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Petrology of Altered Minerals

PLACER DEPOSITS

Handout - 10@ Rosana, MF 2007

y Endapan plaser terbentuk oleh konsentrasimekanik dari mineral-mineral yang tahanterhadap pelapukan (siklus pelapukan-erosi)p p p ( p p )y Umumnya terbentuk sebagai hasil dari

pemisahan secara gravity dan fluktuasi energidari mediumy Faktor yang mempengaruhi pembentukan

endapan plaser :y Durability or resistance of minerals to weathering and

mechanical breakdownHigh specific g a it (densit )

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y High specific gravity (density)y Transporting medium (water, ice or wind)y Site for deposition

y Auy PGEsandSny PGEsandSny Garnet,rutile,diamond,corundum(rubies,sapphires),tourmaline,topaz,beryl(emeralds),spinely Zircon,kyanite,staurolite,basrite,cinabar,spheneIl it it it h it tit y Ilmenite,cassiterite,chromite,magnetite,wolframite

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y Nearthesurface,usuallyinHolocenefluvialorbeachdepositspy Alluvialfany Colluvialy Glaciofluvialy Glacialy Deltaicplacersy Deltaicplacers

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y Accur mainlyalongcratonics marginswherereworking,ofclastic materialhasproduceed forlongg, p gperiodsoftime Finegrained,allochthonous placersy ThemarginsofCenozoicandMesozoicaccretionaryorogenic belts Coarse,autochthonousplacery Volcanicarc,commonlyalongmajorfaultsy PGEbearingdepositcommonlyassociatedwithaccretedandobducted oceanicterranes

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y Coarse(pebbleorboulder),roundedgravels(conglomerate),commonlywithsandyinterbeds or( g ), y ylensesy Garvel ususally imbricated,clast supported,openworkoreith asandymatrix,andtypicallywithabundantresistantrocktypes(quartzite,veinquartz,chert,granite)

l d h k hy Auplacersassociatedwithrocktypeshostingepithermalormesothermal veindeposity PGEplacersaccur withultramafic hostrocks

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y Highlyvariableandlaterallydiscontinousy Paystreak typicallythin(100m often>y Paystreak typicallythin(100m,often>1km)followstrandlinesinshorelineenvironmentsy Usuallyinterbedded withbarrensequences

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y AnomalousconcentrationofAu,As,Fe,Sn,Ti,Zr,REE,Th,Y,U,Hg,Mn orCrinstreamsediments, , , , g,

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y Groundpenetratingradar useful fordelineatingthegeometry,structureandthikness ofdepositswithlowclaycontents especiallyfluvialterraceplacersclaycontents,especiallyfluvialterraceplacersy Shallowseismic,eletromagnetic,IP,resistivityandmagnetometersurveyareusefuly Geophysicalloggingofdrillholeswithapparentconductivity,naturallyoccuring gammaradiationandmagneticsusceptibilitytoolscansupplement

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g p y ppstratigraphic data

y Panningandothermethodsofgravitysortingareusedtoidentifyconcentrationofgold,magnetite,hematite,y g , g , ,pyrite,ilmenite,chromite,garnet,zrcon,rutile andotherheavymineralsy Airphoto andsatelliteimagerydatacanaidexplorationforburiedvalleyplacer

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Endapan pasir besi di Sindang Barang - Cianjur

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KDR 27

BijihBesiFragmental ( magnetitdanhematit)DiKetapang,KalimantanBarat

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Petrology of Altered Minerals

LETERITE DEPOSITS

y Deposit di hasilkan karena proses pelapukan (physical & chemical), eg. Bauksit, clay dan nikel.

y Fe dan Al umumnya mengalami persipitasi bersama-samadgn Fe dominan pada situasi dimana batuan indukdgn Fe dominan, pada situasi dimana batuan induksedikit mengandung Fe yg kemudian mengalamipelapukan, maka Al akan dominan dan hasil endapannyaberupa laterit yang di sebut bauksit.

