Summary:
Gold mineralization at Aurora is divided into four main mineralized zones:
1) Rory’s Knoll;
2) East Walcott and Walcott Hill;
3) Mad Kiss, Mad Kiss West and Mad Kiss South;
4) Aleck Hill and North Aleck Hill.
The Project lies within the Paleoproterozoic Guiana Shield’s greenstone belt, which comprises metamorphosed volcanic and sedimentary rocks. The area features the Barama and Cuyuni Groups, with significant deformation and chemical weathering forming a laterite-saprolite profile.
The Aurora Gold Mine is in a high-strain zone near a granitic batholith, with stratigraphy dominated by metasedimentary and metavolcanic rocks intruded by late stage felsic to mafic suites. The Rory’s Knoll diorite and Mad Kiss quartz-feldspar porphyry are key gold-hosting lithologies due to the brittle deformation imparted on these lithologies.
Gold occurs in four main mineralization zones: Rory’s Knoll, East Walcott/Walcott Hill, Mad Kiss, and Aleck Hill. Mineralization is associated with quartz veining and pyrite, often localized in fold hinges and high-strain zones. Structural controls include foliation, fold interference patterns, and extensional veining.
• Rory’s Knoll (‘‘RK’’): The primary deposit is hosted in a highly altered diorite pipe, with mineralization extending over 2 km below surface, associated with quartz veins and disseminated pyrite.
Most of the gold mineralization occurs within the Rory’s Knoll tonalite, which extends vertically 2,600, approximately 200 m along a trend of 315° west, and plunges steeply to the north-northwest.
• East Walcott/Walcott Hill: Mineralization occurs in fold closures and foliated diorites, extending to a depth between of 420 m and 650 m.
• Mad Kiss (‘‘MK’’): Hosted in a foliated quartz-feldspar porphyry dike, with mineralization extending to a depth of up to 700 m. High-strain corridors control geometry.
The Mad Kiss deposit is located between the Rory’s Knoll and Aleck Hill deposits, approximately 500 m southwest of the Rory’s Knoll deposit. The mineralization occurs as extensional and foliation parallel quartz-ankerite veining hosted by a foliated quartz-feldspar porphyry dike. The dike and related mineralization occur as distinct tabular zones trending 250° and dipping 70° north.
• Aleck Hill (‘‘AH’’)/North Aleck Hill (‘‘NAH’’): Veining is localized in high-strain zones within mafic volcanics and sediments. The area features braided veins and stockworks, with distinct ‘‘pink’’ quartz alteration.
The underground portion of the Aleck Hill (Aleck Hill underground) is located in the northeast corner of the Aleck Hill deposit. The gold mineralization is associated with discrete shear zones and occurs in four distinct sub-vertical quartz-carbonated veins striking approximately 150°. The Aleck Hill underground deposit is approximately 200 m long with a vertical extent of up to 600 m. The thickness of the zone is approximately 55 m, where the width of each domain varies between two metres and 16 m.
Aleck Hill Open Pit Area
In the west and southwest of the Aurora Gold Mine area, the gold mineralization occurs in veins and along vein selvages. The veins are thin, on the centimetre scale, and predominantly quartz. They become more frequent near the hinge of folds. With the axial plane of the folds being sub-vertical, the swarms of gold rich quartz veins tend to produce steeply dipping rodlike structures, with their greatest continuity being along the near-vertical hinge line, their intermediate continuity being parallel to foliation in the horizontal direction, and their shortest continuity being perpendicular to foliation. These ‘‘ribbon vein’’ areas have been modelled using a methodology that differs from the wireframe-based methodology that is appropriate for Rory’s Knoll East Walcott, Mad Kiss, and the underground portion of Aleck Hill.
Two main alteration styles are identified: and both styles are characterized by primarily chlorite, epidote, magnetite and traces of interstitial and veined calcite in the distal zones. Of them, the style 1 alteration consists of pervasive sericite and Fe-carbonate with small component of quartz, and the type 2 alteration is composed of magnetite and calcite with less amount of Fe-carbonate.
All the deposits display an association of gold mineralization with quartz veining and pyrite, locally as much as 10%. Coarse visible gold occurs in quartz-bearing veins and in pyrite-rich fractures.
Mineralization within the volcanoclastic rocks is preferentially developed in fold hinges as silicified, stockwork, and brecciated concentrations of veins exploiting the local, low pressure, extensional environment. Quartz veins typically display crustiform and comb textures; and have been observed in en-echelon sets.
Within foliation dominant fold limbs, gold veining is characterized by narrow, two to twenty-five centimeters, and rarely up to a few meters in width, ribbon line veins that are discontinuous along strike. These veins form anastomosing arrays along a strike of 290° to 305° and dip steeply to the northeast at 70° to 85°.
Alteration is zoned, from distal to proximal to the hydrothermal fluid source. Alteration typically includes silicification, albitization, and pyritization.
The gold mineralization at the Mine exhibits features analogous to mesothermal or ‘‘orogenic’’ gold deposits typified by Archean deposits of the Abitibi region, Canada. Features characteristic of the gold mineralization at the Mine include:
- A strong spatial association to large scale shear zones;
- Relative late timing during active compressional deformation;
- Formed during greenschist metamorphic conditions;
- Association with a propylitic-phyllic alteration assemblages; and
- Principally hosted in quartz-ankerite-pyrite veining.
Whereas the previous interpretation suggested shear-related control, no evidence of strike-slip displacement has been identified in mapping mine exposures. The shear model implied reasonable strike continuity of foliation parallel veins in the volcanoclastic lithologies. Exposures from mining has demonstrated limited strike continuity in the order of five to ten meters of individual veins. It is believed that the mineralization continuity is controlled by the intersection lineation between the earlier foliation and the later, dominant penetrative foliation.