Comparative Study of Gacun and Youre Silver–Lead–Zinc–Copper Deposits in Sichuan, SW China, and their Mineralization Significance

The large Gacun silver-lead-zinc-copper deposit in Sichuan Province is one of the largest volcanogenic massive sulfide （VMS） deposits in China. The deposit consists of western and central ore bodies, which form a vein-stockwork mineralization system corresponding to hydrothermal channels, and eastern ore bodies, which form an exhalative chemical sedimentary system derived from a brine pool in a submarine basin. The Youre lead-zinc deposit, which is currently under exploration and lies adjacent to the southern part of the Gacun deposit, is characterized by intense silicification and vein- stockwork structures and consists of massive silicified rhyolitic volcanics, banded rhyolitic tuff, and phyllitic sericite tuff. From a comparison of their ore-bearing horizons, the Gacun and Youre deposits have a continuous and stable hanging wall （calcareous slate and overlying andesite） and foot wall （rhyolite-dacite breccia and agglomerate）, and the Uthologic sequence includes lower intermediate to felsic rocks and upper felsic rocks. Thus, the Youre deposit, which comprises relatively thinly layered low-grade ore, is regarded as forming a southward extension of the Gacun deposit. A further comparison of the structures of the ore-bearing belts between the two deposits suggests that the Youre ore bodies are similar to the western ore bodies of the Gacun deposit. Moreover, the characteristics of fluid inclusions and stable isotopes in the Youre deposit are also similar to those of the western ore bodies of the Gacun deposit. Genetic models of the deposits are proposed for the Gacun-Youre ore district, and massive concealed ore bodies may occcur in the Youre deposit at depths that are similar to those of the eastern ore bodies of the Gacun deposit.

Distribution characteristics and oil mobility thresholds in lacustrine shale reservoir:Insights from N_(2)adsorption experiments on samples prior to and following hydrocarbon extraction

The pore structure and oil content of shales have an important influence on the oil mobility and enrichment.In this study,the lacustrine shale samples from the Qingshankou Formation(Q1)of Songliao Basin were selected.TOC,pyrolysis,XRD and nitrogen adsorption were performed on the original and extracted shale samples.Then the influence of mineral composition and organic matter(OM)on the development of nano-scale pore,the oil phase states and mobility were analyzed.The Q1 shale samples can be sub-divided into three types according to the isotherm characteristics.Type A samples are characterized by high kerogen content,with oil mainly existing in the free phase state.Type B samples are characterized by medium kerogen content,oil mainly exists in the absorbed phase state.Type C samples are characterized by low kerogen content,with trace oil found in the absorbed phase state.Nano-scale organic pores are well developed in the Q1 Formation.Oil is primarily found in the pore spaces with diameters less than 10 nm,this being the pore size threshold for mobile shale oil.When TOC>2.0 wt%and EOM>1.0 wt%,Q1 Formation shale oil mobility is high,resulting in prospective drilling targets.

Age and Genesis of the Laodaizhanggou Pb-Zn-Ag Deposit in the Fudian Ore Field,Southern North China Craton:Implications for Regional Mineral Prospecting

