Application of an Analytical Spectral Device (ASD) in Alteration Mapping of the Seruyung Project, East Kalimantan, Indonesia Aplikasi Analytical Spectral Device (ASD) pada Pemetaan Ubahan Projek Seruyung, Kalimantan Timur, Indonesia
Majalah Geologi Indonesia, Vol. 26 No. 3 Desember 2011: 155-171
Application of an Analytical Spectral Device (ASD) in Alteration
Mapping of the Seruyung Project, East Kalimantan, Indonesia
Aplikasi Analytical Spectral Device (ASD) pada Pemetaan Ubahan
Projek Seruyung, Kalimantan Timur, Indonesia
Atmasari Rura, Priyo Widekso, Arif Purnomo and Jesse Umbal
PT. Avocet Mining Services,
Jln.. Kol. Sugiono No. 24 Kotobangun Kotamobagu, Bolaang Mongondow, North Sulawesi
ABSTRACTThe Seruyung prospect is an advanced stage project targeting high sulphidation epithermal gold deposits
and mineralized diatreme structures hosted in an interbedded andesitic to dacitic volcanic and
epiclastic sequence, located in the Nunukan District, East Kalimantan Province. This paper describes
briefly the application of an Analytical Spectral Device (ASD) in the alteration mapping conducted
at the Seruyung Project. The aim of the ASD study was to characterize clay alteration assemblages,
their lateral distribution and vertical persistence along drill intercepts, and to identify distribution
patterns that could be used to vector towards potential gold mineralization. The ASD instrument is
a field portable spectrometer that allows the identification of minerals by measuring their vibrational
spectral absorption features. These absorption features are attributable to the variations in composi-
tion, crystallinity, and orientation of the minerals. Samples that can be analysed include rocks and
powder collected from outcrop, soil, drill hole samples, and hand auger samples with flat or a near flat
surface. The method of analyzing samples entails placing the mineral probe on a flat surface of the
material where a spectrometer scan can be taken. The spectral measurements are able to discriminate
low and high temperature species of clay minerals in the studied area. The main alteration minerals
identified in drill hole and surface samples include alunite, silica, quartz, kaolinite, pyrophyllite,
illite-smectite, and minor illite. Oxidation minerals, such as haematite, limonite, and goethite were
also identified. The alteration mineral assemblages exhibit a zoned lateral and vertical distribution
typical of high sulphidation epithermal deposits. The alteration pattern comprises a massive/vuggy
silica core grading outwards into advanced argillic and argillic alteration haloes. Gold mineraliza-
tion is commonly associated with vuggy and massive silica zone, and to a minor extent within the
advanced argillic zone. Pyrophyllite and dickite commonly occur proximal to the margins of hy-
drothermal breccia structures, suggesting their possible use as vectors toward potential high grade
feeder structures masked on the surface by soil and weathered overburden. The presence of discrete
pyrophyllite clays along the peripheral margin of inferred diatreme or maar features suggest that
related arcuate structures are potential path ways for hot ascending acid fluids, and thus, potentially
can be mineralized. The spatial distribution of “hot” clays along these arcuate features should further
be compared with results of detailed geological mapping and geophysical surveys and validated by
trenching prior to drill testing.
Keywords : Analytical spectral Device (ASD), Seruyung, East Kalimantan, sulfidation epithermal gold
SARIProjek Seruyung merupakan proyek tahap lanjutan yang menargetkan deposit emas epitermal sulfida
tinggi dan struktur diatrema termineralisasi dalam runtunan lapisan vulkanik dan epiklastika andesit
- dasitik, di Distrik Nunukan, Provinsi Kalimantan Timut. Makalah ini secara ringkas menjelaskan
aplikasi Analytical Spectral Device (ASD) pada pemetaan ubahan yang dilaksanakan di Proyek
Seruyung. Maksud studi ASD ini adalah untuk mencirikan kumpulan ubahan lempung, distribusi
lateral dan vertikalnya secara terus-menerus selama pengeboran, dan untuk mengidentifikasi pola
Naskah diterima: 03 Januari 2011, revisi terakhir: 12 Desember 2011distribusi yang dapat digunakan pada vektor terhadap mineralisasi emas potensial. Instrumen ASD
Majalah Geologi Indonesia, Vol. 26 No. 3 Desember 2011: 155-171
merupakan spektrometer portable lapangan yang dapat mengidentifikasi mineral dengan mengu-
kur absorpsi spektral vibrasionalnya. Gambaran absorpsi ini diakibatkan oleh variasi komposisi,
kristalinitas, dan orientasi mineral. Percontoh yang dapat dianalisis meliputi batuan dan serbuk
yang dikumpulkan dari singkapan, tanah, percontoh sumur pemboran, dan percontoh dari bor tan-
gan dengan permukaan rata atau hampir rata. Metode analisis percontoh memerlukan penelitian
mineral yang diletakkan pada permukaan rata, dengan demikian scan spektrometer dapat dilakukan.
