Airborne gamma-ray and magnetic anomaly signatures of serpentinite in relation to soil geochemistry, northern California

Serpentinized ultramafic rocks and associated soils in northern California are characterized by high concentrations of Cr and Ni, low levels of radioelements (K, Th, and U) and high amounts of ferrimagnetic minerals (primarily magnetite). Geophysical attributes over ultramafic rocks, which include airborne gamma-ray and magnetic anomaly data, are quantified and provide indirect measurements on the relative abundance of radioelements and magnetic minerals, respectively. Attributes are defined through a statistical modeling approach and the results are portrayed as probabilities in chart and map form. Two predictive models are presented, including one derived from the aeromagnetic anomaly data and one from a combination of the airborne K, Th and U gamma-ray data. Both models distinguish preferential values within the aerogeophysical data that coincide with mapped and potentially unmapped ultramafic rocks. The magnetic predictive model shows positive probabilities associated with magnetic anomaly highs and, to a lesser degree, anomaly lows, which accurately locate many known ultramafic outcrops, but more interestingly, locate potentially unmapped ultramafic rocks, possible extensions of ultramafic bodies that dip into the shallow subsurface, as well as prospective buried ultramafic rocks. The airborne radiometric model shows positive probabilities in association with anomalously low gamma radiation measurements over ultramafic rock, which is similar to that produced by gabbro, metavolcanic rock, and water bodies. All of these features share the characteristic of being depleted in K, Th and U. Gabbro is the only rock type in the study area that shares similar magnetic properties with the ultramafic rock. The aerogeophysical model results are compared to the distribution of ultramafic outcrops and to Cr, Ni, K, Th and U concentrations and magnetic susceptibility measurements from soil samples. Analysis of the soil data indicates high positive correlation between magnetic susceptibilities and concentration of Cr and Ni. Although the study focused on characterizing the geophysical properties of ultramafic rocks and associated soils, it has also yielded information on other rock types in addition to ultramafic rocks, which can also locally host naturally-occurring asbestos; specifically, gabbro and metavolcanic rocks.     

冀北赤城—尚义地区航磁数据处理及其地质意义解析

研究区是我国燕山金矿成矿带的一部分，区内赤城－尚义和常峪口－雕鹗堡两条东西向深断裂带水泉沟东西向碱性杂岩带、燕山期北东向中酸性岩浆活动带是重要的控矿因素。通过对航磁数据的处理与分析，并结合遥感、地质等资料，探讨了区域构造和岩浆岩的发育情况，为金矿成矿预测提供了有益的信息。     

青海巴颜喀拉地区高精度航磁异常查证及金矿找矿效果

中国冶金地质总局地球物理勘查院在青海巴颜喀拉地区开展了1∶5万高精度航磁测量,项目组通过总结测区内的岩(矿)石磁性规律,分析磁场特征及研究区域成矿模型,精选了部分航磁异常来开展地面查证,取得了较好的金矿找矿效果。本文研究了在青海巴颜喀拉地区、利用高精度航磁测量成果快速高效找寻金矿及有色金属矿的技术方法,总结出不同区域的磁场背景与岩(矿)石磁性特征,其构造破碎蚀变带引起的磁异常并不相同,可能是负磁异常,也可能是正磁异常,并认为研究一个地区的磁异常特征,首先要研究区内的岩(矿)石磁性参数特征。     

The Dorn gold deposit in northern Victoria Land,Antarctica: Structure,hydrothermal alteration,and implications for the Gondwana Pacific margin

