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Disparate Tectonic Settings for Mineralisation in an Active Arc, Eastern Papua New Guinea and the Solomon Islands



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Disparate Tectonic Settings for Mineralisation
in an Active Arc, Eastern Papua New Guinea
and the Solomon Islands
R J Holm1, S W Richards2,3, G Rosenbaum4 and C Spandler5
The recent and active magmatic arcs of eastern Papua New Guinea (PNG) and the Solomon
Islands are well endowed with sulde mineralisation and include deposits such as Ladolam,
Panguna and Solwara 1. The majority of the mineral systems in this belt are younger than
four million years old, with some deposits remaining active to the present day as exemplied
by active hydrothermal systems on the island of Lihir. The geodynamic setting that led to the
formation of these deposits is still unresolved, with, for example, both the Pacic and Solomon
Sea plates considered responsible for the formation of the Ladolam deposit on the island of
Lihir under different models. In order to understand and characterise the formation of south-
west Pacic mineral deposits, we must rst reconstruct the complex geodynamic evolution
of the region. New kinematic reconstructions show that while mineral systems in eastern
PNG and the Solomon Islands are hosted within a single arc related to subduction at the New
Britain and San Cristobal trenches, variable and discrete geodynamic settings exist within both
the upper and lower plates throughout this region, which give rise to distinct mineralised
corridors. Most notably, this region is host to:
the Solwara 1 deposit, which occurs within a major transtensional corridor in the eastern
Bismarck Sea
the Ladolam deposit (Lihir), which is interpreted to have formed above but in relation to
tearing of the subducting slab
the Panguna deposit, which is related to structure in the subducting slab marginal to the
actively spreading Woodlark Basin.
An evaluation of the nature and variability of these diverse geodynamic settings through
time can provide us with a better understanding of the relationship between the active south-
west Pacic magmatic arcs and mineral deposit formation. It may also provide insights into the
1. Associate Lecturer, Economic Geology Research Centre, College of Science, Technology and Engineering, James Cook University, 1 James Cook Drive,
Townsville Qld 4811. Email:
2. Head of Geology, Exploration and Geophysics, Citigold, Gregory Highway, Charters Towers Qld 4820. Email:
3. MAusIMM, Adjunct Lecturer, Economic Geology Research Centre, College of Science, Technology and Engineering, James Cook University,
1 James Cook Drive, Townsville Qld 4811.
4. Associate Professor, School of Earth Sciences, The University of Queensland, Brisbane Qld 4072. Email:
5. Senior Lecturer, Economic Geology Research Centre, College of Science, Technology and Engineering, James Cook University, 1 James Cook Drive,
Townsville Qld 4811. Email:
R J Holm et al
diverse array of deposit settings at ancient convergent margins, with applications in mineral
prospectivity studies.
The south-west Pacic has long been the focus of research towards our understanding of giant
ore deposit formation, with New Guinea and the Solomon Islands playing host to deposits
such as Grasberg, Ok Tedi, Freida River, Porgera, Lihir and Panguna (Figure 1; Cooke,
Hollings and Walshe, 2005; Sillitoe, 2010; Richards, 2013). Changes in regional tectonics at
convergent margins have often been viewed as a contributing factor in the formation of
mineral deposits. In particular, changes in the subduction regime are commonly considered
as crucial parameters triggering mineralising events, associated with, for example, terrane
collisions, subduction of slab structure or changes in the slab angle during subduction (Cooke,
Hollings and Walshe, 2005; Rosenbaum et al, 2005; Richards, 2013; Richards and Holm, 2013;
Sillitoe, 2010). At present, Papua New Guinea (PNG) and the Solomon Islands lie within a
tectonically complex zone of oblique convergence between the Australian and Pacic plates,
trapped between the converging Ontong Java Plateau and Australian continent (Figure 1).
Previous tectonic studies provide us with a basic understanding of the geodynamic framework
for much of the south-west Pacic (eg Hall, 2002; Schellart, Lister and Toy, 2006). However,
FIG 1 – Tectonic setting and mineral deposits of eastern Papua New Guinea and Solomon Islands. The modern arc setting related to
formation of the mineral deposits comprises, from west to east, the West Bismarck arc, the New Britain arc, the Tabar-Lihir-Tanga-
Feni Chain and the Solomon arc, associated with north-dipping subduction/underthrusting at the Ramu-Markham fault zone, New
Britain trench and San Cristobal trench respectively. Arrows denote plate motion direction of the Australian and Pacic plates. Filled
triangles denote active subduction. Outlined triangles denote slow or extinct subduction. NBP: North Bismarck plate; SBP: South
Bismarck plate; AT: Adelbert Terrane; FT: Finisterre Terrane; RMF: Ramu-Markham fault zone; NBT: New Britain trench.
