The Tree River property is centred at 670 08’ N 1120 09’ W on NTS 86 P01, 155 kilometres southeast of Kugluktuk in the Kitikmeot Region of Nunavut. The property consists of three Prospecting Permits covering 39,250 hectares and is accessible by float plane or helicopter from Kugluktuk or Yellowknife.
Gold mineralization was discovered in the general area by BHP Billiton (“BHP”) during reconnaissance work in the early 1990’s. Claims were staked in the property area by BHP in 1993 for both diamonds and gold. BHP’s prospecting subsequently located gold in structurally controlled veins, gabbro and within an extensive “siliceous horizon”. The latter quickly became the focus of their efforts when grab samples up to 142 g/t Au were collected near Cracker Lake. After collecting numerous samples greater than 1 g/t Au along a western horizon in addition to the high-grade gold samples returned from an eastern horizon, BHP concluded that the gold-bearing horizon was too extensive to be an exhalate bed and identified the host rock unit as a conglomerate (Tree River Conglomerate or “TRC”). BHP conducted additional prospecting and sampling on the property until 1997. The property was restaked by 4763 NWT Ltd. in 2002 and optioned to Strongbow Exploration Inc. (“Strongbow”). Strongbow conducted airborne horizontal aeromagnetic surveys over the property in the winter and spring of 2002 and made a single visit to the property on August 20, 2002. They collected 7 samples; two of which returned 102.64 g/t Au and 2.357 g/t Au. Subsequent work focused on the uppermost of two conglomerate horizons in a metasedimentary rock unit which Strongbow identified as hosting mineralization similar in style to that found in the Witwatersrand of South Africa. Strongbow subsequently conducted geologic mapping, petrographic studies and channel sampling on the property but allowed it to lapse when they failed to located a large zone of higher-grade gold mineralization.
The property was acquired by Silver Range Resources in February 2018 and is held under three Prospecting Permits covering 39,250 hectares. In August 2018, Silver Range conducted geological mapping, prospecting and sampling on the property. Sampling by Silver Range in 2018 identified two areas of significant gold mineralization. At the Main Zone, the site of historic work by BHP and Strongbow, grab samples collected by Silver Range assayed up to 114 g/t Au and a chip sample returned 0.20 m @ 540 g/t Au. This zone is exposed intermittently over a strike length of 650 m, is strongly silicified and contains visible gold at the highest-grade exposure. The West Zone is 3.6 km along strike and returned grab samples up to 14.05 g/t Au. It is partially exposed over a length of 300 m and remains open along strike in both directions. Mineralization there is characterized by high arsenopyrite (scorodite) content. Background gold content in the TRC as a whole was estimated from an average of 53 basal TRC sample assays, cutting all assays greater than 2.5 g/t Au is very anomalous at 0.203 g/t Au. In April 2020, Silver Range collected >45 kg, 1 m by 1 m panel samples with a rock breaker at both the Main Zone and the West Zone. The sample from the Main Zone returned 36.3 g/t Au and the sample from the West Zone returned 0.29 g/t Au.
The Tree River Property is at the north end of the Anialik Greenstone Belt and is underlain by mafic volcanic rocks overlain by clastic shallow marine sediments, subsequently metamorphosed to greenschist facies. The sedimentary rocks were initially dated at between 2940 and 2700 Ma based on detrital zircons (Jackson, 1997). A zircon sample collected in April 2020, midway between the West and Main Zones, returned maximum ages of 2964 + 9 Ma, conclusively dating the conglomerate unit as Mesoarchean. The TRC is a monomictic, clast-supported quartz pebble conglomerate from 15 to 20 metres thick, approximately 80 m stratigraphically up-section from the volcanic / sedimentary contact. The TRC has been mapped and intermittently sampled for over 7 km but its full extent is unknown.
