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By O Haavisto, N Khajehzadeh, L Koresaar

Online analysis of metal content can be used to optimise the grade and recovery of a flotation circuit. Compared to conventional laboratory analyses, the benefit of an online analyser is the speed of the measurement, therefore disturbances (eg changes in feed grade and decrease in recovery) can be reacted to in real time by an operator or automatic process control system. In some cases traditional X-ray fluorescence analysers can be applied to silica analysis in iron ore when the SiO2 grades are high enough and have a negative correlation with iron; however, generally a laser-induced breakdown spectroscopy (LIBS) analyser is more suitable for low Z elements.The Outotec Courier 8® analyser is a LIBS based analyser able to measure elements starting from lithium (Z = 3), thus being able to measure impurities; eg silica and aluminium directly in iron ore slurries. Besides online elemental analysis, the analyser can be combined with optical measurements for rapid continuous measurement of each sample line. Laboratory testing has also shown potential for mineralogical analysis using the combination of LIBS and reflectance spectroscopy.CITATION:Koresaar, L, Khajehzadeh, N, Haavisto, O and Timperi, J, 2017. Online elemental and mineral measurement of slurries in iron ore beneficiation, in Proceedings Iron Ore 2017, pp 561–566 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Jul 24, 2017

At the present time this plant is treating by direct cyanidation 1,000 tons of ore from Open Cut mining and dumps, and 1,200 tons of current flotation tailing obtained from the Sulphide Mill, per day.The following table shows the average analysis of the two feed products (see Table 1):Precipitation Process.-The pregnant solution obtained by the cyanidation of the above has the following general analysis:-Gold 0·600 dwt. per tonTotal Cyanide 0·050 %Free Cyanide 0·045 %Calcium Oxide 0·015 %KCNS 0·060 %Na2S20S 0·140 %CaS04 0·200 %Cu 0·050 %Zn 0·004 0/0K4Fe (CN) 6 NilThis solution, after standard clarification, is precipitated by means of zinc dust and a soluble lead salt.

Jan 1, 1939

By A Grice

AMC has completed a number of backfill options studies, including several in recent years for large, three-dimensional orebodies involving total extraction by longhole open stoping with backfill. This has led to the development of a thorough review methodology that assists in the selection of the most appropriate backfill system and the most economic mixes to achieve the required performance. A recently developed design tool has simplified the calculation of backfill strengths for various exposure dimensions and automatically determines strengths incrementally over the height of a stope. Using a generic orebody with a plan dimension of 120 m wide by 360 m long, several different stoping layouts have been analysed to determine the effect of stope dimensions and sequencing on backfill strength requirements. This has been done for stope heights from 50 m to 300 m, in 50 m increments. The required strengths have been calculated in slices and summarised for each stope in four equal layers. The average strength requirement for each layout at each stope height has been compared graphically. For all stope heights, a panel stope layout that minimises the proportion of primary stopes results in a lower average strength requirement than grid layouts on a checkerboard pattern. However, for very high stopes, smaller plan dimensions become more economic. The increase in required strength with stope height is non-linear, making taller stopes potentially more attractive. The main cost factor in most backfills is the rate of binder addition, which is closely related to the strength required. By choosing a layout that minimises the backfill strength requirement, significant savings in binder addition rates and overall costs can be achieved. These must, of course, be weighed against other factors to determine the optimum stoping layout and backfill system.

