| Summary: | The Nanisiyik deposits located in Northern Baffin Island, are stratabound dolostone-hosted sulfide deposits which display many similarities to Mississippi Valley Type ore deposits. The sulfide mineralization is contained within the upper member of Proterozoic Society Cliffs Formation which is mainly composed of algal laminated dolostones. The host rocks are pervasively dolomitized. The Society Cliffs carbonate sequence acquired its host capability due to extensive dolomitization and accompanied karstification. This process may have also contributed to the metal content of the pore fluids. The occurrence of fracture-filling luminescent-banded dolomite cements produced by ascending warm fluids prior to and partly concurrent with sulfide occurrence have caused the occlusion of porosity. During mineralization, host rock dissolution occurred in the ore site, indicating addition of acid to the fluids by reduction of sulfate by organic materials at the ore site and in part by sulfide precipitation. As a result, the ore occur in space created by corrosive hydrothermal fluids and also cavities and underground channelways suggestive of karstic processes. White sparry dolomite, the major gangue mineral of main stage mineralization, displays a wide range in σ¹³C indicating carbon contribution from various sources. The isotopic compositions of the dolomite gangue suggest that they were partly a by-product of dissolution-reprecipitation of dolostone host rock and partly due to sulfate reduction processes by hydrocarbons and related organic matter which contributed to the CO₂ production. The σ¹⁸O for the ore dolomite spans a narrower range which is consistent, with the temperature variations of the ore fluids during mineralization. The stable isotopid composition of inclusion fluids of the minerals from the main stage mineralizaiion obtained directly by crushing the minerals, do not appear to differ substantially from one another. The hydrogen and oxygen isotope data from the fluid inclusions are consistent with derivation of metals predominantly by exchange reaction of the fluids with large volume of metalliferous sediments. This indicates that the water/ rock ratio in the hydrothermal system was probably low and isotopic exchange occurred under moderate temperatures. These data point to the similarity between the ore fluids and oil field brines. This similarity suggests, that the hydrothermal lead-zinc deposits are genetically related to oil formation and accumulation of the same age. Based on the sulfur isotope fractionation between co-precipitating sulfide minerals, the temperature of sulfide precipitation is estimated to be in a range 90 to 250ºC. These temperatures are in accordance with data obtained by fluid inclusion geothermometry. The uniform sulfur isotope ratios suggest that all of the sulfur in the deposits was derived from a homogeneous source. The heavy sulfur isotope ratios indicate that seawater acted as the source of sulfide for precipitation of metals. The presence of residual bitumen, and white sparry dolomite precipitation in conjunction with high σ¹⁸O sulfides suggest that reduced sulfur, for precipitation of the sulfide was supplied by in situ inorganic reduction of sea water sulfate. Geological evidence and isotopic features are consistent with the genetic scheme that base metal mineralization was caused by dewatering of the metalliferous sediments in a rift-related basin. The heat needed to derive this system is thought to have been provided by rifting. H₂S needed for mineralizatlon was produced by chemical reduction of seawater sulfate by hydrocarbons. Therefore evidence relating to ore genesis at Nanisivik is in accordance with a mixing model where the fluids carrying metals and hydrocarbons were guided through the faults into the Upper Society Cliffs dolostones and subsequently interacted with sulfate and possibly metal bearing fluids. Sulfide and carbonate gangue precipication occurred when the system became supersaturated with respect to sulfide and carbonates. Doctor of Philosophy (PhD)
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