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Lithos 76 (2004) 565–590

Neoproterozoic ‘anomalous’ kimberlites of Guaniamo, Venezuela:

mica kimberlites of ‘isotopic transitional’ type

Felix V. Kaminskya,*, Sergei M. Sablukova,Ludmila I. Sablukovaa, Dominic M.DeR. Channerb

aKM Diamond Exploration Ltd., 2446 Shadbolt Lane, West Vancouver, British Columbia, Canada V7S 3J1bGuaniamo Mining Company, Centro Gerencial Mohedano, Office 9D, La Castellana, Caracas, Venezuela

Received 27 June 2003; accepted 17 February 2004

Available online 17 July 2004

Abstract

In Venezuela, kimberlites have so far only been found in the Guaniamo region, where they occur as high diamond grade

sheets in massive to steeply foliated Paleoproterozoic granitoid rocks. The emplacement age of the Guaniamo kimberlites is

712F 6 Ma, i.e., Neoproterozoic. The Guaniamo kimberlites contain a high abundance of mantle minerals, with greater than

30% olivine macrocrysts. The principal kimberlite indicator minerals found are pyrope garnet and chromian spinel, with the

overwhelming majority of the garnets being of the peridotite association. Chrome-diopside is rare, and picroilmenite is

uncommon. Chemically, the Guaniamo kimberlites are characterized by high MgO contents, with low Al2O3 and TiO2 contents

and higher than average FeO and K2O contents. These rocks have above average Ni, Cr, Co, Th, Nb, Ta, Sr and LREE

concentrations and very low P, Y and, particularly, Zr and Hf contents. The Nb/Zr ratio is very distinctive and is similar to that

of the Aries, Australia kimberlite. The Guaniamo kimberlites are similar in petrography, mineralogy and mantle mineral content

to ilmenite-free Group 2 mica kimberlites of South Africa. The Nd-Sr isotopic characteristics of Guaniamo kimberlites are

distinct from both kimberlite Group 1 and Group 2, being more similar to transitional type kimberlites, and in particular to

diamondiferous kimberlites of the Arkhangelsk Diamond Province, Russia. The Guaniamo kimberlites form part of a

compositional spectrum between other standard kimberlite reference groups. They formed from metasomatised subcontinental

lithospheric mantle and it is likely that subduction of oceanic crust was the source of this metasomatised material, and also of

the eclogitic component, which is dominant in Guaniamo diamonds.

D 2004 Published by Elsevier B.V.

Keywords: Venezuela; Kimberlite; Neoproterozoic; Mineralogy; Pyrope; Geochemistry; Isotopes

1. Introduction kimberlite groups, namely, ‘basaltoid’ Group 1 kim-

An increasing number of kimberlites have been

described which cannot unambiguously be placed into

any one of the two distinct ‘reference’ South African

0024-4937/$ - see front matter D 2004 Published by Elsevier B.V.

doi:10.1016/j.lithos.2004.03.035

* Corresponding author. Fax: +1-604-925-8754.

E-mail address: felixvkaminsky@cs.com (F.V. Kaminsky).

berlites and ‘mica’ Group 2 kimberlites (Smith et al.,

1985) (essentially ‘kimberlites’ and ‘orangeites’, re-

spectively, after Mitchell, 1995a). Examples of these

include kimberlites of the Arkhangelsk Province, Rus-

sia (Sablukov, 1990;Mahotkin et al., 1997; Beard et al.,

2000), the Koidu kimberlites, West Africa, and the

Aries kimberlite, Australia (Taylor et al., 1994). In this

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590566

contribution we describe the mineralogy, petrography,

major and trace element geochemistry, and isotope

characteristics of the Guaniamo kimberlites. These

kimberlites were discovered in Guaniamo, South-West

Venezuela, by J. Drew, R. Cooper and R. Baxter-Brown

in 1982 (Fig. 1) and, like the previous examples, are not

easily classified among the standard kimberlite types.

Initially the Guaniamo kimberlites were considered

as a group of small dykes, pipes and plugs (Nixon,

1988), but as a result of more detailed exploration,

these kimberlites are now known to form an extensive

series of sheets (Channer et al., 1998, 2001), which

represent the beginning of a new diamondiferous

Fig. 1. Location of the Guaniamo study area, south-west Venezuela.

kimberlite province. The Guaniamo kimberlites have

proven to possess distinctive characteristics that have

widened our appreciation of the structural and com-

positional diversity of kimberlite rocks, their mantle

sources and the ages of kimberlite formation.

2. Area description, methods and materials studied

The materials examined in this study include fresh

and altered kimberlite drill core samples and highly

weathered kimberlite samples collected from shallow

pits. Kimberlite samples from the La Ceniza and Los

Indios sheets were studied in greatest detail, whereas

weathered kimberlite specimens from the Desayuno,

Desengano, La Peinilla, Candado-Julio and Bulla de

las Mujeres sheets were sampled only for mineral

chemistry (Table 1).

The study included a detailed, layer-by-layer pet-

rographic examination of thin and polished sections of

selected kimberlite samples. Kimberlite major element

chemistry was studied by wet chemical analysis, and

trace element concentrations were determined by

instrumental neutron-activation and X-ray fluores-

cence analysis. The samples were studied and ana-

lyzed in Moscow, Russia. Some kimberlite samples

were also analyzed by inductively coupled plasma

spectrometry for major and trace elements by Triad

Laboratories in Venezuela and Vancouver, Canada.

X-ray spectral microanalysis of minerals was per-

formed using a Camebax Microbeam microanalyzer at

U = 20 kVand I= 15–20 nA, with a beam size 3–4 Am.

Rb-Sr and Sm-Nd isotope analyses were carried out

by D.Z. Zhuravlev at the Institute of Geology and

Mineralogy, Moscow with a Finningank MAT-262

multicollector mass-spectrometer in a static measure-

ment mode (Zhuravlev et al., 1983). Blank levels for

Nd, Sm, Sr and Rb were less than 0.3, 0.1, 2.0 and 0.5

ng, respectively. The concentrations were measured in

micrograms per gram of sample weight. Analytical

accuracies were F 1% for concentrations andF 0.2%

for Sm/Nd ratios. Reference standards throughout the

course of analysis gave averaged values of: 87Sr/86Sr =

0.708041F18 (2r, n = 15) for the Eimer and Amend

standard, and 143Nd/144Nd = 0.511840F15 (2r, n =25) for the La Jolla Nd standard. 87Sr/86Sr was nor-

malized for mass fractionation during run time to87Sr/86Sr = 0.1194; 143Nd/144Nd was normalized to a

Table 1

General characteristics of specimens and samples

Sheet Specimen # Log interval, m Sample # Rock

La Ceniza DDH97-64 22.1–22.25

(upper sheet)

DDH97-64/1 fresh micaceous coarse-porphyritic kimberlite

DDH97-64/2 fresh micaceous medium–fine-porphyritic kimberlite

DDH97-65 36.3–36.45

(lower sheet)

DDH97-65/1 fresh coarse-porphyritic kimberlite

DDH97-65/2 fresh fine-porphyritic kimberlite

DDH97-65/3 fresh coarse-porphyritic kimberlite

Los Indios DDH-4 44.6–45.7 G-DD-7068 massive, fresh kimberlite

DDH-4 45.7–46.7 G-DD-7069 silicified kimberlite

DDH-5 55.47–56.69 G-DD-7072 massive, fresh kimberlite

DDH-5 56.69–58.22 G-DD-7073 massive, fresh kimberlite

DDH-6 36.7–38.0 G-DD-7075 silicified kimberlite

DDH-8 63–63.6 G-DD-7097 brecciated, carbonate-rich kimberlite

DDH-8 63.6–63.9 G-DD-7098 brecciated, carbonate-rich kimberlite

DDH-9 46.4–46.9 G-DD-7101 fresh kimberlite with some granite xenoliths

DDH-9 46.9–47.5 G-DD-7102 fresh kimberlite with some granite xenoliths

DDH-44 34.4–35.3 DDH-44/1 silicified micaceous coarse-porphyritic kimberlite

DDH-45 46.8–47.2 DDH-45/1 silicified micaceous fine-porphyritic kimberlite

DDH-49 48.6–56.2 DDH-49/1 silicified micaceous coarse-porphyritic kimberlite

DDH96-41 34.1–34.2 G-R-7283 massive, fresh kimberlite

DDH96-43 46.43–46.53 G-R-7284 massive fresh kimberlite, calcite veins

DDH96-43 46.53–46.63 G-R-7285 massive kimberlite with calcite veins

G-SL-7306 surface G-SL-7306 highly weathered kimberlite

LI34 surface LI34-2 to-6 highly weathered kimberlite

Bulla de las Mujeres LS-SL-7303 surface LS-SL-7303 highly weathered kimberlite

Candado-Julio KPS-130 surface KPS-130 highly weathered kimberlite

Desengano KPS-129 surface KPS-129 highly weathered kimberlite

Desayuno DDH-39 32.6–33.5 DDH-39/2 micaceous fine-porphyritic kimberlite

La Peinilla LP-18 surface LP-18 highly weathered kimberlite

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590 567

value of 143Nd/144Nd = 0.7219. Depleted mantle Sm-

Nd model ages were calculated relative to 147Sm/144Nd = 0.2137 and 143Nd/144Nd = 0.513151. Primary

ratios and values of eSr and eNd were calculated using

modern parameters of model reservoirs: UR (87Rb/86Sr = 0.0825 and 87Sr/86Sr = 0.7045) and chondrite

undepleted reservoir (CHUR) (143Nd/144Nd =

0.512638 and 147Sm/144Nd = 0.1967).

