Submarine deep-sea tailings deposition is an alternative method of the land-based mine-waste disposal, whereby mineral ore is converted into slurry and transported via a pipeline to processing plants located at the coast. The tailings are then discharged into very deep water offshore where they usually cover large parts of the sea bottom. This type of deposition is applied only in few countries: Norway, Papua New Guinea, Philippines, Chile and Turkey. It is, however, a controversial method of mine tailings disposal primarily due to the hazardous chemicals used during processing and deposited metal sulfides which may dissolve and affects the sea environment. The overall objective of this project is to evaluate the copper leaching potential from tailings on the example of the proposed submarine tailings disposal in Repparfjorden discharged from two hydrothermal copper vein type deposits - Nussir and Ulveryggen, exploited by Nussir ASA. The two deposits are 5 km apart and will be processed by the same processing plant. This experiment gives better understanding of the reactivity of sulfide minerals in a saline system during submarine deposition. Planned by Nussir ASA, processing the ore by a flotation system will produce two types of flotation tailings material. These two tailings streams will be mixed with the flotation water, possibly aided by a flocculent, which will be extracted in a thickener afterwards. Seawater will then be added to the material prior to being discharged via a pipeline into the fjord that is at least 90 meters deep. Four kinetic test set-ups containing 0.5 kg of material and 2 L of seawater were prepared for an experiment that runs for 100 days, by recirculation of the leachate (batch experiments). Seawater was routed through pipes at a constant flow rate of 1-2 m/h via a peristaltic pump which arose from about 4 L/h water exchange rate in the column. The columns were run at a temperature of approximately 10°C. The seawater leachate was saturated by oxygen up to 80-100% in order to maintain condition providing sulfide oxidation leaching. Nine leachate samples were collected from each column for analysis of dissolved constituents and physical parameters. Results show that there was a distinct increase in copper concentration while other hazardous elements did not show significant changes. Copper concentrations reached 0.025 mg/L after 100 days. Towards the end of experiments (from 70-100 days), the leaching rate leveled off with time. Subsequent Cu concentrations indicated that there no more copper was released. Reaction was halted likely because sulfide particles have been consumed or encapsulated by iron hydroxides. The leaching reaction rate observed in this test resulted in a much lower copper contamination than previous estimates (Akvaplan-NIVA, 2011). Following the experiment results the estimated reaction rate is approximately 12.7 mg/m2/year, what involves approximately 5.75 kg/year of Cu release for proposed 2.4 km2 of tailings deposition area or approx. 9.75 kg/year in 4.0 km2 after a year from disposal closure. This is significantly less than the annual copper discharge from a river naturally leaching deposits (500-600 kg/year). It was assumed that the copper is being leached only from the surface of the tailings deposit. This is primarily due the saturated condition of the sediments which prohibits oxygen supply through the deposit. Therefore available oxygen is quickly used up, thereby halting sulfide oxidation (Walder & Rzepka 2013). This research revealed that copper leaching is a two-stage process: (1) initial rapid dissolution of secondary Cu minerals formed from previous sulfide oxidation, related to the tailings material disposing period, and (2) long term slow surface-based leaching of sulfide from tailings material settled on the bottom.