Regional & Deposit Geology

Regional Setting

The Back Forty Deposit occurs within the world-class VMS-hosting Penokean Volcanic Belt (PVB) of Northern Michigan and Wisconsin, USA.

The belt is a Paleoproterozoic assemblage of island arc-basin, calc-alkalic metavolcanic rocks and lesser sedimentary units and is part of the Southern Structural Subprovince of the Canadian Shield.

Within the PVB, the Ladysmith-Rhinelander Volcanic Complex hosts volcanogenic base and precious metal deposits including the giant Crandon Deposit (>70 million tonnes), the recently mined Flambeau Deposit, and approximately nine other deposits with greater than 0.5 million short tons (Dematties, 2007). Together the more than 15 VMS deposits and occurrences of the PVB account for greater than 150 million tonnes of mineralization, second only to Flin Flon Belt for Paleoproterozoic VMS tonnage.

Recent lead isotope dating of rocks at the Back Forty Deposit by Schultz and others (2008) places the age at 1874 Ma.  This is consistent with previous studies that have established an age of mostly intrusive rocks in the PVB from 1890 to 1860 Ma (Sims and others, 1989).The PVB has been affected by the Penokean Orogeny, which occurred from 1900 to 1840 Ma and resulted in major folding and faulting, regional metamorphism, and emplacement of granitic intrusions.

The most important deposits in the Penokean Volcanic Belt are associated with large, intensely altered felsic centers in back-arc basins or along the flanks of main volcanic arcs. The Back Forty Deposit occurs in one such felsic center.

Deposit Geology

The Back Forty Deposit stratigraphy consists of a stacked series of lithogeochemically distinct, intensely sericitized and silicified, rhyolite crystal and fragmental tuffs with intermittent fine-grained ash-tuff horizons, overlain by fine-grained, laminated tuffaceous sediments. Massive sulfide (>80% total sulfide) mineralization occurs at two different stratigraphic horizons.

Rhyolites tuffs in the hangingwall and footwall of the main massive sulfide horizon are chemically distinguishable using the immobile element geochemistry. The rocks are arc-associated F-II type rhylites (Ross et al., 2011).

Structurally, the deposit and surrounding host rocks have been folded into a west-southwest striking, southwest plunging, asymetric antiform. Syntectonic, west-southwest striking, steeply dipping quartz-feldspar (QFP) dikes intrude the stratigraphy as have later north-south striking mafic dikes. Early oblique-slip faulting striking west-southwest appears to have emplaced the Pinwheel Zone from depth, presumably from a position near the Hinge Zone, to a higher elevation via an east-west trending oblique slip fault. This fault, along with a QFP dike, also appears to have separated the South Limb from the Deep Zone at depth.  Later north-south faulting have resulted in offsets, including separating the East Zone from the rest of the Main Zone.

The Tuff Zone is still considered a separate, higher stratigraphic horizon from the Main Horizon, above which lie a horizon of fine grained, laminated to coarsely bedded tuffaceous sediments.

Mineralized Zones

The Main Zone massive sulfide horizon is typified by near solid (>80%) pyrite + sphalerite + chalcopyrite + galena mineralization hosted by intensely altered rhyolites.  Gold mineralization from 1 to 3 grams per tonne is ubiquitous throughout the Main Zone. Late folding and faulting has segregated the Main Zone into several spatially distinct zones with differing metal contents. 

East Zone
The East Zone is a near surface zinc and gold rich, generally flat lying, massive sulfide body occupying the hinge of a west-southwest plunging anticline (see geology section). A subcropping cap of gold-enriched hematitic East Gossan overlie the East Zone.

Hinge Zone 
The Hinge Zone is a tectonically thickened body that also occupies the anticlinal hinge to the west of the East Zone.  It is separated from the East Zone by a north-south striking mafic dike and is downdropped by faulting along the dike.

South Limb 
As the name implies, the South Limb massive sulfide lies to the south of the Hinge Zone.  It is detached from the hinge by an east-west striking porphyry dike (QFP). The South Limb dips 65 to 75 degrees to the south.

Pinwheel Zone 
The Pinwheel Zone, so named because of the radial fan of drill holes used to discover it, is interpreted to be an upthrown portion of the north limb of the Main Zone Horizon.  The near surface portion of the Pinwheel Zone is a flat-lying copper-rich massive sulfide capped by an outcropping precious metal and copper-bearing magnetite-hematite Pinwheel Gossan.  Along strike, the Pinwheel Zone dips steeply to the northwest and becomes zinc-rich.

