Transcript Kimberlite

Kimberlite, Carbonatite and Lamproite
Carbonatite: a carbonate rock of apparent magmatic origin,
generally associated with kimberlite and alkline rocks. They
have been variously explained as derived from magmatic melt,
solid flow, hydrothermal solution, and gaseous transfer. A
carbonatite may be calcitic (sovite) or dolomitic (rauhaugite).
Lamproite: A group name for dark-colored hypabyssal or
extrusive rocks rich in potassium and magnesium.
Kimberlite
What is a kimberlite? Based on Clement et al. (1984), kimberlite is a
volatile-rich, potassic, ultrabasic, igneous rock which occurs as small
volcanic pipes, dykes, and sills. It has a distinctively inequigranular
texture resulting from the presence of macrocrysts set in a finergrained matrix. This matrix contains, as prominent primary
phenocrystal and/or groundmass constituents, olivine and several of
the following minerals: phlogopite, carbonate (commonly calcite),
serpentine, clinopyroxene (commonly diopside), monticellite, apatite,
spinel, perovskite and ilmenite. The macrocrysts are anhedral,
mantle-derived, ferromagnesian minerals which include olivine,
phlogopite, picroilmenite, chromian spinel, magnesian garnet,
clinopyroxene (commonly chromian diopside), and orthopyroxene
(commonly enstatite). The macrocrysts and relatively early-formed
matrix minerals are commonly altered by deuteric processes, mainly
serpentinization and carbonatization. Kimberlite commonly contains
inclusions of upper mantle-derived ultrabasic rocks.
Petrology
Kimberlites are divided into Group I (basaltic) and Group II
(micaceous) kimberlites. This division is made along mineralogical
grounds.
The general consensus reached on kimberlites is that they are
formed deep within the mantle, at between 150 and 450 kilometres
depth, from anomalously enriched exotic mantle compositions, and
are erupted rapidly and violently, often with considerable CO2 and
volatile components. It is this depth of melting and generation which
makes kimberlites prone to hosting diamond xenocrysts.
The mineralogy of Group I kimberlites is considered to represent the
products of melting of lherzolite and harzburgite, eclogite and
peridotite under lower mantle conditions. The mineralogy of Group II
kimberlites may represent a similar melting environment to that of
Group I kimberlites, the difference in mineralogy being caused by the
preponderance of water versus carbon dioxide.
Group I kimberlites
Group I kimberlites are of CO2-rich ultramafic potassic igneous
rocks dominated by a primary mineral assemblage of forsteritic
olivine, magnesian ilmenite, chromian pyrope, almandine-pyrope,
chromian diopside (in some cases subcalcic), phlogopite, enstatite
and of Ti-poor chromite. Group I kimberlites exhibit a distinctive
inequigranular texture cause by macrocrystic (0.5-10 mm) to
megacrystic (10-200 mm) phenocrysts of olivine, pyrope, chromian
diopside, magnesian ilmenite and phlogopite in a fine to medium
grained groundmass.
The groundmass mineralogy, which more closely resembles a true
composition of the igneous rock, contains forsteritic olivine, pyrope
garnet, Cr-diopside, magnesian ilmenite and spinel.
Group II kimberlites
Group-II kimberlites (or orangeites) are ultrapotassic, peralkaline
rocks rich in volatiles (dominantly H2O). The distinctive
characteristic of orangeites is phlogopite macrocrysts and
microphenocrysts, together with groundmass micas that vary in
composition from phlogopite to "tetraferriphlogopite" (anomalously
Fe-rich phlogopite). Resorbed olivine macrocrysts and euhedral
primary crystals of groundmass olivine are common but not
essential constituents.
Characteristic primary phases in the groundmass include: zoned
pyroxenes (cores of diopside rimmed by Ti-aegirine); spinel-group
minerals (magnesian chromite to titaniferous magnetite); Sr- and
REE-rich perovskite; Sr-rich apatite; REE-rich phosphates
(monazite, daqingshanite); potassian barian hollandite group
minerals; Nb-bearing rutile and Mn-bearing ilmenite.
