Estimation of undiscovered deposits in quantitative mineral resource assessments—examples from Venezuela and Puerto Rico

Quantitative mineral resource assessments used by the United States Geological Survey are based on deposit models. These assessments consist of three parts: (1) selecting appropriate deposit models and delineating on maps areas permissive for each type of deposit; (2) constructing a grade-tonnage model for each deposit model; and (3) estimating the number of undiscovered deposits of each type. In this article, I focus on the estimation of undiscovered deposits using two methods: the deposit density method and the target counting method.In the deposit density method, estimates are made by analogy with well-explored areas that are geologically similar to the study area and that contain a known density of deposits per unit area. The deposit density method is useful for regions where there is little or no data. This method was used to estimate undiscovered low-sulfide gold-quartz vein deposits in Venezuela.Estimates can also be made by counting targets such as mineral occurrences, geophysical or geochemical anomalies, or exploration plays and by assigning to each target a probability that it represents an undiscovered deposit that is a member of the grade-tonnage distribution. This method is useful in areas where detailed geological, geophysical, geochemical, and mineral occurrence data exist. Using this method, porphyry copper-gold deposits were estimated in Puerto Rico.     

"南水北调"西线工程对地质环境影响和对策

以四川西部高原地质环境脆弱为据，提出了“南水北调”西线工程应引以重视的地质环境问题，并以科学发展观提出了对策。     

边坡工程加固需求度评价及其应用

稳定程度不足的边坡通常都需要进行加固。但在加固系统设计时工程师们必须搞清边坡的哪些部位应进行重点加固 ,哪些部位只需要一般加固 ,甚至不需要加固 ,以便合理地布置加固措施。在工程地质力学综合集成方法论 (EGMS)的基础上 ,作者提出了边坡工程加固需求度 (DRD)的概念及其应用方法。     

Copper–gold occurrences in the Palaeoproterozoic Inglefield mobile belt, northwest Greenland: a new mineralisation style?

Inglefield Land in northwest Greenland is an ice-free 7000 km² region underlain by the Palaeoproterozoic Inglefield mobile belt, composed of quartzo-feldspathic gneisses, meta-igneous and supracrustal rocks. These rocks are unconformably overlain by an unmetamorphosed cover of sedimentary and igneous rocks of the Mesoproterozoic Thule Basin and the Lower Palaeozoic Franklinian Basin. Mineralisation in Inglefield Land is characterised by a copper–gold metal association that can be classified in terms of the hosting rocks, namely: garnet–sillimanite paragneiss-hosted, orthogneiss-hosted and mafic–ultramafic-hosted. The paragneiss-hosted mineralisation, the topic of this paper, is essentially confined within a NE-trending structural corridor and consists of bands of sulphide±graphite-bearing, hydrothermally altered, quartzo-feldspathic gneiss, called “rust zones”. These are commonly parallel to the paragneiss main foliation, suggesting a close relationship. The rust zones have strike lengths from a few metres to more than 5 km, and widths ranging from a few centimetres to 200 m. Sulphides mainly include pyrrhotite, pyrite and chalcopyrite. The sulphides form disseminations, up to 30% by volume, but in places they form massive pods or lenses up to 20–30 m, and about 0.1–0.5 m wide. Graphite contents are up to 5 vol.%. Rust zones typically consist of a quartz–plagioclase mosaic associated with a late generation of red-brown biotite, sericite, chlorite and epidote. Mylonitic or cataclastic textures are locally recognisable. XRD analyses of graphite indicate temperatures of between 650 and 700 °C. Sulphur isotope analyses show δ³⁴S values ranging from −6.2‰ to +9.3‰.An ore genesis model is proposed in which the Palaeoproterozoic precursor sandstone–carbonaceous shale succession is polydeformed and polymetamorphosed to granulite facies quartzo-feldspathic and pelitic gneisses, with transposition of layering to axial plane of folds, followed by ductile shearing and mylonitisation, from which future rust zones were derived. The mylonitic zones were infiltrated by fluids, whose origin can be ascribed to deep-penetrating surface waters and/or external brines. In our ore genesis model, we envisage that brines derived from the overlying Lower Palaeozoic Franklinian succession infiltrated the basement into the structural channels provided by the shear/mylonitic zones. At the regional scale, this infiltration was facilitated by a NE-trending corridor, postulated to be a deep structure.     

Gold occurrence in Central Italy—the Ponte San Pietro mineralization

Gold mineralization related to a sulphide-bearing quartz vein within Triassic phyllites has been identified in a former study of an abandoned Cu–Zn–Pb mine in southern Tuscany–northern Latium. The vein-hosted mineral assemblage includes pyrite, sphalerite, chalcopyrite, galena, arsenopyrite, and tetrahedrite; the gold content reaches up to 20 ppm. The mineralization was related to fluids characterised by moderate temperatures and moderate salinities (5–15% NaCl equivalent) and represents the highest grade of Au in the ore deposits of southern Tuscany. The difference in the estimated values of Te of primary, L+V, pseudo-secondary L+V, secondary L+V fluids inclusions reflects different components present in the solutions.Based on these results one can conclude that euhedral quartz and pyrite deposition was related to fluids having variable salinity and chemical components.The origin of gold in Ponte San Pietro (PSP) deposit may be related to leaching of metapelites/phyllites derived from pelitic rocks containing some amount of sulphur-bearing organic matter; it is worthy noting that quartz–marcasite bearing lenses, interstratified in Triassic phyllites, contain significant amount of visible gold. The Co, Ni, Mn, as well as the δ³⁴S data, suggest a different origin for the phyllite-hosted marcasite, the PSP quartz-vein hosted pyrite, and for gold occurrences.     

