THE DISCOVERY OF EPITHERMAL Au-Cu-Mo PROTEROZOIC DEPOSITS IN THE TAPAJÓS PROVINCE, BRAZIL, by Pedro Jacobi

THE DISCOVERY OF EPITHERMAL Au-Cu-Mo PROTEROZOIC DEPOSITS IN THE TAPAJÓS PROVINCE, BRAZIL

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by Pedro Jacobi

INTRODUCTION

Following a simple principle that a world class deposit should have a world class signature, in 1997 the author and a group of Rio Tinto exploration geologists selected five outstanding gold bearing structural/geological anomalies in the Tapajós Province, Amazon region, northern Brazil. This simple procedure triggered the discovery of several Au-Cu-Mo-Ag mineralized Proterozoic epithermal deposits. The first discovery was a large, low-grade epithermal gold deposit called V3.

THE TAPAJÓS PROVINCE

Tapajós is amongst the largest gold provinces in Brazil. The region is producing gold since 1958. More than 90% of the gold was extracted from thousands of alluvial deposits spread over large areas. A significant number of alluvial districts have produced more than 3Moz of gold during their mining history. Garimpos like Cuiu-Cuiu, Canta-Galo, Abacaxis and Patrocínio are classical examples of quite large sedimentary systems. Alluvial gold deposits are scattered everywhere over a large area in the province. In spite of being a world class gold producing region, mineral exploration efforts started only at the beginning of the 90's and not a single medium to large sized primary gold deposit was known to occur before. The abundance of gold-bearing alluvial deposits induced many exploration geologists to believe that they result from the erosion of former primary deposits. Lack of basic and reliable geological maps, geophysical, geochemical and exploration data contributed to the absence of important primary discoveries in the Tapajós.

Regional geology

The Tapajós Province is situated in Central Amazon and is a part of the large Amazon Craton. In the North, it is bounded by the Amazon Basin near the town of Itaituba, and in the South by the Cachimbo Graben. The main rock units of the Province are attributed to events that took place during the Archaean, Proterozoic and Phanerozoic. Archaean rocks are represented by a few preserved supracrustals. Proterozoic rocks are represented by a huge volume of anorogenic granites and co-magmatic lava flows and pyroclastic rocks belonging to the Uatumã Supergroup (Montalvão et al, 1961), subordinate graben confined sedimentary rocks and mafic, sometimes layered intrusions. Phanerozoic units belong to the Paleozoic/Mesozoic sedimentary rocks of the large intracratonic Amazon Basin and to the Cachimbo Graben.

The main Proterozoic felsic rocks comprise the Parauari granitoids, the Iriri volcanics and late stage granitic intrusions. The Parauari granitoids (1.9-2.2Ga) are frequently rapakivi-type, calc-alkalic, with flat HREE and weakly enriched LREE granites related to the Trans-Amazonian Orogeny. The intrusions occur as large batoliths. The Iriri volcanics (1.7-1.9Ga) consist of rhyolite, dacite, andesite, and trachyte flows and subordinated tuffs, agglomerates, breccias, with minor sedimentary rocks. Late stage, alkalic, granitoids intrude both the Paruari granitoids and the Iriri volcanics are referred to as the Maloquinha-type granite (1.84 Ga). These late stage granitoids are more fractionated than the Parauari rocks and have a Rb/Sr > 1, high Nb/Zr and Gd/Yb ratios. Maloquinha granites are shallow intrusions. They are frequently red, may have a porphyry-like texture and have anomalous contents of F, Zr, REE, Y, Sn, Au and Cu, thus playing an important role in the Tapajós Province mineral economy. No major thermal metamorphism affected the Tapajós Proterozoic units

THE V3 DISCOVERY

V3 is a large epithermal system containing gold and traces of copper, and led to the discovery of more than 5 additional mineralized similar systems in the Tapajós Province. The discovery of V3 started by selecting a conspicuous NE-SW structure closure containing some important alluvial gold occurrences for field investigation. Preliminary exploration consisted of geological mapping and soil geochemistry. At a first interpretation, the results were disappointing because only a few samples from a large grid were anomalous in gold. This fact lead to consider the area as of a lower priority. However, one single sample of rock changed the whole exploration strategy. The sample consisted of a gossan-like, mica-rich rock that, tested by panning, in the bush yield more than 2 g/t of Au. The soils near the sample's site just yield background values. A decision was taken to review the area to better understand the discrepancy. During the subsequent follow-up, a large outcrop of a quartz-pyrophylite-pyrite Au-bearing acid breccia was found. The higher Au values occurred in goethite gossan fragments of the soil's coarser fraction, the - 80# fraction being barren.

