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Brazilian gold rush.

There is greatly increased interest in the gold potential of Brazil

Last year in The Mining Journal's Emerging Markets special supplement (MJ, September 29, 1995, p.6) it was announced that Brazil had jumped from thirteenth to second position in a ranking of emerging markets important to mining investors. This jump reflected the change in the country's constitution which now, once again, allows 100% foreign ownership of mining properties. This change has proved to be the catalyst behind what some believe could be the most significant gold rush in Brazil's history.

Brazil has a long history of gold rushes. One of the world's first began there in the 18th century when Banderanti (the Brazilian counterparts of the U.S.'s 49ers) fought with disease, rugged terrain and hostile Indians to produce up to 16 t/y of gold from its, then, uncharted interior.

More recently from the late 1970s through the 1980s to the present, Brazil experienced a number of new gold rushes. A combination of drought in the rural north-east and high unemployment in the industrial south caused an exodus of people to the now famous gold fields of Serra Pelada, Alta Floresta, Cumaru, Rio Madreia and Tapajos. By early 1983 up to 450,000 garimpeiros (hand-miners) were working in an area stretching from Rondonia in the west to Para in the east and during the early 1980s they were responsible for 90% of Brazil's gold production. For example, in 1983 alone, 50,000 men extracted 13 t of gold from an 80 m deep pit at Serra Pelada.

International interest

However, the participants in the current Brazilian gold rush are not Brazil's poor seeking their fortune, but are the world's international mining companies, seeking to add share-holder value. Along with the soon to be privatised Companhia Vale Do Rio Doce (CVRD), they are conducting exploration programmes in many parts of the country. This reflects the widespread distribution of highly prospective Archaean shield. Of the various shield areas one, the central Brazilian Shield encompassing the region of northern Mato Grosso and southern Para, has attracted the attention of majors such as Anglo American, Barrick Gold, Western Mining and TVX along with a number of juniors, notably Ambrex Mining and Brazilian Resources. All are conducting, or are about to start, target drilling programmes.

Ambrex Mining has concentrated on the formation, through acquisition and staking, of a large land position (295,000 ha) in the region of the Jurena-Teles Pires horst, bound to the north by the Cachimbo graben and to the south by the Caiabas graben and Eugenia Arinos horst. In this region gold mineralisation is associated with continental-scale shearing to the north and south of the Jurena-Teles Pires horst and within the volcanic and metasedimentary unit exposed at the margins of the grabens. The basement horsts consist of the Archaean Xingu Complex which is mainly composed of gneisses and migmatites with intercalated amphibolites and mica schists. During the early Proterozoic, acid to basic metavolcanics and detrital chemical sediments (iron formations) of the Alto Paranaita Formation were deposited over the basement. Later, in the mid-Proterozoic, graben-faulting was initiated and the acid volcanics and chemical sediments of the Uatuma Group were deposited. Subsequently, two phases of late granitic and alkaline plutons including the Teles Pires granites were intruded. The intrusives are confined for the most part to the graben boundary areas.

During the Paleozoic, the Caiabis and adjacent Dardanelos graben were capped with sediments of the Dardanelos Formation which now form the hard sandstones topping the plateaux formed by the resistant grabens.

Gold mineralisation within the region is found in six main settings:

* Quartz vein with pyrite and chalcopyrite.

* Stockworks within granites.

* Shear zones and related silicification, which can also host polymetallic mineralisation

* Shearing associated with doming above late alkaline intrusives.

* Supergene enrichment.

* Alluvial deposits.

It is the alluvial deposits of the region that have proved the most significant to date, not only because they have yielded approximately 6.7 Moz of gold over the past 16 years but also because the garimpeiro workings which are clearly visible by satellite have provided abundant, well-defined exploration targets.

