Mgi 20130102

Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 15-28

15Naskah diterima: 03 Januari 2013, revisi terakhir: 22 Maret 2013, disetujui: 28 Maret 2013

Characteristics of Alteration and Mineralization at Randu Kuning - Wonogiri Project

Karakteristik Alterasi dan Mineralisasi di Projek Randu Kuning, Wonogiri

Abdul Muthi1, I Gde Basten2, I Gede Made Suasta3, and Naomi E. W. Litaay3

1PT. Best Clean Energy (Augur Resources), Jln. Warung Buncit Raya 99, Jakarta 127402Geological Engineering, ITB, Jln. Ganesha 10, Bandung 40132

3PT Oxindo Exploration (MMG), Jln. Ciputat Raya 99, Jakarta 12310

ABSTRACTRandu Kuning prospect is one of the prospective areas within the Wonogiri property which is fully owned by PT. Alexis Perdana Minerals (PT. APM). The area is often reffered to as Selogiri property as it lies within Selogiri Subdistrict. Exploration work in Wonogiri commenced by PT. Oxindo Ex-ploration (MMG) includes regional, detailed mapping, and surface geochemical sampling including rockchip, channel, stream sediment, and grid soil sampling. Ground magnetic survey aided subsurface exploration and a 3D magnetic inversion assisted to define targets for 5 holes scout diamond drilling program to test coincident anomalous soil geochemistry, modelled high magnetic bodies, and exposed sheeted Cu-Au bearing quartz veins. Augur continued the work in the area with surface sampling comprising extensive trenching, diamond drilling, and detailed ground magnetic. Extensive drilling program in 2011 completed 50 diamond holes in 2011 with the total of 15,588.15 m, mostly were drilled into Randu Kuning prospect including the deepest hole (WDD30) that was drilled until 854.95 m. Lithological classification, alteration zonation, and mineralisation have been refined with the new exploration results. Lithological classification differentiates the units based on genesis, relation to mineralisation event, and grain size. Two main groups of lithology are diorites and breccias. Zoned prograde hydrothermal model of Randu Kuning Porphyry system depicts the intersection of diamond holes into outer and inner propylitic zones, porphyry style veins, and core potassic zones. Additional alteration study was done by PIMA work on 102 samples taken from 6 holes of Cross Section TRK01. Clay minerals, illite, and montmorillonite are the main minerals identified in the vein samples. K-Alunite and gypsum in wallrock were identified near the end of WDD030 that suggests signatures of advance argillic alteration and intermediate argillic respectively. Extensive study including alteration mineralogy is required to confirm this. The gold-copper porphyry mineralization in Randu Kuning developed within sheeted and stockwork quartz vein in microdiorite and in the peripheral of micro-diorite intrusion. The system obtained its higher Cu-Au grades from overprinting episodes and block faulting may have caused variations in alteration. Published resource of Randu Kuning is estimated at 90.9 Mt at 0.53 g/t AuEq (0.35 g/t gold and 0.10% copper) using a cut off of 0.2 g/t AuEq.Keywords: Randu Kuning, alteration, mineralization, magnetic survey, prograde hydrothermal, porphyry system, Cu-Au

SARIRandu Kuning merupakan salah satu area prospektif di Wonogiri yang sepenuhnya dimiliki oleh PT. Alexis Perdana Minerals (PT. APM). Daerah ini sering disebut sebagai properti Selogiri karena terletak di Kecamatan Selogiri. Pekerjaan eksplorasi di Wonogiri yang dimulai oleh PT. Oxindo Exploration (MMG) meliputi pemetaan regional dan terperinci, serta pemercontohan permukaan yaitu rockchip, saluran, sedimen sungai, dan pemercontohan tanah grid. Survei magnet permukaan membantu eksplorasi bawah permukaan dan inversi magnet 3D membantu untuk menetapkan target lima lubang pemandu dalam pemboran intan untuk menguji geokimia tanah anomali, model tubuh bermagnet tinggi, dan urat kuarsa pembawa Cu-Au. Augur melanjutkan bekerja di area tersebut


16

INTRODUCTION

Randu Kuning Prospect is a prospective area within the Wonogiri property which is fully owned by PT Alexis Perdana Minerals (PT APM). The area is often referred to as Selogiri property as it lies within Selogiri Sub-regency. The property is located in the southeastern part of Central Java Province and approximately 30 km south of Solo/Surakarta City. An initial exploration activ-ity in this area was conducted by PT Oxindo Exploration (MMG) in partnership with PT APM starting in first semester of 2009. The work and results of this exploration activity is summarized in Suasta and Sinugroho (2011).