The Dorn gold deposit in northern Victoria Land, Antarctica is a fault related gold-only deposit and it represents the first described occurrence of gold mineralization in Paleozoic terranes of the Antarctic continent. The deposit is hosted in lower greenschist facies Middle Cambrian metavolcanic and metasedimentary rocks of the Bowers terrane. Gold-bearing veins are located in a brittle–ductile reverse high-strain zone, which was produced by transpressional deformation that overprints the structures of the Cambrian–Ordovician Ross Orogeny. The vein system is surrounded by a hydrothermal alteration zone that is as much as 300-m-wide, where the host rocks are partially to completely transformed into Fe–Mg carbonate-rich rocks with different degrees of replacement of the original mineralogy and texture.The type of host rock, the temperature estimates for mineralization from 290–320 °C, the quartz dominant vein system with sulfides and Fe-rich carbonates, and the controlling structures linked to a convergent margin tectonic setting together suggest that this mineralized vein system can be classified as an orogenic gold deposit. Close analogies are found with deposits of the Stawell zone in western Victoria, which is consistent with the correlation between units and hydrothermal events in northern Victoria Land and southeastern Australia.     

航磁梯度资料在金矿地质填图和成矿预测中的应用

通过对冀东和冀北地区大比例尺（１：２５０００）航磁梯度资料的分析研究发现，航磁梯度与常规的航磁总场相比，在金矿地质填图中可以更准确地划分不同地层分界、确定火成岩的分布范围以及断裂构造带的平面位置等。总结了金矿床上航磁梯度异常的变化规律。最后，利用包括航磁梯度在内的综合航空物探信息，对冀东渠子地地区进行了金矿预测研究，取得了较好的地质效果。     

新疆北部地区含金地质体遥感信息及光谱特征

新疆北部地区泥盆－石炭纪为地壳拉张期,形成大规模洋壳建造,并形成该地区固体矿产的主要矿源层。本文利用重、磁、波谱资料进一步为此论点提出、佐证。同时,通过对金矿地质体地物波谱的测试和研究,为利用遥感技术直接勘探金矿提出理论依据。     

Mapping Structurally Controlled Alterations Sparked by Hydrothermal Activity in the Fatira–Abu Zawal Area, Eastern Desert, Egypt

The Eastern Desert of Egypt suffered a protracted period of deformation triggered by cratonization of the new juvenile crust known as the Arabian Nubian Shield (ANS), which has been proposed for potential gold discoveries associated with the corresponding tectonic event. The Fatira area, on the border of Egypt''s Northern and Central Eastern Deserts, is covered with metavolcanic rocks twisted by a dextral relocation of the Fatira Shear Zone (FSZ) relative to the Barud magmatic body. The recent study evaluated many deformed post-orogenic granitic intrusions and felsite dikes associated with promising mineralization localities, notably orogenic gold deposits. The combination of various field observations and remote sensing data, followed by the analysis of aeromagnetic enhanced maps, allowed the differentiation of distinct lithologies, structural features, and hydrothermal alterations in the study area. Additionally, the integrated results obtained from the different interpretation techniques are utilized to identify and confirm the previously supposed mineralized localities in the Fatira and Abu Zawal areas and predict other matched localities. The final ASTER, Sentinel 2 hydrothermal alteration, and orientation entropy heat maps demonstrate the association between these mineralized regions and major structures related to the FSZ late stage of deformation rather than other structures studied throughout the area of interest.     

Application of kinematic vorticity and gold mineralization for the wall rock alterations of shear zone at Dungash gold mining,Central Eastern Desert,Egypt

The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number (Wm) is important for quantifying the relative contributions of pure and simple shear in wall rocks alterations of shear zone at Dungash gold mine. Furthermore, it shows the relationship between the gold mineralization and deformation and also detects the orientation of rigid objects during progressive deformation. The Dungash gold mine area is situated in an EW-trending quartz vein along a shear zone in metavolcanic and metasedimentary host rocks in the Eastern Desert of Egypt. These rocks are associated with the major geologic structures which are attributed to various deformational stages of the Neoproterozoic basement rocks. We conclude that finite strain in the deformed rocks is of the same order of magnitude for all units of metavolcano-sedimentary rocks. The kinematic vorticity number for the metavolcanic and metasedimentary samples in the Dungash area range from 0.80 to 0.92, and together with the strain data suggest deviations from simple shear. It is concluded that nappe stacking occurred early during the underthrusting event probably by brittle imbrication and that ductile strain was superimposed on the nappe structure during thrusting. Furthermore, we conclude that disseminated mineralization, chloritization, carbonatization and silicification of the wall rocks are associated with fluids migrating along shearing, fracturing and foliation of the metamorphosed wall rocks.     

