A second birth of prolific base metal camp
Robin Adair (Donner Metals), Gilles Roy and Michel Dessureault (Xstrata Zinc Canada)

The Matagami Lake region holds a rich mining history that dates back to the late 1900s when Robert Bell first conducted exploration in the area. The region is covered by an extensive sheet of thick glacial till that made early exploration difficult and effectively hid the tremendous zinc and copper resources that were to become the prolific Matagami Mining Camp. The advent of airborne electromagnetic surveys in the mid-1950s provided a significant technological improvement to the ability to detect mineral deposits under cover. This new technology and the post-war demand for base metals led to the discovery of the first zinc-copper massive sulphide deposits at Matagami in 1956. With the opening of the first exploration office in 1979, came timely additional discoveries (Isle Dieu and Norita East – 1985, Bell Allard – 1992, Perseverance – 2001) that supported continuous production until 2003 when the last ore was drawn from the Bell Allard Mine and operations ceased due to strongly depressed metal prices. Perseverance, discovered 50 metres below the airport runway and the second largest deposit in the camp, remained undeveloped. In 2006 Donner Metals and Xstrata Zinc joined forces to continue exploration on an extensive land package surrounding the Perseverance deposit with an exceptional exploration team, new geological ideas, strong financial backing, extensive infrastructure and an extremely supportive government. Donner can earn a 50% interest in the extensive Xstrata land package. This coincided with the announcement that Perseverance would be developed. The first discovery of the new joint exploration effort was made within the first ten drill holes of the program.

Geologically, the Matagami camp is characterized by a prolific development of VMS-type zinc and copper sulphides with eleven known deposits identified to date containing an estimated 10.2 billion lbs of zinc and 979 million lbs of copper metal with by-product silver and gold. The geological setting at Matagami is typical of Archean VMS terrains. It is characterized by volcanic sequences that filled a large, regional synvolcanic basin within which second and third order sub-basins were developed and controlled by synvolcanic faulting. Stratigraphy is layer-cake with a marked change from lower most rhyolite/dacite volcanism (Watson Lake Formation) to andesite/basalt volcanism (Wabassee Formation). The first evidence of basin-wide hydrothermal systems that developed loci along synvolcanic faults is demonstrated by the extensive silica-rich Key Tuffite and locally, sulphide-bearing exhalative/replacement systems that are coeval with, or just following the deposition of, the Key Tuffite. This activity produced the majority of the deposits comprising the Matagami Camp and occurred at a prominent break in volcanism marked by the Key Tuffite, which followed the deposition of the Watson Lake Formation. Other tuffite horizons documented within the overlying Wabassee Formation provide new evidence that the sulphide-bearing hydrothermal systems that produced mineralization at the Key Tuffite horizon persisted with the production of vertically stacked sulphide deposits higher in the stratigraphic sequence. The heat source that drove these hydrothermal systems is interpreted to be the large Bell River gabbroic complex which underlies the core of the Matagami Camp. There are general similarities with the Noranda VMS Camp, typified by stacked horizons of tuffite/sulphide deposition, however significant mineralization at Matagami seemed restricted to the Key Tuffite until Xstrata Zinc and Donner Metals discovered the Bracemac Zone in late 2006.

Deposits of the Matagami Camp historically occur at, or immediately below, the Key Tuffite horizon and are typically underlain by strong, hydrothermal plumbing systems developed within the Watson Lake Formation. These plumbing systems, or hydrothermal “pipes”, are characterized by strong chlorite alteration with sulphide stringers proximal to the pipe, and acute chlorite development with magnetite, silica, talc and sulphides in the core of the pipe. The massive sulphide deposits of the Matagami Camp demonstrate a variety of geometries that indicate deposition as traditional exhalites on the sea floor (platter-shaped deposits and pinnacles) and as replacement deposition in the throat of hydrothermal pipes, as demonstrated by substantial sulphide roots developed within the pipe immediately beneath the sea floor interface. Aspects of each type may be present in a single deposit (e.g., Mattagami Lake), whereas others may demonstrate only one specific characteristic (e.g., Perseverance was developed within the hydrothermal pipe). Invariably, there is strong evidence of synvolcanic faulting that was exploited by the hydrothermal systems and along which hydrothermal activity was locally focused to produce sulphide deposits and their underlying pipe systems. Within the core of the sulphide-bearing vents, the Key Tuffite is often disrupted and may be strongly altered. It is also well demonstrated that multiple sulphide deposits occur along the trends of synvolcanic faults (e.g., Mattagami and Isle Dieu deposits).

The new exploration approach used by the Xstrata/Donner team is designed to continue the exploration of the Key Tuffite as well as to investigate potential for vertically stacked mineralized zones in tuffite horizons at higher stratigraphic levels. Moreover, any anomalous mineralization and/or alteration encountered within the Wabassee Formation has strongly positive implications for potential sulphide deposition at the Key Tuffite, following on evidence that hydrothermal cells capable of producing mineralization at the Key Tuffite can persist along reactivated synvolcanic faults. Using this approach, significant sulphide mineralization and an associated alteration pipe was initially discovered in late 2006 at the Bracemac tuffite level, which occurs within the Wabassee basalts 200 metres stratigraphically above the Key Tuffite (note that the sequence has been inflated by an additional 120 metres by later gabbro sills). Follow-up of this mineralization later resulted in the discovery of the Upper Bracemac Zone with its associated alteration pipe a further 100 metres above the Bracemac discovery. Reversing the traditional approach, the exploration team refocused efforts on the Key Tuffite below these upper sulphide occurrences, resulting in the recent discovery of massive sulphides and a related alteration pipe at the Key Tuffite at Bracemac. In addition, the same approach has led to the discovery of a new sulphide zone 1.2 kilometres to the southeast at McLeod.