Geology and Mineralization of Awak Mas: A Sedimentary ... 20120201.pdfmasi Latimojong, merupakan...

Majalah Geologi Indonesia, Vol. 27 No. 2 Agustus 2012: 69-85

69Naskah diterima: 11 November 2011, revisi terakhir: 23 Juli 2012, disetujui 06 Agustus 2012

Geology and Mineralization of Awak Mas:A Sedimentary Hosted Gold Deposit, South Sulawesi, Indonesia

Geologi dan Mineralisasi Awak Mas: Cebakan Emas dalam Batuan Sedimen, Sulawesi Selatan, Indonesia

Cliff D. Querubin and Stephen Walters

PT. Masmindo Dwi Area/One Asia Resources Limited

ABSTRACTThe Awak Mas project has been the object of systematic exploration reaching back from 1987 to the present. Aside from the various studies that have already been completed, drilling within Awak Mas including its satellite prospects, totals 1,012 holes with an aggregate meterage of 118,081.30 m. The Awak Mas deposit, located in the southern section of the Central Sulawesi Metamorphic Belt, is hosted within the Latimojong Formation, a thick sedimentary sequence composed of phyllites, slates, basic to intermediate volcanics, limestones, and schists overlying basement metamorphic rocks, and intruded by late-stage plugs and stocks. East of the metamorphic block is the Mesozoic Lamasi Complex composed of basic intermediate intrusives, pyroclastics, and volcanogenic sediments. Gold at Awak Mas is associated with sulphur-poor, sodic-rich fluids introduced at a relatively late stage in its tectonic history. Albite-pyrite-silica±carbonate alteration generally accompanies gold deposition and overprints the ductile fabric associated with deformation and metamorphism in the older base-ment lithologies. Wall rock alteration consists of albite+quartz+carbonate with the original foliation often preserved. Lawsonite-albite-tremolite-chlorite-epidote mineral assemblages observed from samples define a divariant field stretching from 240° at 3 kb to 360° at 10 kb, conforming to general observations in melange deformation wherein high strain is accompanied by low temperature ranges. This temperature-pressure regime suggests an environment related to either subduction or massive thrusting. Oblique normal faults, extensional shears, and fractures formed in response to extensional deformation, serve as local controls to mineralization. Mineralization styles formed in conjunction with the kinematic framework comprise 1) broad, shallow dipping zones of sheeted and stockwork quartz veining and associated alteration that conform to the shear fabric and 2) steeper dipping zones of quartz veining and breccias associated with high angle faults and/or shear zones cutting both the flyschoid cover sequence and basement metamorphic rocks. These steeply dipping mineralized structures are inferred to represent the main feeder zones to mineralization. The Awak Mas deposit has been subdivided into 5 discrete domains based on the nature and orientation of mineralization and structural boundaries. These mineralized zones have been designated as the Ongan, Mapacing, Lematik, Tanjung, and Rante domains. Current mineral resources for Awak Mas total approximately 2 million ounces gold. The Tarra and Salu Bulo prospects confirm the regional prospectivity of the Latimojong Formation for low-grade, structurally controlled, mesothermal deposits. Anomalous gold geochemistry has been identified within a 5 by 13 kilometer zone extending from north to south of the Awak Mas deposit. Identification of additional mineral resources along this zone is subject to ongoing exploration.Keywords: geology, mineralization, gold deposit, metamorphic belt, Awak Mas, South Sulawesi

SARIProjek Awak Mas merupakan suatu objek eksplorasi sistematis yang berlangsung dari 1987 sampai saat ini. Selain dari berbagai kajian yang telah lengkap, pemboran di kawasan Awak Mas yang me-liputi prospek satelitnya, total 1.012 lubang dengan keseluruhan kedalaman 118.081,30 m. Cebakan


70

INTRODUCTION

The Awak Mas Project is located in the Luwu District of southern Sulawesi, Indo-nesia. It is centered at coordinates 120°5’ E and 3°20’ S lying adjacent to the coast at the head of the Gulf of Bone. The coastal port of Palopo, which is the nearest major town to the project, lies 67 km to the northeast (Figure 1). From Makassar, the provincial capital of South Sulawesi, the road distance is 367 km to the town of Belopa and from thereon, another 41 km to the project site. The project area is generally characterized by moderate to rugged topography with elevations in the range of 800 to 1,400 m asl (Figure 2). Slope gradients are steep-est within the southern to western portions grading to moderate towards the northern and eastern sections (Figure 3).

PROJECT HISTORY

New Hope Consolidated Industries Pty Ltd (NHCI) and PT Asminco Bara Utama represent the original partners in the Awak Mas joint Venture. In September 1991, Battle Mountain Gold Company (BMGC) concluded a farm-in agreement with NHCI thereby earning a 60% equity in the project. In 1994, Lone Star Exploration NL (LSE) negotiated the right to acquire the equity of both BMGC and NHCI. Prior to this, Gasgoyne Gold Mines NIL (GGM) and JCI Limited (JCI) had already structured an agreement to combine the various equities (LSE - 45%, GGM - 45%, JCI - 10%) in the project under the single corporate entity of Masmindo Mining Corporation Limited (MMC). In January 1998, LSE purchased the 45% interest of GGM in the project.

