Browsing University of Alaska Fairbanks by Subject "Gold ores"
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Dynamic simulator for a grinding circuitThe grinding circuit is a primary and indispensable unit of a mineral processing plant. The product from a grinding circuit affects the recovery rate of minerals in subsequent downstream processes and governs the amount of concentrate produced. Because of the huge amount of energy required during the grinding operation, they contribute to a major portion of the concentrator cost. This makes grinding a crucial process to be considered for optimization and control. There are numerous process variables that are monitored and controlled during a grinding operation. The variables in a grinding circuit are highly inter-related and the intricate interaction among them makes the process difficult to understand from an operational viewpoint. Modeling and simulation of grinding circuits have been used by past researchers for circuit design and pre-flowsheet optimization in terms of processing capacity, recovery rate, and product size distribution. However, these models were solved under steady approximation and did not provide any information on the system in real time. Hence, they cannot be used for real time optimization and control purposes. Therefore, this research focuses on developing a dynamic simulator for a grinding circuit. The Matlab/Simulink environment was used to program the models of the process units that were interlinked to produce the flowsheet of a grinding circuit of a local gold mine operating in Alaska. The flowsheet was simulated under different operating conditions to understand the behavior of the circuit. The explanation for such changes has also been discussed. The dynamic simulator was then used in designing a neural network based controller for the semi-autogenous mill (SAG). A two-layer non-linear autoregressive (NARX) neural network with feed to the mill as exogenous input was designed using data generated by the simulator for a range of operating conditions. Levenberg-Marquardt (LM) and Bayesian Regularization (BR) training algorithms were used to train the network. Comparison of both algorithms showed LM performed better provided the number of parameters in the network were chosen in a prudent manner. Finally, the implementation of the controller for maintaining SAG mill power to a reference point is discussed.
Geology and origins of the Mike Lake (Skarn Ridge) gold-copper skarn deposit, Yukon Territory, CanadaThe Mike Lake (Skarn Ridge) deposit has an elemental suite of Cu-Au-Bi-As-Sn and a mineralogy dominated by scapolite, clinopyroxene, and pyrrhotite, with lesser garnet and Fe-axinite (a Ca-borosilicate). This study is the first published description of the deposit. The deposit was studied with techniques including drill core logging and detailed surface mapping (1:5,000 scale), combined with petrographic examination of polished thin sections, X-ray fluorescence and X-ray diffraction analyses, electron microprobe analysis of major minerals, and ⁴⁰Ar/³⁹Ar dating. Ore mineralization styles include vein-controlled, disseminated, and net-textured replacements of clinopyroxene and calcite by electrum, chalcopyrite, pyrrhotite, and arsenopyrite, with variable native bismuth and bismuth tellurides. A strong Au:Bi correlation (R² = 0.74) indicates the two elements were transported and deposited together; however a poor Au:Cu correlation (R² = 0.23) suggests different mineralization events or different modes of Au-Cu transport. The virtual absence of retrograde alteration provides an ideal opportunity to examine metal- and silicate-zoning patterns apparently associated with prograde alteration. Using the ⁴⁰Ar/³⁹Ar dating method, I have determined that the adjacent Mike Lake pluton is younger than the skarn, and hence, genetically unrelated. Through analysis of samples from surface and 72 drill holes, I show systematic zoning in skarn mineralogy and mineral compositions suggesting deposit derivation from an unknown pluton to the southeast at depth.
Gold and base metal mineralization of the Dolphin intrusion-related gold deposit, Fairbanks Mining District, AlaskaThe Dolphin deposit is an intrusion-related gold deposit (IRGD) located approximately 30 km north of Fairbanks, Alaska. The deposit is in--and adjacent to--a composite mid-Cretaceous pluton intruding amphibolite facies metamorphic rocks. An NI43-101 compliant gold resource estimation for the deposit (utilizing a 0.3 g/t cut-off grade) is 61.5 Million tonnes (Mt) at 0.69 g/t indicated (1.36 million oz = Moz) and 71.5 Mt at 0.69 g/t inferred (1.58 Moz). Due to extensive hydrothermal alteration of the intrusion, identifying rock types in hand sample and thin section, as well as by standard compositional techniques (e.g., SiO₂ vs. Na₂O + K₂O), has proven problematic. By plotting wt % TiO₂ vs. P₂O5 obtained from XRF analyses and four-acid digest ICP-MS data, two distinct population clusters appear. By comparison with least-altered intrusive rock analyses from the Fairbanks district, the igneous units were originally granite and tonalite. Because there is no gradational transition through an intermediate granodiorite unit, they were most likely derived from two separate magmatic bodies rather than in-situ fractionation from a single parent. Tonalite is concentrated along the northern and eastern margins of the stock with granite composing the rest of the body. Tonalite xenoliths in granite and granite dikes intruding tonalite prove that tonalite is the older unit. Investigations of hydrothermal alteration (based on chemical analyses, X-ray diffraction, and thin section examination) show albitic and advanced argillic (kaolinite-quartz) alteration are the dominant styles with sericite common throughout. Advanced argillic is a low temperature (<300°C) low pH alteration style that has not been previously identified in intrusion related gold deposits (IRGDs) in interior Alaska. Albitic alteration probably resulted from higher temperature, more neutral pH fluids. Gold investigations show that gold occurs as coarse-grained Au°, aurostibite, and maldonite in quartz + sulfide veins; fine-grained Au° in the oxide zone; and in many forms in disseminated sulfide. These forms include Au° inclusions in pyrite and arsenopyrite; solid-solution Au within compositionally zoned arsenopyrite; and as Au° nanoparticles in pyrite and probably arsenopyrite. Using UAF's JEOL JXA-8530F microprobe, I found that solid-solution gold occurs only in arsenopyrite with strong compositional zoning. Such grains are always small (< 0.2 mm) and commonly have low As cores; gold- bearing mantles with moderate % As; and higher As rims. In contrast, compositionally homogenous arsenopyrite does not contain detectable solid-solution gold. Pyrite is commonly arsenian and carries dissolved gold (if any) near detection limits. Gold mineralization has not been tied to any one lithology or alteration style; however, gold does seem to correlate with abrupt changes in alteration, especially between sericite + albite and kaolinite + sericite alteration. Gold-bearing, zoned arsenopyrite is predominantly associated with advanced argillic alteration and apparently represents a rapid growth, disequilibrium phenomenon.