Jan 1, 2000

In the Ngakuru area, zeolite deposits occur in late Quaternary lake sediments and ignimbrites. The deposits are associated with sinters, hydrothermal eruption breccias, and quartz-adularia altered fault breccias: all indicators of a shallow epithermal environment. Several of the deposits are adjacent to epithermal gold prospects. Zeolites are most abundant in lacustrine tuffs of the Ngakuru Formation, a 100-300 m thick sequence of siltstone, tuff, diatomite, sandstone and conglomerate. The formation overlies the Ohakuri Ignimbrite dated at c. 270 Ka, and is unconformably overlain by alluvial sands and gravels which are younger than 20 Ka. Individual zeolite deposits contain 30-80 per cent zeolite over a thickness of up to 45 m in thinly stratified tuff beds. The tuff beds are locally overlain by diatomite. The tuffs are composed mainly of glass shards and pumice clasts, with minor volcanic plagioclase, quartz and biotite crystals. Glass shards in the tuffs are replaced by the silica-rich zeolites mordenite and clinoptilolite. Amorphous silica (opal A), opal-CT, K-feldspar and smectite are also present. The distribution of the Ngakuru Formation is controlled by northeast trending normal faults, which separate it into the Ngakuru and Guthrie grabens. A silicified (quartz-adularia altered) hydrothermal breccia along one of the fault zones suggests that the faults acted as feeders for heated water that caused the deposition of the zeolite minerals. Some of the zeolite deposits are adjacent to sinters and hydrothermal eruption breccias. The location of the sinter about 20 m above one deposit indicates that the zeolites were formed at a depth of 20 to 70 m below the paleosurface. The hydrothermal eruption breccias contain clasts of altered ignimbrite, which contain geochemically anomalous gold and silver values. By analogy with their occurrence in active geothermal systems in the Taupo Volcanic Zone, mordenite and clinoptilolite in the Ngakuru deposits probably formed at temperatures of 60-150oC from near-neutral pH waters as a result of interaction with glass-rich tuff beds. The Ngakuru zeolite deposits are therefore interpreted as shallow hydrothermal alteration zones within larger epithermal gold-bearing systems.

Jan 1, 2002

By P S. Heithersay, M McGeough

The discovery of Olympic Dam in 1975 and, more recently, Prominent Hill in 2001 and Carrapateena in 2005 demonstrates the potential for world-class IOCG deposits in South Australia. Potential for further discovery of large IOCG mineral deposits in the Gawler Craton and the Curnamona Province is high because of continental-scale pervasive iron-rich alteration, crustal-scale structural preparation and unique magmatic-hydrothermal conditions. Recent exploration successes point to the effectiveness of new exploration tools and technologies to locate this style of mineralisation under cover. The gold endowment of these orebodies located in South Australia exceeds 75 Moz, with further discoveries to come. The lead time between discovery and development of the multi-million ounce Prominent Hill copper-gold mine in less than seven years demonstrates how quickly this style of deposit can be brought into production in South Australia. Clearly these long-life, multi- commodity IOCG deposits mines are an immediate exploration and future growth opportunity for the gold mining industry to replace ever depleting gold inventories.

Jan 1, 2007

IN the recent monograph on Broken Hill, N.S.W., by E. C. Andrews,* two independent petrological reports appear as Appendices 1 and n, by W. R. Browne and F. L. Stillwell respectively. In these considerable differences of opinion occur. Andrews contributes to some of the questions at issue, but no analytical review of the differences appears in the body of the report. Their development may invite question, and the following notes have been prepared partly in explanation and partly as a summary of some of the divergences. The soope of Browne's report extends over the whole district of Broken Hill, while Stillwell's deals with the rocks within 25 chains of the main lode. The more difficult and important rock-types occur in both areas, raising similar questions. In both cases observations extended beyond the limits of the prescribed areas. Broadly, Browne faces his problem as a study of a series of primary igneous rooks, acid and basio, whioh are intrusive into a series of sillimanite gneisses. The development of the sillimanite gneisses is regarded as belonging to an earlier period than the intrusion of the igneous rooks, while the gneissio structures of the latter are features which are incidental to their orystallization, and are not acquired subsequent to their consolidation. He finally produoes a picture in which his views of igneous injection and differentiation takes a prominent place. One direct result is the claim that three separate periods of metamorphism have oocurred in the Broken Hill area. Stillwell primarily examines the rooks as schists and gneisses since their present dominating features have been acquired during metamorphism. The effort is made to understand suoh differences as might arise from differing conditions of metamorphism upon rocks of similar and different oonstitution. The aim is to divest the rocks of those oharaoters which are due to metamorphism before approaching the questions of intrusive relatio'nships between igneous rocks andsediments.