Kimberlitic indicator minerals
Kimberlites are peculiar igneous rocks because they contain a
variety of mineral species with peculiar chemical compositions.
These minerals such as potassic richterite, chromian diopside (a
pyroxene), chromium spinels, magnesian ilmenite, and garnets rich
in pyrope plus chromium are generally absent from most other
igneous rocks, making them particularly useful as indicators for
kimberlites.
These indicator minerals are generally sought in stream sediments
in modern alluvial materials. Their presence, when found, may be
indicative of the presence of a kimberlite within the erosional
watershed which has produced the alluvium.
Kimberlite and some rare xenoliths within it are the principal primary
terrestrial source of the strategic mineral, diamond, and the study
of kimberlite may eventually outline the conditions for the formation
of diamond in nature.
Geochemistry
The geochemistry of Kimberlites is defined by the following
parameters:
Ultramafic; MgO >12% and generally >15%
Ultrapotassic; Molar K2O/Al2O3 >3
Near-primitive Ni (>400ppm), Cr (>1000ppm), Co (>150ppm)
REE-enrichment
Moderate to high LILE enrichment; ΣLILE = >1,000ppm
High H2O and CO2
Kimberlite magmas form "pipes" as they erupt. A tuff cone is at the
surface and formed by base-surge deposits. In the subsurface, a funnelshaped body narrows to a depth of hundreds of meters. The pipe (also
called a diatreme) is filled with kimberlite, with or without diamonds (only
1 in 5 of the pipes at Kimberley contain diamonds). Simplified from
Hawthorne (1975).
Just how many diamonds are needed to make a pipe economical? Some
South African mines operate at 25 carats of diamond per 100 cubic
meters of rock or about 2 grams of diamonds per 100 tons of rock.
Because diamond has a specific gravity of 3.5 grams per cubic
centimeter, 1 cubic centimeter of diamond weighs 16 carats. Picture a
giant 100-ton ore truck full of kimberlite - that truck contains only half of a
cubic centimeter of diamonds! Only about 35% of those diamonds are
gem quality.
Uses for Diamonds
Most diamonds are used in drill bits and diamond tools. A small number are
used for glass cutters and surgical instruments. Only the finest are used as
gems.
World Supply of Diamonds
Australia is currently the world's largest producer of diamonds. Most of these
diamonds are low quality and used for industrial purposes. Most of the
diamonds are from the Argyle diamond pipe in northern Western Australia.
The pipe at Argyle is made of lamproite, not kimberlite. The mine produced
27.8 million carats (1 ct = 200 mg; 5 carats = 1 gram) of low grade diamonds
in 1993-1994. Because of the high rate of production at Argyle, mining
operations will end within the next few years.
The mines at Kimberley, South Africa have produced a total of more than 200
million carats since the 1870s. About half of South Africa’s diamonds are gem
quality.
Most kimberlites are confined to the ancient cratons
(or areas underlain by the cratons), and relatively few
are found in the circum-cratonic fold belts. And, ages
of most kimberlites are in the Late Mesozoic Era
(Jurassic-Cretaceous).
Kimberlite fields with ages (Ma)
Diamond Geology
Kimberlite Pipes
Diamonds form at a depth greater than 93 miles (150 kilometers) beneath
the earth's surface. After their formation, diamonds are carried to the
surface of the earth by volcanic activity. A mixture of magma (molten rock),
minerals, rock fragments, and occasionally diamonds form pipes shaped
like champagne flute glasses as they approach the earth's surface. These
pipes are called kimberlites (see diagram below). Kimberlite pipes can lie
directly underneath shallow lakes formed in the inactive volcanic calderas
or craters.
