Earth’s Continental Crustal Gold Endowment
The analysis of the temporal distribution of gold deposits, combined with gold production data as well as reserve and resource estimates for different genetic types of gold deposit, revealed that the bulk of the gold known to be concentrated in ore bodies was added to the continental crust during the Mesoarchaean gold event at a time (3Ga) when the mantle temperature reached a maximum and the dominant style of tectonic movement changed from vertical, plume-related to sub-horizontal plate tectonic (Frimmel, 2008).
A magmatic derivation of the first generation of crustal gold from a relatively hot mantle that was characterized by a high degree of partial melting is inferred from the gold chemistry, specifically high osmium contents. While a large proportion of that gold is still present in only marginally modified palaeoplacer deposits of the Mesoarchaean Witwatersrand Basin in South Africa, accounting for about 40% of all known gold, the remainder has been recycled repeatedly on a lithospheric scale, predominantly by plate-tectonically induced magmatic and hydrothermal fluid circulation, to produce the current variety of gold deposit types. Post-Archaean juvenile gold addition to the continental crust has been limited, but a mantle contribution to some of the largest orogenic or intrusion-related gold deposits is indicated, notably for the Late Palaeozoic Tien Shan gold province (Frimmel, 2008).
The metallogenic signature and endowment of individual accretionary origins are, at a fundamental level, controlled by the nature, composition and age of the sub-continental lithosphere and a complex interplay between formational processes and preservational forces in an evolving Earth. Some deposit types, such as orogenic gold (e.g. Las Cristinas in Venezuela, Gros Rosebel in Suriname, Omai in Guyana), have temporal patterns that mimic the major accretionary and crustal growth events in Earth history, whereas others, such as epithermal gold-silver deposits, none of which have so far been found in the Guiana Shield, have largely preservational patterns.
Gold in the Guiana Shield
The rich gold endowment of the Guiana Shield has been recognized for many years. The gold bearing areas are spread out over parts of Venezuela, Guyana, Suriname, Brazil and French Guiana, especially in the northern sectors of these countries.
Gold extraction and exploration has been reported in the Guiana Shield since the 1600’s. In this gold metallogenic province, there are two types of gold deposits: (a) lode gold and (b) placer gold.
Lode gold deposits are frequently found in the Precambrian terranes globally. Prolonged weathering of lode gold deposits has produced an overlying residual soil called saprolite, which can be enriched in gold. Under certain climatic conditions, the natural erosion of these soils and underlying rocks, contributes gold to the streams and rivers forming placer gold deposits.
Gold Deposit Types
Hard-rock gold deposits of the Guiana Shield have been divided by Gibbs and Barron (1993) into three main types:
1) Deposits consisting of large auriferous quartz veins in mafic and ultramafic metavolcanic rocks. Examples include El Callao in Venezuela, Wariri, Jubilee Creek and Baramita in Guyana and Lawa in Suriname. Most veins are in rocks metamorphosed to greenschist facies but near rocks of the amphibolite facies.
2) Deposits in auriferous quartz veins near the contacts of granitic porphyry dykes or stocks. Examples of this type are in the Million Mountain, Peters Mine, Eagle Mountain, Aurora, Aranka, Yakishuru and Omai occurrences in Guyana.
3) Deposits associated with tuffaceous, pelitic, carbonaceous and cherty metasediments. These deposits have smaller quartz veins than other types and the quartz may be gray due to graphitic inclusions. Examples include the Eldorado, Kaburi, Honey Camp, Aremu and Tassawini occurrences in Guyana.
Enrichment of gold has occurred as the result of tropical weathering in much of the Guiana Shield. The liberation of gold allows for recovery using low cost gravity and leaching
Gold in Granite – Greenstone Belts
Gold has been won in smaller or larger amounts from every greenstone belt of any size and its occurrence is the principal reason for the early prospecting and mapping of these belts (Boyle, 1991).
The gold is principally in vein deposits cutting basic or intermediate igneous rocks both intrusions and lava flows. The greatest concentration of gold mineralization occurs in the marginal zones of the greenstones belts, near the bordering granite plutons and it decreases towards the centre of the belts. This may suggest that it has been concentrated from the ultramafic-mafic volcanics (typical of island areas) by the action of thermal gradients set up by the intrusive plutons.