Disparate tectonic settings for Mineralisation in an active arc
our current understanding does not provide sufcient insight into the relationships and
feedbacks underpinning the complex tectonic settings. This in turn negates meaningful
comparisons between mineral deposit formation and geodynamic setting. In this study, we
report on the relationships between the tectonic evolution of eastern PNG and the Solomon
Islands established from new reconstructions, and the timing and location of mineral deposit
Plate tectonic reconstructions provide us with a mechanism to test kinematic and dynamic
relationships throughout this structurally complex region. We developed a new plate tectonic
reconstruction for PNG and the Solomon Islands for the Late Neogene and Quaternary using
GPlates software (eg Seton et al, 2012). The reconstructions were generated in one million year
time intervals relative to the hybrid absolute reference frame of Seton et al (2012) and tectonic
plates were assumed to be rigid and non-deforming. Existing data sets of sea oor magnetic
isochrons, subducted slab maps and numerous prior studies into the tectonic history of the
region (eg Hall, 2002; Schellart, Lister and Toy, 2006; Holm and Richards, 2013; Holm, Spandler
and Richards, 2014) were integrated within GPlates as digitised polygon and polyline features
and spatially manipulated to provide constraint on reconstructions at specic geological times.
GPlates then interpolated plate motions about a reconstruction pole between known events to
produce the reconstruction model. A series of iterations were required to ensure a complete and
closed plate circuit. The plate tectonic reconstruction was then correlated with the formation of
mineral deposits in time and space (Figure 2), where deposit ages were derived from Cooke,
Hollings and Walshe (2005) and Singer, Berger and Moring (2008) and references therein.
Active magmatism in eastern PNG and the Solomon Islands is associated with north-dipping
subduction of the Australian plate and several microplates (Figure 1). While the modern-
day active arc appears as a single magmatic arc, it comprises several different geodynamic
settings. In the west, the continental crust of PNG is underthrust beneath the Adelbert and
Finisterre Terranes at the Ramu-Markham fault zone, giving rise to the West Bismarck arc
(Holm and Richards, 2013). Subduction of the Solomon Sea plate at the New Britain trench
results in magmatism of the New Britain arc, Tabar-Lihir-Tanga-Feni Chain and the western
Solomon arc. Subduction of the actively spreading Woodlark Basin and Australian plate at
the San Cristobal trench is related to magmatism of the Solomon arc. Similar to the variation
in arc development, mineral deposits show a comparable degree of variation along the arc
(Figure 1). In the west, there is a distinct lack of reported mineral occurrences, while the eastern
Bismarck, Tabar-Lihir-Tanga-Feni and Solomon arcs are well endowed with mineral deposits,
including the giant Lihir and Panguna deposits. This suggests that there is a relationship
between regional tectonics and ore deposit formation.
A comparison of the location and timing of mineral deposit formation with the tectonic
reconstructions (Figure 2) reveals a strong correlation between deposit emplacement and
the occurrence or passage of major structures in the upper or lower plate respectively. The
Solwara deposits of the eastern Bismarck Sea lie along a major transtensional structure that
accommodates sinistral motion between the North and South Bismarck microplates as well as
the opening of the Manus Basin (Figure 2). This setting is not unique within the Bismarck Sea;
however, this is the only occurrence of a dilational upper plate structure associated with active
subduction at depth. To the east, mineral deposits of the Tabar-Lihir-Tanga-Feni chain are
interpreted to have formed as the North Bismarck plate tracked over a tear in the subducting
R J Holm et al
Solomon Sea plate (Figure 2). The reasons for development of the slab tear are numerous;
however, the two key factors are:
1. accommodating the subduction of a relatively at oceanic plate into a subduction trench
with a 90° bend
2. the left-lateral transpression imposed on the eastern part of the subduction zone caused by
the west-directed motion of the Ontong Java Plateau.
In the Solomon Islands, reconstructions showing the subduction of the active Woodlark
Basin spreading centre and associated extensional basin structures, such as transform faults,
demonstrate a spatial and temporal relationship with the formation of deposits. Specically,
FIG 2 – Selected snapshots from plate tectonic reconstructions with timing of corresponding mineral deposit formation. Reconstruction snapshots
are derived from GPlates reconstructions. Reconstructions shown here include aspects of sea oor spreading in the Woodlark Basin and Manus Basin,
sea oor magnetic isochrons of the Solomon Sea, subducted slab model for the Solomon Sea with 100 km depth contours (Holm and Richards, 2013)
and an associated projection of the subducted slab model area oated to the surface. Oceanic plateaus are shown in dark grey, with the Ontong Java
Plateau to the north-east and the Louisiade Plateau to the south. The arc crust is shown in light grey derived from 1000 m bathymetric contour.
Disparate tectonic settings for Mineralisation in an active arc
the New Georgia Island group hosting the Mase deposit resides above the projected trend of
the subducted Woodlark spreading centre (Figure 1). The reconstructions also show that the
deposit has been located adjacent to the subducting spreading centre since at least the mid-
Pliocene (Figure 2), suggesting a prolonged inuence of ridge subduction on the formation of
the deposit. Finally, the location and timing of the Panguna and Fauro Island deposits of and
adjacent to Bougainville and the Hidden Valley and Koloula deposits of Guadalcanal correlate
well with subduction of lower plate structures marginal to the Woodlark Basin (Figure 2).