Jan 1, 1924

By V Cullinan, D Buchanan

The Brunswick plant is located in Costerfield, approximately 40km east of Bendigo in central Victoria. Crushing, grinding, gravity gold concentration and flotation methods are used to produce a saleable gold concentrate and an antimony-gold flotation concentrate. This paper discusses the improvements made within the processing plant which have resulted in a 99% increase in throughput over nameplate capacity of 6 250 average tonnes per month to 12 445 average tonnes per month and increases of 12% gold (from 2007 average of 78% to 2014 average of 90%) and 16% antimony recovery (from 2007 average of 79% to 2015 average of 95%). This combined effort has resulted in an increase of equivalent gold ounces (AuEq oz) produced from 2010-2014 of 15 872 to 82 574 – a 66 702 increase.AuEq oz= Au oz +Sb t × Sb (US$/t)Au (US$/oz)CITATION:Buchanan, D and Cullinan, V, 2015. Improvements in bi-metal production at the Brunswick Plant, Costerfield, in Proceedings MetPlant 2015, pp 267–272 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Sep 7, 2015

By H Chen

"An EXTENDED ABSTRACT is available for download. A full-length paper was not prepared for this presentation. The Mesozoic iron oxide-copper-gold (IOCG) mineralisation in southern Perú and the northern Chile coastal IOCG belt have emerged as two of the major exploration targets in the Central Andes in the last two decades. These Mesozoic Andean IOCG deposits formed in two mineralisation epochs, the Middle-Late Jurassic (170–150 Ma) and Early Cretaceous (130–110 Ma), with the major Cu-rich IOCG deposits located in the Early Cretaceous mineralisation belt. The early studies in some IOCG centres support a magmtic-hydrothermal model for both Cu-rich IOCG deposits and broadly contemporaneous Cu-poor ‘Kiruna-type’ magnetite deposits. However, recent evidence from field, geochronology, fluid inclusions and stable isotopes show that in the major Central Andean deposits (eg Raúl-Condestable, Mina Justa in southern Perú, La Candelaria-Punta del Cobre and Mantoverde in northern Chile), the incursion of evaporite-sourced basinal brines or sea water may be a prerequisite for economic Cu mineralisation. Moreover, the Cu-poor massive magnetite deposits could be interpreted as a product from iron oxide melt, with minimal intervention by non-magmatic fluids. Therefore, they could represent two contrasted deposit types and contain little genetic relationships. The different roles of magmatism during the Mesozoic are responsible for the variable mineralisation in the Central Andes, including IOCG, magnetite and ‘manto-type’ deposits. The ore modelling for both the Middle-Late Jurassic and Early Cretaceous metallogenic epochs in the Central Andes are reconstructed.CITATION:Chen, H, 2015. The Mesozoic iron oxide-copper-gold (IOCG) mineralisation in the Central Andes – a refined IOCG ore-forming model in the Palaeozoic Continental Margin, in Proceedings PACRIM 2015 Congress, pp 251–254 (The Australasian Institute of Mining and Metallurgy: Melbourne)."

Mar 18, 2015

Water quality standards for some of the substances likely to be discharged to natural waters by the mining industry are discussed. After briefly examining how uses of water can be affected by discharges, standards for suspended solids and turbidity, pH, and toxic materials are considered. Heavy metals are examined in some detail because of recent public concern regarding their discharge. Caution is given in connection with their analysis at the very low concentrations required for management of natural waters. Finally, the issues of what constitutes a water right concentration compliance breach, and the appropriateness of biological monitoring as a condition on a water right are raised.

Jan 1, 1986

Cook Colliery currently mines in the Argo Seam, beneath old workings in an overlying seam. The Argo seam is up to 4.8 m thick and a continuous haulage system behind an ABM25 is used to develop first workings. Current partial secondary extraction utilises floor stripping and pocketing or sumping first working pillars on retreat. The following paper summarises this recent secondary extraction experience, particularly from a geotechnical perspective, but also considering production and ventilation issues. Outcomes and learnings are presented for mining in this challenging and close multi-seam environment. The sealed castor seam workings are within 20 m, and vary from unmined areas to large first working pillars, to partial extraction panels and full extraction panels. High stress remnant pillars and goaf-edges are notable mining considerations. The mining environment is further complicated by faulting, thrust zones and weak laminated roof.

