"Cachimbo" Graben - Alta Floresta-MT - Gold

General Economic Geology Of The High Tapajós Basin (The “Cachimbo” Graben) And Its Boundaries: A Regional Geological Survey With Exploratory Purposes.

Authors: Yokoi, Oscar Y.;  Oliveira, André Luis A . M.; Tachibana, Jorge.

Target Geologia, Mineração e Meio Ambiente Ltda. – oscar_yokoi@targetgeologia.com

Introduction

The so-called Cachimbo Graben or High Tapajós Basin is nearly 500 km long in its NW-SE axe and 150 km wide, being bordered both to N and S by transcurrent faults and informal works for gold exploitation ( the “garimpos”, most of them now inactive). Inside the graben, besides gold along the Teles Pires e São Benedito rivers, there are carbonate/phosphate rocks and diamonds as main mineral resources. There are also forest, military and native indians’ reserves.

Cachimbo Graben is located at northern Mato Grosso, Southern Pará and South-west region of Amazon states.A regional field survey was carried out due to the generalist character of known geological information, on which concrete data were absent, though providing some prospective alternatives.

Discussion of “GRABEN”

Under a geologic point of view, the Cachimbo Graben – according to existing geologic maps–is filled with Paleozoic sediments (Pzat), in conformity with the Geologic Map of Brazil (1981), or with Proterozoic sediments belonging to Beneficiente Group (according to SUDAM-IBGE, 1989). Then, for each considered chronological unit we can elaborate a different geotectonic history  and, consequently, distinct evolutions.

Another point to be considered is related to the concept of “GRABEN”. How can the so-called Cachimbo Graben be characterized in a taphrogenic/extensional model of plate movements that would have happened between 1.9 Ga and 0.9 Ga, and what would be its temporal and spatial relationship with gold mineralization? Would it be only consisted of low-grade metamorphic sediments or could it have volcanic associations? Would be the volcanic rocks that border the Graben really representative of the Iriri Group or could they represent another association?

Tassinari & Teixeira (1978, in Silva et al., 1980), based on oral communication from Cordani, propose an age of 1,400 Ma to the sediments of  “Chapada do Cahimbo” (Cachimbo High Plateau). That value was obtained from siltstones that occur in cuts of the Cuiabá-Santarém Road. Those authors reinforce their opinion observing that the sediments are lying on acid volcanic rocks to which an age of 1,550 Ma was attributed  and also that the sediments are intruded by basic magmatism aged 330 Ma.

In the Sucunduri Dome region, Silva et al. (1980) obtained ages of 1,394 +/- 40 Ma (in syenites) and 1,160 Ma (in phonolites) with the Rb/Sr method for the alkaline rocks that cut the sediments, while the underlying rocks from Uatumã Group resulted in ages of 1,589 +/- 36 Ma with the same method and initial 87Sr/86Sr ratio = 0.707+/- 0.001.

Costa et al. (1992) consider the Cachimbo Graben as being a symmetric structure limited by normal/reverse faults and deriving from successive movement pulses in a listric model. Regionally, the localization of such extensional structures from Middle Proterozoic might be related to reactivation of ancient ductile shear zones and/or granulitic belts. In a drill hole executed by DNPM at the Sucunduri River, the thickness for Beneficiente Group arrived at 432 meters.

The formation of Graben  (probably between 1.7-1.6 Ga), is related to extensional events would have happened, with deposition of Palmeiral Formation ( bottom unit of Beneficiente Group). Several basins were shaped during this second event, notably the Cachimbo Graben and the Purus (Inverted) Graben, which is a continuity of the former, under the Amazonic Basin (Wanderley Filho, 1991, in Wanderley Filho, 1996).

Reconnaissance Survey

Some parts in the inner portion of the Graben were selected after interpretation of satellite images and in those areas it was made a more detailed survey, based on multi-elements geochemistry and ground magnetometry. Assuming that regions showing greater geological disturbance and/or fracturing could represent evidences of sub-outcropping Proterozoic rocks (such as the Beneficiente sediments and/or granites), we carried out a regionally oriented geochemistry campaign, identifying values for gold lower than the detection limits. The anomalous geochemical values for gold were found to be associated to structures frequently filled with basic rocks or limonitic material.

Geochemical Sampling

Soil samples were collected at each 2 km along tracks and roads (top value = 4,601 ppb Au); pan concentrates were collected along the main 1st/2nd/3rd –order tributary rivers (top value = 7,796 ppb Au). The sampling also included rocks samples (top value = 1,075 ppb Au) and shallow-depth drilling using a 500 m X 500 m grid.

Figure 1: Geological Sketch based on compilation and field reconnaissance.Paleozoic in yellow; Palmares in purple at north; Beneficiente Group + Paleozoic cover (photo-interpreted) in orange; Beneficiente Group (photo-interpreted) in green; and basement in blue .Jabuti Lagoon (limestone); Pb Anomaly; Au exploitation (“garimpo”); Diamond exploitation (“garimpo”); Carlos Hotel for sportive fishing.