ISOPLOT v. 3.00 was used in the construction of

isochrons and to determine the ages of the kimberlites.

3. Geology

Kimberlites in the Guaniamo area occur as a series

of gently dipping (5–25j ENE) sheets extending over

at least 10 km, with a NNW strike along the Quebrada

Grande river valley (Channer et al., 2001). The sheets

are accompanied by steeply dipping dykes and recently

discovered intrusive alkaline breccia pipes. These fea-

tures suggest that explosive processes, similar to those

that accompany the formation of kimberlite pipes, have

occurred in Guaniamo. Hence, the potential for kim-

berlite pipe discovery is high. The kimberlites show a

coarsely banded structure with alternating zones of

cumulus-type coarse-porphyritic kimberlites and fine-

porphyritic, nearly aphyric, rocks oriented subparallel

to contact interfaces. This zoning is related not only to

hydrodynamic and gravitational differentiation of the

kimberlite melt in situ, but also to repeated injection of

magmatic melt, already deprived of olivine macro-

crysts through their settling, into still liquid cumulus

zones enriched with olivine macrocrysts. The hetero-

geneous, taxitic rock structure that occurs in the sheets

might be due to rapid intrusion (turbulent flow) and

solidification of the kimberlite melt. Locally, there is

evidence of magmatic liquation that has resulted in the

formation of thin (up to 1 cm) zones and lenses of cross-

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590568

lamellar and lamellar-diverse aggregates of carbonate

crystals in a silicate matrix.

4. Petrography

4.1. La Ceniza sheet

La Ceniza kimberlite specimens from drill core

were taken from two distinct levels of the La Ceniza

sheet, referred to herein as the ‘lower sheet’ and

‘upper sheet’ (Table 1). The specimens are dark grey,

with different structures and a very fresh appearance.

Most of the upper La Ceniza sheet specimen

(DDH97-64/1) comprises coarse-porphyritic, massive,

mica kimberlite with rare country rock xenoliths

(diorite), and rare zones (spots) of medium- to fine-

porphyritic mica kimberlite (DDH97-64/2) (Fig. 2).

Fig. 2. Kimberlite from the La Ceniza sheet. Top—weakly altered,

coarse-porphyritic micaceous kimberlite from the upper sheet

(DDH97-64/1). Bottom—weakly altered, fine-porphyritic kimber-

lite from the upper sheet (DDH97-64/2). Scale bar is 1 mm. Plane-

polarized light.

Macrocrysts are mostly olivine grains representing

two generations (olivine-1 xenocrysts and olivine-2

phenocrysts) and phlogopite laths. Olivine-1 (55%)

occurs as oval or irregular, subangular grains (1–

8 mm, rarely up to 15 mm). This olivine is fresh,

generally with less than 5% serpentinization. Olivine-

1 grains show well-defined fracture bands, implying

deformation prior to its incorporation into the magma.

Olivine-2 (15%) is idiomorphic or subidiomorphic,

some grains with partially ‘fused’ outlines (0.1–1.0

mm, rarely up to 2 mm). Serpentinization of olivine-2

is similar to or greater than that of olivine-1.

Phlogopite macrocrysts (0.5%) form irregular, sub-

angular, light brown laths (0.3–1.5 mm) with weak

pleochroism, commonly having thin red-orange tetra-

ferriphlogopite rims with inverse absorption. Some of

the laths are deformed (are slightly bent) and either

show a wavy extinction or exhibit pronounced cleav-

age. Pyrope occurs as oval or irregularly subangular

(3–9 mm), violet and purplish-red grains. The margins

of all pyrope grains are replaced by thick kelyphitic

rims, which have a radiate or cryptocrystalline struc-

ture. Some of the pyrope grains are completely replaced

by kelyphitic rims. Rock matrix (30%) consists of a

fine-crystalline aggregate of light brown, partially

chloritized phlogopite laths (0.02–0.1 mm) with red–

orange tetraferriphlogopite rims (20%). The space

between phlogopite laths is filled with fine-grained

scaly serpentine (3%), dolomite (3%) and opaque

minerals (3%). Xenomorphic and, more rarely, idio-

morphic opaque mineral grains (0.02–0.06 mm) are

Mg-rich Cr-titanomagnetite and chromian spinel grains

with titanomagnetite rims. Zones and spots of medium-

to fine-porphyritic micaceous kimberlite (DH97-64/2)

have the same types and compositions ofminerals, both

as xenocrysts and phenocrysts, and in the rock matrix,

differing from Sample DDH97-64/1 only in a fine-

porphyritic texture and a higher proportion of phlogo-

pite in the rock matrix.

The lower La Ceniza sheet exhibits a subparallel,

layered structure, where thick (3–9 cm) bands of

coarse-porphyritic kimberlite (DDH97-65/1) alternate

with relatively thin (1 cm) layers of well-sorted fine-

porphyritic kimberlite (DDH97-65/2), lacking olivine-

1 xenocrysts and phlogopite macrocrysts. The thin

layers are inclined at an angle of 10–12j to the

horizontal, with well-defined, sharp, but not cross-

cutting, layer boundaries. Lower sheet rocks are gen-

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590 569

erally similar to upper sheet kimberlite, however, there

is one main difference; they have an almost mica-free

rock matrix composed of a microcrystalline aggregate

of carbonate (dolomite and magnesite) (20%), serpen-

tine (chrysolite and serpophyte) and acicular millerite.

Opaque mineral segregations (3%) are 0.02–0.1 mm in

size, idiomorphic and, more rarely, xenomorphic, Mg-

rich Cr-titanomagnetite and chromian spinel grains,

with titanomagnetite rims. Wall rock xenoliths in these

samples comprise diorite, with a minor proportion of

plagioclase-biotite xenoliths. Gangueminerals occur as

very rare, thin carbonate veinlets.

4.2. Los Indios sheet

The Los Indios samples are generally similar to

upper La Ceniza sheet kimberlites, but with a higher

degree of secondary alteration. The Los Indios rocks

consist of 1–3-cm-thick zones of coarse-porphyritic

mica kimberlite alternating with 5–15-cm-thick fine-

porphyritic mica kimberlite.

Olivine-1 xenocrysts and olivine-2 phenocrysts are

almost completely silicified (quartz and tridymite),

with minor serpentinization in marginal zones and

loop-like veinlets (DDH-45). Tridymite is a rare sec-

ondary mineral in kimberlite, but in the Los Indios

kimberlite it comprises up to 20 vol.% of pseudo-

morphs after olivine. Its presence has been confirmed

by X-ray analysis. Locally, tridymite occurs in associ-

ation with quartz; however, it can be a sole replacement

product after olivine.More rarely, olivine is replaced by

a fine-grained scaly and lamellar serpentine aggregate

with some admixture of carbonate (DDH97-44).

Relicts of fresh olivine are very rare. Phlogopite macro-

crysts (0.5 vol.%) occur as oval or irregular 0.3–1.0-

mm grains, and are commonly replaced by tetraferri-

phlogopite, in their marginal zones. The rock matrix

consists of a finely crystalline aggregate of light brown,

idiomorphic phlogopite grains with red-orange tetra-

ferriphlogopite rims (60%). The space between phlog-

opite laths is filled with a fine-grained aggregate of

carbonate (dolomite, siderite and magnesite; 20%) and

opaque minerals (6%). Opaque minerals occur as

xenomorphic or, more rarely, idiomorphic grains of

magnetite, Mg-titanomagnetite and chromian spinel

with titanomagnetite rims. The rock contains numerous

kelyphitic rim fragments of 1–10-mm size (not less

than 1% of the rock volume). Some pyrope grains

contain oval inclusions of Ni-chalcopyrite, magnesite

and dolomite. In addition, the rock contains strongly

altered wall rock xenoliths (approximately 5% of the

rock volume). Sample DH97-49 shows the highest

intensity of kimberlite alteration, with olivine com-

pletely replaced by quartz and tridymite, and the rock

matrix replaced by chlorite and iron hydroxides.

4.3. Desayuno sheet

The Desayuno sheet rock (DDH-39/2) is an intense-

ly altered mica kimberlite. Olivine grains (10% of the

rock volume) are 0.5–3 mm in size; large grains have a

subangular shape, while small ones (0.5–1 mm) are

mostly subidiomorphic. The cores of the olivine grains

are completely replaced by a fine-grained aggregate of

scaly serpentine. An aggregate of finely crystalline

phlogopite is developed in the marginal zones of large

grains and almost completely replaces small olivine

grains. Phlogopite macrocrysts (1–2%) are subangular,

0.5–1.5 mm, and have a light brown color with a faint

greenish shade and weak pleochroism, many of them

have rims of thin red-orange tetraferriphlogopite. The

rock matrix consists predominantly of an aggregate of

finely crystalline pale brown, idiomorphic phlogopite

grains with red-orange tetraferriphlogopite rims (55%

of the rock volume), and a minor proportion of finely

crystalline carbonate (15%), serpentine (10%), and

chlorite (2%) with dust-like disseminated opaque min-

erals (less than 1%). In addition, the rock contains some

fragments of kelyphitic rims and altered xenoliths of

granite (5%, distributed predominantly along the host

rock border).

