DENVER, May 22, 2013 /PRNewswire/ -- Vista Gold
Corp. ("Vista" or the "Company") (NYSE MKT and TSX: VGZ) today
published the significant results of feasibility-level metallurgic
test work supporting the comminution circuit design and estimated
gold recovery rates for the Batman deposit at the Mt. Todd gold
project in Northern Territory, Australia. The test work was
based on HQ core samples from the Company's 2011 and 2012 drilling
programs and included extensive variability testing. This
test work, combined with the findings of previous test work
programs, supports (1) ore hardness estimates at the Batman deposit
that are consistent and do not change at depth; (2) the selection
of High Pressure Grinding Roll ("HPGR") technology as part of the
comminution circuit; (3) gold recovery rates for freshly mined ore
of 81.7% based on optimized grind size and leach conditions; and
(4) the processing of material from the historic heap leach pad at
the end of the proposed mine life. Results of this test
work are being incorporated into the Company's preliminary
feasibility study ("PFS"), the results of which will be
released prior to market open on May 29,
2013. Vista will host a conference call that
afternoon to discuss the results of the PFS.
Vista's President and Chief Executive Officer, Frederick H. Earnest, commented, "These
metallurgic test results are key milestones in the optimization of
the Mt. Todd gold project. Since purchasing the project in
2006, Vista has completely re-evaluated the entire project with a
methodical, systematic and exhaustive focus on the metallurgical
process. The feasibility-level metallurgic testing program
has added significantly to our understanding of the metallurgical
behavior of the Batman deposit and independently validated more
than six years of technical analysis of the project. We are
confident that material from the Batman deposit can be efficiently
processed by proven, 'off the shelf' technologies to achieve good
gold recoveries."
Full technical reports are available under the "Technical
Reports" header of the Mt. Todd section of the Vista Gold website
at
http://www.vistagold.com/mt_todd.php?subpage=technical_study.
The key findings are summarized below.
Ore Hardness: ore in the Batman deposit is hard, but
consistent and does not change at depth
Samples used for the test work program were sourced from eight
holes from the Company's 2010 and 2011 drilling program that were
oriented to intersect the main Batman deposit beneath the existing
pit and are believed to be representative of the ore within the
limits of the preliminary feasibility pit.
Twenty of the samples were subjected to Bond ball mill work
index ("BWi") tests, the SMC Test (drop-weight and specific gravity
tests) as well as Compressive Strength Tests and Crushing Work
Index ("CWI") tests. The results of the BWi tests show an
average BWi value of 26.2 kWh/t with a maximum value of 28.2 kWh/t
and a minimum value of 23.6 kWh/t.
The results of this test work support two main conclusions: (1)
that the hardness of ore at the Batman deposit is relatively
constant; and (2) that ore at the Batman deposit does not change at
depth.
This test work validates the Company's prior test work and
supports Vista's comminution circuit design, which is designed to
crush and grind ore with an average BWi of 27.4 kWh/t, a 5% factor
of safety above the average BWi and closer to the 75th
percentile of BWi test results.
HPGR Selection: use of HPGRs provides more desirable feed for
ball mills and saves on energy consumption compared to a
Semi-Autogenous Grinding ("SAG") mill
Prior operators of the Mt. Todd project processed approximately
7.5 million tonnes through a five-stage comminution circuit that
incorporated a gyratory crusher and cone crushers as the primary
and secondary-stage crushers. Historic operating data
indicates that the primary and secondary crushers worked well,
although may have been inappropriately sized (too small or
under-powered). The third and fourth-stages of crushing,
which incorporated the use of 19 Barmac crushers, were the
bottleneck of the historic operation.
Vista's proposed comminution circuit incorporates the use of a
large gyratory crusher and two large cone crushers for the primary
and secondary stages, but contemplates the use of HPGRs as the
third-stage of the crushing circuit. Much of Vista's test
work has focused on confirming the use of HPGRs.
Initially, Vista ran a series of parallel tests comparing a
SAG/ball mill circuit with an HPGR crushing and ball mill
circuit. Based on the test work completed, HPGR technology
was selected. Industry experience has shown HPGRs to produce
micro-fracturing in particles that reduce the overall particle
strength and generate a greater distribution of fine material in
the ball mill feed, reducing downstream ball mill energy
requirements. The ore at the Batman deposit consists of
silicified greywackes/shales/siltstones and test work has shown the
HPGRs tend to fracture ore at the Batman deposit along the bedding
planes more than micro-fracturing. The result, however, is
consistent with other industry HPGR applications in that the HPGR
product produces a lower BWi feed for the ball mills. The
test results indicate the SAG mill circuit produced a product with
an average BWi of 26.4 kWh/t compared to the HPGR crushed product
with an average BWi of 24.8 kWh/t, a reduction of over 6%.
Additionally, material crushed in the HPGR test resulted in up
to 10% of the HPGR product being fine enough to by-pass the ball
mills entirely and proceed straight to the leach circuit.
Vista has incorporated this HPGR advantage in its comminution
circuit design.
The test work also assessed the difference in power requirements
between a primary/SAG/ball mill circuit, a conventional 3-stage
crush/ball mill circuit, and a 3-stage HPGR/ball mill circuit to
generate a 90 μm P80 product. The assessment
concluded that the 3-stage HPGR/ball mill circuit has a
significantly lower specific energy requirement than the
primary/SAG/ball mill option and that a finer grind size can be
achieved with the HPGR crushed material compared to conventionally
crushed material ground for the same period of time.
This test work also confirms the Company's prior test work and
supports Vista's comminution circuit design. The use of HPGRs
is anticipated to (a) produce a product that can be ground more
efficiently (lower BWi) in the ball mills; and (b) reduce energy
requirements when compared to a SAG mill design.
