Microscopic digital image analysis of gold ores: a critical test of methodology, comparing reflected light and electron microscopy R. Castroviejo, E. Berrezueta ETSI Minas (Univ. Politécnica de Madrid), Madrid, Spain R. Lastra NRCan CANMET, Ottawa, Ontario, Canada Abstract Automatic digital phase imaging should yield unbiased and representative characterizations of gold ores. However, this is not a simple task due to low gold grade, fine grain size, possible complex mineralogy, nugget effect, etc. Using the same sections of a gold-rich ore, a critical test was performed using an optical microscope (OM) and an electron microscope (EM). In general, the two methods are consistent and useful for ore processing. There are some minor differences, such as the detection of very fine gold grains is enhanced by EM. Key words: Digital imaging, Gold ores, Mineralogy, Electron microscopy, Optical microscopy Introduction sections was scanned. Therefore, differences in the results are Contemporary mining operations commonly deal with very not related to differences in sample preparation methods. The low-grade gold ores that can contain only a few grams of gold studies were performed in an independent manner. The part of per ton of ore. In such ores, the gold often occurs as fine grains the study incorporating image analysis and optical micros- and frequently occurs with complex mineralogy and complex copy was performed at the Madrid Polytechnical University texture. These attributes not only pose challenges for process- (Dept. of Engineering Geology, ETS Ing. Minas, Spain), ing and recovering the gold, but they also complicate the where the instrumentation consists of a Leica Q500MC image mineralogical studies performed to characterize the gold. It is analyzer linked to a Leica DMRXP reflected-light optical on such studies that the ore processing is based. It is not a microscope. The part of the study in which an electron simple task to have a representative gold mineralogy in a microscope was used was carried out at the CANMET- polished section of a few square centimeters. The probabili- MMSL Laboratory (Ottawa, Canada), where the instrumenta- ties of trace minerals, such as gold, appearing at the surface of tion consists of a Carl Zeiss KS400 image analyzer (formerly the polished section are not favorable. Furthermore, depend- Kontron IBAS) linked to a JEOL 733 electron microprobe ing on the magnification used in the microscope, some fine fitted with energy and wavelength X-ray dispersive spectrom- gold grains may be invisible. Nevertheless, for visible gold, eters. The polished sections were first studied in Madrid, the microscope-based methods provide applicable informa- Spain, and then sent to Ottawa, Canada. tion that is directly related to the ore. Therefore, it is important to optimize the methodology to maximize the gold mineral- Studied ores and descriptive mineralogy ogy information and to minimize possible sources of errors. The study was conducted using ores rich in gold. The gold is present in mineralogical phases of simple determination, Instrumentation and objective mostly native gold with variable silver content. In addition, Image analysis, although subject to the above-mentioned the gold grain size is quite coarse. These factors simplify limitations, represents an important advance over traditional imaging and detection of the gold grains. manual techniques for finding and characterizing gold by The samples were taken from two veins at the Nueva microscopy. Naturally, a manual technique is prone to errors Esparta Mine (Los Andes, Nariño, SW Colombia): the Bruja due to human fatigue and is particularly difficult when finely vein and the Gruesa vein. Under the direction of the first ground mineral products need to be examined. It is now author, these samples were the subject of mineralogical stud- possible to integrate digital image analysis with optical or ies for Ph.D. and Master’s theses (Pantoja, 1999; Berrezueta, electron microscopy. This paper presents a comparative study 2000). These studies include a descriptive mineralogical study of image analysis by optical microscopy (OM) and image by reflected and transmitted optical microscopy. Mineraliza- analysis by electron microscopy (EM). In both cases, exactly tion is of a mesothermal vein type with a low Ag:Au ratio, the same polished sections were used and the full area of the related to lower Tertiary Andine magmatism. In the quartz and Preprint number 01-056, presented at the SME Annual Meeting, Feb. 26-28, 2001, Denver, Colorado. Original manuscript accepted for publication January 2002; revisions received April 2002. Discussion of this peer-reviewed and approved paper is invited and must be submitted to SME Publications Dept. prior to Nov. 31, 2002. Copyright 2002, Society for Mining, Metallurgy, and Exploration, Inc. MAY 2002 • VOL. 19 NO. 2 102 MINERALS & METALLURGICAL PROCESSING sulfo-arsenide ore, gold mineralization is associated with both phases and