Awak Mas, yang berada di bagian selatan Jalur Malihan Sulawesi Selatan dan terdapat dalam For-masi Latimojong, merupakan suatu runtunan batuan sedimen tebal yang terdiri atas filit, batusabak, batuan vulkanik basa – intermedier, batugamping, dan sekis yang menindih batuan alas malihan; diterobos oleh sumbat dan stock late-stage. Di sebelah timur blok malihan ini adalah Kompleks La-masi berumur Mesozoikum yang tersusun oleh intrusi basa – intermedier, piroklastika, dan batuan sedimen vulkanogenik. Emas di daerah Awak Mas berasosiasi dengan cairan yang berkomposisi asam, miskin akan sulfur, dan terbentuk relatif pada tingkat akhir kegiatan tektonik. Alterasi albit-pirit-silika+karbonat umumnya menyertai pengendapan emas dan meng-overprint bahan ductile yang berasosiasi dengan deformasi dan pemalihan pada batuan alas lebih tua. Alterasi batuan dinding, dengan foliasi asal yang sering terawetkan, terdiri atas albit+kuarsa+karbonat. Kumpulan mineral lawsonit-albit-tremolit-klorit-epidot yang teramati dari percontoh dan tersebar dari 240o pada 3 kb sampai 360o pada 10 kb, adalah sesuai dengan pengamatan umum dalam deformasi melange dimana strain yang tinggi disertai dengan kisaran temperatur rendah. Rezim temperatur-tekanan ini mem-beri kesan suatu lingkungan yang berkaitan dengan subduksi atau sesar-naik masif. Sesar normal oblique, extensional shear, dan retakan yang terbentuk akibat deformasi ekstensional, berfungsi sebagai kontrol lokal terhadap proses mineralisasi. Corak mineralisasi yang terbentuk dan bersa-maan dengan kerangka kinematik terdiri atas 1) zona kemiringan yang dangkal dan lebar dari urat kuarsa stockwork berlembar dan alterasi yang berasosiasi dan sesuai dengan shear fabric, dan 2) zona kemiringan urat kuarsa dan breksi yang curam, yang berasosiasi dengan sesar bersudut tajam dan/atau zona shear yang memotong baik runtunan tutupan flyschoid maupun batuan malihan alas. Struktur termineralkan berkemiringan tajam tersebut diduga menggambarkan zone feeder utama terhadap mineralisasi. Berdasarkan sifat dan orientasi mineralisasi dan batas struktur, cebakan Awak Mas dapat dibagi ke dalam lima domain mineralisasi, yakni Ongan, Mapacing, Lematik, Tanjung, dan Rante. Sesumber mineral total Awak Mas saat ini untuk emas adalah sekitar 2 juta ounces. Prospek Tarra dan Salu Bulo menunjukkan prospektivitas regional Formasi Latimojong yang berupa cebakan mesotermal level rendah, dan dikontrol struktur. Geokimia anomali emas dalam cebakan emas Awak Mas yang melampar dari utara ke selatan ditemukan dalam zona 5 X 13 km. Identifikasi sesumber mineral tambahan sepanjang zone ini merupakan subjek untuk eksplorasi lanjutan.

Kata kunci: geologi, mineralisasi, cebakan emas, jalur malihan, Awak Mas, Sulawesi Selatan

Geology and Mineralization of Awak Mas: A Sedimentary Hosted Gold Deposit, South Sulawesi, Indonesia (C. D. Querubin and S. Walters)

71

Figure 1. Location map of the Awak Mas Project.

Figure 2. Awak Mas project (looking SE) showing main camp and core storage facility.

In September 1998, Placer Dome Incor-porated (PDI) entered into a joint venture agreement with MMC to acquire 51% interest in the project after spending US$ 10 million. However, in June 1999, after spending US$ 1 million, PDI elected to withdraw from the joint venture stating that the project provided only limited op-portunity to host a resource that would satisfy the company’s internal objectives.

By October 1999, MMC signed an agree-ment with JCI, which basically amounted to a debt for equity swap, wherein JCI acquired 66.6% interest in the Awak Mas project. MMC eventually purchased the interest of JCI in the project thereby again assuming full interest until November 2004, when MMC executed an option to purchase agree-ment with Vista Gold Corporation (Vista). In December 2009, Vista announced that

0 80 120 140 160km40

N

INDONESIA

INDONESIA

KALIMANTAN

BRUNEI

JAWA

SUM

AT

ER

A

Bandung

PalembangBanjarmasin

JakartaCirebon

Surabaya

o0

o-2

o-4

o 120 E

o126 E

o124

o122

Map area

SULAWESIAwak Mas

Project

Medan

MALAYSIA

SULAWESI

Awak MasProject

Tanjungbaliho

Kendari

Baubau

Palopo

Parepare

Makassar

Teku

ManadoCelebes Sea

Gulf of Tomini

Gulf of Bone

Southeast Arm

South Arm

Gulf of Tolo

Palu

North Arm

East Arm


72

its wholly-owned subsidiary, Vista Gold (Barbados) Corporation, has signed a joint venture agreement with One Asia Resources Limited (One Asia) with respect to the de-velopment of the Awak Mas project.

The agreement provides One Asia the op-portunity to earn 80% interest by completingan environmental impact assessment and feasibility study (in compliance with NI 43-101 standards).

RECENT EXPLORATION

In 1987, initial exploration at Awak Mas commenced under a SIPP, which predated the execution of the Contract of Work (CoW). Initial reconnaissance sampling was

completed during this time. From 1988 to 1990, during the two year exploration period associated with the CoW, exploration in-cluded regional stream sediment geochem-istry, photogeological mapping, airborne geophysical survey, GIS database develop-ment, follow-up mapping and stream sedi-ment surveys, regional soil geochemistry, scout drilling programs, detailed structural and geological mapping, and establishment of survey control.

Further sampling, geological mapping, and extensive survey work were conducted in 1991. Soil sampling using Wacca drills were likewise completed. During this time, BMGC conducted a 77 drillhole program directed principally at the Awak Mas target. The feasibility period started in October of

9634000 mN

9632000 mN

9630000 mN

9628000 mN

9626000 mN

9624000 mN

9622000 mN

1760

00 m

E

1780

00 m

E

1800

00 m

E

1820

00 m

E

1840

00 m

E

1860

00 m

E

Figure 3. Digital elevation model of the Awak Mas project area (light blue circle = Awak Mas, red lined boundary = CoW) 1998).