These results determined to change the sampling procedure and bulk soil analysis resulted in the outlining of a wide soil anomaly were previous sampling did not have any success. Subsequent detailed mapping evidenced a large volcanic center containing silicified, brecciated, intensely altered, and sulfide-rich rocks, becoming the first gold-copper-silver Proterozoic epithermal deposit of the Province.

GEOLOGY AND MINERALIZATION OF THE V3 EPITHERMAL SYSTEM

The V3 target sustains a hill that is 300 m above the local regional surface, a feature that also occurs in the other epithermal deposits discovered in the Province. The topographic anomaly is due to the intense silicification of the upper portion of the volcanic centers by hydrothermal acid leaching and destruction of most of the pre-existing minerals and forming a characteristic paragenesis. The interaction between hydrothermal fluids and meteoric water caused the precipitation of a widespread siliceous zone. Silica sealed the fractures and open spaces of the volcanic/pyroclastic pile trapping the remaining hydrothermal solutions underneath the silicified horizon. Latter volatiles and fluids developed several stages of brecciation and stockworks. The silicified upper portion is also characterized by the abundance of cavities and vugs filled with quartz, hematite, carbonates and sulfides. These volcanic centers are likely related to deeper porphyry intrusions, as interpreted in figure 1.

Figure 1 - Sketch section of the V3 epithermal deposit. Click to enlarge

The deposits frequently originate a conspicuous U/K anomaly, reflecting the intense hydrothermal alteration. On the other hand, alteration destroyed the original magnetite resulting in weakly or non-magnetic core. Altered rocks are mainly rhyolitic and dacitic flows and lapilli and ash-flow tuffs. Amigdaloidal andesite and dacite flows and tuffs, as well as sedimentary rocks, in spite of frequent are not related to the mineralization event. Deposits such as the V3, as well as its similar V6 formed in a very shallow environment, near the Proterozoic land surface. Their preservation is due to the silica-rich capping, or to the sedimentary cover that resisted to erosion during later uplift of the region.

The V3 epithermal system occurs near the rim of a large collapsed caldera filled with volcanics and sediments, a structure that has never been described before in the Province. Like other epithermal/porphyry deposits, V3 has a conspicuous chemical fingerprint. The most important elements are Au, Cu, Ag, Mo, Zn, Pb, As, Bi, Sb, Y, La, Zr, Ba, and Te. The gold from V3 is also anomalous in PGE, mostly Pd, which may reach up to 15% in the Au-nuggets.

Hydrothermal alteration styles

The Tapajós epithermal systems are characterized by a widespread and intense multistage hydrothermal alteration that resulted from fluid percolation through a dense array of stockworks. Argillic to advanced argillic represented by quartz, pyrophylite, kaolinite, dickite, and allunite assemblage is the main alteration style. The argillic alteration is followed by a phyllic (quartz-sericite-pyrite) assemblage. Propylitisation (carbonate-epidote-gypsum-chlorite) is confined to the more intermediate volcanics, which occur deeper in the epithermal edifice. Potassic alteration is uncommon and represented by K-feldspar and biotite. The main sulfide is pyrite (5-50%) which occurs in disseminations, blebs and masses within a more than 2 km wide outer shell. This huge amount of pyrite-bearing rocks surrounding the core of some epithermal deposits such as the V3 has never been observed before in other Tapajós primary occurrences. There is a possibility that the late argillic and advanced argillic alteration destroyed an early potassic alteration assemblage.

