Recognition of two contrasting structural-and mineralogical-gold mineral systems in the Youjiang basin,China-Vietnam:Orogenic gold in the south and Carlin-type in the north

The gold deposits in the Youjiang basin,totaling>25 Moz gold,have traditionally been thought to be of Carlin-type,particularly those with extensional structural geometries in the northern basin dominated by platform sedimentary sequences.However,the structural geometries,mineralization styles and alteration types for the Jinya,Gaolong and Nakuang gold deposits in the south-central part of the basin are remarkably similar to those of unequivocal orogenic gold deposits.Structural studies show that gold mineralization in the three gold deposits was controlled by tight“locked-up”anticlines with NW−SE-to E−W-trending and/or concomitant thrusts and/or shear zones,which resulted from NE−SW-to N–S-trending compression or transpression following the Early Triassic closure of the Paleo-Tethyan Ocean.Alteration zones in these deposits are dominated by silicification(quartz),sericitization,sulfidation and carbonation.Zoned pyrites in these deposits comprise Au-poor cores and invisible Au-bearing rims with minor external free gold.Euhedral to subhedral auriferous arsenopyrites also contribute to the gold budget.These features indicate that the three gold deposits are sediment-hosted orogenic gold deposits that contrast markedly with the Carlin-type gold deposits in the northern part of the Youjiang basin in terms of structural geometry and timing,mineralization style and nature of associated alteration.Although additional reliable ages using robust methodologies are still required,the older isotopic ages of the gold deposits in the south-central Youjiang basin are also consistent with earlier formation during transpression that predated extension during orogenic collapse,the period of formation of the Carlin-type gold deposits in the northern Youjiang basin.

Super-enrichment of lithium and niobium in the upper Permian Heshan Formation in Pingguo,Guangxi,China

The upper Permian Heshan Formation in Pingguo,Guangxi,China,is strongly enriched in lithium(Li)and niobium(Nb).The lower bauxite layer contains 0.02–0.04 wt.%Nb_(2)O_(5)(averaging 0.035 wt.%),and the overlying clay rock layer contains 0.06–1.05 wt.%Li_(2)O(averaging 0.44 wt.%),both of which exceed the cut-off grades for independent Li and Nb deposits and are therefore highly prospective.In this study,a preliminary discussion of the genesis of the Li and Nb enrichment is presented to serve as a reference for the investigation,evaluation,and prospecting for clay-and sedimentary-type Li and Nb ores in other regions.The preliminary conclusions are that:(1)The bauxite ore contains abundant anatase,which positively correlates with the whole-rock concentrations of TiO_(2) and Nb,indicating that the Nb is hosted mainly in the anatase;(2)the cookeite content in the clay rock positively correlates with the whole-rock Li concentration,indicating that cookeite is the main carrier mineral of Li,and its genesis can be attributed to reactions between clay minerals(e.g.,pyrophyllite and illite)and Li-Mg-rich underground brine or pore water and groundwater in coastal areas;and(3)provenance analysis of immobile elements(Al,Ti,Nb,Ta,Zr,Hf,and rare earth elements)suggests distinct sources for the Nb-rich bauxite and the overlying Li-rich clay rocks,with the bauxite and Nb originating largely from alkaline felsic rocks in the Emeishan Large Igneous Province,and the clay rocks being derived from peraluminous or moderately fractionated felsic rocks associated with Permian Paleo-Tethyan magmatic arcs.

Ecological risk assessment of Cd and other heavy metals in soil-rice system in the karst areas with high geochemical background of Guangxi,China

Karst areas are widely distributed in China,and the soil derived in these areas is enriched with trace elements and has the typical characteristics of high geochemical background.Therefore,soil ecological risk has gradually attracted extensive attention.In this study,1090 sets of rice grains and paired rhizosphere soils were collected as research objects from the outcropped karst areas of carbonate rocks in Guangxi,and the concentration characteristics and influencing factors of heavy metals,such as Cd and As,in rice grains and the corresponding soils were studied.According to the Chinese Soil Environmental Quality Standard(GB 15618-2018),the proportion of karstic soil samples in Guangxi with heavy metal(As,Cd,Cr,Cu,Hg,Ni,Pb and Zn)concentrations that exceed the standard risk screening values was generally greater than that of surface soils in China or non-karstic soils in Guangxi.Notably,Cd and As were the most prominently enriched heavy metals in soils.According to the Chinese Food Safety Standard(GB 2762-2017),Pb in rice grains from the studied area did not exceed the standard;the exceedance rates of Cr,Hg,and inorganic As were only about 1%;and the exceedance rate of Cd was 12.5%.Soil Cd concentration increased gradually whereas the concentration and exceedance rate of Cd in rice grains decreased gradually with the increase of soil pH.Specifically,the exceedance rate of Cd in rice grains was only 4%when soil pH>6.5.The results indicated that the risk screening values of Cd element in the Chinese Soil Environmental Quality Standard are not satisfactorily applicable to the ecological risk assessment of Cd in karstic soils.Therefore,the recommended threshold values for the safe utilization and risk control of Cd in karstic soils was proposed so as to provide a basis for soil environmental quality hierarchization,pollution risk control and land safety zoning of agricultural land in karst areas in China.