United States

Back Forty Project

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Mine TypeOpen Pit & Underground
  • Gold
  • Silver
  • Copper
  • Lead
  • Zinc
Mining Method
  • Truck & Shovel / Loader
  • Mechanized Cut & Fill
  • Longhole stoping
Backfill type ... Lock
Mine Life... Lock
SnapshotThe feasibility study work for the Back Forty Project in Michigan, USA progressed during 2022.

Once the feasibility study is completed, the Company’s Board of Directors will evaluate the current economic climate and make a decision on how to move forward with the permitting and construction of the Back Forty Mine.


Gold Resource Corp. 100 % Indirect
Gold Resource Corp. (Company) has 100% interest in the Back Forty Project.

On December 10, 2021, the Company successfully completed the acquisition of all the issued and outstanding common shares of Aquila Resources Inc. Aquila’s principal asset is its 100% interest in the Back Forty Project located in Menominee County, Michigan, USA.



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Deposit type

  • VMS


Deposit Types
The zinc-copper-gold-silver bearing sulphide mineralization identified on the Back Forty Property exhibits typical characteristics of VMS mineralization. This deposit type has been well documented in the literature since the early 1970s (Franklin et al. 1981) and the exploration model for the PVB was refined after the discovery of Flambeau (DeMatties et al. 1996).

Mineralized Zones
Mineralization at the Back Forty Deposit consists of discrete zones of: 1) zinc or copper-rich massive sulphide (±lead), which may contain significant amounts of gold and silver, 2) stockwork stringer and peripheral sulphide, which can be gold, zinc, and copper-bearing (±lead/silver), 3) precious metal-only, low-sulphide mineralization, and 4) oxide-rich, precious metal-bearing gossan.

Massive Sulphide Mineralization
The Main Zone massive sulphide, which accounts for the vast majority of massive sulphide mineralization lies at the statigraphic boundary of these two rhyolite units. Rhyolite 1 lies stratigraphically below this sulphide horizon (footwall) while rhyolite 2 lies above the horizon (hanging wall). Another massive sulphide horizon, the Tuff Zone, is located at or near the upper contact of rhyolite 2 and the lower contact of an overlying package of tuffaceous and siliceous sediments. A possible third massive sulphide horizon, the Deep Zone, may represent a lower mineralized zone.

Main Zone Massive Sulphide
The Main Zone is composed of three separate massive sulphide bodies (referred to as the East, Hinge, and South Limb Zones) that form a plunging antiform and are considered the same horizon. These bodies are hosted by Rhyolite 1 (footwall) along and stratigraphically below their contact with Rhyolite 2 (hanging wall). The strata-bound Main Zone is enveloped locally by stockwork and semi-massive sulphide mineralization. Pervasive sericite and disseminated pyrite alteration as well as variable silicification are abundant and extend outward for an undetermined distance. This zone extends along strike for over 450 m in a west-southwest direction; it is up to 100 m wide and subcrops at its eastern end under thin (less than 10 m) glacial overburden and local Paleozoic sandstone. The stockwork-stringer and peripheral sulphide envelope grades outward into a semi-conformable disseminated (less than 10%) pyritic halo that extends throughout the entire altered Rhyolite 1 host unit for an undetermined distance. The zone has been extensively disrupted by variably altered quartz feldspar porphyry (“QFP”) intrusions.

Pinwheel Zone Massive Sulphide
The Pinwheel Zone occupies the northwest portion of the Deposit, located structurally along the gently north-dipping northern limb of the antiform and is truncated to the south by the E-W fault. Limited geochemical data suggests that this unit is in fact located along the contact between rhyolite 1 and rhyolite 2 and is therefore likely the equivalent to the Main Zone massive sulphide and represent a ‘faulted-up’ portion of the north limb of this important massive sulphide horizon. Massive sulphide mineralization on strike of the Pinwheel Zone has been traced for roughly 700 m to the west-southwest where the gentle north-dip of the unit steepens. It should be noted,however, that the massive sulphide mineralization is to some degree discontinuous and often has a ‘stacked’ geometry, and that numerous faults and shear zones have been encountered in the adjacent host rock. The geometry of this zone is likely complicated due to these structures.

Deep Zone Massive Sulphide
The Deep Zone is located north of one of the QFP dykes, juxtaposed against the South Limb Horizon. Recent geological and geochemical data interpretation suggests that the Deep Zone may be the down-dip continuation of the South Limb, where it has been folded and rotated. This interpretation leaves significant spatial potential for further Mineral Resource discovery between the South Limb and the Deep Zone as well as down dip of the Deep Zone.

Tuff Zone Massive Sulphide
The Tuff Zone massive sulphide occurs at the south edge of the Deposit. Stratigraphic and structural data suggest this zone is located at a higher level in the volcanic sequence. In cross sections and three-dimensional models, the zone appears to have a bowl-shaped geometry possibly reminiscent of a small relict depositional basin or local graben structure.

Massive sulphide mineralization of the Tuff Zones appears preferentially developed within coarser grained tuffaceous units at or near the contact of rhyolite 2 and of the overlying tuffaceous and siliceous sediments. Overall sulphide content is less massive than that of the Main Zone (~60%-80%) and is dominated by sphalerite, pyrite, and galena. The zone’s thickness is typically on the order of a couple of metres.