73

1991 and was continued until April of 1996. Aside from the drilling by BMGC, MMCcompleted numerous additional programs from 1993 to 1997. This included the drill-ing of 791 holes (DDH and RC) within Awak Mas, with an aggregate meterage of 94,810 m.

Since 1991, over 150 individual studies have been conducted by external consul-tants covering geology, resource estimation, pit optimisation, geotechnical parameters, groundwater, pit design, mining method, production schedule, earthworks and civil works, metallurgical testwork, process plant design, infrastructure, social impact stud-ies, and environmental management. These studies culminated in a definitive feasibil-ity study, based on a 3 million tonne per annum mining and milling rate, conducted by Minproc Engineers Limited (Minproc). In August 1996, the feasibility study was approved by the Director General of the Department of Mines and Energy. In Oc-tober of the same year, the environmental management plan (AMDAL) was submitted in support of the feasibility study. In January 1997, official government questions were raised and, in February of the same year, the revised AMDAL was re-submitted along with an updated feasibility study.

In late 1997, MMC requested for an exten-sion of the Construction Period due to a fall in gold prices at the time. An extension was granted whereby MMC was given an op-portunity to identify additional higher grade resources and to submit a revised feasibil-ity study and environmental management strategy. Extensive additional regional ex-ploration was thus carried out in 1998. The strategy involved the review of initial stream sediment and float geochemistry, follow-up of additional anomalies, mapping, and dia-mond drilling. The exploration was aimed primarily towards the Tarra, Kandeapi, and Salu Bulo Prospects.

In 1998, PDI continued regional explora-tion with Salu Bulo selected as the highest priority target. This program resulted in the identification of an estimated potential mineralization of approximately 160,000 to 230,000 ounces of gold in the ground. Ad-ditional exploration was also completed in the Tarra Prospect, comprising traverse map-ping, trenching, channel sampling, mapping, and diamond drilling totalling 43 holes with an aggregate meterage of 4,517.90 meters.

In 1999, JCI completed a comprehensive review of the GIS database thereby gener-ating several targets for further assessment including Rutuk, Salu Kanan, and Lelating areas. JCI then conducted grid mapping, trenching, and sampling thereby delineating a broad area of alteration and mineraliza-tion within the Rutuk area measuring 600 by 300 m.

After acquiring the project in 2004, Vista completed re-centralizing and re-catalogu-ing historical data, implementing environ-mental assessments and management plans, re-estimating contained geologic resources in accord with CIM compliant definitions, and initiating an exploration drilling pro-gram. The 2006 program conducted by Vista entailed drilling10 holes within the Rante area with an aggregate meterage of 2,102.10 m and 3 holes within the Lematik area with an aggregate meterage of 470.20 m.

In late 2009, after having assumed the devel-opment for the Awak Mas project, One Asia immediately conducted additional mapping, trench and channel sampling, civil works, and in-house mineral resource calculations prepared prior to technical auditing by an Independent Qualified Person. A drilling program was likewise undertaken with the objective of upgrading the inferred resource category into a measured and/or indicated category. Aside from in-fill drilling, the program likewise included metallurgical


74

sampling and geotechnical assessments. To date, One Asia has completed drilling 75 holes with an aggregate meterage of 5,137.60 m.

In all, the total drilling that has been con-ducted for the Awak Mas project since 1991, including its satellite prospects, is 1,012 holes with an aggregate meterage of 118,081.30 m (Figure 4) as of September 1, 2011.

TECTONICS

Sulawesi lies at the junction of three major tectonic plates, the Eurasian Plate to the west, the Pacific Plate to the East, and the Australian Plate to the South. Its tectonic evolution reflects a history of subduction, multiple collisions, extension, and strike-slip faulting (Figure 5).

The West Sulawesi Arc, representing the western to northern segment of Sulawesi, is a volcanic arc terrane built on pre-Late Cre-taceous accretionary complexes composed of high-pressure metamorphic rocks and ophiolites. Intensely folded Late Cretaceous flysch is overlain unconformably by less-deformed Eocene carbonaceous clastics. These are then overlain by widespread Late Eocene platform carbonates and capped by thick Miocene and younger volcanics. Pres-ent day Sulawesi is dominated by a few large sinistral strike-slip fault zones. These faults accommodate the NW relative movements of parts of Sulawesi (due to the continued convergence of the Pacific, Australian and Eurasian Plates) after the Middle Miocene collision of the Bangai-Sula microcontinent. The currently active NW-SE relative motion of the Sulawesi region also created young accretionary prisms/fold-and-thrust belts

9640000 mN

1750

00 m

E

1800

00 m

E

1850

00 m

E

1900

00 m

E

9635000 mN

9630000 mN

9625000 mN

9620000 mN

Tarra

Awak Mas Kandeapi

Ulu Salu

Salu Bulo

Legend

1991 to 2006 Drillling

One Asia Drilling 2011

CoW

N

Figure 4. Awak Mas project drill hole locations.


75

at the northern and western sections. East Sulawesi is characterized by one of the world’s largest ophiolite complexes. Age of obduction is believed to be Late Eocene to Early Oligocene. Below the relatively flat-lying ophiolite cover is a poorly known, but significant Late Triassic-Cretaceous marine sediment section. It has been suggested that East Sulawesi, Buton, Buru, and Seram were parts of a single microcontinent that separated from Australia in the Jurassic and collided with the Eurasian margin to form

the Sulawesi orogen in the Oligocene (Van Gorsel, 2011).