Reconstructions of the metalogenic Solomon Islands and eastern PNG subduction margins
suggest that the location of favourable structures or the passage of subducting structures
contributes to the localisation of mineralisation corridors. The reasons for this relationship
require further research, but we hypothesise that these structures either promote increased
uid ux within the mantle resulting from a larger exposure of the subducting slab to the
surrounding asthenospheric mantle (Richards and Holm, 2013) or that upper plate structures
may act as preferential conduits to promote high uid ux within the upper crust. Increased
uid ux within either the mantle or upper plate likely promotes the transport of metals,
leading to a favourable setting for mineral deposit formation.
In this extended abstract, we compared the location and timing for ore deposit formation with
new plate tectonic reconstructions for eastern PNG and the Solomon Islands. The comparison
revealed a correlation between deposit emplacement and the location of major structures
through time, where both upper plate and lower plate structures may independently contribute
to prospective settings for deposit formation. Favourable structures seen to contribute to
the localisation of mineralised corridors are varied in nature, including structures within
the subducting slab that comprise slab tearing or slab windows, and possible reactivation
of inherited structures as slab tears; or upper plate structures that promote localisation of
magmatism and uid-ow such as transtensional strain in the eastern Bismarck Sea. These
ndings suggest that a good understanding of geodynamic settings for ore deposit formation
have the potential to contribute to prospectivity studies and the generation of new exploration
targets at regional scales.
Cooke, D R, Hollings, P and Walshe, J L, 2005. Giant porphyry deposits: characteristics, distribution, and
tectonic controls, Economic Geology, 100:801–818.
Hall, R, 2002. Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacic: computer-
based reconstructions, model and animations, Journal of Asian Earth Sciences, 20:353–431.
Holm, R J and Richards, S W, 2013. A re-evaluation of arc-continent collision and along-arc variation in
the Bismarck Sea region, Papua New Guinea, Australian Journal of Earth Sciences, 60:605–619.
Holm, R J, Spandler, C and Richards, S W, 2014. Continental collision, orogenesis and arc magmatism
of the Miocene Maramuni arc, Papua New Guinea, Gondwana Research, doi:10.1016/
Richards, J P, 2013. Giant ore deposits formed by optimal alignments and combinations of geological
processes, Nature Geoscience, 6:911–916.
Richards, S W and Holm, R J, 2013. Tectonic preconditioning and the formation of giant porphyry
deposits, in Tectonics, Metallogeny, and Discovery: The North American Cordillera and Similar Accretionary
Settings, special publication 17, pp 265–275 (Society of Economic Geologists: Littleton).
Rosenbaum, G, Giles, D, Saxon, M, Betts, P G, Weinberg, R F and Duboz, C, 2005. Subduction of the
Nazca Ridge and the Inca Plateau: insights into the formation of ore deposits in Peru, Earth and
Planetary Science Letters, 239:18–32.
R J Holm et al
Schellart, W P, Lister, G S and Toy, V G, 2006. A Late Cretaceous and Cenozoic reconstruction of the
southwest Pacic region: tectonics controlled by subduction and slab rollback processes, Earth-Science
Reviews, 76:191–233.
Seton, M, Müller, R D, Zahirovic, S, Gaina, C, Torsvik, T, Shephard, G, Talsma, A, Gurnis, M, Turner,
M, Maus, S and Chandler, M, 2012. Global continental and ocean basin reconstructions since 200 Ma,
Earth-Science Reviews, 113:212–270.
Sillitoe, R H, 2010. Porphyry copper systems, Economic Geology, 105:3–41.
Singer, D A, Berger, V I and Moring, B C, 2008. Porphyry copper deposits of the world: database and
grade and tonnage models, US Geological Survey open-le report 2008–1155.
... Southward subduction of the Pacific Plate continued throughout the Oligocene, until collision with the oceanic Ontong Java Plateau (OJP) at 20-26 Ma (e.g., Kroenke et al., 1986), which triggered subduction reversal and connected the South Caroline Arc with the Melanesian Arc (Hall, 1997). Holm et al. (2015Holm et al. ( , 2019 suggest that northward subduction then initiated along the Pocklington-Aure Trough and resulting in magmatism along the Maramuni Arc (Figs. 1, 2), which is now part of the New Guinea Mobile Belt (the so-called Pocklington Trough model). ...
... An alternative is that southward subduction instead initiated at the Trobriand Trough (see Holm et al., 2015Holm et al., , 2019. In the Pocklington Trough model, collision of the Maramuni Arc with the Australian continental margin resulted in a first phase of orogeny at around 12 Ma and caused a northward jump of subduction (at maintaining the same polarity) to the modern San Cristobal-New Britain Trench system (Holm et al., 2015). ...
