Lead-isotope and trace-element geochemistry of Paleoproterozoic metasedimentary rocks in the Lead and Rochford basins (Black Hills, South Dakota, USA): Implications for genetic models, mineralization ages, and sources of leads in the Homestake gold deposi07/01/2009
Mixed, silicate–carbonate–sulfide facies, banded iron formations with elevated detrital components were deposited in the Northern Black Hills during middle Paleoproterozoic time, between 2.012 and 1.974 Ga (Homestake Iron Formation and equivalents) and at ≤1.887 Ga (Rochford Iron Formation). These iron formations, deposited atop shales and basalts in intracontinental rifted basins separated in time by ∼80–130 Myr, are the hosts of extensive, late Paleoproterozoic gold mineralization. Gold/sulfide ore bodies occur within dilated segments of late-stage ductile–brittle shears that formed after isoclinal folding related to Wyoming–Superior continental collision. Lead stepwise leaching (PbSL) data for monazite-bearing garnet separated from a sample of Homestake Iron Formation has yielded an isochron age of 1746 ± 10Ma (2σ; MSWD = 0.42), which represents a maximum age for both the isoclinal folding and subsequent gold mineralization. Monazite-bearing garnet from a nearby “Harney Peak”-type granitic pegmatite, believed to be coeval with latest-stage, semibrittle shears and quartz veins that crosscut Homestake ore bodies, has yielded a PbSL isochron age of 1713 ± 10 Ma (2σ, MSWD = 4.1), which probably represents a minimum age for the gold event.
Pb isotopes of sulfides from “replacement ore” and “shear ore” within the Homestake Iron Formation cannot distinguish between these distinct ore types. However, both Pb isotopes and REY (rare earth and yttrium) abundances indicate slight disequilibrium between the ore sulfides and the barren iron formation, thus revealing an epigenetic (sensu stricto) component to the gold mineralization. The Pb-isotope characteristics of the ore further reflect an admixed component with low 232Th/204Pb and variable Th/U ratios in addition to compositions directly derived from BIF-typical leads. This added component corresponds to Pb from the anoxic metasedimentary rocks directly underlying the iron formations and from the nearby pegmatitic granite. The apparent Pb-isotopic match between ore and granite is further consistent with the PbSL data for “shear ore”-type arsenopyrite, which define an isochron age of 1719 +38-45 Ma (2σ, MSWD = 0.2), and by a striking overlap of the Pb-isotopic data fields defined by the Homestake galenas and pegmatitic feldspars. The Precambrian ore leads are therefore best explained as mixtures of magmatic-hydrothermal and host-rock-mobilized leads.
The Lead area also contains significant gold-bearing deposits of Tertiary age, which are hosted in both Paleoproterozoic and Phanerozoic rocks. Tertiary mineralization was temporally and spatially related to coeval plutons emplaced during Laramide uplift of the Black Hills and represents the final mineralization event recognized in the Homestake Mine area. Like the Precambrian galena leads, the leads of Tertiary galena hosted by middle Paleoproterozoic rocks (including the Homestake Iron Formation) define a trend best explained by mixtures of Pb derived from magmatic fluids exsolved from Tertiary intrusives and Pb leached from the same country rocks that contaminated the granite-derived hydrothermal fluids ascending through them during the much earlier shearing-and-mineralization event inferred at ∼1730–1715Ma.