The tectonic and metallogenic setting of cu-au mineralisation in Myanmar : implications for mineral exploration in the Central Magmatic-Volcanic Arc

The Republic of the Union of Myanmar (formerly Burma) is situated between South and Southeast Asia, and has significant endowment of metallic and industrial minerals, gemstones and fossil fuels. Myanmar is broadly classified into two separate terranes: the West Myanmar Block and Sibumasu. A diverse range of ore deposits have been identified in Myanmar, including epithermal, porphyry, volcanic-hosted massive sulphide, intrusion-related, skarn, orogenic, Mississippi valley-type and sedimentary exhalative. Myanmar hosts the giant Monywa high sulphidation Cu and Mawchi Sn-W vein-type deposits, and the supergiant Bawdwin Pb-Zn-Ag VHMS deposit. Despite the diversity of deposits, Myanmar remains poorly explored and understood by modern standards. Furthermore, contemporary tectonic models for the formation of Myanmar are unable to explain many of the geological features observed throughout Myanmar. This thesis aims to account for these discrepancies and to synthesise them into a tectonic model that accounts for the presence of various geological features, explaining the types of mineralisation present in each area, and assessing the prospectivity for base and precious metals within the Central Magmatic-Volcanic Arc of the West Myanmar Block.
The West Myanmar Block is subdivided into two N-trending terranes, the arcuate Indo-Myanmar Ranges in the west and the Central Myanmar Basin in the east. The Indo-Myanmar Ranges consist of Triassic flysch that is unconformably overlain by the 126.6 ± 1.0 Ma to 116.4 ± 2.2 Ma Western Ophiolite Belt, the Paleogene to middle Miocene Indo-Myanmar flysch, and the middle Miocene to Recent Indo-Myanmar molasse. Low-grade metamorphism of the Triassic flysch from the Late Cretaceous to Eocene was related to crustal thickening caused by obduction of the rocks that constitute the Indo-Myanmar Ranges. The Central Myanmar Basin has a middle Cretaceous plutonic basement of granitic to dioritic composition, with local occurrences of metamorphosed granitoids of unknown age. The Central Myanmar Basin is bisected by the Central Magmatic-Volcanic Arc into a fore-arc basin and back-arc basin, each comprised of several sub-basins.
The Central Magmatic-Volcanic Arc is a N-trending geanticline consisting of locally uplifted middle Cretaceous plutonic basement rocks of the Central Myanmar Basin Batholith that were intruded by Cenozoic intrusions and overlain by volcanic rocks of varying ages, which occur in three spatially isolated and temporally distinct inliers within the extensively infilled Central Myanmar Basin. Cenozoic magmatism youngs from north to south, progressing from the 39.48 ± 0.39 Ma to 32.10 ± 0.26 Ma Wuntho Massif, to the 27.05 ± 0.32 Ma to 13.26 Ma Monywa Inlier and the 19.27 ± 0.53 Ma to Recent Mount Popa Inlier. Mineralisation is known to be present in the Wuntho Massif and the Monywa Inlier, whilst the Mount Popa Inlier is only known to contain areas of hydrothermal alteration. The best known and understood example of mineralisation in the Central Magmatic-Volcanic Arc is the Monywa high sulphidation Cu deposits, which consist of the 19.4 ± 0.5 Ma to 18.2 ± 0.2 Ma Letpadaung ore body, the 14.85 ± 0.21 Ma to 13.60 Ma Kyisintaung ore body, and the 13.5 ± 0.2 Ma to 13.26 ± 0.2 Ma Sabedaung ore body. Alteration ages demonstrate that mineralisation in the Monywa deposits was synchronous with adakite-like andesitic magmatism.
The Wuntho Massif contains significant evidence of epithermal and porphyry style Cu ± Au ± Mo mineralisation. The Shangalon district in the southernmost Wuntho Massif contains porphyry Cu-Au?Mo mineralisation at the Sea-Sun-Star mine and Chigyi Taung prospect, both hosted in 108.80 ± 1.09 Ma to 109.41 ± 1.09 Ma quartz diorite porphyry of the middle Cretaceous Kenzachaung Batholith. Additional examples of mineralisation in the Shangalon district occur at Kyungalone and Triocore mines, and Yellowstar prospect, which consist of low- and intermediate sulphidation epithermal Cu-Au mineralisation that is hosted in granodioritic to andesitic intrusions with U–Pb zircon ages of 39.48 ± 0.39 Ma to 37.72 ± 0.40 Ma. A Re-Os molybdenite age of 38.84 ± 0.17 Ma demonstrates that mineralisation at Sea-Sun-Star mine is temporally consistent with the host rocks to epithermal mineralisation.