... An alternative is that southward subduction instead initiated at the Trobriand Trough (see Holm et al., 2015Holm et al., , 2019. In the Pocklington Trough model, collision of the Maramuni Arc with the Australian continental margin resulted in a first phase of orogeny at around 12 Ma and caused a northward jump of subduction (at maintaining the same polarity) to the modern San Cristobal-New Britain Trench system (Holm et al., 2015). Here, subduction initiated in the east along the San Cristobal and eastern New Britain Trench (Hill & Raza, 1999), triggering the resurgence of arc magmatism on Bougainville (Toniva Formation), the Gazelle Peninsula (Nengmutka Volcanics) and potentially northwestern New Ireland (Lumis River Volcanics; Fig. 4). ...
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The Southwest Pacific region, and Papua New Guinea in particular, is spectacularly endowed with mineral resources, including some of the youngest and richest porphyry Cu-Mo-Au deposits in the world. Among them is the giant porphyry-epithermal Ladolam Au deposit on Lihir Island in the Tabar-Lihir-Tanga-Feni (TLTF) island chain, northeast of New Ireland. Its setting within a former forearc basin is very different from most Southwest Pacific porphyry and epithermal deposits. Our synthesis of published and previously unreleased data from ship-based multibeam and seismic studies, satellite gravimetry, geochemistry and geochronology reveals a far more complex crustal structure and composition than is presently understood from the geology of the islands alone. We show that the unique regional Au endowment results from the alignment of various preconditions that are prolific to ore formation: i) hydrous and metal-rich metasomatic veins in the mantle source, ii) second-stage, low volume partial melting due to incipient rifting, iii) high volatile contents and oxygen fugacities of the melts due to preferential melting of hydrous phases in the metasomatic veins, and iv) in the specific case of Lihir, unroofing of the volcanic edifice that led to boiling and rapid metal deposition. This study shows that the location of the Ladolam deposit on Lihir is controlled by large-scale structures that can be traced offshore and are the site of continuing submarine volcanism and epithermal-style Au mineralization. The observed structural framework is dominated by the emergence of trans-lithospheric faults that provided pathways for the melts to the seafloor, near-surface structural focusing of the ascending melts and fluids, and a regional tectonic stress regime that stabilized the conditions over a significant period of time and/or repeatedly. Marine seismic data confirms the complex structure of the TLTF island chain. Each island group sits on tilted blocks that form horst structures separated by half grabens developed due to regional NW-SE-directed extension. Regional compression perpendicular to the extension continues as a result of the transition from subduction to collision at the leading edge of the Ontong Java Plateau. The protracted, transtensional motion between distinct crustal blocks controls the location and timing of magmatism and mineralization. A kinematic link between volcanism at the location of Lihir and the splitting of New Ireland by NE-directed propagation of seafloor spreading in the Manus Basin is suspected. By combining onshore and offshore geology, we propose a new model of the evolution of the New Ireland Basin, magmatism along the TLTF island chain and ultimately ore deposit formation. This study demonstrates the importance of integrating offshore geology and geophysics into models that aim to explain the structural, magmatic, and sedimentary evolution of marginal basins that are host to economic mineral deposits.
... Recent work investigating the petrogenesis of magmatic arcs throughout the Papua New Guinea and Solomon Islands region (Schuth et al., 2009;Woodhead et al., 2010;Holm et al., , 2015b, combined with regional plate tectonic modelling (Holm et al., 2016), provide a framework for us to develop a regional metallogenic model. In this study we build on the preliminary work of Holm et al. (2015a) to test the hypothesis that subduction-related ore deposits have formed under special circumstances, for example, related to terrane collision, ridge subduction or slab tearing. To achieve this, we combine information on subduction processes and arc magmatism with the distribution of mineral deposits, the styles of mineralization, and the timing of mineralization. ...
... Plate tectonic reconstructions allow us to observe and test relationships between major tectonic events and the location and timing of mineral deposit formation. The reconstructions of this study build on the work by Holm et al. (2015aHolm et al. ( , 2016 but are extended back to 30 Ma to encompass the main regional ore-forming events. The plate tectonic reconstructions (Figs. ...