4.4. Other sheets

The examined rock samples from the Desengano,

La Peinilla, Candado-Julio and Bulla de las Mujeres

sheets indicate very strong tropical weathering of

kimberlite. Their indicative minerals (pyrope and chro-

mian spinel) have peculiar morphological and surface

microrelief features that reflect weathering (see below).

5. Mineralogy and mineral chemistry

All the examined kimberlites are similar in miner-

alogy and mineral chemistry, particularly with regard

Table 2

Representative compositions of olivine from the La Ceniza kimberlite (wt.%)

Sample # Grain color SiO2 Cr2O3 FeO MnO MgO CaO NiO Total Mg#

DDH97-64 colorless 41.09 0.05 8.16 0.20 49.63 0.01 0.42 99.14 91.6

colorless 41.13 0.04 8.24 0.09 49.60 0.02 0.32 99.12 91.5

colorless 41.66 0.04 8.43 0.08 48.93 0.01 0.41 99.15 91.2

colorless 40.94 0.00 8.62 0.12 49.45 0.03 0.37 99.16 91.1

yellow 41.29 0.02 11.15 0.15 46.75 0.08 0.20 99.44 88.2

yellow 41.19 0.00 9.43 0.11 48.82 0.03 0.46 99.58 90.2

DDH97-65 colorless 41.15 0.05 8.36 0.09 49.76 0.03 0.31 99.44 91.4

colorless 41.10 0.00 8.27 0.11 49.68 0.02 0.33 99.18 91.5

colorless 41.64 0.08 6.99 0.09 50.20 0.01 0.28 99.01 92.8

yellow 41.32 0.00 10.15 0.22 47.29 0.03 0.32 99.01 89.3

yellow 40.01 0.02 13.61 0.13 45.52 0.03 0.31 99.32 85.6

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590570

to mantle minerals. Fresh kimberlites from La Ceniza

and Los Indios are characterized by very high pro-

portions of mantle minerals, primarily due to the

abundance of olivine macrocrysts. Other kimberlite

indicator minerals (KIM), namely, garnet (pyrope and

pyrope-almandine), chromian spinel and clinopyrox-

Table 3

Representative compositions of phlogopite (wt.%)

Sample # Grain # SiO2 TiO2 Al2O3 Cr2O3 FeO M

Los Indios sheet

DDH-45/1 1core 43.32 0.69 11.22 0.68 4.10 0.0

1rima 42.99 0.23 0.11 0.01 14.81 0.1

2core 40.70 1.55 13.17 0.39 5.15 0.0

2rima 46.76 0.34 1.13 0.01 10.24 0.0

3core 40.16 1.12 14.43 0.01 4.41 0.0

3rima 44.32 0.22 0.08 0.00 13.54 0.0

DDH-45/3 1 42.33 0.89 11.67 1.19 3.98 0.0

2 42.51 3.19 12.28 0.11 3.39 0.0

3 41.92 3.45 12.42 0.19 2.97 0.0

4 41.28 4.08 12.01 0.09 4.35 0.0

5 39.79 8.12 13.08 0.48 4.21 0.0

La Ceniza sheet

DDH97-64/1 1core 42.31 0.74 10.48 0.67 3.84 0.0

1rim 40.75 1.20 11.95 0.06 5.25 0.0

2 40.91 2.15 11.41 0.95 3.13 0.0

3 41.76 0.78 10.59 1.07 4.14 0.0

4 42.19 0.54 11.00 0.97 3.32 0.0

5 41.75 3.06 11.02 0.54 3.11 0.0

DDH97-65/1 1core 42.32 0.27 9.27 0.04 4.83 0.0

1rima 44.54 0.22 0.24 0.04 12.11 0.1

2core 43.25 0.95 11.06 0.65 4.47 0.0

2rim 42.70 0.61 9.73 0.05 6.06 0.0

3 41.88 0.63 10.55 0.73 3.87 0.0

4 41.85 1.78 10.40 0.42 3.69 0.0

5 41.77 3.90 10.96 0.28 3.46 0.0

a Tetraferriphlogopite rims in matrix laths.

ene, occur in the rocks in very minor amounts (single

grains or several tens of grains per kilogram of rock),

and there is scarcity of picroilmenite.

Representative probe data for major mineral phases

are given in Tables 2–7 and include other kimberlite

localities, in addition to Los Indios and La Ceniza. The

nO MgO CaO Na2O K2O Total Mg# Note

8 25.24 0.04 0.15 10.40 95.92 91.7 matrix

2 23.39 0.12 0.23 9.75 91.76 73.8 matrix

7 24.78 0.02 0.30 10.46 96.59 89.6 matrix

9 23.67 0.13 0.53 10.42 93.32 80.5 matrix

2 23.74 0.02 0.00 9.70 93.61 90.6 matrix

8 24.88 0.08 0.40 9.87 93.47 76.6 matrix

3 24.29 0.00 0.15 10.17 94.70 91.6 macrocryst

2 24.81 0.00 0.00 10.91 97.22 92.9 macrocryst

6 24.52 0.06 0.04 10.66 96.29 93.6 macrocryst

8 23.06 0.00 0.04 10.58 95.57 90.4 macrocryst

6 19.55 0.00 0.07 10.28 95.64 89.2 macrocryst

0 24.94 0.00 0.19 9.97 93.14 92.1 matrix

4 24.96 0.00 0.19 9.79 94.19 89.5 matrix

5 23.46 0.00 0.09 10.01 92.16 93.0 macrocryst

1 24.21 0.01 0.12 9.94 92.63 91.3 macrocryst

8 24.81 0.00 0.21 9.71 92.83 93.0 macrocryst

5 22.81 0.03 0.13 9.98 92.48 92.9 macrocryst

8 25.66 0.02 0.08 10.11 92.68 90.5 matrix

4 25.88 0.01 0.18 10.04 93.40 79.2 matrix

8 25.26 0.01 0.20 9.92 95.85 91.0 matrix

7 25.75 0.02 0.07 10.08 95.14 88.3 matrix

6 24.58 0.21 0.13 9.76 92.40 91.9 macrocryst

5 24.14 0.02 0.12 9.61 92.08 92.1 macrocryst

5 22.17 0.00 0.13 9.85 92.57 92.0 macrocryst

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590 571

full data set, covering all data points in the accompa-

nying figures, is available from the principal author.

5.1. Olivine

Olivine macrocrysts in the La Ceniza kimberlites

form two distinct color varieties with different mineral

chemistries (Table 2): (1) colorless olivine (99.9% of

Fig. 3. Compositional variation of phlogopite from the Guani

olivine grains) with FeO contents from 6.99 to 9.62

wt.% and Cr2O3 contents up to 0.08 wt.%, and (2)

greenish-yellow olivine with higher FeO contents of

9.43 to 13.61 wt.%. The proportion of the forsterite

molecule varies from 91% to 93% in colorless olivine

and from 85% to 90% in greenish-yellow olivine, with

corresponding Mg numbers from 91.1 to 92.8 (aver-

age 91.6) and 85.6 to 90.2 (average 88.3). NiO

amo sheet kimberlites (diagram after Mitchell, 1995b).

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590572

contents vary from 0.20 to 0.46 wt.%. The composi-

tion of the colorless olivine is similar to that in mantle

peridotite xenoliths (Mitchell, 1995a).

5.2. Phlogopite

All micas from Guaniamo kimberlites are mag-

matic; they do not show evidence of a metasomatic

origin.

Phlogopite macrocrysts (Table 3) are aluminous

(10.40–13.08 wt.% Al2O3) with widely varying TiO2

contents (0.54–8.12 wt.%), and Mg# from 89.2 to

93.6 (average 92.0). There is no difference in mineral

chemistry between single and deformed crystals of

phlogopite.

Matrix phlogopite is more ferruginous, with Mg#

from 88.3 to 92.1; cores have Mg# from 89.6 to 92.1

(average 90.9) and rims from 88.3 to 89.5 (average

88.9). Matrix phlogopite is aluminous (9.27–14.43

wt.% Al2O3) with lower TiO2 contents (0.27–1.55

wt.%) than macrocrysts.

Table 4

Representative compositions of garnet (wt.%)

Sheet Sample # Grain color SiO2 TiO2 Al2O3 Cr2

Bulla de las LS-SL-7303 violet 41.11 0.41 18.96 5.

Mujeres violet 40.47 0.15 18.71 6.

violet 40.63 0.36 17.74 7.

Candado-Julio KPS-130 violet 40.54 0.30 19.59 5.

violet 40.87 0.25 19.08 5.

violet 39.66 0.17 14.40 12.

violet 39.08 0.12 13.22 13.

Desengano KPS-D-129 orange 39.80 0.48 21.52 0.

orange 41.36 0.13 22.98 0.

purplish-red 41.84 0.26 22.20 1.

La Ceniza DDH97-64 orange 40.07 0.33 21.70 0.

purplish-red 42.35 0.39 22.03 1.

violet 41.62 0.23 20.43 3.

violet 41.20 0.19 18.45 6.

pale pinka 41.92 0.06 21.60 2.

DDH97-65 violet 41.79 0.31 20.34 4.

violet 42.02 0.19 17.73 7.

La Peinilla LP-18 violet 39.95 0.26 16.23 9.

Los Indios DDH-45 purplish-red 41.39 0.07 20.05 4.

violetb 41.09 0.09 18.49 6.

