Ero Copper Corp. indirectly holds approximately 97.6% of the voting shares of NX Gold S.A. (“NX Gold”) through its wholly-owned subsidiary, Ero Gold Corp., incorporated under the BCABC. NX Gold holds a 100% interest in the NX Gold Mine.
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Summary:
The geology of the property can be characterized by strongly deformed volcano-sedimentary rocks altered to greenschist metamorphic grade. Gold mineralization is structurally controlled and hosted in sulphide-bearing, laminated shear veins that cross-cut the previously deformed and metamorphosed volcanic and sedimentary rock. The laminated nature of the veins indicates multiple pulses of quartz intruding a shear zone.
The characteristics of the gold mineralization at Xavantina are similar to orogenic gold described by Groves et al. 1998. Those deposits represent the main source of gold in deformed Precambrian metavolcanic environments and are characterized by high gold grades that range from 5 grams per tonne to over 10 grams per tonne and are hosted in quartz-carbonate veins associated with shear zones. Well-known examples of important gold deposits of this type include the Yilgarn Craton in Australia (Golden Mile and Norseman mines) and the Superior Province in Canada (McIntyre-Hollinger and Kerr-Addison mines).
The rock units present within the Xavantina Operations belong to the Nova Xavantina Volcano-Sedimentary Sequence as defined by Pinho (1990), part of the upper regional Cuiabá Group.
In subsequent geological classification surrounding the mining area, the Nova Xavantina Volcano-Sedimentary Sequence was renamed the Araés Volcano-Sedimentary Sequence and was further subdivided into three lithological associations (Martinelli et al. 1997; Martinelli 1998; Martinelli and Batista 2007; Socio 2008; Martinelli 2010). From the base to the top of the sequence these lithological associations are:
- basic and intermediate metavolcanic association represented by meta-basalt, metaandesite, meta-tuff, and meta-lapilli-tuff;
- chemical metasedimentary rock association containing meta-chert and meta-banded iron formation;
- clastic sedimentary rock association such as meta-sandstone, metasiltite, and metaphyllite.
In more recent work, the rock units of the Xavantina Operations have been re-defined into metavolcanic, metasedimentary, and intrusives by Desrochers (2017). The volcanic units include massive to fragmental basalt with frequent amygdules.
The sedimentary units include (i) debris flow characterized by centimetric subangular to angular fragments of volcanic rock units and fragments of black calcareous phyllite, (ii) siliceous siltstone with poorly developed bedding which may contain pyrite-rich layers parallel to bedding, (iii) siliceous to magnetite-rich chert, (iv) thinly laminated carbonaceous phyllite.
The intrusive units include two types of diorite dyke and one type of felsic dyke. The diorite dyke units can be classified as either foliated or cross-cutting and the felsic dyke units are classified as cross-cutting. Cross-cutting diorite and felsic dykes post-date the main deformation event.
All rock units have been metamorphosed to greenschist facies as indicated by chlorite, sericite and calcite assemblages.
The volcano-sedimentary rock units, and some diorite dykes, are strongly foliated and frequently display intense transposition. There are two main phases of folding recognized at the Xavantina Operations (Campos Neto, 2013, Desrochers, 2017).
Known gold mineralization at the Xavantina Operations is structurally controlled and hosted in four major sulphide-rich quartz veins, with hyaline quartz druse, dipping approximately 40 degrees to the north-northeast and striking between east-west and west-southwest. The veins are hosted in strongly deformed metamorphosed sedimentary rock units and metavolcanic units that trend to the northeast with a 30 to 65-degree dip to the northwest. The veins exhibit a typical laminated pattern in parallel with the vein contacts. The laminations are characterized by alternating centimeter to decimeter quartz bands and foliated host rocks indicating multiple pulses of mineralized fluids during their formation.
The Buracão vein is located on the western part of the property and includes a primary laminated vein measuring 100 meters along strike, dipping 45 degrees to the northwest in the upper portion of the mine and 70 meters along strike dipping 40 degrees to the northwest in the lower portion of the mine. The average thickness of the vein is 4.5 meters with a maximum thickness of up to 6 meters.
The Brás vein is located to the east of the Buracão vein and includes a primary laminated vein measuring 220 meters along strike length in the upper part of the mine and 50 meters along strike in the lower levels of the mine. The average thickness of the vein is 5 meters with a maximum thickness of up to 10 meters.
The Santo Antônio vein is located centrally between the Brás and Buracão veins, within the same structural corridor. The vein has the same geologic characteristics of the Brás and Buracão veins and similar mineralization characteristics, including range of gold grades and sulphide concentrations. The primary difference between the Santo Antônio vein and the historically mined veins of Brás and Buracão, is that mineralization does not outcrop at surface and the dominant plunge direction, to date, is opposite of that the Brás and Buracão veins, particularly within the upper panel of the Santo Antônio vein.
The Matinha vein is located at the eastern extent of known mineralization. The Matinha vein plunges in the same direction as the Santo Antônio vein. Mineralization encountered to date is similar to the other veins. Additional work in this area is planned to further evaluate continuity of grade and thickness.
Gold mineralization in all veins is associated with sulphides that are primarily disseminated within the quartz but can also be associated with minor gold bearing sulphides disseminated within the host rock. The veins generally contain 2 to 15 percent total sulphide represented largely by pyrite and galena, with minor chalcopyrite, bornite, pyrrhotite, and sphalerite.
Higher gold grades are generally associated with galena, chalcopyrite, bornite, and sphalerite.
The veins are typically bordered on the eastern and western edges by discontinuous breccias of less than 2 meters in thickness. Breccias can be described as those with a siliceous matrix containing angular fragments of quartz veins, a matrix containing pyrite and galena typically containing gold grades less than 5 grams per tonne, and as breccias with a carbonaceous matrix containing sub-rounded to sub-angular fragments of meta-volcanic rocks and quartz vein.