Papua New Guinea and the Solomon Islands are in one of the most prospective regions for intrusion-related mineral deposits. However, because of the tectonic complexity of the region and the lack of comprehensive regional geological datasets, the link between mineralization and the regional-scale geodynamic framework has not been understood. Here we present a new model for the metallogenesis of the region based on a synthesis of recent studies on the petrogenesis of magmatic arcs and the history of subduction zones throughout the region, combined with the spatio-temporal distribution of intrusion-related mineral deposits, and six new deposit ages. Convergence at the Pacific-Australia plate boundary was accommodated, from at least 45 Ma, by subduction at the Melanesian trench, with related Melanesian arc magmatism. The arrival of the Ontong Java Plateau at the trench at ca. 26 Ma resulted in cessation of subduction, immediately followed by formation of Cu-Au porphyry-epithermal deposits (at 24-20 Ma) throughout the Melanesian arc. Late Oligocene to early Miocene tectonic reorganization led to initiation of subduction at the Pocklington trough, and onset of magmatism in the Maramuni arc. The arrival of the Australian continent at the Pocklington trough by 12 Ma resulted in continental collision and ore deposit formation (from 12 to 6 Ma). This is represented by Cu-Au porphyry deposits in the New Guinea Orogen, and epithermal Au systems in the Papuan Peninsula. From 6 Ma, crustal delamination in Papua New Guinea, related to the prior Pocklington trough subduction resulted in adiabatic mantle melting with emplacement of diverse Cu and Au porphyry and epithermal deposits within the Papuan Fold and Thrust Belt and Papuan Peninsula from 6 Ma to the present day. Subduction at the New Britain and San Cristobal trenches from ca. 10 Ma resulted in an escalation in tectonic complexity and the onset of microplate tectonics in eastern Papua New Guinea and the Solomon Islands. This is reflected in the formation of diverse and discrete geodynamic settings for mineralization within the recent to modern arc setting, primarily related to upper plate shortening and extension and the spatial relationship to structures within the subducting slab.
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The Maramuni arc represents the only continuous record of the tectonic evolution of Papua New Guinea during the Miocene, and hence provides an opportunity to gain insight into subduction dynamics, orogenesis and crustal processes that operated throughout this dynamic period. We present an integrated U-Pb geochronology, Hf isotope and geochemical investigation of the Maramuni arc utilizing a suite of intrusive rocks from the Kainantu region of the eastern Papuan Highlands that span the Late Miocene from ca. 12 Ma to 6 Ma. The magmatic rocks formed from ca. 12–9 Ma have compositional affinities of subduction-zone magmas, but record increasing incompatible trace element contents and decreasing εHf with time, which we interpret to reflect a progressive increase in the crustal component of the magmas. Porphyry suites emplaced at 7.5-6 Ma are distinct from the older magmatic rocks by their marked HREE-depletion, which reflects a dramatic shift in arc-mantle dynamics. Based on these results we propose a revised geodynamic model for the tectonic evolution of Papua New Guinea involving arrival of the Australian continent at a north-dipping Pocklingon trough from ca. 12 Ma. Continent collision then led to growth of the New Guinea Orogen from 12 Ma aided by underthrusting of the leading continental margin, which contributed crustal material to magma-genesis at ca. 9 Ma. From ca. 7 Ma slab break-off and lithospheric delamination are reflected in a second phase of orogenesis that produced the HREE-depleted geochemical signatures of the contemporaneous magmatic rocks.
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The formation of giant and supergiant porphyry deposits is interpreted to be genetically linked to the subduction of major transform structures on the sea floor. The ultramafic crust and lithosphere associated with oceanic transforms and fracture zones undergo high degrees of metasomatic alteration (serpentinization) aided by ongoing rupture and enhanced fluid flow relative to that experienced by adjacent, structurally homogeneous oceanic crust. When the highly serpentinized (fluid-enriched) oceanic crust and lithosphere formed at these fracture zones subducts at convergent plate boundaries, the hydrous, lithospheric-scale structural weaknesses may locally develop into vertical slab tears. The formation of vertical slab tears leads to localized mantle flow and elevated temperatures at the edges of the slab. The ensuing thermal perturbations along the slab edges enhance serpentinite breakdown reactions and aids in the liberation of aqueous fluids at approximately 600°C and 80- to 100-km depth. This pressure and temperature range is close to the wet melting curve, thereby increasing the potential for slab melting and the generation of magmas such as adakites with relatively high log fO2 > FMQ. Large oceanic transforms such as the Mocha-Valdiva fracture zone in the southeastern Pacific are likely to contain the greatest proportion of serpentinite and therefore carry fluid in the form of serpentinite group minerals into the mantle. Ultimately, the volume of fluid liberated during subduction and dehydration of the serpentinized mantle at these fractures far exceeds the volume of fluid produced at adjacent, weakly altered, and "structureless" oceanic crust. We suggest that the combination of extremely large volumes of slab-derived fluids plus the potential for slab melts to develop in these regions represents a primary control on the formation of giant porphyry deposits. These conditions are met along the eastern Pacific Rim, where more than 10 large transforms are currently being subducte. The endowment of the subducting oceanic crust with transforms and equally impressive mineral endowment contrasts markedly with the western Pacific Rim where only three to four equivalent-sized transforms are found. We propose that the variation in mineral endowment between the eastern and western Pacific and the formation of giant and supergiant porphyry deposits is linked to the formation and subduction of large oceanic fractures.