LI-34-2 violet 40.76 0.05 17.83 8.

violet 39.79 0.36 14.55 11.

violet 39.94 0.21 12.44 13.

violet 39.45 0.22 12.06 14.

a From pyrope peridotite inclusion.b Intergrowth with clinopyroxene.

Important compositional features of the phlogopites

are shown in Fig. 3 (Al2O3–TiO2). Los Indios phlogo-

pites are similar in composition to those from La

Ceniza, although slightly more aluminous. Ground-

mass core compositions are similar to their respective

phenocrysts. Some groundmass rim compositions are

similar to their respective cores but, most especially

from Los Indios, showmuch lower Al2O3 and TiO2 and

higher FeO. These differences may be due to alteration,

which is pronounced in the Los Indios samples.

Most phlogopites lie between, slightly overlapping,

the fields for Group 1 and 2 kimberlites. Some TiO2-

rich phlogopite macrocrysts (up to 8 wt.%) are similar

to lamproite-related micas (Fig. 3).

Rims in some mica macrocrysts and some matrix

laths are compositionally tetraferriphlogopite. These

micas have high iron contents (10.24–14.81 wt.%

FeO) and low Al2O3 (0.08–1.13 wt.%) and Cr2O3

(V 0.04 wt.%) contents. They are similar in composi-

tion to mica from Group 2 kimberlites (Mitchell,

1995b).

O3 FeO MnO MgO CaO Na2O Total Mg# Group

82 7.00 0.38 20.38 5.11 0.03 99.20 83.9 G-9

62 7.91 0.43 19.10 5.77 0.06 99.22 81.2 G-9

77 7.10 0.40 19.64 5.88 0.00 99.52 83.2 G-9

01 7.55 0.39 20.20 4.84 0.05 98.47 82.7 G-9

79 7.17 0.37 20.02 4.98 0.00 98.53 83.3 G-9

03 7.27 0.42 17.90 6.97 0.08 98.90 81.5 G-11

21 8.03 0.53 16.06 9.14 0.02 99.41 78.1 G-12

14 16.48 0.42 12.24 8.06 0.07 99.21 57.0 G-3

19 7.55 0.16 16.23 10.66 0.02 99.28 79.3 G-6

47 9.21 0.38 19.93 4.59 0.00 99.88 79.4 G-9

01 19.66 0.52 11.39 5.99 0.14 99.67 50.8 G-3

77 7.89 0.39 20.50 4.13 0.00 99.45 82.3 G-9

62 7.54 0.38 20.65 4.51 0.01 98.98 83.0 G-9

30 7.54 0.24 20.10 5.11 0.00 99.13 82.6 G-9

08 9.30 0.43 19.31 5.12 0.00 99.82 78.7 G-9

04 7.75 0.36 20.65 4.36 0.01 99.60 82.6 G-9

19 7.32 0.47 20.24 4.82 0.00 99.98 83.1 G-9

94 7.39 0.40 19.02 5.99 0.06 99.24 82.1 G-9

39 8.08 0.33 19.04 5.78 0.00 99.13 80.8 G-9

03 7.74 0.30 18.51 6.60 0.01 98.86 81.0 G-9

01 7.32 0.40 21.50 3.08 0.00 98.95 84.0 G-10

52 6.84 0.41 18.28 6.87 0.07 98.69 82.7 G-11

89 7.66 0.46 15.41 8.68 0.03 98.72 78.2 G-12

44 7.64 0.45 15.79 8.94 0.11 99.10 78.7 G-12

Lithos 76 (2004) 565–590 573

5.3. Garnet

Garnet is the second (after olivine) most abun-

dant mantle mineral (up to 1 vol.%) in Guaniamo

kimberlites, with grain sizes of up to 7 mm.

Garnet is much more abundant than chromian

spinel, but many grains are completely or partially

kelyphitized, such that relicts of fresh garnet occur

rarely (only several tens of grains per kilogram of

rock).

F.V. Kaminsky et al. /

Fig. 4. Plot of CaO vs. Cr2O3 for Guaniamo garnets with fields for harzbu

Sobolev, 1971 and Sobolev et al., 1973) and G-9/G-10 boundary (solid line

is for eclogitic almandine-pyrope garnets.

Grain surfaces have a characteristic, sub-kelyphitic

microrelief with comb-undulating and vuggy features.

Garnet surfaces from strongly weathered kimberlites

show signs of intense dissolution (etch channels,

trigonal pits and droplet features), leading, in extreme

cases, to the formation of a peculiar ‘honeycomb’

structure to the grains.

In terms of color, 45% of garnets are violet and

lilac, 30% purplish-red, and 15% orange, i.e., violet,

lilac and purplish-red pyrope garnets predominate.

rgite (left), lherzolite (center) and wehrlite (right) (dotted lines, after

, after Gurney and Switzer, 1973 and Gurney, 1984). The lower field

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590574

Compositionally, the following groups (after

Dawson and Stephens, 1975) can be distinguished

(Table 4):

1. Orange calcic pyrope-almandine (G-3 group) with

MgO 11–12 wt.%, Mg# 50.8–57.0, FeO 16–20

wt.%, and low Cr2O3 (0.01–0.14 wt.%).

2. Orange pyrope-grossular almandine (G-6 group)

with MgO 16 wt.%, Mg# 79.3, FeO 7.5 wt.%, and

low Cr2O3 (0.19 wt.%), typical of grospydite

xenoliths.

3. Purplish-red and violet chrome-pyrope (G-9 group)

with FeO< 10 wt.%, TiO2 < 0.41 wt.%, and Mg#

78.7–83.9.

4. Violet low-calcium chrome-pyrope of the diamond

association (G-10 group), with a high Cr2O3

content (>8 wt.%), a low CaO content ( < 4 wt.%)

and Mg# 84.0.

5. Lilac uvarovite-pyrope (G-11 group) with MgO

17–19 wt.%, Mg# 81.5–82.7, CaO 6–7 wt.%, and

Cr2O3 11–12 wt.%.

Table 5

Representative compositions of chrome spinel (wt.%)

Sheet Sample # Grain # TiO2 Al2O3 Cr2

Bulla de las Mujeres LS-SL-7303 1 0.54 6.68 63.

2 0.78 7.44 64.

3 1.68 18.19 31.

Candado-Julio KPS-130 1 2.88 6.91 34.

2 1.18 7.41 63.

Desengano KPS-D-129 1 1.57 18.46 38.

2 0.05 53.68 14.

3 0.22 12.24 56.

4 0.76 31.12 26.

La Ceniza DDH97-64 1 0.15 19.62 42.

2a 0.38 28.24 37.

DDH97-65 1 0.39 7.66 59.

2 2.11 8.35 55.

La Peinilla LP-18 1 0.28 9.19 61.

2 0.20 47.91 20.

3 0.14 12.46 59.

4 0.59 6.20 66.

5 0.11 39.45 30.

Los Indios DDH-49 1 0.21 6.62 62.

LI-34-2 1 0.27 22.71 36.

2 0.01 49.99 19.

3 0.83 17.46 49.

G-SL-7306 1 2.23 7.49 59.

a From the pyrope peridotite inclusion.

6. Knorringitic uvarovite-pyrope (G-12 group) with

very high Cr2O3 (>12 wt.%), CaO (8–9 wt.%), and

Mg# 78.1–78.7.

This compositional range occupies the entire

field of ultramafic-suite garnet compositions previ-

ously determined for the Guaniamo area, with the

exception of the most knorringite-rich inclusions in

diamonds from the Quebrada Grande placer (sam-

ples V-1 and V-2, Kaminsky et al., 2000) and the

anomalously Ca-rich, grossular xenocrysts found in

the El Candado and Cordero kimberlites (samples

PHN5750/3(12) and PHN5750/Cord4, Nixon et al.,

1994). Garnet data from Los Indios and La Ceniza

are plotted in Fig. 4 (Cr2O3-CaO), along with

garnet data from other weathered kimberlite local-

ities in Guaniamo (see Table 4). G-9 pyropes are

much more abundant than all other pyrope varieties,

and show a wide variation in Cr2O3 content (1.47–

9.94 wt.%). Pyropes from different kimberlite

sheets are similar in composition. Only pyropes

O3 FeO MnO MgO ZnO V2O5 Total Mg#

05 19.83 0.22 8.08 0.10 0.35 98.85 42.1

53 16.57 0.25 9.40 0.11 0.40 99.48 50.3

47 36.71 0.26 9.63 0.23 0.32 98.49 31.9

64 46.01 0.23 6.81 0.04 0.59 98.11 20.9

62 15.37 0.22 10.45 0.08 0.32 98.65 54.8

05 30.10 0.20 10.33 0.15 0.35 99.21 38.0

36 12.42 0.09 18.76 0.21 0.11 99.68 72.9

92 17.94 0.30 11.13 0.19 0.41 99.35 52.5

80 26.64 0.17 13.84 0.15 0.38 99.86 48.1

22 25.08 0.29 12.53 99.89 47.1

72 19.50 0.13 13.88 99.85 55.9

89 21.20 0.28 10.58 100.00 47.1

02 23.05 0.23 11.09 99.85 46.2

11 18.37 0.22 8.19 0.20 0.45 98.01 44.3

51 15.00 0.10 15.19 0.26 0.21 99.38 64.4

91 16.58 0.24 8.87 0.19 0.35 98.74 48.8

12 15.76 0.17 8.98 0.08 0.31 98.21 50.4

15 15.35 0.15 13.82 0.24 0.24 99.51 61.6

60 16.57 0.36 12.02 98.38 56.4

25 25.24 0.24 12.89 0.17 0.20 97.97 47.7

88 12.92 0.11 15.61 0.27 0.12 98.91 68.3

90 20.94 0.26 9.71 0.17 0.30 99.57 45.3

76 20.84 0.26 8.05 0.16 0.42 99.21 40.8

F.V. Kaminsky et al. / Litho

from Candado-Julio show a wider variation in

Cr2O3 content and contain G-3, G-11 and G-12

pyropes, which are absent in the Bulla de las

Mujeres sheet.

