Oldoinyo Lengai natrocarbonatite revisited: a cognate fluid condensate?

Carbonatite Workshop, 2000 The samples

used in the present

two Nb laterite Republic

deposits:

of Congo

work

Lueshe

and Araxa

come from

state.

In the altered

quartz,

pyrochlore

from Araxa, the as

suggested by the unusually large value of the quadrupole splitting, approximately 80% of this Fe3+ seems to be co-ordinated by only five anions.

and, together

Gorceixite undesired

ore types,

the rare earth deposit I, display with

lateritic

of Corrego

a total measured

do Garimpo,

reserve

Again,

concentration

magnetic

fractionation

The mineralogical containing

CatalZio

with minor barite,

monazite, cerianite,

ore,

small-sized.

spheroids

a few micrometers

the quartz 3.3%

matrix.

density

and

achieved

after

magnetic

Th. after size,

could

image

ore’s liberation analysis,

from the monazite.

very

calculated

poor monazite

and poor gangue

It is possible

performed

ation

spectrum

RE,O,,

to predict,

8.39%

authors

all the

would 30%.

It

concentration using

included

sample’s

carbonatite, mainly

strontianite, Monazite

grade

the liberdesliming and size

containing magnesite,

clinoenstatite,

is the only

6.64% dolomite, pyrite

important

and

RE carrier,

contains 59.7% REO and 4.5% Sr, and occurs as hexagonal prisms with a cavity along the prism. Prediction analysis

of the monazite

states

liberated

only

that

liberation

monazite

at particle

on the other

will

through

image

be reasonably

sizes of about

hand, displays

at 64 mm, and over 90%

50%

1 pm.

Its

liberation

at lpm.

liberation e.g., that

Oldoinyo Lengai natrocarbonatite revisited: a cognate fluid condensate? T.F.D. NIELSEN’,2 and I.V. VEKSLER’ ‘Danish Lithosphere Centre, Copenhagen K, DK-1350, Denmark 2Geological Survey of Denmark and Greenland, Copenhagen NV, DK-2400, Denmark

Oldoinyo magmas

authors

RE,O, and 23.73%

is quite complex

by other

monazite

of only

to simulate

as all the tests

comprises

Natrocarbonatite

Its mineralogy

without

50% monazite

recovery

possible

and so changed

of concentration tests performed by other agree very well with these results. The lateritic ore contains

containing

distributions.

only 24% of the monazite will be recovered for a concentrate containing 50% of the phase. Simulations

Fe,O,.

confirms

be

in the fines,

spectrum,

indicates

from its gangue

only

spectrum

displaying

in 36% of the total mass, concentrates

of the total

tests

gangue,

separation,

of the Fe content

through

of of

in the fines, too.

liberation

up for a monazite

fluor-

as

about

mass

50%

isokite.

RE,O,, besides

in the fines of the ore (< 20pm),

The silexitic liberation

57.6%

ore’s

70%

mass, but 46%

discard

monazite,

appears

of rare earth elements,

half of the elements

but 43% too.

usually

large dispersed

It contains

Sr and no detectable

Concentrations

about

Monazite

displaying

of the total

and in the

from its gangue, but the bad as it is possible to

hem-

apatite and hollandite. Monazite is by far the most important rare earth carrier, as cerianite is uncommon and very

of REE by size, density

concentrates

The mineralised

of the silexitic

RE,O,, is quartz,

atite and goethite,

of

of 4.2%,

could only be achieved

in 44%

has not been

RE,O,.

composition

10.5%

its Sr content

monazite’s poor liberation gangue liberation is not

come

of 30 million tons,

a mean ore grade of 7.56%

with

loss. A concentrate

and silexitic,

hydrobiotite,

may contain up to 1.6% REE, but is an phase for concentration purposes.

The lateritic

distinct

vermiculite,

makes this mineral interesting for recovery as byproducts. Other minor phases, although containing some rare earth elements (REE), are irrelevant.

the Fe content

and J.V. VALARELL12 ‘CETEM - Centre for Mineral Technology, Rua 4, Quadra D, Cidade Universitaria, 21941-590 Rio de Janeiro, Brazil 2Geosciences institute, University of Slo Paulo, PO Box 11348, 05422-970 S6o Paulo, Brazil E-mail: [email protected]

ilmenite, and anatase.

and apatite. Monazite contains 57.1% RE,O,, 3.3% Sr and no detectable Th. Apatite’s RE,O, content is

the elements

R. NEUMANN’

gorceixite

calzirtite, baddeleyite and zircon were also The main rare earth carriers are monazite

fines of the ore (< 20pm),

Technological characterization of the potential RE ores from C6rrego do Garimpo, Catako, Central Brazil

The two

magnetite,

monazite,

priderite,

zirconolite, identified.

1.4%,

all the Fe is present as Fe3+. Spectra of both samples are consistent with presence of all the Fe3+ on the B site. However,

fluorapatite,

goethite,

Pyrochlore,

in Brasil.

In contrast with the assumption that all Fe is present as Fe3+, in the sample from the altered pyrochlore from Lueshe, 47 + 2% of the total Fe is in the Fe2+ oxidation

mainly hematite,

in the Democratic

and comprises

vided conclusive origin

flows

in the crater

of the volcano

Lengai (Tanzania) are the only carbonatite observed to erupt. These flows have proarguments

of carbonatite,

in favour of the magmatic

including

plutonic

varieties.