Petrology, geochemistry and remote sensing data of island arc assemblage along Wadi Abu Marawat, Central Eastern Desert, Egypt

The basement rocks of Abu Marawat area comprise serpentinites (oldest), metavolcanics and their equivalent pyroclastics, intrusive metagabbro–diorite complex, synkinematic granitoids, Hammamat sediments and basic intrusion (youngest). Remote sensing ETM+ data of Abu Marawat area were analyzed, and band ratios technique was applied to discriminate between different varieties of these basement rocks. Serpentinites are represented by lensoidal bodies tectonically incorporated in the metavolcanics. On band ratio 5/7 image, they are characterized by very bright image signature. The metavolcanics comprise basalts, andesite and subordinate dacites together with their equivalent pyroclastics. They were regionally metamorphosed up to the greenschist facies and exhibit dark grey image signatures on band ratio 5/7 image. The metagabbro–diorite complex is made up of metagabbros, diorites and quartz diorites, whereas the synkinematic granitoids are formed of tonalites and granodiorites. The band ratio 5/7 image illustrates tonalites with dark image signature, whereas metagabbro–diorites and granodiorites exhibit grey image signature. The metavolcanic suites are of island arc setting, where metabasalts are of tholeiitic affinity, while the meta-andesites and metadacites are of calc-alkaline character. The metagabbroic and granitoid rocks are of I-type, calc-alkaline affinity and were formed in arc tectonic setting. They are enriched in LIL elements and depleted in Nb and HFS elements, a characteristic feature of subduction-related magmatism. The regular variation trends among the major and trace elements as well as the coincidence of the plotted samples favor the assumption that they are comagmatic and formed by processes such as fractional crystallization.     

Detection of hydrothermal alteration zones in a tropical region using satellite remote sensing data: Bau goldfield,Sarawak, Malaysia

Remote sensing for geology in tropical environments is very challenging, because of the dense vegetation cover and the problem of persistent cloud cover. In this research paper, we have investigated and demonstrated the detection of hydrothermal alteration zones and structural elements associated with intrusion-related gold mineralization using various types of remote sensing data in the Bau gold mining district in the State of Sarawak, East Malaysia, on the island of Borneo. The climate of Bau is tropical with persistent cloud cover and very dense vegetation ground. Geological analyses coupled with remote sensing data were used to detect hydrothermally altered rocks and structural elements associated with gold mineralization in the Bau area. Landsat Enhanced Thematic Mapper⁺ (ETM⁺), Hyperion and Phased Array type L-band Synthetic Aperture Radar (PALSAR) data were used to carry out lithological–structural mapping of the mineralized zones in the study area and surrounding terrain. Hydrothermal alteration zones were detected along the SSW to NNE structural trend of the Tai Parit fault that corresponds with the occurrence of other gold mineralization in the Bau Limestone. The results show that the known gold prospects and potential areas of mineralization are recognizable by the methods used, despite limited bedrock exposure. The approach used in this study is broadly applicable to the detection of gold mineralization using ETM⁺, Hyperion and PALSAR data in tropical/sub-tropical regions.     