GENERAL GEOLOGY

Western Sulawesi contains three major Neogene, N-S trending, tectonic domains consisting of (from west to east): (1) an active fold belt in which Pliocene and Miocene volcanogenic rocks are involved in west-vergent thrusting extending up to

Figure 5. Sulawesi tectonic map (modified after Maulana, 2009).

N

CELEBES SEA

MA

KA

SS

AR

ST

RA

ITS

GORONTALOBAY

TOMINIBAY

BONEGULF

BANDASEA

North Sulawesi Trench

North Arm

MMCPaluFault

Figs.2 & 9

UnaUna

MedianLine

East Arm

Batui Thrust

Balantak F.

Matano F. Banggai-Sula Block

Tolo Thrust

?

?

Southeast Arm

Lawanopo F.

Walanae F.

BC

South Arm 100 km

Tukang Besi-Buton Block

W. Sulawesi Plutono-Volcanic Arc

Central Sulawesi Metamorphic Belt

Banggai-Sula & Tukang Besi Blocks

Pompangeo schists Continental basements and cover

Continental basements below sea level

Active volcano

Major thrust

Major strike-slip fault

Ophiolitic melange

E. Sulawesi OphioliteNeogene & Quartenary sediments

Ultramafic & mafic rocks (undiff.)

Quatenary sediments

Cenozoic volcanics

& plutonic rocks

Cenozoic carbonates

Other Cenozoic and

Late Mesozoic sediments

Mesozoic (?) metamorphic

& ultramafic basement

2 No

0o

2 So

4 So

6oS

120 E o 124 Eo122 Eo


76

the Makassar Strait; (2) a central belt com-posed of a deformed submarine Miocene volcanoplutonic arc built on an Oligocene-Eocene clastic and carbonate platform with Latimojong Mesozoic basement metamor-phic and sedimentary rocks thrust over its eastern margin on west-vergent faults; and (3) an accreted Cretaceous-Paleogene ophiolite (Lamasi Complex) between the Latimojong basement block and Bone Bay (Bergman et.al., 1996).

The Lamasi Complex ophiolite includes dioritic plutons, basaltic sheeted dykes, pil-low lavas, greenstones, tuffs and volcanic agglomerates. Late Miocene to Pliocene ex-trusive and intrusive rocks form a cogenetic volcanoplutonic complex of calc-alkalic to mildly alkalic, potassic, and shoshonitic felsic and mafic magmatic rocks of bimodal composition which were erupted and in-truded during a short episode of Middle Miocene to Pliocene (3 - 18 Ma) lithospheric melting. Major and trace element geochemi-cal data suggest that parental source rocks of the Miocene melts were Late Proterozoic to Early Palaeozoic crustal and mantle litho-spheric assemblages which became heated and melted owing to a continent-continent collision in which west-vergent continental lithosphere derived from the Australian-New Guinea plate was subducted beneath eastern-most Sundaland.

The Makassar Strait is interpreted as a fore land basin bound on both sides by con-verging Neogene thrust belts, in contrast to previous models suggesting Late Tertiary oceanic spreading or continental rifting. West-vergent obduction of a pre-Eocene oceanic, primitive arc, or back-arc crust onto western Sulawesi occurred during Late Oligocene to Miocene times. The Late Miocene western Sulawesi magmatic arc is envisioned as a continent-continent collision product, contrary to previous models in-volving a normal ocean-continent or ocean-

ocean subduction-related magmatic arc or post-subduction rifting. The east Sulawesi ophiolite extends into western Sulawesi, suggesting that Bone Bay resulted from col-lapse of the over-thickened Miocene orogen.

PROJECT GEOLOGY

The Awak Mas project area is situated within the southern section of the Central Sulawesi Metamorphic Belt, a 50 km long, north-northeast trending fault bounded block composed of basement metamorphic rocks and younger sediments. The western margin of this block is represented by an easterly dipping thrust, whereas the eastern margin is defined by a major basement structure (Rozelle, 2007). The main lithologic unit overlying the area is the Late Cretaceous Latimojong Formation, which consists of phyllites, slates, basic to intermediate vol-canics, limestones, and schists representing a platform and/or fore arc trough, flysch sequence (Figure 6). This formational unit overlies basement metamorphic rocks com-posed of phyllites and slates. Intruding the older sequences are late-stage diorite, mon-zonite and/or syenite plugs and stocks. East of the metamorphic block is the Mesozoic Lamasi Complex, which is composed of basic intermediate intrusives, pyroclastics, and volcanogenic sediments.

Within the immediate vicinity or the Awak Mas project, the predominant lithologic unit is a thick sedimentary package that has been subjected to a low-grade, greenschist facies metamorphism (Archibald et al., 1996). The rocks are generally light to dark gray-ish green, foliated, typically fine-grained, with protoliths ranging from mudstones to siltstones to fine-grained sandstones. Included here are thin intervals of fine- to medium-grained, light reddish grey to red-dish brown hematitic schists. Capping this


77

sequence is a foliated to gneissic, coarse-grained, tuffaceous sandstone interval. These coarser-grained deposits occur as a thin veneer overlying the finer-grained sequences and are generally confined along ridge crests. Dark gray, sheared/mylonitic intervals were noted occurring as interbeds oriented parallel to sub-parallel to folia-tion and usually located proximal to major structural features.

A surface layer of consolidated scree and colluvium averaging 3 to 4 m and with a maximum 15 m thickness drapes the de-posit. Weathering has produced a soil profile

that typically grades into saprolite. The base of weak oxidation within the mineralized sequence lies within 20 m of the surface.

