ALBERT

Description: Magmatic differentiation and/or assimilation and related segregation of immiscible sulfide liquid are generally believed to be critical processes in the formation of the majority of orthomagmatic Ni sulfide deposits. In recent years, a new class of Ni sulfide deposits formed by metasomatic and/or hydrothermal modification of peridotites has been recognized. The serpentinite-hosted Avebury Ni sulfide deposit (Tasmania, Australia), the largest known non-magmatic sulfide deposit, provides an unprecedented opportunity to understand sources of metals and fluids responsible for this style of economic mineralization. Our study shows that serpentinization of the Ni-bearing olivine in the Cambrian peridotites of the McIvor Hill complex was followed by metasomatic transformation assisted by heat and fluids supplied by the nearby Late Devonian granite intrusion. The role of the above in the formation of an economic concentration of Ni sulfides is supported by (1) abundant Ni-Fe alloys and sulfides associated with serpentinization of peridotitic olivine, (2) metasomatic olivine containing inclusions of serpentine and metalliferous brines, and (3) the Late Devonian age of the Ni sulfide deposit. The Avebury metasomatic olivine is Ni-depleted and enriched in Mn relative to olivine of similar Fo content in nearby unmineralized peridotites, and to olivine in subduction-related mafic magmas generally. The unusual minor element chemistry of olivine is matched by a unique set of olivine-hosted multiphase inclusions composed of fibrous Mg-silicates and various Na-, K-, Fe-, Ca-, Mn-, and Ba-bearing chlorides/hydrochlorides, sulfides, arsenides magnetite, REE minerals, and Fe-Ni alloys. Peridotite whole-rock Sr-Nd-Pb isotope data and U-Pb dating of metasomatic titanite support earlier suggestions that Ni mineralization is temporally and genetically related with the intrusion of the nearby 360 Ma Heemskirk Granite. It appears that the multiphase inclusions in metasomatic olivine demonstrate chemical signatures of both in situ serpentinites (entrapped alloys, sulfides, arsenides, and magnetite) and distal fluids (enrichment in Pb, Bi, Sn, Sb, Sr, Ba, Rb, Cs, and Ce). We propose that magmatic olivine in large ultramafic bodies provides almost infinite Ni to replacive serpentinites and constitutes a major reservoir of disseminated Ni mineralization. In the case of Avebury Ni was locally redistributed from olivine in the Cambrian peridotites to mainly Fe-Ni alloys and sulfides during serpentinization in the early Paleozoic. In the Devonian reheating and interaction with a granitic fluid in the contact aureole of the Heemskirk Granite led to de-serpentinization and formation of metasomatic high-Mn, low-Ni olivine with inclusions of serpentine and entrapped alloys, sulfides, arsenides, and magnetite, and metalliferous brines rich in "granitic" elements. Nickel released from serpentinite in this process was re-deposited near the margins of the peridotite to form the Avebury Ni orebody. Our model of serpentinization-related release of Ni from magmatic olivine, in situ precipitation of metallic, sulfide, and arsenide Ni-minerals, and their redistribution and recrystallization in hydrothermal conditions represents an alternative to Ni remobilization from magmatic sulfides.

Description: The accumulation of metals and metalloids in diagenetic pyrite framboids is of interest because framboids can be a sink for heavy metal contaminants, a source of metals in ore deposits, and a tool to interpret paleo-ocean chemistry. In this study, we have used laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) to analyze pyrite framboids from both the contaminated Derwent Estuary and the uncontaminated Huon Estuary in Tasmania, Australia. While the enrichment of many trace metals in the Huon Estuary followed expected trends, the trends in the Derwent were quite different. In addition to the expected high contents of Pb, Zn, and Cu in the contaminated interval it was found that several elements are not as strongly incorporated into pyrite within the contaminated zone. It is suggested that this is due to over-competition for adsorption sites on the growing iron sulfides in the contaminated zone resulting in diffusion of several elements to deeper levels in the sediments. This results in an increase of these elements in pyrite below the zone of major contamination. The LA-ICPMS technique also provided the opportunity to obtain accurate data on gold, silver, and tellurium in pyrite, something rarely achieved in sequential leach extractions due to the low concentrations of these metals observed in nature.

Description: Volcanism of Late Cretaceous–Miocene age is more widespread across the Zealandia continent than previously recognized. New age and geochemical information from widely spaced northern Zealandia seafloor samples can be related to three volcanotectonic regimes: (1) age-progressive, hotspot-style, low-K, alkali-basalt-dominated volcanism in the Lord Howe Seamount Chain. The northern end of the chain ( c. 28 Ma) is spatially and temporally linked to the 40–28 Ma South Rennell Trough spreading centre. (2) Subalkaline, intermediate to silicic, medium-K to shoshonitic lavas of 〉78–42 Ma age within and near to the New Caledonia Basin. These lavas indicate that the basin and the adjacent Fairway Ridge are underlain by continental rather than oceanic crust, and are a record of Late Cretaceous–Eocene intracontinental rifting or, in some cases, speculatively subduction. (3) Spatially scattered, non-hotspot, alkali basalts of 30–18 Ma age from Loyalty Ridge, Lord Howe Rise, Aotea Basin and Reinga Basin. These lavas are part of a more extensive suite of Zealandia-wide, 97–0 Ma intraplate volcanics. Ages of northern Zealandia alkali basalts confirm that a late Cenozoic pulse of intraplate volcanism erupted across both northern and southern Zealandia. Collectively, the three groups of volcanic rocks emphasize the important role of magmatism in the geology of northern Zealandia, both during and after Gondwana break-up. There is no compelling evidence in our dataset for Late Cretaceous–Paleocene subduction beneath northern Zealandia. Supplementary material: Trace element compositions of zircons and whole-rock chemical compositions obtained by previous studies are available at: https://doi.org/10.6084/m9.figshare.c.3850975