Regional Geology

The Mine lies within a greenstone belt of the Paleoproterozoic Guiana Shield. The Guiana Shield represents the northern portion of the Amazon Craton, which is surrounded by Neoproterozoic orogenic belts. Most of the belt is comprised of rocks formed during the Transamazonian orogeny that have been deposited onto two Archean nuclei; the Venezuelan Imataca block to the west and the Amapa block to the east.

The central portion of the Guiana Shield comprises three major, west-northwest to east-southeast geotectonic units that young to the Southwest (Kroonenberg, 2016). In general, the terrain to the north is comprised of a tonalite-trondjhemite-granodiorite (TTG) greenstone belt, followed by a granitoid belt in the centre that gives way to Late Paleoproterozoic volcanic, intrusive, and sedimentary rocks in to the south.

McConnell and Williams (1969) subdivided the central portion of the Guiana Shield into the Barama Group conformably overlain by the Mazaruni Group of rocks, both consisting of a pile of metasedimentary and metavolcanics rocks. The Mazaruni Group was further subdivided into the Cuyuni and Haimaraka Formations.

The Cuyuni Formation consists of pebbly sandstones and intraformational conglomerates, intercalated with felsic to mafic volcanic rocks. The Haimaraka Formation is comprised of a sequence of turbiditic mudstones, pelites, and greywackes with lesser volcanic rocks. These rocks are believed to have been formed in a foreland geosynclinal basin on the margin of the Archean craton.

Most of the rocks of the Barama-Marzaruni Supergroup are metamorphosed to lower to middle greenschist facies, however, near the contacts with large granitic complexes, metamorphism increases to upper greenschist to lower amphibolite facies.

Figure 7-1 illustrates the regional geology of Guyana and the Guiana Shield of northeast South America.

Structurally, the rocks in the shield have been subjected to compressional strain imparting a strong northwest-southeast fabric or penetrative cleavage. Large scale ductile shear zones are not common, although a series of major northwest trending faults have been identified including the North Guiana Trough (NGT) in French Guiana and the North Suriname Shear Zone (NSSZ) through north central Suriname. Voicu et al. (2001) suggested that the Central Guiana Shear Zone (CGSZ), identified by geophysical and satellite imagery, forms a major suture across the entre Guiana Shield. This kilometre-wide west-northwest to east-southeast trending structure is known as the Makapa-Kuribrong shear zone (MKSZ) in northern Guyana.

A series of calc-alkaline to intermediate intrusive rocks ranging in composition from granite to granodiorites, diorite and adamelite, called the Transamazonian Granitoids, were emplaced between 2,250M and 1,960M years ago (Gibbs and Barron, 1993). Intrusive rocks in the vicinity of the Mine consist of the Proterozoic-age Iroma–Aranka, Aurora, and Kartuni medium-grained granodiorite and diorite intrusions followed by late-stage basic sills and dikes.

The rocks in the Guiana Shield have been subject to chemical weathering reflecting prolonged exposure to a tropical climate. This has resulted in the formation of a laterite-saprolite profile that can be as deep as 100 m below surface. Chemical weathering results in the formation of stable secondary clay minerals along with iron, magnesium, and aluminum oxides due to the leaching of mobile, alkali elements. In the Aurora area, the depth of chemical weathering varies from as little as 15 m to as much as 75 m, depending on the underlying lithologies.

Figure 7-2 illustrates the major structures and gold deposits in the Guiana Shield.

Local and Property Geology

The Mine is located within a high-strain zone developed along the northeast margin of a granitic batholith. The local stratigraphy comprises metasedimentary and metavolcanic supracrustal rocks metamorphosed to greenschist facies. The supracrustal units have been intruded by suites of phaneritic and sub-volcanic rocks which vary in composition from mafic to felsic phases. Importantly, gold mineralization in the Aurora deposit has been observed in almost all lithologies identified across the GSM.

Figure 7-3 illustrates the property geology and Figure 7-4 illustrates the stratigraphic units on the property.

MAFIC VOLCANICS

These rocks are a package of melanocratic, aphanitic mafic volcanics which are interpreted to be the lowest in the supracrustal sequence. It is commonly a massive, fine grained, magnetite rich, brittle unit, however, compositional and rheological variations have been observed in core as illustrated in Figure 7-5a. Pillowed textures have not been observed in the unit to date. Based on physical observations of mineral constituents, this unit is interpreted to have a high Fe to Mg ratio, and therefore would be classified as a tholeiite.

VOLCANOCLASTIC SEDIMENTS

This package consists largely of fine interbeds of immature sandstones and siltstones, which form texturally unique alternating beds as illustrated in Figure 7-5b. Volcanoclastic sediments comprise an appreciable proportion of greenschist and mafic mineral phases. The sandy and silty units are generally more ductile and can be difficult to identify when highly strained and pervasively altered. In zones where straining is not well developed, subtle grain fining directions can be observed in the coarser beds, although this is difficult to correlate over any distance. Matrix supported units with stretched coarse lithic clasts have been observed in some areas, although these units seem to be narrow and discontinuous.