y Faktor yang penting untuk pembentukan bauksit adalah :y Humid tropical or tropical climate with wet dan dry seasons,

temperature above 20y Suitable parent rock

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y Chemical & organic activityy Topography and topology that permit slow infiltration of

precipitation and promote chemical reactiony Time and preservation of reaction product

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y Lateriticnickelinthisareaareformedbylaterizationprocessoftheultramaficrocksprocessoftheultramaficrocksy Locatedonundulatedtopographicwithmaximumelevationof1,300metery Thethicknessofthelaterite:20m 30m

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Thelaterite profileistypicalofatropicallateritedeposite,thatconsistsofthefollowingsegments,p , g g ,fromtoptobottom:OverburdenLimonite SaproliteBedrock

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CrossSectionofTheNickelLaterite atTj.Buli EastHalmahera

y TheBuli LateriticNickelisformedbylaterization ofCretaceousUltramafic Rock,thatresultedOverburden,Limonite,Saprolite andBedrockLayers

y TheTotalAmountofTheNickellateritey TheTotalAmountofTheNickellaterite Saprolite

OreReserve :31MWmt @Ni:2.4%Min.Resource:84MWmt @Ni:2.4%

LimoniteOreReserve :11MWmt @Ni:1.5%

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OreReserve :11MWmt @Ni:1.5%Min.Resource:120MWmt @Ni:1.4%

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GeologicalMapofHalmaheraIsland

RegionalStratigraphy

TheEasternHalmaheraispredominantlycomposedofOphioliticispredominantlycomposedofOphioliticUnitsassociatedwithOceanicSediment

TheWesternPartisconsistingofVolcanicRock

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SOME FACTORS THAT INFLUENCE THE DEPOSITION OF BAUXITE IS AS FOLLOWS:

1. Source rocks that is rich in aluminium elements

2. Subtropics region with high precipitation environment

3. Daily temperatures > 25oC

4. Undulating topography

5 Stable area (old stadium)

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5. Stable area (old stadium)

6. Formed above the permanent water table

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NODULES OF ANGULAR - AMOEBOID BAUXITE IN SANDY CLAY MASS.

Source rock - rich in Al. Basic-intermediate rocks Sedimentary rocks

Weathering and dissolving processes

BAUXITE DEPOSITION PROCESS

Weathering and dissolving processesby acid water (9 < PH < 4 acidcarbonate +organic solution)

Transporting of dissolved elements, Ca,Na, K, and Mg. Forming of Al2O3 SiO2 H2O residues (hydrous aluminum silica) and salts-Fe

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Settling of hydrate aluminum silicaresidues and salts-Fe at PH 4 9(normalized acid dissolvement)

Dehydration of residues forming bauxite (concretion)

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COMPARISON OF INSITU BAUXITE FROM DIFFERENT SOURCE ROCK

Gabbro

Quartz Monzonite Analysis Result

Compound

Source rock (%)

Insitu bauxite (%)

Source rock (%)

Insitu bauxite(%)

SiO2 48,0 12,9 68,7 29,1 Fe2O3 11,4 18,4 3,52 12,9 TiO2 1 05 1 23 0 36 1 03TiO2 1,05 1,23 0,36 1,03Al2O3 19,6 42,5 15,0 36,0 Na2O 2,73 0,00 3,62 0,00 CaO 10,4 0,04 3,50 0,02 MgO 4,77 0,00 0,9 0,00 P2O5 0,36 0,09 0,12 0,04 K2O 0,26 0,00 2,03 0,00 MnO 0,19 0,04 0,08 0,02 CuO 0,00 0,00 0,01 0,01 Z O 0 02 0 01 0 02 0 01

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ZnO 0,02 0,01 0,02 0,01Cr2O3 0,01 0,03 0,08 0,01 ZrO 0,0 0,00 0,0 0,00 LOI - 24,0 - 20,4

A-Al2O3 - 34,7 - 30,5 R-SiO2 - 5,9 - 5,0

Source: Rodenburg JK,1974.

HIGH SILICA BAUXITE TYPE (SEMERAH MODEL)

HIGH IRON BAUXITE TYPE (SEGELAM MODEL)

( )

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