The Prospecting Licences are located in the Trans-Amazon province of the Guaina Shield, which is part of the Amazonian Craton (Teixeira et al, 1989).
The Trans-Amazonian province is a granitoid-greenstone terrane generated between 2.25 and 2.00 Ga (Gibbs and Olszewski, 1982; Cox et a;., 1993, Santos et al., 2000), whose structural trends broadly parallels the Atlantic coast from Venezuela, through the Guianas to Amapa state in Brazil.
In Guyana, the rocks of the region, collectively known as the Barama-Mazaruni Supergroup, are Paleoproterozoic in age and comprise an east-west trending series of a series of mafic through felsic volcanic flows with intercalated clastic sediments (Gibbs, 1980; Gibbs and Barron, 1993).
Local Geology and Bedrock Gold Potential
Examination of the regional geological and regional aeromagnetic maps of the Exploration Area containing the six PLs reveals that they are underlain essentially by Palaeoproterozoic granite – greenstone belt rocks of the Barama – Mazaruni Supergroup, which have been intruded by dolerite dykes of the Mesoproterozoic Avanavero Suite.
Geologically, Guyana is subdivided into three provinces; Southern, Takutu and Northern. The Northern Province (gold bearing) is further subdivided into three main geological units; greenstone belts, the Roraima Group and recent Tertiary/Quaternary sediments. The greenstone belts of the northern part of the Guiana Shield is largely composed of the Barama-Mazaruni Supergroup (1.9-2.3 Ga) which is divided into three sub-parallel northwest trending belts. From south to north the belts are named: the Mazaruni, Cuyuni and Barama Groups. All belts have similar geology composed mostly of metasedimentary/greenstone terrain intercalated with Archean-Proterozoic gneisses that are intruded by Trans-Amazonian granites.
The Mazaruni Group (best exposed) consists of the Issineru Formation (meta-basalts with layers of dolerite, gabbro, tuffs) followed by the Haimaraka Formation (predominantly metasediments with andesite and conglomerate layers). The Cuyuni Formation (moderately exposed) consists of basal ultramafic and meta-mafic volcanic rocks, overlain by andesitic and subordinate felsic flows and tuffs. These units are locally unconformably capped by metasediments.
conglomerates. The Barama Group (least exposed) consists of a series of ultramafic rocks, porphyritic intrusions, metaconglomerates and metavolcanics.
The Barama-Mazaruni Supergoup formed within a geosynclinal basin locally bordered by an Archean continental foreland. The Trans-Amazonian Orogeny, approximately 2,000 million years ago, resulted in block faulting, crustal shortening, folding, metamorphism and anatexis of the Barama-Mazaruni Supergroup.
The regional metamorphic grade of the Barama-Mazaruni Supergroup is generally lower to middle greenschist facies. Near the contact of some of the larger granitic complexes, the Barama-Mazaruni Supergroup is metamorphosed to upper greenschist to amphibolite facies.
Syn- to late-tectonic calc-alkaline to intermediate intrusive rocks, collectively known as the Trans-Amazonian Granitoids were emplaced during the Trans-Amazonian Orogeny, between 2,250 and 1,960 million years ago. They range in composition from granite to granodiorite, diorite, and adamellite.
The Paleoproterozoic terranes of the Guiana Shield are marked by several large scale shear zones. The most prominent of these structural corridors stretches over 500-600 km in a west-north-westerly direction across most of the Guiana Shield. In Guyana this feature is known as the Makapa-Kuribrong Shear Zone (MKSZ). An interesting observation is that a majority of the known gold mineralization systems are located in thevicinity of these regional tectonic features associated with the Barama-Mazaruni Supergroup
The Quartzstone area has been worked for gold by artisanal workers since 1983. The peak of gold production was between 1893 and 1902 when about 244,000 ounces were produced from placers in the Quartzstone River area. Much emphasis was placed on the development of alluvial gold in the area as systematic work was done by the New York – Alaska dredging Corporation in 1953 and the British Guiana Geological Surveys in 1955.
In 1988, the Eastern Carribbean Company identified an area of significant primary mineralization in some trenches (an average of 2.1 grammes/ tonne gold [0.062 oz/ ton] from 99 samples). They were, however, unable to continue their work due to financial constraints. International Roraima Gold Corporation acquired the property in 1992 and immediately undertook a systematic geological examination over the entire property.
