Selasa, 27 Oktober 2015

Diskripsi Batuan Hasil Alterasi

Dasar Melakukan Deskripsi Batuan Hasil Alterasi

Ketika melakukan survey geologi permukaan ataupun pengeboran inti khususnya yang berhubungan dengan deposit hasil alterasi, terkadang kita dibingungkan dengan diskripsi batuan hasil proses alterasi. Ini disebabkan karena batuan yang teralterasi mempunyai warna, tekstur dan komplesitas mineral yg hadir cukup bervariatif. Dibutuhkan dasar yang kuat untuk bisa  mengenal, mendeskripsi, dan menganalisa batuan hasil alterasi tersebut. dasar mineralogi, petrologi, konsep sistem hidrotermal dan alterasi, struktur dan tekstur batuan asal, sangat  dibutuhkan untuk mengkonfirmasi sebuah data yang diperoleh.

Baca juga postingan dibawah ini yang berhubungan dengan "alterasi dan mineralisasi" :

Macam-macam Batuan Hasil Alterasi

Berikut ini akan saya berikan diskripsi batuan hasil alterasi dari sebuah outcrop ataupun core drilling yang mungkin akan anda jumpai dilapangan, lengkap dengan photographnya. Untuk  mempermudah visualisasi deskripsi, silahkan lihat foto dibawahnya dan disesuaikan dengan nomor masing-masing conto batuan.

1. Silica Vuggy - Silica Sulphide Altered of Dacitic Lithic Tuff

Description: Strongly silicified dacitic lithic tuff, pale grey to grey generally, with spotted reddish brown of oxides, vuggy textured, disseminated very fine grained pyrite; vugs or pits filled by pyrite, limonite, and or sulphure. Specimen explanation: a) pits /vugs, filled by hematitic oxide. b) fine grained dacitic matrix, replaced by fine grained strong silica; disseminated fine grained pyrite. Project: AAA, Location : XXX, Lithology: dacitic lithic tuff, Alteration: Silica vuggy ± Silica Sulphide, Mineralisation: dissemination, Au content: 1.85 g/t.

2. Silica Vuggy Altered of Hydrothermal Breccia

Description: Hydrothermal breccia, poorly sorted of polymictic clast consist of crystal tuff and lithic tuff, subangular to subrounded; dacitic in composition, matrix supported. Matrix composed of altered dacitic volcanic material. Sulphide, mainly represented by fine grained disseminated pyrite.

Specimen explanation: a) subangular dacitic lithic tuff clast, vuggy textured with pyrite and or native sulphure filling. b) Intensely altered of subrounded dacitic crystal tuff, vuggy textured. c) matrix comprises volcanic material, with dusty very fine grained quartz. Project: AAA, Location : XXX, Lithology: hydrothermal breccia, Alteration: Silica vuggy ± Silica Sulphide, Mineralisation: dissemination, Au content: 1.15 g/t.

3. Argillic Illite-Smectite Altered Dacitic Crystal Tuff

Description: Poorly sorted of clastic textured Dacitic lithic tuff, crystal consist of subhedral to anhedral feldspar, quartz eyes, and less common of subhedral hornblende 0.5-2mm; matrix composed by fine grained tuff material altered by illite - smectite. Project: AAA, Location: XXX, Lithology: dacitic crystal tuff, Alteration: Illite-smectite clay alteration, Mineralisation: dissemination, Au content: 0.24 g/t

4. Lithic Alunite Silica Altered 

Description: White, creamy, to brownish dacitic lithic tuff; poorly sorted, matrix supported; matrix comprises crystal tuff, which altered to silica; clast comprised by subangular to rounded lithic (1mm – 1 cm). Specimen explanation: a) subrounded lithic clast, dacitic composition. b) pervasively Alunite over Silica altered. Project: AAA, Location : XXX, Lithology: dacitic lithic tuff, Alteration: Alunite - Silica, Mineralisation: dissemination, Au content: 0.2 g/t.

5. Alunite Silica Altered of Dacitic Lithic Tuff

Description: Fine grained dacitic lithic tuff; pale grey to purplish, matrix comprised tuff material, lithic clast 1 – 5 mm, poorly sorted, subangular to rounded, Altered by Alunite, weak to moderate silicified, hematite oxidation. Specimen explanation: a) Lieze gang ring oxidation. Project: AA, Location : XXX, Lithology: Fine grained dacitic lithic tuff, Alteration: Alunite – Silica, Mineralisation: dissemination, Au content: 0.27 g/t.

