Krueng Isep (also Kr. Isep, modern Krueng Isep) is a river of Atjeh at whose lead-zinc occurrence the highest-grade zinc ore of the colonial Atjeh mineral record was assayed. The Krueng Isep locality is recorded in Van Bemmelen’s *Geology of Indonesia* (1949) in the Lead and Zinc in Sumatra chapter, with an assay of 65.13 % Pb, 8.30 % Zn, 0.89 % Fe, 0.94 % CaO, 0.94 % SiO₂, and 680 gr/t Ag — the highest zinc content of any Atjeh lead-zinc occurrence recorded in the colonial geological literature.
The assay
Van Bemmelen’s Table for the Atjeh lead-zinc occurrences records the Krueng Isep assay as follows (2nd sample, 500 kilogr.):
| Component | Percentage |
|---|---|
| Pb | 65.13 % |
| Zn | 8.30 % |
| S | 10.2 % |
| Fe | 0.89 % |
| CaO | 0.94 % |
| SiO₂ | 0.94 % |
| Ag | 680 gr/ton |
The 8.30 % Zn content is the highest zinc grade recorded for any Atjeh lead-zinc locality, and the 10.2 % sulphur content indicates that the zinc is present as sphalerite (ZnS) rather than as a carbonate or silicate. The 500-kilogram sample size (in contrast to the “small” sample from Krueng Heureueng) indicates that the Krueng Isep assay is based on a more substantial bulk sample, and the actual tonnage and average grade of the occurrence are accordingly better constrained than those of the Krueng Heureueng occurrence.
Place in the Atjeh lead-zinc record
For the wider context of the Atjeh lead-zinc occurrences see the entry on Krueng Heureueng. The Krueng Isep locality is the second of the two Atjeh lead-zinc occurrences recorded in the colonial geological literature, and its higher Zn grade (8.30 % vs 2.2 %) and lower Pb grade (65.13 % vs 76.1 %) place it at the zinc-rich end of the paragenetic zonation inferred for the Atjeh lead-zinc province.
Significance
Krueng Isep is significant for the encyclopedia of Acheh history because it is the site of the highest-grade zinc ore of the colonial Atjeh mineral record — 8.30 % Zn with 680 gr/ton Ag — and accordingly the most attractive zinc-rich lead-zinc prospect recorded in the colonial geological literature on Atjeh. The 500-kilogram sample size also makes the Krueng Isep assay one of the more reliable colonial-era assays for an Atjeh mineral occurrence, since it is based on a substantial bulk sample rather than on a hand-specimen. The fact that the occurrence was never developed is, like the non-development of the other Atjeh mineral occurrences, evidence of the colonial state’s unwillingness or inability to translate geological occurrences 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), p. 163 (Fig. 35 key, Atjeh lead-zinc occurrences, with assay table).