Nov 25, 2010

By D Barclay, S Mercer, S Wright

Mine water management is often an afterthought in mine planning processes and can introduce significant risks to projects when studied as a stand alone component at a later stage in the mine design. Incorporation of a mine dewatering and pore pressure dissipation strategy in the early stages of mine planning is imperative in order to minimise constraints on the production schedule, to match designed pit slopes and maximise project value for an open cut mine option such as Olympic Dam. A risk-based mine planning approach was adopted during the project selection phase study whereby water management was introduced into mine planning, scheduling and the development of an operational strategy. Hydrological and hydrogeological studies and outputs were successfully integrated into the various iterations of the mine planning cycle during early selection phase to manage the potential water management risks associated with pit slope depressurisation and access to dry working conditions during rapid advancement of mining phases.An extensive network of vibrating wire piezometer sets and long-term depressurisation testing characterised aquifer conditions and demonstrated that hydraulic connectivity exists within the geological profile due to the presence of structures and resource drill holes which enhance vertical connectivity. This connectivity, along with underdrainage from mining activities contributes up to 100 m of depressurisation in low permeability units.A pit dewatering strategy incorporates the mine planning schedules and accounts for operational constraints as to the location and development of the dewatering wells, storages and pipelines. Numerical modelling generated a range of likely pit inflows and dewatering requirements with the average initial dewatering discharge likely to be 150 L/s and reduce quickly to 50 L/s after five years, depending on the premine schedule.Coupled hydrogeological and geotechnical assessments were also carried out as part of the mine planning cycle and indicate that successful dewatering will go a long way to reduce the pore pressures within key geotechnical units during the rapid mining phase where around 300 m of cover sequence is stripped in five years. However, the residual storage in the primary aquifer and the residual pore pressure in underlying low permeability units will need to be dissipated (in-pit horizontal holes) as the pit floor advances. Pit slope design angles are sensitive to depressurisation and dewatering and up to 70 per cent depressurisation of key geotechnical rock units are required. Whilst an accelerated program of aquifer dewatering supported by lithostatic unloading should drive rapid depressurisation in the lower permeability units, intervention is anticipated in key geotechnical units such as the Tregolana Shale and volcaniclastics. The extent of induced drainage will ultimately depend in part upon the mining rate and geotechnical stability requirements. Practical operability testing addressed the potential for operational constraints to mine depressurisation activities such as congestion and mining schedule development within the pit and identified that depressurisation drilling can be achieved even with reduced access availability. Mine operations and planning groups established design criteria and considered the likely inoperable periods following rainfall events of seven to 14 days to manage the base surge and minimise impact on production. In-pit surface water management options to minimise operational disruptions during and after significant rainfall events were estimated used a stochastic approach.

Jan 1, 2009

State ownership of mineral in the ground is not newthe Regalian Right is an ancient concept. And the compulsory purchase of land and the minerals beneath it by the State or a body approved by the State (Fitzgerald, 1952) was a principle well established and enforced in practice over a hundred years ago, when such powers became a practical necessity during the period of railway building in Britain and elsewhere:The acquisition of mineral properties which stand in the way of public construction projects still goes on. But in recent years nationalization is being increasingly undertaken as part of the planned use of national resources, for the closer regulation of economic activities, or merely, so it would seem, as a matter of political expediency or national pride. For instance, the unsevered coal in Britain came under State ownership in 1938 in an attempt to simplify a complex pattern of freehold titles which were impedingplanned mining operations. Later, unsevered minerals became a factor in negotiations arising under various town and country planning acts. The acquisition of the Rio Tinto mines by the Spanish Government implies political aspirations. Or State purchase may even be accidental: it was the easiest course for the Sierra Leone Government consequential upon its failure to prevent widespread trespass over legally-held diamond privileges in 1955.This paper has been prompted by problems that have arisen when properties containing unsevered mineral have been expropriated by the Crown in New Zealand: expropriations, it is true, that have been on a small scale in rela...

Jan 1, 1957

A study of literature on the epidiorite dykes which transgress the Broken Hill lode and its wall rocks reveals a considerable diversity of thought regarding the time of emplacement of such dykes. In this paper it is deduced that the epidiorite dykes were formed at a late stage and were probably post-ore. Detailed mapping, sampling, and correlation of data from over 100 exposures substantiates this deduction. Physical disruption of the epidiorite dykes is indicated and some interaction of the epidiorite and ore at the time of intrusion is inferred.Further petrological study may confirm this.

Jan 1, 1968

A number of mineral deposits occur in close proximity to the Pacific plate fronts. Among these deposits are gold-silver and the base metal sulphide deposits of copper, zinc and lead. Gold deposits that commonly occur in these areas are of hydrothermal origin. Of these the epithermal deposits have attracted the attention of the exploration geologists as well as the metallurgists. One vital aspect of the activity of the geologists is the development of models for epithermal gold mineralisation in the Pacific Rim. These models can also be used by metallurgists for predicting gold extraction processes. This paper reviews the mineralogy, fineness and the host minerals of gold and examines the data in the light of gold mineralisation in the southwest Pacific Rim. The gold mineralisation models proposed for the Pacific Rim have been used to predict gold extraction processes suitable for the various mineralisation types. Discussed also are the mineralisation and treatment processes of some selected epithermal gold deposits and gold-rich porphyry copper deposits in Indonesia, Papua New Guinea, Solomon Islands, Fiji and New Zealand.