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The Bismarck Sea region of Papua New Guinea is marked by recent arc–continent collision giving rise to a highly dynamic tectonic environment, characterised by complex plate interactions that are yet to be fully understood. We present a new crustal and upper mantle crustal architecture model for northeastern Papua New Guinea and western New Britain that reveals complex tectonic geometries of overprinting slab subduction and partial continental subduction, resulting in a unique setting in which to investigate along-arc magmatic variation. Earthquake hypocentre databases are combined with detailed topography and seafloor structure together with geology and regional-scale gravity to unravel the sub-surface structure of northeastern Papua New Guinea. These data are used in conjunction with an updated 3-D slab map of the region to propose a new interpretation of the area whereby Australian continental crust extends as an underthrust block beneath the accreted Finisterre Terrane. The subducting continental crust combined with slab stagnation has resulted in a complex pattern of arc-related geochemical signatures from east to west along the Bismarck arc. In the east, where the Solomon Sea plate is subducting beneath New Britain, the sedimentary component is low, whereas in the west, the arc volcanics exhibit a greater sedimentary component, consistent with subduction of Australian crustal sediments. As a result, a new plate reconstruction is provided for the region together with a forward-looking reconstruction of the Papuan peninsula, the Solomon Sea plate and New Britain that illustrates that the same process will likely be repeated in some 5–10 m.y.
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Global plate motion models provide a spatial and temporal framework for geological data and have been effective tools for exploring processes occurring at the earth's surface. However, published models either have insufficient temporal coverage or fail to treat tectonic plates in a self-consistent manner. They usually consider the motions of selected features attached to tectonic plates, such as continents, but generally do not explicitly account for the continuous evolution of plate boundaries through time. In order to explore the coupling between the surface and mantle, plate models are required that extend over at least a few hundred million years and treat plates as dynamic features with dynamically evolving plate boundaries. We have constructed a new type of global plate motion model consisting of a set of continuously-closing topological plate polygons with associated plate boundaries and plate velocities since the break-up of the supercontinent Pangea. Our model is underpinned by plate motions derived from reconstructing the seafloor-spreading history of the ocean basins and motions of the continents and utilizes a hybrid absolute reference frame, based on a moving hotspot model for the last 100 Ma, and a true-polar wander corrected paleomagnetic model for 200 to 100 Ma. Detailed regional geological and geophysical observations constrain plate boundary inception or cessation, and time-dependent geometry. Although our plate model is primarily designed as a reference model for a new generation of geodynamic studies by providing the surface boundary conditions for the deep earth, it is also useful for studies in disparate fields when a framework is needed for analyzing and interpreting spatio-temporal data.
Giant ore deposits contain anomalously large quantities of metal and are priority targets for mineral exploration companies. It is debated whether these giant deposits have a unique mode of formation. Alternatively they may simply represent the extreme end of a spectrum of deposit sizes, formed by an optimum coincidence of common geological processes to build unusually large accumulations of metal. If formed by unique processes, the occurrence of giant ore deposits may be difficult to predict. Conversely, if formed by common processes, understanding the mechanisms that lead to optimum circumstances for giant metal deposits could help with exploration. A review of several giant porphyry copper–molybdenum–gold and epithermal gold–silver deposits reveals that many have characteristics consistent with formation during the optimization of normal ore-forming processes. In several cases, the large size of the deposit reflects specific factors, such as distinct tectonic configurations, reactive host rocks or focused fluid flow, that are not unusual by themselves but have helped to enhance the overall process. Thus, I suggest that effective exploration for giant deposits should seek distinct conditions within fundamentally prospective geological settings that might lead to enhanced ore-forming processes.
More than half of the 25 largest known porphyry copper deposits, defined in terms of contained copper metal, formed during three time periods: the Paleocene to Eocene, Eocene to Oligocene, and middle Miocene to Pliocene. These giant deposits are clustered within three provinces, central Chile, northern Chile, and southwest Arizona-northern Mexico. Other giant deposits occur in Montana, Utah, Panama, Peru, Argentina, Irian Jaya, Mongolia, and Iran. Compressive tectonic environments, thickened continental crust, and active uplift and erosion were associated with the formation of many of these deposits. Calc-alkalic magmas are most favorable for the formation of giant porphyry copper deposits, although several of the largest systems are associated with high K calc-alkalic intrusions, The 25 largest gold-rich porphyry depos its are concentrated in the southwest Pacific and South America, with other occurrences in Eurasia, British Columbia, Alaska, and New South Wales. Many of the deposits formed in the last 13 m.y. The largest of the deposits are associated with high K calc-alkalic intrusions. Many calc-alkalic porphyritic intrusions have also produced giant gold-rich porphyries. In the last 2 0 m.y., the formation of giant porphyry copper-molybdenum and copper-gold deposits in the circum-Pacific region has been closely associated with subduction of aseismic ridges, seamount chains, and oceanic plateaus beneath oceanic island and continental arcs. in several examples, these tectonic perturbations have promoted flat-slab subduction, crustal thickening, uplift and erosion, and adakitic magmatism coeval with the formation of well-endowed porphyry and/or epithermal mineral provinces. Similar tectonic features are inferred to be associated with the giant porphyry copper-molybdenum provinces of northern Chile (Eocene-Oligocene) and southwest United States (Cretaceous-Paleocene). Topographic and thermal anomalies on the downgo ing slab appear to act as tectonic triggers for porphyry ore formation. Other factors, such as sutures in the overriding plate, permeability architecture of the upper crust, efficient processes of ore transport and deposition, and, in some cases, formation and preservation of supergene enrichment blankets are also vital for the development of high-grade giant ore deposits. A low-grade geochemical anomaly may be the final product of mineralization, if ore-forming processes do not operate efficiently, even in the most favorable geodynamic settings.
