DENVER,
Nov. 27, 2017
/PRNewswire/ -- Vista Gold Corp. (the "Company," "we" or
"our") (TSX and NYSE American: VGZ) today issued the results of its
previously announced metallurgical testing program for the Mt Todd
gold project located in the Northern Territory, Australia.
The test work confirms that the inclusion of automated sorting and
a re-designed, 2-stage grinding circuit will enable the project to
achieve a finer grind size, higher gold recoveries/higher gold
production, and lower processing costs with no material increase in
project capital.
Automated Sorting
The Company's automated sorting program (see the Company's
July 24, 2017 press release for full
details) is now complete, with assays for all size fractions.
The test work confirmed that the combination of x-ray transmission
("XRT") and laser sorting on the +5/8" (16mm) screened HPGR crushed
material (approximately 18% of the run-of-mine feed) enables the
Company to reject approximately 10% of the run-of-mine feed as
below cut-off grade material (this uneconomic material is hereafter
referred to as "waste"). This results in an approximate 8%
improvement in estimated mill feed grade (life-of-mine average 0.91
grams gold per tonne "g Au/t" compared to the 0.84 g Au/t
life-of-mine reserve grade) with an estimated gold loss of only
1.3%. With the installation of an automated sorting circuit,
the Company expects to reduce total grinding, leaching and tailings
handling costs by approximately 10%.
Newly Designed 2-Stage Grinding Circuit
Past metallurgical studies at Mt Todd demonstrate a strong
correlation between finer grind size and higher gold
recovery. To efficiently achieve a finer grind size and
higher recovery, the Company has optimized the crushing and
grinding circuits. Excess capacity in the HPGR crusher will
now be used to produce finer feed for the grinding circuit.
Each grinding module is now comprised of a primary ball mill and
three, small fine-grinding mills. The combination of the
reduced material volume (as a result of automated sorting), smaller
mill feed size, and 2-stage grinding, lowers the total estimated
project power requirements by approximately 13% and enables the
Company to cost-effectively achieve a nominal P80 grind
size of 60µm. Leaching tests on material ground to 60µm in
the 2-stage grinding tests show improved estimated gold recoveries
of 86.2% (weighted average, net of solution losses) compared to
previous gold recovery estimates of 81.7% calculated on the same
basis.
Vista's President & CEO, Frederick H. Earnest, commented, "Mt Todd is one
of the largest, advanced-stage gold development projects in
Australia. Last year we initiated a comprehensive program to
optimize the Mt Todd project and demonstrate its viability at a
US$1,250 per ounce gold price.
We have now completed test work confirming our ability to achieve a
significant improvement in feed grade to the grinding circuit while
reducing our grinding, leaching and tailings handling costs.
Our newly designed, 2-stage grinding circuit is expected to
generate a finer product (with lower power consumption), which we
have confirmed results in higher gold recovery and a 4.1% increase
in gold production. We intend to include these process area
improvements in an updated Mt Todd preliminary feasibility study,
targeted for completion in Q1 2018. We expect these
improvements, along with current foreign exchange rates, and
updated capital, labor and operating costs estimates to demonstrate
that Mt Todd will be a long-life, low-cost, significant gold
producer, with robust project economics at today's gold
prices."
Metallurgical Testing Results Details
Automated Sorting
The bulk automated sorting tests were four, five-tonne
composites prepared from 3.75" drill core. Three of the
composites contain predominately the sulfide mineralization and one
composite contains mixed oxide/sulfide material that is encountered
on the periphery of the deposit. The drill core was HPGR
crushed and screened at 16mm at the facilities of Thyssen Krupp
Industries ("TKI") near Dusseldorf, Germany. The +16mm material
was sent to the test facility of Tomra Sorting Solutions ("Tomra")
near Hamburg, Germany where this
material was initially sorted using XRT sorting. A total of
12 sorting tests were completed. The XRT rejects were then
subjected to laser sorting to produce a final reject. All
material (-16mm HPGR crushed, XRT product, laser product and
sorting reject) was sent to the metallurgical laboratory of
Resource Development Inc. ("RDi") in Wheatridge, Colorado for subsequent sample preparation,
assaying and additional metallurgical testing.
