The Last Hope Property is located within the Churchill Structural Province of the Canadian Shield, lying within the southern portion of the Lynn Lake Greenstone Belt (see figure below). It consists of tholeiitic to calc-alkaline mafic volcanic and volcaniclastic rocks with minor rhyolite and dacite (Jones, et. al. 2005).
The Lynn Lake Greenstone Belt, comprised of the North and older South Belts, is part of a larger litho-structural unit which extends in a north-easterly direction from the La Ronge Greenstone Belt in Saskatchewan. The rocks in the South Belt consist of lens-shaped volcanic and sedimentary units which have been interpreted as representing overlapping edifices with flanking aprons of volcaniclastic rocks (Gilbert et al. 1980). This linear feature has been termed the ‘Johnson Trend’. The former Burnt Timber open pit deposit (Au) is contained within this trend.
Structurally, the most significant feature in the South Belt is the east-west trending Johnson Shear Zone (“JSZ”), a wide zone of intense brittle-ductile deformation, characterized by faulting, shearing, mylonization and associated silica and carbonate alteration and sporadic gold mineralization. The JSZ is host to at least 26 gold prospects and showings over a 44 km strike length.
The North Belt is a north-facing homocline and consists of rhyolite, overlain by andesite and basalt, sedimentary rocks and an upper basaltic unit. The upper basalts include high alumina and subordinate high magnesia tholeiites. Both the MacLellan deposit (Au, Ag) and the Farley Lake deposit (Au) are located within this belt occurring in a metallotect termed the ‘Rainbow Trend’.
The Last Hope Property is underlain by a west-northwest-striking layered succession. From south to north, this succession is comprised of quartz-feldspar porphyry, mafic tuff, quartzite, mudstone, magnetite-bearing quartzite and feldspathic quartzite. The deposit consists of two shallow plunging ore shoots within a steep, tabular quartz vein that averages 1.5 m in width.
Two parallel quartz veins cut the quartzite, the South Vein and the Mandole Vein, both hosting gold bearing sulphide mineralization while the North Vein is barren.
The Mandole Vein outcrops for approximately 225 m and strikes northwest, dips 80 degrees southwest and fills a fracture in thinly bedded impure quartzite. The wall rocks around the veins are altered for approximately 2.5 cm. The north boundary of the vein is a felsite dyke and is schistose at the contact. The south boundary is a hornblende schist and cherty feldspathic quartzite. Minor amounts of chlorite are present.
The Mandole Vein is 0.3 to 1.2 m wide and can be divided into two units:
- A southern white massive quartz unit; and
- A northern grey aphanitic, siliceous unit with disseminated grains and stringers of pyrite and trace chalcopyrite.
The average sulphide content of the south vein is 5% (local variation up to 15%). The best gold values are found in the highly altered, quartz-pyrite rich footwall of a fault which lies on the periphery of an intrusive.
The Last Hope Deposit can be classified as a mesothermal lode gold deposit in a Proterozoic setting. Mesothermal lode gold deposits typically occur in metamorphosed, supracrustal rocks, most commonly in tholeiitic basalts and komatiites but also in felsic volcanic rocks. Discrete veins occur in deformation zones in greenschist metamorphic domains where brittle or brittle-ductile fracturing is dominant. Veins are emplaced in cross-cutting or layer-parallel shear zones, extensional zones and more rarely in saddle reefs (Klien and Day, 1994). Gold is associated with disseminated sulphide minerals. Gold-bearing sulphide minerals are controlled by minor fractures, and occur in irregular patches in quartz, in the wall rock adjacent to the vein, or as disseminations or replacements in zones of highly altered and deformed rocks. Ore bodies tend to be tabular or rod-shaped formed by persistent or discontinuous veins and irregular bodies of gold bearing quartz. Quartz veins are typically surrounded by haloes of silicification and carbonate minerals.