STRUCTURES

Three major NNE-SSW trending, parallel to sub-parallel, sub-vertical fault zones transect Awak Mas. These have been arbi-trarily designated as the Chinese, Garlic, and Discovery Faults (Figure 7). Based on local kinematic indicators, the latest displace-ments along these faults indicate a dextral sense of motion. This perceived movement

Figure 6. Generalized geologic map of the Awak Mas project (after Archibald et al., 1996).

Legend:

Lamasi Complex

Latimojong Formation

Highly magmatic late stage intrusives

Inferred shallow intrusives

Major decollement fault structures

Secondary faults

Contract of work boundary

Project/prospect areas

Awak Mas

Tarra

N

9620000 mN

9635000 mN

9630000 mN

9625000 mN

175000 m

E

180000 m

E

185000 m

E

190000 m

E

Salu Bulo


78

correlates well with findings by previous workers suggesting that thrusting within this part of the region is directed WSW to SW (EHW, 1996 and 1977a).

However, the five domains within Awak Mas are segregated by major structures as represented by the Chinese, Garlic, and Discovery Faults. The displacement exhibited by each domain relative to its ad-jacent domain is in the magnitude of a few hundred meters and appear to have been sinistral. This is exemplified by the relative positions of broad, shallow dipping zones of sheeted and stockwork quartz veining and associated alteration conforming to the shear fabric within the Ongan and Mapacing Domains. In addition, the most significant mineralization at Lematik is described as being localized in N-S trending, steeply dip-ping shear zone segments. The configuration and orientation of vein systems within this specific area strongly resemble features similar to a dilational jog. This would sug-gest that extension within this domain was initially directed E-W. The steeply dipping

mineralized structures in Lematik are in-ferred to represent the main feeder zones to mineralization at Ongan, Mapacing, and Rante.

DISCUSSION

Petrography

The petrographic analysis of samples from selected drill cores show that rocks within Awak Mas have undergone low tempera-tures with fairly high pressure metamor-phism (England, 1992a,b). Assemblages of lawsonite-albite-tremolite-chlorite-epidote define a divariant field stretching from 240° at 3 kb to 360° at 10 kb, where glaucophane-lawsonite replaces albite-chlorite- tremolite. These pressures are equivalent to a possible depth range of from 3 to 35 km and conform to general observations in melange defor-mation wherein high strain is accompanied by low temperatures. This combination of intense tectonism and low geothermal gra-dient suggests that the environment may be

Figure 7. Mineralization and structural styles across Awak Mas. Locations of sections A-D shown.

Ongan9.627.900 mN

17

90

00

mE

17

92

00

mE

17

94

00

mE

17

96

00

mE

17

98

00

mE

18

00

00

mE

18

20

00

mE

9.627.700 mN

9.627.500 mN

9.627.300 mN

9.627.100 mN

B

Mapacing

Rante

Tanjung

Lematik

CH

IN

ES

E

FA

UL

T

A A’

B’

C C’

D

N

GA

RL

IC

F

AU

LT

Legend

D’

Broad shear zone activeduring mineralization

Area of exposed quartz vein localized along syn-mineral fault surface

Mineralized quartz vein with strike & dip

Mineralized quartz vein >1 m wide

Distant breccia, quartz-albite-pyrite altered wallrock clasts in mineralized quartz matrix

Quatrz-albite-pyrite altered wallrock

Quartz veinlets stockworks

Late syn-mineral fault

Late syn-mineral fault active

Gold grade relatively continousbetween drillholes and sections

Post-mineral fault, gritty, poorlyconsolidated plug and cataclasite

Post-mineral normal fault

Interlayered metabasite & basement rocks

Cover sequence schists

Recently mapped structures

Gold grade generally discontinuous& patchy,sharp variations between drillholes & sections.


79

related to either a subduction zone or to mas-sive thrusting. The albitization of samples is inferred to have been caused by seawater expelled from the sediments and focussed through the melange zone.

Deposit Type and Mineralization

Awak Mas is a mesothermal, metamorphic-hosted gold deposit. The gold is associated with sulphur-poor, sodic-rich fluids intro-duced at a relatively late stage in its tectonic history. Albite-pyrite-silica±carbonate alter-ation generally accompanies gold deposition and overprints the ductile fabric associated with deformation and metamorphism in the older basement lithologies (Gustavson, 2007).

Mineralization is typically hosted within a flysch sequence that generally dips 15° to 50° north. Gold is usually associated with

abundant quartz veining and silica-albite-pyrite alteration. Oblique normal faults as well as extensional shears and fractures, formed in response to extensional deforma-tion, serve as local controls to mineraliza-tion although the orientation of the local host foliation likewise acts as an essential component.

In conjunction with the kinematic frame-work, two main styles of mineralization have been developed. The first represents broad shallow dipping zones of sheeted and stockwork quartz veining and associated alteration that conform to the shear fabric (particularly within the mudstones), and the second consists of steeper dipping zones of quartz veining and breccias associated with high angle faults and/or shear zones cutting both the flyschoid cover sequence and basement metamorphics (Figures 8 to 11). The steeply dipping mineralized faults

ColluviumFault actives during mineralization

Broad shear zone actives during mineralization, includes local cataclasis,brecciation, silica clay, silica albite-pyrite alteration and stockwork quartz veins

Mineralized quartz vein > 1m wide

Gold grade inferred to be relatively continuous between drillholesand sectionsGold grade generally discontinuous and patchy, sharp variations between drillholes and sections