CARBONACEOUS SEDIMENTS

This group consists of alternating layers of thin beds of carbonaceous shales and fine grained, well graded argillaceous sandstones and siltstones/greywackes as illustrated in Figure 7-5c. This is a relatively ductile unit and can become indiscernibly deformed in high strain domains. Thicker beds of silica-rich, feldspathic sandy layers have been occasionally observed. Due to both the chemistry and ductile nature of these rocks, they are generally unmineralized.

CARBONACEOUS SEDIMENTS

This group consists of alternating layers of thin beds of carbonaceous shales and fine grained, well graded argillaceous sandstones and siltstones/greywackes as illustrated in Figure 7-5c. This is a relatively ductile unit and can become indiscernibly deformed in high strain domains. Thicker beds of silica-rich, feldspathic sandy layers have been occasionally observed. Due to both the chemistry and ductile nature of these rocks, they are generally unmineralized.

LATE BASIN SEDIMENTS

Although this package has not been observed at the GSM, it is still considered significant due to its regional spatial extent as inferred from historical mapping. The unit consists of a clast supported, polylithic conglomerate and interbeds of siltstones and sandstones as illustrated in Figure 7-5d. The lithic clasts consist of lower supracrustal greenstone sequences with lesser volcanics, sediments and subvolcanics. Fining sequences are well preserved in the sandstones and siltstones, and evidence of primary depositional textures have been recorded on layer boundaries.

INTRUSIVE SUITES

Intrusive emplacement in the supracrustal units of the Cuyuni Belt in the Aurora area is currently understood to be linked to two main events: the regional Transamazonian orogeny and post-orogenic dike intrusions related to the Cretaceous Atlantic rifting.

The most volumetric of these are the Transamazonian batholiths, which are mostly equigranular granitoids of a felsic to intermediate composition. The Aurora deposit is located on the n margin of one of these batholiths. Smaller stocks, sills, dikes, and irregular plutons of varying composition are also interpreted to have been emplaced during this orogenic event.

DIORITE

The Rory’s Knoll diorite is the most significant host to gold mineralization at Aurora. When unaltered, it appears as a medium grained phaneritic unit but with increasing alteration becomes almost unrecognisable as all the primary minerals and textures have been obliterated. As a fresh rock it consists of interlocked crystals of quartz, feldspar, and hydrous ferromagnesian silicates and have been observed with traces of magnetite and sericite in lesser altered occurrences as illustrated in Figure 7-6a. The unit varies from unstrained to highly strained with pervasive continuous foliation characterized by significant stretching lineations on the remnant ferromagnesian crystals. It is notable that there are occurrences of diorites outside of Rory’s Knoll, particularly at Walcott Hill which are not as significantly mineralized. The main difference noted with the lesser endowed occurrences is the lack of ductile straining which appears to precede the mineralizing event.

QUARTZ FELDSPAR PORPHYRY

This is the main host to gold mineralization at the Mad Kiss deposit. It is a porphyritic unit with medium grained phenocrysts of quartz and feldspar within a fine grained to aphanitic siliceous matrix. The unit has been observed with tightly spaced disjunctive foliations which have elongated quartz and feldspar crystals (Figure 7-6b). In the Mad Kiss deposit, the unit occurs as a tabular body which trends northeasterly and dips steeply to the northwest. Other smaller intrusives have been observed at Aleck Hill, West Mad Kiss, and Powis Hill.

UNDIFFERENTIATED MAFIC INTRUSIVES

These fine grained phaneritic rocks are composed of interlocked feldspars and ferromagnesian minerals, or as feldsparphyric mafic porphyries, illustrated in Figure 7-6c. They are generally brittle units and have not generally been observed with evidence of significant ductile straining. Although volumetrically they are not a major constituent of the rock package at the mine scale, they have been identified as a host of mineralization at Aleck Hill and East Walcott. Their occurrence has been noted on the layered contacts in sediments and volcanics, and on stratigraphic contacts at all levels in the Barama supracrustals.

MINERALIZATION

Gold mineralization at Aurora is divided into four main mineralized zones; Rory’s Knoll, East Walcott and Walcott Hill, Mad Kiss and Mad Kiss West, and Aleck Hill and Aleck Hill North (Figure 7-14).

All the deposits display an association of gold mineralization with quartz veining and pyrite, locally as much as 10%. The auriferous veins developed relatively late in the deformational history and occur as brittle stockworks in more competent host rocks, e.g., Rory’s Knoll diorite and lesser quartz-feldspar porphyry dikes, and as foliation parallel, ribbon-like veins that vary in width from a few centimetres to rarely up to one to two metres wide.

For more information on Aurora’s geological setting and mineralization, please read the Roscoe Postle Associates Inc. Technical Report entitled, “March 2019 – Technical Report on the Aurora Gold Mine, Guyana” found under Technical Reports.