Summary:
In the Superior Province, major gold deposits are spatially associated with large-scale regional deformation zones and associated Timiskaming-type metasediments (LeClair et al., 1993). These regional structures are interpreted as zones of transpressive terrain accretion.
Typical greenstone-hosted, mesothermal gold deposits are associated with structurally controlled quartz-carbonate veins hosted by moderately to steeply dipping shear zones displaying brittle to ductile deformational features in low-grade (greenschist) metamorphic rocks. In contrast, the Sugar Zone gold deposit is hosted in medium-metamorphic-grade (amphibolite) rocks that exhibit ductile deformation. The Sugar Zone host rocks have been intruded by felsite and porphyry sills, and exhibit strong association of gold with silica-sulphide-potassic alteration.
The Sugar Zone is interpreted as an orogenic, mesothermal gold deposit located in the SDZ, which is an area of high strain. The auriferous zones of the Sugar zone are interpreted to be highly strained altered mafic flows, intermediate to felsic porphyritic intrusions, and boudinaged auriferous quartz veins. Alteration consists primarily of silicification, biotization, and sulphidization. Hydrothermally altered basalt is recognized as a key component of mineralized zones. Commonly in contact with porphyries within mineralized zones, it is strongly silicified biotitic basalt containing significant amounts of pyrrhotite, and pyrite.
The Upper, Lower, and Footwall mineralized subzones range in thickness from 0.2 m to 6 m, strike at 140° and dip between 65° and 75° to the west.
The mineralized Upper, Lower, and Footwall Subzones of the Sugar Zone lie within the SDZ. They occur within a highly strained assemblage consisting of variously altered mafic volcanic flows, intermediate porphyritic intrusions, and boudinaged auriferous quartz veins. The three zones range in true thickness from about 0.2 to 6 m, and are separated by 20 to 30 m of un-mineralized metavolcanics.
Each Subzone is made up of one or more porphyritic intrusions, flanked by altered basalt, and hosting conformable quartz veins. Alteration within the mafic metavolcanic portions of the subzones consists primarily of silicification that occurs both as pervasive alteration, and as quartz veining, diopsidation, and biotization. The porphyry units of the zones exhibit biotite and silica alteration, but lack diopside alteration.
Gold mineralization mostly occurs in quartz veins, stringers, and quartz-flooded zones predominantly associated with porphyry zones, porphyry contact zones, hydrothermally altered basalts and, rarely, weakly altered or unaltered basalt within the Upper, Lower, and Footwall Subzones. Fine to coarse-grained specks and blebs of visible gold are common in the Sugar Zone quartz veins, usually occurring within marginal, laminated, or refractured portions of the veins. The visible gold itself is often observed to be concentrated within thin fractures, indicating some degree of remobilization. Quartz veins and floods also contain varying amounts of pyrrhotite, pyrite, chalcopyrite, galena, sphalerite, molybdenite, and arsenopyrite. The presence of galena, sphalerite, and/or arsenopyrite is a strong indicator of the presence of visible gold. Pyrite, chalcopyrite, and rarely molybdenite form a minor component of total sulphides, and do not appear to be directly related to the presence of gold mineralization.
The Middle Zone, located between the Sugar and Wolf Zones, may represent the extension of the Sugar Zone to the north. The zone occurs within a highly strained package of massive and pillowed flows exhibiting various degrees of biotite alteration. Gabbro sills and flows are common about the zone. Similar to the Sugar Zone, a weak Upper and Footwall Zone may locally be developed. The zone typically ranges from 0.2 to 6 m in thickness. Middle Zone gold mineralization is often associated with quartz veins and veinlets hosted within a package of altered mafic volcanic and feldspar porphyry. The gold mineralization is often accompanied by 1%–5% pyrrhotite, and pyrite with local sections of minor galena and sphalerite. Galena is normally indicative of higher grades and the presence of visible gold.
The auriferous Wolf Zone lies along strike of the Sugar Zone, and may represent the northern extension of the SDZ. It is defined as highly strained packages consisting of variously altered mafic volcanic flows and gabbros. The zone ranges in true thickness from 0.2 to 3.0 m. The Wolf Zone is made up of highly sheared mafic metavolcanics and a network of intrusive, intermediate quartz-feldspar porphyry dykes/sills. Alteration in the mafic volcanic and gabbro units consists mainly of silicification (both pervasive and quartz veining), diopside alteration, and magnesium-rich, brown biotite alteration. Alteration within the intermediate porphyry units consist of mostly silicification, with small amounts of magnesium-rich brown biotite, and no diopside. The zone is observed in trenches to pinch and swell over 30 m. Wolf Zone gold mineralization mostly occurs in quartz veins, stringers, and quartz-flooded zones predominantly associated with porphyry zones and hydrothermally altered basalts and gabbros. Fine-grained specks of visible gold are occasionally observed in the Wolf Zone quartz veins. The visible gold itself is often observed to be concentrated within thin fractures, indicating some degree of remobilization. Quartz veins and floods also contain varying amounts of pyrrhotite, pyrite, and occasional galena. The presence of galena is a strong indicator of the presence of visible gold. Pyrite and pyrrhotite form most of the total sulphides, but do not appear to be directly related to the presence of gold mineralization.