Major meta-basite bodiesInterlayered meta-basite & “basement” rocks

Lower Rante discontinuous rockwork documented mineralizationPost-mineral fault

Labels for fault and mineralized zones respectively

Foliation form-lines in cover sequence rocks


Structural features

metres1 : 500

0 15 20 30 40 50

1792

50 E

1793

00 E

1793

50 E

1794

00 E

1794

50 E

1795

00 E

1795

50 E

1796

00 E

1796

50 E

1797

00 E

1797

50 E

1300 RL

1250 RL

1200 RL

1150 RL

1100 RL

1050 RL

1000 RL

950 RL

900 RL

98 m@ 1.63 g/t Au

76 m@ 1.71 g/t Au

21 m@ 2.57 g/t Au

59 m@ 1.63 g/t Au

AMD 291

AMD 246

144.20141.60

130.10

4750

86.50

246.60

245.00

28.00

100.00

76.00

80.38

AMD 152

AMD 104

AMRC027AMRC030

AMD 251AMD 219

AMRC028

AMRC091

AMD 113

AMD 206

AMD 149

Lematik east shear zone 020

LEMATIK SEC C-C’ N 9627269

207.50

100.00

110.00

164.40

40.52

39.90

153.39

49.00

Lematik east shear zone #1

Lematik east shear zone #4?

Basalt Shear

Suplay of Basalt Shear

Lematik east shear zone #1

AMD 227

AMD 207

AMD 089

Figure 8. Section N 9627269 across Lematik Domain showing selected mineralized intercepts.


80

Figure 9 . Section N 9627681 across Ongan Domain.

900 RL

850 RL

800 RL

0 10 20 30 40 50

1 : 500Metres

1000 RL

1792

50 E

1793

00 E

1793

50 E

1794

00 E

1794

50 E

1795

00 E

Colluvium

Structural Features

Fault actives during mineralizationMineralized quartz vein > 1m wide

Gold grade inferred to be relatively continuous between drillholes and sections

Gold grade generally discontinuous and patchy, sharp variations between drillholes and sections

Major meta-basite bodiesInterlayered meta-basite & “basement” rocks

Lower Rante disscontinuous rockwork documented mineralization

Post-mineral fault




1150 RL

1100 RL

1050 RL

Broad shear zone actives during mineralization, includes local cataclasis,brecciation, silica clay, silica albite-pyrite alteration and stockwork quartz veins

Ongan domain foliation parallel zone 2

Ongan domainfoliation parallel zone 1

950 RL

750 RL

deformed? remnants of syn-mineral/fault in post mineral cataclasite

Fault inferred from displacement and structure contour

Lematik East Shear Zone #2

Lematik East Shear Zone #1

AMD 517

AMD 325

AMD 279AMD 413

AMD 311AMRC 103

57.00

77.55129.60

144.10

115.50

82.70

07.5004.30

154.50

50.05

AMD 280

AMD 405

AMD 397AMD 392

AMD 204

“Terminator 2" fault brace

“Terminator 2" fault”

“Terminator 1" fault brace

Chinese Workings Fault Zone

syn & post mineralisation movement

ONGANSEC A-A’ N 9627681

1795

50 E

1796

00 E

1796

50 E

1797

00 E

1797

50 E

Figure 10. Section N 9627481 across Mapacing Domain.

1050 RL

1000 RL

950 RL

900 RL

850 RL

1200 RL

1150 RL

1100 RL

metres

1 : 500

0 10 20 30 40 50

Mapacing SEC B-B’ N 9627481

Normal fault projected from outcrop of 20000E

Normal fault oFoliation 45 / 276

oFoliation 40 / 200 oFoliation 40 340

containing o

veins ... 60 310

AMD 553AMD 657

AMD 484

AMD 415

AMD 402

AMD 470

“Basement”“Basement”

“Basement”

AMD 046

AMD 479 AMD 322

AMD 451

AMD 356AMD 023

Chinese Working

Fault

Colluvium

Structural Features

Fault actives during mineralization

Broad shear zone actives during mineralization, includes local cataclasite,brecciation, silica clay, silica albite-pyrite alteration, and stockwork quartz veins


Gold grade inferred to be relatively continuous between drillholes and sections

Gold grade generally discontinuous and patchy, sharp variations between drillholes and sections

Major meta-basite bodies

Interlayered meta-basite & “basement” rocks

Lower Rante discontinuous stockwork documented mineralization

Post-mineral fault




Albite alteration and weak mineralization (....and orientation) not constrained

54.50

94.50

60.00

73.00

50.0054.20 54.00

107.30

112.30

59.20

21.3020.00

11.00

Fault Zone ......


81

and shear zones, which are associated with repeated silica flooding and brecciation, possibly represent the main feeder zones to mineralization where higher grade gold intercepts are closely related to elevated values of silver and other metals.

The Awak Mas deposit has been subdivided into 5 discrete domains based on the nature and orientation of mineralization. These domains have been designated as the Ongan, Mapacing, Lematik, Tanjung, and Rante domains. Steep faults commonly form the bounding structures separating these do-mains (Figure 12).

Resource Estimate

The gold resources for the Awak Mas deposit (Figure 13) were estimated using normal, whole-block kriging procedures. These gold resources were then classified

into measured, indicated, and inferred cat-egories. Based on evaluations made, the Awak Mas deposit was estimated to contain a current Measured and Indicated Mineral Resource amounting to 45.1Mt at 1.30 g/t Au utilizing a cut-off grade of 0.50 g/t Au, and additional Inferred Mineral Resources of 6.5Mt at 1.13g Au/t based on the same cut-off grade for a total resource of 2.13 Mozs gold (Kirchner and Gossage, 2004). The resource estimate is reported in accor-dance with JORC code (Tetra Tech 2011).

Resource Potential

Aside From Awak Mas, the Salu Bulo prospect located approximately 2 km ESE on-trend from Awak Mas, represents the most advanced exploration area within the CoW (Cox, 1998). This area lies adjacent to a major tectonic feature as represented by the NW-SE trending suture between the

Figure 11. Oblique Section E 180037 across Tanjung-Rante Domain.

