Summary:
The gold deposits in the Val-d’Or area are consistent with the orogenic gold model. Orogenic gold deposits are typically distributed along first-order compressional to transpressional crustal-scale fault zones that mark the convergent margins between major lithological and/or tectonic boundaries such as the Larder Lake–Cadillac Fault Zone.
Most orogenic gold deposits in Archean terranes are hosted in greenstone belts. In the Lamaque Project area, competency contrasts are the most important localizing host rock control. At Lamaque and Triangle the intersection of shear zones with late diorite to granodiorite plugs host the main gold-bearing veins, whereas at Sigma and Ormaque a syn-volcanic diorite (the C-porphyry) hosts mineralization near the sheared contact with the surrounding volcaniclastic rocks.
Orogenic gold deposits develop in response to shear failure, extensional failure and/or hybrid extensional shear failure. In the Lamaque Project area both shear veins and extension veins are widely recognized, and their identification is important to constrain vein geometries and ore shoots. In the Triangle deposits the main C vein structures are steeply dipping shear veins and host the bulk of the resource, whereas in the Ormaque deposit gently dipping extension veins contain most of the ore.
Current gold resources at the Lamaque Complex are defined in the Triangle, Plug No. 4, Parallel and
Ormaque deposits, with most resources occurring in the Triangle and Ormaque deposits.
Historic Sigma Mine
Auriferous veins at the Sigma mine consist of quartz and tourmaline with lesser carbonates, muscovite, pyrite, scheelite, chlorite and chalcopyrite (Robert and Brown, 1986a, b). Four types of veins can be distinguished based on their host rock associations and geometries: (1) steeply to moderately dipping fault-fill veins within shear zones; (2) subhorizontal extensional veins; (3) arrays of subhorizontal extensional veins hosted within the feldspar porphyry dykes, referred to as dyke stringers; and (4) moderately north-dipping extensional-shear veins, referred to as the North Dipper veins.
Historic Lamaque Mine
Gold mineralization at Lamaque consists of quartz-tourmaline-carbonate-pyrite veins, stringers and stockworks. These gold-bearing veins occur within all rock types but are most abundant in late intrusions. Roughly 85% of the gold mined historically at Lamaque was from veins hosted by the Main Plug. The quartz-tourmaline-carbonate veins, as at Sigma, are associated with brittle-ductile reverse shear zones, and occur in multiple orientations and styles. The veins were classified into three main types namely shear veins, flat veins, and stringer veins (Karvinen, 1985). The shear veins are generally thick (up to 10 m) and have strike lengths of over 500 m. They are best developed in the volcanic rocks away from the plugs in the upper parts of the mine. Typical thicknesses of flat veins are less than 0.5 m but these also extend for hundreds of meters in strike. They dip gently (10 to 20°) to the west and intersect shear veins at 75 to 85 °. Stringer veins are essentially stockwork veins and veinlets found predominantly in the two productive plugs and in several zones these veins were so numerous that bulk mining was possible. These stockwork zones were very prolific at the Lamaque Mine and produced most of the ore. They were developed usually near the intersection of sub-vertical south dipping shear zones with the intrusion. The resulting geometry created large zones of gold bearing veins and veinlets that could be as large as the intrusion itself and therefore were very economic to extract.
Triangle Mine
Gold mineralization in the Triangle deposit occurs primarily within quartz-tourmaline-carbonate- pyrite veins in the Triangle Plug and adjacent massive mafic lapilli-blocks tuffs. The veins are localized in a series of shear zones and fractures that cut both units. The thickest and most continuous veins have an east-west strike and developed within ductile-brittle reverse shear zones that dip 50-70° to the south. They are classified as C-veins (shear veins) and at least twelve discrete C-veins (C1 to C10 and C8b and C9b) have been identified to a depth of greater than 1500 m. Smaller extensional shear veins form splays to the C-veins and dip 20-45° south and are commonly associated with sub-horizontal extension veins. The bulk of the gold occurs within the quartz veins, but lower grades also occur in the strong muscovitecarbonate-pyrite alteration selvages. In and below C8, C9 and C10 veins a lower faulted portion of the Triangle Plug contains intense stockwork veins similar in style to zones within the Lamaque mine that were previously bulk mined.
Plug No. 4 Deposit
The Plug No. 4 deposit is located 550 m north of the Triangle deposit and 1 km southwest of the historical No.3 Mine shaft, to which it is connected by drifts on the 450’ and 700’ levels. Plug No.4 is a fine- to medium-grained magnetic gabbro intrusion measuring roughly 100 to 150 m in diameter. It is enveloped by an older syn-volcanic diorite and diorite breccia similar to the C- porphyry. These intrusions are cut to the west by a fine-grained porphyritic felsic diorite dyke which extends northwest towards the No. 3 Mine area. Gold mineralization at Plug No. 4 is restricted to the gabbro intrusion.
Parallel Deposit
The Parallel deposit is located 650 m northwest of the No.3 Mine and 900 m east-southeast of Lamaque Mine main shaft. Gold mineralization at the Parallel deposit is hosted within fine- to medium-grained Cporphyry diorite. The diorite is medium greenish-gray and contains 1 to 5% disseminated pyrite commonly in a chlorite-silica pervasive alteration. The ore zones at the Parallel deposit occur as subhorizontal extension veins at shallow depths (70 m to 200 m) and as east-west striking shear veins dipping approximately 30° south at deeper levels. The mineralized veins consist of quartz and carbonate with lesser amounts of tourmaline, chlorite, and sericite. Fine pyrite within the vein commonly amounts to 1-3% and rarely up to 5 %. Traces of chalcopyrite occur locally. In wider veins, pyrite and gold are typically confined to vein margins and/or vein contacts, especially in veins composed mainly of quartz and carbonate. The sub-horizontal veins are laterally extensive (up to 300 m) and occur in en-echelon arrays that exhibit pinch and swell geometries. Adjacent wall rocks contain carbonate-albite-muscovite-pyrite alteration. In general, the veins form stacked sets which are 10-25 m thick and contain up to 8 individual veins. Shear veins also occur in clusters. Typically, up to four en-echelon south dipping veins occur within a 75 m wide vertical corridor that cuts across the porphyritic diorite. The shear veins most commonly range in width from 15 cm and 1.5 m but can be up to 2.6 m thick locally.
Ormaque Deposit
The Ormaque vein system occurs within the C-porphyry at the contact with volcaniclastic rocks to the north. Gold mineralization occurs dominantly in gently south-dipping quartz-tourmaline-carbonate extension veins and localized breccia zones. Individual extension veins have widths ranging from several cm up to 2 m and have east-west strike lengths of up to 650 m. The veins form vertically stacked clusters from a depth of about 150 m to at least 600 m below surface.