Summary:
The geology in the Mara Rosa District is principally delineated by three northeast striking, moderately to steeply northwest dipping belts of metamorphosed volcano-sedimentary and associated intrusive rocks. These belts, referred to as the Western, Central, and Eastern Belts, are separated by broad zones of tonalitic orthogneiss.
The Eastern Belt is bounded to the southeast by the Rio dos Bois fault, which also defines the southeastern limit of the GMA.
Amarillo’s land position within the Mara Rosa District primarily covers the Eastern Belt greenstone assemblage with some coverage of the Western and Central belts as well. The Eastern Belt, has a maximum thickness of 6 km, generally strikes to the northeast and dips moderately to steeply to the northwest. Surface topography is characterised by moderate relief and locally dissected drainages that follow lithologic or structural weaknesses. Depth to fresh bedrock is generally shallow, ranging from 0 to 15 m. The upper portion of the weathered profile consists of clay-rich latosol and saprolite derived from the underlying bedrock.
Rocks of the Eastern Belt are locally intruded by quartz-feldspar-muscovite and biotite granitic rocks and associated aplite and pegmatite dykes, small stocks and dykes of hornblende, biotite and magnetite diorite, and, in its north-central portion, a large body of hornblende-plagioclase gabbro. All units exhibit varying degrees of foliation that typically range from weak to moderate, and generally intensify along sheared contacts. The tonalitic orthogneiss that separates the Eastern and Central Belts is composed of coarse-grained plagioclase, hornblende, and biotite with localised patches of biotite schist near its contact with the Eastern Belt.
Structurally the Eastern Belt is dominated by well-developed, penetrative foliation that strikes 30° to 50° and dips 40° to 70° north-west – an orientation subparallel to stratigraphy. Major structural systems include 50° to 65° striking shears and thrusts and associated drag folds. Shears are most commonly developed along zones of elastic disparity such as lithologic contacts. Shear sense is typically reverse-dextral oblique although a sinistral sense is locally observed. A second set of structures consist of late cross cutting north-west to east-northeast striking brittle faults and fractures that locally offset stratigraphy in apparent dextral strike-slip sense.
Several significant mineral deposits occur in the Mara Rosa region. These include the Posse gold deposit, the former Zacarias gold-silver-barite deposit, and the Chapada copper-gold deposit.
Posse Deposit
Posse Deposit is shear-hosted mesothermal lode-gold.
The geophysical, geological and geochemical data available demonstrate that the Posse Deposit occurs within a 50 km long shear zone with potassium alteration and lower order gold-copper- molybdenum mineralization. The Posse Deposit has a metamorphosed granodiorite traditionally called a grey gneiss or “Biotite gneiss” in the hanging wall of the fault and amphibolite, “greenstone” in the footwall. Shearing and hydrothermal alteration, of the meta granodiorite has resulted in the formation of mylonitic zones that form a distinct lithologic unit, a quartz- feldspar-mica schist, known as the Posse Schist that is characteristic of the Posse ore zone. This unit has been identified in several other areas including the Posse footwall and on strike extensions of the Posse Ore Zone to the northeast. Shearing is most intense in the footwall. It is speculated that the rheological contrast between the hanging wall and footwall rock types captured the regional thrust (movement west to east) for a 2 km segment of the shear. It is also possible that the chemical contrast between the hanging wall and footwall rocks may have aided in focusing mineralizing fluids. Observations from drill core suggest that an earlier potassic event with quartz veining, chalcopyrite, molybdenum, biotite and K-feldspar was followed by a later phyllic (sericite) event with pyrite, iron-telluride, and gold. Gold occurs as native gold and also with telluride and pyrite.
In general, mineralization at Posse is developed along a 050° to 065° striking fault zone. Mineralization tends to be strongest within mylonitic zones that follow more northerly striking (approximately 030° to 050°) shear strands and dilatant jogs that obliquely transect the contact between the hanging wall and footwall rocks.
The mineralization envelope at Posse is about 30 m thick and over 1 km long. It has a mylonitic appearance that is most noticeable adjacent to the footwall where shearing is the most intense. Higher intensity of shearing is associated with increased sulphide mineralization (up to about 4%), and a slight increase in metamorphic grade from greenschist to high greenschist facies in the hanging wall through to high greenschist/low amphibolite facies in the footwall (biotite flakes and garnet alteration). Higher gold values are associated with increasing intensity of shearing and higher levels of silicification and sulphide mineralization.