SILICIFIED-HEMATITE-PYROPHYLITE RICH BRECCIAS

Hematite-bearing breccias cap the sulfide-rich breccias of the V3 deposit, and have been found from the surface to a depth of 100 m. Likely tonnage is in excess of 100Mt. Hematite formed as a result of the oxidation of sulfur-gold-copper-silver bearing fluids, during the Proterozoic. Most of the fragments of the breccia were totally replaced by an assemblage of pyrophylite, hematite and microcrystalline silica. Hematitic breccia hosts the main gold and silver at V3. Specks of native copper are common. These breccias suggest that some of the Tapajós Province Proterozoic deposits may belong to the important family of Iron oxide Au-Cu deposits. Hematite content varies from 20 to nearly 100%. Several populations of hematite were observed. Late stage specularite filling fractures (picture) are common.

Figure 2 -Silicified-hematite-breccia. Clasts (some rounded) cemented by hematite and silica. An especularite, late stage, veinlet cuts the rock (click to enlarge)

SULFIDE-SILICA-PYROPHYLITE RICH BRECCIAS

These breccias occur underneath the hematite-bearing breccias after a few-meters transition zone, and are characterized by an intense silicification and sulfidation of the clasts. After initial sealing, late stage fluids broke trough the rock developing a typical crackle breccia with unusual amounts of gold and some silver. Pyrite (10-60%) occurs disseminated as individual crystals, blebs and fine-grained masses. Other minerals comprise andalusite, ilmenite, chalcocite, covelite, and rare rutile, apatite and chalcopyrite.

Figure 3 -Silicified-sulfide-pyrophylite-breccia. Silicified clasts are cemented by finely disseminated pyrite (dark green). Open spaces were filled by masses of pyrophylite (light colours) :click to enlarge

Most of the epithermal deposits of the Tapajós Province occur within a volcanic center with a core made up of brecciated and intensely hydrothermally altered quartz porphyry. The core is rimmed by ash flow tuffs and ignimbrites frequently brecciated and altered. Dacite and andesite flows and pyroclastics are widespread in the region but are unrelated to the mineralized volcanics.

SEDIMENTARY ROCKS

The volcanic pile frequently contains intercalations of Proterozoic sedimentary rocks. These comprise arkoses and sandstones, such as those of the V6 target and likely represent intra-crater sedimentary facies. However, the relationships between some of these rocks and the underlying volcanics are difficult to determine. Sedimentary units may host copper-gold mineralization.

GRANITOIDS

Red granites and gray granodiorites constitute the "basement" of most volcanic centers. These rocks have been eroded and appear to be unconformably capped by the ash flow tuffs. Red granitic intrusions may have porphyry-like texture and potassic alteration consisting of an assemblage of quartz, K-feldspar, chlorite and green mica in general restricted to fracture selvages. Veinlets of K-feldspar and masses of chlorite probably formed after biotite represent the alteration style in V6. Fluorite and calcite in fracture fillings are interpreted as the result of the circulation of late-stage hydrothermal fluids. So far, not a single typical porphyry intrusion was detected in association with one of these shallow epithermal systems.

MINERALIZATION

Until today the Tapajós epithermal deposits were not properly tested. Just a few were drilled by Rio Tinto. Economic concentrations of Au, Cu, Ag, and Mo were found in at least three ( V3, V6 and V7) volcanic centers. Other targets as V4, V8, V9, V12 and V13, have, so far, not been detailed. Apparently, these targets group into two categories of deposits. The V3-type, which has economic grades of Au-Ag and sub-economic grades of Cu, Zn, and Mo, and the V6-type, a low-grade Cu deposit with Mo, Au and Ag as by-products. Galena and sphalerite were recorded as rare dissemination. Data about grade and tonnage of both types are not available for publication yet.

The main gold and copper reserves of these targets are supergene. The primary Tapajós deposits underwent deep rain forest weathering during the Cenozoic, developing a large Cu-Au-Ag rich supergene blanket.

At V3, the blanket is sub-horizontal and sustains the topography roughly at an altitude of 300 m. The primary mineralization was intersected in the deeper portions of the volcanic center and correlates well with the pyrite-rich zones. Estimated reserve is larger than 100 Mt of low grade ore that may be mined as an open-pit as soon as infrastructure and prices rise to profitable levels.

