Placer mining involves washing stream gravel to separate and save free gold particles. Usually a placer mine must discharge excess effluent water, which contains some residual suspended fine silt and clay particles, from a settling pond, back into the stream. This study examines the applicability of introducing placer effluent through a diffuser to lessen environmental impact. A diffuser is a mechanism which introduces effluent fluid into the main current of a receiving stream so that mixing and dispersion are facilitated. A diffuser usually consists of a header pipe with one or more discharge ports. Effluent is ejected from the ports at high velocity into the receiving water. Performance of the diffuser depends on the diameter, spacing, and angle of the ports in relation to the water column. Diffusers are used effectively in many industrial applications. Computer programs have been developed which predict the effluent plume from a diffuser, using input data from both the receiving stream and the effluent discharge. We used the CORMIX model, which was developed for the U.S. Environmental Protection Agency. This program can also be used in the design of diffusers. We modelled two series of scenarios using receiving water data for the Fortymile River. In one series, we varied the concentration of suspended solids in the effluent. In the other series, we varied the flow rate of the effluent. In alll cases from both of these series of models, the CORMIX program predicted that the effluent diffused into the receiving water rapidly. A water quality objective of 12.5 mg of suspended sediment per litre of water was achieved in a very short distance downstream of the diffuser. We also used the CORMIX model to predict effluent behaviour in some hypothetical receiving streams. These scenarios showed that the larger the receiving stream, the better the diffusion rate. It also showed that diffusers would not be as effective on small streams where large effluent concentrations are discharged. Dilution is limited by the flow rate of the receiving water. The CORMIX model cannot predict whether sediment from the effluent plume will settle out on the streambed. Our team hydrologist examined the question of potential sedimentation by using the following techniques:: comparing background to introduced sediment levels, calculating the shear/fall velocity ratio for the sediment, comparison to flume testing, and comparison to other field investigations. He predicted that in the example of the Fortymile River using our mine effluent parameters, there would be little or no sediment deposited on the streambed from a diffuser installation. The use of diffuser systems to disperse placer effluent into receiving water appears to be an effective way of mitigating the impact of placer mining on the aquatic environment, providing there is adequate flow in the receiving stream. Effluent is dispersed into the stream flow, reducing the possibility of sedimentation of the stream substrate.

In the last five years, eroding gold prices, increasing production costs and the depletion of reserves have resulted in a dramatic increase in the use of drilling to evaluate placer deposits. Accurate sampling and deposit evaluation would enable planning for cost-effective mining and reclamation. However, sampling placer gravel accurately is an extremely difficult task due to the nugget effect (inclusion or loss of a single particle of gold) and any errors are compounded by the small size of the drill samples. Additional sampling errors result from contamination, splitting and fire assaying. More placer mine failures can be attributed directly to improper sampling and sample processing practices during property evaluation than to any other cause. There is very little impartial, accurate information available to guide the selection of modern drills. Drillers and their equipment are often selected for their penetration rate or cost-per-foot rather than for sampling accuracy or gold recovery. A brief description of several types of drills including churn, auger, rotary tricone, reverse circulation, Becker hammer, down-the-hole hammer and Sonic drills is summarized in Section 6 from references. Three solid auger drills and two types of reverse circulation (R/C) drills were evaluated under typical Yukon field conditions using radioactive placer gold as tracers (radiotracers). A frozen cylindrical core of compacted gravel containing four sizes (-10+14, -20+28, -35+48 and -65+100 mesh) of radiotracers was placed in 35 drill holes and the holes were redrilled. Hand-held scintillometers were used to track gold losses during drilling, sample recovery and sample processing. Radiotracers lost due to spillage and blow-by around the collar (top) of the hole, and those trapped in drilling equipment (carry-over) were easily located. The results of these tests are summarized Table 1. There was no significant difference between the recovery of the four sizes of gold particles with any of the reverse circulation or auger drills tested. Observations and down-hole scintillometer records indicate that the radiotracers did not follow the bit down the hole and were either carried out of the hole or forced onto the sides of the hole at or above the depth at which the radiotracer core was positioned.