Fig. 5. Composition of chromian spinel from the Guaniamo kimberlites:

Kharkiv et al. (1989).

5.4. Chromian spinel

Chromian spinel occurs predominantly as small

(0.2–0.5 mm, rarely up to 0.7 mm) fragments and

s 76 (2004) 565–590 575

TiO2 vs. Cr2O3 (top) and Al2O3 vs. Cr2O3 (bottom). Trends from

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590576

fractions of grains with differing morphology. Most of

the grains (80%) are fragments with step–like frac-

tures, numerous joints, and a corrosion–related micro-

relief, imparting a ‘collective’ lustre to their surfaces.

Other morphological types include intact, flat-faced

octahedral grains with varying degrees of distortion,

and combination-type crystals with blocky structures

(4%). A few chromian spinel grains have very thin

titanomagnetite rims. Compositionally, these rims are

similar to titanomagnetite from the rock matrix, and it

is likely that they formed during one of the latest

stages of magma crystallization.

Chromian spinels from weathered kimberlites are

characterized by the presence of numerous micro-

cracks, and by the occurrence of grains with zonal

internal structures, where MgO and Al2O3 contents

decrease from core to margin, and TiO2 and FeO

contents increase.

Chromian spinels from fresh kimberlite in La

Ceniza are rich in MgO (10–16 wt.%), and have a

wide range of Al2O3 and Cr2O3 contents (6–43 wt.%

and 22–64 wt.%, respectively). Data for these spinels

and other Guaniamo kimberlite localities (Table 5) are

shown in Fig. 5 (Al2O3 and Cr2O3) where they form a

well-defined peridotitic isomorphic Cr3 +–Al3 + trend

(after Kharkiv et al., 1989). The chromian spinels are

Table 6

Representative compositions of ilmenite (wt.%)

Sheet Sample # Grain # TiO2 Al2O3 Cr

Bulla de las Mujeres LS-SL-7303 1 50.07 0.42 5.2

Candado-Julio KPS-130 1 45.51 0.04 0.0

2 50.55 0.06 0.0

3 54.04 0.11 0.0

4 50.58 0.00 0.0

5 50.76 0.00 0.0

6 54.02 0.09 0.0

La Ceniza DDH97-64 1 43.50 0.11 0.1

DDH97-65 1 48.62 0.00 0.0

2 48.39 0.04 0.0

3 50.20 0.00 0.0

Los Indios DDH-45 1 51.35 0.19 0.0

DDH-49 1 53.77 0.08 0.0

2 53.26 0.13 0.0

DDH-45/1 1–1 50.03 0.17 0.0

1–2 53.41 0.25 0.0

1–3 54.04 0.21 0.0

LI-34-2 1 56.96 0.29 0.9

G-SL-7306 1 50.96 0.07 0.0

2 51.81 0.03 0.0

3 51.24 0.03 0.0

related to different depth facies, from the spinel-

pyroxene facies (7–17 kbar) to the diamond-pyrope

facies (>40 kbar), and are notable for the high

proportion of high-Cr grains (with Cr2O3>55 wt.%)

from the diamond association (Sobolev, 1974). When

plotted on a Ti/(Ti + Cr +Al) vs. Fe2 +/(Fe2 +Mg) dia-

gram (after Mitchell, 1986), the chromites and titano-

magnetite rims plot within the field of worldwide

kimberlite spinels.

5.5. Ilmenite

Two types of ilmenite were recognized in Guaniamo

kimberlites: manganese ilmenite and picroilmenite.

The first most common type forms irregular, angular

grain fragments, 0.3–0.6 mm in size. These contain

significant concentrations of MnO (0.73–2.55 wt.%)

and are herein described as ‘manganese ilmenite’. In

contrast to magnesian ilmenite (picroilmenite), which

is common in kimberlites, themanganese ilmenite has a

lowMgO content (0–1.39 wt.%), as well as low Al2O3

(0–0.19 wt.%) and Cr2O3 (0–0.10 wt.%) contents

(Table 6). It has very low Mg# from 0 to 4.7. Similar

Mn-ilmenites were previously found as inclusions in

Guaniamo diamonds (Kaminsky et al., 1997, 2000;

Sobolev et al., 1998). Such low-Mg, Mn-rich (up to 11

2O3 FeO MnO MgO ZnO V2O5 Total Mg#

5 31.02 0.27 11.89 0.02 0.50 99.44 40.6

0 50.30 1.01 1.39 0.11 0.14 98.50 4.7

3 45.76 2.24 0.05 0.00 0.00 98.69 0.2

2 39.67 2.03 0.11 0.05 0.00 96.03 0.5

5 46.77 2.33 0.00 0.04 0.00 99.77 0.0

3 47.28 0.82 0.28 0.00 0.00 99.17 1.0

3 41.25 2.18 0.16 0.01 0.00 97.74 0.7

0 55.25 0.10 1.01 100.07 3.2

0 47.67 1.95 0.06 98.30 0.2

0 47.57 2.55 0.09 98.64 0.3

2 47.27 0.73 0.35 98.57 1.3

0 46.14 1.82 0.27 99.77 1.0

0 40.84 1.43 4.17 100.29 15.4

5 37.87 1.46 6.28 99.05 22.8

3 44.43 1.64 1.02 97.32 3.9

0 35.29 1.14 8.72 98.81 30.6

0 31.63 1.03 11.97 98.88 40.3

0 27.68 0.29 13.79 0.00 0.12 100.03 47.1

0 46.45 1.60 0.00 0.07 0.16 99.31 0.0

1 46.41 1.23 0.09 0.11 0.05 99.74 0.3

3 46.70 0.97 0.14 0.08 0.16 99.35 0.5

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590 577

wt.%MnO) ilmenites are in other areas associated with

ferropericlase, majoritic garnet and CaTi-perovskite,

and are believed to belong to the superdeep association

(see Kaminsky et al., 2001, and references therein).

Some grains of manganese ilmenite from the Los

Indios kimberlite are characterized by a sharply het-

erogeneousMn distribution, withMnO content varying

from 1.02 to 11.97 wt.%, within a single grain. In Fig.

Fig. 6. Composition of ilmenite in Guaniamo sheet kimberlites: TiO2 vs.

composition of different areas in a single grain. Manganese ilmenite occup

worldwide picroilmenite are from I.P. Ilupin (personal communication, 20

6, Guaniamo ilmenite data are shown in comparison

with worldwide picroilmenite data. Most Guaniamo

ilmenites plot at lower MgO values on a TiO2 vs. MgO

diagram and at higher MnO and FeO values on a MnO

vs. FeO diagram (see Fig. 6).

The second type of ilmenite in Guaniamo kimber-

lites (found in the Los Indios and the Bulla de las

Mujeres sheets) is picroilmenite, found as very small

MgO (top) and MnO vs. FeO (bottom). The lines connect points of

ies the area distinct from picroilmenite in both plots. Data points for

02).

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590578

(0.2–0.3 mm) grains with a very peculiar, flat-faced

myriohedral habit. Picroilmenite with this habit occurs

rarely, predominantly as microinclusions in other

minerals. Chemically, these grains are similar to

kimberlite-related picroilmenites (Table 6, Fig. 6).

5.6. Chromian diopside

Chromian diopside (chrome-diopside) is rare and

occurs as single grains intergrown with pyrope.

Al2O3, Na2O and Cr2O3 contents range from 1.5 to

2.5 wt.% (Table 7). However, one grain exhibits

higher Al2O3 (6.0 wt.%), Na2O (3.0 wt.%) and

Cr2O3 (3.2 wt.%) and a lower magnesium index

(Mg# = 88.9 vs. 93.4–93.9 in other grains).

The calcic index (Ca#) of the clinopyroxenes varies

from 45.9 to 49.3. Assuming equilibrium of chrome-

diopside with orthopyroxene, this range of values cor-

responds to a formation temperature range of 900–

1000 jC, using methods described in Sobolev (1974).

5.7. Mantle xenoliths

Mantle rock xenoliths occur in the examined

samples very rarely, as intergrowths of several

olivine grains with a common, uneven boundary

(dunite microxenoliths). Detailed examination of

polished sections of kimberlite sample DDH97-64

revealed a single xenolith of pyrope peridotite. This

sub-angular xenolith, 2 cm in size, consists of

partially altered olivine, phlogopite, pale pink py-

rope and chromian spinel. The pyrope garnet is of

G-9 type with a low Cr2O3 content (2.08 wt.%) and

a high FeO content (9.30 wt.%; Mg# = 78.7) (Table

4). The chromian spinel is a Mg-alumochromite of

the grospydite depth subfacies (22–34 kbar, after

Sobolev, 1974), with a moderate Cr2O3 content

(37.72 wt.%) and a high Al2O3 content (28.24

wt.%) (Table 5).