Exploration work on this project continued with further surface and drilling program by Augur Resources from Australia (Augur) that added more knowledge and understanding of the mineralization and prospectivity of the area. Much works have been concentrated on Randu Kuning area to follow up interesting results in initial exploration program. This paper will discuss the results of recent work carried out by Augur Resources on Wonogiri

project with emphasis on characteristics of al-teration and mineralization at Randu Kuning.

REGIONAL GEOLOGY

The Sunda-Banda volcanic arc developed during subduction of the north-moving Indo-Australian Plate beneath the Asian continental plate margin. The Sunda - Banda arc of Middle Miocene to Pliocene age is thought to have initiated by subduction re-versal following an Oligocene compressive event that was associated with the northward emplacement of ophiolite and island arc assemblages onto the Sunda margin and associated formation of melanges, ophiolite fragments, and deformation zones offshore from western Sumatra (Daly et al., 1991; Harbury and Kallagher, 1991 in Hellman, 2011). The initiation of northward subduc-tion beneath the Sunda - Banda arc migrated eastward following this collision event. The western segment of the arc, west of central Java, developed on continental crust on the southern margin of Sundaland whilst the arc

dengan pemercontohan permukaan yang terdiri atas pemaritan ekstensif, pemboran intan, dan pemagnetan permukaan terperinci. Program pemboran ekstensif pada tahun 2011 menyelesaikan 50 lubang intan dengan total 15.588,15 m, terutama di prospek Randu Kuning, meliputi lubang (WDD30) sampai kedalaman 854,94 m. Klasifikasi litologi, zonasi dan mineralisasi alterasi telah diperbaiki dengan hasil eksplorasi baru tersebut. Klasifikasi litologi membedakan unit-unit berdasarkan genesis, hubungan dengan mineralisasi dan ukuran butir. Dua kelompok litologi tersebut adalah diorit dan breksi. Model hidrotermal prograde sistem Porpiri Randu Kuning yang dizonakan menggambarkan interseksi lubang intan ke zona propilitik luar dan dalam, urat tipe porpiri, dan zona potasium inti. Studi alterasi tambahan dilakukan oleh PIMA terhadap 102 percontoh yang diambil dari 6 lubang Cross Section TRK01. Mineral lempung ilit dan monmorilonit merupakan mineral utama yang teridentifikasi di dalam percontoh urat. K-Alunit dan gipsum di dalam wallrock teridentifikasi dekat ujung WDD030 yang menandakan alterasi argilik lanjut dan argilik intermediet. Studi ekstensif yang meliputi mineralogi alterasi diperlukan untuk mengkonfirmasikan hal ini. Mineralisasi porpiri emas-tembaga di Randu Kuning berkembang dalam urat kuarsa stockwork dalam mikrodiorit dan dalam tepi intrusi mikrodiorit. Sistem yang memperoleh kadar Cu-Au yang lebih tingginya dari episode pertimpasan dan pensesaran blok ini mungkin menyebabkan variasi dalam alterasi. Sumber daya Randu Kuning yang dipublikasikan diperkirakan sebanyak 90,9 Mt pada 0,53 g/t AuEq (0,35 g/t emas dan 0,10 % tembaga menggunakan cut off 0,2 g/t AuEq.Kata kunci: Randu Kuning, alterasi, mineralisasi, survei magnetis, hidrotermal prograde, sistem porfiri, Cu-Au

Characteristics of Alteration and Mineralization at Randu Kuning - Wonogiri Project(Abdul Muthi et al.)

17

east of Central Java developed on thinner island arc crust (Carlisle and Mitchell, 1994 in Hellman, 2011).

There are also variations in dominant styles of mineralization along the arc. In northern Sumatra in the Aceh Province, mineraliza-tion is characterized by porphyry Cu-Mo systems and high-sulphidation deposits (e.g. Miwah and Martabe). In contrast, southern Sumatra, west Java, and central Java are typified by a lack of known porphyry sys-tems but an abundance of low-sulfidation epithermal deposits or prospects with vein systems. Examples include Tambang Sawah, Rawas, Lebong Donok, Lebong Simpang, and Seung Kecil in southern Sumatra, also the Cikotok and Jampang districts, Gunung Pongkor and Cikondang in west Java, and Trenggalek in central Java. Further the east, in east Java and then through Lombok and Sumbawa, there is a reappearance of porphyry and high sulfidation epithermal systems along the eastern arc segment, including the the Tumpangpitu high-sul-fidation epithermal and porphyry system on Intrepid’s Tujuh Bukit project, The Selodong

high-sulfidation and porphyry district in-cluding the Motong Botek porphyry system on Lombok, and the Batu Hijau porphyry Cu - Au system on Sumbawa (Hellman, 2011). Location of the Wonogiri Project with respect to other porphyry projects is shown in Figure 1.

