DENVER, Colo., Nov. 28, 2016 /PRNewswire/ -- Vista Gold
Corp. ("Vista", the "Company," "we" or "our") (NYSE MKT: VGZ) (TSX:
VGZ) today announced the results of process area optimization
studies that have the potential to enhance the project economics
for the Mt Todd gold project in Northern Territory, Australia. The anticipated economic
improvements are the result of process flowsheet changes that are
expected to produce higher gold recoveries and lower process area
operating costs. Testing indicates that these objectives can be
accomplished through selectively screening and rejecting
sub-economic, coarse crusher product prior to grinding; resulting
in opportunities for improved gold recovery through finer grinding
and improved operating costs down-stream of the grinding
circuit.
A conference call with management to discuss the results of the
Mt Todd gold project optimization studies is scheduled for
Tuesday, November 29, 2016 at
2:00 pm MST.
Summary of Process Area Optimization Results
Earlier this year, Vista completed an independent review of the
Mt Todd gold project. The combined input of the independent
consultants and the project management team resulted in additional
comminution and metallurgical recovery evaluations. These
evaluations assessed opportunities to selectively remove below
cut-off grade material from the process circuit after
crushing. This results in less material being processed in the
grinding and leaching circuits. The reduction in material results
in excess grinding capacity which can be used to achieve a finer
grind size in the final product and a corresponding increase in
gold recovery for the remaining portion of the leach feed.
Achieving the finer grind size is not expected to materially change
the total grinding costs. Most importantly, the higher gold
recovery is expected to increase cash flow, while the leaching,
cyanide detoxification and tailings management costs are expected
to decrease as a result of the reduction in the volume of material
processed.
We completed size distribution and assay analysis of the high
pressure grinding rolls ("HPGR") crusher product. This
indicates that screening the HPGR crusher product at 5/8" produces
an oversize product that accounts for 15-20% of the weight, but
contains only 7-9% of the gold. Subsequently, we completed test
work with Tomra Sorting Solutions, Inc. ("TOMRA") at their
facilities in Germany to evaluate
the recovery of sulfide/quartz/calcite-bearing material using Laser
Reflection/Fluorescence and X-ray Transmission ("XRT") automated
sorting techniques. This test work indicates that approximately 85%
of the gold in this oversize fraction can be efficiently recovered
with a return of 30-40% of the weight. We expect this to translate
into an 11.5% reduction in material to be processed in the
grinding/leaching circuits with a 1.6% loss in gold coming from the
elimination of sub-economic material from the process stream.
If there is an 11.5% reduction in material passing to the
grinding circuit, we expect to achieve a final grind size of 80%
passing ("P80") 75 microns ("μm"), approximately 17% finer than the
final grind size used in our May 2013
PFS of 90 μm. Test work confirms that at a P80 75 μm grind
size, we can expect to achieve gold recoveries in the range of
83-84%, up from the 81.7% reported in the May 2013 PFS.
Vista's President & CEO, Frederick
H. Earnest commented, "The process improvements that we have
identified have the potential to generate lower operating costs and
better gold recoveries without adding materially to the capital
costs of the project. These improvements, coupled with the current
US dollar to Australian dollar exchange rate have the potential to
produce a significant improvement in project economics. We are
now evaluating an update of our preliminary feasibility
study."
Detailed Discussion of Metallurgical Testing Program
Results
Mt Todd Deposit Geology
The Mt Todd deposit is hosted by a silicified sequence of
greywackes, siltstones and shales. This host is cross-cut (nearly
perpendicular to the bedding planes) by two different series of
quartz and calcite veins. The injection of quartz- and
calcite-rich solution provided the transportation mechanism for the
gold and other sulfide minerals found in the deposit. The gold
in the Mt Todd deposit is associated with the veining and
occurrence of sulfide minerals, and is found only in small
concentrations (likely the result of micro-fracturing) in the host
rock. The quartz/calcite veining is pervasive throughout the
deposit with vein thicknesses running from a couple of millimeters
to several centimeters. Vein density can vary from 3-4 to 10-15
veins per meter. Selective assaying was completed to assess
the presence of gold exclusively in the veins and/or sulfides
compared to the host rock. Selective samples submitted to Pine
Creek Labs in Pine Creek, Northern Territory, Australia indicated that carefully selected
veins and sulfide samples contained 0.16-41.4 grams Au/tonne of
sample compared to 0.05-0.46 grams Au/tonne of sample for the host
rock. The relatively small proportions of vein/sulfide material
account for the average grade of the deposit, and at the same time
highlight the value of efforts to separate the host rock from the
gold-bearing veins and sulfides.