6. Silica Vuggy of Hydrothermal Breccia

Description: Brown, creamy, matrix of silica, clast 1mm-3cm, very angular-subrounded, poorly sorted, open fabric, locally point contact, clast of SV, SM. Strong silica alteration with vuggy texture. Specimen explanation: a) Silica massive clast. b) Silica vuggy clast. c) Vuggy filled by limonite. Project: AAA, Location : XXX, Lithology: hydrothermal breccia, Alteration: Silica vuggy ± Silica Massive, Mineralisation: dissemination, Au content: 3.8 g/t.
Deskripsi batuan alterasi
Macam-macam batuan hasil alterasi.

7. Alunite Silica of Dacitic Lithic Tuff

Description: White, creamy, to brownish dacitic lithic tuff; poorly sorted, matrix supported; matrix comprises ash tuff, which altered to alunite and weak silica; clast comprised by subangular to rounded lithic (1mm – 1 cm). Partly Alunite altered to kaolinite – haloysite due to weathering process (Petrology report).

Specimen explanation: a) white, soft, powdery kaolinite – haloysite filled up fracture. Project: AAA, Location : XXX prospect Drillhole YYY, 7.7 m, Lithology: Dacitic lithic tuff, Alteration: Alunite-silica, Mineralisation: disseminated, Au content: 24.2 g/t.


8. Silica Clast of Hydrothermal Breccia

Description: Polimictic clast of vuggy silica and massive silica, sub rounded – angular shape, 2 mm – 3 cm clast size, poorly sorted, set in chalcedonic silica + sulphide cement, moderate – strong oxidized.

Specimen explanation: a) vughy silica clast. b) massive silica clast. c) oxidized clay cavity filling (supergene clay?). d) oxidized  silica contain rich sulphide. Project: AAA, Location : XXX, Drillhole YYY, 11.2 m depth, Lithology: hydrothermal breccia, Alteration: Silica vuggy ± Silica Sulphide, Mineralisation: dissemination, Au content: 21.6 g/t.

9. Dacitic Lithic Tuff

Description: Dacitic lithic tuff, light grey and locally darker in strong Sulphide zone, lithic clast 10 – 15 % of volume, polymictic clast of dacitic ash tuff and dacitic crystal tuff, sub angular – angular clast, poorly sorted set in dacitic crystal matrix supported. The alteration is Vughy Silica dominant with locally lithic altered to massive silica.

Specimen explanation: a) Dacitic ash tuff clast, altered to massive silica and rimmed by sulphide. b) Dacitic crystal tuff clast altered to vughy silica. c) Chalcedonic veinlet contain rich sulphide. Project: AAA, Location : XXX, Drill hole YYY, 40.1 m depth, Lithology: Dacitic Lithic Tuff, Alteration: Silica vuggy ± Silica Sulphide, Mineralisation: dissemination, Au content:  18.95 g/t.


10. Potassic Altered Diorite Porphyry Intrusive

Description: Pale green with slightly greenish diorite porphyry, porphyritic textured plagioclase as main phenocryst, fine grained and subhedral. The rock is strongly altered to sericitic, disseminated pyrite, spotted secondary biotite. Project: AAA, Location : XXX, North-West of YYY main ridge, Lithology: Diorite porphyry, Alteration: Potassic, Mineralisation: unmineralized, Au content: unknown.

11. Phyllic (Si-Cy-Py) Altered Dacitic Crystal Tuff

Description: Grey to pale grey dacitic crystal tuff, crystal of feldspar, quartz, 0.5-2mm, anhedral; matrix comprised of volcanic material which are altered to Silica – clay – pyrite assemblage (Phyllic alteration). Project: AAA, Location : XXX, hole YYY at 108.8 m depth, Lithology: dacitic crystal tuff, Alteration: Phyllic (Silica-clay-pyrite), Mineralisation: dissemination, Au content: 0.57 g/t.

12. Silica Alunite – Chalcedonic Altered Brecciated Lithic Tuff

Description: Dark brown to yellowish brown colour, silica alunite altered brecciated lithic tuff, chalcedonic vein swelled within open space fracture patchy silica vuggy. Pits/open space filled by alunite and oxides, completely oxidized (COX).

Specimen Explanation:  1) Alunites fill vugs texture / open space. 2) Chalcedonic Vein. 3) Sub angular silica vuggy clast. Project : BBB, Location : XXX, Lithology : Brecciated Lithic Tuff , Alteration : Silica Alunite (SA) dominant  patchy Silica Vuggy (SV), Mineralisation : Dissemination and chalcedonic vein, Au Content/Grade : up to 5 g/t Au.
 

Outcrop maupun sampel core drilling diatas diperoleh pada sebuah prospek area (maaf tidak bisa saya sebutkan disini, yang pasti lokasinya di Indonesia). Dari hasil tersebut silahkan anda menganalisa sendiri dan mengambil kesimpulan (walaupun secara umum), menyangkut tipe deposit, alterasi apa saja yang berkembang di daerah tersebut, bagaimana sistem/skema hidrotermalnya, "host rock" nya apa, dan sebagainya, mudah kan!!.