Near-infrared effectiveness on degraded core in tropical climates
- Authors: Jansen, Nicholas , Cooke, David , Harris, Anthony , McKnight, Stafford
- Date: 2017
- Type: Text , Journal article
- Relation: Economic Geology Vol. 112, no. 4 (2017), p. 1011-1019
- Full Text: false
- Reviewed:
- Description: In tropical climates, postdrilling oxidation of sulfide-rich core can severely degrade drill core, producing lowtemperature iron oxyhydroxides, sulfates, and clays. Variable growth of these secondary minerals in exposed drill core, combined with the hydration and degradation of primary hydrothermal minerals, may lead to the production of spurious results in near-infrared (NIR) spectroscopic studies. However, the NIR technique can remain an effective tool in assessing hydrothermal alteration, even in extremely degraded core. We have assessed the usefulness of the NIR technique on degraded core at the Ladolam gold deposit, Papua New Guinea. Here, we seek to determine whether the primary alteration mineralogy had been significantly transformed by postdrilling oxidation over several years of weathering. In doing so, the study tested whether NIR analysis can be an effective tool in the discrimination of primary hydrothermal minerals in degraded core. Our study was made possible using semiquantitative X-ray diffraction (QXRD) analyses of a drill hole in 2004, where samples were collected at 50-m intervals. We subsequently repeated NIR and QXRD analyses on the same drill core in 2012. After nine years of storage, the drill core had degraded considerably, with the growth of jarosite and other sulfates. Despite this, XRD results from 2004 and 2012 show no major differences in the primary alteration mineralogy. Closely spaced NIR analyses were conducted at 1-m intervals to increase the chance of obtaining a spectrum of the primary mineralogy and to exclude secondary oxidation minerals. The drill core, where possible, was broken immediately prior to analysis to obtain a fresh surface. On average, over a 10-m interval, approximately 25% of the NIR spectra did not contain secondary minerals and relict primary alteration minerals could be detected. The remaining spectra were affected by the occurrence of secondary jarosite, gypsum, and/or residual water, but in most cases, the primary alteration mineralogy could be determined. We conclude that NIR analyses remain an effective tool in the construction of geological deposit models when logging degraded historic core, even for sulfide-rich core that has degraded in tropical environments. © 2017 Society of Economic Geologists, Inc.
Mineralogical domains within gold provinces
- Authors: Hughes, Martin , Phillips, Neil
- Date: 2015
- Type: Text , Journal article
- Relation: Transactions of the Institutions of Mining and Metallurgy, Section B: Applied Earth Science Vol. 124, no. 3 (2015), p. 191-204
- Full Text: false
- Reviewed:
- Description: Mineralogical domains use hypogene minerals (i.e. minerals not modified by weathering) and related geochemical characteristics of mineral occurrences, not only ore deposits, to subdivide large mineralised regions. Their use in the Victorian gold province is described using readily available historical data and field checking, and this is a scheme that has not required modification since 1997.The Victorian province is typical of sediment-hosted hydrothermal ores in metamorphic terrains (often termed orogenic gold deposits). Five distinctly different mineral assemblages are used to subdivide all Victorian gold occurrences into eight domains up to hundreds of kilometres in length and tens of kilometres in width. These parallel the regional structural trend and most are closely associated with, or sharply bounded by, major regional-scale faults. Seismic work has shown these faults to be listric thrusts, which flatten into a zone of duplexed greenstones overlying older basement rocks in the deeper crust. Although not defined genetically or temporally, mineralogical domains provide an additional variable related to fluid flow to assist genetic interpretation such as the scale at which a combination of processes operates, permitting predictions as to the origin of the fluids and their pathways. The variations in mineralogy in Victorian gold occurrences indicate that ore fluid compositions differed significantly between adjacent domains, and between areas overlying different regions of deeper crust. The pattern of domains gives clues to the existence of multiple mineralising events and to the degree of overprinting of these events. Domains also assist genetic comparisons by projection into similar adjoining regions to create new domains, for example Tasmania (Mathinna domain and Lefroy sub-domain), NSW (Cobar domain) and New Zealand (Reefton domain). The domainal pattern has application to mineral exploration, metallurgy and environmental issues. Mineralogical domains could be applied elsewhere, particularly in the study of difficult-to-subdivide sedimenthosted gold ores and Archaean greenstone-hosted gold, and possibly for other commodities, especially those that occur as hydrothermal ores. © 2015 Institute of Materials, Minerals and Mining and The AusIMM.
