(Reprinted with permission from the April 2000 issue of JOM, © 2000 by The Minerals, Metals, and Materials Society (TMS), Warrendale, PA 15086 USA.) The beneficiation of gold ores begins with an examination and classification of the types of gold occurrences and recovery methods. Measurements can provide the necessary grind size for liberation and determine the sizes and associations of gold with gangue materials. In this article, the textural features of several gold occurrences are described and compared.

Cangalli Gold Alluvial Deposit, Bolivia- The Cangalli gold deposit is located in the Tipuani District, about 100 km north of La Paz. The district lies within a major down-faulted graben called the Apollo-Caranavi Trench along a valley formed by the Tipuani River and its tributaries. The Cangalli formation represents a unique conglomerate deposit of auriferous gravel and sand that has filed up the paleovalley of the Tipuani River. The Tipuani tributaries continued to erode new river channels along the sides of the valleys between the Tertiary conglomerate and Ordovician phyllitic bedrock. The enormous size of this deposit is indicated by its dimensions within the Tipuani basin, ranging up to 25 km in length, 2.5 km wide, and about 500-2,500 m thick.¹⁶

Gold mining in the Cangalli area has occurred for more than 1,000 years, resulting in reported amounts of gold exceeding 30 million ounces produced by pre-Inca natives, Incas, and Spaniards, as well as both Bolivian companies and companies from outside of the country. Numerous surface workings are visible from the bottom to the top of the conglomerate, suggesting that the conglomerate horizons are auriferous. Average gold grades 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 alluvial conglomerate area know in Bolivia.¹⁶

A recent report by Behre Dolbear and Company¹⁷ confirms the widespread gold mineralization. They collected and assayed 73 samples from six widely separated areas over much of the deposit owned by Golden Eagle International. The results of the sampling campaign indicated that more than 90 percent of samples contained significant amounts of visible alluvial gold. The variations in grades were erratic and nonpredictable, which is typical of most alluvial deposits. The erratic mineralization is expected, due to nugget effect from sample to sample, but confirms that coarse gold occurs widely throughout the deposit in interstitial sand and silt between pebbles and boulders in the conglomerate. This megatextural distribution of coarse gold presents a problem for representative bulk sampling, assaying, mining, and metallurgical treatment.^{18 4}

In a visit to the sluice operations near the main shaft of the Cangalli mine in 1997, it was observed that the visible gold particles in gravity concentrates were morphologically very thin sheaths, classified as leaf gold, ranging from about a millimeter up to approximately a centimeter in average diameter. The grades of coarse gold (greater than 200 microns) recovered by this batch/sluice method were reported to average 3-4 g/t. Significant amounts of fine to medium gold (less than 200 microns) are estimated to be lost in the process, but may be recovered by more efficient gravity methods and /or flotation or cyanidation of gravity tails.

The variable distribution of coarse gold throughout the deposit requires large bulk sampling. Sluice processing of samples from a number of small open pits could provide recoverable gold grades and would evaluate grade continuity to justify the development of a large open pit for production.