Mineralizations in the Graben

Diamonds ( with subordinate gold) - Garimpo “Paraíso” : informal exploitation works developed in alluvial site that is located in the São Tomé Syncline, at the right margin of Juruena River. The source of diamonds may be related to conglomerates of Paleozoic age belonging to Bararati Formation and/or directly to neo-Proterozoic kimberlites existing in the zone of influence of Sucunduri Dome (west).

Figure 2: Open cut of the Garimpo Paraíso.Horizon A consists of whitish, sandy, non-cohesive material, while Horizon B is rich in organic matter and Horizon C corresponds to the diamondiferous gravel. Horizon D is the arenitic substract.
A.        Carbonate Rocks

Figure 3: Cavern in Jabuti Lagoon area Stalactites formation from dissolution of carbonates along fractures.

Gold Mineralization of the Cachimbo Graben – Primary and Secondary Gold Deposits

 Secondary gold mineralization: nowadays (1997), gold production comes from the Teles Pires River through the use of gravimetric methods. Recovering is carried out in dredging boats that suck gravel from the bed of the river up to 25 m deep.Gravel occurs as reminiscences (banks) along the Teles Pires River and consists of poorly sorted rock fragments, mainly quartzites/sandstones and some volcanic rocks. Gravel also fills fractures in the underlying arenitic rocks.Apparently, such gravel has been deposited on a fractured arenitic bed, locally forming “pans” like those observed in the confluence of São Benedito and Teles Pires rivers. Later on, the gravel were lateritized.

Figure 4: At Teles Pires river Lateritized auriferous gravel adsorbed on the wall of sandstones.The arenilitic rocks do not show any sedimentary or tectonic feature which could indicate that they are possible sources for mineralization.

Source of Alluvial Gold

The primary source is not yet known. Macroscopically, it is extremely difficult to differentiate Paleozoic and Proterozoic rock fragments. Until now, even without wellfounded data, it is possible to state that the main source of alluvial gold in the Teles Pires River  are Proterozoic rocks (Beneficiente Group) that occur in the south border of the Graben, volcanic rocks of Iriri Group and intrusive granitoids in a “shear zone/gash veins” context, as it is illustrated  in the photograph, taken in the Vermelho Reserve (delimited zone for informal mining works), city of Paranaíba (State of Mato Grosso).

Figure 5: Alta Floresta region - Underground informal mining works in Paranaíta State of Mato Grosso – showing the auriferous zone consisting of quartz veins/veinlets with banded massive sulfides (Chalcopyrite and pyrite mainly). In this case, the mineralization strikes N35ºW/45ºNE).

Present data and potentially prospective correlations

1. The Castelo dos Sonhos (“Dream castles”) area, formerly considered as being consisted of Beneficiente rocks, is basically a plateau (open syncline) structure sectioned by tectonic foliation N30ºE and strongly dipping NW and SE, with a 150 km2 surface , representing the source of alluvial deposits that are exploited in surrounding areas.The region consists of green schist/amphibolite facies metamorphic rocks (that the authors think to be local/thermal, due to intrusions) represented by quartzites, conglomeratic quartzites and polymictic meta-conglomerates (these are poorly sorted and contain recrystallized quartz, hematite/limonite and muscovite/sericite. The pack is mylonitized in some places and intruded by acid /basic sub-volcanic rocks that would have contributed as local thermal sources for gold reconcentration.Gold mineralization is mainly disseminated in meta-conglomerates and conglomeratic quartzites (with grains varying from 5 to 200 microns in size) and exhibits supergene enrichment where iron concentration is found. “In hematite-rich fractured zones, gold grades are more consistent, suggesting that a later remobilization related to oxidizing (meteoric?) fluids has occurred, with hematitization of sulfides, ilmenite and magnetite”.

2. By its turn, in the Abacaxis (“Pineapples”) Garimpo, Tapajós, gold mineralization occurs in a “net” of quartz veins which are hosted by clastic sediments (Prosperança Formation, according to some autors, or Jacareacanga Formation, to others).No matter the primary or secondary nature of known mineralizations, both borders of the Cachimbo Graben/High Tapajós Basin would be potential targets for mineral prospecting, for the geological survey in those areas suggest that gold and base metals geochemical anomalies are associated with Paleozoic cover rocks.

