GEOLOGICAL SOCIETY OF THE PHILIPPINES
Metamorphic-Magmatic Connections and Crustal Scale Mineralisation: Evidences from Kontum Massif -Sepon Basin, Indochina Region
Joel S. Aquino*
The Sepon Basin, located in SE' Laos, is a >5 kms thick tilted marine sedimentary package that has been deposited during Ordovician to Late Permian periods. On its southern part is a proterozoic metamorphic terrane, known as Kontum Massif, that extends all the way to Central Vietnam. Both Sepon Basin and Kontum Massif are currently interpreted by Sepon geologists and available literature as two separate terranes that either has an unconformable relationship or has been juxtaposed by the Truong Son Fault system. However, an alternative hypothesis is the possibility that these two terranes are transitional to each other comprising a >26 km thick continental slab and where the granite/rhyodacite 'porphyry intrusions observed in Sepon Basin are partial melts derived from the granulite facies of the Kontum Massif. This continental slab has a current crustal exposure of >600 kms that exhibits varying petrological gradations, increasing age, increasing metamorphic grades and deepening structural levels of different mineralisation styles from north to south.
Notwithstanding the later displacements of the Truongson Fault, a crustal scale mineral anomaly is thus opened up to explorationists. At its shallowest and uppermost structural levels (northern Sepon Basin), hydrocarbon deposits are envisaged at Boulapha Sector. Down a bit to the south and slightly deeper structural level, is the Mississippi-Valley Type Pb-Zn mineralisation in Anamite sector, followed by the highly mineralised porphyry Mo-Cu-Au field in Padan- Thengkham sectors. Going deeper to the mid- crustal levels, metamorphosed VHMS and mesothermal gold veins are interpreted to be the mineralisation styles present at Sebanghiang-Leloy sectors, southern Sepon Basin. These deposit types are expected to continue on the low grade metamorphics at the Vietnam side. Finally, towards the deepest structural levels to the south (Central Vietnam), diamond-bearing gemstones and diamond- bearing intrusive pipes are the prospective commodities; with the diamond prospect east of Play Cu, Vietnam as an example.
*Former Senior Geologist, Oxiana Resources NL
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