Diamonds in the kimberlite
Mineralogical and geochemical signatures of kimberlites in glacial
sediments, Kirkland Lake, Ontario
Kimberlite Indicator Minerals
Several minerals, when found in glacial sediments, are useful indicators of the
presence of kimberlite, and to a certain extent, in evaluation of the diamond
potential of kimberlite. These minerals are far more abundant in kimberlite
than diamond, survive glacial transport, and are visually and chemically
distinct. Cr-pyrope (purple colour, kelyphite rims), eclogitic garnet (orange-red),
Cr-diopside (pale to emerald green), Mg-ilmenite (black, conchoidal fracture),
chromite (reddish-black, irregular to octahedral crystal shape), and olivine
(pale yellow-green) are the most commonly used kimberlite indicator minerals
in drift prospecting, although in rare cases, diamond is abundant enough to be
its own indicator. Kimberlite indicator minerals are recovered from the medium
to very coarse sand-sized fraction of glacial sediments, and analyzed by
electron microprobe to confirm their identification. For this study, indicator
minerals were picked from the 0.25-0.5 mm, 0.5-1.0 mm, and 1.0 to 2.0 mm
non-ferromagnetic heavy mineral concentrates of 10 to 20 kg glacial sediment
samples and analyzed at the GSC using a four spectrometer Cameca SX50
electron microprobe.
Mg-ilmenite
Kimberlitic ilmenites can be
distinguished from other ilmenites
by their high MgO content, typically
containing >4 wt.%. Ilmenites from
Diamond Lake kimberlites (red dots)
contain 4 to 15 wt.% MgO, and are
low in Cr2O3, most grains contain
<1 wt.% Cr2O3. Glacial sediments
(blue dots) contain kimberlitic
ilmenite as well as ilmenite form
other sources. Each kimberlite has a
distinct Cr2O3 versus MgO
signature that is mimicked by the
glacial sediments and kimberlite
boulders collected down-ice, as
shown in the Cr2O3 versus MgO
plots.
Pyrope
Subcalcic harzburgitic garnets are associated with
diamondiferous kimberlites. They can be
differentiated from other lherzolitic, harzburgitic or
dunitic garnets by plotting CaO versus Cr2O3. The
diagonal line separating lherzolitic and harzburgitic
garnets is the 85% line defined by Gurney (1984).
Garnets that fall below the 85% line, i.e. low-Ca Crpyropes, are "subcalcic" or G10 garnets derived
from harzburgite. The high chrome content gives
these garnets a distinct lilac purple color. The
vertical line separates Cr-poor, orange-red garnets
having <2 wt. % Cr2O3 from the purple peridotitic
garnets. Garnets from Diamond Lake kimberlites
(red dots) are mostly G9 (lherzolitic), and only a
few are subcalcic-G10 garnets. Garnets in the
glacial sediments, in general, have similar
compositions to those in the kimberlites. The low
abundance of G10 garnets in the kimberlites and
glacial sediments is consistent with the low
diamond grades of these pipes.
The G10 field is better described as “diamond in” field. It will contain lherzolitic
garnets (>45 Kb) which could be diamond bearing (garnets in G9 field may be
harzburgitic that will not be high pressure enough to be diamondiferous).
Chromite
Chromite associated with diamonds
has a high Cr2O3 content (>60 wt. %)
and moderate to high level (12-16
wt.%) of MgO. The compositions of
chromites from Diamond Lake
kimberlites represent a "poor"
chromite population with only a few
chromite xenocrysts approaching the
diamond inclusion field. This is
consistent with the trace quantities of
diamonds found in these pipes.
Chromite grains in glacial sediments
show similar compositions to those
from the kimberlites, suggesting many
of the grains are from kimberlite.
Cr-diopside
Cr-diopside Cr-rich (>0.5 wt.% Cr2O3)
diopside is easily identified by its
distinctive green color. It indicates the
presence of kimberlite but provides little
information on the presence of diamonds
in the kimberlite. Cr-diopsides in the
Kirkland Lake area occur in ultramafic
rocks as well as kimberlites, although only
kimberlites contain very Cr-rich (>1.5 wt.%
Cr2O3) diopsides. The Diamond Lake
kimberlites contain pale green (0.5 wt.%
Cr2O3) to bright green (1.0 to 4.0 wt.%
Cr2O3) Cr-diopside. The presence of Crdiopside is not a useful kimberlite indicator
on its own, because of the ubiquitous
distribution of Cr-diopside grains in glacial
sediments across the Kirkland Lake region
and the difficulty in distinguishing between
Cr-diopsides from kimberlite and those
from other rocks.