The Sunda - Banda arc comprises both Miocene to Pliocene volcanics and younger Quaternary volcanics. The arc has migrated not only from west to east over time but also from south to north (Whitford et. al., 1979; Katili, 1989; and Claproth 1989 in Hellman, 2011). This migration is clearly evidenced by the east-west alignment of deeply dis-sected Miocene to Pliocene volcanic centers along the south coast of Java, Lombok and Sumbawa, and a parallel E-W alignment of juvenile and active Quaternary volcanoes that define the present active arc further north along central Java and northern Bali, Lombok, and Sumbawa (Figure 2).

The Sunda-Banda arc is segmented by a se-ries of arc-normal structures that trend NNE and which are evident in topographic-data set. Tectonic factors appear to have local-

Figure 1. Locality map of Wonogiri Project with respect to SundaArc and other major deposits (Augur Re-sources, 2012).

Wonogiri Project LocationJava, Indonesia

Augur’s Wonogiri Project

Significant Project/deposit

Quaternary Sunda Arc

Oligocene Miocene ArcI N D O N E S I A

Wonogiri JV

0 300

km

Tujuh Bukit (Intrepid)1.700 Mt @ 0.41% Cu

and 0.46 g/t AuBatu Hijau (Newmont JV)

914 Mt @ 0.53% Cuand 0.40 g/t Au

Elang (Newmont)

S u m a t r a

Jakarta

o0

o10

BandungJ a v a

J A V A S E A

F L O R E S S E A

I N D I A N O C E A N

Bangka

Belitung Banjarmasin

Mak

asar

Sumbawa

BanyumasBali Lombok

B o r n e oS u l a w e s i

MalangFlores

Timor

Selodong (Soutern arc)

F L O R E S S E A

N


18

ized volcanic centers of the Miocene arc at positions near the southwestern margins of these transfer structures. Contemporaneous continental to ocean deep clastic sediments were deposited on the margins of the vol-canic centers (Hellman, 2011).

The project location is surrounded by several Quartenary Volcanoes, such as Gu-nung Lawu, Merapi, Merbabu, and others. Stratigraphically the Wonogiri Project area consists of from young to old: Alluvium, Merapi Volcanic Rock, Lawu Volcanic Rock, Wonosari-Punung Formation, Oyo Formation, Nglanggran Formation, Semi-lir Formation, Mandalika Formation and Gamping Wungkal Formation (Figure 3).

From this regional geological map, Wonogi-ri Project area is dominated with Mandalika Formation, which consists of dacite-andesit-ic lavas and dacitic tuff with dioritic dykes. South area is covered by Semilir Formation comprising tuff, dacitic pumice breccia, and tuffaceous sandstone and shale. North area is covered by alluvium deposits composed

of loose material of sands and clays with varieted of grain size. Local geology shows that the project area consists of volcanic breccias, lithic tuff, andesite cut by quartz diorite and microdiorite intrusions, and in parts is overlain by alluvial deposits. Local geological structures in the project area are dominated by northeast southwest strike-slip fault and east-west thrust fault that appears to be the effect of the north south subduc-tion movement from Australian Plate at the Eurasian Plate collision.

METHODOLOGY AND RECENT EXPLORATION

Surface work in Wonogiri by PT Oxindo Exploration (MMG) includes regional and detailed mapping and surface geochemi-cal sampling including rock chip, channel, stream sediment, and grid soil sampling. Ground magnetic survey aided subsurface exploration and a 3D magnetic inversion assisted to define targets for 5 holes scout

Figure 2. Regional physiography map of Java (after Imai et al., 2007).

Physiography

Alluvial plain

Selogiri Research Area

0

o112 E

o114 E

o110 E

o112 E

o114 E

o7 S

o7 S

o8 S

o8 S

75 150km

Quaternary volcano

Rembang-Madura anticlinorium

North Serayu-Kendeng zone

Domes and ridges

Depression

Southern Mountain Range

N


19

Figure 3. Geology and broad stratigraphic succession of the area as defined on the 1:100,000 geological map of the Surakarta and Giritontro (Surono et al., 1992).

diamond drilling program to test coincident anomalous soil geochemistry, modelled high magnetic bodies and exposed sheeted Cu-Au bearing quartz veins.