Deep Zone 
The Deep Zone is a relatively steeply plunging lens interpreted to be the continuation of the South Limb where it has been refolded at depth.

Tuff  Zone
The Tuff Zone is a second, distinct horizon stratigraphically above the Main Zone.  It occurs to the south of the South Limb massive sulfide and has a similar dip. Tuff Zone massive sulfide is localized at the contact of finely bedded tuffaceous and siliceous sediments and underlying rhyolite fragmental rocks.

Stringer Sulfide Zones

The Stringer Zone consists of rhylolite-hosted sulfide stockwork-stringers that occur in the footwall of the East and Main Zone massive sulfides. Recent positive metallurgical tests indicate that the Stringer Zone may represent a recoverable resource of copper and gold.

Gold Mineralization

Numerous zones of gold mineralization occur in close proximity to the massive sulfide zones. The close association of felsic dominated lithologies and hydrothermal geochemical signatures are suggestive of an epithermal overprint scenario.

East Gossan 
The East Gossan is a flat subcropping hematitic zone directly overlying the East Zone.  It is the result of supergene weathering of massive sulfide.  Gold has been greatly concentrated in the East Gossan while all other metals have been depleted.

Pinwheel Gossan 
The outcropping Pinwheel Gossan is magnetite and hematitic rich with supergene enrichment of gold, silver, and minor copper. It caps the updip (eastern) extent of the Pinwheel Zone. The Pinwheel Gossan is also thickened along a major east-west fault.

90 Zone
The 90 Gold Zone is a subcropping, tabular zone of disseminated gold and silver mineralization ranging from 7 meters to greater than 20 meters of estimated true thickness.  It extends from surface to a depth of 70 meters and along strike for 400 meters. Mineralization is hosted by tuffaceous sediments and underlying fragmental rhyolites (the updip equivalent position of the Tuff Zone).  Mineralization consists of 5%-10% disseminated pyrite with lesser arsenopyrite, chalcopyrite, and galena hosted in siliceous tuffaceous sediments and rhyolite (R2). Gold mineralization is associated with variable silicification and may be structurally controlled, sub parallel and slightly crosscutting the section.

Porphyry Margin (PM) Zone
Locally bonanza grade gold mineralization occurs along the margin and within a subvertical porphyritic dike that has intruded the central anticline and separates the Hinge Zone from the South Limb. Named the Porphyry Margin, or PM Zone, it is defined by 15 intercepts spanning 140 meters of strike length and 230 meters down dip. 

Sulfide mineral assemblages include fine to medium-grained, pyrite + arsenopyrite + galena + sphalerite + chalcopyrite + pyrrhotite and locally visible free gold.

Below: Native gold with galena from drill core (LK-80)

NS Zone
The “Near Surface” or NS Gold Zone consists of numerous significant gold intercepts in an area immediately south of the Pinwheel Zone.  It is not included in the current resource and represents yet another potentially significant shallow precious metal resource.  Mineralization consists of trace disseminated sulfide in highly sheared crystal tuff and ash tuff and appears to be focused along a major east-west fault that truncates the southern edge of the Pinwheel Zone.  The NS Zone has been traced for a strike length of 125 meters.

References:
Dematties, T. A.,  2007, An evaluation of the Back Forty volcanogenic massive sulfide (VMS) deposit, Menominee County, Michigan, U.S.A.:  Technical report prepared for Aquila Resources Inc., 52p.

Ross, C., Hudak, G., Morton, R., Quigley, T.O., and Mahin, B., 2011, Preliminary stratigraphy and physical volcanology associated with the Paleproterozoic Back Forty VMS deposit, Menominee County, Michigan: Proceedings of the Institute on Lake Superior Geology, v. 57, Part-1 Programs and Abstracts, 70-71

Schultz, K.J., Nicholson, S.W., and Van Schmus, W.R., 2008, Penokean massive sulfide deposits: Age, geochemistry, and paleotectonic setting: Proceedings of the Institute on Lake Superior Geology, v. 54, Part 1-Programs and Abstracts, 76-77.

Sims, P.K., Van Schmus, W.R., Schultz, K.J., and Peterman, Z.E., 1989: Tectono-stratigraphic evolution of the early Proterozoic Wisconsin magmatic terranes of the Penokean orogen: Canadian Journal of Earth Sciences, v.26, p.2, 145-2, 158.