Olivine
Olivine has hitherto rarely been used as
kimberlite indicator mineral since it occurs
in abundance in basalts and other
ultramafic rocks and is not unique to
kimberlite. It is, however, the most
abundant mineral in the upper mantle (main
component of peridotite) and occurs as a
macrocryst and phenocryst phase in
kimberlite. It is often the most abundant
mineral in heavy mineral concentrates from
kimberlites. Olivine from kimberlite and
peridotite is MgO-rich (close to the
forsterite end member of the olivine solid
solution series) and is colorless to pale
yellow or pale green. Its chemistry is
characterized by high Mg-number
(Mg/(Mg+Fe) between 84 and 95 and
notable traces of NiO.
Diamond Mining - Extraction Methods
Diamonds and other precious and semi-precious gemstones are extracted
from the earth via three main types on mining. These diamond extraction
methods vary depending on how the minerals are deposited within the
earth, the stability of the material surrounding the desired mineral, and the
peripheral damage done to the surrounding environment. The principle
methods of extraction for diamonds are:
Artisanal Mining
Hard Rock Mining
Marine Mining
Open Pit Mining
Placer Mining
Hard-Rock Mining
The term "Hard Rock Diamond Mining" (top of page, left) refers to
various techniques used to mine gems, minerals, and ore bodies by
tunneling underground and creating underground "rooms" or "stopes"
supported by timber pillars of standing rock. Accessing the underground
ore is achieved via a "decline" or a "shaft". A "decline" is a spiral tunnel
which circles the flank of the ore deposit or circles around the deposit. A
"shaft" is vertical tunnel used for ore haulage and runs adjacent to the
ore. A "decline" is generally used for personnel and machinery access to
the ore.
Open Pit Mining
Open-Pit diamond mining or "Open-Cast Mining" (top of page, middle) is
a method of extracting rock or minerals from the earth by removal from
an open pit or burrow. Open pit mines are used when deposits of
minerals are found near the surface or along kimberlite pipe. Open pit
mining is used when the "overburden," or surface material covering the
deposit, is relatively thin and/or the minerals are imbedded in structurally
unstable earth (cinder, sand, or gravel) that is unsuitable for tunneling.
"Pit lakes" tend to form at the bottom of open-pit mines as a result of
groundwater intrusion.
Placer Mining
Placer Diamond Mining, also known as "sand bank mining" (top of
page, right) is used for extracting diamonds and minerals from alluvial
secondary deposits. Placer Mining is a form of open-pit or open-cast
mining used to extract minerals from the surface of the earth without
the use of tunneling. Excavation is accomplished using water pressure
(a.k.a. hydraulic mining), mechanized surface excavating equipment,
or digging by hand (artisanal Mining).
Marine Mining
Marine mining technology only became commercially viable in the early
1990s. Marine diamond mining employs both "vertical" and "horizontal"
techniques to extract diamonds from offshore placer deposits. Vertical
marine mining uses a 6 to 7 meter diameter drill head to cut into the
seabed and suck up the diamond bearing material from the sea bed.
Horizontal mining employs the use of Seabed Crawlers (remotely
controlled, CAT-tracked underwater mining vehicles) move across the sea
floor pumping gravel up to an offshore vessel.