内蒙古维拉斯托地区高精度航磁调查与应用研究

维拉斯托地区位于大兴安岭成矿带内,区域内构造发育丰富,成矿条件优越,继续找矿的潜力巨大.采取向上延拓、小波分析、边界识别等技术对维拉斯托地区的航磁数据进行处理,结合已有地质资料,进一步完善了维拉斯托地区的地质构造,梳理出维拉斯托、拜仁达坝矿床的形成过程.通过航磁异常视磁化率成像反演结果提取出维拉斯托北侧地下的隐伏岩体信息.结果表明:（1）维拉斯托地区的北东向左行剪切性质断裂破碎带整体控制了该区的岩浆活动和矿体运移,同时该断裂带也是东南负磁异常与西北正磁异常之间的磁性过渡带,因此已知矿点与研究区构造分布、磁性强弱分布密切相关.磁异常反演结果显示维拉斯托北侧地下深部可能存在隐伏岩体.据此,本文划分了3处成矿远景区,可为后续找矿工作提供一定参考.（2）向上延拓与小波分析结果显示白音查干-达青牧场断裂（F1）和助力可河断裂（F2）形成时间早、向地下延伸深,是控制维拉斯托地区岩浆活动和构造发育的主断裂.结合倾斜角法等边界识别结果,对研究区内断裂与磁性岩体分布进行了较为精细的拟定,共识别出14条断裂与11处磁性岩体,根据其形成与分布发现维拉斯托是研究区的成矿中心,来自深部的成矿流体经F1与F2在维拉斯托附近上升富集,再由西向东,沿中部北东向断裂破碎带（F3、F4、F5、F6、F7、F10等）运移至拜仁达坝,最终逐渐形成大兴安岭地区两个典型的多金属矿床.      

地磁参数在航磁岩性填图中的应用——以青阳—芜湖地区为例

安徽青阳—芜湖地区是长江中下游成矿带重要地区之一.为进一步寻找新的矿产地,2017年在该区进行了 1 ∶ 5万航空磁法测量.在岩性填图工作中,开展了系统的岩(矿)石的磁化率调查,经分类统计实测资料,总结区内地层、侵入岩等各类岩(矿)石的磁性特征.结果表明:沉积岩一般呈弱磁性或无磁性;火山岩不发育,仅在白垩系、奥陶系个别...     

Exploration of new gold occurrences in the alteration zones at the Barramiya District,Central Eastern Desert of Egypt using ASTER data and geological studies

The use of ASTER data and fieldwork supported by mineralogical and geochemical analyses enabled exploration of new gold occurrences in the alteration zones in the ultramafic–mafic successions at the Barramiya district. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) band ratios (4/8, 4/2 and 8/9 in R, G and B channels, respectively) helped in recognising of two listwaenite alteration zones (areas 1 and 2) promising for gold in the north-east and south-east of the Barramiya gold mine. Mineralogical studies and X-ray diffraction analysis revealed that areas 1 and 2 are characterized by variable concentrations of talc, ankerite, magnesite, quartz and calcite. Ore microscope studies revealed the sulphide minerals carry gold within these alteration zones; moreover, goethite and malachite are also observed. Fire assay results show Au contents in the range of 5.04 ppm in the graphite schist, 4.02 ppm in the quartz veins and 3.76 ppm in the listwaenite alteration. The atomic absorption analysis (AAS) of samples from area 1 yields an average Au content in the quartz-veins of 2.4 ppm, Ag content is 8.0 ppm and Cu content is 2.4 wt%. The listwaenite alteration gives an average Au content of 4.4 ppm and a Cu content of 2.8 wt%. In area 2, the AAS of the quartz-veins revealed an average Au content of 2.6 ppm, 6.2 ppm Ag and 1.9 wt% Cu. The listwaenite alterations of area 2 grade 3.5 ppm Au and 2.4 wt% of Cu. The Barramiya District is made up of ophiolitic ultramafic belts of serpentinites, talc carbonates and talc graphite schists, mainly thrust over the metavolcanic sequences. They include highly strained and tectonised parts enriched in sulphides, iron oxides and carbonates, with developed listwaenite alterations along the thrust contacts. Gabbro and granitic intrusions were intruded in the ultramafics and metavolcanic rocks. ASTER data are an accurate and helpful tool for detecting and mapping alteration zones for gold exploration.     