Pengukuran spektral dapat membedakan spesies mineral lempung suhu rendah dan tinggi di daerah
penelitian. Mineral ubahan utama yang teridentifikasi pada percontoh sumur bor dan permukaan
meliputi alunit, silika, kuarsa, kaolinit, pirofilit, ilit-smektit, dan ilit minor. Mineral oksidasi, seperti
hematit, limonit, dan goetit juga teridentifikasi. Kumpulan mineral ubahan menunjukkan distribusi
vertikal dan lateral yang terzonasikan, khas deposit epitermal sulfidasi tinggi. Pola ubahan terdiri
atas inti silika masif/vuggy bergradasi keluar ke argilik lanjutan, dan halo ubahan argilik. Mineral-
isasi emas biasa berasosiasi dengan zona silika masif dan vuggy, dan sampai tingkat minor berada
pada zona argilik lanjut. Pirofilit dan dikit biasa terdapat dekat batas struktur breksi hidrotermal,
mengesankan kemungkinan penggunaannya sebagai vektor terhadap struktur feeder tingkat tinggi
di permukaan tertutup oleh tanah dan lapisan lapuk. Kehadiran lempung pirofilit yang berbeda di
sepanjang batas periferal diatrema atau maar terduga mengesankan bahwa struktur arkuat terkait
merupakan jalur yang potensial bagi cairan panas asam yang mengalir naik, dan karena itu poten-
sial dapat dimineralisasikan. Distribusi spasial lempung panas di sepanjang arkuat ini hendaknya
dibandingkan dengan hasil pemetaan geologi terperinci dan survei geofisika serta divalidasi dengan
trenching sebelum uji pemboran.
Kata kunci: Analytical Spectral Device (ASD), Seruyung, Kalimantan Timur, emas epitermal sulfidasi
INTRODUCTION
An understanding of the type and distribu- tion of various alteration mineral assem- blages is an important component for the successful exploration of hydrothermal ore deposits. Conventional geological mapping tools and techniques are able to provide qualitative descriptions of the alteration minerals by their physical characteristics (e.g. kaolinite, alunite, etc.), but are con- strained and limited in defining important compositional and attendant crystallinity variations by virtue of their fine-grained nature. The application of spectral geology offers a more specific quantitative analytical solution to these limitations. Spectral geology is a specialized field that applies the science and techniques of reflec- tance spectroscopy to identify spectrally distinct and physically significant features of different rock types and surface materi- als, their mineralogy and their alteration signatures. Spectral signatures are mea- sured using specialized sensors that record either solar or artificially provided radiation reflected from the surface of materials.
Many materials absorb radiation at spe- cific wavelengths that appear as diagnostic troughs in a spectral curve. It is this characte r istic absorption spectral features and pat- terns that enable discrimination between individual materials (Kruse, 1994). The range of wavelength most suitable for the discrimination of geological materials and oil slicks includes those in the visible and near-infrared (VNIR), short- wavelength infrared (SWIR), and the mid or thermal infrared (TIR) wavelengths, while the characteristic fluorescence of hydrocarbons occurs in the ultra-violet (UV) spectral region (Figure 1). The positions of the wavelength are deter- mined by the cation and the bond length. The ionic radius of the cation determines the length of the chemical bond between the cations and to the molecule (e.g. such as water, hydroxyl, and carbonate) to which
Application of an Analytical Spectral Device (ASD) in Alteration Mapping of the Seruyung Project,
East Kalimantan, Indonesia (A. Rura et al.)it is attached. Spectroscopy measures the vibrational energy of the bond, which vi- brates at discrete wavelengths specific to the mineral bond length. For instance, the Al-OH bonds lie within the 2200 μm re- gion, the Fe-OH bonds within 2280 - 2295 μm for clays, and the Ca-CO3 bonds at around 2334 μm.
The basic principle of spectral identification is pattern and wavelength recognition and in particular, the presence of a specific feature or minima at a certain wavelength or band.
