Geology and gold mineralization of the Jacobina Mine and Bahia Gold Belt, Bahia, Brazil and a comparison to Tarkwa and Witwatersrand

This paper is a contribution to the understanding of the regional and local geological
framework of the Jacobina area, situated in the northern region of Bahia state,
Brazil. The focus will be on how gold mineralization relates to the Paleoproterozoic
geological evolution (deformational events, metamorphism, intrusive rocks, and
hydrothermal alteration) of the area. The contribution is based on two and one-half
years of exploration carried out by Desert Sun Mining Corp. (Desert Sun) along the
170 km long by 8–10 km wide Bahia Gold Belt, held by Desert Sun, which comprised
mapping, petrographic studies, rock geochemistry, geophysical surveys, and an extensive
drilling program at the Jacobina mine area, as well as at several exploration targets
along the Bahia Gold Belt. Recently released regional geological maps of the area
by the Geological Survey of Brazil (CPRM) and airborne geophysical surveys by the
State Government of Bahia (CBPM) have contributed significantly to this work.
Gold mineralization at the Jacobina mine occurs within extensive quartz pebble
conglomerate horizons, know as “reefs,” following South African nomenclature,
within the Serra do Córrego Formation, which forms the base of the Paleoproterozoic
Jacobina Group metasediments. The Serra do Jacobina mountains have been mined
for gold since the 18th century with numerous old workings (garimpos) dug by artisan
miners (garimpeiros) following the ridges of the mountain chain. Three former
mines (Canavieiras, Itapicuru and João Belo) produced approximately 700,000
ounces of gold, mostly from 1983–1998. Desert Sun is currently re-activating the
Jacobina Mine (formally João Belo) and expects to be in production at the rate of
100,000 ounces per year by mid-2005. Gold in the conglomerates occurs as finegrained,
native gold with pyrite and/or hematite, predominantly in the matrix of
coarser conglomerates. Fuchsite is ubiquitous, but not all conglomerates with fuchsite
contain gold. Individual reefs are typically 5 to 25 m wide and extend for up to kilometres
in strike length.
Past exploration was focused primarily in the mine area. However, Desert Sun
has expanded exploration to new target areas in the northern part of the Bahia Gold
Belt, where gold mineralization is associated with zones of strong silicification and
quartz veining related to the major Pindobaçu fault system, especially along the contact
of Jacobina Group metasediments and the Archean Mundo Novo Greenstone rocks. This gold mineralization occurs in a structural setting, but the mineralogy and
geochemical signature is very similar to that of gold mineralization in the conglomerates
in the south suggesting a genetic link. There is evidence of a major regional ultramafic
geochemical signature of gold mineralization hosted by both the Jacobina
Group and Mundo Novo Greenstone rocks. Field and petrographic evidence suggests
that the major gold mineralization event, and its related hydrothermal alteration,
took place between the peak of regional metamorphism, which varies from greenschist
to amphibolite facies, and prior to the emplacement of late- to post-tectonic peraluminous
felsic intrusions. A hydrothermal origin for gold mineralization with
porosity and permeability (diagenetic and/or structural) being the primary control
for deposition is favored, although the possibility of an original paleo-placer contribution
for some of the gold in the conglomerates cannot be totally ruled out.
The deposits of the Jacobina basin have geological characteristics that resemble
those of both the Tarkwa belt and Witwatersrand basins in Africa. Both the Jacobina
basin and the Tarkwa belt are inverted rifts of Paleoproterozoic age filled with
oligomictic, well-sorted conglomerates of both fluvial and alluvial fan origin. Both
were affected by oxidizing and reducing hydrothermal fluids and underwent metamorphism
up to medium grade. The basement of both the Jacobina and Witwatersrand
basins is Archean granite-greenstones with the potential for development of
extensive fluvial drainage systems. “Carbon” is present in both the Witwatersrand
and Jacobina sediments possibly representing hydrocarbons derived from nearby
organic-rich marine shales. Therefore in many ways Jacobina can be regarded as a
combination of both the Witwatersrand and Tarkwa models of conglomerate-hosted
gold deposits.