Augur continued the work in the area with surface sampling comprising extensive trenching which are assayed for 10 ele-ments. Total of 9,783 m of trenching was completed in 2011, mainly within Randu Kuning and the surrounding area with the aim to understand the distribution of Au and Cu in this area (Figure 4). This surface program was also aimed to define epithermal

vein targets which were not evaluated by MMG. The trenches were sampled using 2 or 4 m composite channel samples with total of 2,931 samples for the whole project. These trenches were mapped and aided the interpretation of surface and sub-surface geology and drilling program. In addition, a detail ground magnetic survey completed over approximately 50% tenement area.

Extensive drilling program in 2011 complet-ed 50 diamond holes in 2011 with the total of 15588.15 m. They were mostly drilled into Randu Kuning prospect including the

Tms

TommQa Alluvium: Clay, mud, silt, sand, gravel, pebble, marbble.

Lawu Volcanic Rocks: Volcanic breccia, lava, and tuff

Semilir Formation: Tuff, dacitic pumice breccia, tuffaceous sandstone and shale

Mandalika Formation: Dacite-andesitic lavas and dacitic tuff with dioritic dykes

Pendul Diorite: DioriteQvl Tpdi

Wonogiri Project Location

o110 45' E o110 50' E

o110 45' E o110 50' E

o7 45' S

o7 50' S

o7 45' S

o7 50' S

N

Legend:


20

deepest hole (WDD30) that was drilled until 854.95 m. These drillholes together with 5 previous holes by MMG contributed to a better understanding of mineralization and alteration of Randu Kuning prospect.

GEOLOGY OF RANDU KUNING

The areas of interest in the Wonogiri pro-ject have a high topographic relief trending NW-SE (Figure 5). This area occurs in the northern part of tenement boundary comprising a series of hills extends to the north at lower elevation interpreted to form a series of intrusion body. The area has been sub-divided into several prospects based on geology and type of mineralization (Figures 4 and 5).

Locally, geology of the Wonogiri project comprises a series of multiple diorite in-

trusion intruding into the early volcanic sequence and with dominant structures of NE-SW strike-slip fault and E-W thrust fault. The surface geology of Wonogiri project is dominated by hydrothermal al-teration (clay-chlorite-magnetite±epidote and carbonate). This alteration is typical of a propylitic alteration that is overprinted by later argillic-phyllic alteration.

Most of the work has been concentrated on Randu Kuning area to follow up on encour-aging results in the previous work. A total of 38 drill holes including two historical holes of MMG were completed in this area. These drill holes have confirmed that the Randu Kuning prospect is a typically porphyry system with Cu-Au mineralization.

The prospective area occurs as a steep west dipping NS trending zone characterized by polyphasal porphyry intrusions separated by intrusive and fault contact parallel to the

Figure 4. Trenching location on geological map (modified after Corey, 2010).

Bukit Piti

Randu Kuning

South Randu Kuning

Geblak

Jangglengan

485.800

9.13

7.00

0 9.

137.

200

mN

mN

9.13

7.40

0 m

N 9.13

7.60

0 m

N 9.13

7.80

0 m

N

9.13

8.00

0 m

N

9.13

8.20

0 m

N 9.13

8.40

0 m

N 9.13

8.60

0 m

N

9.13

8.80

0 m

N

mE 486.000 mE 486.200 mE 486.400 mE 486.600 mE 486.800 mE 487.000 mE 487.200 mE 487.400 mE

Legend:

Alluvial

Breccia

Microdiorite

Diorite

Volcanic Breccia

Tuff-Sediments

Trench Line

Drill Holes Collar

N


21

mineralized trend. Many intrusion and fault contacts were recognized between different intrusive bodies, with some diorite and mi-crodiorite intrusion types are recognized and standardized in geology logging of drilling. Most contacts are between different phases of diorite and microdiorite with prominent mafic phenocrysts herein summarized in cross section TRK01 (Figure 6).

Lithology

Lithological classification of the Wonogiri Project has been modified in more detail to differentiate the units based on genetic and grain size. The plates showing textural varia-tion of the lithology can be seen in Figures 7 and 8. In summary, the intrusive phases that are currently recognized at Randu Kuning prospect, described from oldest to young-est, are:

Coarse Grain Diorite (CDIO)Pre-mineral, coarse-grained, porphyritic, >1 mm subhedral - anhedral of feldspar phenocrysts set in fine-grained or crystalline groundmass, finer size of mafic phenocrysts, non to poorly mineralized.

Medium Diorite (DIO)Pre-mineral, dominantly medium-grained, <1 - 2 mm plagioclase or mafic phenocrysts (variably fine-to slightly coarse-grained within intensely altered and intrusive/crackle breccia zone), equigranular to sub- porphyritic, non to poorly mineralized (in microdiorite margin).