Carbonatite:
Carrier for Nb, Ta, REE
鈮鉭及稀土金屬礦物種類繁多，其中重要的工業礦物有：
REE:
Monazite
獨居石 (Ce,La)[PO4]
Bastnaesite 氟碳鈰礦 Ce[CO3]F
Parisite
氟碳鈣鈰礦 Ce2Ca[CO3]3F2
Rinkolite
綠層矽鈰鈦礦 CeNa2Ca4Ti[Si4O15]F3
Nb and Ta:
Niobite
鈮鐵礦 (Fe,Mn)Nb2O6
Tantalite
鉭鐵礦 (Fe,Mn)Ta2O6
Pyrochlore
燒綠石 (Ca,Na)2(Nb,Ti)2O6F
Microlilte
細晶石 CaNa(Ta,Ti)2O6(OH,F)
Fergusonite 褐釔鈮礦 Y(Nb,Ta)O4
Loparite
鈦鈮鈣鈰礦 (Ca,Na,Ce)(Ti,Nb)O3
Euxenite
黑稀金礦 (Y,Ca,Ce,U,Th)(Nb,Ta,Ti)2O6
Samarskite
鈮釔礦 (Y,Ce,U,Ca,Pb)(Nb,Ta,Ti,Sn)2O6
Simpsonite
鉭鋁石 Al4(Ta,Nb)3O13(OH,F)
Tantalite
Niobite
IDENTIFICATION
SYNONYMS: Nephelinitic and ultramafic carbonatite-hosted deposits.
BYPRODUCTS: Niobium, tantalum, REE, phosphate, vermiculite, Cu,
Ti, Sr, fluorite, Th, U magnetite (hematite, Zr, V, nickel sulphate,
sulphuric acid, calcite for cement industry).
EXAMPLES:
Magmatic: Aley (REE, niobium), St. Honor?(niobium, Quebec, Canada),
Mountain Pass (REE, California, USA), Palabora (apatite, South Africa).
Replacement/Veins: Rock Canyon Creek (Fluorite, REE), Bayan Obo
(REE, China), Amba Dongar (fluorite, India), Fen (Fe, Norway),
Palabora (Cu, vermiculite, apatite, South Africa).
Residual: Araxa, Catalao and Tapira (niobium, phosphate, REE, Ti,
Brazil), Cargill and Martison Lake (phosphates, Ontario, Canada).
GEOLOGICAL CHARACTERISTICS
Carbonatites are igneous rocks with more than 50% modal carbonate
minerals; calcite, dolomite and Fe-carbonate varieties are recognized.
Intrusive carbonatites occur commonly within alkalic complexes or as
isolated sills, dikes, or small plugs that may not be associated with other
alkaline rocks.
Carbonatites may also occur as lava flows and pyroclastic rocks. Only
intrusive carbonatites (in some cases further enriched by weathering) are
associated with mineralization in economic concentrations which occur as
primary igneous minerals, replacement deposits (intra-intrusive veins or
zones of small veins, extra-intrusive fenites or veins) or residual
weathering accumulations from either igneous or replacement protores.
Pyrochlore, apatite and rare earth-bearing minerals are typically the most
sought after mineral constituents, however, a wide variety of other minerals
including magnetite, fluorite, calcite, bornite, chalcopyrite and vermiculite,
occur in economic concentrations in at least one carbonatite complex.
TECTONIC SETTING: Carbonatites occur mainly in a continental
environment; rarely in oceanic environments (Canary Islands) and are
generally related to large-scale, intra-plate fractures, grabens or rifts that
correlate with periods of extension and may be associated with a broad zones
of epeirogenic uplift.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Carbonatites
intrude all types of rocks and are emplaced at a variety of depths.
AGE OF MINERALIZATION: Carbonatite intrusions are early Precambrian to
Recent in age; they appear to be increasingly abundant with decreasing age.
HOST/ASSOCIATED ROCK TYPES: Host rocks are varied, including calcite
carbonatite (sovite), dolomite carbonatite (beforsite), ferroan or ankeritic
calcite-rich carbonatite (ferrocarbonatite), magnetite-olivine-apatite ?
phlogopite rock, nephelinite, syenite, pyroxenite, peridotite and phonolite.
