ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉High-angle annular dark-field scanning transmission electron microscopy is a powerful 〈span〉Z〈/span〉-contrast technique able to depict the structural motifs in Pb-(Bi-Sb)-sulfosalts. Using two homologs from the kobellite homologous series, a group of “chessboard derivative structures,” represented by Bi-, and Sb-rich pairs of natural phases (the kobellite-tintinaite isotypic series and giessenite-izoklakeite homeotypic series), we visualize the slabs underpinning crystal structural modularity for the 〈span〉N〈/span〉 = 2 homolog kobellite and the 〈span〉N〈/span〉 = 4 homolog, in this case a Bi-rich izoklakeite [Sb/(Sb+Bi) = 0.35]. The homolog number, 〈span〉N〈/span〉, can be readily calculated as 〈span〉N〈/span〉 = n〈sub〉1〈/sub〉/6 – 1 and 〈span〉N〈/span〉 = n〈sub〉2〈/sub〉/4, where n〈sub〉1〈/sub〉 and n〈sub〉2〈/sub〉 are the numbers of atoms in the PbS- and SnS-motifs, respectively. Atom-scale imaging of thinned foils extracted in situ from samples for which compositional data are available also reveals syntactic unit-cell scale intergrowths on [001] zone axis with 〈span〉a〈/span〉〈sub〉kobellite〈/sub〉 || 〈span〉b〈/span〉〈sub〉izoklakeite〈/sub〉. These are as small as half-unit cells of 〈span〉b〈/span〉〈sub〉izoklakeite〈/sub〉 and one-unit cell 〈span〉a〈/span〉〈sub〉kobellite〈/sub〉. Replacement relationships are also observed as irregular slabs of kobellite “intruding” into izoklakeite. Both banded and irregular intergrowths account for the compositional fields measured at the micrometer scale.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Compositional data, comprising electron probe microanalysis and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace-element data, are presented for common (Cu)-Pb-Sb sulfosalts (bournonite, jamesonite, tetrahedrite, and boulangerite), subordinate semseyite, heteromorphite, robinsonite, and (Cu)-Pb-Bi-Sb sulfosalts, as well as for accompanying base metal sulfides (BMS) in auriferous gold veins from the Gutaishan Au-Sb deposit, southern China. The objectives of the study were to identify whether these sulfosalts represent overlooked hosts for precious metals and other trace elements of petrogenetic or economic interest, establish partitioning trends among coexisting sulfosalt species and between sulfosalts and BMS, and to seek evidence for a genetic link between the abundance of (Cu)-Pb-Sb sulfosalts and the high-fineness of native gold in the deposit. All (Cu)-Pb-Sb sulfosalts analyzed were found to be remarkably poor hosts for gold and thus do not contribute to the overall mineralogical balance for gold. Trace yet measurable concentrations of Au are, however, noted in the (Cu)-Pb-Bi-Sb sulfosalts, in agreement with published data indicating that (Cu)-Pb-Bi-Sb sulfosalts may be minor Au-hosts in some ore systems. Silver is preferentially partitioned into tetrahedrite at the expense of other sulfosalt phases, and tetrahedrite is thus the major host for Ag in the Gutaishan deposit. LA-ICP-MS trace element mapping allows partitioning relationships among different sulfosalt and BMS phases to be determined for several trace elements. Jamesonite concentrates Fe, Zn, Bi, Cd, Ag, Ni, and In over coexisting bournonite, yet boulangerite is the better host for As, Ag, Sn, Se, and Te than jamesonite. Cd and Co are typically enriched in sphalerite relative to any sulfosalt, and when present, pyrite is always enriched in Au and Co relative to all other phases. A high Au/Ag ratio in the ore-forming fluid, the presence of abundant tetrahedrite that has sequestered silver during mineral precipitation, and a lack of evidence for cooling-driven precipitation may be significant reasons for the formation of high-fineness gold throughout the deposit. Two generations of native gold are documented whereby the first is coarse-grained, Ag- and Bi-bearing, and is associated with the main (Cu)-Pb-Sb sulfosalts (bournonite, jamesonite, tetrahedrite, and boulangerite). The second generation is fine-grained and has the highest fineness. Increase in the complexity of sulfosalt assemblages, re-distribution of Ag within coarse native gold and dissolution-reprecipitation reactions among the sulfosalt-gold association increase the gold fineness. The present study shows that linking petrographic aspects at the micrometer-scale with minor/trace element distributions in complex sulfidesulfosalt assemblages can track a complex history of Au deposition and enrichment.〈/span〉

Description: 〈span〉NASA's 〈span〉MESSENGER〈/span〉 spacecraft orbited Mercury from 2011 to 2015 and has provided new insights into the interior of the innermost planet. Mercury has a large metallic core ~2,000 km in radius covered by a thin layer of rock only ~420 km thick. Furthermore, a surprisingly large fraction of this outer layer was produced by melting of deeper rocks, forming a light crust ~35 km thick. The core is now known to produce a magnetic field that has intriguing similarities and differences compared to Earth's field. Some rocks near the surface are magnetized, and the strongest magnetizations are likely to be 〉3.5 billion years old. This new understanding of Mercury's interior is helping reveal how rocky planets operate.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Pyroxene exsolutions and associated Fe–Ti oxides and spinels are described in a sample of olivine gabbro representing the Middle Zone of the Panzhihua layered intrusion, Southwest China, part of the Emeishan LIP. High-angle annular dark-field scanning transmission electron microscope imaging, electron diffraction, and energy dispersive spectroscopy reveal complex multi-stage exsolution relationships in the host clinopyroxene. The studied assemblage is common in gabbroic rocks and comprises subcalcic diopside and lamellar clinoenstatite (〈1 wt% Ca). Two sets of exsolved clinopyroxene lamellae are observed. Only one is, however, well developed as lamellae oriented approximately parallel to (801) of diopside, making an angle of ~10 to 11° with the (100) planes, or the 〈span〉c〈/span〉 axis, of both phases. These are the so-called “100” lamellae with a perfect fit along 〈span〉a〈/span〉-crystallographic axes when viewed down to [010] zone axis. Crosscutting exsolutions of Fe–(Ti) oxides are relatively common throughout the same host clinopyroxene. Apart from ilmenite and magnetite with variable Ti-content, hercynite is a minor yet ubiquitous phase. The nanoscale study indicates a sequence of fine-scale processes: from higher-〈span〉T〈/span〉 (~1030–1100 °C): (I) (clino)enstatite exsolutions in low-Ca diopside; followed by (II) slightly Ca-richer diopside overgrowths and high-〈span〉T〈/span〉 titanomagnetite exsolution in diopside; to lower-〈span〉T〈/span〉 (〈450 °C) (III) titanomagnetite exsolutions into ulvöspinel + magnetite; followed by (IV) sub-solidus re-equilibration in clinopyroxenes and among Fe–Ti oxides + hercynite. Using exact phase boundary theory, pressures of lamellar exsolution within the host diopside are estimated as ~2 GPa with an error of ± ≤1 GPa. The present study of complex exsolutions in clinopyroxene demonstrates that a nanoscale approach can help constrain 〈span〉P-T-X〈/span〉 evolution during formation of layered intrusions.〈/span〉