Logging Of Drill Holes Research Articles

Geochemical vectors for targeting hydrothermal gold mineralization in the Damang area, southwestern Ghana: Constraints from petrography, multi-element geochemistry and multivariate statistical analysis

The Damang Gold Mine in southwestern Ghana sits on two kinds of mineralization: the Tarkwaian paleoplacer and a hydrothermal type found within the Tarkwaian Group's meta-sedimentary rocks. Geochemical vectors related to the hydrothermal gold mineralization in the area are not well known. Therefore, this research was conducted to identify pathfinders for hydrothermal gold mineralization in the Damang area. A total of fifty (50) rock samples were collected from three diamond drill holes focusing on mineralized zones of the holes. The mineralogy of the host rocks was established using petrographic studies of twelve (12) selected representative drill core samples. Multi-element geochemical analysis of the host rocks (n = 50) involving Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) technique was conducted to quantify the trace element concentrations in the host rocks. The multi-element geochemical data was analyzed using multivariate statistics. The petrographic studies showed the abundance of quartz, biotite, pyrite, pyrrhotite, sericite, and minor amounts of feldspars (plagioclase and orthoclase). Al, Mg, Fe, and Ca dominated the geochemical data. The highest Au concentrations were found in Birimian quartz veins and the intrusive lithologies compared to the other lithologies in the region. Most of the data failed the Shapiro-Wilk and Kolmogorov-Smirnov tests. The deviations from normality were shown graphically by Q-Q plots. The centered log-ratio transformation was used to correct this, and the data tested again for normality. Spearman rank correlation showed a positive correlation between Te, Bi, U, Ba, Ag, and Au. When hierarchical cluster analysis was run, three clusters emerged with Au in the same cluster with Te, Ag, and Bi. Factor analysis served as a confirmatory test with Au in the same factor with Bi, Ag, and Te. Datamine Studio RM was used to create drillhole logs showing the anomalies of the three elements and Au with different lithologies downhole, which showed a similitude in the abnormal occurrences. Therefore, hydrothermal gold mineralization in the Damang area is related to Bi-Te-Ag-Au multi-element association.

Multiscalar 3D temporal structural characterisation of Smøla island, mid-Norwegian passive margin: an analogue for unravelling the tectonic history of offshore basement highs

Abstract. Smøla island, situated within the mid-Norwegian passive margin, contains crystalline-basement-hosted intricate fracture and fault arrays formed during a polyphase brittle tectonic evolution. Its detailed study may strengthen correlation attempts between the well-exposed onshore domain and the inaccessible offshore domain, further the understanding of the passive margin evolution, and provide useful constraints on petrophysical properties of fractured basement blocks. A combination of geophysical and remote sensing lineament analysis, field mapping, high-resolution drill hole logging, 3D modelling, petrographic and microstructural studies, and fault gouge K–Ar geochronology made it possible to define five deformation episodes (D1 to D5). These episodes occurred between the post-Caledonian evolution of the regional-scale Møre–Trøndelag Fault Complex (MTFC) and the Late Cretaceous and younger crustal extension preceding the final stages of Greenland–Norway break-up. Each reconstructed deformation stage is associated with different structural features, fault and fracture geometries, and kinematic patterns. Synkinematic mineralisations evolved progressively from epidote–prehnite, sericite–chlorite–calcite, chlorite–hematite, hematite–zeolite–calcite, to quartz–calcite. K–Ar geochronology constrains brittle deformation to discrete localisation events spanning from the Carboniferous to the Late Cretaceous. Multiscalar geometrical modelling at scales of 100, 10, and 1 m helps constrain the extent and size of the deformation zones of each deformation episode, with D2 structures exhibiting the greatest strike continuity and D1 features the most localised. Overall, the approach highlighted here is of great utility for unravelling complex brittle tectonic histories within basement volumes. It is also a prerequisite to constrain the dynamic evolution of the petrophysical properties of basement blocks.

