ORCID Profile
0000-0002-6998-236X
Current Organisation
University of Oxford
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Publisher: Elsevier BV
Date: 03-2008
Publisher: Geological Society of London
Date: 07-07-2023
DOI: 10.1144/JGS2023-004
Abstract: Ophiolitic peridotites in Myanmar (Burma) occur along three major tectonic zones: the Kaleymyo–Nagaland suture along the Indo-Burman Ranges, the Jade Mines belt and the Tagaung–Mytkyina belt. These belts all show harzburgite–lherzolite–dunite peridotites, but the Hpakan-Taw Maw region (Jade Mines belt) also hosts jadeitites, including pure jadeite, mawsitsit (Cr-rich jadeite) kosmochlore (Cr-rich clinopyroxene) and albitite. Jadeitites with high Na and Al contents require either very unusual Al-rich, Si-poor protoliths or extensive fluid metasomatism, or both. The Hpakan jadeitites formed by Na-, Al- (and Si-) metasomatic alteration of pyroxenite–wehrlite intrusions into harzburgite–dunite from widespread fluid alteration. The fluids could have been derived from a mid-Jurassic intermediate pressure subduction event during ophiolite formation and emplacement. In the Lake Indawgyi area, normal ophiolitic peridotites, including harzburgite and dunite with pyroxenite veins, have not been jadeitized. Gabbros related to the Jade Mines ophiolite gave a U–Pb zircon age of 169.71 ± 1.3 Ma (MSWD 2.2), a similar timing to the Myitkyina ophiolite (173 Ma) to the east, suggesting that the ophiolite belts were originally continuous. The jade ‘boulders’ in the Uru conglomerate beds at Hpakan have also resulted from normal in situ serpentinization weathering processes, followed by limited fluvial mass transport processes along the Uru River. Supplementary material: U–Pb zircon data are available at 0.6084/m9.figshare.c.6655269
Publisher: Society of Economic Geologists
Date: 09-2019
DOI: 10.5382/ECONGEO.4676
Abstract: Carlin-type gold deposits are one of the most important gold mineralization styles in the world. Despite their economic importance and the large volume of work that has been published, there remain crucial questions regarding their metallogenesis. Much of this uncertainty is due to the cryptic nature of the gold occurrence, with gold occurring as dispersed nanoscale inclusions within host pyrite rims that formed on earlier formed barren pyrite cores. The small size of the gold inclusions has made determining their nature within the host sulfides and the mechanisms by which they precipitated from the ore fluids particularly problematic. This study combines high-resolution electron probe microanalysis (EPMA) with atom probe tomography (APT) to constrain whether the gold occurs as nanospheres or is dispersed within the Carlin pyrites. APT offers the unique capability of obtaining major, minor, trace, and isotopic chemical information at near-atomic spatial resolution. We use this capability to investigate the atomic-scale distribution of trace elements within Carlin-type pyrite rims, as well as the relative differences of sulfur isotopes within the rim and core of gold-hosting pyrite. We show that gold within a s le from the Turquoise Ridge deposit (Nevada) occurs within arsenian pyrite overgrowth (rims) that formed on a pyrite core. Furthermore, this As-rich rim does not contain nanonuggets of gold and instead contains dispersed lattice-bound Au within the pyrite crystal structure. The spatial correlation of gold and arsenic within our s les is consistent with increased local arsenic concentrations that enhanced the ability of arsenian pyrite to host dispersed gold (Kusebauch et al., 2019). We hypothesize that point defects in the lattice induced by the addition of arsenic to the pyrite structure facilitate the dissemination of gold. The lack of gold nanospheres in our study is consistent with previous work showing that dispersed gold in arsenian pyrite can occur in concentrations up to ~1:200 (gold/arsenic). We also report a method for determining the sulfur isotope ratios from atom probe data sets of pyrite (±As) that illustrates a relative change between the pyrite core and its Au and arsenian pyrite rim. This spatial variation confirms that the observed pyrite core-rim structure is due to two-stage growth involving a sedimentary or magmatic-hydrothermal core and hydrothermal rim, as opposed to precipitation from an evolving hydrothermal fluid.
Location: United Kingdom of Great Britain and Northern Ireland
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