The Xinghai-Zeku area of western China is one of the most important Au polymetallic metallogenic belts in Qinghai Province, China. To guide the mineral exploration in this area, log and isometric log-ratio (ilr) transformations of stream sediment data are evaluated using principal component analysis (PCA) combined with geological data to study the relationship among different elements. In addition, Mean + 2 standard deviations (Mean + 2STD), Median + 2 median absolute deviations (Median + 2MAD), and concentration-area (C-A) models are applied to identify pathfinder threshold values and geochemical patterns are decomposed using a spectrum-area (S-A) model. The results show that: (1) PCA for the ilr-transformed data can accurately describe three different geochemical assemblages, Au-As-Sb, that represent Au-As-Sb mineralization in fracture zones; (2) anomalies of Au + As + Sb are more suitable for targeting Au-As deposits than those of the single element Au; (3) the C-A model is useful for indentifying geochemical anomalies associated with mineralization because results obtained by the C–A fractal model and the geological characteristics are well correlated; (4) background and anomaly maps for Au + As + Sb from the S-A model in conjunction with the Mean + 2STD successfully identify weaker anomalies by reducing anomalous areas; and (5) using Au + As + Sb anomalies identified by C-A in conjunction with the S-A model and Mean + 2STD method are effective in exploration for Au deposits in the area.
The podiform chromitite found within the Luobusha ophiolite comprises characteristic nodules and massive chromitites. However, the exact origin of these formations remains a topic of ongoing debate. In this study, the microstructures of olivine and chromite are investigated to unravel their formation processes and shed light on the associated geodynamic mechanisms. EBSD analysis provides insights into chromitite and host peridotite deformation mechanisms. Olivine grains in the host dunite and nodular chromite exhibit crystallographic preferred orientations (CPOs) with D-type fabrics, which show a girdle distribution in the [010] and [001] axes, normal to the foliation plane of the sample. The massive and disseminated chromitite displays B-type and C-type olivine fabric, with a concentration of [001] axes parallel to the lineation of the sample. Crystal plastic deformation can be observed in the Luobusha chromite grains, highlighting intercrystalline deformation processes. Small grains lacking misorientation observed in the massive chromitite are likely attributed to heterogeneous nucleation. Chromite nodules are found to be a patchwork of subgrains with various orientations and high-angle boundary misorientation. The formation of Luobusha chromitite involves deep-seated crystallization, followed by amalgamation, and subsequent deformation within the mantle peridotite. These findings distinguish Luobusha chromitite from other ophiolitic chromite deposits, offering valuable insights into the deformation history and formation processes.
The Yuka gold deposit, located in the western part of northern Qaidam, contains Au orebodies hosted in early Paleozoic metamorphic basic volcaniclastic rocks. The Yuka mineralization can be divided into three stages: early quartz-pyrite (stage-I), middle quartz-gold-polymetallic sulfide (stage-II), and late quartz-carbonate (stage-III). Gold deposition is primarily contained within stage-II. Three types of fluid inclusions were identified in the vein mineral assemblages using petrography and laser Raman spectroscopy: H 2 O-CO 2 -NaCl (C-type), H 2 O-NaCl (W-type), and pure CO 2 (PC-type). Stage-I fluids record medium temperatures (205.2°C to 285.5°C) and H 2 O-CO 2 -NaCl±CH 4 fluids with variable salinities (0.6–8.5 wt.% NaCl equiv.). Stage-II fluids evolved towards a more H 2 O-rich composition within a H 2 O-CO 2 -NaCl±CH 4 hydrothermal system at medium temperatures (193.1°C to 271.1°C), with variable salinities (0.4–11.7 wt.% NaCl equiv.). Stage-III fluids are almost pure H 2 O and characterized by low temperatures (188.1°C to 248.5°C) and salinities (0.4–16.1 wt.% NaCl equiv.). These data indicate that ore-forming fluids are characterized by low to medium homogenization temperatures and low salinity and are evolved from a CO 2 -rich metamorphogenic fluid to a CO 2 -poor fluid due to inputs of meteoric waters, which is similar to orogenic-type gold deposits. The average δ18OW of quartz varies from 3.3‰ in stage-I to 2.1‰ in stage-II and to 1.4‰ in stage-III, with the δD values ranging from −41.6‰ to −58.5‰, suggesting that ore-forming fluids formed from metamorphic fluids mixed with meteoric waters. Auriferous pyrite δ34S ranges from 0.5 to 7.4‰ with a mean value of 4.43‰, suggesting that fluids were partially derived from Paleozoic rocks via fluid-wall rock interactions. Auriferous pyrites have 206 Pb/ 204 Pb of 18.238–18.600 (average of 18.313), 207 Pb/ 204 Pb of 15.590–15.618 (average of 15.604), and 208 Pb/ 204 Pb of 38.039–38.775 (average of 38.1697) and stem from the upper crust. Basing on geological characteristics of the ore deposit as well as new data from the ore-forming fluids, and H-O-S-Pb isotopes, the Yuka gold deposit is best described as an orogenic-type gold deposit.
There is a lack of information on the geochemical characteristics and genesis of ferromanganese nodules and crusts from the Central Rift Seamounts Group of the West Philippine Sea (CRSGWPS). This study presents the major, trace, and rare earth contents and 10Be isotopic data for ferromanganese nodules and crusts from the CRSGWPS to determine their genetic origin, classification, and growth rate. The Mn/Fe ratio of the crust is 0.96–3.97, and the Co + Ni + Cu content is 4499–6324 ppm. The crust is rich in rare earth elements (REE), with significant positive Ce and negative Y anomalies. The Mn/Fe ratio of the nodule is 0.90–1.52, and the Co + Ni + Cu is 1617–9549 ppm. Samples near the core of the nodule have a relatively low REE content, with significant positive Ce and negative Y anomalies. 10Be specific activity and thickness of the nodules and crusts show that growth rates were 2.26 mm/Ma and 2.5 mm/Ma, respectively, indicating that proliferation of nodules and crusts in this area began in the mid-Miocene (9.32–12.03 Ma). A comprehensive study of the growth rate, mineralogy, structure, and chemical characteristics of nodules/crusts in the study area indicates that their formation was mainly hydrogenic, and the early growth of nodules is affected by hydrothermal action.
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