Lithostratigraphy of a portion of the Neoproterozoic Rosh Pinah formation: Host rocks of the Western Orefield, Rosh Pinah mine, southern Namibia

The Rosh Pinah Zn–Pb–Ag deposit in southern Namibia comprises a significant sulphidic Zn–Pb resource (19.94 Mt @ 7.3 wt% Zn, 1.83 wt% Pb and 27.7 g/t Ag) occurring mainly as sphalerite and galena. The ore minerals as well as pyrite are hosted by the Ore Equivalent Horizon (OEH), a part of the Neoproterozoic Rosh Pinah Formation. Details on original rock types and depositional environment of the OEH units remain unclear despite past work. Here, we combine detailed drillhole logging data, drillhole logging database and drillcore photographs, underground and near-mine outcrop observations coupled with detailed petrographic observations to elucidate original rock types and depositional environments. The OEH is comprised of three mappable lithofacies (carbonaceous argillite and siltstones, volcanic/volcaniclastic rocks, and carbonate rocks) enclosed within a sequence of arkose. The lateral transition from coarse-grained arkose to the fine-grained carbonaceous rocks of the OEH reflects subaqueous fan type deposition. The presence of diagenetic textures in the carbonate rocks in form of cloudy, isopachous fibrous dolomitic cements point to subaqueous depositional environments. The geometry of carbonate bodies in the OEH at the Rosh Pinah mine and in nearby surface outcrop exposures suggest local accumulations with a pronounced topographic relief above the surrounding sediments, reminiscent of carbonate buildups. The OEH appears to represent a limited interval of marine or lacustrine deposition that may be related to syn-sedimentary extensional conditions.

Gravity and magnetic models at Rangitoto Volcano, Auckland Volcanic Field, New Zealand: Implications for basement control on magma ascent

In small-volume volcanism, pre-existing crustal structures can influence magma ascent processes. Rangitoto volcano in the Auckland Volcanic Field, New Zealand, provides a possible example of magma-structure interaction as this volcano was emplaced adjacent to an outcrop of a regional-scale basement fault, the Islington Bay Fault. In this study, we evaluated the magmatic plumbing system of Rangitoto using gravity and magnetic data acquired over the volcano and the adjacent, non-volcanic Motutapu Island. We modelled the Rangitoto internal architecture and magma plumbing using 2.5D forward and 3D inverse modelling methods. Both models are constrained by petrophysical data, while drill hole logs are only used to constrain the 2.5D model. Our model endmembers suggest that parallel magma pathways are present below the Rangitoto summit cones. In 3D magnetic models, this is evidenced by a fault-aligned pair of high-susceptibility bodies. Interpreting our models in conjunction with previously published geological and geophysical models allows us to hypothesise that the fault-parallel alignment of Rangitoto magma pathways reflects the primary influence of the Islington Bay Fault over the Rangitoto magma ascent. Magma diversion at shallow levels by other finer structures intersecting and adjacent to the fault could explain why Rangitoto erupted 3.5 km west of the Islington Bay Fault surface trace.

Multiple fluid sources in skarn systems: Oxygen isotopic evidence from the Haobugao Zn-Fe-Sn deposit in the southern Great Xing’an Range, NE China

Abstract Diverse fluid sources and complex fluid flow paths in skarn systems appear to be well documented. Nevertheless, in situ microanalysis of oxygen isotopes by secondary ion microprobe (SIMS) in skarn minerals can provide further high spatial resolution information on this complexity and the formation of skarns and associated ore deposits. In this study, we investigated the Haobugao skarn Zn-Fe-Sn deposit (0.36 M tonnes Zn) in the southern Great Xing’an Range, northeast (NE) China, and the associated Early Cretaceous Wulanba biotite granite. Based on drill hole logging, four early skarn phases are recognized: proximal red-brown garnet-hedenbergite exoskarn, central green garnet exoskarn, light brown garnet-diopside exoskarn, and distal pyroxene skarn. Oxygen isotope analyses of garnet, pyroxene, and other minerals from skarn, oxide, and quartz-sulfide stages were carried out using SIMS to determine the origin and evolution of the skarn-forming hydrothermal system. Garnet from exoskarn has a much wider range in δ18OVSMOW, between –8.1 and +6.0‰, than other stages and minerals. The estimated δ18O values of fluids in equilibrium with the Haobugao skarn vary widely from –5.1‰ to +8.9‰, suggesting that the skarn formed via episodic flux of magmatic fluid and meteoric water. Low δ18O values of cassiterite and quartz from quartz-sulfide stage rocks are +1.2 to +3.6‰, and +5.7 to +5.9‰, respectively, indicating significant contributions of meteoric water during deposition of Pb-Zn sulphides. Therefore, meteoric fluids were periodically present throughout most of the stages of skarn formation at Haobugao.