Gold Recoveries: gold recovery rates expected to be 81.7% for
freshly mined ore and not negatively impacted by presence of copper
in ore
Vista's focus was to solve the high reagent consumption, poor
gold recovery and copper leaching issues encountered by previous
operators. Historic core samples indicated the presence of
cyanide soluble secondary copper mineralization (chalcocite and
bornite) in ore at the Batman deposit, and as such, Vista's initial
focus was to develop a flowsheet that incorporated the production
of a copper concentrate.
However, Vista's drill programs from 2007–2012 indicated a
significant change in the mineralogy of ore at the Batman deposit
with depth with copper mineralogy changing from cyanide soluble
secondary copper to non-cyanide soluble primary copper
mineralization (chalcopyrite). The change in mineralogy
occurs at approximately 40 meters below surface and the majority of
the ore containing cyanide soluble secondary copper was mined by
previous operators. As a result, more than 96% of ore at the
Batman deposit contains low-to-non-cyanide soluble primary copper
mineralization. Therefore, Vista's recovery circuit has been
simplified and focuses only on recovering gold from ore at the
Batman deposit through a conventional Carbon in Leach ("CIL") circuit.
The remainder of Vista's test work relating to gold recovery
focused on optimal grind size, pre-conditioning of ore with lime
(to reduce cyanide consumption), the identification of a reagent to
suppress copper leaching (lead nitrate was selected), and optimal
cyanide concentration.
After determining the optimal leach conditions, 99 samples
covering a range of head grades from throughout the Batman deposit
were subjected to leach tests resulting in gold extraction between
75% and 85%, with an average of 81.7%, net of solution
losses. Cyanide consumption was estimated at 0.77kg/t and
lime consumption was estimated at 0.91kg/t.
This test work validates the Company's prior recovery estimates
(82%), indicates little gold recovery variability throughout the
Batman deposit, and supports Vista's recovery plant design
utilizing a conventional, industry-proven, CIL circuit.
Deepak Malhotra, President of
Resource Development Inc., who is a "qualified person" within the
meaning of Canadian National Instrument 43-101 – Standards of
Disclosure for Mineral Projects, has verified and approved the data
and approved the scientific and technical information disclosed in
this press release.
Existing Heap Leach Pad: material to be processed through
mill at end of mine life
In addition to analysis of freshly-mined ore from the Batman
deposit, Vista has analyzed the potential to incorporate nearly
13.35 million tonnes of material on the existing heap leach pad
into the Mt. Todd gold project. The original Mt. Todd project
started as a heap leach operation with historic records indicating
that the average grade of material placed on the pad was 0.96
g Au/t. Although the material was partially leached in the
mid-1990s, Vista has drilled 24 air-rotary holes into the heap
leach pad and assayed 361 samples, and created a 3D resource model
that has an average grade of 0.54 g Au/t.
Initial evaluation efforts focused on re-starting the heap leach
pad. Bottle roll and column tests were completed, both of
which supported the leachability of the material with gold recovery
rates around 35%. However, poor in situ permeability rates
caused Vista to ultimately abandon plans to re-start the heap.
Vista subsequently submitted two heap leach variability
composites and two drill hole composites from the leach pad for CIL
cyanidation leach test work. The samples were ground to
P80 passing 90 μm and pre-treated with lime and 100g/t
of lead nitrate to suppress copper leaching. The material was
then leached for 24 hours. These results support recovery
rates of 70% for this material when processed through the CIL
plant.
Vista's upcoming pre-feasibility study will incorporate
processing this material through the plant at the end of the mine
life. The heap leach material will be listed as a separate
resource from the Batman deposit. A separate technical report
relating to the heap leach CIL cyanidation test work has been
completed and posted to the Vista website along with the additional
reports referenced above.
About Vista Gold Corp.
Vista is focused on the development of the Mt. Todd gold project
in Northern Territory, Australia,
to achieve its goal of becoming a gold producer. Vista has
completed a preliminary economic assessment on its Guadalupe de los Reyes gold/silver project in
Mexico and has granted Invecture
Group, S.A. de C.V. a right to earn a 62.5% interest in the Los
Cardones gold project, in Mexico.
Vista's other holdings include the Awak Mas gold project in
Indonesia, subject to One Asia
Resources Ltd.'s right to earn an 80% interest, and the Long Valley
gold project in California. 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 Vista expects or
anticipates will or may occur in the future, including such things
as, the completion and release of the PFS as scheduled, the
expected efficiencies from the use of HPGR's in Vista's plant
design, Vista's expected gold recovery rate at the Mt. Todd
project, Vista's ability to recover gold using a CIL recovery
circuit and Vista's goal of becoming a gold producer 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: timing
and completion of the preliminary feasibility and feasibility
studies, timing and the ability to obtain the necessary permits,
keeping expenditures at a minimum while taking advantage of the
estimated mineral resources at the Mt. Todd gold project, the
amount of expenditures required to increase the plant capacity and
timing and schedule of consultants' work and other such
matters are forward-looking statements and forward-looking
information. 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 Vista 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 estimates,
estimates of results based on such resource estimates; risks
relating to completing metallurgical testing; risks relating to
cost increases for capital and operating costs; risks related
to the timing and the ability to obtain the necessary permits,
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
Vista's 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
Vista's Annual Report Form 10-K as filed on March 14, 2013 and other documents filed with the
U.S. Securities and Exchange Commission and Canadian securities
regulatory authorities. Although Vista has 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, Vista assumes no
obligation to publicly update any forward-looking statements or
forward-looking information; whether as a result of new
information, future events or otherwise.
SOURCE Vista Gold Corp.