preprints.org > environmental and earth sciences > geophysics and geology > doi: 10.20944/preprints202306.0547.v1

Preprint Article Version 1 Preserved in Portico This version is not peer-reviewed

Version 1 : Received: 7 June 2023 / Approved: 7 June 2023 / Online: 7 June 2023 (11:57:26 CEST)

The acquisition of regional, 1 metre resolution LIDAR in the PNG Highlands combined with 3D modelling in MoveTM has revolutionized our understanding of the evolution of the Porgera gold mine and areas of new potential. The new 3D model demonstrates active pull-apart basins along a regional transfer during ongoing fold and thrust deformation. When overlain on regional aeromagnetic data, new potential is revealed. The Porgera gold mine is one of the richest in the world and lies in a wide valley at an elevation of 2800m surrounded by mountains up to 4000m high in the middle of the PNG Highlands. It lies along a major lineament, the Porgera Transfer Zone (PTZ), cutting across the orogen that is associated with a 50 km offset of the ophiolite belt. To the NW of the mine there is an extensive belt of low- to high-grade metasediments that formed in deep water during Jurassic rifting, but were metamorphosed during Eocene subduction to the north. Subduction ceased due to accretion of the Sepik Terrane which caused inversion and mild erosion of the Porgera area. The Late Oligocene onset of wrenching in the Mobile Belt to the north placed that area into extension, emplacing metamorphic core complexes, and led to regional subsidence in the Early Miocene. Collision of the margin with the Melanesian Arc in the Middle Miocene caused Late Miocene to Pliocene orogenesis creating the broad mountain belt that we see today, that is still active, as shown by the 2018 MW 7.5 earthquake. Regional mapping of the area using high resolution LIDAR in association with limited field mapping, analysis of gravity and magnetics data and drilling of 300-500m deep core-holes has allowed development of a detailed 3D structural model. The Porgera valley is constrained laterally by the Eastern Boundary and Western Boundary dextral strike-slip faults that lie along the PTZ cutting across the orogeny. However, whilst the position and nature of the faults is clear, due to bending, fracturing and offset of major synclines and anticlines, the faults do not crop-out as significant through-going features. Rather, the dextral offset of basement is soft-linked to the Tertiary limestones at surface due to intervening thick, ductile Cretaceous shales. Fold and thrust structures are ubiquitous in the Tertiary limestones, but the youngest structural features are large extensional faults, particularly around the Porgera valley. The intrusive underlying the Porgera ore-body was emplaced at 6.0+-0.3 Ma, the time of maximum compression during orogenesis. At this time the Eastern Boundary fault and Western Boundary fault were both active allowing a pull-apart basin to form locally along the PTZ at the south-eastern boundary of the Jurassic metasediments. This enabled emplacement of the intrusive. The gold-bearing fluids from the intrusive and metasediments were brought up through the Mesozoic muds to the contact with the Paleogene carbonates where the Porgera ore body was emplaced.

Fold belt; Metasediments; Ore-body; Orogenesis; Pull-apart basin

Environmental and Earth Sciences, Geophysics and Geology

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