The Sepon gold deposits, Laos: Exploration, geology and comparison to Carlin-type gold deposits in the Great Basin

The Sepon project area is located in south-central Laos and has a long history of
artisanal panning, but hard rock gold potential was not recognized until 1990 when
field reconnaissance identified high grade rock chip samples and wide spread alteration.
Large scale exploration commenced in 1993, leading to the discovery of the
Nam Kok and Discovery deposits. Continued detailed work in the Sepon mineral district
(SMD) has resulted in the discovery of seven additional gold ore-bodies totaling
83 Mt @ 1.8 g/t Au for 4.75 Moz Au, plus the Khanong (27 Mt @ 4.3 percent Cu) and
Thengkham South copper deposits (9.8 Mt @ 2.3 percent Cu).
The SMD is situated in the Sepon basin within the Truongson Fold Belt, an elongate
NW trending belt of Paleozoic sedimentary and metamorphic rocks, which forms
part of the Indochina terrane of SE Asia. The Sepon basin comprises Devonian to
Carboniferous age sedimentary rocks deposited during the formation of an E-W oriented
pull-apart basin. These alternating sequences of calcareous shale, dolostone,
limestone, sandstone, and siltstone underwent a major compressional event, likely
associated with the Indosinian orogeny. The major basin forming structures were
inverted and became conduits for mineralizing fluids. Intrusion of rhyodacite porphyry
dikes post-dated late diagenesis but occurred before the conclusion of compressional
tectonism.
The dominant style of mineralization in the SMD is micro-disseminated, sedimenthosted
gold in structural, rheological and chemical traps. Gold mineralization is
closely associated with decalcification of calcareous shale and irregularly distributed
silica replacement (jasperoid) bodies. The geometries of the deposits are controlled by
the interplay of shallow-dipping lithological contacts and high-angle faults, forming
tabular to rod-like ore bodies. Gold mineralization is associated with elevated arsenic,
antimony and locally thallium. It can be argued that the Sepon district is a zoned district-
scale system associated with intrusion of the rhyodacite porphyry suite. In this
model porphyry Mo-(Cu) systems are present at the core and mineralization zones
outward through skarn and carbonate replacement Cu deposits to the Au-Ag dominated
sediment-hosted deposits.