Journal of African Earth Sciences A-2 7

Carbonatite

Workshop, 2000

However, eruption temperatures of 544-593’C, and drastic compositional and mineralogical differences question the genetic relationship to intrusive, generally

carbothermal

calcite-rich,

of the deposits is to contain rare earth element (REE) and Th minerals, fluorite, apatite and barite in economic

genesis

carbonatite

and the currently

of natrocarbonatite

favoured

by silicate

melt,

car-

bonatite melt separation referred to as liquid immiscibility. The authors agree with those who argue that the experimental a liquid immiscibility

results provide evidence origin of Oldoinyo

against Lengai

natrocarbonatite. The authors advocate, as previously suggested and supported by our recent melt inclusion studies and hydrothermal Lengai

experiments,

natrocarbonatite

flows

that Oldoinyo

are expulsions

of

processes,

related

with

fluids

en-

riched in CO,, Cl, F, PO,, H,O, is the absence of sulphide mineralisation. A common characteristic

grade.

The main

REE minerals

are carbonates

phosphates. Carbonatites are associated with alkaline rocks: quartz syenite, feldspathoid-bearing

and

silicate syenite

(miaskite, foyaite), phonolite and trachyte. Carbonatites consist of calcite-carbonatite in the Kizilcaoren and Sofular Districts. Carbonatite in the Felahiye District

has not been characterised

mobile, alkaline and CO,-rich vapour conIn contrast to all others, this genesis for

of a calcite

Oldoinyo

Lengai

Two varieties of carbonatite have been distinguished in the Kizilcaoren and Sofular Districts: (i) REE

view

natrocarbonatite

in agreement

with

the

is in the authors’ experimental

data,

geochemical characteristics and observations. All experimental investigations of natural carbonatite and silicate rock systems fail to reproduce Oldoinyo Lengai natrocarbonatite. The equilibrated carbonatite liquids are all alkali-bearing but too calcic to cross the thermal barrier to natrocarbonatite composition. The lack of equilibrium between natrocarbonatite and appropriate Ca-bearing phases that could have driven experimentally equilibrated alkali-bearing carbonatite liquids to natrocarbonatite compositions make the origin of Oldoinyo melts

Lengai natrocarbonatite

by liquid immiscibility

unlikely.

densates

1 O-20%

of compositions

natrocarbonatite. carbonatite

alkalis would

produce

similar to Oldoinyo

The origin of Oldoinyo

as condensate

from

carbonate;

deposit,

the carbonatite

generally

subordinate

In the Sofular

type.

associated

with fluorite

formation

magmas

series.

attenuation

by

suture

oceanic

at

and carbonatites

at each of the three localities lithospheric

of the juxtaposition Plates along

of a northern

branch

of the a wellof the

crust.

carbonatites Dissolution of apatite ore from Angico DOS Dias Carbonatite Complex (Brazil) and recovery of rare earth elements

Department of Geology, University of Dokuz Eylijl, Izmir, Turkey Carbonatites and related fluorite and rare earth mineralisation have been identified at three different localities in Turkey. These are the Kizilcaoren (Eskisehir), Sofular (Malatya) and Felahiye (Kayseri) Districts. Ore processes and mineral parageneses of the

F. PEREIRA’, E. BILAL’, J. MOUTTE’, F.L. LAPIDO*, J.J. GRUFFAT’ and B. ALBERT’ ‘Spin-Geochimie, Ecole des Mines, 158 Cours Faurel, St Etienne, France 2CETEM Centre for Mineral Technology, Rua 4, Quadra

D, Cidade Universitaria,

21941-590

Rio de Janeiro,

deposits display obvious differences from the wellknown hydrothermal, pegmatitic and pneumatolytic processes related with acidic rocks. The most

The sulphuric

distinctive

fertiliser

deposits

governed

by

acid process,

industry

a

on ore

and Middle Miocene.

and Tauride

zone

place

inclusion studies reveal that the

post-collisional

as a result Anatolide

suggest that

took

rocks

The carbonatite generated

veins are

350-500%.

Lengai natro-

I. OZGENC

A-28 Journal of African Earth Sciences

crystallisation

was between

NeoTethysian

of the

studies on carbonatites

Late Cretaceous

were

is of the REE

britholite

veins.

The ages of the alkaline

known

feature

Monomineralic

temperature of 55O’C. Fluid fluorite, apatite and britholite

and the disequilibrium

of Turkish

the carbonatite

phosphate

Pontide,

Characteristics

to the late stage carbonatite.

deposit,

carbonatite

type.

are barren. Bastnaesite is and barite in veins that are

range between

fluid

REE are contained

is of the REE carbonate

Early stage carbonatites associated with fluorite

corroborates with the isotopic signatures, the trace element distributions, including the Cl-F distribution in the Th-U decay

and (ii) REE phosphate.

Lengai

co-magmatic

euhedral

in bastnaesite (Th-rich REE carbonate), brockite and britholite (Th-Ce-rich REE phosphate). In the Kizilcaoren

the

fluid con-

and coarse-grained

crystals.

Fluid inclusion

Hydrothermal

experiments show that the condensate of the comagmatic fluid phase has three times the alkali content of its parental melt. Alkali-bearing carbonatite melts with

sanidine

groundmass

yet; it consists

cognate, densate.

generally

for the production

Brazil

used by the of phosphoric