Summary:
Navidad mineralisation is epithermal, as demonstrated by widespread open space-filling crustiform and cockade textures in the gangue minerals (carbonate, barite) and sulphide assemblages. The abundance of base metals, combined with carbonate-rich gangue, suggests that the deposit is intermediate, rather than low, sulphidation in style. Typical high sulphidation sulphides and gangue minerals are absent, but there is rare late stage kaolinite and minor hydrothermal alunite that implies late ingress of a hypogene acid fluid.
The principal metal association is Ag-Pb. Other associations include Ag-Pb-Cu and Cu-Ag and, more rarely, Ag-Zn. Occasionally there is Ag only, or Cu-Pb-Zn or simply isolated occurrences of these base metals. This suggests that deposition occurred through successive pulses of mineralisation. Gold appears to be totally absent from the system.
Mineralisation is mostly hosted in the upper latite, but important mineralisation occurs in the lower latite at Galena Hill. In a few places the underlying andesite also hosts high grade mineralisation. Deposits with mostly latite-hosted mineralisation include: Loma de La Plata, Valle Esperanza, Calcite Hill, and Galena Hill. Sedimentary rocks and volcaniclastics that overlie or are laterally equivalent to the upper latite also host significant mineralisation. Deposits where the mineralisation is dominantly hosted by these rock types include Calcite NW, Navidad Hill, Barite Hill, and Connector Zone.
High grade mineralisation mostly occurs in permeable host rocks. Examples of primary porosity include coarse volcaniclastic rocks and autobrecciated lava flows. Secondary porosity occurs as crackle brecciation of the brittle lava flows, hydrothermal eruption breccias, and tectonic breccias. At both Valle Esperanza and Loma de La Plata, the autobrecciated upper latite acted as an aquifer, sealed by overlying organic-rich sedimentary rocks (mudstones, limestones). The sediments were unconsolidated and are commonly slumped. Mixing of reduced water, derived from the organic-rich sediments, and rising metal- laden hydrothermal fluid probably triggered sulphide precipitation.
To date, the Navidad Project comprises eight individual mineral deposits in three separate mineralized trends (Navidad, Esperanza and Argenta trends). The six deposits of the Navidad Trend occur along strike over a distance of about 5.8 km and are essentially continuous. They comprise, from northwest to southeast: Calcite NW, Calcite Hill, Navidad Hill, Connector Zone, Galena Hill, and Barite Hill. The Valle Esperanza deposit occurs on the east flank of the Esperanza Trend and is found approximately 400 m south-southwest of Galena Hill. The Loma de La Plata deposit occurs in the north part of the Argenta Trend, approximately 2.2 km southwest from Calcite Hill.
Mineralisation at Calcite NW takes the form of three long and tabular to slightly synformal bodies. The main body lies from the surface to a depth of 130 m below surface and has an average overburden thickness of approximately 60 m. It has a strike length of 1,825 m towards the northwest, a width between 350 m to 500 m, and a thickness between 10 m and 80 m. The mineralised body plunges gently to the northeast with a dip between 1º to 5º. The base of the main body is normally identified by the Galena Marker. Towards the south-eastern end of the deposit, a smaller lens lies close to the surface parallel to the main body and about 80 m above it. It has a regular shape 275 m long, up to 250 m wide and between 20 m and 40 m thick. Another elongated lens of mineralisation lies between 15 m to 50 m below and parallel to the northern end of the main body. The body is 1,000 m long, between 200 m and 350 m wide, and ranges between 10 m and 30 m in thickness.
Mineralisation at Calcite Hill forms an irregular body with a narrow upper portion outcropping towards the western end of Calcite Hill, which merges with a larger mineralised lens. Mineralisation outcrops and extends to a depth of around 250 m below surface. It forms a relatively flat surface 600 m long, ranging from 270 m to 600 m in width. The lower portion of the body has an irregular shape resulting from two nearly separate lenses that merge into one lens having a variable thickness between 150 m to 20 m. The body plunges to the southwest with a - 5º dip.
Mineralisation at Navidad Hill trends for 520 m towards the northwest and forms an irregular globular shape ranging from 270 m to 470 m wide and 10 m to 175 m thick. The mineralised zone has a shallow dip to the southwest and lies at the subsurface along the ridge crest to around 50 m depth along the southern flank.
The mineralisation at Connector forms two intersecting, but distinct bodies, which combined, are 670 m in strike length, and between 240 m and 590 m wide. The mineralisation lies from the surface to a depth of 330 m.
The extent of mineralisation at Galena Hill is long and wide with a strike length of roughly 900 m and a width of between 250 m and 700 m. In section views orientated at 030° to 210°, the mineralised body as defined by values approaching 50 g/t AgEQ forms a roughly strataform body with a slight dip to the southwest. This body resembles an inverted shield with a flat top and a thicker central portion that thins to the margins. On nearly every section the mineralisation is affected by postmineralisation movement on the northwest to southeast trending block faults resulting is displacements of roughly 10 m to 50 m. Those portions of the mineralisation located above the horst are partly eroded whilst those portions to either side are preserved in their entirety. The mineralised zone ranges from a few metres thick at the extreme margins to over 200 m thick in the central portions of the deposit.
Mineralisation at Barite Hill forms three lenses. The northern lens is about 230 m long along strike, between 170 m and 430 m wide in the dip direction and between 5 m and 30 m thick. The southwest dip varies between 3° where the body outcrops in the north to 25° in the southwest where the body lies approximately 120 m below surface. The second lens is found towards the southern end of Barite Hill. Its dimensions are approximately 300 m long by 350 m wide with thicknesses ranging from 4 m to 32 m. It occurs at the subsurface on the crest of the ridge and plunges to the southwest. The third mineralised body, characterised by high Ag values, forms an irregularly shaped mass around 350 m long, between 100 m and 400 m wide, and between 7 m to 100 m thick. It lies between 50 m and 200 m below the second lens in southern Barite Hill and has a dip of 30° to the west-southwest.
Two distinct mineralised bodies are present at Loma de La Plata. The main deposit is 850 m long with a north-south strike, between 600 m to 1,200 m wide and 40 m to 50 m thick. It covers a surface area of 74 ha. The second body is considerably lower in grade and is located approximately 60 m beneath the main deposit. It has approximately the same surface area as the upper main body but with an average thickness of only 5 m. The area with the highest grade mineralisation is located in the central and western side of the upper Loma de La Plata deposit; overburden thickness varies from 0 m to 50 m. The dimensions of the high grade zone are 500 m north-south by 170 m east-west.