In the last five years, eroding gold prices, increasing production costs and the depletion of reserves have resulted in a dramatic increase in the use of drilling to evaluate placer deposits. Accurate sampling and deposit evaluation would enable planning for cost-effective mining and reclamation. However, sampling placer gravel accurately is an extremely difficult task due to the nugget effect (inclusion or loss of a single particle of gold) and any errors are compounded by the small size of the drill samples. Additional sampling errors result from contamination, splitting and fire assaying. More placer mine failures can be attributed directly to improper sampling and sample processing practices during property evaluation than to any other cause. There is very little impartial, accurate information available to guide the selection of modern drills. Drillers and their equipment are often selected for their penetration rate or cost-per-foot rather than for sampling accuracy or gold recovery. A brief description of several types of drills including churn, auger, rotary tricone, reverse circulation, Becker hammer, down-the-hole hammer and Sonic drills is summarized in Section 6 from references. Three solid auger drills, two types of fully cased normal circulation (N/C) drills and two types of reverse circulation (R/C) drills were evaluated under typical Yukon field conditions using radioactive placer gold as tracers (radiotracers). A frozen cylindrical core of compacted gravel containing four sizes (1.2-1.7, 0.60-0.84, 0.3-0.42 and 0.15-0.21 mm) (-10+14, -20+28, -35+48 and -65+100 mesh) of radiotracers was placed in 44 drill holes and the holes were redrilled. Hand-held scintillometres were used to track gold losses during drilling, sample recovery and sample processing. Radiotracers lost due to spillage and blow-by around the collar (top) of the hole, and those trapped in drilling equipment (carry-over) were easily located. The results of these tests are summarized Table 1. There was no significant difference between the recovery of the four sizes of gold particles with any of the fully cased nomal circulation, reverse circulation or auger drills tested. Observations and down-hole scintillometre records indicate that the radiotracers did not follow the bit down the hole and were either carried out of the hole or forced onto to the sides of the hole at or above the depth at which the radiotracer core was positioned.

Even though standard churn drilling techniques may be recommended in placer gold sampling programs, other methods may prove to be more satisfactory. It is necessary that other methods also be considered in an attempt to lower costs. The time required to drill a hole must be considered too, especially in the evaluation of potentially large economic auriferous gravel deposits. Each placer deposit may have certain characteristics and ultimate requirements which will prevent any one technique from being considered "best". Consequently, several types of evaluation methods are described.

This report is an account of our work testing the viability of floater dredging for placer gold in permafrost. Floater dredging is a method of mining which uses a hydraulic excavator to feed placer gravel to a floating gravel processing plant. We wanted to see if the digging face of a dredge pond dug in permafrost would thaw fast enough to supply gravel for a production-scale mining operation. We stripped the vegetation and muck from an area on a permafrost terrace above the Fortymile River. We dug a pit in the stripped area and pumped it full of water to form a pond in which to operate our floater dredge. Water from the pond seeped into the surrounding gravel, thawing as it penetrated. We confirmed that when frozen gravel is exposed to water, it thaws faster than when exposed to air. We experimented with various methods of accelerating the rate of thaw at the dredging cut face. We found that sloping the cut face at 30° to the vertical exposed more surface area to the thawing action of the water, without sacrificing digging efficiency of the excavator. We found that equipping the excavator with spade-type abrasion teeth gave the best combination of penetration, digging efficiency, and wear. Towards the end of the experiment, we found that water had penetrated at least 12 feet back from the cut face. We believe that the artificial water-table, created by seepage from the pond, may have started thawing large blocks of the permafrost. Further work is required to test this theory. We found that the cut face thawed back at a rate of at least 2 feet in 24 hours. With a long enough face, this would provide enough thawed gravel to support a production-scale operation. A 600-foot cut face is required for a 100 yd/hour operation over a 10-hour sluicing day. The gravel did not thaw at an even rate along the face, and we dug more gravel in the thawed areas, causing the face to become uneven. This made mining difficult because the cut became disorganized. The solution to this problem was to even the cut face by using a dozer to push thawed material from the stripped area in front of the face into the sections of the cut where the deepest advances had been made. Using this technique, we were able to dredge the frozen ground successfully on a sustainable basis. Floater mining offers an option to miners working permafrost deposits. However, because there are factors limiting its applicability, a careful analysis should be made before committing resources to this type of operation.