The characteristic features of a mineral spectra are influenced by its composition, degree of crystallinity, and orientation. In this respect, a mineral spectral signature can be useful to assess the mineral compo- sitional and crystallinity variations, as well as possible mixing with associated mineral assemblage.
Measurements of the spectral properties of a mineral were previously conducted in a laboratory. In the last decade, however, several field portable spectral measuring de- vices had been developed and successfully tested in the field and are now commercially available for use by exploration companies.
Spectrum 542 446 minerals with spectral signatures within 578 400 500 600 700 meters Visible 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 1 10 10 2 10 3 10 4 10 5 10 6 10 7 10 8 VIS
0.4 0.7 1.1 1.4 1.7 2.0 2.3 2.63 6 9 12 15 H
2 O
H 2 O H H 2 O 2 O H H 2 O 2 O CO 2 CO 2 CO 2 CO 2 CO 2 CO 2 O 2 OWavelength (pm) Wavelength (pm)
2 TV/Radio Rays Wavelength in micrometres X-Rays Ultraviolet Thermal infrared Near&Mid infrared Soil AI-OH mineral Water Vegetation 60 40 20% Reflectance Blue Green Y ellow
Orange
Red Purple MicrowaveFigure 1. This plot shows the region of the electromagnetic spectrum, which is of interest to reflectance spectros-
copy. The visible (VIS), near infrared (NIR), and short wave infrared (SWIR) wavelength values are delimited.
The horizontal scale is in microns and nannometers, the vertical scale is percent reflectance as calibration to
the reflectance standard (adapted from Hauff, 2005).At present, field portable spectrometers provide spectral information on fine-grained
Majalah Geologi Indonesia, Vol. 26 No. 3 Desember 2011: 155-171
Figure 2. ASD instrument set showing spectrometer, mineral probe, laptop, spectralon standard, and petrie dish.
ASD measurements on drill core and powder samples.
the visible (0.4 - 0.7 μm), near (0.7 - 1.3 μm), and shortwave infrared (1.3 - 2.5 μm) wavelengths. Within this wavelength range, a multitude of minerals germane to the exploration and assessment of mineral deposits can be identified.
There are currently several commercially available field portable spectrometers, with two of the most popular and widely used brands being the Analytical Spectral Device (ASD®) and the Portable Infrared Mineral Analyzer (PIMA®). Both devices have been evaluated by Avocet, eventually deciding on acquiring an ASD in 2008, which it has successfully integrated into its exploration projects ever since. This paper briefly describes the application of ASD techniques in mapping the alteration zones at Seruyung Project in Kalimantan to complement exploration data acquired from geological mapping, geochemistry, geophysical (i.e. IP and ground magnetic), and scout drilling. The aim of the ASD study was to characterize clay alteration minerals, their associated assemblages, their lateral distribution and vertical persistence, and to identify distribution patterns that could be used to vector towards potential gold mineralization.
METHODOLOGY Equipment and Set-up
The ASD TerraSpec® Pro is a portable spectrometer designed for field analyse of soil, rock, and drill (i.e. core and RC) samples. The device consists of three main components - a spectrometer, mineral probe, and laptop. A typical equipment set-up is shown in Figure 2. The spectrometer is the main analytical component encased in a durable, light- weight, and portable casing. It can process a full spectral range between 350 - 2500
μm
at a spectral resolution of 6 - 7
μm and is
capable of rapid data collection at 1/10th of a second per spectrum. The instrument uses both DC and AC power supply and is very tolerant of voltage and frequency fluctuations. The mineral probe is a handled and work- station component of the equipment and connects to the spectrometer unit using a fiber-optic cable. It features a built-in light source that enables it to measure solid, paste, powder, rock, and granular media directly or inside a clear and transparent plastic bag, pet- rie dish, or in the accompanying sample dish attachment. The sensor window of the probe
Application of an Analytical Spectral Device (ASD) in Alteration Mapping of the Seruyung Project,
East Kalimantan, Indonesia (A. Rura et al.)is composed of sapphire, thus, is generally resistant to scratching by most materials. It comes with a spectralon - an optical standard reference, for calibration purposes.
The equipment has a dedicated laptop linked to the spectrometer using an Eth- ernet cable and has four pre-installed ASD TerraSpec® Pro supported computer soft- ware programmemes - High Contrast Field
SpecPro RS3, ViewSpecpro, SpecWin, and SpecMin. High Contrast FieldSpecPro RS3
is the software programmeme for collect- ing reflectance measurements directly from the spectrometer and translating the spec- trum data into an ASD file (i.e. as an .sco extension) format (Figure 3). The spliced data are analysed using the ViewSpecPro software programmeme. For generating, viewing, manual analysis and interpreta- tion of spectral data, the SpecWin software programmeme is used (Figure 4). Individual spectral images can be referenced from an ASD spectral database catalogue accessible using the SpecMin software programmeme.
