Iron ores in Atjeh

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tagsconcept, iron, atjeh, colonial, mineral-resource
aliasesAtjeh iron ore, Iron ore in Atjeh, Iron-ore occurrences of Atjeh, Atjeh iron deposits

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The Iron ores in Atjeh is the corpus of colonial-era geological observations that document the occurrence of iron-ore deposits in the Residency of Atjeh, recorded in the canonical 1922 survey of Jansen, Lindberg and Wolvekamp and synthesised by Van Bemmelen’s *Geology of Indonesia* (1949). Eight Atjeh localities are recorded, of which the Kr. Geuenteuet occurrence (32 km South of Kota Radja) carries the largest documented colonial-era reserve estimate: 2,600,000 tons of ore with an average of 58–64 % Fe, estimated in 1920 and “suspended on account of low market-prices”.

Geological model: contact-metamorphic origin

The geological model for the Atjeh iron-ore occurrences is the contact-metamorphic model established by Hövig (1918) and Tobler (1919) for the northern-Sumatran iron-ore deposits in general. In this model, the magnetite-hematite ores are formed at the contact zone of an eruptive intrusion (typically a granite, in Tobler’s interpretation, rather than a diabase) and the surrounding limestone country rock, with the diagnostic formation of garnet and silicified limestone at the contact. The same model applies to the Muara Sipongi deposits just south of the Atjeh border in Tapanuli, where Tobler “explicitly indicated the contact-metamorphic origin of the ore” and argued that “contact-metamorphism was effected by the granite and not by the diabase”.

The Atjeh iron-ore localities — Kr. Geuenteuet (with “much pyrite and large boulders of magnetite containing a core of pyrite”), Gle Mon Ampeuet (with limestone, amphibole andesite, garnet and silicified limestone), Kr. Ligan (with iron ore, pyrite and pyrolusite), Kr. Rigaih and Alu Talu (with scattered ore-fragments at eruptive-rock contacts, with garnet) — all conform to this contact-metamorphic model.

The eight Atjeh localities

The eight Atjeh iron-ore localities recorded in Van Bemmelen’s Fig. 41 key are:

  1. Kr. Geuenteuet (No. 1): 32 km South of Kota Radja. “In the Al. Besu or A. Pantom Raba, a tributary of the Kr. Geuenteuet, much pyrite and large boulders of magnetite containing a core of pyrite were found. An analysis showed 10 gr Au and 33 gr Ag per ton.” 1920 reserve estimate: 2,600,000 tons of 58–64 % Fe ore.
  2. Gle Mon Ampeuet (No. 2): 37 km South of Kota Radja. “Numerous fragments of iron ore, with a core of pyrite, on the slopes. In the vicinity limestone and an eruptive rock (amphibole andesite) were observed. The presence of garnet and a silicious limestone indicates contact-metamorphic origin. The thickness of this occurrence is some metres.”
  3. Kr. Ligan (Babah Loh) (No. 3): 113 km NW of Meulaboh. “Over a length of 700 m iron ore, pyrite and pyrolusite were observed. Thickness irregular from 8 to 17 m. Probably also contact-metamorphic origin.”
  4. Kr. Rigaih (No. 4): 88 km NW of Meulaboh. “The deposits consist of scattered fragments of ore, formed at the contact of eruptive rock (dikes) and sediments. Locally garnet has been observed.”
  5. Alu Talu (No. 5): 78 km NW of Meulaboh. Same description as Kr. Rigaih.
  6. Kwala Boe (No. 6): 30 km NW of Meulaboh. Bog-iron (swamp-ore) deposit. “Beach deposits: Scattered fragments of ore at the surface. Below 40 cm reddish-yellow sand, containing lumps of ore.”
  7. Tjot Plaj (No. 7): 12 km NW of Meulaboh. Bog-iron (swamp-ore) deposit. Same description as Kwala Boe. The combined Kwala Boe–Tjot Plaj bog-iron reserve is estimated at 300,000–400,000 tons of lump-ore.
  8. A. Pentuwa Gade (No. 8): 68 km SE of Meulaboh. “No particulars available.”
  9. Kuto Buloh (No. 9): 27 km North of Tapa Tuan. “No particulars available.”

The Kr. Geuenteuet reserve

The 1920 reserve estimate for the Kr. Geuenteuet occurrence — 2,600,000 tons of ore with an average of 58–64 % Fe — is the largest documented colonial-era iron-ore reserve on Atjeh territory. The estimate is recorded by Van Bemmelen with the additional note that “Investigation was suspended on account of low market-prices.” The 58–64 % Fe grade is in the range of medium-grade iron ore, suitable for blast-furnace smelting; the gold-and-silver byproduct content (10 gr Au/ton, 33 gr Ag/ton) recorded in the 1922 Jansen-Lindberg-Wolvekamp report would, in other circumstances, have made the Kr. Geuenteuet occurrence a candidate for combined iron-and-precious-metal mining.

The Kwala Boe and Tjot Plaj bog-iron deposits

The Kwala Boe and Tjot Plaj bog-iron deposits (Nos. 6/7) are the only Atjeh iron-ore localities classified in the colonial geological literature as bog-iron (swamp-ore) deposits rather than as contact-metamorphic magnetite-hematite deposits. The 300,000–400,000-ton lump-ore reserve estimate is the only quantitative bog-iron reserve estimate for Atjeh in the colonial geological literature. Philippi’s thermal-origin hypothesis — proposed on the basis of a few samples from a single pit — is supported, in Van Bemmelen’s judgment, by the “peculiar structure and the comparatively high Zn content (3–4 %)” of the deposits, which are indeed more consistent with a hydrothermal than with a purely surficial bog-iron origin.

Significance

The Iron ores in Atjeh is significant for the encyclopedia of Acheh history because it is the corpus of colonial-era geological observations that document the occurrence of iron-ore deposits in the Residency of Atjeh — a corpus that includes the largest documented colonial-era iron-ore reserve on Atjeh territory (the 2,600,000-ton, 58–64 % Fe Kr. Geuenteuet deposit) and the only documented colonial-era bog-iron reserve estimate for Atjeh (the 300,000–400,000-ton Kwala Boe–Tjot Plaj deposit). The non-development of the Atjeh iron-ore occurrences — the “suspension [of investigation] on account of low market-prices” at Kr. Geuenteuet — is itself historically significant: it is the documentary evidence that the colonial state, despite its substantial investment in the geological survey of Atjeh, was unwilling or unable to translate geological reserves into productive mines.

See Also

Source

The Geology of Indonesia, by R. W. van Bemmelen, Vol. II: Economic Geology (The Hague: Government Printing Office; Martinus Nijhoff, 1949), pp. 205–211 (iron ores in Sumatra, Fig. 41 and key, with detailed locality descriptions).

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