Igneous breccia system of the Shand porphyry Cu-Mo deposit, Northern Mongolia

The Shand Cu-Mo deposit is located in the Orkhon-Selenge depression, Northern Mongolia. It lies near the Erdenetiin Ovoo porphyry Cu-Mo deposit, which together define one of Northern Mongolian’s most economically significant metallogenic belts. In the Shand Cu-Mo (Au?) deposit, several ore related breccia types are associated with the porphyritic granodiorite intrusions, and they contain pre-, synand post-mineralized porphyry stocks, magmatic-hydrothermal and intrusive breccia. There are genetically at least two type of hydrothermal breccias have recognized in Shand deposit, i.e. magmatic-hydrothermal breccia and intrusive breccia. Magmatichydrothermal breccia is presented spatially associated with intrusions but extending sub vertical which characterized by angular fragments/clasts supported or infilled by minerals commonly indicative of high temperature and salinity (e.g. tourmaline, feldspar), silicas, carbonates and sulphides (Cu, Mo, (Au)) matrix derived from hydrothermal fluids precipitation. May grade downwards into cupolas of intrusive with or without intrusive breccia and pegmatite where occur at approximately deep from 250-1300 m depth. Intrusive breccia is mostly occurred in contact between margin intrusions at shallow depth which is mainly composed by granodiorite porphyry, granodiorite and dacite. Our drillhole relogging and petrographical observations are granodiorite hosted breccia and granodiorite porphyry hosted breccia. Here, we present an integrated study involving detailed drillhole logging, and petrographical observations to elucidate the genetic relationship and evolution of the Shand deposit for magmatic-hydrothermal breccia and intrusive breccia. Also, we propose that the magmatic breccia types indicate emplacement of igneous rocks from initially dacitic magma composition.

Objective Domain Boundaries Detection in New Caledonian Nickel Laterite from Spectra Using Quadrant Scan

Modelling of 3D domain boundaries using information from drill holes is a standard procedure in mineral exploration and mining. Manual logging of drill holes can be difficult to exploit as the results may not be comparable between holes due to the subjective nature of geological logging. Exploration and mining companies commonly collect geochemical or mineralogical data from diamond drill core or drill chips; however, manual interpretation of multivariate data can be slow and challenging; therefore, automation of any of the steps in the interpretation process would be valuable. Hyperspectral analysis of drill chips provides a relatively inexpensive method of collecting very detailed information rapidly and consistently. However, the challenge of such data is the high dimensionality of the data’s variables in comparison to the number of samples. Hyperspectral data is usually processed to produce mineral abundances generally involving a range of assumptions. This paper presents the results of testing a new fast and objective methodology to identify the lithological boundaries from high dimensional hyperspectral data. This method applies a quadrant scan analysis to recurrence plots. The results, applied to nickel laterite deposits from New Caledonia, demonstrate that this method can identify transitions in the downhole data. These are interpreted as reflecting mineralogical changes that can be used as an aid in geological logging to improve boundary detection.

Improving geological logging of drill holes using geochemical data and data analytics for mineral exploration in the Gawler Ranges, South Australia

Geochemical data are frequently collected from mineral exploration drill-hole samples to more accurately define and characterise the geological units intersected by the drill hole. However, large multi-element data sets are slow and challenging to interpret without using some form of automated analysis, such as mathematical, statistical or machine learning techniques. Automated analysis techniques also have the advantage in that they are repeatable and can provide consistent results, even for very large data sets. In this paper, an automated litho-geochemical interpretation workflow is demonstrated, which includes data exploration and data preparation using appropriate compositional data-analysis techniques. Multiscale analysis using a modified wavelet tessellation has been applied to the data to provide coherent geological domains. Unsupervised machine learning (clustering) has been used to provide a first-pass classification. The results are compared with the detailed geologist’s logs. The comparison shows how the integration of automated analysis of geochemical data can be used to enhance traditional geological logging and demonstrates the identification of new geological units from the automated litho-geochemical logging that were not apparent from visual logging but are geochemically distinct. KEY POINTS To reduce computational complexity and facilitate interpretation, a subset of geochemical elements is selected, and then a centred log-ratio transform is applied. The wavelet tessellation method is used to domain the drill holes into rock units at a range of scales. Several clustering methods were tested to identify distinct rock units in the samples and multiscale domains for classification. Results are compared with geologist’s logs to assess how geochemical data analysis can inform and improve traditional geology logs.

