McFauld's Lake Project
Jeremy Niemi, Vice President, Geology for Noront Resources is the qualified person who prepared or supervised the preparation of the scientific and technical information disclosed herein.
Blackbird One
Geological and Mineralization Model
In the winter of 2008, Noront encountered massive chromitite mineralization in boreholes drilled to test airborne anomaly AT2 on the Double Eagle Property. The AT2 anomaly is a coincident magnetic and conductive feature that was recognized as paired linear AeroTEM conductors striking parallel to the margins of a highly magnetic body several kilometres long. The magnetic body is connected directly to the Eagle One magmatic massive sulfide deposit and as such was considered a prime target for further discoveries of peridotite-hosted magmatic sulfide mineralization. Diamond drilling into the conductive anomalies confirmed that the magnetic anomaly corresponds to a large body of metadunite and metaharzburgite that has been altered to magnetite-rich tremolite serpentinite and talc-carbonate rock. Drill holes encountered extensive Ni-rich sulphide mineralization hosted by shear zones parallel to the contact between the ultramafic rocks and their felsic plutonic (granodiorite, sensu lato) host rocks. The sulphide deposit at the AT2 anomaly area was named the Eagle Two deposit.
Below the Eagle Two shear-hosted sulphide deposit the drilling unexpectedly intersected chromite mineralization. The chromite mineralization has been named the Blackbird One Deposit. Blackbird One mineralization consists of massive chromitite layers interbedded with chromite-rich metadunite, now entirely replaced by talc carbonate minerals, chromite, and minor ferrochrome overgrowths.
Several drill holes intersected massive chromitite mineralization. The layers vary widely in thickness, from centimetres on the margins of the Blackbird One deposit to continuous massive chromitite intersections approaching true thicknesses of 30 metres at its central axis. The mineralization is thickest along an axis plunging steeply to the northwest from a near-surface expression near the southeast extremity of the drill pattern. To the southwest of this axis the mineralization thins rapidly to nothing; along the northeast side there are insufficient data to tell the shape of the body at the time of writing.
The chromite mineralization at Blackbird One shows several different textural and structural styles. In the host ultramafic silicate rocks there is abundant chromite which stands out in drill core as isolated or disseminated submillimetric black euhedra in the white talc-carbonate host rock. The modal abundance of chromite varies from less than 1% to as much as 25% and locally shows evidence of primary layering. When chromite abundance reaches 25% the rock typically shows antinodular texture, with submillimetre chromite grains forming a closely packed matrix around larger pseudomorphs of olivine. Within domains showing disseminated or antinodular texture there are common cognate xenoliths of chromitite or dunite up to several centimetres in size. Where xenoliths are larger than the diameter of the core they may become difficult to distinguish from primary layers, however, most primary layering preserves some combination of centimetric layering and fine laminations, which make identification unequivocal.
The chromitite mineralization does not have a notably strong magnetic susceptibility, compared with serpentinized dunite and peridotite which are both common in the area around Eagle One, Eagle Two, and the Blackbird One Deposits. Chromite is an electrical insulator hence there is no EM expression from the chromite deposit despite the presence of traces of interstitial sulfide minerals in the massive chromitite.
A useful characteristic of chromite is its high density, around 4.5, which is similar to that of magnetite and pyrrhotite. Massive chromite therefore has an anomalously high density compared even with ultramafic rocks and is detectable by gravity survey when it exists in sufficient tonnages.
The demonstration of a detectable gravitational response to the massive chromitite at the Blackbird One Deposit has been used to infer the presence of another body of chromitite some900 metres long striking 60 away from the AT2 area. A single diamond drill hole through the highest part of the density anomaly at line 4000E cut two bodies of massive chromitite, each with apparent thicknesses of approximately 20 metres. The new mineralized zone has been named the Blackbird Two Deposit.
Massive layers of chromitite vary in thickness from centimetres to apparent thicknesses exceeding 70 metres and true thicknesses probably exceeding 30 metres. Within massive layers there are occasional xenoliths and thin beds of talc after olivine. The chemical composition of the chromitite mineralization has been assessed by three methods; whole-rock assay by instrumental neutron activation analysis (INAA), electron microprobe analysis of individual chromite grains (EMP), and hand-held semi-quantitative X-ray fluorescence spectrometry (HXRF). Assay results through the chromitite zone in drillhole 1G17 are shown in Figure 7.3. Cr was determined by INAA and is not subject to matrix effects; the results are therefore considered to be quantitative. The main mineralized zone remains well above 40% Cr2O3 for more than 30 metres, and includes a zone greater than 5 metres wide that averages above 45% Cr2O3. Similar results have been reported in other holes, and further assay results are pending. The Cr/Fe elemental weight percent ratio in the whole rock is consistently above 2 and averages about 2.6 over the massive chromitite interval.
Electron microprobe analyses were conducted on polished thin sections of samples taken at intervals through several chromitite intersections. Ten grains were analyzed at random from each thin section. Notable results include the observations that the Cr/Fe ratio of the chromite ranges from 1.6 to 2.2, and that the Cr2O3 concentration in a chromite mineral concentrate would exceed 52%.