Jan 1, 1999

Core samples, which were collected by the author from the Busang prospect, East Kalimantan, reflect a progressive evolution of a large magmatic-related hydrothermal system that is comparable to that encountered in many similar systems elsewhere in the Pacific region. The initial, localized deposition of sheeted porphyry quartz-molybdenite veins took place at high temperatures and fluid salinities, and at relatively deep levels. This was followed by an extensive phase of phyllic (sericite-quartz-pyrite ¦ tourmaline-apatite) wallrock replacement and vein formation. Propylitic (epidote-chlorite-carbonate-quartz) alteration was formed marginal to the phyllic assemblages. These porphyry-related events were centred in the Southeast Zone, and are probably associated with the emplacement of a felsic intrusion at depth. This event contributed only trace gold to the system. The early stages of hydrothermal activity were followed by an episode of quartz (¦ adularia) û carbonate û base metal sulfide veining, which was accompanied by argillic (illitic-kaolin clay) wallrock alteration. This event was sulfide-rich in the Central Zone and carbonate-dominated in the Southeast Zone. Gold mineralisation was observed in Central Zone drillcore to be associated with both sulfide, as well as carbonate, deposition. A final stage of epithermal-style quartz ¦ stibnite û realgar veins are, in the Central Zone, locally gold-bearing. The latter two events are interpreted to be associated with the late-stage exsolution of metal-bearing brines from the felsic intrusion that formed the earlier porphyry quartz-molybdenite veins and widespread phyllic-propylitic alteration.

Jan 1, 2002

The dissolution of gold in cyanide has been studied, using both chemical and electrochemical techniques. The electro-chemical studies have confirmed that gold may become passive in cyanide solution if its potential becomes more positive than -O· 5 V v. saturated calomel electrode (S.C.E.). The results of the conventional solution rate tests suggest that, although the solution rate is controlled by the rate of diffusion of oxygen at low oxygen and agitation levels, when these are increased the gold may become passive and its solution rate approach a steady value, or even decrease with increasing oxygen level. The passivity of gold has been shown to be of importance in the solution of small spheres of this metal, and in the extraction of gold from a calcined auriferous pyrite concentrate.INTRODUCTIONAlthough it has been appreciated for many years (McLaurin, 1893, 1895) that dissolved oxygen is a necessary adjunct to the solution of gold by the cyanide process, attempts to quantitatively assess its effect have led to conflicting results. For example, Barsky, Swainson, and Hedley (1934) reported that an increase in oxygen concentration improved the solution rate, but Beyers (1936) claimed that such an increase would reduce the rate at which the gold dissolved.By using an electrochemical approach Kudryk and Kellogg (1954) showed that the diffusion rate of either the cyanide or the oxygen was a controlling factor, and that, if the mole ratio of cyanide to oxygen concentration was greater than 10, the solution rate was proportional to oxygen concentration.These results, although they represent some of the most careful work available, were in direct contradiction to the findings of Mills (1951), who found that gold may become passive in cyanide solution, with a resultant decrease in solution rate.