The Peters Mine is located in the central part of northern Guyana at approximately latitude 6˚15’ North and longitude 59˚22’ West. It is situated on the west bank of the Puruni River approximately 45 kilometres above its confluence with the Mazaruni River; it is some 145 kilometres southwest of the capital city of Georgetown. A road, 80 kilometres in length, connects Peters Mine to Bartica. The nearest towns are Issano, about 40 kilometres up the Mazaruni River from the mouth of the Puruni, and Bartica, the main departure point for much of the northern part of the country. The total distance by road from Georgetown to the mine is about 180 kilometres.
The Peters Mine property is underlain by greenstones, volcaniclastic sediments and intrusives (Barama-Mazaruni Supergroup), which are locally unconformably overlain by conglomerates and sandstones probably the middle Proterozoic Roraima Supergroup. These are basic metavolcanic rocks in the west; a broad shear zone, Mango Trend Deformation Zone (MTDZ), in the centre; and felsic intrusive rocks in the east.
Mineralization on the Peters Mine property is typically hosted in quartz veins characterized by sericite alteration. Gold is typically free milling and associated with pyrite, sphalerite and molydenite. Gold is also present in the saprolite zone.
The Mineral Resources of the Peters Mine property are:
|Tonnes||Au g/t||Tonnes||Au g/t|
Discovery found at the eastern edge of its Toroparu Gold Deposit located in the Republic of Guyana. The
most recent mineral resource estimate for the Toroparu Gold Deposit contains 240.89 million tonnes @
0.78 g/t Au for 6,034,000 ounces in the Measured and Indicated categories and 179.18 million tonnes @
0.69 g/t Au in the Inferred category contained within an optimized pit at a cutoff grade of 0.28 g/t Au.
|Resource Classification||Tonnes (000’s)||Au (g/t)||Au oz (000’s)||Cu %||Cu (M lbs)|
|(All rock types)|
|Measured & Indicated||240891||0.78||6034||0.08||420|
Million Mountain Gold Project
The Million Mountain Gold Project operated by Sacre Coeur Minerals Limited encompasses an area of about 88.5 km2 (34 square miles or 21769 acres). This area is located on the Puruni River, 6 km north of Peters Mine.
The Million Mountain project area is underlain by rocks of the Cuyuni Formation which is part of the Barima-Mazauni Supergroup, which with granitoid bodies make up the Northern Guyana Greenstone Belt, which consists of three district island-arc systems (North-west District, Cuyuni and Mazaruni greenstone belts) which accreted during the Paleoproterozoic Trans-Amazonian orogeny.
The region surrounding the Million Mountain ridge is underlain by two major types of geological terrain separated at the ridge by a major structural discontinuity known as the Million Mountain Break (Barron, 1967).
Barron (1967) recognized the Million Mountain Break as an abrupt structural break between a granite pluton to the south and low-grade metamorphosed sedimentary and acid to intermediate volcanic rocks of the Cuyuni Formation to the north.
Aurora Gold Deposits
The Aurora property operated by Guyana Goldfields Inc. is located in the Cuyuni greenstone belt of the Guiana Shield in the Amazon Cartaon. It consists of a number of gold deposits on the eastern side of the Aurora zoned intrusion. These deposits are about 35 km northwest of the Cuyuni River Resources Exploration Area.
Gold was produced from underground operations at Aleck Hill and the Mad Kiss Mines between 1940 and 1948 and the gold deposits were interpreted as fracture-controlled, mesothermal veins. The gold veins are accompanied by ankeritization, pyritization and sericitization of the wall rocks. Where the gold veins are closely spaced, gold values approach commercial values.
- At a 1 g/t Au cut off the Total Indicated Resources is 1.86 Million Ounces and the Total Inferred Resource is 2.54 Million Ounces for a total of 4.40.
|Grade||Tonnes (millions)||Contained Ounce (millions)||Grade||Tonnes (millions)||Contained Ounce (millions)|
- At a 0.50 g/t Au cut off the Total Indicated Resource is 1.97 Million Ounces and the Total Inferred Resource is 2.68 Million Ounces for a total of 4.65 Million ounces.
|Grade||Tonnes (millions)||Contained Ounce (millions)||Grade||Tonnes (millions)||Contained Ounce (millions)|