The Fudian ore field in the southern North China Craton hosts the giant Donggou porphyry Mo deposit and several Pb-Zn-Ag vein deposits. Ore genesis of the Pb-Zn-Ag deposits and their relationships with the Donggou porphyry-related system are still controversial, which further restricts the regional prospecting and exploration. The Laodaizhanggou Pb-Zn-Ag deposit in the northwest of the ore field was focused in this study, to investigate its ore-forming age and genesis, and further to explore the implications for regional prospecting of Pb-Zn-Ag and Mo. The Pb-Zn-Ag veins at Laodaizhanggou are structurally controlled by the east-striking fault zones transecting the host volcanic rocks of Proterozoic Xiong’er Group. Field observations and textural relationships indicate that there are four paragenetic stages during ore-forming process, including the quartz-pyrite veins(stage I), siderite-polymetallic sulfide veins(stage II), ankerite-polymetallic sulfide veins(stage III), and quartz-calcite veins(stage IV). Ore-related sericite 40 Ar/39 Ar dating shows that the Pb-Zn-Ag mineralization at Laodaizhanggou was formed at 124.7±1.2 Ma. Carbonate minerals(siderite, ankerite, and calcite) have δ13 CPDB values of-9.1‰ to-3.9‰ and δ18 OSMOW of 12.1‰ to 15.6‰, corresponding to calculated values for the ore fluids of-8.0‰ to-2.8‰ and 4.9‰ to 10.1‰, respectively. These isotope values are in accordance with those of magmatic fluids. Sulfide minerals at Laodaizhanggou have δ34 S values of 5.3‰ to 10.1‰, and galena separates have 206 Pb/204 Pb ratios of 17.380 to 17.458, 207 Pb/204 Pb ratios of 15.459 to 15.485, and 208 Pb/204 Pb ratios of 38.274 to 38.370. Both S and Pb isotope data of Laodaizhanggou are consistent with those of the Donggou porphyry Mo deposit and distal Sanyuangou and Wangpingxigou Pb-Zn-Ag deposits, suggesting they share a similar magmatic origin. However, the Laodaizhanggou deposit was not the distal product of the giant Donggou porphyry-related magmatic-hydrothermal system, as the former is about 7 Ma older than the latter. The ore-forming age of Laodaizhanggou is consistent with that of the phase 1 magmatism of Taishanmiao batholith, indicating the Laodaizhanggou deposit is genetically related to ca. 125 Ma magmatism in the area. Combined the geochronological and geochemical data on Laodaizhanggou and the regional geological setting, we propose that the fracture systems in the northeast of the Taishanmiao batholith are potential sites for prospecting Pb-Zn-Ag deposit and the deep part among Laodaizhanggou, Xizaogou, and Liezishan is a target for prospecting porphyry Mo deposit.

Geology,Mineralogy,and Isotopic Characteristics of the Zhonghe Ag-Pb-Zn Deposit,Western Henan Province,Central China

Located in Luoning County,western Henan Province,Central China,the Zhonghe Ag-Pb-Zn ore field,is a newly discovered deposit in the Xiaoshan District.Ore bodies controlled by NNW Faults occur as veins within the Paleoproterozoic Xiong’er Group or the Early Cretaceous porphyritic granite.Given that the Zhonghe deposit has been covered by thick Quaternary sediments,the paragenetic mineral assemblage was determined mainly by microscopic observations,including the quartzsiderite-pyrite alteration (StageⅠ),polymetallic sulfide precipitation (StageⅡ),silver mineralization(Stage Ⅲ),and quartz-carbonate stage (StageⅣ).The host minerals for silver are diverse,such as freibergite,pyrargyrite,polybasite,argyrodite,canfieldite,argentite,and native silver,whereas those for Pb and Zn are galena and sphalerite,respectively.In order to constrain the ore-forming components of the Zhonghe deposit,a combined in-situ analysis was conducted on represented sulfides from StageⅡand Stage Ⅲ.In-situ δ^(34)S values of the analyzed sulfides,including the pyrite,chalcopyrite,sphalerite,and galena,display a relatively narrow range (0.90‰-5.0‰),which is close to magmatic sulfur source.The ^(206)Pb/^(204)Pb and^(207)Pb/^(204)Pb ratios show a narrow range (17.140-17.360 and 15.385-15.490),whereas the ^(208)Pb/^(204)Pb ratios exhibit a broad variation (36.601-37.943),indicating a contamination of the Xiong’er Group.Synthesis of geochronological and geochemical data from the Xiaoshan District,we contend that the ore-forming materials of the Zhonghe Ag-Pb-Zn deposit are originated from the lower crust,which has presumably been influenced by the large-scale lithospheric delamination of the eastern North China Craton during the Early Cretaceous.In consideration of the geological setting,mineralogy,and geochemical compositions,we suggest that the Zhonghe Ag-Pb-Zn deposit is characterized as intermediate sulfidation type epithermal deposit,and may be a potential exploration target for porphyry Mo-Cu deposits.

Druse Calcite Crystals Formed by Mesoproterozoic Paleo-Earthquake Activity in the Northern Margin of the North China Craton

