Discovery and characterization of the Martabe epithermal gold deposit, North Sumatra, Indonesia


The Martabe mineral district near Sibolga in North Sumatra, Indonesia has been
evaluated using a multidisciplinary exploration approach, integrating geochemistry,
geophysics, mapping, drilling and geological interpretation since its discovery in 1996.
The gold deposits are located in the western Sunda-Banda magmatic arc, within and
adjacent to a porphyritic dacite-andesite dome and breccia complex that was
emplaced into a Miocene volcano-sedimentary sequence comprising sandstone, siltstone,
carbonaceous mudstone and andesite lava flows. The deposits are proximal to
a series of fault splays that form part of the Sumatra Fault System. Episodic fault
activity, related to wrench tectonics associated with oblique subduction guided multiphase
volcanic, phreatomagmatic and hydrothermal events.
There is no history of mining or prospecting in the area. Gold anomalies were
first discovered during a regional reconnaissance stream sediment sampling program
using BLEG (Bulk Leach Extractable Gold) analytical techniques undertaken to
follow-up on anomalous base metal samples collected by the British Geological Survey
in the 1980s. Follow-up of a 14 ppb gold BLEG anomaly identified mineralized
float assaying up to 20 g/t Au and 76 g/t Ag, which was traced to a series of prominent
silica ledges. Subsequent geological mapping, soil sampling and geophysical methods
and drilling have identified numerous prospects over a seven-kilometer strike length.
Helicopter-supported diamond drilling commenced in October 1998 on the Purnama,
Pelangi and Baskara prospects. As of late 2004, more than 390 holes have been drilled
resulting in the identification of a resource estimated at the end of 2004 to be 40M
tonnes grading 2.29 g/t Au. More than 90 percent of the drill inferred resource is
within the Purnama deposit with the remainder in Baskara.
All known economic and sub-economic gold-silver mineralization and related alteration
is high-sulfidation epithermal in character. However at Pelangi, low-sulfidation
quartz veins and vein stockworks, as well as silicified sedimentary rocks with local
gold mineralization were intersected in drill holes. The Purnama deposit consists of
disseminated gold-silver mineralization distributed sub-horizontally within and adjacent
to the western perimeter of a complex breccia body that is interpreted to be a
diatreme. The thicker breccia units show distinct facies variation. Lithic-rich facies
pass upwards and inwards through a finer grained, matrix-supported, facies containing
accretionary lapilli up to a late-stage facies containing abundant wispy-textured juvenile magmatic clasts. At Baskara, mineralization is sub-vertical and closely associated
with breccia bodies emplaced along NE-striking fault and fracture zones.
Baskara breccia is coarse to finely fragmental, but juvenile magmatic clasts have not
been identified and the cause of fragmentation is interpreted to be mostly phreatic
and hydrothermal.
Oxide and transitional mineralization grades downward into sulfide mineralization
containing grains of enargite-luzonite-covellite-pyrite in veins and open spaces.
The depth of oxidation varies significantly. Alteration zones consist of a core of brecciated,
massive and vuggy residual quartz that grades outwards through an advanced
argillic zone comprised of quartz-alunite and quartz-dickite-kaolinite, to pervasive
chlorite-smectite alteration. Mineralization is dated at 2.0 to 3.3 Ma based on 40Ar-39Ar
dating of hypogene alunite from Purnama.
Interpretation of soil geochemical data suggest both mechanical and chemical
transport of gold and pathfinder elements have produced anomalies that are commonly
displaced from the deposit source. At Purnama, the strongest gold-in-soil
anomaly occurs southwest of the deposit and is associated with scree material. Multielement
data indicates gold correlates with Ag, Sb, As, Pb and Cu.
In 2004, a large pole-dipole Induced Polarization (IP)/Resistivity survey was conducted
over the core of the district to discover additional resources. This outlined
zones of electrically resistive silica alteration, and also assisted in identifying a new
zone at East Baskara. Due in part to the success of the IP survey, an airborne electromagnetic
survey (HoistEM) was also undertaken to cover most of the Contract of
Work. The known deposits are clearly identified as areas of low conductivity and several
new anomalies have been selected for further work.

SKU: 2005-60 Category:

Additional information


Primary Author

J. Bruce




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Geochronological Method

Alteration Type