Table 7

Representative compositions of chrome-diopside (wt.%) from the Los Ind

Grain # SiO2 TiO2 Al2O3 Cr2O3 FeO MnO

1a 52.98 0.02 6.05 3.21 2.89 0.12

2 54.55 0.10 1.57 1.99 2.03 0.01

3 54.73 0.16 2.34 2.29 1.92 0.12

4 55.37 0.19 2.22 2.37 1.88 0.06

a Intergrowth with pyrope.

6. Major and trace element geochemistry

Major and trace element data are presented in Table

8 for representative samples from La Ceniza and Los

Indios. All major petrographical varieties of kimber-

lite (fine-, medium–fine- and coarse-porphyritic kim-

berlites) are represented.

Contamination, either by weathering or xenoliths,

is a common problem in the interpretation of kimber-

lite geochemical data. The contamination index

(Clement, 1982) where C.I.=(SiO2 +Al2O3 +Na2O)/

(2K2O+MgO) is useful in this respect. Taylor et al.

(1994) used a maximum C.I. of 1.7 for olivine lamp-

roites and micaceous kimberlites, and a maximum of

1.5 for non-micaceous kimberlites.

The compositions of some samples are affected

by weathering and/or silicification. Samples DDH-

44/1, DDH-49/1 and G-R-7069 have SiO2 contents

greater than 50 wt.%, clearly suggesting silicifica-

tion, which is common in the Guaniamo kimberlites.

In particular, silicified kimberlite (DDH-49/1) has a

very low MgO content (2.43 wt.%) but has relative-

ly unaltered Cr (1520 ppm), Ni (3080 ppm) and

Co (188 ppm) contents, which are of typical kim-

berlitic values. These values are even higher than

average, in the olivine cumulate zone, where Ni,

previously disseminated in olivine, has partially al-

tered to millerite. The same holds true for a number

of other components with relatively low mobilities.

Other samples, such as DDH-39/2 and G-DD-7102,

have elevated SiO2 contents from 40 to 50 wt.%

and high K2O values above 4 wt.%, due to the

occurrence of xenoliths of granite in the kimberlite

samples.

Fresh kimberlites have low SiO2 (23.6–36.4

wt.%), high and variable MgO (18.5–37.2 wt.%),

low Al2O3 (1.38–5.05 wt.%) and TiO2 (0.38–1.41

wt.%), and variable FeO (6.1–12.6 wt.%) contents.

There is no significant difference between the Los

ios sheet, sample DDH-45

MgO CaO Na2O K2O Total Mg# Ca#

12.98 17.69 2.98 0.02 98.94 88.9 49.3

16.23 20.89 1.52 0.09 98.98 93.4 48.2

15.64 19.66 2.02 0.07 98.95 93.6 47.7

16.09 19.20 1.95 0.05 99.38 93.9 45.9

Table 8

Chemical compositions of kimberlites from Guaniamo

Componenta La Ceniza sheet Los Indios sheet

DDH97-64/1 DDH97-64/2 DDH97-65/2 DDH97-65/3 DDH-44/1 DDH-45/1 DDH-45/3

Major elements (wt.%)

SiO2 32.64 26.39 33.49 53.94 48.79 44.06

TiO2 0.87 1.41 0.56 0.41 0.88 0.74

Al2O3 2.40 1.50 1.38 1.09 3.22 1.22

Fe2O3 8.85 12.60 8.52 5.40 6.54 7.00

FeO 1.96 2.77 1.24 1.25 5.66 6.82

MnO 0.20 0.24 0.19 0.18 0.35 0.51

MgO 36.05 33.49 37.25 17.85 13.62 14.11

CaO 3.49 5.88 5.10 4.68 3.73 5.76

Na2O 0.27 0.34 0.47 0.54 0.17 0.20

K2O 1.68 0.42 0.42 2.70 2.70 1.53

P2O5 0.02 0.08 0.12 0.05 0.25 0.32

LOI 10.78 14.38 10.78 12.00 13.29 16.92

Total 99.21 99.50 99.52 100.09 99.20 99.19

H2O� 0.30 0.26 0.34 1.38 0.89 0.97

H2O+ 4.42 5.03 3.77 5.01 2.67 3.36

CO2 6.02 9.32 6.75 3.14 9.00 12.61

Stot. 0.07 0.04 0.04 0.09 0.20 0.21

C.I. 0.90 0.82 0.93 2.39 2.74 2.65

Trace elements (ppm)

Ni 1972 1889 1178 2641 1290 1100 1400

Cr 1960 2989 3442 1005 460 2700 2400

Co 114 110 102 114 73 85 127

Sc 15 21 22 9.3 27 20

V 27 72 60

Pb 6.7 7.2 12 7.3 18 25 24

Cu 52 177 67

Zn 52 55 58

Zr 22 26 62 28 71 52 37

Hf 0.6 2.1 2.0 0.5 1.5 0.7

Nb 88 66 350 140 444 237 219

Ta 6.5 7.5 20 6.4 21.0 13.0

Th 5.2 6.7 28 16 53 13.5 9.6

U 1.0 1.0 3.2 1.2 21 2.4 2.2

Sr 430 500 960 1000 94 630 734

Ba 779 1316 1015 803 53 960 523

Rb 50 85 23 26 184 156 92

Cs 1.0 2.0 1.1 0.04 2.9 2.4

Y 1.0 1.0 2.7 5.5 5.5 5.8 7.4

La 112 171 250 213 44 395 394

Ce 121 186 275 245 102 455 470

Nd 28 34 65 65 20 80 101

Sm 1.9 2.8 3.8 4.5 2.1 7.2 7.9

Eu 0.4 0.9 0.5 0.8 1.9 2.0

Tb 0.8 0.4 0.4 0.5 0.4 0.6

Yb 0.3 0.3 0.4 0.5 0.9 0.2

Lu 0.03 0.1 0.05 0.08 0.02 0.07

a Major oxides—wet silicate analysis; Ni, Cr, Co, Sc, Hf, Ta, Th, U, Cs, La, Ce, Nd, Sm, Eu, Tb, Yb and Lu—neutron activation analysis; V,

Pb, Cu, Zn, Y, Zr, Nb, Sr and Rb—XRF analysis.

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590 579

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590580

Indios and La Ceniza samples. High LOI values

(10.7–22.2 wt.%) reflect high volatile contents,

particularly in dolomite in both the groundmass

and in veins. High Ni (426–2641 ppm), Cr (195–

3442 ppm), and Co (32–114 ppm) values attest to a

mantle origin for the kimberlite magma.

Fig. 7. Major element chemical composition of Guaniamo kimberlite sheet

and group 2 kimberlites (kimberlite fields after Mahotkin et al. 1997); com

of depleted mantle after Sablukov et al. (2002).

In detail there is clear correspondence between

geochemistry and kimberlite petrography. Mica-

ceous kimberlite from La Ceniza (DDH97-64/2)

has higher Al2O3 and K2O than non-micaceous

samples (DDH97-65/2 and DDH97-65/3). These

differences are shown in Fig. 7 on diagrams of

s compared with fields for other kimberlite types, including group 1

position of primitive mantle after Jagoutz et al. (1979), composition

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590 581

Al2O3–TiO2 and K2O–TiO2, where the micaceous

kimberlite samples overlap the field for Group 2

kimberlites. Less micaceous samples show slight

overlap with the Group 1 kimberlite field. Alkali

ratios (Na2O/K2O) are approximately 1 in non-

micaceous kimberlite, while they are much less

than 1 in micaceous kimberlite. Among the trace

elements, Y, Nb, Pb, Th, Sr, La, Ce, Nd, and Sm

values are lower in micaceous vs. non-micaceous

Fig. 8. Sc–Ta and Zr–Nb plots for Guaniamo kimberlites, compared with

Sample symbols are the same as in Fig. 6. Fields for Arkhangelsk kimberl

kimberlite, while the opposite is true for Rb which

correlates with K2O and Al2O3, suggesting this

element resides in phlogopite.

In non-micaceous coarse-porphyritic kimberlite Ni/

Cr = 2.6, reflecting the high olivine content, while in

fine-porphyritic varieties Ni/Cr = 0.34 as a result of

the higher proportion of chrome spinel. This is also

responsible for higher MnO and Sc in fine-porphyritic

kimberlite. Fine-porphyritic kimberlite has higher Th,

primitive and depleted mantle, and other ultramafic compositions.

ites after Sablukov (1990), other fields after Mahotkin et al. (1997).

Fig. 9. Primitive mantle (Jagoutz et al., 1979; McDonough et al., 1992) normalized plot for kimberlites from Guaniamo.

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590582

U, Nb, Ta, Zr, Hf, Ti and LREE, and lower Y than

coarse-porphyritic kimberlite. These differences are

shown in Fig. 8 for Sc-Ta and Zr-Nb. The kimberlites

show a wide range of Nb values (28–350 ppm) at low

Zr (22–132 ppm).

MgO correlates negatively with increasing CaO,

and CO2 positively, both reflecting increase of car-

Table 9

Trace element ratios for Guaniamo kimberlites compared with other kimb

Element ratio Guaniamo Group 1

kimberlite

Group 2

kimberlite

Olivin

lampr

n= 10 r n= 17 r n= 14 r n= 60

P2O5/Ce (�E+ 04) 18.1 17.9 58 16 33 6 34

Nb/Zr 4.0 2.2 1.1 0.8 0.48 0.29 0.2

Nb/U 60.7 37.7 42 11 25 6 95

Ba/Rb 19.9 13.2 26 14 19 15 20

Nb/La 1.0 0.8 1.8 0.5 0.7 0.2 0.8

U/Th 0.2 0.1 0.21 0.05 0.17 0.04 0.11

Ce/Sr 0.3 0.1 0.24 0.12 0.32 0.1 0.31

Ni/MgO 43.5 14.4 40 8 49 12 42

Sc/Al2O3 7.8 3.8 7.2 2.3 6 2.1 5

Comparative data from Taylor et al., 1994, and Beard et al., 2000.

bonate and phlogopite as olivine and serpentine de-

crease. Sr and Ba, to a lesser extent, correlate

positively with CaO.

