Laos covers the junction between three large blocks of continental crust; the South China Terrane to the north, the Indochina Terrane to the south and the Shan-Thai Terrane to the west. These blocks are composed of metamorphic and intrusive rocks of Silurian to Precambrian age.
During Carboniferous to Permian time, shallow seas opened between the three continental blocks. Large volumes of limestone and related carbonate rocks were deposited in these seas. During early Permian time, the continental blocks moved together producing subduction zones with associated magmatic arcs. Numerous porphyry style mineralisation systems were generated in these magmatic arcs. Interaction between the porphyry systems and carbonate rocks produced many skarn style mineral deposits (Phu Kham Cu Au). Several examples of Michigan style (native copper) mineralisation have been identified in the Intermediate to mafic volcanics of these magmatic arcs, but this style has attracted very limited exploration effort.
A number of Mississippi Valley style lead-zinc deposits have been identified in the Permo-Carboniferous limestones. A few of these deposits have been exploited on a small scale for oxide zinc minerals, but the sulphide zones remain generally unexplored.
The continental blocks ultimately collided during Permian time, producing in the Truongson and Loei fold belts. Several orogenic quartz-gold vein style deposits known in Laos probably formed during this time. Previously formed deposits were deformed.
Several significant ultramafic complexes with Pt-Ni-Cr-Cu mineralisation occur along crustal scale structures in Laos. The age of these rocks is unknown, but radiometric dates from the Nan-Uttaradit ultramafic complex in Thailand suggest a Carboniferous age.
A second episode of extension during early Triassic time resulted in another episode of shallow marine carbonate sedimentation and acid magmatism. Interaction between these intrusives and the host rocks produced epithermal and porphyry mineralisation styles (Chatree low sulphidation vein Au and Sepon carlin style Au + manto style Cu). Several clusters of granitoid intrusions with associated tin and polymetallic mineralisation systems may also have intruded at this time.
From Jurassic to Cretaceous time, much of Laos was covered by a vast inland basin that filled with oxidised "redbed" style sediments. The basin has subsequently been dissected, but remnants contain several thick evaporite sequences including halite, gypsum and potassium rich salts that have been exploited for industrial uses. The lower parts of this sequence have also developed some "redbed style" sedimentary copper-silver deposits that have had very limited exploration.
Another episode of compression in the Truongson belt during Paleogene (Tertiary) time produced numerous thrust faults that complicate the geology and dismember some ore deposits.
Two fold belts with porphyry Cu-Au environment cover 70% of the country
Proven mineral endowment with three world class Cu-Au districts (Sepon, Phu Kham and Ban Houayxai).
Numerous skarn and carbonate replacement (Carlin) style occurrences related to porphyry centres
Numerous unexplored red-bed style Cu-Ag occurrences