Summary:
Coffee is interpreted to represent a shallow-level (epizonal) structurally controlled orogenic gold deposit (Buitenhuis et al., 2015; Allan et al., 2013). Mineralization textures are described below for the four deposits which comprise the Coffee resources and reserves, namely Supremo, Latte, Double Double, and Kona.
Supremo.
Narrow gold-bearing brittle fault structures with gold hosted in intense fracture zones, immature clast supported breccia and in zones of most intense deformation matrixsupported breccia. Mineralization commonly on the margin of and within dacite dykes which intruded along the fracture zones premineralization. Gold mineralization and accompanying quartz-sericite-pyrite alteration associated with later reactivation of structures. Complete oxidation up to 250m below surface.
The gold mineralization at Supremo is generally characterized by two distinct styles: brecciated mineralization and biotite replacement mineralization. The highest grades are associated with polyphase hydrothermal breccias.
Breccia textures range from mature matrix-dominant phases with rounded fragments to immature wall rock crackle breccias. Matrix compositions range from incompetent limonite-clay material to strongly silicified material. Angular-to-subrounded clasts range from 0.5 to 3 cm in diameter and consist predominantly of highly silicified fragments and subordinate altered wall rock and dacite porphyry fragments. Brecciated clasts occur locally, indicating multiple phases of brecciation.
The lower grade gold mineralization is associated with pervasive hydrothermal alteration of non- brecciated gneissic host rock. Biotite is pseudomorphously replaced by pyrite and the hydrothermal alteration is characterized by an overall removal of potassium and aluminum with the addition of sulphide, carbonate, and silica.
Latte.
Gold is hosted in zones of brecciation and strong fracturing as well as areas with pervasive sericite alteration and strongly disseminated sulphides. Some high-grade zones associated with quartz vein breccias. Dolomite-illite/sericite-arsenian pyrite sulphidizes foliaform biotite laths. Potential remobilization of gold to other structures. Complete oxidation up to 75m below surface.
Latte consists of a stacked set of moderately to steeply south-southwest dipping, east-southeast striking brittleductile structures, whereas the Latte North structure splays off from the main Latte structure and dips moderately to steeply to the southeast with a north-easterly strike. No shear fabric or observable high strain indicators are visible in association with the steep and mineralized Latte structures.
The western portion of the Latte zone is dominated by broad regions of disseminated mineralization found throughout a wide panel of biotite schist. The western structures strike approximately 100° and contain five or more mineralized shoots which merge and separate along strike. The structures continue to the east and eventually merge into the Connector zone, where the Supremo north-south structures and the east-west Latte and Double Double structures converge. Total traceable length of the mineralized Latte structure is currently in excess of 2,100 m.
Mineralization at Latte consists of disseminated gold-bearing arsenian pyrite, overprinted by later brecciation and late fluid ingress. Mica-rich rocks are the main host for gold, with a three phase mineral reaction resulting in gold precipitation. Gold-bearing mineralizing fluid rich in CO2-As-Sb and S reacted strongly with Fe-bearing biotite within the biotite schist at Latte. A sulphidation reaction proceeds, in which Fe within the biotite is leached to form finegrained arsenian pyrite, illite, and dolomite which pseudomorphously replace the parent biotite grain. Titanium within the parent biotite is removed and incorporated within hydrothermal illite and rutile.
Double Double.
Narrow gold-bearing brittle structures hosted in matrix-supported breccia including dacite porphyry fragment breccia. Anastomosing quartz vein networks and microbreccia associated with high-grade. Local intense silicification and strong disseminated sulphide mineralization. Complete oxidation up to 350m below surface.
The Double Double zone trends east-northeast with a known strike length of 600 m. It dips steeply to the north and consists of a number of discrete, high-grade strands of mineralization up to several metres wide. Host rocks are augen-bearing gneissic rocks with interleaved biotite-feldspar-quartz (± muscovite ± amphibole) schist. The gold mineralization at Double Double is structurally controlled, and may be associated with a north-easterly trending splay off the main Latte structure.
Gold-rich intervals at Double Double are characterized by relict schistose to mylonitic textures overprinted by mottled silica and sericite alteration in addition to limonite-filled micro fracture networks and oxidized pyrite cubes. Breccia domains locally exceed 50% by volume within gold zones, characterized by silicified fragments as well as strongly altered wall rock and porphyry dyke clasts.
Kona.
Gold hosted in quartz-sericite altered granite. Iron oxides after disseminated pyrite, pyrite veinlets, stockworks and sooty-pyrite rich shear zones.
The Kona zone is hosted within coarse grained equigranular biotite monzogranite, and consists of 1-3 east-northeast trending, steeply southdipping fault structures. The gold structures are associated with narrow, less than 5 m wide, sparsely feldspar phenocrystic to aphanitic andesite to dacite dykes.
Alteration typically consists of sericite, clay and limonite, with illite being detected during reconnaissance PIMA work at Kona. Sulphide is dominated by sooty arsenian pyrite, which typically replaces ferromagnesian minerals, and also occurs as veins/veinlets or fracture fill, and in sulphide- matrix fault breccias.