Porphyry Cu systems host some of the most widely distributed mineralization types at convergent plate boundaries, including porphyry deposits centered on intrusions; skarn, carbonate-replacement, and sedimenthosted Au deposits in increasingly peripheral locations; and superjacent high- and intermediate-sulfidation epithermal deposits. The systems commonly define linear belts, some many hundreds of kilometers long, as well as occurring less commonly in apparent isolation. The systems are closely related to underlying composite plutons, at paleodepths of 5 to 15 km, which represent the supply chambers for the magmas and fluids that formed the vertically elongate (>3 km) stocks or dike swarms and associated mineralization. The plutons may erupt volcanic rocks, but generally prior to initiation of the systems. Commonly, several discrete stocks are emplaced in and above the pluton roof zones, resulting in either clusters or structurally controlled alignments of porphyry Cu systems. The rheology and composition of the host rocks may strongly influence the size, grade, and type of mineralization generated in porphyry Cu systems. Individual systems have life spans of ∼100,000 to several million years, whereas deposit clusters or alignments as well as entire belts may remain active for 10 m.y. or longer. The alteration and mineralization in porphyry Cu systems, occupying many cubic kilometers of rock, are zoned outward from the stocks or dike swarms, which typically comprise several generations of intermediate to felsic porphyry intrusions. Porphyry Cu ± Au ± Mo deposits are centered on the intrusions, whereas carbonate wall rocks commonly host proximal Cu-Au skarns, less common distal Zn-Pb and/or Au skarns, and, beyond the skarn front, carbonate-replacement Cu and/or Zn-Pb-Ag ± Au deposits, and/or sediment-hosted (distal-disseminated) Au deposits. Peripheral mineralization is less conspicuous in noncarbonate wall rocks but may include base metal- or Au-bearing veins and mantos. High-sulfidation epithermal deposits may occur in lithocaps above porphyry Cu deposits, where massive sulfide lodes tend to develop in deeper feeder structures and Au ± Ag-rich, disseminated deposits within the uppermost 500 m or so. Less commonly, intermediatesulfidation epithermal mineralization, chiefly veins, may develop on the peripheries of the lithocaps. The alteration-mineralization in the porphyry Cu deposits is zoned upward from barren, early sodic-calcic through potentially ore-grade potassic, chlorite-sericite, and sericitic, to advanced argillic, the last of these constituting the lithocaps, which may attain >1 km in thickness if unaffected by significant erosion. Low sulfidation-state chalcopyrite ± bornite assemblages are characteristic of potassic zones, whereas higher sulfidation-state sulfides are generated progressively upward in concert with temperature decline and the concomitant greater degrees of hydrolytic alteration, culminating in pyrite ± enargite ± covellite in the shallow parts of the lithocaps. The porphyry Cu mineralization occurs in a distinctive sequence of quartz-bearing veinlets as well as in disseminated form in the altered rock between them. Magmatic-hydrothermal breccias may form during porphyry intrusion, with some of them containing high-grade mineralization because of their intrinsic permeability. In contrast, most phreatomagmatic breccias, constituting maar-diatreme systems, are poorly mineralized at both the porphyry Cu and lithocap levels, mainly because many of them formed late in the evolution of systems. Porphyry Cu systems are initiated by injection of oxidized magma saturated with S- and metal-rich, aqueous fluids from cupolas on the tops of the subjacent parental plutons. The sequence of alteration-mineralization events charted above is principally a consequence of progressive rock and fluid cooling, from >700° to <250°C, caused by solidification of the underlying parental plutons and downward propagation of the lithostatichydrostatic transition. Once the plutonic magmas stagnate, the high-temperature, generally two-phase hypersaline liquid and vapor responsible for the potassic alteration and contained mineralization at depth and early overlying advanced argillic alteration, respectively, gives way, at <350°C, to a single-phase, low- to moderatesalinity liquid that causes the sericite-chlorite and sericitic alteration and associated mineralization. This same liquid also causes mineralization of the peripheral parts of systems, including the overlying lithocaps. The progressive thermal decline of the systems combined with synmineral paleosurface degradation results in the characteristic overprinting (telescoping) and partial to total reconstitution of older by younger alteration-mineralization types. Meteoric water is not required for formation of this alteration-mineralization sequence although its late ingress is commonplace. Many features of porphyry Cu systems at all scales need to be taken into account during planning and execution of base and precious metal exploration programs in magmatic arc settings. At the regional and district scales, the occurrence of many deposits in belts, within which clusters and alignments are prominent, is a powerful exploration concept once one or more systems are known. At the deposit scale, particularly in the porphyry Cu environment, early-formed features commonly, but by no means always, give rise to the best orebodies. Late-stage alteration overprints may cause partial depletion or complete removal of Cu and Au, but metal concentration may also result. Recognition of single ore deposit types, whether economic or not, in porphyry Cu systems may be directly employed in combination with alteration and metal zoning concepts to search for other related deposit types, although not all those permitted by the model are likely to be present in most systems. Erosion level is a cogent control on the deposit types that may be preserved and, by the same token, on those that may be anticipated at depth. The most distal deposit types at all levels of the systems tend to be visually the most subtle, which may result in their being missed due to overshadowing by more prominent alteration-mineralization.
