ORDOVICIAN AND EARLY MISSISSIPPIAN IRON FORMATIONS ASSOCIATED WITH VOLCANIC-HOSTED MASSIVE SULFIDE DEPOSITS, BATHURST, NEW BRUNSWICK AND FINLAYSON LAKE AREA, YUKON: CONSTRAINTS ON DEPOSITIONAL SETTING AND MINERALIZING CONDITIONS

Spatial and temporal relationships exist between certain iron formations (IF) and many massive sulfide deposits. Such IFs are generally thought to be fossil sediments deposited from seafloor hydrothermal vent fluids. Particulates were carried upward in a buoyant hydrothermal plume until they reached neutral density and were dispersed laterally from the vent site. The sulfide, carbonate, sulfate, silicate, or oxyhydroxide plume particulates were sedimented largely due to gravitational settling and were dispersed on a scale of km to tens and even hundreds of km. The Bathurst Mining Camp (BMC), New Brunswick, is a Middle Ordovician bimodal volcanosedimentary sequence thought to have formed in a continental back-arc basin. Here, IF generally overlies many of the massive sulfide deposits and extends laterally away for over 12 km of strike length. Together with fluid inclusion evidence from some of the massive sulfide deposits, this suggests that the hydrothermal fluids formed buoyant plumes. Three distinct types of IF are recognized in the BMC: carbonate- (siderite), oxide- (magnetite and/or hematite) and silicate- (chlorite) predominant. Geochemical data indicate that in addition to direct hydrothermal input of certain elements, these IFs contain variable contributions from clastic detritus and seawater. The hydrothermal component is generally greater than 40 wt.%. Element associations indicate that Fe, Mn, Pb, and Zn are generally of hydrothermal origin, whereas Al and Ti originate from detrital clastic material. Similar signatures are recorded in IF associated with Early Mississippian massive sulfides in the Finlayson Lake Area (FLA), Yukon and which are also thought to have been formed in a continental back arc setting. Evidence for anoxic conditions in the BMC and in the FLA comes from heavy sulfur isotope values of massive sulfides. The redox conditions of the ambient water column in the BMC back-arc influenced the nature of the IF particles deposited. Anoxic to transitional conditions favored the precipitation of iron carbonates, whereas oxygenated conditions favored oxides. Negative Ce anomalies in some IFs also indicate more oxic conditions. Positive Eu anomalies reflect high-temperature venting. However, not all IFs possess such anomalies, and those lacking such anomalies likely reflect lower temperature (<250°C) venting.