Examples of mineralisation in the Wuntho Massif outside of the Shangalon district include Mahar San, Htawara and Shwe Kaba (Kyaukpazat) mines, which are mostly hosted in the andesitic to basaltic volcaniclastics and minor intrusions of the Mawgyi Group. The age of the Mawgyi Group and its relationship to the Kenzachaung Batholith are uncertain. Based on the available geochronological data it is inferred that the Mawgyi Group may be Late Cretaceous, post-dating the formation of the middle Cretaceous Kenzachaung Batholith. Mahar San mine is reported to be a Cu-Au polymetallic VHMS deposit of unknown age. Host rocks are cryptic due to extensive supergene alteration and post-mining weathering. Htawara Au-Cu mine is hosted in fine-grained, chlorite-altered andesite of the Mawgyi Group. Mineralisation is characterised by quartz-pyrite-chalcopyrite veins, consistent with an intermediate sulphidation system. Shwe Kaba Au mine exploits quartz ± carbonate veins in highly sheared andesites of the Mawgyi Group, and is interpreted to be of magmatic-hydrothermal origin. A nearby pyroxene-dominant endoskarn yielded a U–Pb zircon magmatic age of 99.45 ± 0.99 Ma, making it the oldest known occurrence of mineralisation within the Wuntho Massif, and the only known example of middle Cretaceous mineralisation in the Central Magmatic-Volcanic Arc.
Investigations into the prospectivity of the Central Magmatic-Volcanic Arc using whole rock igneous geochemistry indicated high potential for future mineral discoveries. Prospectivity investigations showed a strong association with areas of known mineralisation and alteration. Targets were ranked based on their prospectivity, spatial and temporal associations with known mineralisation and alteration, which identified three areas prospective for porphyry and epithermal mineralisation that have not previously been recognised as having mineral potential (Myay Lin, Leikontaung and Twinywa), which included rocks of the middle Cretaceous Kenzachaung Batholith. Alteration geochemistry is consistent with the styles of alteration and mineralisation present in the Wuntho Massif and the Monywa Inlier.
A new tectonic model for Myanmar is proposed, focussed on the evolution of the West Myanmar Block. The proto-West Myanmar Block consisted of Triassic flysch that originated off the western Australian coast and rifted in the Late Cretaceous as a deep-water oceanic ridge ahead of the Indian Plate. This interpretation contrasts with previous models that argued that the West Myanmar Block was part of Southeast Asia prior to the Mesozoic. The new tectonic model also proposes that the middle Cretaceous plutonic basement rocks of the Central Magmatic-Volcanic Arc formed as part of the Trans-Himalayan Batholith, which also included the Lohit, Ladakh and Gangdese batholiths. The Triassic flysch of the Indo-Myanmar Ranges accreted with the Central Myanmar Basin Batholith in the Late Cretaceous, which initiated low-grade metamorphism in the Triassic flysch and formed the composite West Myanmar Block. Subduction associated with this accretion event may be responsible for the andesitic to basaltic magmatism of the Mawgyi Group, and back-arc spreading that formed the Mahar San polymetallic VHMS deposit in the Wuntho Massif. A period of magmatic quiescence ensued in the West Myanmar Block until the Eocene. During this time, the accretion of the West Myanmar Block with the western margin of Sibumasu initiated, leading to regional low-pressure metamorphism of the Mogok Metamorphic Belt. Subduction-related magmatism in the Wuntho Massif occurred in the Eocene which was caused by asymmetric transpressional subduction of the northeast Indian Plate, leading to emplacement of the Shangalon intrusives of the southern Wuntho Massif, which host epithermal Cu-Au mineralisation, and the mineralising intrusions responsible for porphyry Cu-Mo ± Au mineralisation at the Sea-Sun-Star mine and Chigyi Taung in the Shangalon district. The Eocene also saw continued crustal shortening and uplift of the West Myanmar Block, the final stages of metamorphism of the Indo-Myanmar Ranges, and high-grade regional sillimanite-grade metamorphism of the Mogok Metamorphic Belt. The Auk Thitaya trachyte is the youngest intrusion in the Wuntho Massif. It was emplaced in the early Oligocene and is interpreted to have formed through low-degree partial melting of thickened crust. The Oligocene also saw the earliest intrusions emplaced in the Monywa Inlier and the final stages of regional sillimanite-grade metamorphism of the Mogok Metamorphic Belt. These features imply that subduction of the Indian Plate was still active, but may have locally jammed up around the Wuntho Massif. Miocene subduction-related magmatism continued in the Monywa Inlier, leading to the formation of the giant Monywa high sulphidation Cu deposits, and magmatism was initiated contemporaneously in the Mount Popa Inlier.
Diachronous cooling and uplift of the Mogok Metamorphic Belt commenced in the south at around 27 Ma, younging to the north and ceasing around 14 Ma. Uplift of the Mogok Metamorphic Belt led to the formation of the Shan Boundary Fault System and the juxtaposition of metamorphic, igneous and sedimentary rocks of western Sibumasu.
The new tectonic model links the diachronous cooling with the SE-directed extrusion of Indochina, which was facilitated by the activation of the Mae Ping Fault system of western Thailand and the Red River-Aliao Shan Fault Zone of Vietnam. The dextral transpressional Sagaing Fault formed around 5 – 4 Ma in the vicinity of the boundary between the West Myanmar Block and Sibumasu, ....