The mining newspaper for Alaska and Canada's North
The mining newspaper for Alaska and Canada's North
NW BC operation produces 152,484 oz gold during first six months; output expected to improve with better grade control North of 60 Mining News – February 1, 2018
Pretium pours the first gold at its Brucejack, a high-grade underground mine that produced 152,484 oz gold during the first six months of operation.
The mill at Brucejack has exceeded its 2,700 tpd nameplate capacity since the July start of commercial operations at the high-grade gold mine in B.C.'s Golden Triangle.
High-grade underground gold mines have a lot of moving parts and ramping a new one up to commercial production is a period often interrupted by downtime as the operator finds things not working quite as well as planned. This has not been the case at Pretium Resources Inc.'s bonanza grade Brucejack gold mine in northwestern British Columbia.
Achieving commercial production on July 1, the Brucejack Mine produced of 152,484 ounces of gold during the first six months of operation. This roughly 305,000-oz-per-year pace is despite the fact that lower grade was fed into the mill early on.
Once running at full capacity, this underground mine in B.C.'s Golden Triangle region is expected to produce 7.3 million oz of gold over an initial 18-year span, or roughly 404,000 oz annually.
Going into 2017, Valley of the Kings deposit at Brucejack hosted 15.6 million metric tons of proven and probable reserves averaging 16.1 grams per metric ton (8.1 million oz) gold.
Mill outperforms
One sign that the production ramp-up at Brucejack is going smoothly is that the brand-new mill has outperfomed its 2,700-metric-ton-per-day capacity over the first six months of operation.
During the third quarter, the mill processed 261,262 metric tons of ore, which equates to roughly 2,840 metric tons per day. The material processed averaged 10.5 grams per metric ton gold and recovery was 96.5 percent. As a result, the operation produced 82,203 oz of gold and 83,233 oz of silver during the third quarter.
The mill continued to outperform during the fourth quarter, processing 271,501 metric tons of ore, or 2,951 metric tons per day while recovering 95.8 percent of the gold fed into it during the final three months of 2017.
While the mill processed roughly 4 percent more ore and the recovery was only down slightly, gold production dropped by nearly 15 percent during the fourth quarter, compared to the first three months of operation. This indicates a drop in the grades of the ore fed into the mill during the quarter.
This lower than expected gold grade has been one of the only hitches to a nearly perfect startup at Brucejack.
The ore fed into the mill from Aug. 1 to the end of 2017 has only run about 75 to 80 percent of what the reserve model anticipated.
The company also said the system for bagging concentrates has required more mill downtime and maintenance than expected. The company plans to either upgrade or replace the bagging system.
Grade control
Pretium attributes some of the lower than modelled gold grades being fed into the mill last year to ore that was mined from an area that "had a lower drill density than stopes on other levels of the mine."
As the mining moves into areas more thoroughly drilled, the company expects the ore reaching the mill to be closer to the grades expected.
During and prior to the second half of 2017, underground development at Brucejack focused on opening areas in support of the long-term mine plan.
Pretium said access to the broadest possible range of stopes will maximize stope blending and is expected to improve the management of production grades as the ramp-up continues at Brucejack.
With multiple areas opened up for mining and a grade control program expected to be operational in the coming weeks, the company expects to have better control of ore grades going into the mill moving forward.
A sample splitting station, which is a key component of the grade control program, has been re-engineered to better handle day-to-day mining operations. When complete, this station will provide a better idea of the ore grades so that will allow the blending of ore in a way that will allow for more consistent ore grades at the mill.
Modifications have been completed and commissioning of the sample splitting station is underway. The upgraded and more robust sample splitting station is expected to be fully operational by the middle of this quarter.
The grade control program originally planned to be operational early in the fourth quarter of 2017 is now expected to be operational in the middle of the first quarter of 2018. When fully operational, the grade control program is expected to provide an estimated grade for each ring blasted within a long-hole stope. This information will allow the blending of ore from the various stopes on a ring-by-ring basis to smooth out head grade to the mill.
Another component of the grade control program is additional infill drilling prior to mining.
In February, Pretium will begin testing the use of reverse circulation drills for this infill drilling. The company said that reverse circulation drilling will provide a larger sample per meter and should be faster and more cost effective than core drilling, which has been used for infill drilling to date. If successful, reverse circulation drilling will be adopted for long-term grade control infill drilling.
Bigger, more efficient
With the mine operating smoothly so far, Pretium has applied for permits to increase Brucejack production to an average rate of roughly 3,800 metric tons per day, which is roughly 40 percent higher than current.
The application submitted in December reflects a production rate increase to an annual average of 1.387 million metric tons from the currently approved 0.99 million metric tons per year. The approval process is expected to take approximately six to twelve months.
Based on preliminary engineering, Pretium estimates the capital cost to increase the mill capacity to be less than US$25 million. The company will update this cost when the engineering process is complete.
In the meantime, Brucejack is expected to churn out a steady stream of gold in 2018.
Pretium is currently estimating the high-grade underground mine to produce 150,000 to 200,000 oz of gold during the first six months of 2018, which would put the first full year of production – July 2017 through June 2018 – at 302,000 to 352,000 oz.
All-in sustaining costs – which include indirect costs such as depreciation, depletion and amortization – are expected to range from US$700 to US$900 per oz of gold sold during the first half of 2018.
As operations continue to ramp-up at the Brucejack Mine through 2018, Pretium will shift its focus on operational efficiency to bring costs down.
Over his more than 16 years of covering mining and mineral exploration, Shane has become renowned for his ability to report on the sector in a way that is technically sound enough to inform industry insiders while being easy to understand by a wider audience.
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