A Cenozoic tectonic reconstruction is presented for the Southwest Pacific region located east of Australia. The reconstruction is constrained by large geological and geophysical datasets and recalculated rotation parameters for Pacific–Australia and Lord Howe Rise–Pacific relative plate motion. The reconstruction is based on a conceptual tectonic model in which the large-scale structures of the region are manifestations of slab rollback and backarc extension processes. The current paradigm proclaims that the southwestern Pacific plate boundary was a west-dipping subduction boundary only since the Middle Eocene. The new reconstruction provides kinematic evidence that this configuration was already established in the Late Cretaceous and Early Paleogene. From ∼ 82 to ∼ 52 Ma, subduction was primarily accomplished by east and northeast-directed rollback of the Pacific slab, accommodating opening of the New Caledonia, South Loyalty, Coral Sea and Pocklington backarc basins and partly accommodating spreading in the Tasman Sea. The total amount of east-directed rollback of the Pacific slab that took place from ∼ 82 Ma to ∼ 52 Ma is estimated to be at least 1200 km. A large percentage of this rollback accommodated opening of the South Loyalty Basin, a north–south trending backarc basin. It is estimated from kinematic and geological constraints that the east–west width of the basin was at least ∼ 750 km. The South Loyalty and Pocklington backarc basins were subducted in the Eocene to earliest Miocene along the newly formed New Caledonia and Pocklington subduction zones. This culminated in southwestward and southward obduction of ophiolites in New Caledonia, Northland and New Guinea in the latest Eocene to earliest Miocene. It is suggested that the formation of these new subduction zones was triggered by a change in Pacific–Australia relative motion at ∼ 50 Ma. Two additional phases of eastward rollback of the Pacific slab followed, one during opening of the South Fiji Basin and Norfolk Basin in the Oligocene to Early Miocene (up to ∼ 650 km of rollback), and one during opening of the Lau Basin in the latest Miocene to Present (up to ∼ 400 km of rollback). Two new subduction zones formed in the Miocene, the south-dipping Trobriand subduction zone along which the Solomon Sea backarc Basin subducted and the north-dipping New Britain–San Cristobal–New Hebrides subduction zone, along which the Solomon Sea backarc Basin subducted in the west and the North Loyalty–South Fiji backarc Basin and remnants of the South Loyalty–Santa Cruz backarc Basin subducted in the east. Clockwise rollback of the New Hebrides section resulted in formation of the North Fiji Basin. The reconstruction provides explanations for the formation of new subduction zones and for the initiation and termination of opening of the marginal basins by either initiation of subduction of buoyant lithosphere, a change in plate kinematics or slab–mantle interaction.
A large number of ore deposits that formed in the Peruvian Andes during the Miocene (15–5 Ma) are related to the subduction of the Nazca plate beneath the South American plate. Here we show that the spatial and temporal distribution of these deposits correspond with the arrival of relatively buoyant topographic anomalies, namely the Nazca Ridge in central Peru and the now-consumed Inca Plateau in northern Peru, at the subduction zone. Plate reconstruction shows a rapid metallogenic response to the arrival of the topographic anomalies at the subduction trench. This is indicated by clusters of ore deposits situated within the proximity of the laterally migrating zones of ridge subduction. It is accordingly suggested that tectonic changes associated with impingement of the aseismic ridge into the subduction zone may trigger the formation of ore deposits in metallogenically fertile suprasubduction environments.
A plate tectonic model for the Cenozoic development of the region of SE Asia and the SW Pacific is presented and its implications are discussed. The model is accompanied by computer animations in a variety of formats, which can be viewed on most desktop computers. GPS measurements and present seismicity illustrate the high rates of motions and tectonic complexity of the region, but provide little help in long-term reconstruction. Plate boundaries shifted rapidly in the Cenozoic. During convergence of the major plates, there were numerous important episodes of extension, forming ocean basins and causing subsidence within continental regions, probably driven by subduction. Within eastern Indonesia, New Guinea and the Melanesian arcs, there are multiple Cenozoic sutures, with very short histories compared to most well-known older orogenic belts. They preserve a record of major changes in tectonics, including subduction polarity reversals, elimination of volcanic arcs, changing plate boundaries and extension within an overall contractional setting. Rapid tectonic changes have occurred within periods of less than 5 Ma. Many events would be overlooked or ignored in older orogenic belts, even when evidence is preserved, because high resolution dating is required to identify them, and the inference of almost simultaneous contraction and extension seems contradictory.