Cava do Expedito

Ambrex has five properties in the region. The Expedito concession, near the town of Aripuna, is the most advanced. During the 1980s garimpeiros produced between 8-10 t of gold, primarily from alluvial sources on the property. However, decreasing recoveries and inexperience with hard-rock sources forced most of the garimpeiros to move on. At Expedito, Ambrex has the right to earn a 70% interest in the project from Consolidated Madison by spending $US4 million over a 42-month period and has begun a diamond drilling programme using two drill rigs, to test an altered polymetallic gold-bearing shear zone. The shear zone was identified from exposure in a now-abandoned hard rock garimpeiro working known as Cava do Expedito, and by a large geochemical and geophysical anomaly.

The Cava Do Expedito working brought the property to the attention of Anglo American in 1993 when its Brazilian subsidiary Unigeo began a soil geochemical survey in the area, and defined anomalous zones 150 m to the south-east and 300 m to the northwest of the Expedito pit. However, it was unable to come to an agreement with the garimpeiros working in the area and staked the surrounding areas along strike to the northwest and to the south. Currently, approximately 100 m northwest of the Expedito property boundary, Unigeo is diamond drilling, on 50 m grid, a mixed sulphide target said to contain signifcant gold and silver along with copper, lead, zinc and molybdenum.

After Anglo's investigations, Western Mining's Brazilian subsidiary gained the right to conduct geochemical and geophysical surveys over the 2,000 ha concession and confirmed strong EM and IP anomalies with associated polymetallic anomalies along the strike of the shear zone. However, WMC withdrew from western part of the region and focused on the south-eastern part of the Cachimbo graben near to Ambrex's Cabeca property. In 1995 Ambrex acquired its option to the property from Consolidated Madison and shortly after, all of the WMC data.

Ambrex then proceeded with a geochemical survey using hand augers to sample weathered bedrock at one to two metres depth along with IP and magnetic geophysical surveys. The surveys, conducted on a larger grid, have extended the original WMC anomaly. This has prompted Ambrex to begin a 2,500 m diamond drill programme to test mineralisation beneath the Expedito pit and then test the strongest anomalies between the pit and the property boundary with Anglo American's concession, to the northwest. The gold mineralisation occurs in a high grade zone of up to 11.87 g/t within the shear zone and 0.43 g/t - 3.73 g/t in silicified bands up to 50 m from the main shear.


At Ambrex's Triaro property 420 km to the east of Aripuna, and 70 km north east of the city of Alta Floresta, the gold mineralisation occurs in a different setting to that at Aripuna. On the southern edge of the Cachimbo graben Ambrex's geochemical reconnaissance programme has identified the source of alluvial gold for the extensive alluvial workings in the area as a quartz stockwork hosted by a granite which now forms a prominent hill.

The stockwork appears to be the result of regional faults parallel to the graben boundaries causing the development of cataclasites within the granite. These are characterised by intense quartz veining, sericitisation and the development of sulphide rich zones, mainly pyrite and chalcopyrite. To date. preliminary geological mapping has identified a number of cataclasite zones on the 35,000 ha property, which Ambrex plans to investigate further, using geophysics and geochemical sampling.

Castello dos Sonhos

Approximately 150 km north east of Ambrex's Triaro prospect, in the Tapajos area, is Barrick's aptly named Castello dos Sonhos (Castle of Dreams) project. The deposit came to the attention of Barrick after garimpeiros working the streams draining this conical hill produced around 10 t of gold. Barrick has placed this quartz stockwork deposit at the core of its $US10 million Brazilian exploration programme and has three drilling rigs investigating the its hardrock potential.

If the prospects identified by companies such as Ambrex, Anglo American, Barrick, TVX and Western Mining are typical of what companies could expect to find in Brazil, then coupled with the increased investor interest in Brazil caused by the planned privatisation of the giant CVRD, it would seem likely that this year Brazil will be in the running to assume the number one position as the prefered emerging market for mining investment.
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Author:Thomas, Andrew
Publication:Mining Magazine
Date:Aug 1, 1996
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