Carbonatite lava flows and pyroclastic rocks are not known to contain
economic mineralization.
DEPOSIT FORM: Carbonatites are small, pipe-like bodies, dikes,
sills, small plugs or irregular masses. The typical pipe-like bodies
have subcircular or elliptical cross sections and are up to 3-4 km in
diameter. Magmatic mineralization within pipe-like carbonatites is
commonly found in crescent-shaped and steeply-dipping zones.
Metasomatic mineralization occurs as irregular forms or veins.
Residual and other weathering-related deposits are controlled by
topography, depth of weathering and drainage development.
TEXTURE/STRUCTURE: REE minerals form pockets and fill
fractures within ferrocarbonatite bodies. Pyrochlore is disseminated;
apatite can be disseminated to semi-massive; bastnaesite occurs as
disseminated to patchy accumulations; fluorite forms as veins and
masses; hematite is semi-massive disseminations; and chalcopyrite
and bornite are found in veinlets.
ORE MINERALOGY [Principal and subordinate]:
Magmatic: bastnaesite, pyrochlore, apatite, anatase, zircon,
baddeleyite, magnetite, monazite, parisite, fersmite.
Replacement/Veins: fluorite, vermiculite, bornite, chalcopyrite and
other sulphides, hematite.
Residual: anatase, pyrochlore and apatite, locally crandallite-group
minerals containing REE.
GANGUE MINERALOGY [Principal and subordinate]: Calcite,
dolomite, siderite, ferroan calcite, ankerite, hematite, biotite, titanite,
olivine, quartz.
ALTERATION MINERALOGY: A fenitization halo (alkali metasomatized
country rocks) commonly surrounds carbonatite intrusions; alteration
mineralogy depends largely on the composition of the host rock. Typical
minerals are sodic amphibole, wollastonite, nepheline, mesoperthite,
antiperthite, aegerine-augite, pale brown biotite, phlogopite and albite. Most
fenites are zones of desilicification with addition of Fe3+, Na and K.
WEATHERING: Carbonatites weather relatively easily and are commonly
associated with topographic lows. Weathering is an important factor for
concentrating residual pyrochlore or phosphate mineralization.
ORE CONTROLS: Intrusive form and cooling history control primary
igneous deposits (fractional crystallization). Tectonic and local structural
controls influence the forms of metasomatic mineralization. The depth of
weathering and drainage patterns control residual pyrochlore and apatite
deposits, and vermiculite deposits.
GENETIC MODELS: Worldwide, mineralization within carbonatites is syn- to
post-intrusion and commonly occurs in several types or stages:
1) REE-rich carbonatite and ferrocarbonatite, magmatic magnetite,
pyrochlore
2) Fluorite along fractures
3) Barite veins
4) U-Th minerals + silicification
5) calcite veining and re-precipitation of Fe oxides (hematite)
6) Intense weathering may take place at any later time.
Magmatic mineralization may be linked either to fractional crystallization or
immiscibility of magmatic fluids. Metasomatism and replacement are
important Not all mineralization types are associated with any individual
carbonatite intrusion. In general, it is believed that economic Nb, REE
and primary magnetite deposits are associated with transgressive (late)
igneous phases, but understanding of the majority of deposits is not
advanced enough to propose any general relationship of timing.
mineralization at St. Honor? for example, is probably relatively earlyformed.
ASSOCIATED DEPOSIT TYPES: Nepheline syenite and nepheline
syenite-related corundum deposits and sodalite. REE and zircon placer
deposits can be derived from carbonatites. Wollastonite occurrences are
in some cases reported in association with carbonatites. Fluorite deposits
are known from the roof zones of carbonatite complexes. Kimberlites and
lamproites (common host-rocks for diamonds) may be along the same
tectonic features as carbonatites, but are not related to the same
magmatic event.
