ALBERT

Beschreibung: The Ok Tedi porphyry Cu-Au deposit in Papua New Guinea is a world-class mineral deposit. In its pre-mining stage, it contained 5.5 Mt Cu and 18.1 Moz Au; skarn accounts for ~80% of the current 1.9 Mt Cu and 7.7 Moz Au in total resources. Two composite felsic stocks intruded a tectonostratigraphic sequence of clastic and carbonate rocks at ~1.16 ± 0.10 Ma, in a postcollisional tectonic setting. Ensuing magmatic-hydrothermal mineralization occurred within a geologically short time span of ~200 k.y. On the mine scale, heterogeneous metal grades (0–2.5% Cu) reflect contrasting styles of mineralization, conforming to classic skarn- and porphyry-type mineralization. A three-dimensional geologic model, alteration distribution, mineralization styles, and grade distribution indicate that fluids, exsolved from a deeper magma chamber, used shallow stocks as their preferred permeable pathway upward. Where limestone was present at depth, high-grade skarn formed, with the permeable stock having very low (〈0.1% Cu) grade. In contrast, where the intrusions were encapsulated solely by impermeable silicified siltstone, economic grade (avg 0.6% Cu) developed within the intrusion, showing concentric patterns interpreted to represent individual magmatic pulses within the stock. The rare opportunity to visualize this in three dimensions and interact with the model is provided in the interactive electronic supplementary material.

Beschreibung: Although circumstantial evidence from ore deposit mineralogy and geochemistry can imply potential sources for fluids and metals, rarely is direct evidence for metal leaching from source rocks seen in the vicinity of deposits. Here, we investigate the source of metals for a series of fault zone– and shear zone–hosted uranium occurrences in the Mount Isa inlier, Australia. As well as containing uranium, these deposits are enriched in Zr and rare earth elements (REEs), requiring that unusual fluids were responsible for addition of these typically immobile elements. During the Isan orogeny, highly saline metamorphic fluids infiltrated the sheared margin of a highly evolved granite intrusion, which contains elevated U, Th, F, Zr, and REEs. Synchrotron X-ray fluorescence spectroscopy and high-resolution electron microprobe mineral mapping show that this radioactive element–rich characteristic caused zircon crystals to become highly metamict, allowing the elements therein to become mobile. Thus, when orogenic fluids pervasively infiltrated along shear zones ~100 m.y. after granite intrusion, their unusually saline character allowed enhanced dissolution of regional carbonates and fluorite from the granite, providing the ligands needed for transport of uranium and the normally immobile elements from the metamict zircons.

Beschreibung: Orogenic gold deposits are unevenly distributed in geologic time. The richest period of orogenic gold deposit formation was the Phanerozoic, which followed a 〉1-b.y. period of relatively meager deposit generation during the Proterozoic. This Proterozoic low-gold period developed despite continent-building episodes, which should have been appropriate for orogenic gold genesis. Here, a brief review of the changes in global geochemistry brought on by evolving interactions between plate tectonics and the biosphere is conducted before the consequences for gold uptake into sedimentary pyrite are discussed. It is suggested that the extensive oxygenation of the deep oceans during the second Great Oxidation Event, over the period 635 to 510 Ma, produced conditions where gold was soluble in the deep oceans from that point on. Regions where bacterial sulfate reduction drove formation of sedimentary pyrite also destabilized this soluble gold, promoting its uptake into pyrite, and allowing formation of metallogenically enhanced sedimentary sequences. These sediments would have become ideal source rocks for gold deposits during subsequent collisional tectonics and metamorphism. This biogenic influence on source region enhancement may explain the association during the Phanerozoic eon between some of the world's most gold productive regions and sedimentary sequences that contain pyritic carbonaceous rocks.

Beschreibung: The Hidaka Metamorphic Belt, in southeastern Hokkaido, Japan, provides insights into how magmatic sulfide deposits may form through magma mixing deep within arcs. Here, a near-complete cross-section of arc crust is exposed, with large mafic igneous complexes preserved at deeper levels. Magmatic sulfide mineralization occurs within the Opirarukaomappu Gabbroic Complex (OGC), which preserves a record of crustal contamination of mafic magmas via assimilation and magma mixing involving introduction of crust-derived tonalite. Assimilation–fractional crystallization modelling suggests that the gabbro evolved through a combination of approximately 10 wt % mixing and 14 wt % fractional crystallization. Magmatic sulfides and associated gabbros, diorites and tonalites at this locality contain graphite, with carbon isotope signatures consistent with derivation from the surrounding partially melted carbonaceous shales. This indicates that crust-derived carbon was added to the mafic magma through the magma mixing and assimilation process. Sulfur isotope data suggest that sulfur was also added from crustal sources during assimilation and magma mixing. The relationships observed in the OGC suggest that intrusion of basalt into a segment of deep arc crust drove partial melting of carbonaceous metamorphic rocks, producing graphite-bearing felsic magmas with high reducing potential. Redox budget modelling shows that mixing of only small proportions of these magmas is sufficient to lower the oxidation state of oxidized basaltic magmas enough to induce sulfide saturation and consequent exsolution of immiscible sulfide melt. Magmatic sulfide deposits are likely to form by this reduction-induced sulfide saturation mechanism deep within other arcs where magma mixing is thought to be common.