The distribution of major and trace elements in

the La Ceniza and Los Indios sheets, normalized to

primitive mantle composition (McDonough et al.,

1992; Jagoutz et al., 1979), are shown in Fig. 9.

erlite groups

e

oite

Koidu

kimberlite

Aries

kimberlite

Tres

Ranchos

Arkhangelsk

r n= 22 r n= 3 r n= 1 n= 9 r

14 18 9 9 3 18 99 45

0.08 1.7 0.8 4.2 0.6 1 0.45 0.11

46 62 16 100 31

7 24 8 12 2 35 16 9

0.2 1.4 0.3 1.7 0.1 0.8 1.6 0.7

0.06 0.16 0.05 0.1 0.04

0.12 0.57 0.11 0.74 0.37 0.38 0.12 0.05

6 42 5 61 14 49 46 5

1.9 6.7 0.7 6.2 0.8 6.2 3.3 1.3

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590 583

The Guaniamo kimberlites are strongly enriched in

incompatible elements and LREE, with approximate-

ly normal values of HREE and major elements. In

general, the kimberlites are characterized by negative

K, Rb, Cs, U, Hf, Zr and Y peaks and positive Th,

Nb, Ta and LREE peaks. The Al, Fe, Nb, Ta, Sc,

Fig. 10. Results of multi-element discrimination analysis for: A—major

Fields and factors after Taylor et al. (1994).

LREE and CO2 contents of the La Ceniza and Los

Indios sheets are close to Group 1 kimberlites,

whereas their Ti, K, Ca and H2O+ contents are more

similar to Group 2 kimberlites. The most distinctive

geochemical characteristics of the La Ceniza sheet

are its very high Nb/Ti ratio and very low P, Zr, Hf

element oxides, and B—major element oxides and trace elements.

okim

berlites

Sr

87Sr/86Sr (I)

e Sr

Sm

(ppm)

Nd

(ppm)

147Sm/144Nd

143Nd/144Nd

143Nd/144Nd(I)

e Nd

T(D

M)

T(CHUR)

03F19

0.704872

17.2

2.083

19.94

0.06315

0.511790F8

0.511496

�4.4

1.377

0.968

76F17

0.704965

18.5

5.840

68.55

0.05150

0.511856F6

0.511616

�2.1

1.216

0.821

43F19

0.704761

15.6

6.498

72.85

0.05392

0.511875F7

0.511624

�1.9

1.217

0.815

62F17

0.704903

17.6

53F15

0.704863

17.0

67F14

0.704476

11.5

3.690

46.1

0.04839

0.511839F6

0.511614

�2.1

1.209

0.822

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590584

and Y contents relative to Groups 1 and 2 kimber-

lites (Smith et al., 1985).

In terms of trace element ratios (Table 9), average

values of U/Th, Ce/Sr, Ni/MgO, and Sc/Al2O3 for

Guaniamo kimberlites are similar to Group 1 kimber-

lite average values. Ba/Rb and Ce/Sr average values

are similar to Group 2 kimberlites and olivine lamp-

roite average values. P2O5/Ce and Nb/U average

values are similar to those for the Koidu dykes in West

Africa, while P2O5/Ce and Nb/Zr values are similar to

the Aries kimberlite of north-west Australia. Com-

pared with Arkhangelsk kimberlites from the Zolotitsa

field (Beard et al., 2000), Ba/Rb and Ni/MgO ratios are

similar. When the ranges of values for the trace

element ratios are considered, there is more overlap

between Guaniamo and other kimberlite groups. It is

noteworthy that, at the 1r level, there is still no overlap

of Nb/Zr values between Guaniamo and Groups 1 or 2

kimberlites. This ratio is very similar to the Aries

kimberlite and there is strong overlap.

Discriminant plots based on results of multigroup

discriminant analysis for major element oxide compo-

sitions (after Taylor et al., 1994) show that Guaniamo

kimberlites overlap fields for Group 1 kimberlites (Fig.

10). Applying the same analysis to both trace andmajor

element oxides, a more complex picture emerges.

Group 1A, 1B, and 2 kimberlites form separate, well-

defined fields (Fig. 10), as doKoidu andAries in Taylor

et al. (1994). Guaniamo samples form a broad field

overlapping Group 1A, Koidu, and Aries kimberlite

fields. The Guaniamo field comes close to the Group 2

kimberlite field due to the influence of sample DDH97-

64/2, which is a micaceous kimberlite.

Table

10

Rb-SrandSm-N

disotopecompositionoftheGuaniam

Sam

ple

Rb

(ppm)

Sr

(ppm)

87Rb/86Sr

87Sr/86

LosIndiossheet

DDH-44/1

190.1

85.47

6.477F3

0.7705

DDH–45/1

162.2

617.4

0.7610F3

0.7126

DDH-45/3

97.0

713.6

0.3930F2

0.7087

LaCenizasheet

DDH97-64/1

47.9

406

0.3414F18

0.7083

DDH97-64/2

83.9

514

0.4727F18

0.7096

DDH97-65/2

22.5

954

0.0682F5

0.7051

Initialratiosreferto

710Ma.

7. Isotope characteristics and age

In order to gain more precise information on the

nature of the mantle source of the Guaniamo kimber-

lite samples and to assess their emplacement age, the

Sm-Nd and Rb-Sr isotope characteristics of the La

Ceniza and Los Indios kimberlites were determined

(Table 10).

The age of the Los Indios sheet, as determined

from a three-point Rb-Sr isochron, is 710.3F 6.5 Ma

(IR = 0.70486F 0.00028; MSWD=0.65; 87Rb/86Sr%

errors = 0.5; 87Sr/86Sr% errors = 0.05). This dating

agrees well with the age of La Ceniza sheet kimber-

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590 585

lites as determined from a three-point Rb-Sr isochron

at 783F 83 Ma (IR = 0.70444F 0.00040; MSWD=

0.55; 87Rb/86Sr% errors = 0.5; 87Sr/86Sr% errors =

0.05). The isochrons, with data-point error ellipses

as 2r, are presented in Fig. 11. The emplacement of

both the La Ceniza and Los Indios sheet kimberlites

occurred in the Neoproterozoic.

The average age of all Guaniamo kimberlites, as

determined from a six-point Rb-Sr isochron (samples

from La Ceniza and Los Indios sheets; Fig. 11), is

711.6F 5.9 Ma (IR = 0.70478F 0.00017; MSWD=

1.13; 87Rb/86Sr% errors = 0.5; 87Sr/86Sr% errors =

0.05). By excluding sample DDH-44/1, which has

an anomalously low Sr content (85.5 ppm) and high87Rb/86Sr ratio (6.477), the five-point Rb-Sr isochron

Fig. 11. Rb-Sr isochron diagrams

gives an age of 757F 49 Ma (IR = 0.70452F0.00032; MSWD = 0.39; 87Rb/86Sr% errors = 0.5;87Sr/86Sr% errors = 0.05). We accept the value of

712F 6 Ma as the most probable age of emplacement

of the Guaniamo kimberlite sheets; it includes all

determinations and has the lowest error value. This is

much younger than the Paleoproterozoic age (1730

Ma), which was previously reported for Guaniamo

kimberlites by Nixon et al. (1994), who did not have

access to fresh drill core, only to highly tropically

weathered kimberlite clays. The Sm-Nd model ages

(TDM= 1377–1209 Ma; Table 10) indicate the possi-

ble age of mantle metasomatism.

In the eSrt–eNd

t diagram corrected for an emplace-

ment age of 710 Ma (Fig. 12), the Guaniamo kim-

for Guaniamo kimberlites.

Fig. 12. Sr-Nd isotope diagram of the Guaniamo kimberlites. 1–4, kimberlites and related rocks of the Arkhangelsk area: 1—kimberlite, 2—

kimmelilitite, 3—olivine melilitites, 4—kimberlite and kimpicrites (Mahotkin et al., 1997). Group 2 kimberlite after Smith et al. (1985).

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590586

berlites lie between fields for Group 1 and Group 2

kimberlites, although closer to the Group 1 field. They

plot close to the field for transitional type kimberlites

(Skinner et al., 1994). However, the Guaniamo sam-

ples coincide closely with the field for Al-series

diamondiferous kimberlites of the Arkhangelsk dia-

mond province (Mahotkin et al., 1997; Beard et al.,

2000).

8. Discussion and conclusions

8.1. Classification of the Guaniamo kimberlites

The La Ceniza and Los Indios kimberlite sheets are

separate horizons, with the Los Indios unit approxi-

mately 70 m above the La Ceniza layer (Channer et al.,

2001). This study has not found any significant

mineralogical, geochemical, or isotopic difference

between the two sheets. Therefore, it is likely that

the two sheets, and probably also the other kimberlite

sheets in Guaniamo, formed as part of the same

intrusive event from the same source region.