Basin-Scale 3D Sedimentary Modelling: An Approach to Subdivide Baltic Sea Onshore Sediments for Land use and Construction

Understanding the local stratigraphy and geometry of sediment units is necessary for successful 3D modelling and the prediction of ground behaviour and engineering-geological properties for urban land use and construction. A detailed 3D model of the main sediment units for the Rastaala basin in southern coastal Finland was constructed, combining a conceptual geological model and information from local drill-hole logs and geotechnical drilling. The 3D modelling was based on a cross-section approach, where sediment units were defined on the basis of lithological boundaries that are of major geotechnical importance for land use and construction. The conceptual geological model was based on a sequence of events that characterize the late- and postglacial evolution of the Rastaala basin. The present study combines the allostratigraphy and lithostratigraphy of unconsolidated glacial, late-and postglacial sediment units in southern coastal Finland. It also investigates the appearance and distribution of sediment units in the Rastaala basin and compares modelling and visualization results for unit surfaces and solids using two independent modelling software packages. Finally, information on the geotechnical properties of different sediment units in the Rastaala basin is provided so that they can be applied to other onshore regions with similar geological settings in the coastal Baltic Sea Basin.

Interpretation of buried basement in the southwestern Athabasca Basin, Canada, from integrated geophysical and geological datasets

Recent discoveries of basement-hosted uranium deposits in the Patterson Lake corridor in the southwestern Athabasca Basin of Canada have brought vigorous exploration interest to the region. New lithostratigraphic constraints, geochronology and airborne geophysical surveys have dramatically improved the understanding of the host basement geology, warranting a re-examination of the remote predictive mapping and geophysical responses of the buried basement rocks. This study took a two-step approach to examine the regional basement geology and architecture. First, a mosaic of the long-wavelength response of potential field (gravity and magnetic) datasets was examined to divide the basement into regional domains based on bulk physical property variations. The interpretive geological model was then refined using textural and lineament analysis of new airborne gravity and magnetic datasets, geological drill hole logs and magnetic susceptibility measurements. The new basement map identifies and updates major features including a crustal-scale structure that separates the southern Tantato Domain from the newly defined eastern Taltson Domain. This structure may have played a role in localizing fluid flow in the Patterson Lake corridor, defining the spatial extents of structurally controlled buried felsic intrusions, and redefines the boundaries of the Taltson, Clearwater and Tantato Domains. In addition, the potential field enhancements delineated significant regional faults that controlled the geometry of Paleoproterozoic cover sequences and have implications for understanding the crustal architecture of the southern Rae Province. These new interpretations shed light on the tectonic history of the region to support on-going exploration activities and delineate regionally prospective areas in this understudied area of the Canadian Shield. Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

The Miocene Brahma porphyry Cu-Mo prospect in Central Chilean Andes (35°45′S): Geology, geochronology (U-Pb, Re-Os) and geochemistry

The Brahma prospect is a Neogene porphyry copper deposit located in the Chilean Andes at ~ 35°45′S, beyond the southerly extension of the previously defined Mio-Pliocene metallogenic belt of Central Chile. We present the results of a comprehensive study of this prospect based on drill hole logging, petrography, geochemistry, and geochronology, with the aim of constraining its paragenesis and formation within the metallogenic background of the region. The prospect is hosted by dioritic and granodioritic equigranular units mineralized by fluids from at least two hydrothermal pulses in turn related to the emplacement of porphyritic intrusives and hydrothermal breccias. Existing exploration drill holes only intercepted the phyllic and argillic zone of the system, consistent with the paucity of potassic alteration and the dominance of pyrite, sphalerite, chalcopyrite and molybdenite mineralization. Zircon U-Pb determinations of the main igneous units present yielded ages of ~ 17 Ma, 14 Ma, and 9 Ma. Mineralization ages of ~ 14 Ma, 12 Ma, and 10 Ma were obtained by Re-Os determinations on molybdenite from samples of different paragenetic stages. These data indicate that (i) the prospect is composed of at least three main magmatic events, (ii) early mineralization is associated with the ~ 14 Ma magmatic pulse, and (iii) economic mineralization occurred at ~ 10 Ma and lasted until at least 9 Ma, as indicated by the presence of a younger mineralized porphyritic unit of this age. The obtained ages are comparable to those reported for the Los Pelambres deposit, a Cu-Mo porphyry located at 31°43′S and belonging to the Mio-Pliocene Metallogenic Belt of Central Chile. For the main intrusive units recognized in the Brahma prospect, whole rock geochemical analyses reveal arc affinities, juvenile signatures, and the involvement of amphibole in the fractionating assemblages. In a whole, our results support the existence of a complex deep magmatic reservoir that fed the magmatic-hydrothermal system for a long period, of at least 4 to 5 m.y., a common characteristic in other Miocene-Pliocene deposits in Chile and also in porphyry Cu deposits worldwide. In addition, our results highlight an interesting exploratory potential for the Brahma prospect, a long-lived Cu-Mo porphyry deposit of early Miocene age which southerly expands until ~ 36°S the Mio-Pliocene Metallogenic Belt of Central Chile.