Microdiorite (MDR)Syn-mineral, fine-grained, <1 mm pheno-crysts size, aphanitic (?), variation of fine feldspar porphyry to prominent mafic, vari-ably to strongly mineralized.

Figure 5. Distribution of prospects of the Wonogiri project on topographical relief.

Bukit Piti

Randu Kuning

South Randu Kuning

Gawe

Geblak

Jangglengan

486.000

9.1

37.0

00 m

N

9.1

37.5

00 m

N

9.1

38.0

00 m

N

9.1

38.5

00 m

N

mE 486.500 mE 487.000 mE 487.500 mE

N

0 50 100

mScale 1:10.000


22

Figure 6. Cross section TRK01 showing lithology and significant intersections in the centre of Randu Kuning Prospect.

Figure 7. Textural differences between dioritic intrusions in Randu Kuning (Muthi, 2012).

65.6 m @ 1.24 g/t Au & 0.29% Cu

WD

D20

WD

D10

WD

D05

TRK01938165mN W

DD

01

WD

D48

WD

D30

129.5 m @ 0.83 g/t Au & 0.24% Cu

123.5 m @ 1.34 g/t Au & 0.22% Cu

89 m @ 0.56 g/t Au & 0.13% Cu

6 m @ 0.59 g/t Au & 0.06% Cu

24 m @ 0.58 g/t Au & 0.06% Cu

98 m @ 0.33 g/t Au & 0.1% Cu

9 m @ 0.23 g/t Au & 0.3% Cu

27 m @ 0.66 g/t Au & 0.1% Cu

79 m @ 0.57 g/t Au & 0.14% Cu

5 m @ 0.28 g/t Au & 0.04% Cu81 m @ 0.87 g/t Au & 0.15% Cu

67 m @ 0.68 g/t Au & 0.19% Cu

18 m @ 0.31 g/t Au & 0.07% Cu

118 m @ 0.75 g/t Au & 0.13% Cu

19 m @ 0.23 g/t Au & 0.03% Cu

9.1 m @ 0.38 g/t Au & 0.02% Cu

5 m @ 0.21 g/t Au & 0.03% Cu

27 m @ 0.20 g/t Au & 0.11% Cu

12 m @ 0.34 g/t Au & 0.13% Cu

6 m @ 0.20 g/t Au & 0.14% Cu

6 m @ 0.28 g/t Au & 0.09% Cu

5 m @ 0.22 g/t Au & 0.04% Cu

5 m @ 0.20 g/t Au & 0.05% Cu

3 m @ 0.20 g/t Au & 0.03% Cu

Microdiorite

Medium Grained Diorite

Coarse Grained Diorite

Intrusive Breccia

Hydrothermal Breccia

Fault zone

485.900mE 486.000mE 486.100mE 486.200mE 486.300mE 486.400mE

485.900mE 486.000mE 486.100mE 486.200mE 486.300mE 486.400mE

-500

mR

L

-4

00m

RL

-3

00m

RL

-200

mR

L

-100

mR

L

0mR

L

100m

RL

200m

RL

-500

mR

L

-4

00m

RL

-3

00m

RL

-200

mR

L

-100

mR

L

0mR

L

100m

RL

200m

RL


23

Porphyritic Plagioclase Diorite (PDIO)Post mineral, coarse-grained, porphyritic, >2 mm plagioclase, plagioclase rich, non-mineralized, (Not common in Randu Kun-ing but observed from intercepted drill holes in surrounding Randu Kuning prospect).

In addition of multiple intrusions as de-scribed above, Randu Kuning drill holes intersected two types of breccia that are referred to as Intrusive Breccia (IBX) and Hydrothermal Breccia (HBX). Intrusive breccias are commonly observed in drill core in contact with, and immediately above the deep central microdiorite intrusion. They ex-hibit a distinct fragmental texture; generally increasingly polymictic clast assemblages at higher levels. This breccia is mostly comprised of diorite and microdiorite clasts which deformed during the intrusion phase. Occasionally, the presence of clasts of mafic dominant rock with disseminated pyrite is noted. The source of these clasts is not yet known.

Hydrothermal breccia was identified in some drill intercepts with increasing silica and clay alteration. This breccia is also characterized by additional sulphide (pyrite) and partly rimmed clasts. It is interpreted that this unit is structurally controlled; hy-drothermally generated silica and sulphide deposited in the fault zone.

Corbett (2011) noted lithological variations; include fine and coarser grained equivalents

as well as different styles of alteration; be-tween different phases of diorite porphyry with prominent mafic phenocrysts as seen in the photos below. Shales, skarn type of alteration and intrusive dykes in the eastern part were intersected in WDD15 which ap-pear to be the basementof thestratigraphy.

