My apologies if this has previously appeared on the thread:
(also for the length).
sec.gov
NOTES ON SITE VISIT:
CANGALLI GOLD DEPOSIT, BOLIVIA
Golden Eagle International, Inc.
By Donald M. Hausen, Ph.D.
September 1997
INTRODUCTION
Technical Advisory Board members Don Hausen and Max Staheli were invited by
Golden Eagle International, Inc. to visit the United Cangalli's gold placer
concessions in the central portions of the Tipuani Mining Districts during the
week of September 2-9, 1997. The placer concessions are significantly large and
owned by the United Cangalli Gold Mining Cooperative, and are under contract by
Eagle Mining of Bolivia, Ltd., a subsidiary of Golden Eagle International, Inc.,
for development and exploitation. The visit to the mining operations and placer
deposits was scheduled for September 6 and 7, accompanied by Terry Turner,
President of the Golden Eagle International, Inc., along with several executive
board members and staff, including Mary Erickson, Rene Velasquez, and Guido
Paravicini. Field observations and geologic interpretations are discussed in
this report with emphasis on the potential reserves and mining aspects of the
gold placer occurrences.
GENERAL DESCRIPTION OF THE CANGALLI/TIPUANI DISTRICT
Gold mining in the Cangalli area has occurred over a period of 1000 years
or more, resulting in reported amounts of gold exceeding 30 million ounces
produced by pre-Inca natives, Incas and Spaniards, as well as both Bolivian and
foreign companies.
According to Guido Paravicini (1997), average gold grades found in the
Cangalli/Tipuani District vary from a few grams to several ounces per cubic
meter; this district is acknowledged as the largest and richest gold-bearing
placer area known in Bolivia.
The Tipuani District lies in the middle of a major down faulted graben
called the Apollo-Caranavi Trench along a valley formed by the Tipuani River and
its tributaries. The bedrock consists of undifferentiated Ordovician shales (or
phyllites), sandstones and quartzites, which are mostly covered within deep
valleys by the Cangalli formation conglomerates of late Tertiary age. The
Cangalli's deposits are located along the Tipuani River valley, notably
outcropping in the vicinity of the small village of Cangalli. This thick
auriferous formation consists of alternating beds of gravel, sand and clays that
fill the Tertiary deep canyons along the Tipuani River and nearby tributaries.
The enormous size of this placer deposit is indicated by its dimensions within
the Tipuani basin, ranging in length up to 25 km (15 miles), average width of 25
km (1.5 miles) and approximate thicknesses of 500 to 2500 meters (1,640 to 8,200
ft).
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Subsequent to this massive Tertiary deposition of auriferous gravel and
sand, the regional rivers, including the Tipuani River, continued to erode new
river channels along the sides of the valleys between the conglomerate and the
phyllitic clay host rock.
Approximately 3 to 9 meters above the bottom of the paleo-canyon, a very
abrupt change in conditions occurred. Massive boulders up to several tons in
size were noted just above the mud "clay/silt zone", and locally buried into the
mud. This sudden change was apparently initiated by major environmental
variations in climate or orogenic disturbances, resulting in abrupt increases in
stream flow The valley was subsequently filled with poorly sorted gravel, sand
and clays, containing significant amounts of gold derived from unidentified
upstream source(s). This Tertiary geologic event initiated the deposition of the
Cangalli conglomerate, which filled the deep canyon with hundreds of meters of
auriferous gravels along with interstitial fine sands, silts and clays.
Unsorted cobbles and pebbles are well rounded by stream action, indicating
that they were carried for some distance from the Cordillera on the west. Most
of the coarse cobbles were identified as metaquartzites, associated with minor
amounts of granitic rocks. Most of the gold is contained in t he finer matrix of
sand and clay between the cobbles, rather than within the coarse gravel.
EXPLORATION AND DEVELOPMENT CONSIDERATIONS
Over hundreds of years, the natives have achieved rich placer recoveries of
coarse gold from multiple surface locations throughout the Cangalli conglomerate
at high and low elevations, wherever sufficient water was available for simple
sluice methods. Their recovery methods were primitive and a ffective mostly in
collecting coarse gold. Total gold contents are significantly higher than the
amounts recovered by gravity methods. The spotty distribution of coarse and fine
gold throughout the bedded gravels and sandy to clayey seams poses the problem
of "How to sample and determine the grades of gold throughout the Cangalli
deposit!".