Improving Automated Geological Logging of Drill Holes by Incorporating Multiscale Spatial Methods

Manually interpreting multivariate drill hole data is very time-consuming, and different geologists will produce different results due to the subjective nature of geological interpretation. Automated or semi-automated interpretation of numerical drill hole data is required to reduce time and subjectivity of this process. However, results from machine learning algorithms applied to drill holes, without reference to spatial information, typically result in numerous small-scale units. These small-scale units result not only from the presence of very small rock units, which may be below the scale of interest, but also from misclassification. A novel method is proposed that uses the continuous wavelet transform to identify geological boundaries and uses wavelet coefficients to indicate boundary strength. The wavelet coefficient is a useful measure of boundary strength because it reflects both wavelength and amplitude of features in the signal. This means that boundary strength is an indicator of the apparent thickness of geological units and the amount of change occurring at each geological boundary. For multivariate data, boundaries from multiple variables are combined and multiscale domains are calculated using the combined boundary strengths. The method is demonstrated using multi-element geochemical data from mineral exploration drill holes. The method is fast, reduces misclassification, provides a choice of scales of interpretation and results in hierarchical classification for large scales where domains may contain more than one rock type.

Assessment of an ore body internal dilution based on multivariate geostatistical simulation using exploratory drill hole data

Dilution can best be defined as the proportion of waste tonnage to the total weight of ore and waste in each block. Predicting the internal dilution based on geological boundaries of waste and ore in each block can help engineers to develop more reliable long-term planning designs in mining activities. This paper presents a method to calculate the geological internal dilution in each block and to correct the ultimate grade of each geological block according to the internal dilution values that have already been calculated for each one of them. In this regard, the input data is first indexed based on the lithological logs of drill holes. The occurrence probabilities of ore and waste in each block are calculated via 100 realizations using the sequential indicator simulation. Dilution is computed as the ratio of waste rock tonnage to the total tonnage of ore and waste. Furthermore, joint simulation of the continuous variables is performed for each mining block using the minimum/maximum auto-correlation factors. In the next step, for each block, the final grade variables including iron and iron oxide are computed by considering the calculated internal dilution. These analyses are applied to the Gohar Zamin iron ore deposit, and the actual internal dilution calculated based on the lithological logs of blast holes is compared with the same values obtained based on the proposed method in each block. The results obtained were found to be satisfactory.

A GIS-aided resource estimation of coalfields in Kangal basin, Sivas province, Turkey

This paper presents the procedures used for determining and defining the tonnage and grade of the coalfields of Kangal basin from the developed GIS-aided block model. In this work, firstly, all the lithological logs of drill holes and chemical analysis data of core in the basin were analyzed with the help of geostatistics, and then the digital raster maps of each one of the attributes such as the thickness, calorific value (LCV), ash content (AC%), moisture content (MC%), and surface maps of lignite seams were mapped in GIS environment. In the second stage, quantities of the overburden and resources with different categories were calculated on the basis of field-based quality and volume queries with the help of the digital maps on GIS platform. As a result, it was estimated that the Kalburcayiri field had a tonnage of 116 Mt of lignite with an LCV of 1308 kcal/kg, the Hamal field had a tonnage of 30 Mt of lignite with an LCV of 987 kcal/kg, and the Etyemez field had a tonnage of 48 Mt of lignite with an LCV of 1282 kcal/kg. Also it was estimated that almost 24,278,151 tons of lignite in the Hamal and Etyemez fields had a quality of less than 950 kcal/kg that could be directly fired without the blending process in the power plant. As a consequence, the Hamal and Etyemez fields should go into production as soon as possible and be fired in the power plant after being mixed with the lignite in the Kalburcayiri field so that they can be redounded to economy.