RESULT AND DISCUSSION

Alteration

Hydrothermal alteration and zonation is modelled by Corbett (2011) as s hown in Figure 9. Outer propylitic zone is typified by magnetite-chlorite ± epidote alteration. Interestingly, this relatively weak alteration hosts elevated Cu-Au grade mineralisation (Hole DDH10IWG002) and the barren sheeted laminated quartz-magnetite veins (Hole WDD19 and WDD20).

The zone of magnetite-epidote ± chlorite at lower temperature or actinolite towards the heat source is characteristic of inner propylitic alteration which is best displayed in hole WDD22. Fracture epidote within chlorite alteration in the upper portion pass-ing down hole to pervasive epidote flooding of host rock.

Potassic alteration is the most prevalent type of alteration in Randu Kuning min-eralisation and it is indicated by pervasive magnetite - biotite ± secondary K-feldspar.

Figure 8. Two types of breccia in Randu Kuning (Muthi, 2012).


24

Crackle brecciation developed in this al-teration zone as an overprint comprising magnetite ± actinolite ± chalcopyrite. Strong positive magnetic anomaly as identified by detail ground magnetic survey results is a reflection of this magnetic flooding of the hydrothermal alteration and brecciation. The Randu Kuning porphyry Cu-Au hydro-thermal system is localized within an 800 m long NW- NNW trending magnetic feature which declines in intensity to the SE.

The zone of silica-sericite-pyrite is a char-acteristic of phyllic alteration herein is adja-cent to mainly the later stage porphyry style B veins, collapsing down structures and low temperature stage epithermal quartz-sulphide veins.

Total of 102 PIMA samples were taken from six holes of Cross Section TRK01. These samples are collected from veins and per-vasive or in the wallrock alteration zones as well as clasts and matrix of the breccia to see if there is any pattern in alteration assemblage. Clay minerals illite and mont-morillonite are the main minerals identified in the vein samples throughout the section

suggesting structurally controlled argillic alteration. Minor siderite (carbonate) oc-curs at depth. NH-Alunite is identified in one of the sample and is present at centre of the magnetic anomaly.

In wallrock of altered samples, apart from illlite and montmorillonite, chlorite, kaolinite and tourmaline were also identi-fied in WDD030. K- Alunite and gypsum were identified at depth of 743 m and 792 m respectively. The presence of alunite indicates more acid fluid environment and typical of advance argillic alteration (Corbett and Leach, 1998). Gypsum after anhydrite in porphyry setting is a charac-teristic of intermediate argillic as exampled by Middleton et al. (2004). The presence of these minerals might suggest another zone(s) of porphyry or proximity of major structure that channels hot acid fluid. How-ever, more extensive study including altera-tion mineralogy is required to confirm this.

Mineralization

The gold-copper porphyry mineralization in Randu Kuning area developed within

Possible settings of the barrenquartz-magnetiteveins in WDD19 and WDD20

dilatant structure

WDD22

bottom of WDD15

apophyses

stock

magmatic source

POTASSIC

INNER PROPYLITIC

OUTER PROPYLITIC

zeolites

chlorite

epidote

actinolite

A vein

M vein

magnetite biotite k-feldspar

Figure 9. Zoned prograde hydrothermal alteration model (Corbett, 2011).


25

sheeted and stockwork quartz veins along microdiorite and in the peripheral of mi-crodiorite intrusion (Figure 10). Overprint-ing events and style of mineralization are clearly visible in Randu Kuning. The Cu-Au grades developed as an accumulated affect of these as depicted in Figure 11.

Style of mineralization can be grouped as disseminated chalcopyrite, early quartz veins, most quartz veins with sulphides, and barren laminated and sheeted quartz magnetite (Corbett, 2011). The dissemi-

nated chalcopyrite is associated with clots of magnetite and/or chlorite - epidote and contains low grade Au and and some elevated (?) Cu. It is interpreted to be lo-cally derived from cooling intrusion (Cor-bett, 2011). The early quartz veins are the veins generally known as the A veins with sinusoidal gradational vein margins and occasional contains disseminated chalco-pyrite. Saccharoidal quartz texture and minor magnetite are common features of these veins. Majority of quartz veins with

Figure 10. Cross Section TRK01 at the center of Randu Kuning showing alteration and mineralization.

