Summary:
The Fruta del Norte (FDN) deposit is a completely concealed, well-preserved Late Jurassic epithermal deposit. Although the geologic features found at FDN place it firmly in the epithermal category, its classification as either a low- or intermediate-sulphidation deposit is not straightforward (cf. Hedenquist et al., 2000).
The alteration and mineralization at the FDN deposit has a strike length of approximately 1.3 km along the eastern edge of the Suárez Basin. At its northern extremity, the FDN deposit is approximately 80 m wide, expanding to between 100 m and 125 m wide for a distance of more than 300 m in the central high- grade core, and then widening incrementally southward: first to about 160 m and then to almost 300 m on approach to its southern limit. In this wider southern zone, the vein array and stockwork are more open containing lower-grade mineralization. The most intense alteration, veining, and brecciation, greatest mineralogic complexity, and highest grades occur in the 300 m long, high-grade core, which contains most of the current Mineral Resource.
The deposit comprises four distinct mineralogical and geochemical mineralization types. In the southern zone, the deposit is characterized by manganoan carbonates, with intensely mineralized zones having greater than 1% Mn and composed of sheeted to stockwork veins and vein breccias, with finely banded, crustiform-colloform quartz, carbonates, and sulphides.
Above and to the north of the manganoan carbonate zone, it changes to a mineralization characterized by intense silicification, chalcedony-marcasite veining and breccia cement, extremely fine-grained, disseminated marcasite ± pyrite, and the absence of carbonate minerals. The host-rock lithologies are typically obscured, but interbedded fine-grained tuffaceous sediments and andesite are discernible locally. This zone thickens toward and ends abruptly at the West fault and thins from greater than 100 m in the high-grade core of the deposit to only a few metres in the south.
In the central part of the deposit, there is a tabular, flat-lying mineralized zone, up to 20 m thick, characterized by intense silicification, locally abundant illite, and notably lower sulphide and iron contents. The zone directly underlies either Suárez conglomerate or the sinter horizon. The northern part of FDN comprises large volumes of white quartz and/or chalcedony and calcite ± adularia veins, typically with only minor sulphide and manganese contents. This style of veining extends northward and is also concentrated along the western, steeply west-dipping contact of the feldspar porphyry. In this zone, crustiform-colloform texture is not as developed as farther south, but is commonly observed as faint, centimetre-scale bands in large quartz-chalcedony veins and as finer laminations in quartz veinlets. Quartz is partially to completely pseudo-morphosed after irregular lattices of bladed calcite, which are particularly common, and manganoan carbonates are absent. The veining in this zone is mineralogically much simpler and typically lower in average grade than the other three types of mineralization, but nonetheless of significant volume.
The FDN deposit is characterized by preserved examples of classic epithermal vein textures and contains two main styles of precious metal mineralization: the massively to finely banded, crustiform-colloform quartz and/or chalcedony ± sulphide ± carbonate ± adularia veins, stockworks, and breccia cements; and intensely silicified host rocks with disseminations of fine-grained marcasite ± pyrite cut by crustiform-colloform chalcedony and quartz ± marcasite veins and stockworks. The distribution of these two styles and predominant vein mineralogy are mappable and contribute to the subdivision of the deposit into the four distinctive types of mineralization as defined above.
The intensity of veining at FDN varies from millimetre-scale veinlets to composite vein intervals tens of metres wide, with the veins comprising multiple gangue and sulphide generations. In the northern part of the deposit, stockwork and sheeted veins of semi-massive to massive, saccharoidal quartz and chalcedony make up to 80% to 100% of the rock volume over widths of up to 100 m. Stockwork veining in the upper part of the deposit appears entirely random and resembles a giant crackle breccia when visualized at a scale larger than drill core. At depth in the manganoan carbonate-rich zone; however, sheeted veins are more common, with dips varying from sub-horizontal, to shallow through to steeply west and steeply east. Underground mapping indicates a paragenetic sequence of steep easterly veins through steep westerly veins to moderate westerly veins and lastly the sub-horizontal orientated veins.
The dominant sulphide gangue minerals are marcasite and pyrite, which occur together or separately as bands in veins and veinlets. Marcasite dominates the sulphide disseminations in the upper silica zone, whereas disseminated pyrite prevails in the quartz / chalcedony-illite alteration in the main parts of the deposit. Bands of sphalerite and galena, locally with chalcopyrite, are common in manganoan carbonate veins, being most abundant in the deepest veins, commonly below the deposit. Sphalerite and, less commonly, galena also occur as drusy euhedral crystals lining open spaces and, at shallow levels, in late fractures. The sphalerite varies from black to dark brown in crustiform bands in quartz- carbonate-sulphide veins at depth to honey-coloured crystals in open spaces at shallow levels. Traces of tetrahedrite accompanying visible gold in manganoan carbonate veins were identified at depth, but only rarely elsewhere. Other volumetrically minor metallic minerals identified with a hand lens or microscopically include pyrrhotite, proustite-pyrargyrite, acanthite, native silver, freibergite (argentiferous tetrahedrite), boulangerite, and jamesonite. To these may be added alabandite (MnS), seen exclusively at depth below the manganoan carbonate-rich zone, and stibnite and arsenopyrite, which locally accompany marcasite in the silicified conglomerate above the deposit.
Most gold at FDN is microscopic occurring in veins and veinlets, but coarse, visible gold is a distinctive and integral feature of the deposit. Visible gold is associated with a range of minerals, including quartz, chalcedony, carbonates, marcasite, sphalerite, and tetrahedrite, and where present, the highest-grade zones (>15 g/t Au). Individual gold grains range from isolated specks less than 0.1 mm in size to “broccolilike”, dendritic crystals greater than 10 mm across. Preliminary and limited electron microprobe analysis found that gold fineness ranges from approximately 750 (electrum) in the north to greater than 900 (native gold) in the central, high-grade core. The sulphosalts that commonly coexist with visible gold are argentiferous, resulting in significantly higher silver than gold values and Au/Ag ratios typically less than 1:2. The gold and silver values are generally roughly equal (Au/Ag ~1) in the upper and central parts of the deposit, with silver tending to increase relative to gold with depth. The Au/Ag ratios are less than 0.1 in the deepest parts of the deposit. The mineralization level with roughly equal gold and silver values gradually deepens to the south in the manganoan carbonate-rich zone.
Metallurgical tests show that most of the gold, particularly gold contained in the vein-dominated zones, is free milling. In contrast, the disseminated mineralization in the upper silicic part of the deposit is moderately refractory, with approximately 40% of the total gold endowment locked in sulphide minerals, mainly marcasite and pyrite.