Geology of the Sturgeon Lake copper-zinc-lead-silver-gold deposit

CIM Bulletin, Vol. 75, No. 846, 1982

P.W.A. SEVERIN, Senior Exploration Geologist, Corporation Falconbridge Copper, Winnipeg, Manitoba

The Sturgeon Lake deposit, a partly eroded lens of massive sulphides with an underlying sulphide stringer zone, is one of five orebodies that occur in the Sturgeon Lake area of northwestern Ontario within volcaniclastic rocks of the Archean Wabigoon Subprovince in the Superior Province of the Canadian Shield.The Sturgeon Lake Cu-Zn-Pb-Ag-Au deposit was discovered in October 1970 as a result of an I. P. survey. Production from this open-pit mine began in March 1974 and continued until the ore was exhausted in March 1980.The Sturgeon Lake property is underlain by basaltic volcanics, minor amounts of mafic lapilli tuff and rhyolite volcaniclastics. A quartz crystal-bearing rhyolite occurs at the top of the felsic volcaniclastics and is "capped" by the Sturgeon Lake massive sulphide deposit. The felsic volcanics have been intruded by a crudely layered sill-like, carbonatized mafic intrusion. Recent work suggests that the felsic rocks that underlie the Sturgeon Lake deposit are lateral equivalents of those in the vicinity of the Mattabi Mine and are not correlative with the dacitic volcaniclastics that underlie the Lyon Lake and Creek Zone deposits. Hydrothermal processes that resulted in the formation of the Sturgeon Lake orebody have resulted in a depletion of Na2O, CaO and FeO and an increase in MgO, K2O, copper and zinc. In the vicinity of the sulphide stringer zone, the rhyolites are "bleached" due to silicification and serialization. The rocks lying immediately below the massive sulphides are locally strongly chloritic.The Sturgeon Lake deposit may have formed on a topographic high which prevented mafic flows and dacitic volcaniclastics that underlie the Lyon Lake and Creek Zone deposits from inundating the Sturgeon Lake fumarolic vent from the west. Continued or resurgent activity from the Sturgeon Lakefumarole may have resulted in the formation of the Lyon Lake and Creek Zones as more "distal" deposits in topographic lows. The Sturgeon Lake, Lyon Lake and Creek Zone deposits were subsequently overlain by mafic tuffs and flows.The Sturgeon Lake massive sulphides exhibit metal zoning and the sulphide stringers or veinlets form a ragged net-like pattern within the footwall rhyolites and are most heavily developed immediately below the central portion of the massive sulphides.A narrow carbonatized mafic sill/dyke intrudes the hang-ingwall contact of the massive sulphides and crosscuts the north sector of the deposit. Strong foliation in this dyke and intercalated tuff within the massive sulphides at both extremities of the deposit has provided channelways for ground-water circulation that resulted in minor to moderate amounts of oxidation of sulphides and local development of strong chlorite and kaolinite. These alteration products, located in the north and east areas of the deposit, were a source of metallurgical problems during the early stages of production.
Keywords: Geology, Economic geology, Sturgeon Lake deposit, Bell Lake mafics, Beidelman Bay Complex, Massive sulphides, Sulphide deposits, Copper, Zinc.