Properties and roles of brines and hydrocarbons during sphalerite deposition at Pine PointTuesday, November 19, 2019 - 4:20pm to 4:40pm Theatre One
The chemical mechanisms of lead- and zinc-sulfide mineralization in carbonate-hosted ("Mississippi-Valley type") deposits is still contentious. Debate has extended over the past several decades, and is still ongoing. Some workers have suggested that mineralization is controlled by acid-base neutralization; others have argued against this interpretation, in favor of redox reactions. Similarly, some workers have interpreted these chemical processes as driven by fluid mixing, whereas others favor fluid-rock reaction as the primary cause.
The Pine Point system, located near Hay River, NWT, is an excellent natural laboratory to investigate these processes. In addition to widespread zinc and lead mineralization, the system shows unique features such as occurrence of bitumen and native sulfur. Using the new samples collected from drill core and from open pits, we investigated the properties of sphalerite-forming fluids at Pine Point in order to gain insight into the origins of the fluids, and the chemical processes that drove mineralization.
The sphalerite crystals from Pine Point commonly contain primary inclusions of aqueous brine, as well as primary hydrocarbon inclusions composed of liquid petroleum and gas. Microthermometry of the brine inclusions shows that they are highly saline brines (~30 wt% NaCl) and enriched in CaCl2, signatures that suggest derivation from evaporated seawater or fluids that have interacted with marine evaporites. Raman analysis of the petroleum inclusions shows that they contain bitumen and light hydrocarbons. Laser ablation analyses of the brine inclusions shows that they are highly enriched in lead, up to several thousand ppm. The petroleum inclusions, in contrast, contain only trace metals below detection limits. Petrographic analysis shows that both inclusion types (brine and hydrocarbon) were trapped as the sphalerite grew, which in turn indicates that both metal-rich brine and petroleum were present during the sphalerite deposition.
Together, these observations broadly support a genetic mechanism involving chemical interaction between saline brines and petroleum, and provide new insight into the chemical processes of mineralization in this system.