Where does all this gold come from?
By: Paul Collar
|Paul is a hydrogeologist and environmental engineer and the owner/operator of Osa Water Works. You may reach him at firstname.lastname@example.org|
All that is gold does not glitter
Not all those who wander are lost
The old that is strong does not wither
Deep roots are not reached by the frost.
From 1981 to 1989, during the Osa Peninsula’s famous gold rush, some 4800 kilograms of the precious metal was mined from its placers. This compares to an estimated 1800 kg from the Tilarán/Aguacate gold district in the same time frame1. The Cerritos District in the Curtis Region of San Carlos near the Nicaraguan border is the third known occurrence of gold inside Costa Rica. Cerritos has never been mined and is in fact the subject of an ongoing World Bank arbitration between the Canadian company Infinito Gold, Ltd and the nation of Costa Rica2. Infinito seeks reimbursement of its $93 million investment (down from its original demand of $1 billion in lost projected profits) after Costa Rica declined the exploitation concession required to legally mine. Osa, Tilaran/Aguacate, Crucitas: nowhere else in Costa Rica is gold known to occur in greater than trace amounts.
In both the Tilarán/Aguacate and Cerritos districts gold occurs in low-temperature primary quartz veins that fill fractures or in primary veinlets disseminated throughout hard rock. The hydrothermal mineralization of both is thought to result from the heat and metal source of a cooling pluton in the tectonically active subsurface. In both cases, mining requires the breaking of solid rock and physical extraction of ore and its subsequent physical and chemical processing. In the Tilarán Cordillera, hand miners used drift tunnels underground to follow ore veins during artisanal mining that dates from the early XIXth Century. Recent mining operations include the Bellavista Mine, an open-pit mine that operated from 2005 to 2007. In both tunnel and open-pit mining, primary hard-rock gold ore, whether extracted with chisels and sledges or dynamite and excavators, must be first crushed and milled and then concentrated chemically with cyanide for leaching gold from ore heaps and tailings piles and native mercury to subsequently recover and concentrate the leached gold. Two more toxic substances than cyanide and mercury are rarely mentioned in a single sentence whose subject matter is not toxicity itself. Hard-rock gold mining and refinement have catastrophic environmental consequences, no matter where it is practiced. It is not an activity compatible with any definition of environmental sustainability, and it never will be.
At Crucitas, the disseminated nature of the gold meant that strip mining by Infinito was to be the only viable extraction method. With Costa Rica’s environmental patience already pushed to its breaking point over the 2007 landslide and cyanide spill at Bellavista, the Crucitas venture never got off the ground and never had a chance. In fact, Costa Rica’s law forbidding strip mining dates to 2002, and the only reason Bellavista was allowed to open in 2005 was that the extraction concession had been issued years before the new law was passed. With Costa Rica’s future joined at the hip with the environment and eco-tourism, hard-rock mining and its environmentally destructive methods of both mining and chemical extraction made it unlikely then and make it unlikely today to ever be jumpstarted again.
The gold that forms Costa Rica’s most important gold region, the Osa / Burica gold district, is, however, not primary gold, but secondary, placer gold. It is native gold that has already eroded from a primary source and been concentrated by water and topography. The gold mined in the Osa is the native element itself and requires no further refinement during mining. Gold is, after all, impervious to chemical alteration or dissolution, so it stays in the environment forever. It is the densest substance that occurs naturally on the earth’s surface, so it prefers to sink into low spots and is for this reason readily concentrated by natural geomorphic processes. No chemicals are needed or used, and mining methods include the use of sluice boxes and wash plants to mechanically concentrate the loose gold in soils, regolith, ancient river and beach terraces, channel deposits, flood plains, active river channels and beach sands. Neither hard-rock tunneling nor strip mining is required, and placer gold mining carries none of the environmental threats of hard-rock gold.
Another key difference that has left a long impact on Osa society and culture is that placer gold can be worked by a single miner or in small groups of partners or cooperatives and does not absolutely require machinery or substantial capital investment to achieve results. In Tilarán, collective effort, capital, and ingenuity were required to extract and concentrate gold even in the early days. In the Osa all that was needed was a single man’s determination, grit and obstinacy in the face of overwhelming nature.
The primary gold districts of Tilarán and Crucitas carry placer gold in their local rivers and streams just like primary gold occurrences do everywhere. The converse is also usually true: nearly all placer districts are accompanied by a known primary gold source from which the placer gold was eroded.