While the equipment is tolerant of varying temperature conditions, overheating could affect the quality of the spectra acquired. Thus, it is advisable to have the equipment shaded from direct sunlight when working outdoors and the unit should be given time to cool down after several hours of continu- ous measurements. Rooms with fluorescent light illumination should be avoided when working indoors as this may cause major interference in the spectra. It is advisable to have the equipment run for half an hour to stabilize prior to use and connecting to the laptop. Oversaturation can occur and in this situation recalibration using the optical standard is required.
Figure 3. High Contrast FieldSpecPro RS 3 software screen view for preliminary viewing and translating measured spectrum data into ASD file (.sco extension) format.
Majalah Geologi Indonesia, Vol. 26 No. 3 Desember 2011: 155-171
Sampling Design and Sample MaterialsThe ASD samples collected for this study comprised soil (i.e. both matrix and frag- ments), rock, and core samples. These samples constitute fresh, weathered, and altered rocks found within the prospect. All samples were sun-dried to allow evapora- tion of excess water prior to measurement. A total of 685 samples were collected for this study comprising of 64 surface (i.e. soil, outcrops, and floats) and 621 core samples.
Rock and core samples with facet faces having a minimum flat or near flat surface area of 20 mm x 20 mm dimension (i.e. cor- responding to the probe’s sensor window area) were selected to avoid getting noisy spectra during measurement. Rock sam- ples were taken from all known outcrops and boulder floats in the project site and cleaned of adhering weathered materials prior to measurement. Core samples were taken at preselected intervals in each drill hole to be as representative as possible of alteration types and changes. In the case of polymictic breccias, collection was undertaken to get a representative sample of all the component fragments as well as the matrix. This is to ensure that possible differences and/or commonality in the al- teration minerals inherent in the breccias are captured.
Soil samples were collected both selectively and randomly along grid fence-lines, roads, and foot trails crisscrossing the project area. Soil samples were presegregated into its weathered matrix and clast fragments. The dried clast fragments were cleaned of possible weathered material clinging into its outer surface. Both soil matrix and clast fragments were measured.
Figure 4. Spectral curve generated for analysis using the SpecWin software.
Application of an Analytical Spectral Device (ASD) in Alteration Mapping of the Seruyung Project,
East Kalimantan, Indonesia (A. Rura et al.)Measurement Procedure and Data Compilation
Sample measurements were conducted in- doors in the Seruyung camp office and core- shed facilities, and at the Kotamobagu office, North Sulawesi. All samples were cleaned of potential contaminants and sun- dried.
Prior to measurement, each rock, core or soil clast was examined for homogeneity of its components and alteration. Samples which components and alteration appear to be heterogeneous had each of its component and/or alteration facets separately marked for measurements. Soil material was placed in a petrie dish in quantities sufficient to cover the entire bottom of the dish. This is to ensure that no extraneous light source enters the sen- sor window that can degrade the spectra acquired.
Measurement is made by placing the probe sensor window in contact with the sample medium (i.e. rock, core, and soil). At least two measurements are taken for each sample or target sample facet surface for rock and core samples and at least two measurements each for soil material and its associated clast types. Results were compiled and analyzed using a combination of the ASD preinstalled software programmemes.
The final results were tabulated into a Mi- crosoft Excel spreadsheet (Figure 5). The results were later converted into ArcGIS shapefiles for graphical representation and ease in spatial analysis.
GEOLOGICAL BACKGROUND Regional Setting
The Seruyung prospect is an advanced stage project targeting high sulphidation
Figure 5. ASD Excel worksheet show tabulated data and interpretation.
Majalah Geologi Indonesia, Vol. 26 No. 3 Desember 2011: 155-171
Figure 6. Project location map.Explanation N
Major river Province boundary line Border line Indonesia Malaysia Brunai Seruyung KP Kilometres 300
West Kalimantan East Kalimantan Central Kalimantan South Kalimantan 105 105 o E 120 o E 135 o E o E 120 o E 135 o E o o Seruyung KP
epithermal gold deposits and mineralised diatreme structures hosted in an interbedded andesitic to dacitic volcanic and epiclastic sequence. The prospect is located in the dis- trict of Nunukan, East Kalimantan Province (Figure 6).