Drilling and sampling programs have been considered to establish the grades
and reserves of the Cangalli conglomerate deposit. Shallow churn drill methods
have been used to estimate the gold content of alluvial material along gravel
benches and beneath flood plains near rivers where abundant wa ter is available.
Such methods would appear to be impractical in deep drilling through hundreds of
meters above river beds where the terrain is mountainous in relief.
Reverse circulation drilling programs into bedrock deposits usually provide
representative sample cuttings for assaying. However, deep RC or diamond
drilling methods to evaluate placer deposits have not been conventionally used
by others for several reasons:
(1) The spotty nature of coarse gold in placer deposits require very large
samples (multiple cubic meters).
(2) Preparation and splitting of large samples containing coarse gold
would require special screening and fine grinding, resulting in high
costs for each sample.
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(3) Drilling through gravel containing coarse cobbles adjacent to fine
sand, silt and clays would probably not collect representative samples
for assaying.
(4) Deep drilling (greater than 300 meters) through conglomerate would
require extensive casing, and would be costly.
VISIT TO THE MINE SITE AND SLUICE OPERATIONS
The two-day visit to various facilities at the mine site included the two
sluice operations: (No. 1) the current main operation at the village at Cangalli
consisting of a head frame, an underground shaft and a sluice operation; and
(No. 2) a recently installed sluice system at the nearby Cueva Playa.
The sluice operation near the main shaft at Cangalli was visited during the
afternoon of the first day. Basal portions of the conglomerate mined below the
300 meter level of the shaft were transported and dumped by truck through a
water-fed conveyer system onto a conventional sluice. At the completion of each
batch load, the sluice panels were removed and the concentrates upgraded and
recovered by means of manual washing and selective removal of coarse pebbles,
sands and clays. Coarse gold was visually observed, mostly at the upper end of
the sluice. Concentrates were collected and taken to another building where
further concentration of coarse and fine fractions was accomplished by
additional gravity (hand panning and tabling) methods. The overall process was
labor intensive, using batch rather than continuous methods. However, the
expected amounts of medium fine to coarse gold were recovered from the basal
zones of the conglomerate from the lower levels of the mine workings.
The visible gold particles occurred morphologically as thin sheaths,
classified as "leaf" gold, ranging mostly from about a millimeter up to
approximately one centimeter in average diameter. The grades of gold recovered
by this batch/sluice method are reported to average about 3-4 grams per met ric
ton.
Significant amounts of fine gold are estimated to be lost in the process,
but may be recovered by more efficient gravity methods and/or by cyanidation of
gravity tails.
GEOLOGIC OBSERVATIONS ALONG DRIFTS NEAR THE BOTTOM OF SHAFT
Several areas of the underground workings were inspected during the evening
of the first day. Wall rock in most of the drifts consisted mostly of Ordovician
gray/green shales (or slickensided phyllites) located below the valley floor.
Intersections of drifts and stopes with the bottoms of Pale o-valleys were most
interesting, revealing the paleo-environmental conditions prior to the
deposition of the Tertiary conglomerates. The valleys (or canyons) were narrow
with steep V-shaped walls, indicating rapid stream erosion of the soft clayey
bedrock Valley fill at this time was essentially clay fragments (or phyllitic
breccia) derived from the clay bedrock.
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Locally, small lenses of well sorted sand-silt were noted, indicating that
the conditions of sedimentation were gradual, resulting from normal stream flow.
This facies may be termed the mud "clay/silt zone".
For these reasons, it is not recommended to schedule a drilling program at
Cangalli without contacting drilling contractors who have experienced this type
of drilling. We must be aware that the Cangalli deposit is possibly unique in
its origin, its considerable thickness and variable distributi on of gold
throughout its extent.
The most economical and representative means to obtain large samples for
analysis would be from surface exposures using front-end loaders and dump
trucks. Such large samples could be transported to sites of sluices (or other
efficient means of obtaining gravity concentrates) for comparative tes twork.
