Abstract The Cenozoic alkali basalts are widely exposed in the Jingpohu Volcanic Field, NE China. Previous volcanology and geochronology research has revealed that they were formed in three periods in the Miocene ( c. 29.23–13.59 Ma), Pleistocene ( c. 83.7 ka) and Holocene ( c. 5.5–5.2 ka BP). The Miocene and Pleistocene basalts consist of alkali olivine basalts, while the Holocene basalts are composed of alkali olivine basalts and leucite tephrites. Petrogenetic studies reveal that the primary magmas of the Miocene and Pleistocene alkali olivine basalts originated from partial melting of EM2-like garnet peridotites, and those of the Holocene alkali olivine basalts were derived from melting of EM1- and EM2-like garnet peridotites with higher garnet proportions. In contrast, the primary magmas of the Holocene leucite tephrites were derived from the melting of eclogites and peridotites. Combined with previous research, we suggest that melting of the mantle source region to generate Jingpohu alkali basalts was triggered by decarbonization and dehydration of the slabs stagnated in the mantle transition zone.

Abstract Carlin-type Au deposits in Guizhou Province, China, have similarities to and differences from the Carlin-type Au deposits in Nevada, USA. The Shuiyindong and Jinfeng deposits, located in the Guizhou Province of southern China, are compared with the Getchell and Cortez Hills Carlin-type Au deposits of Nevada in terms of ore paragenesis and pyrite chemistry. The Guizhou deposits formed in a tectonic setting similar to Nevada with the deposition of passive-margin sequences in a rifted cratonic margin context with subsequent deformation. In both districts, orebodies are preferentially hosted in limestone and calcareous siltstone and are related to faults, gold is invisible and ionically bound in arsenian pyrite, and ore-stage minerals include quartz and illite with late ore-stage minerals, including calcite, realgar, orpiment, and stibnite. Despite major similarities, however, the Guizhou deposits have characteristics that contrast with those of Carlin-type deposits of Nevada. Significant differences include the following: Guizhou ore-stage pyrite is commonly subhedral to euhedral, and typical Nevada fuzzy ore pyrite is absent. Guizhou ore pyrite contains significantly less Au, As, Hg, Tl, Cu, and Sb than the Nevada ore pyrite. Decarbonatization in Nevada deposits is expressed by extensive removal of calcite, dolomite, and Fe dolomite. In contrast, decarbonatization in the Guizhou deposits results in loss of most primary calcite, but Fe dolomite was instead sulfidized, forming ore pyrite and dolomite. This alteration is a key process in the formation of ore pyrite in the Guizhou deposits. Silicification in Nevada deposits is characterized by jasperoid replacement of calcite, dolomite, and Fe dolomite, whereas in the Guizhou deposits jasperoid replaced mainly calcite but not Fe dolomite or dolomite. Minor vein quartz, which formed during the early ore stage in Guizhou deposits, has not been identified in Nevada deposits. Clay alteration in the Nevada deposits is characterized by formation of significant illite and variable kaolinite/dickite; however, in the Guizhou deposits, trace to minor illite is present and kaolinite is uncommon. Late ore-stage arsenopyrite and vein quartz are common in Guizhou deposit but are rare in Nevada deposits. Guizhou ore fluids contained significantly more CO 2 and were higher in temperature and pressure compared with the ore fluids in Nevada deposits. To date, magmatism spatially or temporally associated with the Guizhou deposits has not been recognized. Conversely, the Nevada deposits coincide in time and space with the southward sweep of Eocene magmatism and related extension. Dolomite-stable alteration in Guizhou formed from less acidic, CO 2 -rich ore fluids at higher temperature and pressure compared with Nevada deposits, reflecting similarities between Guizhou deposits and orogenic systems. Study results are consistent with Guizhou deposits having formed in a transitional setting between typical orogenic gold and shallow Carlin-type deposits, as indicated by estimated pressure-temperature conditions at the time of gold deposition and ore-forming fluid chemistry.

Abstract The Nadaleen trend is a 25-km-long alignment of recently discovered Carlin-type gold prospects located along the northern margin of the Selwyn basin in east-central Yukon Territory, Canada. These prospects are among the closest analogues to the large, Carlin-type gold deposits found in Nevada. The Nadaleen trend is bound structurally to the south by the regional Dawson thrust and to the north by the Kathleen Lakes fault. The Dawson thrust marks the boundary between dominantly Neoproterozoic to Paleozoic slope and basin facies carbonate, siltstone, and clastic rocks of the Selwyn basin and strata of the Mackenzie platform. The Nadaleen trend contains numerous Carlin-type prospects, with the three largest being Conrad, Osiris, and Anubis. Carlin-type prospects of the Nadaleen trend are hosted in silty limestone and calcareous siliciclastic rocks along with isolated gabbroic dikes. Gold mineralization at Nadaleen is inferred to have accompanied decarbonatization of host limestone and subsequent silicification and/or brecciation. Typically, this was followed by late, open-space calcite, realgar, and orpiment. The prospects exhibit both structural and stratigraphic controls, with zones located near prominent fault and fold features. Gold is associated with elevated As, Hg, Sb, and Tl in mineralized zones. Several types of arsenian pyrite are found in mineralized zones, typically as rims around earlier barren pyrite cores or as <10- μ m disseminations and aggregates. Evidence from the Conrad zone suggests that Carlin-type gold mineralization occurred between 74.4 and 42 Ma. The tectonic and magmatic setting in this remote part of the Yukon during gold mineralization is poorly understood, with little or no evidence for contemporaneous regional magmatism or tectonism. While deposit-scale processes responsible for gold mineralization appear very similar for Carlin-type prospects in the Yukon and Carlin-type gold deposits in Nevada, whether the crustal-scale processes that formed these systems are similar remains enigmatic.