Precious metal-bearing, structurally controlled vein system centered on a rhyolite dome complex in the Sierra Madre Occidental in NW Durango, 270km northwest of Durango City.
The Buenavista Project is located in northwest Durango State, 270 km northwest of Durango City, at elevations of between 2,600m to 2,900m in the Sierra Madre Occidental (SMO). The region is largely covered by the ignimbrite sheets of the Upper Volcanic Series (UVS). Prospero is exploring under the terms of an earn-in with Exploraciones de Altiplano, a privately held company that owns concessions covering 1,177ha.
Access is via Parral (Chihuahua) along the paved road to Guadalupe y Calvo, to a turn-off 34km west of El Vergel, from where a 50km gravel road passes through San Julian and Piedra Bola to reach the project area (Figure 1).
Buenavista occurs in a window into the lower portions of the UVS and possibly the top of the Lower Volcanic Series (LVS) a common host for precious metals in the SMO. Veining is structurally controlled and centered on rhyolite domes. Alteration at surface is high-level argillic suggestive of the presence of an epithermal deposit at depth, possibly hosted in LVS andesites.
Between 1996 and 2008, 11 drill holes were completed at Buenavista to explore for relatively shallow mineralization. Highlights of these drill programs include:
In 2018, Prospero completed an 11 hole, 2,811m program with strategic partner Fortuna Silver Mines Inc.
The project area is underlain by the lower portion of the Upper Volcanic Series (UVS), intruded by numerous rhyolite and felsite dikes and domes that form a northwest-trending dome complex covering an area of 20km2 (Figure 2). Local erosional windows expose reddish to greenish porphyritic andesites likely related to the upper portions of the Lower Volcanic Series (LVS). The green andesites are propylitically altered.
The dikes, domes, and veins that intrude the volcanic stratigraphy exhibit both northwest (NW30°SE), and northeast (NE50°SW) orientations. Northwest trending structures dip predominately to the SW, whereas northeast trending structures dip predominantly steeply both to the NW and SE. The dikes are pre-vein in age; veins and alteration occur both at their margins as well as within the intrusive rocks (Figure 2).
Figure 2. Buenavista project geology map.
The Buenavista project exhibits typical structurally controlled dome-centered geology with argillic alteration caps present within dikes, domes and host rocks at elevations between 2,700m to 2,900m above sea level (ABSL). The high level argillic alteration, and the stratigraphic position of the mineralization suggests the presence of a preserved epithermal system at depth.
The most attractive target zones -the El Pajaro- Buenavista NW trending altered and mineralized corridor- are related to the presence of N40- 50°E, N30-40°W, and N50°E dikes, veins, and stockworks distributed within a core area 3km long and 500m wide.
Buenavista Vein “U shaped target”: A set of dikes and veins emplaced as a “U” shaped pattern reflecting an intersection zone of the NNW and NE structural trends. The dikes are silicified by veins and stockworks zones up to 30m wide. Argillic alteration (kaolinite) predominates in the higher extension of the vein.
The 1996 and 2008 drill programs explored the immediate extension of this mineralized zone over a length of 300m to depths of 100-250m indicating the presence of sub-economic mineralization in veins and stockworks over widths of 1.5m to 30m. Drill holes 96-1, BVE-4, BVE-5, and BVE-6 indicate a mineralized zone raking to the SW, which remains open to the west and at depth. Surface sampling returned highly anomalous silver values along the longitudinal section with some sections averaging silver values of 98ppm Ag, and 27ppm Ag (Figure 3).
Figure 3. Longitudinal section of the “U” structure
Buenavista South Stockwork: This zone was identified in the early 2012 mapping campaign, and consists of a 100m by 100m silicified area hosted within a semi-circular quartz porphyry dome-like intrusive. Preliminary sampling of the silicification returned highly anomalous silver (from trace up to 166ppm) with several samples assaying 30 to 90ppm Ag. Gold assays in the order of fractions of a ppm. The South Stockwork may be analogous to the San Julian mine, where the largest orebody of the district is hosted in the central portion of a dike/dome, and not in veins at the contact of the dike and enclosing volcanic rocks.
El Pajaro vein extension to the NW: The El Pajaro trend consists of a parallel set of dikes, faults, and associated quartz veins and veinlets with pyrite and weak Ag-Au geochemistry, with N30°-40°W strikes and dips of 50° to 75° both to the NE and SW. The precious metal values diminish at higher topographic elevations within an argillic alteration cap which persists for a length of 1.3 km to the NW. The tilt of approximately 15° to the south of the argillic cap is congruent with the general tilting of the volcanic strata on a district wide level.
