Summary:
The geology of BlackRock Property is dominated by the anorthositic Lac Doré Complex (LDC), with minor late gabbro, diorite and felsic intrusions and late Proterozoic diabase dykes.
All units form an unfolded homoclinal sequence striking roughly from N30° to N50° and dipping between 45° to 80° to the southeast. Stratigraphic tops face to the southeast.
On surface exposures and in drill cores, the anorthosite zone is dominantly composed of anorthosite layers, with minor gabbro, anorthositic gabbro and gabbroic anorthosite. As expected, and verified by drilling, gabbro and magnetite bearing gabbro increase within the top 150 m of the zone near the contact with the layered zone. The magnetite as well as the vanadium and titanium content increases accordingly.
The layered zone, is the main host of the vanadium-titanium-magnetite (VTM) Deposit and is well exposed in the area of the Southwest Deposit. It is a rhythmic layered sequence dominated by pyroxenite and anorthosite beds. The thickness of the layered sequence ranges from 150 m to 900 m and the thickness of individual beds ranges from a few centimetres to a few metres.
The granophyre zone of the LDC has been identified by previous regional mapping. It can be traced all along the southeastern boundary of the main block outside the limit of the proposed Armitage and Southwest Pits.
Locally, the rocks were plastically deformed during regional metamorphism. There is also some post-genetic brittle fracturing and faulting. As a consequence, the units may be fairly difficult to distinguish in mapping. Although the favourable stratigraphy is relatively well exposed in trenches in the Southwest Deposit area, it is not the case over the remainder of the Property. In fact, because of the poor exposure, it has been impossible to distinguish the various magnetite bearing units over most of the Property. In the southwestern half of the Property in the Armitage Lake area, the favourable stratigraphy is almost entirely covered by overburden or under water.
The VTM mineralization in the Lac Doré Complex on the BlackRock Property is one of the most important mineral occurrences in the Chibougamau region. It is a magmatic-type iron mineralization related to layered mafic intrusions. It was discovered in the 1950's and it has been sporadically explored for vanadium since then. The economic potential of the iron and titanium has been identified and delineated by BlackRock Metals over the last five years.
The Lac Doré Complex
As described by Girard and Allard (1998), the Lac Doré Complex (LDC) T is a differentiated ultramafic-mafic sill of the Archean age, which is observed in the heart of the Chibougamau anticline. Its flanks outcrop on the north and south shores of the Chibougamau Lake. It was emplaced during the arc-magmatic and collisional stages of orogenesis at 2.727 Ga. Shortly after emplacements, the LDC was folded into a broad anticline during continued compressive accretion of the Abitibi-Wawa Terrane between 2.698-2.690 Ga.
The anorthosite zone is dominantly composed of anorthosite layers, with minor gabbro, anorthositic gabbro and gabbroic anorthosite. Gabbro and magnetite bearing gabbro increase within the top 150 m of the zone near the contact with the layered zone. The iron (magnetite) as well as the vanadium and titanium content increases accordingly.
The layered zone is the main host of the VTM Deposit. It is a rhythmic layered sequence dominated by pyroxenite and anorthosite beds. The thickness of the layered sequence ranges from 150 to 900 m and the thickness of individual beds ranges from a few centimetres to a few metres.
The Granophyre Zone of the LDC has been identified by previous regional mapping. It can be traced all along the southeastern boundary of the main block outside the limit of the proposed Armitage and Southwest Pits.
Within the LDC, the VTM mineralization occurs at the top of the anorthosite zone and the base of the Layered zone. Allard (1967) has previously defined three main VTM mineralized horizons called P1-P2-P3. Girard and Allard (1998) have partly revised this stratigraphy and have implemented a P0 unit stratigraphically below the former P1.
The layered sequence has been subdivided into various units, separated by discontinuous anorthosite screens. It is noted, from the base to the top of the mineralization:
- P0: Anorthosite sequence interspersed by thin layers of magnetite (10-15% magnetite);
- P1: Dominant anorthositic sequence interspersed with numerous layers of magnetite of metric thickness (15-40% magnetite). This sequence is considered a low-grade magnetite source but one that is of high-grade vanadium. This resource is probably economic.
- P2: Sequence dominated by layers of magnetite and magnetite ferrogabbro (>40% magnetite) with a high-grade of vanadium. This sequence constitutes the major part of the vanadiferous magnetite deposit;
- P3: Magnetite and ilmenite ferrogabbro sequence (20-40% magnetite).
Generally, the three main mineralized units, P1 to P3, are separated in the field by rocks carrying no, or very low amounts of magnetite. The magnetite is usually well layered with sharp contacts, but some is disseminated in the fringed host rocks. The limits for each unit are seldom clear but the gangue rock is distinctive.
Magnetite and ilmenite are the main iron and titanium economic minerals. As described by Allard (1967), magnetite mineralization occurs as an alternation of layers of solid titaniferous magnetite, magnetite rich gabbro, magnetite rich pyroxenite, gabbro and anorthositic gabbro. The solid magnetite layers range from centimetres to over a metre in thickness. The magnetite rich band is everywhere at the same stratigraphic horizon, but each magnetite layer is discontinuous and exhibits marked changes in thickness and character along strike.
As expressed by the ground magnetometer survey and the recent aeromagnetic survey These horizons extend, almost continuously, for 20 km in length.
Layered Mafic Intrusions
Layered Mafic Intrusions (LMI) are one of the most complex and well-studied geologic systems. This has been driven in part by economic interests seeking predictive models for locating valuable mineralization that can range from sulphides (PGMs, Ni, Co and Cu) to oxides (Cr, Fe, Ti and V). These deposits have also been exploited for use in ceramics, refractories, or in cases of high Anorthite (An) plagioclase like Lac Doré, for their aluminum resources.
The primary deposition of magnetite and ilmenite as ore-bearing minerals in these bodies is termed orthomagmatic; they represent a direct crystallization and deposition from the parent magma. Some of the processes responsible for this are directly analogous to sedimentary processes.