Gold-bearing rocks of Mangalur greenstone belt,Gulbarga District,Karnataka

The Mangalur greenstone belt of Dharwar Craton, South India, is an Archaean schist belt dominated by metavolcanic rocks. The gold mineralization occurs within the metavolcanics and the fabric, mineralogy and geochemistry of these host rocks indicate that they were tholeiitic basalts regionally metamorphosed under medium to low-grade greenschist facies. The basic metavolcanic rocks occur as tholeiitic metabasalts and amphibolites. The rocks have undergone some fractionation and appear to be derived from melts generated by 10 to 25% melting of the mantle at depths 30 to 35 km around temperature 1200°C and pressure 12 kb. The source of gold is mainly in the basalts and not in the surrounding granites.     

内蒙古中部地区岩矿石磁化率特征

为了解内蒙古中部的锡林浩特、巴林左旗等地区岩矿石磁性特征,更好地对该地区的航磁异常进行综合解释,使用航遥中心研制的ZH-1磁化率仪,于2009~2011年间先后四次在该地区进行了野外岩矿石磁化率实测。并在统计、分析实测磁化率数据的基础上,研究了该地区岩矿石磁化率分布情况,分别叙述了区内沉积岩、侵入岩、火山岩、变质岩、矿石及围岩等各种类型岩矿石的磁化率特征,对岩矿石磁化率与航磁、矿产间的联系进行探讨,为磁法勘查解释、地质找矿提供了重要依据。     

Origin of wehrlite cumulates in the Moho transition zone of the Neoproterozoic Ras Salatit ophiolite,Central Eastern Desert,Egypt: crustal wehrlites with typical mantle characteristics

Ultramafic cumulates, mainly crustal true wehrlites, were discovered and described in the mantle–crust transition zone (MTZ) and the extremely lower layered gabbro sequence of the Ras Salatit ophiolite, Central Eastern Desert, Egypt. They form either boudinaged lensoidal tabular bodies or interdigitated layers often concordant with the planolinear fabrics of the Ras Salatit ophiolite rocks. The contact between wehrlites and the host MTZ dunite or layered gabbro is razor sharp, lobate and/or sinuous, without chilled margins or any visible deformations. The Ras Salatit wehrlites are orthopyroxene-free and composed mainly of olivine and clinopyroxene. They are texturally equilibrated and show a characteristic poikilitic texture. Crystallization order of the Ras Salatit wehrlites is olivine/spinel followed by clinopyroxene with the absence of plagioclase. Olivine and clinopyroxene of the Ras Salatit wehrlites are compositionally uniform and conspicuously high in Mg#, mostly around 0.93 and 0.92, respectively. Moreover, the clinopyroxene shows low Ti and Al contents coupled with marked depletion in LILE. The calculated melt in equilibrium with clinopyroxene from the Ras Salatit wehrlites is largely similar to lavas from the Izu-Bonin forearc. Given the above characteristics, the Ras Salatit wehrlites were produced by crystal accumulation from a hydrous depleted basaltic/tholeiitic melt corresponding to temperatures between 1,000 and 1,100°C at the oceanic crustal pressure (~2 kbar). The involved hydrous tholeiitic melt has been probably formed by fluid-assisted partial melting of a refractory mantle source (similar to the underlying harzburgites) in a somewhat shallow sub-arc environment.     

内蒙古赤峰柴胡栏子金矿及周边地区构造形迹的“米字型”分布及其控矿特征

柴胡栏子金矿位于中国东部大兴安岭北北东向构造带与华北地块北缘近东西向构造带的交汇部位。该交汇区经历了多期构造-岩浆活动,产出了柴胡栏子、莲花山、红花沟等多个大-中型金矿床。航磁和遥感资料解译结果表明,矿田内发育北北西向、北西西向、北北东向和北东东向共4个方向的构造形迹,呈现“米字型”结构的展布特点,分别控制了晚古生代岩浆岩(脉岩)和主要金矿床的展布。在柴胡栏子金矿区,4个方向的构造形迹也组成了“米字型”结构,控制了矿体(矿化带)和化探异常的展布。矿田和矿区级别的构造形迹均呈现“米字型”分布特点,是新华夏构造体系分级控岩控矿规律的具体表现。上述认识为矿床成因研究和矿田成矿预测提供了重要的依据。     

