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Issues affecting heap biooxidation of low-grade refractory gold ore: Formation of secondary sulfates, ore lithology, alteration and sulfide mineralogy at Gold Quarry, Carlin, Nevada
AuthorSherlock, Wesley Kingston
AdvisorThompson, Tommy B.
Geological Sciences and Engineering
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The Gold Quarry mine is located in the Maggie Creek District in the northern section of the southern half of the Carlin Trend, 11 km north of the town of Carlin, Eureka County, Nevada. The primary metal of interest is gold. The majority of the ore is sulfidic-refractory consisting primarily of homogenously distributed extremely small gold particles, generally ranging in size from colloidal to approximately 50Å, hosted primarily as a solid solution within the structural lattice of arsenian pyrite rims which surround some pre-ore pyrite crystals (Arehart et al., 1993). Arsenian pyrite is also commonly found as discrete fine-grain disseminated crystals or in local fine-grain masses or clouds. High-grade sulfidic refractory ore is processed via roasting methods where as, the low-grade ore cannot be economically processed through the roaster and is instead oxidized by a cost mediated biological heap method on three nominal 800,000 ton pads. The oxidized low-grade ore is then utilized as supplementary mill feed. Recycling of the biooxidation fluid over time has resulted in a solution highly saturated in sulfate and various metals of which iron and aluminum are the largest contributors to sulfate formation. Consequently, local areas within the heap pad that experience dehydration may experience substantial secondary hydrous sulfate mineral accumulations. These sulfate mineral accumulations locally reduce permeability and may channel the oxidizing solution and induced airflow as well as limiting diffusion within the aggregate itself resulting zones of reduced oxidation and therefore reduced gold recovery. A significant percentage of the heap, visually estimated during visits to unloading pads in 2005 and 2006 at 25 to 30%, was locally isolated from a balanced biosolution and air flow mix due to channelization or the formation of overlying sulfate umbrellas and was consequently only minimally oxidized. There are many complex factors affecting the efficiency and effectiveness of the heap style biooxidation process. It is clear however, that sulfate formation throughout heap plays a very significant negative roll in the oxidation process. Elevated heap core temperatures, large fluxuations in peripheral heap zone temperatures, and extreme low pH values may temporarily serve to decimate local populations of iron oxidizing microbes. The temperature in the core area of the heap periodically exceeds 87°C. This is well above the maximum temperature survivable by most bacteria and may also be above the upper limits for the archaea strains present. Both in-situ columns and laboratory columns were utilized for the study. The laboratory columns were operated under parameters intended to simulate extreme conditions present locally within the heap both spatially and temporally rather than under optimal conditions as is normally done in process labs. This allowed for phenomena such as sulfate channeling of air and fluid to be replicated. Many sulfates were identified including several iron sulfates thought to be the primary contributors to the sulfate formation. Aluminum was found to also be an important element in the formation of sulfate second only to iron. Aluminum occurs as aluminum sulfate (alunogen) as well as in several aluminum-iron sulfates. This complicates solution management as aluminum precipitation requires a higher solution pH than iron. Lithology, specifically as related to permeability and alteration primarily as related to silicification and argillazation, are also major controlling factors regulating overall sulfide oxidation percentage within the heap. Given certain lithologic parameters, oxidizing fluids may only penetrate those sulfides directly exposed at the surface of the aggregate or along fractures. The average sulfide size rarely exceeds 1 mm resulting in a very narrow oxidation halo in low permeability lithologies. Petrographic examination of thin sections from the biooxidized samples commonly display little or no oxidation below the aggregate surface. The most porous and least silicified samples collected rarely display oxidation deeper than 2 to 3 mm below the aggregate surface. A detailed understanding of sulfide mineral paragenesis and corresponding gold mineralizing events is important in determining the optimum duration of the biooxidation cycle for a specific lithologic host and its associated ore mineral assemblage. Multiple arsenian mineralization events may not all coincide with the gold mineralizing event and therefore may require more or less complete oxidation than currently perceived. Increasing silicification in conjunction with waning ore mineralization may also have caused varying degrees of late stage silica encapsulation or entrainment thereby locally inhibiting biooxidation. This study has also recognized the apparent coprecipitation of silica with the arsenian pyrite rims. Scanning electron microscope and microprobe analysis of arsenian pyrite rims revealed that some of the rims contain silica within the arsenian pyrite matrix. Locally extensive extremely fine-grain arsenian pyrite is common in certain lithologies or structural zones such as breccias or debris flows and may also be more easily isolated due to silica encapsulation.