El Pajaro-Stockwork with NE structural controls: A window of older andesites exhibits a broad 50m-wide zone of quartz veins and veinlets striking predominantly N30°E with base metal sulfides, which outcrop along a creek bed. Preliminary sampling of some of the veins indicates the presence of elevated silver values (samples #78078 and #78077 with 734ppm Ag and 1280ppm Ag respectively).
Camelón vein: This vein outcrops further to the east along the same creek that exposed El Pajaro NE stockwork. It is located very close to the top of the basal andesites, and exhibits relatively high silver assays in the range of 20ppm to 212ppm. The outcrop only persists 150m longitudinally, it is associated with a rhyolite dike, and exhibits a shallow dip (30°) to the west.
Hydrothermal Alteration and Structures
The host rocks at Buenavista exhibit predominately illite and NH4-illite alteration. At higher topographic elevations alteration is predominantly silicification of the host rocks and development of kaolinite and lesser NH4-illite. The NH4-illite likely reflects the presence of Guerrero Terrain carbonaceous Jurassic (?) sediments which occur in windows at deeper erosional levels in SMO.
Where El Pajaro extends to the north, at higher elevations it exhibits a well developed argillic alteration cap over a distance of 1,300m (Figure 4), with weakly anomalous precious metals, fluid inclusion temperatures of <200°C, and abundant vapor-rich inclusions.
Figure 4. El Pajaro structure, longitudinal section.
The Buenavista mineralization outcrops at an elevation of 2800m ABSL, 250m topographically higher than El Pajaro, and is hosted in rhyolites in the footwall and the banded dome in the hangingwall. The vein shows discrete bands of microcrystalline quartz with pyrite and argentite. The volcanic stratigraphy, as well as the unaltered upper unit of the UVS are generally tilted ~20° to the south and suggest the mineralized zone is tilted to the south as well, explaining the difference in elevation between the top of mineralization at El Pajaro and Buenavista.
Drill programs were carried out by Exploraciones de Altiplano in 1996 (5 holes completed, 857m), Esperanza Silver de Mexico in 2008 (6 holes completed, 1,203m) amd Prospero in 2018. Three holes were completed on the El Pajaro vein and the remainder targeted the Buenavista vein. Most of the holes cut narrow zones of moderate to low grade silver. Highlights of the Altiplano and Esperanza drilling results are tabulated below. Drill sections and a drill collar plan for the 201'8 drilling are thumbnailed on the left hand side of this page.
|Hole #||From||To||Width||Au ppm||Ag ppm|
Prospero’s drilling in 2018 returned the following intersections:
|BVF-18-01 final depth 225.1m|
|BVF-18-02 final depth 326.4m|
|BVF-18-03 final depth 216.5m|
|BVF-18-04 final depth 241.1m|
|BVF-18-05 final depth 206m|
|BVF-18-06 final depth 330.9m|
|BVF-18-07 final depth 127.8m|
|BVF-18-08 final depth 270.3m|
|BVF-18-09 final depth 270.25m|
|BVF-18-10 final depth 364.65m|
|BVF-18-11 final depth 225.2m|
Base metal assays have been rounded to the nearest ppm and only grades >100pm Zn or Pb have been reported. Cross sections for all holes, and a table of drill collar locations, dips and azimuths are available on Prospero’s website here: http://www.prosperosilver.com/s/BuenavistaDurangoState.asp and here: http://www.prosperosilver.com/i/pdf/Drill-Collar-Coords-and-Orientations.pdf
Holes BVF-18-01 & 2
These two parallel holes cut an intrusive dome phase with a shallow dipping footwall towards the north (see section on Prospero’s website). The deeper portion of the holes intersected rhyodacitic tuff. Hole 18-02 reached an andesitic agglomerate in its lower portion. Silica veining is common within the dome phase and the pyroclastics. Silver geochemistry is weak in the near-surface BVF-18-01 intersection (highs of 12.9 to 15.4 ppm Ag), but increases in hole BVF-18-02, which returned 0.57m of 815 ppm Ag, and cut deeper stratigraphic levels.
Holes BVF-18-03 & 4
These holes explored the Buenavista NE dike and vein which had not been explored in earlier drilling campaigns (see plan view and section on Prospero’s website). Hole BVF-18-03 intersected vein material 75 vertical meters below the surface outcrops, at a structural contact. The best interval returned from hole 18-03 was 151ppm Ag over 90cm from 91.2m downghole. BVF-18-04 intersected a new body of the felsite dome phase which does not appear to outcrop. Two vein zones were intersected in hole 18-04; the first within the dome phase where a zone of narrow quartz-sulfide veinlets returned 50 ppm Ag over 16.85m from 144.85m downhole, and another at the footwall contact of the dome from 194.3m to 199.6m which carries 121 ppm Ag and 0.14 ppm Au over a width of 5.3m. As in the case of holes BVF-18-01 and BVF-18-02, this vertical section also shows an increase in silica veining and mineralization with depth.