A Data Mining Approach to Validating Drill Hole Logging Data in Pilbara Iron Ore Exploration

A Data Mining Approach to Validating Drill Hole Logging Data in Pilbara Iron Ore Exploration

Geology, fluid inclusion, and stable isotope systematics of the Dongyang epithermal gold deposit, Fujian Province, southeast China: Implications for ore genesis and mineral exploration

The Dongyang gold deposit is a recently discovered large gold deposit located within the Dehua–Youxi–Yongtai mining camp in central–east Fujian Province, southeast China. The deposit contains two individual ore sections: the northwestern Jitou section and the southeastern Dadongkeng section. Most of the mineralization is hosted by rhyolite porphyry and volcanic rocks of the Nanyuan Formation. The locations of subvolcanic intrusions in this area are controlled by NE–SW and NW–SE trending faults that are also closely related to mineralization. Detailed drillhole logging and adit mapping indicates that the deposit contains massive, polymetallic quartz–sulfide veins, and cryptoexplosive breccia type ores. Ore minerals consist of gold, electrum, silver minerals, pyrite, marcasite, and arsenopyrite with lesser amounts of sphalerite and chalcopyrite. The mineralization is hosted by quartz, sericite, illite, chalcedony, and calcite gangue and is associated with sericite and argillic alteration as well as silicification. Fluid inclusion microthermometry indicates that the inclusions within quartz from auriferous quartz–sulfide veins formed during the main stage of mineralization and homogenize at temperatures (Th) of 172 °C–217 °C (average of 192 °C) and have salinities of 0.4–2.6 wt% NaCl equivalent. These low to moderate temperatures and low salinities suggest that the mineralization occurred in an epithermal environment. The precipitation of gold mineralization at Dongyang was most likely caused by the mixing of a large amount of meteoric fluid with a smaller amount of magmatic fluid. Quartz-hosted fluid inclusions have H and O isotope compositions (δDH2O = −63.1 to −54.2‰; δ18OH2O = −5.7‰ to −3.8‰) that are indicative of derivation from a predominantly meteoric source. The S isotope values of pyrite vary from a narrow range of −3.4‰ to 0.9‰, suggesting that the S within the deposit was derived from a homogeneous magmatic source. The mineralization and alteration, gangue and ore mineral assemblages, fluid inclusion and H–O–S isotope compositions indicate that the Dongyang gold deposit is a low-sulfidation epithermal-type deposit. The estimated ore formation depth of Donyang is nearly identical to its actual depth, indicating minor denudation in the region. These results imply a great prospecting potential for other epithermal mineralization at the Dehua–Youxi–Yongtai mining camp.

Geotechnical Investigations and Support Design of Underground Pump House Cavern: A Case Study from Lift Irrigation Project

For better rock mass classification and support design of supporting system, geotechnical investigations were carried out for the proposed 196.80 m long, 50.30 m high and 25.24 m wide underground pump house cavern of a lift irrigation scheme. Geotechnical site investigations, rock support design, excavation and related works and modification of supports design according to the observational construction method are the principal activities for the construction of underground rock cavern. The investigation includes engineering geological mapping, geological logging of drill holes, in situ permeability test and laboratory testing on core samples. Detailed geotechnical investigations carried out supporting the feasibility of underground excavations of cavern within the hillock. Support system recommended is the rock bolt and steel fibre reinforced shotcrete based on empirical approaches and their capacity is determined. Capacity of support system was conducted to evaluate the efficacy of the proposed support system.

Unravelling an allochthonous, subaqueously deposited volcanic–epiclastic to subaerial andesitic lava assemblage in Hong Kong: age, stratigraphy and provenance studies of the Middle Jurassic Tuen Mun Formation

The Middle Jurassic Tuen Mun Formation, Hong Kong, is a fault-bounded block of rare andesitic and related rocks that preserve a snapshot of the developing SE China continental arc system during the late Mesozoic. The forearc depositional setting is considered to have been dominated by an emergent andesitic volcanic massif, which shows a transition to a fluvial-dominated volcanic plain, and then to an offshore marine environment. The youngest laser ablation inductively coupled plasma mass spectrometry U–Pb detrital zircon ages obtained from the three packages define a coherent group ( n = 396; MSWD = 1.8) suggesting a maximum depositional age of 169.5 ± 0.3 Ma. The dominance of large euhedral and concentrically zoned zircons reflects a common volcanic provenance. Hf isotope data ( ε Hf ( t ) = 0 to −11) on the youngest representative zircon grains imply derivation of magmas from dominantly recycled crustal sources. The detrital zircon age signatures and Hf isotope data show no indication of magmatic interaction with adjacent late Palaeozoic and Early to Middle Jurassic sedimentary rocks, or Middle Jurassic to Early Cretaceous magmatic rocks of Hong Kong. On the basis of the structural, zircon age and isotope data, the Tuen Mun Formation is interpreted to be an allochthonous block, emplaced in the Hong Kong region during the late Middle Jurassic. Supplementary material: A full dataset of U–Pb dating and Hf isotope analysis of detrital zircons, stratigraphic drillhole logs and a description of the terminology used in this paper are available at https://doi.org/10.6084/m9.figshare.c.3736972

