Summary:
Mineralization at the Santa Luz Project is consistent with an orogenic gold deposit type. Orogenic gold deposits are distributed along major compressional to transpressional crustal-scale fault zones, in deformed greenstone terranes commonly marking the convergent margins between major lithological domains, such as volcano-plutonic and sedimentary domains.
In the Project area, gold deposits are hosted within the Paleoproterozoic aged Rio Itapicurú Greenstone Belt (RIGB), which was deformed and metamorphosed during the Trans-Amazon Orogeny (approximately 2 Ga). Gold mineralization mainly occurs with fault-related quartz-sulphide and quartz-carbonate-sulphide veining and quartz-sulphide breccia. Alteration includes sericitization, carbonate alteration, albitization, sulphidation, and silicification.
Gold deposits and prospects in the Project area occur in silicified breccia zones at or proximal to the faulted contact of the volcanic and sedimentary domains of the RIGB. Significant gold targets and deposits in the Santa Luz trend include the C1 (formally called Maria Preta and including Antas 1), Antas 2, Antas 3, the Mansinha Trend (South, including M11 and M3-M4, and North, including M16 and M17), and the Mari Deposit.
The C1, Antas 2, Antas 3, Mansinha South (including the M11 and M3-M4 zones), Mansinha North (including the M16 and M17 zones), and Mari deposits are hosted in a set of three parallel, gently to moderately dipping, reverse and/or transcurrent fault zones, which extend for over 20 km along a north-south trend, and are thought to lie within an extensive reverse fault system.
Mineralization is best developed in areas where the faults deviate in orientation, and where the faults comprise the contact between host rocks having contrasting geochemical and/or rheological properties.
The dominant fault system at C1 exhibits a steep dip at surface, which flattens down dip. Down, dip stretch lineations and slickensides suggest that fault movement was dominantly vertical. Asymmetric folds and folded quartz veins demonstrate reverse movement. An increase in breccia thickness and gold grade is observed with the flattening of the fault. Mineralized zones range in thickness from a few metres to tens of metres and have relatively long strike and dip lengths.
At the Project, gold mineralization is closely related to quartz-carbonate-sulphide veining and breccia filling hosted in sheared and hydrothermally altered rocks. There are two main types of quartz veins. The most common type is foliation-parallel, white to grey coloured, north–south striking, and generally <1 m wide. These may be deformed and boudinaged, or may appear undeformed. Where deformed, they are associated with stretch lineations related to transcurrent or thrust movements. The second type of commonly observed quartz vein is most likely related to late northeast-striking structures. These structures are in brittle or ductile-brittle faults that crosscut all other structures, including foliation, and display a general dextral sense of movement. The veins associated with these structures are normally black or dark grey, and are thicker than the north-south striking veins. Fluid assisted hydraulic breccia is commonly associated with these veins.
Mixed veins, characterized by black and milky-coloured quartz, are found in strongly broken zones, mainly at the intersections of the north-south structures with the northeast-southwest structures. These intersections host voluminous and high-grade gold quartz veins and silicified breccia.
The host rocks of the gold mineralization exhibit weakly to moderately strong hydrothermal alteration. The most common type is quartz-sericite alteration. Also common are locally-pervasive albitization, carbonatization, and sulphidation.
Mineralized zones range in thickness from a few metres to tens of metres and have relatively long strike and dip lengths. Mineralization at C1 has a north-south strike length of approximately 1,550 m, width of approximately 1,400 m orthogonal to strike, and a depth extension of at least 840 m. Mineralization ranges in thickness from 15 m to 55 m with good continuity along strike and down dip. Dacitic mineralization accounts for approximately 45% of the deposit, with carbonaceous mineralization accounting for 55% of the deposit.
Mineralization at Antas 3 has a north-south strike length of approximately 1,575 m, width of approximately 200 m to 400 m orthogonal to strike, and depth extension of 150 m. Mineralization ranges in thickness from <5 m to 15 m with good continuity along strike and down dip. Dacitic mineralization accounts for approximately 67% of the deposit, with carbonaceous mineralization accounting for 33% of the deposit.
Mineralization for C1 and Antas 3 is open at depth beyond the current limits of drilling and along strike, although it becomes relatively weaker.
QEMSCAN analysis found that gold is mainly found in association with pyrite and arsenopyrite as fine-grained inclusions, on grain boundaries and on fractures of sulphide grains. Gold grains may also be found in and on the surfaces of gangue minerals, especially hydrothermal quartz or carbonate. Gold grains commonly range from <5 µm to over 30 µm in size, with increasingly rare occurrences at progressively larger sizes.