The Hemlo gold deposit, Ontario, Canada: principal deposit characteristics and constraints on mineralization

Abstract

The Archean, world-class, Hemlo lode gold deposit has been overprinted by deformation and metamorphism resulting, in part, in several proposed depositional models for its emplacement. No single model clearly accounts for all of the characteristics of the deposit. Most proposed models are inappropriate in terms of timing with respect to host rock, deformation, intrusion, or metamorphism, or require inappropriate geometry, metal tenors or metal ratios. The deposit is spatially associated with: high-strain zones; a restraining bend in the Hemlo greenstone belt; and the volcanic–sedimentary contact of the Moose Lake volcanic complex (MLVC). The restraining bend correlates with changes in the style of mineralization and/or alteration, hence, the deposit is subdivided into two segments. The West Segment is west-striking and consists of numerous, low-grade, erratically mineralized, west- to west–northwest-striking zones. Mineralization is locally dominantly fracture-controlled or disseminated. The Main Segment strikes approximately 290°, contains the majority of the ore and consists of two main tabular zones of dominantly disseminated mineralization: the Main Mineralized Zone, within the Lake Superior shear zone, and the Lower Mineralized Zone, within the Moose Lake fault zone. A broad zone of alteration, about 4 km long and as much as 400 m wide, asymmetrically surrounds the deposit. There is ample evidence, at a variety of scales, of D 2 structural control on the various mineralized zones. Tabular zones of alteration and/or disseminated mineralization are parallel to the predominant S 2 fabric in the West Segment, and a composite mylonitic S 2–S 2M–S 3C fabric in the Main Segment. Tabular mineralized zones of fracture-controlled alteration and pervasive alteration are paraconcordant with lithotectonic units. The deposit is ductilely deformed by D 2 and D 3 strains, which are associated with sinistral and dextral transpression, respectively. The deposit was emplaced after F 2 folding, synchronous with the incipient development of high-strain zones during mid-D 2 strain. Post-deposit overprinting by deformation and regional metamorphism has resulted in remobilization and modification of some of the original deposit characteristics. The Hemlo gold deposit is interpreted as an atypical, mesozonal-orogenic, disseminated-replacement-stockwork deposit, broadly synchronous with D 2 and “middle” stage granitoid plutonism, prior to or synchronous with peak regional metamorphism, and involving magmatic±metamorphic fluids. Its emplacement is poorly constrained between 2694 and 2677 Ma. U–Pb systematics are consistent with field evidence for a history of protracted, episodic, progressive strain, intrusive activity, and an evolving hydrothermal system. The chief emplacement controls specifically appear to have been the restraining bend, competency contrast at a major lithologic contact, and permeability in a fragmental unit, with general, more broadly developed, steady-state permeability in ductile zones. Enigmatic barite associated with the deposit is considered an intrinsic product of the hydrothermal system. The orientation of the major part of the deposit is consistent with statistically preferred orientations for large gold deposits in other gold camps of the Superior Province. Other examples of alteration and mineralization in the Hemlo greenstone belt share some common attributes with the Hemlo deposit. These factors provide a basis for selecting favourable exploration targets in the greenstone belt.