65.6 m @ 1.24 g/t Au & 0.29% Cu

WD

D20

WD

D10

WD

D05

48

-500

mR

L

-4

00m

RL

-3

00m

RL

-200

mR

L

-1

00m

RL

0m

RL

100m

RL

2

00

mR

L

-500

mR

L

-4

00m

RL

-3

00m

RL

-200

mR

L

-1

00m

RL

0m

RL

100m

RL

200

mR

L

5.900mE 486.000mE 486.100mE 486.200mE 486.300mE 486.400mE

485.900mE 486.000mE 486.100mE 486.200mE 486.300mE 486.400mE

No Qtz vein

Propylitic

Potassic

Phyllic

Base of Weathering

Base of Oxidation

Low Density Qtz Vein (0.1-1%)

Low - Medium Density Qtz Vein (1-2.5%)

Medium Density Qtz Vein (2.5-5%)

High Density Qtz Vein (>5%)

Qtz-Mt “M-Vein” Zone

Qtz-Mt “A-Vein” Zone

Stockwork Vein

Sheeted Vein

WD

D01

WD

D48

WD

D30

129.5 m @ 0.83 g/t Au & 0.24% Cu

123.5 m @ 1.34 g/t Au & 0.22% Cu

89 m @ 0.56 g/t Au & 0.13% Cu

6 m @ 0.59 g/t Au & 0.06% Cu

24 m @ 0.58 g/t Au & 0.06% Cu

98 m @ 0.33 g/t Au & 0.1% Cu

9 m @ 0.23 g/t Au & 0.3% Cu

27 m @ 0.66 g/t Au & 0.1% Cu

79 m @ 0.57 g/t Au & 0.14% Cu

5 m @ 0.28 g/t Au & 0.04% Cu

81 m @ 0.87 g/t Au & 0.15% Cu

67 m @ 0.68 g/t Au & 0.19% Cu

18 m @ 0.31 g/t Au & 0.07% Cu

118 m @ 0.75 g/t Au & 0.13% Cu

19 m @ 0.23 g/t Au & 0.03% Cu

9.1 m @ 0.38 g/t Au & 0.02% Cu

5 m @ 0.21 g/t Au & 0.03% Cu

27 m @ 0.20 g/t Au & 0.11% Cu

12 m @ 0.34 g/t Au & 0.13% Cu

6 m @ 0.20 g/t Au & 0.14% Cu

6 m @ 0.28 g/t Au & 0.09% Cu

5 m @ 0.22 g/t Au & 0.04% Cu

5 m @ 0.20 g/t Au & 0.05% Cu

3 m @ 0.20 g/t Au & 0.03% Cu


26

sulphides were the result of cooling of the system at depth including M veins, massive stockwork of A veins, linear A veins, and later the AB veins taking over the earlier A veins with deposition of chalcopyrite. The barren laminated and sheeted quartz magne-tite veins in WDD19 & 20 resemble M veins and sheeted A style and both generally has no sulphide content.

Overprinting of low temperature epithermal veins on the earlier porphyry system can be summarized in Figure 12, showing two pos-sible mechanisms: cooling magmatic source and later magmatic event.

Randu Kuning South

Randu Kuning South is located about 300 m south of main Randu Kuning prospect

Figure 11. Plates showing different veining style and mineralization of Randu Kuning (Corbett, 2011).

Figure 12. Alternative models for the origin of late stage epithermal veins (Corbett, 2011).

Epithermal veins

later magmatic event

x x x xx x x xx x x xx

x x xx xx x#

#

#

#

#

#

##

##

##

+++ +

+++++++

++

+++

+

Epithermal veins

cooling magmatic source

x x x xx x xx xx x x

++

+

++++

++

+

++

+

+

+++

+

++

+

+


27

and was tested by four drill holes. Crackle brecciation is common in this area filled mostly by sulphide. Alteration on this area is dominated by early prophylitic alteration which is overprinted by phyllic altera-tion, possibly controlled by intense faultinge.g. in WDD25. Infill clay and sulphide in the crackle breccia in WDD25 is believed to be a signature of epithermal system which is controlled by fault zone with typical as-semblage of quartz-carbonate and pyrite veins. This area is interpreted as a fault controlling extension of porphyry system in Randu Kuning.

CONCLUSIONS

Based on the data grained from geology and recent drilling, the mineralization at Randu Kuning indicates extensive gold copper porphyry mineralization from surface to 400 m depth below surface. It is related to near vertical gold copper porphyry deposits within a large eroded volcanic centre, ap-pear to be related to a northward migrating Oligocene to Miocene volcanic arc.

