Summary:
Deposit Types
Mineralization at La Plata is defined as a gold-rich polymetallic volcanic massive sulphide (VMS) deposit, which shows similarities to Noranda / La Ronde / Kuroko type Cu–Pb–Zn (±Au, Ag) massive sulphide deposits. Mineralization generally occurs as massive to semi-massive sulphide lenses within a calc-alkaline bimodal arc succession.
At La Plata, the VMS mineralized horizons are stratiform and internally highly deformed with no discernible sulphide zonation. There is often a low-grade stockwork immediately below, and sometimes above this horizon, and is a result of secondary shearing and sulphide and metal remobilization. Barite generally occurs within and peripheral to the VMS and stockwork mineralization. A laminar chert (siliceous and hematite-rich layer) generally occurs several metres above the VMS horizon. This can be used as a marker horizon.
One aspect of the La Plata deposit is that it is relatively Au-rich, compared to typical Noranda / La Ronde / Kuroko Cu–Pb–Zn massive sulphide deposits. This may be due to a secondary porphyritic mineralizing event, partially overprinting the deposit. Further exploration and investigation are needed to confirm this hypothesis.
Mineralization
With the exception of preferentially sulphide-replaced clasts, and matrix sulphide-flooding, there are no primary sulphide textures at La Plata within the massive sulphide unit, and grain sizes are very fine.
Well-developed planar layering in the sulphide bodies is tectonic in origin. Furthermore, the destruction of nearly all primary depositional features is manifested in several zones of the massive sulphide bodies, which is also exacerbated by the multiple pulses of mineralization imposing on pre-existing sulphides.
La Plata mineralization occurs at the contact between hanging wall basaltic to andesitic volcanics and the footwall andesitic rocks. The massive sulphide zone is usually conformably overlain by a narrow unit of jaspilitic exhalite which is the stratigraphic marker horizon.
The VMS system at La Plata Project is composed of three (3) lenses or orebodies: The South Block, the North Block and Guatuza deposits. Mineralization in the La Plata South Block and North Block orebodies occurs as massive to semi-massive lenses with thicknesses varying from several decimetres to a maximum of 12 m, averaging 6 m. The mineralization occurs mainly as bedded layers and, to a lesser extent, as massive bodies with cryptic and no apparent textural expression, no geometrical zoning and no consistent barite cap have been observed within the sulphide’s horizon.
Additionally, geochemical zonation is not a distinctive feature of the deposit, with zinc-rich and copper-rich zones often intercalated indicating multiple pulses of mineralization.
The new drill holes completed on the area called Guatuza between 2020 and 2021 intercepted significant mineralization showing several meters of massive sulfides and semi-massive sulfides, which continue downhole into a stockwork zone located immediately below.
The main minerals present in the sulphide horizons are chalcopyrite, pyrite, sphalerite, bornite, and galena, with subordinate fahlore (tennantite), covellite, digenite, chalcocite, and native gold. Gangue minerals consist in barite and accessory quartz.
La Plata Deposit Geometry, Deformation Style and Deposit Architecture
The La Plata VMS deposit comprises of the La Plata South block, the La Plata North block and Guatuza zone, both striking in a north-south orientation. The South and North VMS orebodies essentially dip between 80º and 10º towards the east, while the Guatuza orebody has a dip between 35º to 50º towards the west. The overall N-S strike length of the La Plata deposit is about 1,050 m, and an average ‘flattened’ width of ~250 m. More precisely, the South Block, North Block and Guatuza Block have lateral extensions of 150 meters, 190 meters and 80 meters, respectively.
The deposit is crosscut by reverse faulting and shearing, with some late, re-activated normal faulting. Late felsic dyke and sill intrusive (rhyodacitic in composition) and a few mafic sills and dykes utilize some of these structures, particularly in the La Plata North Block and the Guatuza Block just to the north-east.
Outcrops at La Plata are scarce due to overlying ash deposits masking most outcrop. However, those exposing primary layering in volcano-sedimentary rocks commonly show brittle and ductile features consistent with contractional deformation. Such features include folds (locally expressed as fault-propagation folds) and faults that have accommodated reverse movements (Davis, 2017).
The style of deformation is inferred as being characteristic of thin-skinned tectonics/deformation, which typically involves the development of a shallow, gently-dipping detachment horizon composed of rheologically weak rocks. Thin-skinned tectonics cause rocks in the hanging wall to the décollement to accommodate significant strain by shearing along lithological layering. Dislocation of layering sequences produces structural dislocation, ramping and the development of low-angle faults (Davis, 2017).
Rocks immediately beneath the basal detachment level, including the crystalline basement, remain undeformed and become shortened elsewhere, i.e. in the more internal parts of the orogen.
Creeks at the El Gordo prospect locally expose well-developed soft sediment structures, including convolute lamination and water escape structures.
These features indicate that the sequence youngs to the east, which is consistent with the interpreted stratigraphic younging of the sequence hosting VMS mineralization.
Current cross-sectional interpretations of the geometries of mineralized zones show them to be steeply dipping parallel to host lithologies. Structural ‘roll-overs’ are interpreted that produce open fold closures at the upper terminations of mineralization, akin to fault propagation folds (Davis, 2017).
Consequently, any fold closures in the deposit are likely to be asymmetric, due either to formation as thrust horses and/or fault propagation folds. Beyond this, any folds would be expected to comprise broad, open flexures, or zones of strain accumulation adjacent to competent bodies (Davis, 2017).
Thrust repetition / dislocation of sulphide bodies during progressive, thin-skinned deformation could result in stacking of mineralized bodies. Ongoing shortening would produce further dislocation and/or open folding of the thrust-repeated mineralization.
The complex geo-tectonic history of the Macuchi Belt, hosting the La Plata deposit and at least two (2) more known similar VMS deposits, is reflected onto the micro scale. Deciphering the local structural regime is the key component in defining the present-day morphology of the inferred and yet to be discovered high-grade VMS horizons in La Plata property.
The La Plata deposit is hosting at least three known mineralized tectonic blocks separated by a complexly arranged zones of sheared rocks. The two blocks, La Plata North and La Plata South, are hosting the bulk of the high-grade polymetallic ore. The third block immediately to the north is known as the Guatuza prospect. All three (3) blocks are affected by a prominent syn-volcanic dextral fault cutting through the district. This structure is parallel to the regional Chimbo-Toachi Fault, the crustal scale suture, bounding the Macuchi belt to the east which is inferred to be situated close to the eastern margin of the La Plata concession.