It is therefore recommended that multiple small open-pit locations be
developed at various strategic locations at high and low elevations throughout
the conglomerate deposit. It is also recommended that a simple gravity feed
system be used to screen out coarse pebbles and cobbles prior to deliv ery to
the sluices. The upgraded gold concentrates would then be fed directly to the
sluices for test recovery of coarse gold. The gravity fine tails should then be
split and sampled for fire assay and/or cyanide bottle tests. In this manner,
both fine and coarse gold can be determined for the potential total gold
recoveries.
It is recommended that several widely spaced sites of surface bulk sampling
be tested during the coming year. In the interim, the recovered gold can be sold
on the market as "cash flow", in the same manner that the Cangalli sluice
operation is now processing the gold from the bottom of the shaft. This "cash
flow" should defer a portion of the cost for the development program.
MINING AND METALLURGICAL CONSIDERATIONS
The prime purpose of gravity testwork on bulk samples would be to provide
the necessary preliminary information on the distribution of recoverable gold
values through a significant part of the Cangalli conglomerate deposit. If
values are sufficiently high and continuous to favor the development of a viable
mining venture, the next step would be to locate a mill site where gravity
tailings can be further processed by cyanide leaching, and disposed at low cost.
It is recommended that the Cueva Playa, located along the Tipuani River, be
considered for the following reasons:
(1) This site is located near a large meandering floodplain from which the
river was diverted in past years. This site would also provide space
for the dumping of much of the coarse gravel separated by screening,
prior to the gravity gold recovery system.
(2) A large section of the Cangalli conglomerate is exposed at higher
elevations above the meandering floodplain, which could be benched at
multiple levels, and possibly provide gravity feeding to the mill
site.
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(3) A favorable location within the Cueva Playa should be considered for
the installation of a small cyanidation plant to process fine gold
unrecoverable by gravity methods.
CONCLUSIONS AND RECOMMENDATIONS
In my opinion, the Cangalli conglomerate formation can be classified as a
very large low-grade placer deposit, with the potential reserves of millions of
ounces of gold. The viable development and exploitation of similar deposits
world-wide require low-cost large-scale methods of mining and pro cessing. It is
therefore recommended that large open-pit mining methods and cheap (low capital)
continuous volume gravity processing methods be developed for this type of
deposit.
The spotty distribution of coarse gold throughout the deposit requires
large bulk sampling to provide representative dependable estimates of gold
grades, during early development. A large bulk sampling program of the Cangalli
conglomerate concessions is therefore proposed: (1) to confirm the c ontinuity
of viable gold values through much of the conglomerate; (2) to optimize methods
of mining and gravity concentration of coarse gold; and (3) to provide some
interim cash flow.
Again, it is emphasized that large tonnage placer deposits require low-cost
open-pit mining operations and cheap continuous gravity methods for the recovery
of medium to coarse gold on a large scale. It is also recommended that the fine
gold be extracted from the gravity tails by means of cyani de dump leaching to
improve the overall gold recovery. However, the location and operation of a dump
leaching facility requires more study.
The underground operations should be discontinued as soon as possible, and
future bulk samples be collected from multiple small open-pit sites, widely
spaced over accessible exposures across the conglomerate deposit. It is also
recommended that continuous, rather than batch methods of gravity c
oncentrators, be tested for collection of leaf-type coarse gold. This should
include testwork using various types of jigs, spirals, strakes, Knelson bowls,
etc, for upscaling volume production without excess manual labor.
From the viewpoint of a process mineralogist, the amounts, morphology and
size range of the reported fine gold be determined from several random large
samples (~500 lbs each) of gravity tails from the Cangalli sluice operation.
Each bulk sample must be screened first into a number of fractions; i.e. plus 4
mesh, -4/28 mesh, -8/+28 mesh -28/+100 mesh, -100/+200 mesh, -200/+325 mesh, and
minus 325 mesh slimes, followed by assaying for gold by fire assays (FA) and
cyanide extraction assays (CNAA). This should account for the potential losses
of fine gold at Cangalli. Screen fractions containing highest concentrations of
gold losses would be selected for detailed mineralogic examination.
Hope this may answer some drilling questions.
Later: Rick |
