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.

KimberliteWhat 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 finer-grained 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.

PetrologyKimberlites 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 kimberlitesGroup 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 kimberlitesGroup-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 mineralsKimberlites 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 funnel-shaped 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 PipesDiamonds 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 MineralsSeveral 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 Cr-pyropes, 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 lherzoliticgarnets (>45 Kb) which could be diamond bearing (garnets in G9 field may beharzburgitic 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 Cr-diopside 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 MiningThe 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 MiningOpen-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 MiningPlacer 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 MiningMarine 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]FParisite 氟碳鈣鈰礦 Ce2Ca[CO3]3F2Rinkolite 綠層矽鈰鈦礦 CeNa2Ca4Ti[Si4O15]F3Nb and Ta:Niobite 鈮鐵礦 (Fe,Mn)Nb2O6Tantalite 鉭鐵礦 (Fe,Mn)Ta2O6Pyrochlore 燒綠石 (Ca,Na)2(Nb,Ti)2O6FMicrolilte 細晶石 CaNa(Ta,Ti)2O6(OH,F)Fergusonite 褐釔鈮礦 Y(Nb,Ta)O4Loparite 鈦鈮鈣鈰礦 (Ca,Na,Ce)(Ti,Nb)O3Euxenite 黑稀金礦 (Y,Ca,Ce,U,Th)(Nb,Ta,Ti)2O6Samarskite 鈮釔礦 (Y,Ce,U,Ca,Pb)(Nb,Ta,Ti,Sn)2O6Simpsonite 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 fractures3) Barite veins4) U-Th minerals + silicification5) 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 early-formed.

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 medium-grained 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 REE-minerals. 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 carbonatitealkaline complexes in Araxa.

They are Late Cretaceous volcanic activities occurred onthe Archean to upper Proterozoic formations.

Back scatterelectron image(BSEI)

Precambrian mica schistand quartzite basement

Glimmerlite

Carbonatitecomplex

Nb concentrate(pyrochlore)

REE concentrate

P concentrate

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K-Ar 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 Fe-oxide-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. Fe-oxide-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 Jurassicgranitic rocks in S China--an example of U-enrichment