COMMENTS: Carbonatites should be evaluated for a variety of the
mineral substances as exemplified by the exceptional Palabora
carbonatite which provides phosphate (primary and possibly
hydrothermal), Cu (hydrothermal), vermiculite (weathering) and also Zr, U
and Th as byproducts. While extrusive carbonatite rocks are known to
contain anomalous REE values, for example the Mount Grace pyroclastic
carbonatite in British Columbia, they are not known to host REE in
economic concentrations.
Rare Elements in Araxa Carbonatites
In recent years there has been an increasing economic
interest in the REE found in the laterites（紅壤） formed on
REE-rich alkaline complexes. REE contents of up to 42%
have been described in laterites on the Mt Weld alkaline
complex in Western Australia. Those in the Longnan and
Xunwu in the Jiangxi province in south China can also be
considered to be laterites. When searching for high REE
contents, the most promising laterites are likely to be those
developed on top of alkaline complexes; the RE2O3 contents
of laterites found on top of alkaline rocks, such as nepheline
syenites, melteigites, ijolites and carbonatites, often amount to
10-25%. The most famous example is the laterites formed on
top of the alkaline complexes of Araxa and Catalao, Brazil.
The Araxa or Barreiro alkaline complex is situated 2 km
south of the town of Araxa (Minas Gerais State) within a fold
belt of Brasiliano age. It is intruded into quartzites and mica
schists of the Precambrain metasedimentary sequences of the
Araxa Group and is a circular complex of about 4.5 km
diameter. Drilling has proved the central area of the complex
to be principally alkaline rocks with an outer collar of
glimmerites. The alkaline rocks are coarse- to mediumgrained sovites (calcite carbonatites) and rauhaugites
(dolomitic carbonatites). The glimmerites are the products of
an alteration of original pyroxenites and peridotites by an
interaction with metasomatic alkali-rich fluids released from
the carbonatitic magma.
Catalao is a plug of about 6 km in diameter comprising alkaline,
ultramafic and carbonatitic rocks. It is emplaced in the same
Brasiliano-age fold belt as Araxa, i.e. in mica schists and quartzites
of the Precambrian Araxa Group. Extensive drilling has indicated
that a carbonatitic core is surrounded by pyroxenites, serpentinized
peridotites and glimmerites. Lateritic cover overlying the intrusive
rocks of the Catalao complex reaches depths of 15-250 m and
contains variable amounts of phosphates, pyrochlore, Ti- and REEminerals.
Lateritization is extensively developed and the process
dramatically changes the mineralogical and chemical composition
of the primary rocks. The lateritic mantle concealing the alkaline
complex consists of an upper lateritic soil (2-30 m) and an
intermediate residuum of goethite, magnetite, barite, monazite and
gorceixite（磷鋇鋁石）. Bariopyrochlore and cerian pyrochlore
are worked as a source of niobium.
There are five carbonatite
alkaline complexes in Araxa.
They are Late Cretaceous
volcanic activities occurred on
the Archean to upper
Proterozoic formations.