On a material mass basis, the combined XRT and laser
sorting tests confirmed the Company's expectation that it can
reject 10% of the run-of-mine feed waste (test results range from
6.8% to 11.0%). The average grade of the rejected material is
estimated to be 0.12 g Au/t (results range from 0.06 g Au/t to 0.23
g Au/t) compared to the mine cut-off grade of 0.4 g Au/t, resulting
in a gold loss from the rejected waste of approximately 1.3%.
The improvement in mill feed grade is expected to be approximately
8%, resulting in run-of-mine average mill feed grade of 0.91 g Au/t
compared to the life-of-mine reserve grade of 0.84 g Au/t. We
now expect grinding, leaching and tailings handling costs, which
are dependent on the volume of material processed, to be
approximately 10% less.
Grind Size and Leach Optimization Tests
RDi prepared samples of the individual components received
from Tomra and TKI and sent splits to Florin Analytical Services
("Florin") in Reno, Nevada for
assay. RDi then recombined the components (excluding the
sorter reject material) to create representative samples of each
composite. Single-stage grind optimization tests were
completed on the representative samples from each composite.
Leach optimization tests were undertaken on these samples at
various grind sizes. Subsequently, 2-stage grind and leach
optimization tests were undertaken on representative material from
the composites ground to a nominal P80 grind size of
60µm. Additionally, 2-stage grind and leach optimization
tests were undertaken on high-grade samples left over from previous
Mt Todd testing undertaken at RDi. In total, 41 grind and
leach tests were completed on samples representing low-grade, near
deposit average grade and high-grade material from the Mt Todd
deposit. The leach tests were designed and undertaken to
simulate the proposed process flow sheet leach circuit.
Bottle roll leach tests consisted of four hours of pretreatment
with lime and air, 24 hours of cyanide leaching and six hours of
carbon loading. Head and tail assays were completed by
Florin. This testing program indicates that 2-stage grinding to a
nominal P80 grind size of 60µm can be expected to result
in gold recoveries up to 91.0%. Taking into consideration the
relative proportions of the different ore types to be processed,
the leach test results indicate that a recovery of 86.2% (weighted
average, net of solution losses) can be achieved.
The results of the grinding/leaching tests have been
transmitted to the process design team at TetraTech Proteus
("Proteus") who are completing the process area designs as part of
the updated preliminary feasibility study. Proteus and Vista
have worked closely with the engineers at TKI to assess the
capacity of the HPGR crushing circuit. We have also consulted
with the engineers at Glencore relative to our selection of
IsaMills as the preferred fine grinding equipment.
The removal of waste by automated sorting is complemented
by changes in the operation of the HPGR crushing circuit and the
design of a new 2-stage grinding circuit. The new process
flow sheet incorporates work-sharing between the HPGR and grinding
circuits. Excess capacity in the HPGR circuit will now be
used to produce a ball mill feed product of approximately
P80 1/8" (nominally 3.5 mm), a 42% reduction from past
studies. Due to the final product size, wet screening will be
incorporated and the HPGR crusher circuit product will now report
to a sump and cyclone bank. In each grinding circuit module,
the new 2-stage grinding circuit replaces each previously planned
large, single ball mill (designed to produce a P80 90µm
product) with a ball mill and three IsaMills to produce a final
product with a P80 grind size of 60µm. Due to the
reduction in the volume of ore to the grinding circuit and
efficiencies resulting from the combination of mill type, ball
charge size and feed size, the implementation of automated sorting
and 2-stage grinding is expected to consume approximately 13% less
power, produce a final product that is 32% finer, and produce 4.1%
more gold from the same run-of-mine feed.
Updated Preliminary Feasibility Study
Guidance
The Company expects to complete an updated preliminary
feasibility study in the first quarter of 2018. The updated
preliminary feasibility study will include the more favorable
current US dollar vs. Australian dollar exchange rate, gold price
of US$1,250 per ounce, automated
sorting, improved gold recovery, redesigned and more efficient HPGR
crushing/grinding circuit, and various other capital and operating
cost improvements, all of which are expected to result in a
technically improved project with higher gold production, lower
operating costs, and strong project economics.
Qualified Persons
Dr. Deepak Malhotra,
Metallurgist and President of RDi and John
Rozelle, Senior Vice President of Vista, independent
Qualified Persons as defined by Canadian National Instrument 43-101
("NI 43-101"), prepared or supervised the preparation of
information that forms the basis for the scientific and technical
information disclosed herein and have reviewed this press release
and have consented to its release. For additional information on
the Mt Todd gold project, see our technical report entitled "NI
43-101 Technical Report – Mt Todd Gold Project, 50,000 tpd
Preliminary Feasibility Study, Northern Territory, Australia" amended and restated July 7, 2014.