At V6, a chalcocite rich blanket was developed. However, this target differs from V3 in its prevailing potassic alteration, lesser amounts of pyrite, lower gold grades and higher Cu and Mo grades. The V6 chalcocite supergene enrichment zone is a classic example of oxidation of primary disseminated Cu-sulfide deposit. From the surface to deeper levels, the supergene blanket consists of an iron oxide rich zone overlying a copper-rich zone. Primary sulfides include chalcopyrite, pyrite, rare galena and sphalerite. Molibdenite was not recorded during the early exploration stages, but soil geochemistry indicate sub-economic to economic grades of the metal within areas that do not coincide with the main copper anomaly.

CONCLUSIONS

The Tapajós epithermal deposits have several features in common to the Proterozoic Iron oxide-Cu-Au deposits such Olympic Dam. The most important features are:

1. its geochemical fingerprint: Au, Cu, Ag, Pb and As, Mo, Bi, Sb, Y, La, Zr, Ba, U, REE and Te,

2. age (1.8Ga),

3. geological control by major intracratonic extensional structures,

4. the considerable volume of hematite-bearing breccias,

5. alkaline affinity of the volcanic rocks,

6. hydrothermal alteration styles

7. proximity to a magnetic, deeper source.

The V3 target has also similarities to other iron oxide epithermal deposits as Igarapé Bahia Candelaria and Ernest Henry. Its discovery opens completely new horizons for the exploration of other large epithermal Au deposits in the Tapajós Province, as well as in the Amazon region. It is the author's feeling that some will be mined in the forthcoming decade. It also clearly renews the economic potential of some Brazilian areas that have not merited reliable basic mapping and proper geochemical and geophysical exploration.

Acknowledgments

To my fellow geologists and technicians, whose dedication and competence made these discoveries possible. To Hardy Jost for the excellent revision and contributions.

REFERENCES

Bezerra, P.E.L, Cunha, B.C. C., Del'Arco, J.O, Drago, V.A., Montalvão, R.M.G.(1990): Projeto Zoneamento das Potencialidades dos Recursos Naturais da Amazônia Legal Convênio IBGE/SUDAM

Einaudi, M.T. and Oreskes, N. (1990): Progress Toward an Occurrence Model for Proterozoic Iron Oxide Deposits - A Comparison Between the Ore Provinces of South Australia and Southeast Missouri; in The Midcontinent of the United States - Permissive Terrane for an Olympic Dam Deposit?, Pratt, W.P. and Sims, P.K. Editors, U. S. Geological Survey, Bulletin 1392, pages 589-69.

Hauck, S.A. (1990): Petrogenesis and Tectonic Setting of Middle Proterozoic Iron Oxide- rich Ore Deposits; An Ore Deposit Model for Olympic Dam Type Mineralization; in The Midcontinent of the United States - Permissive Terrane for an Olympic Dam Deposit?, Pratt, W.P. and Sims, P.K. Editors, U. S. Geological Survey, Bulletin 1932, pages 4-39.

Hildebrand, R.S. (1986): Kiruna-type Deposits: Their Origin and Relationship to Intermediate Subvolcanic Plutons in the Great Bear Magmatic Zone, Northwest Canada; Economic Geology, Volume 81, pages 640-659.

Jacobi, P.S.L.: unpublished reports

Lestra, A.D: unpublished reports

Oreskes, N. and Einaudi, M.T. (1990): Origin of Rare Earth-enriched Hematite Breccias at the Olympic Dam Deposit, Roxby Downs, South Australia; Economic Geology, Volume 85, pages 1-28.

Reeve, J.S., Cross, K.C., Smith, R.N. and Oreskes, N. (1990): Olympic Dam Copper- Uranium-Gold-Silver Deposit; in Geology of the Mineral Deposits of Australia and Papua New Guinea, Hughes, F.E., Editor, The Australasian Institute of Mining and Metallurgy, pages 1009-1035.

Roberts, D.E. and Hudson, G.R.T. (1983): The Olympic Dam Copper-Uranium-Gold Deposit, Roxby Downs, South Australia; Economic Geology, Volume 78, pages 799-822.

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