The Seruyung prospect is situated along the SW extension of an ENE to NE trending volcanic arc that passes through Tawau in Malaysia into the Tawi-tawi-Sulu volcanic island group in the Philippines. This vol- canic arc is associated with Miocene to Recent
(?) south-eastward subduction of the Sulu Sea Plate. The oldest rocks are a Pre-Tertiary to Pa- leogene sedimentary sequence intruded by Cenozoic intrusions that comprise most of the Central Highland region of Kalimantan. The eastern foot slopes of the highlands are bounded by N-S to NNE trending basin margin faults. The flat estuarine terrain to the east of the mountain range is underlain by Late Paleogene to Neogene sedimentary sequences associated with sedimentation within the Tarakan Basin. The northern and southern margins of the basin are bound by NW-trending regional arc-transverse faults referred to as the Sempurna and Maratua- Mangkalihat Fault Zones, respectively. Miocene and younger Pliocene volcanic rocks sporadically outcrop within the broad estuarine tidal flats that are for the most part covered by extensive Quaternary alluvium. The main regional structure is an ENE fault referred to as the Sembakung Linea- ment (Figure 7). This fault can be traced for nearly 100 kilometres along the Sem- bakung River.
Application of an Analytical Spectral Device (ASD) in Alteration Mapping of the Seruyung Project,
East Kalimantan, Indonesia (A. Rura et al.) Figure 7. Regional geology map. Plio-Pleistocene Volcanics Mio-Pliocene Intrusives Miocene Volcanis Early to Middle Miocene Sediments Oligo-Miocene Intrusives Eocene to Early Miocene Sediments Pre-Tertiary Basement Lithology Units Fault Lineament Legend Seruyung Kilometres 5 10 15 20 50'0"E 117 25 116 o o 0'0"E 117 o 10'0"E 117 o 20'0"E 117 o 30'0"E 116 o 50'0"E 117 o 0'0"E 117 o 10'0"E 117 o 20'0"E 117 o 30'0"E 40'0"N 3 o 50'0"N 3 o 0'0"N 4 o 4 o 10'0"N 3 o 40'0"N 3 o 50'0"N 4 o 0'0"N 10'0"N 4 oN Prospect Geology
The prospect area is underlain by sub-hor- izontal to gently-dipping bedded andesitic to dacitic tuffs interbedded with minor epi- clastic and breccia units (Williams, 2000; PT Sago, 2007; Bautista and Munajat, 2007; Hardjana et al.
, 2009). Crystal-, ash-, and lithic-tuffs comprise the individual units within the volcanic sequence. The ages of these volcano-epiclastic units are poorly known, but could well range from Mio- cene to Pliocene. Young and fresh-looking porphyritic basaltic andesites crop-out in nearby hills and may well correlate with Quaternary basaltic andesites regionally found in East Kalimantan. The youngest units are recent unconsolidated colluvial, alluvial, and swamp deposits. The areal distribution of these rock units is shown in Figure 8.
Several breccia units were mapped in out- crops and recognized in drill cores. Their textural characteristics indicate a wide range of origin that include hydrothermal breccias, fault breccias, and diatreme brec- cias.
The structural fabric is dominated by roughly E-W trending and cross-cutting NW to WNW trending dextral faults and lineaments. The most prominent of these structures is an E-W dextral fault, infor- mally referred to here as the Seruyung
Fault, which forms a linear cliff face along the northern slopes of Mount Seruyung
The interplay of these various structural sets suggests a genetic association with the regional Sembakung Lineament and accounts for the rough E-W elongation of Mount Seruyung. In addition, there are two geomorphologically pro minentar cuate/cir- cular features recognized in the prospect: a
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Figure 8. Geological map of the Seruyung Prospect. 0 50 100 200 300 400 metres Legend StructureLineament Depression Dome
Feature Topo contour 5m
Lithology Colluvial/alluvial deposits Breccia unit 1 Breccia unit 2 Lithic tuff
Crystal tuff Ada Raye Southern area
Main silica cap Northern colluvium N
400m-wide semicircular depression located in the western portion and a 200m-wide oblate dome-like feature at the summit area of Mount Seruyung.
The general alteration mineral assemblages and their distribution patterns are typical of high sulphidation epithermal deposits. Hy- drothermal alteration mineral assemblages comprise a massive silica and vuggy silica core (i.e. Main Silica Cap) partially envel- oped by an advanced argillic alteration halo (alunite- pyrophyllite) that grades outward towards a broader marginal intermediate alteration halo that is kaolinite-dominated. Sulphides, mostly pyrite, are typically fine- grained and commonly oxidised. Enargite is present, but has been oxidised to scorodite (Sinclair Knight Merz, 2009; Latanzi et al., 2008).