Jan 1, 1963

By J Merrells

Fluor, together with our clients, has yielded significant reductions in the total cost of mining projects by employing a systematic and methodical process of cost reduction. This paper outlines this structured approach to reducing the total cost of projects through the gathering of information, the evaluation of the zero base design, and the development of the execution strategy for the project in the study phase.Over the recent iron ore boom the cost of growth projects escalated rapidly due to a variety of factors including the engineering cost of re-designing facilities, construction in remote areas and an increase in the ‘perceived’ capital requirements for maintainable and operable facilities.Fluor has made strategic steps towards safely reducing the capital intensity of projects, by working with clients to develop zero base designs which utilise the minimum technical requirements in order to meet the function of the plant and then add cost for safety, maintenance and operability only where clearly justified.This paper has set forth a structured three phased approach to focus the team on reducing capital cost while maintaining a low increase in engineering effort.The initial ‘gather’ phase focuses the team on acquiring information and identifying decisions that will need to be made so that the team can prioritise and focus the attention on areas that will provide the greatest benefit. Within this phase the identification of zero base design requirements, review of lessons learnt and analysis of capital cost levers, allows the team to identify the key decisions on the project that will affect the capital cost.In the subsequent ‘evaluation’ phase the team established the zero base design, using the hierarchy of cost reduction as a guide to focusing the team’s attention on high yielding cost saving opportunities.In the final phase the team develops the execution strategy which will yield the least cost using strategies and tools such as modularisation, standardisation, replication, strategic sourcing and 3D design.CITATION:Merrells, J, 2017. Reducing total project cost in the study phase using zero base design, in Proceedings Iron Ore 2017, pp 621–626 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Jul 24, 2017

By P Valenzuela

Joint mapping is a fundamental input for rock mass characterisation and slope stability analysis. The usual way to do it is using a compass in the field or digitising structures on a screen, with a software package.In this paper a method to identify and map joints automatically along a rock face is presented; it is called the Concentric Shells (CS) algorithm. The method scans a point cloud sequentially, tests if selected points are ‘true pivots’ within an inner shell, and then it searches for points inside a wider sphere (outer shell).The algorithm, developed by the author, was applied in real data. It reproduces the main features obtained from conventional (compass) and digital mapping (software package). Execution times are approximately 45 seconds for two point sets holding of 3.1 million points. Obtaining the same results via digital mapping takes approximately 3–4 working days.Key advantages of this method are: significant time reductions; elimination of mapping bias; and the ability to manage very large data sets (>2 million points) without the need of scanning the cloud on the screen. These advantages make this method an attractive tool for engineers and geologists wanting to increase productivity.CITATION:Valenzuela, P, 2017. Automated joint mapping using the concentric shells algorithm, in Proceedings Tenth International Mining Geology Conference 2017, pp 257–262 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Sep 20, 2017

By R Shaw, M Hidding

The design of primary sample cutters for high-capacity conveyors is critical to achieving a correct sample, both in collecting the sample and ensuring smooth sample discharge flow. Modelling usually occurs at the design stage and requires extensive experience to achieve the correct result. However, it is difficult to confirm the effectiveness of the cutter design until after installation. Even with high-speed video, confirmation of complete sample capture is difficult, especially if the material characteristics change over time. Recent investigation using discrete element method (DEM) simulation to model the flow of the ore into the cutter and observe how it interacts with the cutter has revealed some interesting design requirements not observed in the past. By including DEM modelling, the correct design and optimisation of the cutter can be more accurately determined prior to commissioning.CITATION:Hidding, M and Shaw, R, 2017. Modelling ore flow in the design of high-capacity sample cutters, in Proceedings Eighth World Conference on Sampling and Blending , pp 145–150 (The Australasian Institute of Mining and Metallurgy: Melbourne).

May 9, 2017

Gold in the stibnite veins of Costerfield, Victoria, was described in 1922 1, prior to any mineragraphic examination, as (a) free yellow gold (b) free rusty gold and (c) combined gold. The free yellow gold occurs in quartz and will amalgamate with mercury. The free gold of dull, rusty or copper-red colour occurs in massive stibnite and has been stated to resist amalgamation with mercury. The dull colour is only skin deep and the colour is bright when the rusty gold is scratched. The appearance of the rusty gold is spongy or cellular and it generally occurs near small cavities in the stibnite. The suggestion that the cellular character indicated the removal of a certain amount of stibnite leaving a residue of se.condary gold cannot be sustained.It now appears probable that the rusty gold is formed by the primary dissociation of a gold antimonide and is distinct in origin from the normal yellow gold which is deposited with quartz or stibnite from auriferous mineral solutions.The combined gold referred to in the earlier paper was the gold contained in the stibnite as revealed by assay. Such gold was at first thought to be either dissolved in the stibnite or else present as free interstitial particles too minute for observation in transmitted light in thin sections.Subsequent examination of numerous polished,surfaces disclosed the small particles of gold occurring in the grain boundaries in aggregates of fine grained stibnite 2. The assumption that all the gold in the stibnite, as revealed by assay, occurred in this way is now known to be incorrect, because limited amounts of a gold antimonide have been found in the veins.

Jan 1, 1952