The Meso-neoproterozoic Bayan Obo rift is located along the northern margin of the North China Craton,and was associated with the break-up of the Columbia supercontinent.During rift evolution,syn-sedimentary deformation occurred due to tectonic activity and earthquakes.Seismic events are recorded in the Jianshan Formation of the Bayan Obo Group,Inner Mongolia,as soft sediment deformation structures in the central Bayan Obo rift.Druse calcite crystals and collapse breccias in the Jianshan Formation may provide information on the rift evolution.The druse calcite crystals are idiomorphic-columnar in shape and associated with graphite,pyrite,and quartz.δ^(13)C values of the graphite are−20‰,indicative of biogenic deoxygenation and formation in water.The druse calcite crystals are inorganic in origin and formed in water at a temperature of 55℃,based on calciteδ^(13)C andδ^(18)O data.The calcite grew in paleo-caves containing fault breccias,with heat derived from faulting.As such,the druse calcite crystals are important evidence for seismic events.The collapse breccias(i.e.,fault breccias)and other indicators of slip show that displacement occurred from NE to SW,which is different from the paleocurrent direction in the Jianshan Formation.The thickness of the collapse breccia is∼200 m,which represents the height of the fault scarp.The strike of the fault scarp was NE-SW,based on the distribution of the collapse breccia.The Bayan Obo and Yanliao rifts experienced rapid NW-SE extension,and developed similar deformation structures at ca.1.6 Ga related to break-up of the Columbia supercontinent.

中蒙边界地区成矿区带划分

中蒙边界地区成矿区带划分是近年来该区成矿规律研究中的重要科学问题之一。以板块构造-地球动力学理论为指导,在中蒙合作完成的该区1∶100万建造-构造图和成矿规律图的基础上,对中蒙边界地区成矿区带进行了统一厘定和划分。该区Ⅰ级成矿域隶属于古亚洲成矿域和滨太平洋成矿域。古亚洲成矿域可划分为阿尔泰、准噶尔-南蒙古和塔里木3个成矿省、12个成矿带和21个成矿亚带。滨太平洋成矿域划分出大兴安岭和华北陆块2个成矿省、4个成矿带和15个成矿亚带。研究提出,蒙古的欧玉陶勒盖-查干苏布尔嘎大型-超大型斑岩型矿床成矿亚带向西与中国的东天山-北山成矿亚带相连,为该区寻找同类型斑岩型矿床指明了方向。

An integrated ore prospecting model for the Nyasirori gold deposit in Tanzania

The Nyasirori gold deposit,located in the middle-western end of the Musoma-Mara Archean greenstone belt in Tanzania,is a tectonic altered rock type gold deposit controlled by shear tectonic zone.This work conducted high-precision ground magnetic measurements to delineate fault structures and favorable prospecting targets,utilized induced polarization(IP)intermediate gradient to roughly determine the distribution and extension of the tectonic altered zone and gold ore(mineralized)bodies,and further carried out IP sounding and magnetotelluric sounding to locate the tectonic altered zone and gold ore(mineralized)bodies.The anomalous gradient belt of the combination of positive and negative micromagnetic measurements reflects the detail of shallow surface tectonic alteration zone and gold mineralization body.Micromagnetic profile anomalies indicate the spatial location and occurrence of concealed tectonic alteration zone and gold(mineralized)ore bodies.Soil geochemical measurements indicate that the ore-forming element Au correlates well with As and Sb,and As and Sb anomalies have a good indication to gold orebodies.Based on the multi-source geological-geophysical-geochemical information of the Nyasirori gold deposit,this work established an integrated prospecting model and proposed a set of geophysical and geochemical methods for optimizing prospecting targets.

Detection of Geochemical Element Assemblage Anomalies Using a Local Correlation Approach

As direct prospecting data,geochemical data play an important role in modelling prospect potential.Geochemical element assemblage anomalies are usually reflected by the correlation between elements.Correlation coefficients are computed from the values of two elements,which reflect only the correlation at a global level.Thus,the spatial details of the correlation structure are ignored.In fact,an element combination anomaly often exists in geological backgrounds,such as on a fault zone or within a lithological unit.This anomaly may cause some combination of anomalies that are submerged inside the overall area and thus cannot be effectively extracted.To address this problem,we propose a local correlation coefficient based on spatial neighbourhoods to reflect the global distribution of elements.In this method,the sampling area is first divided into a set of uniform grid cells.A moving window with a size of 3×3 is defined with an integer of 3 to represent the sampling unit.The local correlation in each unit is expressed by the Pearson correlation coefficient.The whole area is scanned by the moving window,which produces a correlation coefficient matrix,and the result is portrayed with a thermal diagram.The local correlation approach was tested on two selected geochemical soil survey sites in Xiao Mountain,Henan Province.The results show that the areas of high correlation are mainly distributed in the fault zone or the known mineral spots.Therefore,the local correlation method is effective in extracting geochemical element combination anomalies.