甘肃省肃北县马庄山金矿成因分析

马庄山大型金矿床形成于古生代岛弧火山机构中。该矿床与晚石炭世中酸性火山岩及次火山岩在空间上、成因上密切相关。矿石和脉石REE配分图和微量元素特征与岩浆岩相似 ,说明成矿物质来自系统内岩浆源。成矿流体具有中低温、浅成、偏碱性、弱还原特点。成矿条件参数 pH值、氧逸度、硫逸度、Eh值及δ34 S值均随着还原参数Hy和温度有规律变化。成矿机制是在岩浆演化—火山机构—构造空间—地下水共同作用下 ,形成岩浆热液—地下水混合流体 ,使系统内物理化学条件发生改变 ,导致成矿物质沉淀成矿     

甘肃省迭部刀扎—舟曲憨班一带金矿找矿新认识

迭部刀扎—舟曲憨班一带已发现金矿床(点)与桑坝推覆体构造前缘的采古逆冲断裂具有密切的关系。该断裂不仅控制了含矿地层、岩浆岩的分布,而且是区内重要的控矿、容矿断裂。有规模的金矿体一般产在岩体的接触带上。金矿化与硅化、黄(褐)铁矿化、黄铜矿化关系密切。由此而提出了在该区带进一步找矿的主要地质标志。     

Application of geostatistical methods in gold geochemical anomalies identification (Montemor-O-Novo, Portugal)

The study described herein concerns the application of geostatistical methods to data soil from Montemor-O-Novo area (Southern Portugal). In the area, the gold mineralised zones (Banhos, Caeiras, Falés, Gamela, Malaca and Monfurado) are characterised by different geological settings and mineralogical assemblages. A total of 1211 soil samples were collected in Montemor-O-Novo area and analysed for Cu, Pb, Zn, As, Ba and Au by atomic absorption spectrometry.To account for spatial structure, simple and cross variograms were computed for the main directions of the grid sampling. From the experimental variograms a linear model of coregionalization composed of three structures, a nugget effect and two anisotropic spherical structures, was fitted to each of the six variables. The coregionalization matrices deduced from the theoretical model show the relationships between the variables at different scales. These matrices were compared with those obtained by principal component analysis (PCA).This methodology was the basis for estimating the corresponding spatial components (Y0, Y1 and Y2) using factorial kriging analysis (FKA). Maps of raw data, Y0, Y1 and Y2 were made for each variable.The use of multivariate analysis permit the study of the spatial structure intrinsic to geochemical data and the identification and refinement of significant anomalies related to Au-bearing mineral deposits.     

The Dawson Cut Forest Bed in the Fairbanks area, Alaska, is about two million years old☆

The Dawson Cut Forest Bed lies in the lower part of thick, late Cenozoic loess deposits in the Fairbanks area. It is associated with several distal tephra beds that provide age control and offer the opportunity of its recognition elsewhere in central Alaska. EC tephra (named herein) occurs in the uppermost part of the Dawson Cut Forest Bed and its petrographic and chemical properties point to a co-magmatic relationship with PA tephra, which has not been found in direct association with the forest bed. Both tephra beds are pink and have unusually high Cl in their glass shards, which readily separates them from all other tephra beds in the Fairbanks area. They were produced by discrete eruptions, closely spaced in time. PA tephra has a glass-fission-track age of 2.02 ± 0.14 myr, indicating that the Dawson Cut Forest Bed must be about 2 million years old. The Palisades tephra (named herein) has very similar properties to these two tephra beds, suggesting that the buried forest bed just above it at the Palisades site on the Yukon River, about 250 km west of Fairbanks, correlates with the Dawson Cut Forest Bed.     

北山地区照壁山金矿床地质特征及成因

照壁山金矿床是北山地区中部金矿集中区最具代表性的金矿床之一,金矿化主要在黑云母花岗岩株内或沿其与志留系公婆泉群火山-沉积岩的接触带产出,黑云母⁴⁰Ar-³⁹Ar同位素年龄为296±5 Ma.金矿体主要由含金石英脉,网脉和细脉浸染状块体组成,金属矿物主要有黄铁矿、方铅矿、闪锌矿、黄铜矿、自然金和银金矿,脉石矿物为石英和绢云母.围岩蚀变自含金矿脉(体)向外,分别为硅化、绢云母化和绿泥石化;另外,沿岩体与火山-沉积岩接触带常可观察到黄铁绢英岩化.微量元素和硫、氧、氢及铅同位素数据表明:黑云母花岗岩是同碰撞造山期深源岩浆与地壳物质相互作用的结果,成(岩)矿物质主要来自壳幔混合源,含矿热液是岩浆热流体与大气降水混合的产物.海西期花岗质岩浆活动不仅为金矿床的形成提供了物质和热力来源,而且是成矿热液对流循环的"发动机",成矿体系温、压降低是导致金沉淀与富集的重要控制因素.