The main alteration minerals identified by ASD from surface (i.e. rocks and soils) and drill hole samples include alunite, silica, quartz, kaolinite, and pyrophyllite. Illite- smectite and illite tends to be minor altera- tion constituents. Similarly, dickite occurs as another minor constituent with kaolinite and pyrophyllite. The spatial distribution of surface samples and those from cores are shown in Figure 9.
Alteration Minerals
The representative alteration minerals in the prospect include alunite, quartz, silica, kaolinite, pyrophyllite, illite, illite-smec- tite, and dickite. The characteristic spectral signatures of some of these common miner- als are shown in Figure 10.
Application of an Analytical Spectral Device (ASD) in Alteration Mapping of the Seruyung Project,
East Kalimantan, Indonesia (A. Rura et al.) 200 100 200 Metres NClay alteration minerals (ASD) SRD060 SRD044 SRD045 Geomorphology Interpretation EXPLANATION Depression outcrops, auger, and drill holes Drill Hole Alteration Minerals Na-alunite K-alunite Western breccia SRD042 K-alunite SRD052 SRD047 SRD051 SRD053 SRD054 SRD046 SRD043 Main Silica Cap Outcrop Alteration Mineral (second) Kaolinite Illite-smectite Dickite Na-alunite Dome Contour Kaolinite-mod Kaolinite-low Dickite-well Illite Dickite-mod Goethite Dickite-low Kaolinite-well Illite-smectite SRD066 Outcrop & Auger Alteration Mineral Dickite-well Dickite-mod Dickite-low K-alunite Pyrophyllite Silica Illite-smectite Silica Quartz Pyrophyllite Silica Sry_dril_col Quartz Prophyllite Kaolinite-well Kaolinite-mod Kaolinite-low Figure 9. Location map of surface and drill hole samples and results of ASD alteration identification. 10 8 pyrophyllite 6 kaolinite alunite dickite 2 4 illite 0.0 500 1000 1500 2000 2500 illite-quartz Figure 10. Spectra of common alteration minerals in the Seruyung prospect.
The spectral signature of alunite is recogniz- minima range from 1425 - 1439 and from able by the presence of a diagnostic doublet 1478 - 1495. The wavelength at the 1478 - feature at the 1425 - 1439 μm and 1478-1495 1495 μm minima can be used to discriminate
μm wavelength and triplet feature within
between the potassium (i.e. 1478 - 1486 μm) the 2200 and so dium
μm wavelength region. The two (i .e . 148 8 - 1494 μ m) al
Majalah Geologi Indonesia, Vol. 26 No. 3 Desember 2011: 155-171
un it e varieties. Alunite is one of the most common alteration minerals measured in the samples collected. Its presence sug- gests temperature of formation ranging from <100oC – 300oC and acidic conditions (i.e. pH<4). The dominant alunite species is potassium alunite (i.e. hypogene), which is commonly associated with magmatic- derived acidic hydrothermal fluids. Sodium alunite was recognized mostly from surface samples, but is minor and generally subor- dinate to potassium alunite. The presence of a low water feature in the spectral signature in some of sodium alunite observed may suggest a supergene origin or the presence of an associated clay (Hauff, 2005). Alunite occurs generally in association with kaolin- ite, pyrophyllite, silica, and quartz. It was also commonly observed as in-fill in vugs and open cavities. Similar to alunite, kaolinite is ubiquitous throughout most of the outcrops and cores samples measured. It is recognizable by its characteristic doublet features at the 1392 - 1402 μm and 2162 - 2168 μm wavelengths. Both high and low crystalline varieties were observed. Crystalline kaolinite (i.e. identified on the basis of small minima within the
1810 - 1848 nm spectral wave- length) typically occurs in association with alunite, pyrophyllite, and to a minor extent dickite. In contrast, the low crystalline va- riety was commonly observed in surface samples and in shallow core intercepts commonly in association with illite and illite-smectite clays.
Pyrophyllite is a high temperature clay alteration mineral that forms under low pH (i.e. <4) conditions similar to alunite. Pyrophyllite was observed in most drill hole samples and outcrop samples taken mostly within the Main Silica Cap and Western Breccia Zone, but is less common than alunite, kaolinite, and silica. Pyrophyllite occurs mostly in association with potassium alunite, silica, quartz, and crystalline ka- olinite. Discrete occurrences of pyrophyllite were observed only in outcrops mapped in association with local structures or inferred lineaments.
