TYPE CEBAKAN MINERAL 2

“CEBAKAN SEKUNDER”

Klasifikasi

• Cebakan yang berhubungan dengan Proses Sedimentasi Kimia

• Cebakan yang berhubungan dengan sedimentasi klastis

• Cebakan yang berkaitan dengan proses pelapukan

Cebakan Sedimen Kimia

• Logam – dasar (Cu-Pb-Zn) ; contoh adalah : Zambian Copperbelt

• Iron deposits (BIF) ; contoh : taconine di Lake Superior, Itabirite di Brazil, jaspilite di Australia, dan banded iron di BC (Canada).

• Mangan ; (nodul)

• Fosfat

Cebakan Sedimen Klastik

• Cebakan Placer Emas (Au)• Cebakan Plaser Timah (Sn)• Cebakan plaser Kromit (Cr)• Cebakan Pasir Besi (Fe)• Cebakan Plaser Ti• Cebakan Plaser Intan

Cebakan Akibat Proses Pelapukan

• Laterite Bouxite

• Laterite Ni

• Laterite Fe

CEBAKAN NIKEL LATERITE

• The term laterite is derived from Latin word later, means brick. This term was firstly introduced by Hamilton in 1807 for the earthy iron crusts that were being cut into bricks for building material by the people of south-central India.

• Currently, the term laterites is used for soil that are rich in iron and aluminium, formed under the influence of chemical weathering with special ground water condition.

• Laterite are the residual products of chemical weathering of rock at surface of the earth (Elias, 2002).

• Nickel laterites are the product of laterisation of Mg-rich or ultramafic rocks which have primary Ni contents of 0.2 - 0.4 %

Pembentukan Laterit

• Laterisation processes are controlled on regional and local scales by :

• Rock type and structure• Climate• Topografi• Acidity of ground water

Skema pembentukan bijih nikel pada penampang laterit di atas batuan ultramafik

Air Hujan yg Kaya CO2 dari Atmosfir

ZONA LIMONIT

Sedikit pelindian zona limonit Di musim hujan.

Konsentrasi residu Penguapan dan Dari Fe dan Cr Pengendapan Si, Al

Selama musim kering Fe-hidroksida (+Ni,Al) Al-hidroksida Naiknya air tanah Mineral lempung akibat gaya kapiler Mn-hidroksida (+Co)/ Cr- spinel

Pengurangan Larutan pembawa Ni, Mg, Si

ZONA PELINDIAN Silikat yang mengandung

Ni terurai. Mg, Si, dan Ni larut

Penambahan Larutan Pembawa Ni, Mg, Si

ZONA SAPROLIT

Pengendapan Kembali sebagian Ni, Mg, dan Si pada rekahan Mis. Sbg : - garnierite - krisopras

BATUAN DASAR

Sebagian Mg mengendap Kembali pada rekahan di Batuan dasar. Mis: - gel garnierite - serpentin PERIDOTIT - SERPENTINIT

Serpentinisasi

BATUAN ULTRAMAFIK

General process Effects in ultramafic rocks

1. Leaching of mobile elemnets :alkalis, alkaline earth

2. Formation of stable secondaryminerals : Fe and Al oxides, clay

3. Partial leaching of less mobilecomponents : silica, alumina, Ti

4. Mobilitation and partialreprecipitation of redox-controlledcontituents : Fe,Mn

5. Retention and residualconcentration of resistant minerals: zircon, chromite, quartz

Breakdown of olivin, pyroxene, serpentineand leaching of Mg, Ni, Mn, Co

Goethite formation, smectite formation,adsorption of Ni from solution

Leaching of silica in rainforest and moistsavana climates

Precipitation of Mn oxides and adsorptionof Ni and Co from solution

Residual chromite solution

Reaksi-reaksi kimia yang terjadi pada proses pelapukan batuan ultrabasa

• 4 Mg2SiO4 + 10 H+ Mg3Si4O10(OH)2 + 5 Mg2+ + 4 H2O (forsterite) (saponite)

• 4 Fe2SiO4 + 8 H+ + 4 O2 Fe2Si4O10(OH)2 + 6 FeO(OH)(fayalite) (Fe-smectite) (goethite)

• Mg2SiO4 + 2 H+ SiO2 + 2 Mg2+ + H2O(forsterite) (quartz)

• Fe2SiO4 + 2H+ + O2 SiO2 + 2 FeO(OH)(fayalite) (quartz) (goethite)

Reaksi pertukaran ion Ni dan Mg pada mineral serpentin

Mg3Si2O5(OH)4 + 3Ni++(aq) Ni3Si2O5(OH)4 + 3 Mg++

(aq)

Serpentin Ni-serpentin

Mineral Serpentin Grup :- Crysotile- Antigorite- Lizardite

Posisi atom Ni pada struktur kristal clay mineral

Schematic laterite profile developed on ultramafic rock in a tropical climate

Alteration

Zone

Ni Fe SiO2 MgO Co Ni/MgO Fe/MgO LOI SM Olivine

1 1,01 6,9 33,3 26 0,003 0,04 0,26 11,01 1,28 -12,6

2 2,01 6,7 34,5 27,1 0,003 0,08 0,25 11,35 1,27 -13,5

3 2,65 6,9 35,3 27,9 0,004 0,09 0,25 11,82 1,26 -15,2

4 3,11 9,0 35,6 25,4 0,009 0,12 0,35 11,28 1,40 -23,3

Average 2,19 7,37 1,30

Tipe-tipe profile laterite

• There are three main types of profiles on the basis of the dominant mineralogy :

• Oxide laterite• Clay laterite• Silicate laterite

Schematic comparison of principal laterite profiles

Contoh inti bor urat kuarsa-garnierite

Ni = 1.25 % Co = 0.004 % Fe = 5,12 % SiO2 = 57.48 % MgO = 20.24 %

Klasifikasi Cebakan Bijih Berdasarkan Koefisien Variasi (Carras, 1987) : CV = SD x 100/Mean

• Type A : Koefisien Variasi Rendah

Type A1 – Geometri dan distribusi kadar sederhana. Contoh : Batubara, besi, bauxite, nikel laterite dan tembaga stratabound/stratiform

Type A2 – Geometri sederhana dan distribusi kadar kompleks. Contoh : Cu terdisseminasi, gold stockwork, Witwatersrand gold.

• Type B – Geometri kompleks dan distribusi kadar sederhana dengan koefisien variasi rendah. Contoh : Cebakan basemetal (skarn Cu deposits)

• Type C – Geometri kompleks dan distribusi kadar kompleks dengan koefisien variasi tinggi. Contoh : Archean gold (misal : Kalgoorlie dan Canada).