Colluvium

Structural Features

Fault actives during mineralization

Broad shear zone actives during mineralization, includes local cataclasite,brecciation, silica clay,silica albite-pyrite alteration, and stockworkquartz veins


Gold grade inferred to be relativelycontinuous between drillholes and sections

Gold grade generally discontinuous and patchy,sharp variations between drillholes and sections

Major meta-basite bodies

Interlayered meta-basite & “basement” rocks

Lower Rante discontinous stockworkdocumented mineralization

Post-mineral fault

Labels for fault and mineralized zonesrespectively



1250 RL

1200 RL

1150 RL

1100 RL

1050 RL

1000 RL

950 RL

900 RL

850 RL

1050 RL

1000 RL

950 RL

900 RL

850 RL

metres1 : 500

0 15 20 30 40 50

Projected position of fault controlling eastern edge of Rante Orebody

Stockwork veins. Discontinuous pods.Large body containing breccia .................occur approx. 5405E and contains approx 2-3 m thick layers of mudrock.becoming discontinuous towards 5600E.internal layers should have a geometry.

possible E+W faultin Upper units

breccia stockworks

body of composite layer parallel veins and stockworkmay not be a continuous in ........... grade mineralization as ......

Discontinuous stockwork domined mineralizationsome mineralized horizons continuous ...........across drill-holes on basis of style and have relatively shallow apparent dips on this section

Stockwork-dominated mineralizationbecomes more discontinuous

126.40127.40

113.75

125.45

74.20

“Basement”

205.50

123.57 108.00

53.00

71.00

57.40

AMD 503

AMD 161

AMRC 018AMD 132

AMD 081

AMD 138AMD 115 AMD 042

AMD 142

AMD 161AMRC 017

AMRC 013

AMD 042B

AMD 024

AMD 108

bottom 7 cm of this hole average approx 1.05 g/t

173.60

AMD 162

AMD 064

oxidased vein and breccia

D7 or C”

stockwork veins and minor composite foliation parallel and breccia

stockwork composite veins breccia and

composite veins

breccia pods

Breccia and stock composite ...... parallel veins

? Possible basement .........

composite veins

TANJUNG - RANTESEC D-D’ E 180 037


82

Lamasi Complex and the Latimojong For-mation. From 1998 to 1999, Placer Dome conducted an exploration program which entailed 2,956 m of channel sampling and trenching including 4,517.90 m of diamond drilling. From the results obtained from

this program, Placer Dome estimated the potential mineralization to be in the range of from 160,000 to 230,000 ounces of gold in the ground (Gustavson, 2007). Drill re-sults at Salu Bulo include [email protected] g/t Au, [email protected] g/t Au and [email protected] g/t Au.

9628000 mN

9627500 mN

9627000 mN

178500 mE 179000 mE 179500 mE 180000 mE 180500 mE

Pre 2011 Drill hole traces2011 Drill hole tracesMineralized domainsCross section line

Ongan

Lematik

Tanjung

RanteMapacing

N

Metres

0 150 300

Figure 12. Location of Mineralized Domains across Awak Mas showing line of Section in Figure 13.

Mapacing Domain

Ongan Domain

Lematik Domain

Tanjung Domain

Rante Domain

Drillhole Trace

1400.0Y

1200.0Y

1000.0Y

1790

00.0

E

1792

00.0

E

1794

00.0

E

1796

00.0

E

1798

00.0

E

1800

00.0

E

1802

00.0

E

MapacingOngan

Lematik

TanjungRante

A

A’

Legend :

Figure 13. Cross section showing mineralized domains across Awak Mas deposit.


83

The Tarra prospect, located approximately 3 kilometers N-NW of Awak Mas, represents another area exhibiting considerable poten-tial. In a review conducted by Etheridge, Henley, and Williams (1997b,c) of data from drillcores and surface exposures, it was estimated that approximately 1Mt with a tenor of 2.20g Au/t occurs in a zone above a planar basal fault sitting underneath 100 to 150 m of relatively un-mineralized rock. Preliminary drill results from Tarra have re-turned up to [email protected] g/t Au. Additionally, the potential for mineralization is present in a shallow plunging shoot at depths approxi-mately 40 m from the surface. Determining the continuity of mineralization within this particular area, however, requires further drilling.

Exploration Potential

The Tarra and Salu Bulo prospects confirm the regional prospectivity of the Latimojong Mountains for low-grade, structurally con-trolled, mesothermal deposits. A regional stream sediment sampling completed over the majority of the CoW has identified a broad area of anomalous gold geochemistry, which is 5 km wide and extends to a length of over 13 km to the north and south of the Awak Mas deposit (Figure 14).

The majority of currently identified re-sources and prospects lie within the northern 40% of this anomalous area. However, little follow-up exploration has been undertaken within the remaining 60%. It is considered highly likely that additional mineralized oc-

Sewatu Prospect Trench 27m at 2.82 g/t Au

West Tarra Prospect Trench 36m at 2.78 g/t Au

North West Tarra Prospect Trench 50m at 1.79 g/t Au

Tarra Prospect Drilling 57m at 2.54 g/Au

Salu Kombong Prospect Trench 15.0m at 2.51 g/t Au

Puncak Utara Prospect Trench 16.0m at 2.95 g/t Au

Lelating Prospect Trench 40.0m at 2.99g/t Au

Salu Bulo Prospect Drilling 30m at 2.95 g/t Au

Awak Mas 2.13 M oz

9628000 mN

9626000 mN

1 Km

180

000

mE

1820

00 m

E

1840

00 m

E

9630000 mN

9632000 mN

178

000

mE

N

Figure 14. Location of Awak Mas deposit and principal regional prospect areas.