Approaches to reporting grade uncertainty in high nugget gold veins
- Authors: Dominy, Simon , Edgar, W.
- Date: 2012
- Type: Text , Journal article
- Relation: Transactions of the Institutions of Mining and Metallurgy, Section B : Applied Earth Science Vol. 121, no. 1 (2012), p. 29-42
- Full Text: false
- Reviewed:
- Description: High nugget effect gold veins are generally considered to be one of the most challenging of deposit types to evaluate and exploit. To potential investors and mining companies, they are viewed as high risk because of the associated uncertainties in the grade estimate and general paucity of reserves at production start-up. Despite the well-known risks, these deposits are often high grade and have a grade upside which makes them potentially attractive. The reporting of resource grade is of importance and perhaps even more so is the reporting of the associated uncertainty attached to the grade estimate. The quoting of the estimated grade within a grade range is recommended to achieve more complete and useful disclosure. The definition of the grade range can be somewhat subjective, ranging from opinion-based through to data-based estimates and geostatistical conditional simulation. Whichever approach is used, the over-riding issue must be the provision of enough data of appropriate quality and full disclosure to the public of relevant uncertainties. © 2012 Institute of Materials, Minerals and Mining and The AusIMM.
- Description: 2003010578
Gold mineralisation and ore controls at the Clogau mine, Dolgellau, north Wales, United Kingdom
- Authors: Dominy, Simon , Platten, Ian
- Date: 2012
- Type: Text , Journal article
- Relation: Transactions of the Institutions of Mining and Metallurgy, Section B : Applied Earth Science Vol. 121, no. 1 (2012), p. 12-28
- Full Text: false
- Reviewed:
- Description: At a recorded production of 2·5 t Au and mill recovered grade of 17 g/t Au, Clogau is the largest and richest mine in the Dolgellau gold-belt and is the most prolific gold producer in the United Kingdom. The main period of production commenced in 1861 and ended in 1911, with intermittent production between 1983 and 2007. The principal gold-quartz bearing Main reef can be traced for about 3 km along strike, trending approximately NE-SW and dipping between 60uSE and 90uSE. The reef system pinches and swells, changes strike, and splits into a number of major and minor branches. Reef width is variable, ranging from 6 m to a few centimetres. The reef is internally complex and composed of swarms of narrow sub-parallel quartz veins separated by sheets of country rock. The gold-bearing vein segments within the composite reef package are discontinuous, though may locally possess bonanza grades running at 1000s g/t Au. The predictability of the gold-rich segments is poor, but has been shown to be related to presence of the Clogau Formation (black-shale), interactions of the reef fault with greenstone sills, and/or reef splitting to form discrete ore shoots. © 2012 Institute of Materials, Minerals and Mining and The AusIMM.
- Description: 2003010576
Possible intrusion-related gold systems in the western Lachlan orogen, southeast Australia
- Authors: Bierlein, Frank , McKnight, Stafford
- Date: 2005
- Type: Text , Journal article
- Relation: Economic Geology Vol. 100, no. 2 (2005), p. 385-398
- Full Text:
- Reviewed:
- Description: Several gold deposits occurring in the western Lachlan orogen have geological, geochemical, and geochronological characteristics that distinguish them from typical vein-hosted orogenic gold deposits of the central Victorian gold province. The later are responsible for more than 90 percent, of primary (hard-rock) gold production from this region and are generally considered to represent the only economically significant type of gold deposit in the western Lach an orogen. Atypical gold occurrences at Malmsbury, Myrtle Creek, Mount Piper, and the Wonga deposit in the Stawell goldfield are characterized by a close spatial and temporal association with posttectonic felsic intrusions, disseminated to stockwork-style mineralization, alteration dominated by sericitization, sulfidation, silicification, carbonatization and tourmalinization, and associated complex An ± Mo-W-Bi-Te-Cu. The deposits have a number of features in common with intrusion-related gold deposits elsewhere in Phanerozoic orogenic belts. Although production from this type of gold mineralization in the western Lachlan orogen has been small compared to orogenic gold deposits, the possible existence of intrusion-related gold deposits has potentially important implications for exploration in this region and also provides significant clues to the tectonic framework and Paleozoic metallogeny of eastern Australia. © 2005 Society of Economic Geologists, Inc.
- Description: C1
- Description: 2003001065