About Vista Gold Corp.
The Company is a well-funded gold project developer. Our
principal asset is our flagship Mt Todd gold project in Northern
Territory, Australia. Mt Todd is one of the largest
undeveloped gold projects in Australia. For more information about our
projects, including technical studies and resource estimates,
please visit our website
at www.vistagold.com.
For further information, please contact Connie Martinez at (720) 981-1185.
Forward Looking Statements
This
press release contains forward-looking statements within the
meaning of the U.S. Securities Act of 1933, as amended, and U.S.
Securities Exchange Act of 1934, as amended, and forward-looking
information within the meaning of Canadian securities laws.
All statements, other than statements of historical facts,
included in this press release that address activities, events or
developments that we expect or anticipate will or may occur in the
future, including such things as our plan to implement automated
sorting and our belief that automated sorting and a re-designed,
2-stage grinding circuit (each module consisting of a ball mill and
three IsaMills to produce a final product with a P80
grind size of 60µm) will enable the project to achieve a finer
grind size, higher gold recoveries/higher gold production, and 10%
lower processing costs (comprised of grinding, leaching and
tailings handling) with no material increase in project capital;
our plan to use excess capacity in the HPGR crusher to produce
smaller grinding circuit feed of approximately P80
1/8" (nominally 3.5 mm); our plan to
include these process area improvements in an updated Mt Todd
preliminary feasibility study, targeted for completion in Q1 2018;
our expectation that an average recovery of 86.2% (net of solution
losses) can be achieved; our plan to use wet screening with the
HPGR crusher circuit product reporting to a sump and cyclone bank;
our expectation that automated sorting and 2-stage grinding will
consume approximately 13% less power, produce a final product that
is 32% finer, and produce 4.1% more gold from the same run-of-mine
feed; our expectation that all of these improvements, along with
current foreign exchange rates, and updated capital, labor and
operating costs estimates will demonstrate that Mt Todd will be a
long-life, low-cost, significant gold producer, with robust project
economics at today's gold prices and other such
matters are forward-looking statements and forward-looking
information. The material factors and assumptions used to develop
the forward-looking statements and forward-looking information
contained in this press release include the following: our approved
business plans, exploration and assay results, results of our test
work for process area improvements, mineral resource and reserve
estimates and results of preliminary economic assessments,
prefeasibility studies and feasibility studies on our projects, if
any, our experience with regulators, and positive changes to
current economic conditions and the price of gold. When used
in this press release, the words "optimistic," "potential,"
"indicate," "expect,""intend," "hopes," "believe," "may," "will,"
"if," "anticipate," and similar expressions are intended to
identify forward-looking statements and forward-looking
information. These statements involve known and unknown
risks, uncertainties and other factors which may cause the actual
results, performance or achievements of the Company to be
materially different from any future results, performance or
achievements expressed or implied by such statements. Such
factors include, among others, uncertainty of resource and reserve
estimates, uncertainty as to the Company's future operating costs
and ability to raise capital; risks relating to cost increases for
capital and operating costs; risks of shortages and fluctuating
costs of equipment or supplies; risks relating to fluctuations in
the price of gold; the inherently hazardous nature of
mining-related activities; potential effects on our operations of
environmental regulations in the countries in which it operates;
risks due to legal proceedings; risks relating to political and
economic instability in certain countries in which it operates;
uncertainty as to the results of bulk metallurgical test work; and
uncertainty as to completion of critical milestones for Mt Todd; as
well as those factors discussed under the headings "Note Regarding
Forward-Looking Statements" and "Risk Factors" in the Company's
latest Annual Report on Form 10-K as filed on February 22, 2017 and other documents filed with
the U.S. Securities and Exchange Commission and Canadian securities
regulatory authorities. Although we have attempted to
identify important factors that could cause actual results to
differ materially from those described in forward-looking
statements and forward-looking information, there may be other
factors that cause results not to be as anticipated, estimated or
intended. Except as required by law, we assume no obligation
to publicly update any forward-looking statements or
forward-looking information; whether as a result of new
information, future events or otherwise.
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SOURCE Vista Gold Corp.