Summary:
Deposits within the Cerro Negro mine operations are low sulfidation, epithermal gold/silver vein deposits.
Vein mineralogy depends on the location of veins relative to the Eureka Volcanic-Subvolcanic Complex. Veins within the Complex (Eureka, San Marcos and the Marianas) contain higher silver and gold grades, and the Eureka veins contain abundant adularia and ginguro-style banding. Veins outside the dome and hosted by the Cerro Negro Ignimbrite (Bajo Negro and Vein Zone) contain lower silver grades, coarse pyrite rather than fine sulphides in ginguro bands, and a higher percentage of chalcedony and less adularia and carbonate in the gangue.
Vein textures typical of low-sulphidation epithermal systems include colloform and crustiform banding, cockade, and manganese/iron-oxide matrix breccias.
Eureka
Outcrops of the Eureka vein system can be traced for 4.6 km between post-vein cover rocks to the northwest and hot-spring deposits to the southeast. Higher-grade mineralization has a known strike extent of about 1,500 m. The entire mineralized zone, including stockwork and vein material, can reach 100 m in width; however, economic widths are substantially less, locally as wide as 27.8 m, but averaging about 8.4 m. The Eureka vein system strikes northwest to east–west and dips southwest to south. Host rocks are typically intrusive andesite in the hanging wall and a sequence of andesitic rocks and more felsic porphyries and ignimbrites in the footwall.
Mariana Norte and Mariana Central
The oldest unit in the Mariana area consists of fine-grained andesitic volcanic rocks, exposed to the south of the Mariana Central vein. This andesite is correlated with the lowermost unit of the Eureka Volcanic-Subvolcanic Complex and is host to all known mineralization at Mariana Sur, Mariana Central, and San Marcos.
The andesite is overlain by a rubbly collapse breccia (BAFU) which, at Mariana Central and Mariana Norte, consists predominantly of andesite clasts and blocks, some of which may be tens of metres in size. BAFU contains vein quartz clasts near the base of BAFU where it overlies veins.
Mariana veins formed within dilation zones on major northwest-trending normal faults and/or splays, with predominantly dip-slip displacement.
Continued movement during and after mineralization has resulted in footwall collapse and deposition of the post-mineral breccias. Veins of Mariana Norte and Mariana Central are in a half-graben, with the stratigraphic succession dropped down stepwise to the north.
The two Mariana vein systems are about 250 m to 500 m apart, depending upon location, and strike 280º (Mariana Norte) and 305° (Mariana Central). Both veins have northerly dips, but the Norte vein dips at about 60º, while the Central vein has a steeper dip of approximately 65º.
Mariana Central comprises a main vein, a hanging wall split that separates from the main vein near the east end of the deposit and then rejoins the main vein 300 m farther to the west–northwest, and a separate, roughly parallel, secondary hanging wall vein that occurs about 100 m into the hanging wall of the main and hanging wall split veins. A third, roughly parallel, hanging wall vein occurs about 100 m from the main, interior to the hanging wall split vein. All of the defined veins have small, discontinuous subparallel veins. The main vein strikes west–northwest at about 305º and dips rather consistently at about 65º to the north.
Overall dimensions of the main vein are 800 m long by 300 m high by an average of over 7.1 m thick, reaching a maximum modeled thickness of 23.0 m. Southeasterly extensions, known as Mariana Southeast and Emilia, have recently extended the length of the known mineralization by more than 1 km. This is the highest-grade vein in the district.
Mariana Norte consists of a main vein, with an adjacent, discontinuous, high-grade subparallel footwall vein. A hanging wall split separates from the main vein near the east end of the deposit at an angle of about 20º. This hanging wall split dips very steeply northeast at over 80º. The main vein strikes west–northwest at about 280º and dips consistently at about 60º to the north.
Overall dimensions of the main vein are 700 m long by 400 m high, averaging 7.4 m wide; the widest part of the vein is approximately 26.9 m. Recently discovered mineralization at the east end of Mariana Norte extends the known length of the system by several hundred metres.
Bajo Negro
The Bajo Negro vein is hosted by a relatively uniform sequence of weakly to moderately welded dacitic ignimbrites assigned to the Cerro Negro Ignimbrite. To date, the Bajo Negro vein has been defined by drilling over a strike length of almost 1,200 m, with an average true width of 7.4 m, maximum width of 16.0 m and a known vertical extent of as much as 300 m.
The vein is essentially a single structure that strikes approximately 330º and dips at 65º to 75ºNE in its central half, but jogs to 310º and flattens slightly in its northwestern and southeastern extensions. Several smaller parallel veins occur on the western end of the structure.
The Bajo Negro vein is a single structure consisting of chalcedonic to crystalline quartz plus well-crystallized pyrite or (more commonly) iron oxide after pyrite.
Vein Zone
The Vein Zone deposit as presently defined is approximately 500 m long, occurs over a vertical extent of close to 400 m, and excluding the footwall vein, is as thick as 80 m. Mineralization, consisting of complex discrete veins and stockworks, is hosted by a relatively uniform sequence of welded rhyodacitic ignimbrites.
Both discrete veins and stockwork-like zones appear to have been emplaced in a complex north- to northeast-dipping fault zone that forms the boundary between strongly welded and moderately welded ignimbrites.
The Vein Zone consists of a west–northwest-trending arcuate system of quartz veins, sheeted veins, and breccia zones, in which the veins have easterly, northwesterly, or northerly strikes and steep to sub-vertical dips; the system dips to the north and northeast. The main deposit dips at about 60° northeast; the footwall structure dips about 80° northeast.
Gold is associated with oxidized pyrite and manganese oxide along with hematitegoethite, minor sphalerite, kaolinite, illite, and adularia. Arsenic, manganese, and barium are locally anomalous. Platy quartz that is a pseudomorph of carbonate, colloform banding, and open or clay-filled vugs accompanies the gold. Mineralization occurs within an extensive envelope of kaolinitic alteration that changes sharply to illite alteration in the footwall.
San Marcos
A west–northwest-trending fault/breccia vein separates fine-grained andesite of the basal member of the Eureka Volcanic-Subvolcanic Complex to the north from rhyodacitic ignimbrite of the Complex, to the south.
The mineralized east-trending vein at San Marcos is a braided system dominated by two primary veins, along with two separate sub-parallel veins and a hanging wall split. The two primary veins—main hanging wall vein and main footwall vein—are more continuous and predictable than the subsidiary veins.
The main hanging wall and footwall veins strike east–west and are defined over a strike length of 750 m. These veins, and the subsidiary hanging wall and footwall veins, are typically vertical, though in places the dip is as low as 80º. The main hanging wall vein, which is on the north side, averages 6.5 m thick and has a maximum thickness of 20.4 m; the main footwall vein, which is on the south side, averages 4.7 m thick and has a maximum thickness of 9.8 m.
Mineralization consists of white quartz veins with abundant coarsely crystalline pyrite, vein breccias, and some black banding.