The system obtained its higher Cu-Au grades from overprinting episodes as seen in the overprinting of earlier chalcopyrite within disseminations and magnetite-bearing fracture by sheeted quartz veins and later dilatant fractures filled with chalcopyrite. Some intrusions are particu-larly enriched with early sulphides. Barren laminated and sheeted quartz-magnetite veins within chlorite- magnetite altered diorite seems to be the upper unmineralized portion of a porphyry and the potential for vertical extension of these veins at depth is unknown. Current resources of Randu Kun-ing is estimated at 90.9 Mt at 0.53 g/t AuEq (0.35 g/t gold and 0.10% copper) using a cut off of 0.2 g/t AuEq (Augur Resources, 2012).

E-W trending block faulting may cause vari-ations in alteration. Pervasive K-feldspar is common in central Randu Kuning while the drilling shows extensive epidote alteration to the north with epidote flooding intersected in WDD22. This may be indicative of buried porphyry mineralized system. Some higher Au grade mineralization is related to epith-ermal veins. Encounter of older basement of sedimentary sequence defines the boundary of this mineralization system in the east. To the west, the system is not closed off and it requires more work to define as bornite veins have been intersected in hole WDD24 further west than expected.

Further work is required to define the west-ern limit and postulated buried porphyry as well as investigation of advance argillic and intermediate argillic alteration at depth.

ACKNOWLEDGMENTS

The paper has been presented in the MGEI, Banda and East Sunda Seminar 2012. The authors would like to acknowledge the MGEI Committee who give a permission to publish the paper in MGI.

REFERENCES

Augur Resources Ltd., 2012. ASX News Release, 10 July 2012.

Corbett G.J. and Leach T.M., 1998. Southwest Pa-cific Rim Gold-Copper Systems; Structure, altera-tion, and mineralisation. SEG Special Publication, 6, 236pp.

Corbett, G.J., 2011. Comments on the exploration potential of the Wonogiri porphyry Cu-Au Project, Central Java, Indonesia, Gorbett Geological Ser-vices Pty. Ltd. 2011. Internal report, unpublished.

Corey, Mike., 2010. Scout Drilling Program 2010 Summary Report, Porphyry Copper-Gold Project, Wonogiri, Central Jawa, Indonesia. Internal report, unpublished.

Daly, M.C., Cooper, M.A., Wilson, I., Smith, D.G., and Hooper, B.G.D., 1991. Cenozoic plate tecton-


28

ics and basin evolution in Indonesia. Marine and Petroleum Geology, 8, p.2-21.

Hellman, P.L., 2011. Tujuh Bukit Project, Report on Mineral Resources, Located in East Java, Indonesia. Hellman & Schofield Pty Ltd.

Imai, A., Shinomiya, J., Maung T.S., Setijadji, L.D., Watanabe, K., and Warmada, I.W., 2007. Porphyry-Type Mineralization at Selogiri Area, Wonogiri Re-gency, Central Java, Indonesia. Resource Geology, 57(2), p.230-240.

Katili, J.A., 1989. Evolution of Plate Tectonic Concepts and its Implication for the Exploration of Hydrocarbons and Mineral Deposits in South-east Asia. PANGEA UNESCO, distinguished lecture.

Middleton, C., Buenavista, A., Rohrlach, B., Gonzalez, J., Subang, L., and Moreno, G., 2004. A Geological Review of the Tampakan Copper-Gold Deposit, Southern Mindanao, Philippines. PACRIM Proceedings, AUSIMM, p.173-187.

Muthi, A., 2012. Geology Overview of Wonogiri Project, PT. Best Clean Energy, Wonogiri, Central Java, Indonesia. Internal report, unpublished.

Suasta, I.G.M. and Sinugroho, I.A., 2011. Occurence of Zoned Epithermal To Porphyry Type Cu-Au. Mineralisation at Wonogiri, Central Java. Proceedings JCM Makassar 2011, The 36th HAGI and 40th IAGI Annual Convention and Exhibition.

Surono, Toha, S.B., and Sudarno, I., 1992. Geological map of the Surakarta-Giritontro Quadrangles, Java, Indonesia. Pusat Penilitian dan Pengembangan Geologi, Bandung 1992.

Warmada, I.W., Soe, MT., Sinomiya J., Setijadji L.D., Imai, A., and Watanabe, K., 2007. Petrology and Geochemistry of Intrusive Rocks from Selogiri Area, Central Java, Indonesia. Proceedings of 2nd International Symposium on Earth Resources Engineering and Geological Engineering Education, p.163-169. Whitford, D.J., Nicholls, I.A., and Taylor, S.R., 1979. Spatial variations in the geochemistry of Quaternary lavas across the Sunda Arc in Java and Bali. Contributions to Mineralogy and Petrology, 70, p.341-356.

Mgi 20130102

Documents

Transcript of Mgi 20130102