Back scatter
electron image
(BSEI)
REE concentrate
Precambrian mica schist
and quartzite basement
P concentrate
Nb concentrate
(pyrochlore)
Glimmerlite
Carbonatite
complex
Bayan Obo Fe-REE-Nb deposit
內蒙古白雲鄂博鐵-鈮-稀土礦床
礦區位於包頭之北的白雲村，寬溝大斷裂與烏蘭寶力格深
大斷裂的交匯處，分佈著中元古代（約1500 Ma）的一套
白色石英岩、板岩、灰岩和白雲岩，總厚度大於9000公尺。
礦區東西長約18km，南北寬約5km，白雲岩是主要含礦層，
礦石顯著富集的元素有鐵、鈮、鉭、輕中稀土、釔等，以
往主要是開採鐵礦，但目前已知是世界最大的稀土礦床。
白雲鄂博之熱液交代作用
原有礦床：中元古（1500-1400 Ma，獨居石年齡）的
沈積變質白雲岩。
來源：海西期的酸性岩漿及其熱液（270 Ma，雲母KAr及易解石*Th-Pb等實線年齡），包括先期的輝長－
閃長岩和後期大規模的黑雲母花崗岩兩期侵入（花崗
岩最晚為230Ma）。
1. 接觸交代：發育在礦區東部的白雲岩與花崗岩的接
觸帶，導致鎂矽卡岩之產生，包括：矽鎂石**－金雲
母矽卡岩，透輝石－金雲母矽卡岩，透閃石－矽鎂石
矽卡岩
* Eschynite (Ce,Ca,Fe,Th)(Ti,Nb)2(O,OH)6
** Humite (Mg,Fe2+)7(SiO4)3(F,OH)2
2. 鈉、氟交代：是本礦床發育最廣泛的交代作用，
為鈮和稀土元素的主要成礦階段。
鈉交代→鈉閃石、霓石、黑雲母＋鈮稀土鐵礦石及
鈮稀土礦石
氟交代→螢石、氟碳酸鹽礦物＋條帶狀鈮稀土鐵礦
石
3. 充填交代：形成晚期不同礦物組合的細脈：
燒綠石霓石脈，易解石鈉閃石脈，氟碳鈰礦螢石脈
最近有人認為屬於火成碳酸岩岩漿的侵入，與花崗
岩無關，理由為：
1. 尖山東坡有貫入式的碳酸岩 (白雲岩)。
2. 白雲岩礦體deltaS34平均值為0.52，接近隕石硫。
3. 礦區中含鈮、稀土元素白雲岩鐵礦床與貫入式碳
酸岩的87Sr/86Sr初始值一致。
The world's largest REE deposits are those at Bayan Obo in China, with
grades of 3 to 6% REO (Rare Earth Oxide) and reserves of at least 40
million metric tons, and at Mountain Pass in California with a grade
averaging 9.3% REO and reserves of 20 million metric tons.
THE BAYAN OBO FE-REE-NB DEPOSIT, INNER MONGOLIA, CHINA:
COMPARISONS WITH CARBONATITE-RELATED AND FE-OXIDE-TYPE
DEPOSITS
Along with other factors the Fe-oxide-Cu-Au deposit class is commonly
characterized by alkali silicate alteration and enrichments in the REE and other
high field strength elements. For this reason a link has been proposed with Feoxide-rich deposits associated with carbonatites (most notably Palabora, S.A.).
The Bayan Obo Fe-REE-Nb deposit, Inner Mongolia, China has been compared
with both carbonatite-related deposits and with the hydrothermal Fe-oxide deposit
class (with inferences of an A-type granite source rock for metals). The
carbonatite-related model is supported by the geochemistry of the deposit, stable
isotope data, and the fenite-like silicate alteration assemblage. Sm-Nd and Pb
isotope systematics, however, indicate an enriched and possibly crustal source for
metals. Both textural and fluid inclusion studies indicate that the deposit was
formed and then modified by multiple stages of fluid circulation, possibly linked to
deformation.
An overview of available geochronological data suggest that the most
obvious ways to reconcile these differences are either the remobilization of
an old (Proterozoic) alkaline igneous- or carbonatite-related deposit by
fluids produced during Caledonian orogenic activity, or through the
derivation of carbonatites from enriched upper mantle. The inferred
carbonatite source for metals at Bayan Obo is indicative of distinct
differences between Bayan Obo and the deposits of the Fe-oxide class.
Most notably the differences in alteration are probably a result of the fact
that hypersaline brines were not involved in the genesis of the Bayan Obo
deposits. The evolution of fluids from carbonatite magmas is just one
mechanism that can generate low sulphur, chloride-rich brines, capable of
transporting large masses of Fe. Distinctions must begin to be drawn
between Fe-oxide rich deposits that may form a common class (e.g. Feoxide-Cu-Au deposits associated with regional Na-Cl alteration) and those
which, despite having common features, are linked to distinct fluid and
metal sources. This distinction may have important implications for
exploration and prospectivity.
Lamprophyric dikes in Jurassic
granitic rocks in S China--an
example of U-enrichment