Greystar makes progress at Angostura

High in the majestic Andes of northern Colombia, Greystar Resources (GSL-T) is preparing to launch an independent prefeasibility study on an epithermal gold-silver deposit considered amenable to open-pit mining methods.

Situated 35 km northeast of here, the Angostura project covers 1,750 hectares divided among five adjoining exploration concessions. Greystar has the option of acquiring these concessions either from the government or from local vendors.

On a recent visit to the site, The Northern Miner noted that the region is characterized by rugged hills and river valleys, with the property itself varying from 3,000 to 3,500 metres in elevation. Angostura is roughly a two-hour drive along a mountainous road from the city of Bucaramanga.

A number of small gold mines operate on, and near, the Angostura property.

The operations are small in scale, run by local families and focused on high-grade clayey material that is easy to crush.

Greystar envisages a 50,000-tonne-per-day open-pit and heap-leach operation at Angostura. To realize this goal, the junior must overcome problems associated with the region’s alpine topography and social instability, but perhaps its most difficult task will be to prove the deposit can support a mine.

To that end, Greystar recently commissioned KD Engineering of Tucson, Ariz., to undertake a prefeasibility study. The study, which is expected to be completed within three months, will include reserve calculations, preliminary mine designs, cost analyses and flow-sheet models. Nevada-based Mine Development Associates has been subcontracted to focus on the mining and geological portion of the work.

Greystar President David Rovig, speaking from his Montana office, said the study will encompass geotechnical features, site locations, resource and reserve calculations, preliminary metallurgical work and infrastructure needs.

Resources at Angostura are estimated at 34 million tonnes grading 1.65 grams gold per tonne, based on a cutoff grade of 0.5 gram gold. When the cutoff grade is lowered to 0.3 gram, the resource jumps to 105 million tonnes averaging 0.95 gram gold.

Both resource figures were calculated in late 1997 by an independent mining consultant, and were based on 55 of the 66 core holes completed at that time. The stripping ratio is 3.5 to 1 at the higher cutoff grade and 0.5 to 1 at the lower cutoff grade.

Since drilling began in 1995, Greystar has concentrated most of the completed 134 holes in the resource area. In 1997, the company also drove an adit into the southern portion of the deposit. A 640-kg sample of unoxidized material was taken for preliminary metallurgical tests and stations were marked for future underground drilling.

The mini-bulk sample, which assayed 3.86 grams gold and 39.87 grams silver, released 76.6% of its gold and 91.86% of its silver after 102 days of leaching in a bottle-roll test carried out by Metcon Research. Although the company considers the sample representative of the area from which it was taken, it does not consider it representative of the deposit as a whole.

Geologically, Angostura is part of the Angostura-California gold province, a belt of gold deposits associated with the northeasterly trending Rio La Baja fault. The fault cuts pre-Devonian-aged granitoids and migmatites and Cretaceous-aged metasediments, quartzites, limestones and argillites.

At the property, mineralization appears to be controlled by fault, shear and shatter zones near the margins of quartz monzonite stocks. The structures typically dip steeply to the northwest and strike northeasterly to east-northeasterly, though in one portion of the property they strike north-south.

Gold is hosted by pyritic quartz-feldspar porphyry, especially where it has been altered to silica, sericite, alunite and argillite, and by intercalated, altered mafic gneiss and pyritic, vuggy, brecciated chert, which marks the top of the epithermal system. The company believes silver concentration increases toward the south portion of the property, where the mineralization tends to be more porphyritic.

Overburden is ubiquitous in the area, but generally extends no more than 5 metres below the surface. A zone of oxidized rock can extend, in places, to 100 metres below the overburden; it is followed by primary mineralization.

Cross-sections through the deposit show it to grade generally less than 1 gram gold, but greater than 1 gram in a number of enriched pockets. The best intersections from two recently completed holes exemplify this pattern of mineralization: Hole 12 returned 62 metres (366-428 metres downhole) averaging 3.62 grams gold and 10.87 grams silver, including 4 metres (387-391 metres) of 15.15 grams gold and 8.85 grams silver and 9 metres (419-428 metres) of 9.75 grams gold and 19.45 grams silver. Hole 13 returned 92 metres (35-127 metres) averaging 4.3 grams, including 13 metres (75-88 metres) grading 14.3 grams and 5 metres (118-123 metres) grading 8.6 grams.

Both holes were drilled from the same location, but in opposite directions.

In the case of hole 12, the two smaller intervals account for 58% of the total; in hole 13, the two smaller intervals contain 66% of the gold.

A team of six men log the drill core on site, though the company plans to shortly begin a relogging program using only one man. This should ensure that any inconsistent interpretations are corrected.

“We are very satisfied with the information we have,” said Rovig. “The idea is to get a senior [geologist] in there who may pick up some subtleties we’ve missed, but we haven’t missed anything of major consequence.” Asked why such an action has taken so long, Rovig explained that, at first, the company was not sure if it even had a viable project at Angostura. “Only in the last year have we established [Angostura] as a project rather than just a prospect. So we’re probably remiss by a few months in not having someone down there, but not by three years.”

Greystar’s procedure on site is to saw off one-quarter or one-half of the core and ship it to British Columbia for analysis. Samples are taken at every metre (more recently, every two metres); an exception to this procedure is the taking of separate samples where identifiable structures, such as chert beds, are intersected.

The unsplit core is stored in a locked facility in the nearby town of California. For security reasons, only a few of the assay results are sent to Colombia, but all are sent to the company’s Vancouver office.

While downhole surveys have not been completed on three-quarters of the holes drilled to date (due to malfunctioning survey equipment), appreciable deviations from preset paths were not noted in the holes that were surveyed.

As such, Rovig says the company is not concerned with the unsurveyed holes.

Greystar’s immediate objective is to complete nine holes in the southwestern portion of the deposit, where data is scare, and six holes in a newly discovered area to the east, dubbed QP. Each set of holes will be spaced on 50-metre centres.

The QP zone was uncovered earlier this year when a drill road was extended eastwards from the resource area. Results from the first hole drilled, QP98-1, were released a month ago and, according to the company, suggest a southeastern extension to the deposit. Gold values ranged from 1.03 grams over 13.1 metres (189-193 metres) to 15.37 grams over 2 metres (265-267 metres). The longest interval was 32 metres (20-52 metres) averaging 2.1 grams gold.

The hole bottomed out about 100 metres northeast of hole SI91-1 and intersected the same mineralized zones displayed along the road. Results from chip samples along that road cut were generally less than 1 gram gold and the best results from the earlier hole were 3 metres (53.5-58.5 metres) of 1.96 grams gold and 4.5 metres (73.5-79 metres) of 0.72 gram.

The second of the six stepout holes was getting under way at the time of the Miner’s visit.