Eu/Eu*=2.63); 4) the LFC from a large vein in the hydraulic

E .... R

t4 t5 ... Ivf

t2

~""""

...

~

F

tl ... , ^{.... il.. t6 t3 Im}

^c

^If

181 D~ \ Altered host dolomite

0

-11

White sparry dolomite

+

Dark replacement dol. (lvf, 9)

• Dark replacement dol. (If, 11) IJ Dark replacement dol. (Im, 9) D White spany dolomite (II, 115) IHI Late filling dolomite (IIld, 10) 0 Late filling calcite (Ille, 25)

~ Evapolite pseudomorph (dol, 7)

<> Evaporite pseudomorph (cal, 11)

-10 -9 -8 -7 -6

o18Q (%0 PDB)

Fig. 3. Median isotopic composition of the different generations and sub- generations of host and gangue carbonates from the San Vicente deposit in the ()¹⁸0 versus /)¹3C space.

breccia (IIIBc) shows a pronounced enrich.ment in MREE and HREE. This suggests that the late post-ore filling calcites precipitated from a more evolved fluid enriched in uncomplexed heavy REE (ore precipitation decreases the pH of the and destabilizes the REE-complexes) .

DISCUSSION AND CONCLUSIONS

1. The incoming mineralizing fluid was slightly acidic and oxidizing, probably due to migration along the underlying Red Sandstones and other detrital rocks at the base of the Pucar:i basin.

2. The regionally uniform isotopic and ele- mental patterns, coupled with the mineralogi- cal and petrographic similarities among the dif- ferent MVT occurrences, reflect that similar mineralizing processes occurs in the entire San Vicente belt. This implies the existence of an interconnected mineralizing hydrothermal sys- tem. In addition, the limited isotopic and ele- mental equilibration between the incoming fluid and the Pucar:i rocks indicates that access of the corrosive ore fluid to the mineralized zones was likely by permeable channel-ways (faults, basement highs).

3. Two mechanisms were responsible for precipitation of the ore-stage carbonates and associated sphalerite: a) Temperature-depen- dent interaction of the mineralizing fluids and the carbonate host rocks explains the alteration and replacement of the dark dolomite (I) and precipitation of early sphalerite (I) and (II).

Subsequently, a second generation of hydrothermal white sparry dolomite (II) and sphalerite (II) formed overgrowths on the altered dolomite (I) by filling secondary porosi- ty under fluid-buffered conditions. This main mineralizing stage was the result of prolonged fluid-rock interaction coupled with minor

co2

degassing. b) Pressure fall leading to outgassing of C0₂ and consequent increase in the pH of the ore fluid caused precipitation of late sparry carbonates (dolomite and calcite III) and, like- ly, sphalerite (III).

4. The Fe-Mn covariations combined with the Eu anomalies of the hydrothermal carbon- ates are consistent with a change from a reduc- ing ore-forming stage to oxidizing conditions following ore deposition. The REE enrichment, the Mn depletion, and the positive Eu anom-

Fig. 4. Median REE patterns of the

Uncush limestone, and the hydrothermal carbonates normal- ized to contents in the very fine-grained DRD (host dolomite).

Abbreviations as in figu.re 3.

298

z

0

a:

101~

Q1 Ql

a. E _Ill

Cl)

0.01 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

alies of the late-stage vug-filling carbonates indicate that the post-ore "residual" acidic flu- ids were again oxidizing due to continuous influx of fresh basinal waters.

5. Late REE-rich dolomite III (or calcite) and associated sphalerite (III) formed from the slightly acidic ore fluid during

col

degassing, which was caused in turn by an enhanced hydrothermal porosity. The widespread hydraulic brecciation, upward "escape" veins, and other structures indicate that the fluid was overpressured. An abrupt pressure drop caused by increase dissolution openings could favor

co2

degassing and therefore may play a major role in the ore precipitation during the late hydrothermal events in San Vicente.

ACKNOWLEDGMENTS

This study is supported by the Swiss National Science Foundation (Gram No.

2000.040575.94). This is a contribution to IGCP project No. 342 (Age and Isotopes of South American Ores}. We are grateful ^to San Ignacio de Morococha S.A. Mining Company and the staff of the Geology Deparrment of San Vicente mine for their help in the field work. The authors thank]. Hunziker and Z.

Sharp (stable isotopes}, Ph. Thelin (XRD- analyses} and F. Bussy (Electron microprobe} of the University of Lausanne for their coopera- tion during the analytical work.

REFERENCES

Anderson, G.M., 1983, Some geochemical aspects of sulfidc precipitation in rarbonate rocks, in: Kisvarsanyi, G., Grant, S.K., Pratt, WP., and Koenig, J.W, eds., International Conference on Mississippi Valley type lead-zinc deposits. Proceedings volume : Rolla, Un.iv. Missouri - Rolla.

Press, p. 61-76.

Anderson, G.M., 1991, Organic maturation and ore precipitation in southeast Missouri:

Econ. Geol., v. 86, p. 909-926.

Barnes, H.L., 1983, Ore-depositing reactions in Mississippi Valley-type deposits, in:

Kisvarsanyi, G., Grant, S.K., Pratt, WP., and Koenig, J.W, eds., International Conference on Mississippi Valley type lead- zinc deposits. Proceedings volume : Rolla, Univ. Missouri - Rolla. Press, p. 77-85.

Beales, ^F.~ (1975) Precipitation mechanisms for Mississippi Valley-Type ore deposits:

Econ. Geol., v. 70, p. 943-948.

Fontbote, L., and Gorzawski, H., 1990, Genesis of the Mississippi Valley-type Zn- Pb deposit of San Vicente, Central Peru:

geological and isotopic (Sr, 0, C, S) evi- dences: Econ. Geol., v. 85, p. 1402-1437.

Fontbote, L., Spangenberg, J., Oldham, L., Davila, D.,and Febres, 0., 1995, The Mississippi Valley-type district of San Vicente, eastern Pucara basin, Peru. Inter-

national Field conference on carbonate- hosted lead-zinc deposits. June 3-6, 1995, St. Louis, Missouri, USA. (this volume}

Spangenberg,

J.,

1995, Geochemical {elemen- tal and isotopic} constraints on the genesis of the Mississippi Valley-type zinc-lead deposit of San Vicente, central Peru: [Ph.D.

dissertation], University of Geneva, Switzerland (in press}

Spangenberg,]., Sharp, Z., & Fontbote, L., 1995, Apparent carbon and oxygen isotope variations of carbonate gangue minerals Mississippi Valley-type Zn-Pb of San Vicente deposit, central Peru: the effect of organic matter and sulphides: Mineralium Deposita., v. 30, p. 67-74.

Sverjensky, D.A., 1981, The origin of a Mississippi Valley-Type Deposit in the Viburnum Trend, Southeast Missouri:

Econ. Geol., v. 76, pp. 846-1872.

Sverjensky, D.A., 1986, geneis of Mississippi Valley-type lead-zinc deposits. Ann. Rev.

Earth Planet. Sci., v. 14, pp. 177-199.