HPGR Product Screen-size and Gold Distribution Analysis
In 2012, we prepared two bulk samples representative of the Mt Todd
deposit using metallurgical core dispersed across the deposit
(identified as M1 and M3). These two samples were sent to
Thysen Krupp Industrial Solutions' HPGR testing facilities in
Germany for HPGR crushing
tests. A total of approximately two tonnes of material between
the two composites was crushed. The HPGR crushed material was
returned to Vista for subsequent metallurgical testing. Using HPGR
crushed material from these two composites, we undertook studies to
evaluate the amount of material and amount of gold contained in
various screen size fractions. The results of this analysis
are summarized in the following table. This work (including
the subsequent sample prep) was completed by Resource Development
Inc. of Wheatridge, Colorado
("RDi") with assaying completed by Florin Analytical Services in
Reno, Nevada.
|
M1
Sample
|
M3
Sample
|
|
Wt%
|
Au
(g/mt)
|
%
Distribution
Au
|
Cumulative
Au Passing
|
Au (g/mt)
in Screen
Undersize
|
Wt%
|
Au
(g/mt)
|
%
Distribution
Au
|
Cumulative
Au Passing
|
Au (g/mt)
in Screen
Undersize
|
Feed
|
100
|
0.92
|
100
|
-----
|
-----
|
100
|
0.79
|
100
|
-----
|
-----
|
+7/8"
|
6.6
|
0.71
|
5.1
|
94.9
|
0.93
|
4.3
|
0.67
|
3.7
|
96.3
|
0.79
|
7/8" x
5/8"
|
13.1
|
0.53
|
7.5
|
87.4
|
1.00
|
11.7
|
0.4
|
6
|
90.3
|
0.85
|
5/8" x
3/8"
|
19.8
|
1.19
|
25.8
|
61.6
|
0.93
|
17.2
|
0.87
|
19.1
|
71.2
|
0.84
|
3/8" x
1/4"
|
12.6
|
0.92
|
12.7
|
48.9
|
0.93
|
12.7
|
0.72
|
11.7
|
59.5
|
0.87
|
-1/4"
|
47.9
|
0.93
|
48.9
|
-----
|
-----
|
54.1
|
0.87
|
59.5
|
-----
|
-----
|
Based on this data and with some understanding of the minimum
size requirements for selective automated sorting we determined
that the 5/8" screen oversize provided a meaningful volume of
material for our testing to evaluate selective sorting and
rejection. This size was also selected because it is reasonable to
expect that screening can be effectively achieved at this size in a
full scale operation. Physical observation of the +5/8" material
indicated visually identifiable portions of vein material and
sulfide material. It was noted that the majority of the 5/8"
material showed no visual indications of veining or
sulfides.
Automated Sorting Testing
Automated sorting is employed by many industries, and in the gold
sector the Coeur Kensington mine is successfully operating an
automated sorting circuit to recover higher grade ore before
wasting the rejects. Many different sensors are used
today in sorting, including but not limited to: Optical, Laser,
XRT, Near Infrared, Visible light, Electromagnetic, and
Color. A sample of +5/8" oversize material was sent to TOMRA's
mining sorting division for testing. Testing was performed
under the supervision of Outotec personnel using XRT and Laser
sorting sensors. Testing indicates that a combination of XRT
and Laser sorting methods could effectively identify the material
containing quartz/calcite veining and/or sulfide material for
subsequent removal using automated air sorting technology. A
larger sorting test is planned to verify the promising indicative
results. The following table summarizes the results of test work
completed by TOMRA under Outotec supervision at TOMRA's mining
sorting division in Germany.