Silica and quartz are differentiated here as different crystalline forms of SiO2. They can be discriminated from each other by the presence of broad trough feature in their spectral signatures at wavelengths of 1420 - 1426 μm and 1443 - 1447 μm, respectively. Silica and quartz occur as the dominant alteration minerals in outcrops and drill hole intercepts located within the Main Silica Cap area. The prominent peak and resistant nature of Mount Seruyung is attributed to the pervasive silica alteration at its summit. Observed gold - sulphide mineralization is also commonly associated with volcanic units having vuggy silica and silicified breccia alteration textures. Elsewhere in the prospect, these two minerals occur as subordinate alteration minerals in associa- tion mostly with alunite and kaolinite. Dickite can be recognized by the pres- ence of a diagnostic doublet features with minima at 1378 - 1386 μm and 2180 - 2186
μm wavelength. The presence of another
very small minima at the 1798 - 1836 μm wavelength is indicative of well developed crystallinity in dickite. Dickite primarily occurs as minor alteration constituent as- sociated with kaolinite and pyrophyllite in several drill holes samples and surface samples. Crystalline dickite was found as- sociated with crystalline kaolinite and/or pyrophyllite. Low crystallinity dickite was observed to be generally associated with low crystallinity kaolinite and to a minor extent with illite- smectite.
Illite can be recognized by the presence of a sharp feature at the 1408 - 1414 μm and 2190 - 2220 μm wavelengths. The pres- ence and depth of a small minima at the
Application of an Analytical Spectral Device (ASD) in Alteration Mapping of the Seruyung Project,
East Kalimantan, Indonesia (A. Rura et al.)2110 μm indicates the illite is crystalline. Illite-smectite can be differentiated by a broad feature at the 1404 - 1415
μm and
2188 - 2192 μm wavelengths. Minor illite was observed mostly in some of the drill holes and typically is of the poorly crystal- line variety. It occurs mostly in association with kaolinite. Illite - smectite showed similar association with kaolinite and in most instances was observed in deeper drill hole samples. In outcrops, illite - smectite clays were observed only on the northwest margin of the prospect.
Chlorite is recognisable by the presence of a broad feature at 2242 - 2262
μm and 2314
Oxide minerals include haematite, limonite, and goethite. In addition, scorodite, an hy- drated iron arsenate, was identified petro- graphically (Sinclair Knight Merz , 2009) and probably originated as the oxidised alteration mineral of enargite with the iron provided by dissolution of pyrite (Lattanzi et al., 2008).
Alteration Mineral Assemblages and Patterns
The ASD results were subsequently adopted to further refine the alteration boundaries delineated from previous geologic mapping activities. At least three main alteration zones are recognized on the basis of altera- tion mineral associations and assemblages: vuggy and massive silica, advanced argillic, and argillic zones. The distribution of these alteration zones is shown in Figure 11.
- 2368 μm wavelengths. Chlorite is the least minor alteration mineral identified and was only found deep in one drill hole in associa- tion with illite and illite-smectite.
NE
Northern collovium
SW Main silica capSouthern area Ada Raye Legend
Topo contour 5m Conceptual line ssection Layer
Illite-smectite Kaolinite Alunite-silica Silica-alunite Massive silica/vuggy silica Metres 0 50 100 200 300 400 N
Western breccia Figure 11. Alteration map of Seruyung prospect.
Majalah Geologi Indonesia, Vol. 26 No. 3 Desember 2011: 155-171
The outermost propylitic alteration zone having the chlorite- illite±illite-smectite alteration assemblage was encountered only at the base of one hole. The presence of potassium alunite, together with pyro- phyllite and high crystalline kaolinite, suggests a magmatic origin for the acidic hydrothermal fluids.
The vuggy and massive silica zone has quartz-silica±alunite±sulphides as the main alteration minerals. It is the primary altera- tion style in the Main Silica Cap area in the summit of Mount Seruyung and on the large boulder floats that litter the Northern Col- luvium area of the prospect. Gold miner- alization is commonly associated with this alteration zone. The dominant alteration minerals in the ad- vanced argillic zone comprise silica-alunite- pyrite±pyrophyllite±kaolinite±dickite. It envelops the vuggy/massive silica zone and forms discrete patches of alteration associated with local dilational structures in the prospect area. The kaolinite and dickite minerals occurring in this zone are typically of the crystalline variety.