The known auriferous portion of the Tree River Conglomerate is 5 to 8 metres thick and occurs at the bottom of the succession. This sequence contains abundant pyrite within the matrix with 1-2% in background and up to 25% in localized pods. Arsenopyrite, chalcopyrite, stibnite and sphalerite together with a green mica identified as fuchsite also occur in this interval. Thin and polished section work by the Saskatchewan Research Council described the pyrite as detrital with grains rounded to “muffin-shaped” (Muntiner, 2006). The matrix contains up to 5% carbonaceous material in layers parallel to bedding, a feature noted in Archean gold-bearing conglomerates in the Witwatersrand and Pilbara districts. The presence of visible gold along fractures in one zone, together with recrystallized quartz and sulphides, and some sulphide-bearing fractures indicate that the mineralization is at least partially remobilized. There is no evidence of any major nearby structure and surrounding rocks are metamorphosed to greenschist facies, suggesting remobilization is likely local and related to regional metamorphism. The 2020 panel sampling at the high grade Main Zone returned results comparable to grab and chip samples suggesting that the TRC here is uniformly mineralized and the gold present is likely of syngenetic origin.
The zircon sample collected in 1997 by noted northern regional mapping geologist Valerie Jackson for age dating yielded two suspected diamonds but the lab results were never verified or formerly reported. In 2018, Drs. Graham Pearson and Jesse Reimink of the University of Alberta and Pennsylvania State University conducted follow-up work where the 1997 zircon sample was collected. They collected two 10 kg separate samples from the general area of the 1997 sampling and recovered 2 diamonds from one sample and a third diamond from the second sample (Timmerman et al, 2020). They reported that:
“The three recovered diamonds (< 210 μm) have (cubo-)octahedral morphological features and have nitrogen contents ranging from <10 ppm (n=2) up to 1770 ppm N (Type IaA), the latter having positive δ15N (+3.1 to +4.4‰). Carbon isotope compositions of the 3 diamonds range from -3.5 to -0.3 ‰, similar to the heaviest values found in microdiamonds on the Slave Craton and potentially indicating an oxidized source. The Type IaA diamond contains a high-Mg olivine inclusion, representing a lithospheric origin.”
The diamond-bearing samples were collected from the basal unit of the TRC which contains abundant pyrite and anomalous to high-grade gold. The samples were collected on IOL parcel CO-69 about 300 metres from the boundary with the Tree River Property.
Work to date suggest that the mineralization at the Tree River property is similar in setting and style to gold mineralization recently discovered in the Pilbara district of Australia and fits currently proposed models for Witwatersrand-style gold mineralization:
- Gold is hosted in a regionally extensive, fluvial to shallow marine conglomerate.
- Pods of high-grade gold mineralization are present within a much larger envelope of lower grade material.
- Primary carbonaceous material is present.
- Pyrite is abundant and a significant proportion is reported to be rounded and detrital in origin
- The TRC is dated as Mesoarchean, placing it in the favourable time window for the Great Gold Deposition Event (Frimmel & Hennigh, 2015).
Silver Range intends to conduct systematic sampling, geological mapping and focused prospecting at Tree River in the coming season, building on recent results; the solid technical data collected by Strongbow; and incorporating the latest insights into the developing Witwatersrand 2.0 deposit model. The work program will focus on evaluating the large-scale gold and diamond potential of the Tree River Conglomerate.
Frimmel, H. and Q. Hennigh. 2015. First Whiffs of Atmospheric Oxygen Triggered Onset of Crustal Gold Cycle. Miner Deposita, Vol. 50, p 5-23.
Jackson, V. 1997. Preliminary geology of the Tree River volcanic belt. DIAND NWT Division: EGS 1997-14.
Muntiner, C. 2006. Report on exploration activities. Nunavut Mining Recorder: AR085021
Timmerman, S., D. G. Pearson, F. Nestola, A. Banas, T. Stachel, R. A. Stern, A. Vezinet, J. R. Reimink, A Lelpi, C. Mircea and V. A. Jackson. 2020. Diamond-bearing sediments point to thick, cool lithospheric root established by the Mesoarchean beneath parts of the Slave craton (Canada). American Geophysical Union 2020 Annual General Meeting Abstracts.