3. The Cachimbo Graben: in Teles Pires and São Benedito rivers, sandy clastic sediments showing  horizontal bedding and crossbedding (fluvial environment) are predominant. Secondarily,  limestones and carbonatic pelites/psamites (marine environment) outcrop in the area. At the time of field works, no certainty was held about the age of that sedimentary pack.CPRM (the governmental organ for geological survey), approximately 20 years ago, carried out a reconnaissance  along the northern part of the confluence of Teles Pires and Juruena rivers and identified the so-called Palmares Formation (considered as being of Proterozoic age) and, laid on it, clastic and chemical sediments (Paleozoic age, based on some fossils). Macroscopically, such sedimentary formations lying on the Palmares Formation do not show differences in relation to those rocks that outcrop along São Benedito and Teles Pires rivers (but for the amount of kaolin). Therefore, a Paleozoic age is attributed to the sediments along  these last rivers.On the other hand, there are some Paleozoic sites where gold anomalies might be associated to Tertiary sediments and/or Proterozoic rocks covered by Paleozoic sediments. Description of thin sections have shown the existence of explicit sedimentary assemblages such as limestones, dolomites, carbonatic conglomerates (with phosphatic matrix), as well as quartz-mica schists (sample EF-42) of green schist facies ( north-east borders of the graben) in a “Paleozoic” environment. Under these lights, Tachibana (oral communication) considers that this could be a reminiscence of Castelo dos Sonhos Formation, which is considered to be older than Beneficiente Group.

References

ALMEIDA, F. F. M. & NOGUEIRA FILHO, J. V. (1959).  In: Geologia do Brasil. Brasília: DNPM, p. 106, 1984.

ARANEDA, R. and others (1998). Características Geológicas e Natureza das Mineralizações Auríferas de Alguns Prospectos da Província do Tapajós-PA. Barrick do Brasil, internal report, p. 1-21.

BIZINELLA, G. A. et al. (1980). Projeto Tapajós-Sucunduri : Final Report,. Manaus: CPRM/DNPM, Vol. I b, p. 357-700.

CAPUTO, M. V.; RODRIGUES, R. and VASCONCELOS, D. N. N. (1971). Litoestratigrafia da Bacia do Rio Amazonas. Belém: Petrobrás, Internal Technical Report N. 64.

COSTA, J. B. S. and HASUI, Y. (1996). Evolução Geológica da Amazônia. In: Contribuição à Geologia do Amazonas. Belém: FINEP/SBG-Núcleo Norte, 1997, p. 15-90.

COUTINHO, M. G. de N.; ROBERT, F. and SANTOS, R. A. (1998) In: Anais do 40º Congresso Brasileiro de Geologia. Belo Horizonte: SBG, p.160.

DNPM (1995). Principais Depósitos Minerais do Brasil. Brasília: Convênio DNPM/CVRD, Vol. 1.

FARACO, M. T. L.; CARVALHO, J. M. de A. and KLEIN, E. L. (1997). Carta Metalogenética da Província Aurífera do Tapajós. In: Contribuição à Geologia do Amazonas. Belém: FINEP/SBG-Núcleo Norte, 1997, p.423-437.

FUZIKAWA, K. (1985). Inclusões Fluidas: Métodos Usuais de Estudo e Aplicações. In: Contribuição à Geologia e à Petrologia. Belo Horizonte: SBGM, p. 29-44.

MELO, A. F. F. et al. (1980). Projeto Tapajós-Sucunduri: Final Report. Manaus: CPRM/DNPM, Vol I a, p. 1-356.

PESSOA, M. R. and others (1977). Projeto Jamanxim: Final Report. Manaus: CPRM/DNPM, Vol I a, 218 p.

QUADROS, M. L. do E. S. and others (1999). In: Anais do 6º Simpósio de Geologia da Amazônia. Manaus: SBG, p. 566-569.

SANTIAGO, A. F.; SANTOS, J. O. S. and MAIA, R. G. N. (1980). Estratigrafia Preliminar da Bacia Sedimentar do Alto Tapajós. In: Anais do 31º Congresso Brasileiro de Geologia. Camboriú: SBG, Vol. 2, p. 786-795.

SANTOS, D. B. et al. (1975). Projeto RADAM: Folha SB.21-Tapajós, Geologia. Rio de Janeiro: DNPM, Vol. 7, p. 15-116.

SCHOBBENHAUS, C. and others (1984). Geologia do Brasil. Brasília: DNPM, p. 93-113.

SENA COSTA,J.B., Borges, M.S., Araujo,J.B.-(1998) A História de reativação da Provincia Aurífera do Tapajós. XL Congresso Brasileiro de Geologia. 11 a 16 de outubro de 1998. Belo Horizonte/MG

SILVA, G. G. et al. (1974). Projeto RADAM: Folha SB.22-Araguaia and part of Folha SC.22-Tocantins, Geologia. Rio de Janeiro: DNPM, Vol. 4, p. 1-143.

TARGET Geologia, Mineração e Meio Ambiente Ltda. (1995-1998). Internal Technical Reports. Belo Horizonte.

YOKOI, O. Y. (1990-1994). Visit Technical Reports  (unpublished). Belo Horizonte.