Fathom Geophysics Newsletter 26

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Ore model news: Gold deposits offshore of New Ireland are focus of new map

THE FIRST remote predictive map of onshore and offshore geology of the New Ireland Basin and its surrounds has been compiled by researchers from Germany and Canada to help explorers better understand porphyry-epithermal gold systems.

"Offshore, the New Ireland Basin represents a highly permissive tract for gold mineralization," the researchers said in their journal paper. [1]

To create their map of the the 600 kilometers long by 150 kilometers wide New Ireland Basin, they synthesized multiple data repositories including submarine multibeam echo-sounder survey data, Advanced Spaceborne Thermal Emission and Reflection (ASTER) data, reinterpreted seismic reflection profile data, various published onshore geology maps, marine gravity modeling, and data from offshore rock samples and sediment core samples obtained via scientific expeditions.

Tectonically speaking, the whole complex of island chains to the northeast of Papua New Guinea's Papuan Peninsula has it all — subduction, collision, thrust faulting, strike-slip faulting, arc magmatism, back-arc extension, and rifting.

Wedged between the westward-moving Pacific Plate and the northward-moving Australian Plate is a patchwork of tectonic features: the Solomon Sea Microplate, the Woodlark Microplate, the North and South Bismarck Microplates, the Adelbert Microplate, the Manus Micropate, the Trobriand Block, the Pocklington Rise, the Malaita Anticlinorium, the Solomon Island Arc, and the New Britain Trench.

The Tabar-Lihir-Tanga-Feni (TLTF) island chain is situated immediately northeast of New Ireland.

The TLTF chain is located along a former forearc sedimentary basin that's now experiencing the onset of rifting and faulting and is therefore now producing shoshonitic volcanism, which involves potassium-rich trachyandesites enriched in mantle-incompatible elements. Rifting began after active subduction largely ceased along the Manus-Kilinailau trench when the Ontong Java Plateau met the North Bismarck Microplate.

Mineralization is found on each of the TLTF chain's island groupings. But the Ladolam deposit hosted by Lihir's Luise volcano is the largest by far.

Ladolam's huge size and high grades are thought to be a product of multiple favorable geological processes that included (1) involvement of anomalous melting of subduction-modified mantle, (2) local tectonically controlled focusing of melts and hydrothermal fluids, (3) evolution from a porphyry-producing regime to epithermal conditions, and (4) the host volcano's sector collapse about 190,000 years ago.

"Unroofing of the volcano removed material with a vertical thickness of 600 to 900 meters, [which] caused the release of magmatic volatiles, boiling of the hydrothermal system, and triggered rapid high-grade gold mineralization," the researchers said.

Sector collapse of the Luise volcano also uplifted the floor of its crater and its fringing limestone terraces by at least 38 meters. This amount of crater uplift suggested that the Ladolam ore body initially began forming at or below sea level.

The researchers said that one thing complicating the process of identifying the regional factors that can lead to major mineralization at some locations in the TLTF chain but not at other locations is that less than 10 percent of the area extending between New Britain, New Ireland, Bougainville, and the TLTF chain is above sea level.

Hence their new map.

Among their offshore-mapping findings was that in the deeper waters between the Lihir and Tanga island groupings, large areas of the seafloor host what's known as sediment waves — huge dune-like bedforms of sediment that build up when volcanic eruptions trigger turbidity currents, or submarine landslides, or both. The tallest sediment waves created up to 20 meters of seafloor relief.

"Integrating new offshore data with a review of the geological setting of one of the world's largest porphyry-epithermal gold deposits highlights its unique setting," they said.

"However, all volcanic centers in the New Ireland basin are likely to [prolifically] host gold-rich mineral systems because of similar geological preconditions and tectonic processes."

They said that their integrated onshore/offshore map enabled them to propose a new model about how the New Ireland Basin evolved and how magmatism and ore deposits developed along the TLTF island chain.

References

[1] P.A. Brandl, M.D. Hannington, J. Greersen, S. Petersen, and H.-H. Gennerich (June 2020) "The submarine tectono-magmatic framework of Cu-Au endowment in the Tabar-to-Feni island chain, PNG", Ore Geology Reviews, 121, 103491.

About Fathom Geophysics

In early 2008, Amanda Buckingham and Daniel Core teamed up to start Fathom Geophysics. With their complementary skills and experience, Buckingham and Core bring with them fresh ideas, a solid background in geophysics theory and programming, and a thorough understanding of the limitations of data and the practicalities of mineral exploration.

Fathom Geophysics provides geophysical and geoscience data processing and targeting services to the minerals and petroleum exploration industries, from the regional scale through to the near-mine deposit scale. Among the data types we work on are: potential field data (gravity and magnetics), electrical data (induced polarization and electromagnetics), topographic data, seismic data, geochemical data, precipitation and lake-level time-lapse environmental data, and remotely-sensed (satellite) data such as Landsat and ASTER.

We offer automated data processing, automated exploration targeting, and the ability to tailor-make data processing applications. Our automated processing is augmented by expert geoscience knowledge drawn from in-house staff and from details relayed to us by the project client. We also offer standard geophysical data filtering, manual geological interpretations, and a range of other exploration campaign-related services, such as arranging surveys and looking after survey-data quality control.