Summary:
The Gramalote Ridge, Trinidad, and Monjas West deposits are examples of an intrusion related gold deposits.
Mineralization at the Gramalote Project is an unusual example of the deposit type in that the Gramalote Ridge, Trinidad, and Monjas West host intrusion has a different oxidation state to that of a typical host intrusion, which is reduced. This is reflected in the scarcity of ilmenite, and the common presence of primary magnetite.
The Project is located in the northern portion of Colombia’s Central Cordillera. The terrane is primarily comprised of a metamorphic basement complex and the Antioquia Batholith. The Cajamarca–Valdivia basement terrane (Cediel and Ciceres, 2000 and Cediel et al, 2003) consists of early Paleozoic metamorphic rocks; as well as ophiolitic, oceanic, volcanic, and intrusive rocks.
Gramalote Ridge
The Gramalote Ridge deposit has dimensions of 1,300 x 1,500 x 700 m. The mineralization has been drill tested to a depth of about 865 m.
Mineralization at Gramalote Ridge is hosted in a uniform medium-grained tonalite– granodiorite body within the regional quartz diorite and diorite that makes up most the Antioquia batholith.
Alteration is structurally-controlled and occurs as both broad zones and narrow selvedges around veins. At Gramalote Ridge, vein selvedges range from a few millimetres up to as much as 10 cm.
Gramalote Ridge mineralization occurs within several zones that periodically coalesce both along strike and down-dip. Zones vary in width from 10–150 m in true width with vertical to sub-vertical dips to the south–southeast. These east–northeast-trending mineralized corridors are made of a subtle to moderate increase in vein density, coalescing alteration selvedges, and sulphide percent, with internal veinlets trending in three main directions (northwest, north–south and northeast).
The deposit remains open at depth.
Mineralization is vein hosted, either in sheeted veinlets or in local stockworks, and is structurally-controlled. The mineralized zones form magnetic lows in the magnetic maps, due to the alteration halos associated with the mineralized veinlets.
The following paragenetic stages are identified and associated with vein and alteration types:
• Quartz–carbonate veinlets with weak potassic alteration halos and fine-grained pyrite;
• Quartz–calcite–pyrite or chalcopyrite–gold veinlets; related to well-developed Kfeldspar or carbonate alteration. Associated with elevated gold grades. Gold occurs in fractures in pyrite together with chalcopyrite. Sulphides can include molybdenite and, more rarely, sphalerite;
• Pyrite–white mica veinlets with minor calcite or quartz. Associated with moderate to strongly developed white mica alteration in host rocks. Rarely mineralized;
• Coarse grained pyrite veinlets and lenses. Commonly associated with the highest grade gold values. Preferentially located at edges or centerlines of quartz– carbonate veinlets.
Cross cutting relations can be very variable, and most vein types, especially Types 1–4 may also be synchronous. Anomalous gold mineralization is associated with Type 1, Type 2 and Type 5 vein types, with Type 1 and Type 2 being the most prevalent. Type 7 veins are associated with the intrusive fabric, not the mineralizing systems.
The silver to gold ratio is approximately 1:1. Free gold occurs as argentiferous gold coeval with several tellurides and bismuth sulphosalt minerals. The primary sulphide mineralization consists of pyrite and chalcopyrite.
Trinidad
The Trinidad deposit has dimensions of approximately 1,500 x 500 m. Mineralization has been drill tested to a depth of about 521 m.
The Trinidad mineralization is hosted in similar rock types to those described for Gramalote Ridge.
Mineralized zones at Trinidad are 10–80 m thick and are continuous along strike for up to 1,500 m and down dip for >500 m. As at Gramalote Ridge, such zones periodically coalesce both along strike and down-dip. The deposit remains open at depth and along strike, especially to the west.
Mineralization is associated with stockwork veinlets and alteration along their margins. Veinlets have been classified into seven types, and gold mineralization is associated with Type 1 (quartz with fine pyrite), Type 2 (quartz with iron carbonate) and Type 5 (quartz with granular pyrite) veins.
Monjas West
The Monjas West deposit has dimensions of approximately 1,000 x 500 m. Mineralization has been drill tested to a depth of 400 m.
Mineralized zones at Monjas West are 10–80 m thick and are continuous along strike for up to 700 m and down dip for >350 m. As at Gramalote Ridge, such zones periodically coalesce both along strike and down-dip. The deposit remains open at depth and along strike, especially to the west.
Mineralization is associated with stockwork veinlets and alteration along their margins. Veinlets have been classified into seven types, and gold mineralization is associated with Type 1 (quartz with fine pyrite), Type 2 (quartz with iron carbonate) and Type 5 (quartz with granular pyrite) veins.