related to partially brecciated, vein-fill textures. The main minerals (>5% by volume) are quartz, carbonates, arsenopyrite and sericitic/clay minerals. The observed acces- sory minerals (<5% by volume) are tetrahedrite/freibergite, sphalerite, galena, pyrite, native gold/electrum, chalcopyrite and traces of sulfosalts/telluride. Gold occurs locked in quartz or associated (locked or exposed) with the sulfide minerals (arsenopyrite, galena, sphalerite, freibergite or sulfosalts). The gold content is variable and can be up to 600 g/t. Figure 1 shows a typical area, rich in coarse gold grains. From a genetic and morphologic perspective, two aurifer- ous generations can be distinguished. The early native gold generation is the most abundant and has a granular morphol- ogy. A later generation occurs as fillings in microfissures in the minerals listed. Most of the gold grains are fine (<10 µm). However, the major gold content is contained in the coarser µ Figure 1 — A typical area rich in coarse gold grains in a grain sizes (>7 m). Gruesa vein sample. Minerals shown are arsenopyrite (asp), galena (gn) gangue (ga) and gold (Au). This is a Digital image analysis by optical microscopy reflected-light photomicrograph of a polished section from and electron microscopy a fragment of unground ore. The basic methodology (e.g., image digitalization, clean- ing of binary images) is very similar in the case of both Before acquisition of the OM images, the equipment was techniques. There are some minor differences related to the allowed to warm up for one hour to obtain a stable response image analysis software available at each laboratory. The from the video camera. Then, a calibration of the white main difference lies in the fact that in one technique the image balance of the video camera was carried out using a white source is an optical microscope (OM) and in the other an LEITZ reflectance standard. electron microscope (EM). The operational and technological The acquired images from the polished sections were seg- differences between an optical microscope and an electron mented with the image analyzer into three binary images: one microscope yield quite different images, which result in for siliceous gangue, a second for all sulfide minerals (arse- differences in the image analysis routines used in each of the nopyrite, pyrite, sphalerite, galena, chalcopyrite, tetrahedrite, two techniques. etc) and a third for gold. The clear difference between the Complete details on the instrumental setup for the OM reflectance of these phases made it possible to segment using method are part of a Master’s thesis in Spanish (Berrezueta, the color of the image rather than different values for each 2000). Therefore, a brief description of the method is called separated RGB signal. The typical low and high set values for for in this paper. the segmentation were: for gangue: R(0-40), G(0-40), B(0- As indicated, the instrumentation for the gold search by 40); for sulfides: R(60-200), G(60-200), B(60-200) for sul- optical microscopy technique consists of a Leica Q500MC fides; and for gold: R(210-255), G(210-255), B(210-255). image analyzer linked to a Leica DMRXP reflected-light In general, the work procedure consisted of: optical microscope. An additional stabilized power source was used to provide constant and reproducible power to the • the manual selection of the field using a stage with a illumination lamp of the optical microscope and to the entire stepping grid to perform an orderly scan of a polished system. A video camera (CCD Javelin JE-3622X) was con- section of the sample, nected to the microscope. The video camera has a 0.5-in. • the manual focusing of the image, (12.7 mm) pick-up element, an active picture element of 752 • image acquisition, x 562 pixels and a resolution of 480 TV lines. The Y/C output • image segmentation with operator supervision to check of the video camera was connected to a frame grabber in the and allow for fine adjustments and host computer along with the image analyzer. The micro- • automatic measurement of parameters. scope was fitted with an air plan-corrected objective of 20X magnification. There was no ocular lens between the video As indicated, the geometric scale of the acquired image camera and the objective lens. In addition, the C-mount corresponded to 0.41 µm/pixel. It should be emphasized that adapter between the video camera and the microscope had no this geometric scale applies to a digitally magnified image lens. The video camera reproduction was close to 1:1. How- from the objective lens. Therefore, it does not represent the ever, the image was digitally magnified to obtain a 400 x 400 resolution for the method. The theoretical optical resolution pixel image (TIF format) with a geometric scale correspond- (R) for the 20X objective lens used is a function of its ing to 0.41-µm/pixel, equivalent to a virtual system magnifi- numerical aperture (NA = 0.45) and the wavelength of the cation of ~300X.