|
|
|
Grade
|
%
Distribution
|
|
|
Wt
%
|
Gold (g
Au/mt)
|
Total Sulfur
(%)
|
Gold
|
Sulfur
|
Average Feed
(Calculated)
|
100%
|
1.98
|
0.84
|
100%
|
100%
|
Average
Product
|
35.6%
|
4.80
|
2.10
|
86.1%
|
88.6%
|
Average
Waste
|
64.4%
|
0.43
|
0.15
|
13.9%
|
11.4%
|
Grind-size/Recovery Testing
As part of the comprehensive metallurgical testing completed in
2012, we completed grind size optimization studies. Those
studies indicated that a P80 of 90 μm was the optimum grind
size. At that time we recognized that finer grinding could
result in higher gold recoveries, but the additional capital (i.e.
larger mills) and power could not be justified to achieve the
incremental increase in gold recovery. With the successful
completion of the screening and automated sorting tests, we
evaluated the relative advantages of down-sizing the mills to match
the expected throughput compared to maintaining the mill size and
grinding to a finer size.
As part of the recent testing, we completed grinding tests to
achieve an initial product with a P80 of 90 μm. This product
was subsequently screened to assess the relative proportion of
material at size fractions from 100 mesh to minus 400
mesh. This screen analysis indicated that 40-50% of the ground
material was minus 400 mesh and 10-20% of the material was plus 200
mesh. RDi completed an analysis of the energy requirements to grind
to a finer size and determined that the ball mills selected for the
project in the 2013 PFS, operating with 11.5% less feed from the
crushers would be able to successfully produce a product with a P80
of 75 μm without any additional power demand. Based on grind-size
and leach recovery work completed in 2012 and work recently
completed by RDi, we expect that total recoveries in the range of
83-84% can be achieved with mill product with a P80 of 75
μm.
Evaluation of Potential to Incorporate 2-Stage HPGR Crushing
In addition to the foregoing work, we prepared additional core
composite samples and sent them to KCA in Reno, Nevada for additional HPGR testing. We
conducted a testing program to evaluate the potential to achieve
significant additional size reduction through 2-stage HPGR
crushing. The objective of the program was to evaluate the
potential to achieve sufficiently fine ball mill feed that even
finer grinding, and hence higher gold recoveries, could be
achieved. At this time we are not convinced that the additional
capital and operating costs of 2-stage HPGR crushing can be
justified by the results, but additional testing is
contemplated.
Qualified Person
Dr. Deepak Malhotra, of Resource
Development Inc., is a Qualified Person as defined under Canadian
National Instrument 43-101 and has reviewed the scientific and
technical information in this news release.
Management Conference Call
A conference call with management to discuss the results of the
Mt Todd gold project optimization studies is scheduled for
Tuesday, November 29, 2016 at
2:00 pm MST.
Event Title: Vista Gold's Mt Todd Gold Project
Optimization Update
Toll-free in North America:
1-866-233-5249
International: 416-642-3300
This call will also be web-cast and can be accessed at the
following web location:
http://event.on24.com/r.htm?e=1322195&s=1&k=B2530005D32B12FE6F9AE93198FAF931
This call will be archived and available at www.vistagold.com
after November 29, 2016. Audio
replay will be available for 21 days by calling toll-free in North
America: 1-888-203-1112, passcode 1698128.
If you are unable to access the audio or phone-in on the day of
the conference call, please email questions to Connie Martinez, Manager – Investor Relations
(email: connie@vistagold.com), and we will try to address these
questions prior to or during the conference call.
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 where we are
seeking approval of our final environmental authorization and
evaluating an update of our preliminary feasibility study. We also
hold approximately 4.4% of the outstanding common shares of Midas
Gold Corp., non-core projects in Mexico and the
United States and royalty interests in Indonesia. 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 the
potential to enhance the project economics for the Mt Todd gold
project in Northern Territory, Australia; the anticipated economic
improvements that the process flowsheet changes that are expected
to produce; selectively screening and rejecting sub-economic,
coarse crusher product prior to grinding; our expectation that the
capital costs will not add materially to the capital costs of the
project; the effect on the projects economics of the current US
dollar to Australian dollar exchange rate; our plan to finalize a
schedule for the completion of an updated preliminary feasibility
study; and our expectation that we will announce our plans for its
completion in the near future 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, mineral resource and
reserve estimates, results of optimization studies and results of
preliminary economic assessments, technical studies,
pre-feasibility 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; 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 26, 2016 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.