Holes BVF-18-05 & 6
These holes were drilled to explore the continuation to the west of the Buenavista vein and dike (see plan view on Prospero’s website). The holes cut an intrusive dome phase in the hanging wall of the vein which does not outcrop at surface. Hole BVF-18-05 intersected two closely spaced veins which correlate with two outcropping veins. The entire vein zone in hole 18-05 returned 48.95m @ 43 ppm ppm Ag and 0.13 ppm Au, with better grade in the footwall vein which carries 353 ppm Ag and 0.42 ppm Au over 1.55m from 185.6m downhole. Hole BVF-18-06 indicates the footwall of the dome phase flattens slightly, and returned a best intersection of 3.1m @ 31 ppm Ag from 188.8m downhole.
Hole BVF-18-07 was drilled under the Camaleon vein, and intersected a felsitic dike hosting quartz veining in the footwall zone of the felsites (see section on Prospero’s website). The visually best mineralized vein (with pyrite, sphalerite and suspect gray sulfides) occurs between 81.95m to 82.80m (0.85m width) in an inetrval that returned 4.3m @ 22.6ppm Ag from 80.4m to 84.7m, with a strong silicified fault in the hangingwall.
Hole BVF-18-08 was the first hole to explore at depth the Pajaro Stockwork zone at surface. It intersected 46 meters of similar mineralization from 84.40m to 111.70m, 60m below the outcrop with several intervals of low grade Ag-Pb-Zn mineralization, the best interval with 1.65m of 81 ppm Ag.
Given the uncertain strike of the Pajaro Stockwork zone, this hole was drilled at right angles to BVF-18-08 to gain a better idea of bearing and dip on the broad mineralized zone. It intersected a broad zone of veining from 105.00m to 145.35m, from 50m to 80m below the present surface, with a best interval of 1.60m with 189 ppm Ag and 0.18 ppm Au. This intersection seems to confirm that in plan view the stockwork zone belongs to the northeast striking vein trend.
This hole was drilled 90m south of hole BVF-18-04 and 55m deeper to test laterally the two mineralized intersections of BVF-18-04 that occur both within and in the footwall of the dome phase. BVF-18-10 also intersected the dome phase and indicated the footwall flattens sharply downwards from the line of section of BVF-18-03 and BVF-18-04 where it shows a dip of 60°, to 45° on the line of section of Hole BVF-18-10. The structural flattening appears to be responsible for diminished veining and geochemistry.
Hole BVF-18-11 represents a 80m offset of hole BVF-18-05, and 25m deeper to investigate if the mineralization in the former hole persists further to the southwest. Diminished quartz veining and lower grades were encountered in this intersection, indicating a possible limit to the southwest of the Buenavista vein mineralized zone which was previously drilled a continuous 400m to the northeast.
Fluid Inclusion Work
Fluid inclusion petrography has been carried out on samples from all the veins and targets at Buenavista. Inclusions were identified in quartz from the main veins and in sphalerite from the el Pajaro stockwork. Homogenization temperature values are in the order of -or below- 200°C to 220°C. In the deeper erosional windows, such as the El Pajaro and Camaleon veins, or in the outcrop of the Buenavista vein, temperatures in the order of 240°C to 250°C were measured. These zones are within or close to the outcrops of the stratigraphically deeper andesites, and are associated with the higher precious metal and base metal values. Most of the temperatures measured in crystalline quartz from the Buenavista targets correspond to a position above or close to the top of a typical Mexican epithermal deposit.
A common characteristic of crystalline quartz present in the high level epithermal environment consists of the presence of ubiquitous all vapor-rich inclusions. Nearly all the samples studied host abundant all vapor-rich inclusions again suggestive of a high level epithermal environment for Buenavista.
Figure 5. Buenavista fluid inclusion results & interpretation
Stockwork mineralization hosted occurs at Buenavista within a dome-shaped intrusive, and is geologically similar to the San Julian district located 20km to the north. Surface outcrops at Buenavista appear to represent the upper portions of a preserved epithermal mineralized zone at depth. The presence of base metal sulfides in the NE striking Pajaro stockwork, which corresponds to the deepest exposed geologic level, indicates that significant polymetallic mineralization could also be present a depth.
Following 3 drill programs, Prospero and its partner, Fortuna Silver, are evaluating the potential for additional drilling.