84

currences will be identified within this area with further systematic exploration.

CONCLUSION

Awak Mas is a mesothermal gold deposit hosted in a flysch sequence (Latimojong Formation) that generally dips 15° to 50° north. The gold is associated with sulphur-poor, sodic-rich fluids introduced at a relatively late stage in the regional tectonic history. Gold mineralization is closely re-lated to abundant quartz veining and silica-albite-pyrite alteration.

Lawsonite-albite-tremolite-chlorite-epidote mineral assemblages define a divariant field stretching from 240° at 3 kb to 360° at 10 kb, conforming to general observations in melange deformation wherein high strain is accompanied by low temperature range. This temperature-pressure regim suggests an environment related to either subduction or massive thrusting.

Oblique normal faults as well as extension-al shears and fractures formed in response to extensional deformation, serve as local controls to mineralization. Mineralization styles formed in conjunction with the ki-nematic framework are 1) broad shallow dipping zones of sheeted and stockwork quartz veining and associated alteration that conform to the shear fabric, and 2) steeper dipping zones of quartz veining and brec-cias associated with high angle faults and/or shear zones cutting both the flyschoid cover sequence and basement metamorphic rocks. These steeply dipping mineralized structures are inferred to represent the main feeder zones to mineralization.

Based on the nature and orientation of min-eralization as well as structural boundaries, the Awak Mas deposit has been subdivided into 5 discrete domains, namely: the Ongan, Mapacing, Lematik, Tanjung, and Rante domains.

Current mineral resources for Awak Mas total approximately 2.13 million ounces of gold. In terms of resource potential, the Salu Bulo and Tarra prospects have a combined potential resource of approximately 400,000 ounces gold based on current limited drill-ing.

The Tarra and Salu Bulo prospects confirm the regional prospectivity of the Latimo-jong Formation for low-grade, structurally controlled, mesothermal deposits. Anoma-lous gold geochemistry has been identified within a 5 by 13 km zone extending from north to south of the Awak Mas deposit. With a systematic exploration, it is pos-sible for additional mineral resources to be identified.

ACKNOWLEDGEMENTS

The article has been presented in the Seminar of the Sulawesi Mineral Resources, Manado, 2011, and compiled in the proceedings of the seminar. The authors acknowledge the Committe. By improving the content, the article is published in the Majalah Geologi Indonesia - IAGI. Therefore, the authors thank to IAGI.

REFERENCES

Archibald, N.J., Power, W.L., Ketelaar, P.B., Utley, D.C., Panizza, N., and Nichols, S.N., 1996. Geol-ogy of the Awak Mas prospect area, south central Sulawesi, Indonesia. Fractal Graphics for PT Mas-mindo Eka Sakti, 24pp.

Bergman, S.C., Coffield, D.Q. Talbot, J.P., and Gar-rard, R.A., 1996. Tertiary tectonic and magmatic evolution of western Sulawesi and the Makassar Strait, Indonesia: Evidence for a Miocene continent-continent collision. The Geological Society, London.

Cox, S., 1998. Review of Salu Bulo, W Tarra, NW Tarra and Sewatu prospects, Sulawesi, Indonesia. Steffen, Robertson and Kirsten (Australasia) Pty Ltd, trading as SRK Consulting, 25pp.

England, R.N., 1992a. Brief petrographic notes on 23 samples from south Sulawesi. prepared for Katchan., Battle Mt. Inc., c/o PT Masmindo Eka Sakti.


85

England, R.N., 1992b. Petrographic notes on 22 samples from south Sulawesi; prepared for Katchan, G., Battle Mt. Inc., c/o PT Masmindo Eka Sakti

Etheridge, Henley, and Williams (EHW), 1996. Structural geology for mineral exploration. EHW for Placer Exploration Ltd., 100pp.

Etheridge, Henley, and Williams (EHW), 1997a. Fault activity at Rante Bella, Sulawesi, Indonesia; EHW for Lone Star Exploration NL, 13pp.

Etheridge, Henley, and Williams (EHW), 1997b. Geometry and continuity of mineralised zones at Awak Mas, Sulawesi, Indonesia. EHW for Lone Star Exploration NL, 61pp.

Etheridge, Henley, and Williams (EHW), 1997c. Gold mineralization and structure at Tarra prospect, Sulawesi, Indonesia; EHW for Lone Star Explora-tion NL, 36pp.

Etheridge, Henley, and Williams (EHW, 1997d. Ore zone orientation and continuity at Awak Mas (scoping report); EHW for Lone Star Exploration NL, 19pp.

Kirchner, I. and Gossage, B.L., 2004. Mineral re-source estimate for the Awak Mas deposit, Sulawesi Indonesia. RSG Global for Vista Gold Corp.

Maulana, A., 2009. Petrology, geochemistry andmetamorphic evolution of the South Sulawesi base-ment rock complexes, Indonesia. M.Phil. Thesis, Aust Nat Univ, Canberra.

Rozelle, J.W., 2007. Report NI 43-101, Technical report on the Awak Mas gold project, Sulawesi, Indonesia. Gustavson Associates for Vista Gold Corp., 104pp.

Tetra Tech, 2011. Awak Mas Gold Project Su-lawesi, Indonesia. Independent Geologists Report. Prepared for One Asia Resources Limited.

Geology and Mineralization of Awak Mas: A Sedimentary ... 20120201.pdfmasi Latimojong, merupakan...

Documents

Transcript of Geology and Mineralization of Awak Mas: A Sedimentary ... 20120201.pdfmasi Latimojong, merupakan...