Mineralization Encountered at Depth
Mineralization has been encountered in two separate stratigraphic horizons which have tentatively been called the ‘Upper Deep’ and ‘Lower Deep’ zones. LK-479, drilled to the southwest of known mineralization and to a total depth of 911 m, encountered each of the two zones and represents the ‘discovery’ hole for each.

The upper mineralized section in drill hole LK-479 intercepted ~60 m of strongly altered rhyolite and tuffaceous and siliceous sediments with variable sulphide mineralization in the form of disseminated and stringer sulphides as well as two small intervals of high-grade massive sulphide. The mineralization was also cut by a quartz-feldspar porphyry dyke that contained PM-style gold mineralization. Assayed intervals (drilled thickness) within the section include: 12 m (366-278 m) of 1.25 g/t Au, 57.9 g/t Ag, 0.41% Pb, and 1.0% Zn and 12.8 m (407.3-420.1 m) of 4.27 g/t Au, 189.1 g/t Ag, 1.4% Pb, and 4.2% Zn. The metal content of this zone, as well as the spatial relationship to the tuffaceous and siliceous sediment unit would suggest that this zone may be related to the Tuff Zone type sulphide mineralization, however, limited and preliminary geochemical data indicates that the mineralization may reside at the contact of rhyolite 1 and 2 which would imply that this zone is related to Main Zone mineralization. It is also interesting that the high-grade massive sulphide encountered in this section appears to cross-cut bedding which would indicate that higher-grade mineralization may be related to a cross-cutting structure. A number of drill holes attempted to follow-up on this intercept and encountered similar mineralization in altered rhyolite and tuffaceous and siliceous sediments, but grades were typically less substantial than that of LK-479. The zone lies roughly 80 m along strike, to the southwest of the south limb of the Main Zone massive sulphide. The lower mineralized section in drill hole LK-479 intercepted roughly 68 m of massive sulphide overlain by stockwork stringer type mineralization. Metal content of the massive sulphide was generally zinc-poor with a relative



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Mining Methods


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Crushers and Mills


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CommodityProductUnitsAvg. AnnualLOM
Silver Payable metal koz 6,260
Copper Payable metal M lbs 86
Copper Concentrate kt 223
Lead Payable metal M lbs 26
Lead Concentrate kt 37
Zinc Concentrate kt 794
Zinc Payable metal M lbs 801
Gold Equivalent Payable metal koz 1281,500

Operational metrics

Daily processing capacity 2,800 dmt of sulfide *
Daily processing capacity 350 dmt of oxide *
Waste tonnes, LOM 53,069 kt *
Ore tonnes mined, LOM 15,849 kt *
Total tonnes mined, LOM 68,918 kt *
Tonnes processed, LOM 15,858 kt *
* According to 2019 study.

Production Costs

All-in sustaining costs (AISC) Gold Equivalent USD 926 / oz *  
All-in sustaining costs (AISC) Gold USD -397 / oz * **  
C1 cash costs Gold Equivalent USD 733 / oz *  
C1 cash costs Gold USD -82 / oz * **  
Assumed price Lead USD 0.91 / lb *  
Assumed price Zinc USD 1.08 / lb *  
Assumed price Copper USD 3.05 / lb *  
Assumed price Silver USD 18.2 / oz *  
Assumed price Gold USD 1,485 / oz *  
* According to 2019 study / presentation.
** Net of By-Product.

Operating Costs

OP mining costs ($/t mined) USD 3.03 *  
UG mining costs ($/t mined) USD  ....  Subscribe
OP mining costs ($/t milled) USD  ....  Subscribe
UG mining costs ($/t milled) USD  ....  Subscribe
Processing costs ($/t milled) USD  ....  Subscribe
G&A ($/t milled) USD  ....  Subscribe
Total operating costs ($/t milled) USD  ....  Subscribe
* According to 2019 study.

Project Costs

MetricsUnitsLOM Total
Initial CapEx $M USD  ......  Subscribe
Sustaining CapEx $M USD  ......  Subscribe
Closure costs $M USD  ......  Subscribe
OP OpEx $M USD  ......  Subscribe
UG OpEx $M USD  ......  Subscribe
Processing OpEx $M USD 310
G&A costs $M USD 46
Total OpEx $M USD  ......  Subscribe
Mining Taxes $M USD  ......  Subscribe
Income Taxes $M USD  ......  Subscribe
Royalty payments $M USD  ......  Subscribe
Gross revenue (LOM) $M USD  ......  Subscribe
Pre-tax Cash Flow (LOM) $M USD  ......  Subscribe
After-tax Cash Flow (LOM) $M USD  ......  Subscribe
Pre-tax NPV @ 6% $M USD  ......  Subscribe
After-tax NPV @ 6% $M USD  ......  Subscribe
Pre-tax IRR, %  ......  Subscribe
After-tax IRR, %  ......  Subscribe
After-tax payback period, years  ......  Subscribe

Heavy Mobile Equipment


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Mine Management

Job TitleNameProfileRef. Date
....................... Subscription required ....................... Subscription required Subscription required Jul 5, 2023
....................... Subscription required ....................... Subscription required Subscription required Oct 14, 2019
....................... Subscription required ....................... Subscription required Subscription required Oct 14, 2019
....................... Subscription required ....................... Subscription required Subscription required Oct 14, 2019

Total WorkforceYear
...... Subscription required 2019


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