In order to facilitate exploration and mining of placer gold in historic placer regions increasingly innovative mining and exploration methods must be employed. Using a combination of historic and modern exploration tools, it is possible to delineate placer gold deposits in areas which were previously thought to be exhausted of reserves. These placer settings are sometimes referred to as ‘technogenic’, a term used to describe original gravel remnants and gold-bearing tailings which formed as a consequence of inefficient technology or poor mining techniques. These deposits may now be exploited using modern exploration and mining methods.

A reconnaissance study of the composition of gold from several placer streams in the Stewart River map area was carried out to characterize the likely style(s) of lode mineralization from which the placer gold in each stream was derived. Results of the study indicate that placer gold from Eureka and Black Hills creeks, as well as gold grains from colluvium in exploration pits at the head of Eureka Creek, have relatively low fineness, low copper contents and high mercury contents. These compositions are consistent with both the gold in colluvium and most of the placer gold having been derived from epithermal sources in the Eureka Dome or Henderson Dome area. Gold in placers in the Moosehorn Range is likely derived from intrusion-related, gold-bearing quartz veins exposed in the headwaters of the placer creeks, and is characterized by relatively high fineness, high copper contents and low mercury contents. Placer gold in Thistle, Kirkman and Blueberry creeks is very similar to that from streams in the Moosehorn Range, suggesting that an undiscovered intrusion-related gold deposit is present within the Thistle/Kirkman drainage basin.

Placer gold is widely distributed throughout the western Yukon; however, lode sources for most of these deposits remain unknown. Previous studies of gold compositions in this region using scanning electron microscope (SEM) and electron microprobe (EMP) methods showed 1) that there are consistent differences in average composition (although with considerable overlap) between gold from different styles of lode gold mineralization; and 2) the composition(s) of placer gold can be matched with specific lode sources, or the most likely style of lode source can be identified. In the current study we employ SEM and EMP methods together with laser ablation ICP-MS trace element analysis and study of the micro-inclusion suite(s) to more completely characterize the major, minor and trace element composition of the gold as well as the mineralogy of the lode sources themselves. We also report new data for placer and lode gold, mainly from the Klondike District.

This report reviews methods and criteria for placer mining settling pond design. The mining and processing of placer gold generates wastewaters containing high concentrations of fine sand, silt and clay. Reduction of sediment discharges is required primarily to minimize impacts of sediment and turbidity on the aquatic environment and fish. Sediment discharge control to avoid sedimentation of water supply intakes of downstream mining operations and to allow recycling of process water in water short areas are secondary factors.

This project undertook a literature review on particle generation when soil or rock material is subjected to dredging processes, the introduction of the particles into the water column (dredge-induced resuspension) and the early stages of dredge plume development. Knowledge in these areas is required to specify suspended sediment source terms for input to dredge plume prediction models that are used in environmental impact assessment of dredging proposals. The report focuses on the two types of hydraulic dredgers that are most commonly employed in major dredging projects in Australia, namely the trailing suction hopper dredger (TSHD) and the cutter suction dredger (CSD). The report responds to Task 2.1 of the WAMSI Dredging Science Node Science Plan (Masini et al. 2011). This report presents a review of available knowledge on the generation/development of sediment particle size characteristics by dredging processes, and the mass release rates (resuspension rates) from dredging activities. This report responds to Task 2.1 of the WAMSI Dredging Science Node Science Plan (Masini et al. 2011).