Beschreibung: This paper describes the new mineral nataliyamalikite, the orthorhombic form of thallium iodide (TlI), from high-temperature fumaroles from the Avacha volcano, Kamchatka Peninsula, Russia. We also present some chemical analyses showing extreme enrichment of Tl in the volcanic gases at the Avacha volcano, and a review of thallium geochemistry that highlights the fascinating processes that led to the formation of nataliyamalikite. Nataliyamalikite occurs as pseudo-cubic nanocrystals (≤0.5 μm) within vacuoles in an As-(Te)-rich amorphous sulfur matrix and rarely as irregularly shaped aggregates up to ~50 μm in diameter within the amorphous sulfur matrix. Associated minerals include an unidentified Tl-As-S mineral, barite, and rare inclusions of a Re-Cu-bearing phase. The mean empirical composition based on four EDS analyses is Tl 1.00 (I 0.95 Br 0.03 Cl 0.02 ), corresponding to the ideal formula TlI. Nataliyamalikite crystallizes in the orthorhombic system, space group Cmcm , which is consistent with the low-temperature (〈175 °C) synthetic TlI polymorph. EBSD data reveal that some grains retain the cubic symmetry ( Pm m ) of the high-temperature polymorph, although most analyzed grains display the orthorhombic symmetry. Single-crystal X-ray studies of material extracted by the focused ion beam-scanning electron microscopy (FIB-SEM) technique, and carried out on the MX2 macromolecular beamline of the Australian Synchrotron, determined the following cell dimensions: a = 4.5670(9), b = 12.803(3), c = 5.202(1) Å, V = 304.2(1) Å 3 , and Z = 4. The six strongest calculated X-ray reflections and their relative intensities are: 3.31 (100), 2.674 (73), 3.20 (43), 2.601 (28), 2.019 (21), and 2.284 Å (19). The combination of EBSD analysis (providing an efficient test of the crystallinity and crystal symmetry of a population of micrometer-sized grains) and synchrotron single-crystal X-ray micro-diffraction (beam size ~7.5 μm) on micro-aggregates extracted using FIB-SEM opens the way to the characterization of challenging specimen—in this case, the sulfur matrix is highly beam sensitive, and the nataliyamalikite grains could not be isolated using optical microscopy. The high-temperature (〉600 °C) sulfidic (~1.2 wt% S) vapors at Avacha are extremely enriched in thallium; with 34 ppm, they contain an order of magnitude more Tl than the richest volcanic gases analyzed to date and ~100 x more Tl than most metal-rich fumarolitic fluids associated with volcanic arcs. The formation of nataliyamalikite illustrates the complex processes that control thallium geochemistry in magmatic arc systems. Thallium minerals have now been reported in andesitic (Avacha), basaltic (Tolbachik, Kamchatka), as well as rhyolitic (Vulcano, Eolian Islands, Italy) volcanoes. Ultimately, these thallium minerals result from the transfer of thallium from subducted sediments to volcanic gases in arc volcanoes. We suggest that the extremely thallium-enriched vapors from which nataliyamalikite formed result from complex and transient interactions between Tl-rich sulfosalt melts and magmatic vapors, a process that may be important in controlling metal distribution in boiling epithermal systems.

Beschreibung: It is generally thought that orogenic gold deposit formation is associated, in many cases, with metamorphic fluids. These fluids are mainly liberated by chlorite breakdown in the upper greenschist facies, and there is some debate as to whether the pyrite-to-pyrrhotite desulfidation reaction precedes or coincides with this fluid release. Since pyrite is an important metal host in metamorphic rocks, its breakdown to pyrrhotite may allow complexation of Au with liberated sulfur. Thus, the question of the temperature-pressure conditions required for pyrite desulfidation is important because it determines which tectonic processes and which parts of the crust are relevant to gold deposit genesis. Here, we investigate metal redistribution during pyrite breakdown using laser ablation-inductively coupled plasma-mass spectrometry and bulk-rock analysis on greenschist to amphibolite facies rocks of the Ballachulish metamorphic aureole, Scotland. Results suggest that the pyrite-to-pyrrhotite transition field spans a broader temperature range than indicated by recent modeling, and that pyrite breaks down simultaneously with chlorite. Gold, As, Bi, Sb, Mn, and W were likely removed from the rock in chlorite-derived fluid. Nickel, Co, Ga, Ge, Mo, Tl, V, and Cr are retained in the rock by uptake in pyrrhotite. Copper is retained by growth of chalcopyrite as temperature increases, whereas Pb and Zn are retained by progressive uptake in nonsulfide minerals; these elements are not depleted during amphibolite facies metamorphism. The tendency of S to react with silicate-hosted Fe and be retained in the rock implies that Fe-poor pyritic sedimentary rocks may produce more S- and metal-rich fluids in source regions.

Beschreibung: We describe a unique occurrence of a world-class Proterozoic ore deposit, Century, affected by an Ordovician meteorite impact, the Lawn Hill impact structure. The meteorite excavated a complex crater with a Proterozoic core surrounded by an annulus filled with Cambrian carbonate rocks. The Century deposit is located at the southwestern edge of the crater and is bounded by postore faults that indicate an originally larger orebody. It is overlain by breccias, which contain evidence of impact-related textures that we interpret as fallback suevite. (Suevite is a rock consisting partly of melted material, typically forming breccia containing glass and crystal or lithic fragments, formed during an impact event.) Above the suevite are slumped Cambrian carbonate and Proterozoic shale, including a million-tonne block of Century ore detached from the main orebody and contained in the carbonate breccias. The Cambrian rocks were partially consolidated prior to impact, became fluidized on impact, and then resurged into the crater from surroundings areas. The resurge process resulted in a fivefold thickening of carbonates in the annulus, preserved to 600-m depth. Beneath the suevite, in the more competent orebody, fracturing and slab formation were the dominant responses to the impact. A restoration of the displacements on the impact-related faults suggests that the orebody was moved toward the crater. The inference is that, prior to impact, the deposit continued for several hundred meters beyond the current faulted northern boundary of the orebody and that this missing segment was displaced and may be buried within the annulus.