The Guaniamo kimberlites cannot readily be clas-

sified as either ‘basaltoid’ Group 1 or ‘mica’ Group 2

kimberlites (Smith et al., 1985). Based on petrograph-

ic and mineralogical characteristics, the rocks being

studied show the following similarities to Group 2

kimberlites: sharp predominance of the ultramafic

chromian mineral association (forsterite, pyrope and

chromian spinel), a subordinate role of eclogitic suite

minerals (pyrope-almandine), and the almost com-

plete absence of ultramafic iron-titanian association

minerals (picroilmenite and orange titanian pyrope).

Quantitatively, garnet is much more abundant in the

examined rocks than chromian spinel. In addition,

phlogopite is abundant in the groundmass and both

clinopyroxene and perovskite are rare. A peculiar

feature of the Guaniamo kimberlites is the presence

of manganese ilmenite, which is also known to occur

as inclusions in Guaniamo diamonds (Kaminsky et

al., 2000). In contrast, the abundance of olivine

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590 587

macrocrysts is a feature more commonly found in

Group 1 kimberlites.

Chemically the Guaniamo kimberlites are equally

difficult to categorize. The least altered La Ceniza

sheet kimberlites have Al, Fe, K, Nb, Ta, LREE and

CO2 contents close to those of Group 1 South African

kimberlites, and Ti, Ca and H2O+ contents similar to

those characteristic of Group 2 kimberlites (Smith et

al., 1985). Trace element ratios such as U/Th, Ce/Sr,

Ni/MgO, and Sc/Al2O3 are similar to Group 1 kim-

berlite average values, while Ba/Rb and Ce/Sr are

closer to Group 2 kimberlite average values. The most

prominent geochemical features of the Guaniamo

kimberlites are their very high Nb/Zr ratio and very

low P, Zr, Hf and Y contents. The high Nb/Zr ratio of

the Guaniamo kimberlites is similar to the geochem-

ically distinctive Aries kimberlite of Australia (Taylor

et al., 1994). Taylor et al. (1994) suggested the

existence of a compositional spectrum between non-

micaceous Group 1 kimberlites, through mica-bearing

Koidu kimberlites, to the Aries end member. Using

multidiscriminant analysis of geochemical data (after

Taylor et al., 1994), the Guaniamo kimberlites plot in

the region between Group 1 kimberlites and Aries,

overlapping the Koidu kimberlites, and supporting the

ideas of Taylor et al. (1994).

Guaniamo kimberlites have etNd < 0 while etSr val-ues are similar to Group 1 kimberlites (Fig. 12).

Isotopically, the Guaniamo kimberlites show close

correspondence to Al-series kimberlites of the

Arkhangelsk Province (Sablukov, 1990). Transitional

kimberlites (Clark et al., 1991) have higher etSr andslightly higher etNd than Guaniamo kimberlites. Group

2 kimberlites have much higher etSr and much lower

etNd than the Guaniamo kimberlites. The Tres

Ranchos kimberlite in Brazil (F. Kaminsky and S.

Sablukov, unpublished data) has an isotopic signature

which overlaps the field for transitional kimberlites

and which is quite close to the Guaniamo signature.

Hence, the Guaniamo kimberlites, while petro-

graphically similar to Group 2 kimberlites, on chem-

ical and isotopic grounds, are clearly part of a

compositional spectrum which appears to exist be-

tween the standard reference Group 1 and 2 kimber-

lites, and other possible extreme end members such as

the Aries kimberlite.

The Guaniamo kimberlites show some similarities

with the Snap Lake kimberlite dyke in Canada

(Pokhilenko et al., 2001; Kirkley et al., 2003), most

obviously in morphology and high diamond grade.

Mineralogically, they both have low KIM content, the

presence of pyrope and chromian spinel with an

almost complete absence of picroilmenite, and the

presence of pyropes of lherzolite (and wehrlite) para-

genesis with Cr2O3>12 wt.%. However, there are also

sharp differences between the two kimberlites. The

majority of Snap Lake diamonds are peridotitic

(Pokhilenko et al., 2001), in contrast to Guaniamo.

Isotopically, the Snap Lake kimberlite is clearly a

Group 1 kimberlite with an age of 522.9 Ma (Kirkley

et al., 2003).

8.2. Nature of the mantle source

The sub-calcic pyrope garnets in the Guaniamo

kimberlites are derived from depleted harzburgitic and

lherzolitic subcontinental lithospheric mantle

(SCLM), which in the case of harzburgite is unique

to the Archean (Griffin et al., 2003). The limited

geochemical and isotopic data available for igneous

rocks of the Cuchivero Province of the Guyana Shield

show that this province is primarily juvenile Protero-

zoic crust (Tassinari and Macambira, 1999), but the

presence of sub-calcic garnets in the Guaniamo kim-

berlites is a clear sign that Archean SCLM has been

preserved to some extent beneath the Guyana Shield.

The high ultramaficity of the Guaniamo kimber-

lites (high Ni, Cr, Mg# and low Al2O3 and CaO) is

further evidence of the derivation from a depleted

harzburgite mantle source. However, the high concen-

trations of volatiles, incompatible elements and LREE

enrichment show that mantle metasomatism had oc-

curred before or syn-kimberlite melt formation. The

Mesoproterozoic model Nd age range for the Gua-

niamo kimberlites (TDM = 1377–1209 Ma) gives an

indication of when this metasomatism may have

occurred, producing incompatible element enriched

mantle peridotite. The Guaniamo kimberlites repre-

sent low degree partial melts of this metasomatised

mantle (Mitchell, 1986). The incompatible trace ele-

ment ratios in the Guaniamo kimberlite reflect the

original signature of the metasomatised mantle source,

plus any fractionation effects caused by residual

phases during partial melting.

The mantle beneath the Guaniamo area, judging by

an unusually high proportion of eclogitic-type inclu-

F.V. Kaminsky et al. / Lithos 76 (2004) 565–590588

sions in diamonds, was affected by subduction of

oceanic crust (Sobolev et al., 1998; Kaminsky et al.,

2000). Further support for this hypothesis was recent-

ly found in extremely elevated d18O values (from

+ 10.2x to + 16.4x) in coesite included in Gua-

niamo diamonds (Schulze et al., 2003a,b). Additional

support comes from the elevated oxygen isotope ratio

(up to + 9.26 x d18O) found in eclogitic garnet xen-

ocrysts from the La Ceniza kimberlite (Schulze et al.,

2003c).

It is possible that subduction beneath the Guyana

Shield at around 1200 Ma, the age of the Nickerie

event in the Guyana Shield, dragged K-metasomatised

peridotites from the mantle wedge down into the

mantle, along with the oceanic crust. As suggested

by Taylor et al. (1994), these peridotites would have

high Nb and F, and low Sr and P. This mechanism

provides both the metasomatising material plus the

carbon for eclogitic diamond genesis, and the oceanic

crust for eclogite formation.

8.3. Age of the Guaniamo kimberlites

In previous studies, Guaniamo kimberlite sheets

have been dated at 1730 Ma (Nixon et al., 1994), an

age which could suggest possible ingress of diamonds

into Proterozoic Roraima Supergroup sedimentary

rocks (1900–1550 Ma; Gaudette et al., 1996; Santos

et al., 2003), from Guaniamo area kimberlite sheets or

kimberlites of similar age. Whether diamonds

reported in Roraima Supergroup (Reid and Bisque,

1975) come from older, still undiscovered primary

sources, or from younger kimberlites intruded into the

Roraima, remains to be determined.

The results of this study, according to which local

kimberlite sheets were re-dated at 712F 6 Ma, allow

the identification of a new, Neoproterozoic kimberlite

formation epoch in the Guyana Shield.

Neoproterozoic kimberlites are not common, but

kimberlite rocks dated at approximately 700 Ma do

occur in the West African Craton (Bardet and Vach-

ette, 1966), which, according to plate tectonic recon-

structions, occurred during the Paleoproterozoic

integral to the Guyana Shield (see Zhao et al., 2002).

Zhao et al. (2002) review evidence for the existence of

a Paleo-Mesoproterozoic supercontinent, named Co-

lumbia, which broke up between 1.6 and 1.2 Ga, a

time of abundant mafic magmatism in the Guyana

Shield. This period was then followed by the assembly

of another supercontinent, known as Rodinia, which

finally broke up around 800–700 Ma (see Meert and

Torsvik, 2003 for a recent review), at the time when

the Guaniamo kimberlites were emplaced. The relative

positions of the continents are, however, less well

constrained during this time, due to lack of reliable

paleomagnetic data (Meert and Torsvik, 2003).

The presence of high diamond grade kimberlites in

a ‘non-traditional’ setting, combined with the chemi-

cal and isotopic evidence for a distinctive source, in

which subduction of oceanic crust played a key role,

is of significant importance with regard to identifica-

tion of new prospective areas, which might previously

have been regarded as uninteresting.

Acknowledgements

The authors are thankful to D.Z. Zhuravlev for the

Rb-Sr and Sm-Nd isotope analyses of the kimberlites

and to K.R. Ludwig for supplying us with program

ISOPLOT-3.00, which we used for calculation of

kimberlite ages. We thank our reviewers, Peter Nixon

and Jacques Letendre, for their valuable, constructive

comments, and the editor, Larry Heaman, for his

careful work with the manuscript. D. Schulze kindly

presented us with his unpublished manuscripts. I.

Coulson helped us with editing the manuscript.

Guaniamo Mining Company and its President, R.E.

Cooper provided financial support for this research.

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