Drill Targeting with 3-D Seismics for Volcanogenic Massive Sulfide Exploration in the Flin Flon Mining Camp

A 17-km2 3-D seismic survey was conducted in 2007 within the Flin Flon volcanogenic massive sulfide (VMS) mining camp as part of the Targeted Geoscience Initiative-3. The resultant 3-D images show strong reflectivity associated with the ore-hosting mine horizon and prominent reflections attributed to the known ore zones. Structural repetition of the mine horizon is clearly identified in the seismic images with at least three levels identified. The 3-D seismic images have been used in conjunction with detailed geologic and geophysical drill hole logs to identify new exploration drill targets. Results from the first three seismic-based exploration drill holes are presented. The drill holes encountered reflective geologic contacts at depths within 3 to 8% of those predicted from the 3-D seismic images. In the first case, a gabbro of no economic significance was encountered at the target depth. In the second case, a highly altered mafic volcanic panel with stringer mineralization was identified sitting blind beneath surface exposures of sandstone. In the third case, the target turned out to be an additional structural level of the mine rhyolite, which provides a new horizon for further exploration. A small massive sulfide ore lens was discovered during follow-up drilling around the third target. These results demonstrate that seismic methods can be successfully applied for mineral exploration in areas of existing infrastructure and cultural noise.

Zircon U–Pb and Molybdenite Re–Os Ages of the Lakange Porphyry Cu–Mo Deposit, Gangdese Porphyry Copper Belt, Southern Tibet, China

AbstractThe Lakange porphyry Cu–Mo deposit within the Gangdese metallogenic belt of Tibet is located in the southern–central part of the eastern Lhasa block, in the Tibetan Tethyan tectonic domain. This deposit is one of the largest identified by a joint Qinghai–Tibetan Plateau geological survey project undertaken in recent years. Here, we present the results of the systematic logging of drillholes and provide new petrological, zircon U–Pb age, and molybdenite Re–Os age data for the deposit. The ore‐bearing porphyritic granodiorite contains elevated concentrations of silica and alkali elements but low concentrations of MgO and CaO. It is metaluminous to weakly peraluminous and has A/CNK values of 0.90–1.01. The samples contain low total REE concentrations and show light REE/heavy REE (LREE/HREE) ratios of 17.51–19.77 and (La/Yb)N values of 29.65–41.05. The intrusion is enriched in the large‐ion lithophile elements (LILE) and depleted in the HREE and high field‐strength elements (HFSE). The ore‐bearing porphyritic granodiorite yielded a Miocene zircon U–Pb crystallization age of 13.58 ± 0.42 Ma, whereas the mineralization within the Lakange deposit yielded Miocene molybdenite Re–Os ages of 13.20 ± 0.20 and 13.64 ± 0.21, with a weighted mean of 13.38 ± 0.15 Ma and an isochron age of 13.12 ± 0.44 Ma. This indicates that the crystallization and mineralization of the Lakange porphyry were contemporaneous. The ore‐bearing porphyritic granodiorite yielded zircon εHf(t) values between −3.99 and 4.49 (mean, −0.14) and two‐stage model ages between 1349 and 808 Myr (mean, 1103 Myr). The molybdenite within the deposit contains 343.6–835.7 ppm Re (mean, 557.8 ppm). These data indicate that the mineralized porphyritic granodiorite within the Lakange deposit is adakitic and formed from parental magmas derived mainly from juvenile crustal material that partly mixed with older continental crust during the evolution of the magmas. The Lakange porphyry Cu–Mo deposit and numerous associated porphyry–skarn deposits in the eastern Gangdese porphyry copper belt (17–13 Ma) formed in an extensional tectonic setting during the India–Asia continental collision.