The argillic zone has kaolinite ± quartz ± illite ± illite-smectite ± dickite as its alteration mineral assemblage. Kaolinite and dickite are typically of low crystal- linity. The zone distribution is restricted to the NW edge of the prospect and is en- countered mostly at the bottom intercepts of drill holes.
These alteration assemblages form a general zonal distribution pattern comprising a mas- sive/vuggy silica core progressively grading outward into enveloping regions character- ised by advanced argillic and argillic altera- tion zones, respectively. On plan (Figure 11), the general alteration pattern is asymmetric with the enveloping main advanced argillic zone forming an irregular “tadpole” shape halo suggesting that ascending acidic hydro- thermal fluids were strongly influenced by fault structures and the presence of favour- able lithology. On SW-NE section (Figure 12), the alteration outline mimics closely the classical zoned mushroom- shaped alteration pattern commonly observed in structurally intact high sulphidation epithermal deposits with a narrow, downward tapering “stalk“ corresponding to the geometry of the hydro-
- 100RL -200RL
- 300RL Legend Rock Units Diatreme Hydrothermal breccia Dacite tuff Andesitic volcanics Alteration Massive/vuggy silica Advance argillic Argillic Intermediate argillic Propyllitic Sry_xsec_conceptual_struct 526600E 432000N 526800E 527000E
Figure 12. Sectional view of the alteration in Seruyung prospect.
Western breccia zone area Main silica cap area 100RL 200RL
Application of an Analytical Spectral Device (ASD) in Alteration Mapping of the Seruyung Project,
East Kalimantan, Indonesia (A. Rura et al.)thermal breccia unit (i.e. feeder structure) intersected by drill holes.
Mineralization
The mineralization style in Seruyung is typical of a replacement style, high sul- phidation epithermal system, where gold grades out from high-grade feeder structures to a lower- grade advanced argillic (quartz- alunite-pyrite) alteration halo.
Comparison of gold tenor and alteration type from outcrops and drill core samples suggests that gold values are commonly el- evated within rock units that exhibit vuggy and massive silica alteration. In particular, significant gold grades are commonly ob- served in silicified breccia units interpreted as structural feeders.
Discrete elevated gold values are also noted within the advanced argillic altered rocks, but are of lower grade. More commonly, gold within this zone occurs where alunite is dominant and there is attendant localized silicification along structures.
In contrast, gold within the argillic zone is commonly absent. Where elevated gold values are found in discrete patches within this zone, they are commonly associated with minor dilational features along local fault structures.
The presence of pyrophyllite and dickite appears to have no direct bearing on gold grade, though its presence is commonly observed near and immediately peripheral to the margins of drill hole intercepts and outcrop samples that have significant gold grades. This spatial relationship highlights the potential use of identifying these “hot” alteration clay minerals for vectoring into possible high grade structures.
The presence of pyrophyllite along the inferred trace and margin of a diatreme structure in the Western Breccia Zone sug- gests the possibility of a nearby structural feeder. This concept is currently being tested - the presence of pyrophyllite and a coincident IP resistive signature, by initially trenching across and eventually drilling this inferred margin of a diatreme structure. If successful, this relationship can be useful for prospecting areas where outcrops are relatively sparse and there is significant overburden that could mask potential high sulphidation epithermal deposits.
Application of ASD equipment and tech- niques has been useful in alteration mapping and in understanding the mineralization at the Seruyung prospect. Previously, mapped alteration zone boundaries were further refined with a better understanding of the compositional and crystallinity variation of associated alteration minerals.
The common alteration minerals identified are alunite, quartz, silica, kaolinite, and py- rophyllite. Dickite, illite, and illite-smectite tend to be minor with traces of chlorite identified deep in one drill hole. Oxides include haematite, limonite, and goethite. The presence of potassium alunite and pyro- phyllite suggests a magmatic origin for the acidic hydrothermal fluids associated with the alteration and mineralization.
Three main alteration zones were recog- nized on the basis of alteration mineral associations and assemblages. These in- clude a central vuggy and massive silica zone (quartz-silica ± alunite ± sulphides) enveloped by an advanced argillic zone (silica-alunite- pyrite ± pyrophyllite ± ka- olinite ± dickite) that grade outward into a broad outer argillic zone (kaolinite ± quartz ± illite ± illite-smectite ± dickite). The pro- pylitic zone (chlorite-illite- illite-smectite)
Majalah Geologi Indonesia, Vol. 26 No. 3 Desember 2011: 155-171