高精度航磁资料在新疆西天山地区 金属找矿中的应用

新疆西天山地区海拔高,地形切割大,开展地面矿产普查工作难度很大。充分利用高精度航磁资料,以区域成矿规律为基础,结合地质、矿产、化探、遥感等资料,研究不同规模、形态和强度的磁异常,通过地面查证,可以直接发现强磁性磁铁矿体,同时通过发现和圈定中弱磁性的中酸性岩体进而找到与之密切相关的多金属矿产。利用上述找矿思路,在西天山地区取得了很好的找矿效果,发现了铁、铜、金等不同种类金属矿产。因此高精度航磁资料在该地区找矿过程q-可以发挥非常重要的作用,能有效指导地面矿产勘查。     

Syn-metamorphic gold mineralisation,Invincible Vein,NW Otago Schist,New Zealand

The Invincible Vein fills a fault zone which strikes northeast and dips steeply southeast in the lower Rees Valley, NW Otago. The vein cuts north striking foliation in lower greenschist facies Otago Schist. Structures associated with the fault zone are both brittle and ductile, and the fault zone has had a complex history of post-mineralisation reactivation. Mineralised vein material filling parts of the fault zone consist of quartz, albite, muscovite, chlorite, calcite, pyrite, arsenopyrite and minor gold. These minerals have been strained and locally recrystallised during ductile deformation. Fluid inclusion homogenisation temperatures (140–175°C) and ice melting temperatures (0 to –1°C) indicate that the mineralising fluid was low salinity, low CO₂ water with a density between 0.88 and 0.93 g/cm₃. Arsenopyrite geothermometry implies a temperature of mineralisation of 370 ± 70°C. Mineralisation pressure lay between 2 and 5 kbar. Mineralisation pressure-temperature conditions and mineralogy are essentially the same as for metamorphism of the host schist. Vein calcite oxygen isotope ratios (+12 to +15 per mil) are similar to host schist values. Carbon isotope ratios of vein calcite (– 3 to –5 per mil) are distinctly different from ratios in host schist (–7 to –10 per mil). Elevated vein Cr contents, and isotopically depleted carbon data, are consistent with some degree of equilibration with metavolcanic rocks. It is inferred that metavolcanic rocks of the underlying Aspiring Terrane were a significant source for mineralising fluid and metals. Invincible mineralisation occurred in the latter stages of metamorphism, and is the earliest recognised gold-bearing vein system in the Otago Schist.     

内蒙古西部银根-额济纳旗盆地航磁 异常特征及地质意义

在对银根-额济纳旗(银-额)盆地出露的岩体的磁性特征和航磁异常特征分析的基础上,结合局部重力异常资料和前人在区域地质方面的研究成果,讨论了航磁异常与岩浆岩之间的联系,定量拟合解释了盆地东部地区的一条重磁剖面,进而探讨了岩浆岩的分布规律。研究结果表明,银根-额济纳旗盆地各类岩体磁性分布不均匀,哨马营东南侧的中性岩、银根附近的中性和酸性岩磁性较强,而其它地区出露的岩体的磁性则为弱磁性或无磁性,并且基性岩总体表现为弱磁性;区域磁异常反映了磁性基底的特征,额济纳旗西侧、苏亥图—乌力吉—苏红图一带区域磁力高异常是古元古界深变质岩系的反映,额济纳旗—绿园—特罗西滩—因格井和银根—查干德勒苏一带区域磁力低异常是前二叠系中浅变质岩系的反映;额济纳旗、哨马营北—特罗西滩、苏亥图西—因格井—乌力吉—查干德勒苏一带局部磁力高异常主要由中酸性岩所引起,哨马营—陶勒特—特罗西滩东一带局部磁力高异常主要由中性岩体所引起,绿园—务桃亥—因格井一带局部磁力高异常主要由沉积盖层中磁性物质所引起;区域性大断裂控制着银-额盆地磁性侵入岩的发育。