But the Osa / Burica gold district is an exception. Here placer gold is abundant and has been mined by modern man for ninety years and in pre-Colombian time for at least 2000 years without a single primary hard-rock gold vein ever known to be discovered. Here we have lots of secondary gold but no trace of the primary. This begs a question that has been keeping economic geologists and exploration geochemists awake at night for the past 90 years.
Where does all this gold come from?
See my literature review3 on the subject for details, but to summarize what we think we know:
Shatwell4 points out that porphyry-type polymetallic deposits are common in the Talamanca Mountains, and that these are the most likely source for the Osa gold, that it was eroded from the Talamancas and transported by river systems before the opening of the Golfo Dulce to where we find it today. This theory suggests that Osa and Tilarán/Crucitas gold have similar hydrothermal origins from cooling plutons of an intermediate lithology type, somewhere between basalt and granite. Kriz4 and Berrangé5 say “not so fast; the gold couldn’t have come from that far away.” Berrangé pointed to crystalline gold with intertwined quartz crystals in Osa nuggets that could not have traveled far or they would be broken apart and rounded. He offered up the kilogram-plus mega-nuggets of Violines Island as proof positive that the gold was local and could hardly have rolled down rivers from a source 20 or 30 miles away. Kriz insisted that the gold came instead from a rock overlying today’s land surface that has since been completely eroded away. Berrangé did not agree, insisting the gold had to come from quartz veins in the Nicoya Complex rocks, the actual basement rock upon which these placers form today. So, Kriz says the primary source is eroded away; Berrangé says it remains undiscovered in hard rock beneath 30 meters or so of regolith and soils.
Both theories have holes: if all the gold is derived from rock that is beneath our feet, then why after ninety years of mining has the primary never been found? And if the entire primary source was in an overlying unit that was completely eroded away, then why do we recover freshly crystallized vein material in some upland nuggets in the Rancho Quemado region and other places on the peninsula?
By comparing the ratio of gold to silver in nugget and vein gold from Osa, Tilarán, and Crucitas ore bodies, Berrangé showed that the gold in the Osa has a dramatically higher Au:Ag ratio and is quite different from the native alloy found in Tilarán and Crucitas ore. The latter two he concluded were quite similar, as would be expected from their common metallogenesis from intermediate rocks inland from the leading edge of the overriding Caribbean plate. Osa gold is dramatically more pure and has a different trace metal distribution, evidence of an origin from the hydrothermal veins of not intermediate plutons like Talamanca and Tilaran rocks at all, but mafic basalt-family rocks, consistent with the composition of both the Nicoya Complex oceanic rocks thrust on land—ophiolites to geologists—as well as the relatively young basalt occurrences shown in blue in the Costa Rica geologic map of Tournon and Alvarado7.
Whatever the ultimate primary source of gold may be, Berrangé made the most comprehensive classification of the Osa placer types. It shows a series of placer types likely familiar to most of those with local hand- or commercial mining experience.
While most prospecting is done with bar, pick, shovel, pan, and sluice box, Violines Island is one of the few places in the world—like parts of Australia—where loose gold occurs in nuggets large enough that it may be successfully prospected with metal detectors. A friend of mine, Ray Smith, formerly of Golfito and now guiding back-country big-game hunts in Montana, found a 76-gram nugget in the late nineties using a metal detector in the Guerra region, right across the Sierpe River from Violines. But for those that tire of the search for native gold, rumor has it that Sir Francis Drake left a considerable treasure buried along the shores of the bay along the Osa’s northwest coast that today carries his name. So long as gold remains within reach of ordinary men, its presence—whether natural or hidden by pirates—shall likely always inspire some to defy the mosquitoes and leaches, to suffer the rain and disease, to weather the heat and oppressive wetness, and to gallantly suffer all the miseries attendant upon its pursuit to continue prospecting and hand-mining the gold of paradise.
1 La Nacion, 2013: Península de Osa: el lugar donde mas abunda el oro en Costa Rica.
2. Tico Times, 2015, July 15: Gold mining company that sued Costa Rica files for bankruptcy
- Collar, 1994: The Geology and Distribution of Gold, Osa Peninsula, Costa Rica, a Literature Review; MUDESA private report, reprinted: http://www.soldeosa.com/editorial/07-21-gold-literature-review.htm
4 Shatwell, 2004: Subducted Ridges, Magmas, Differential Uplift, and gold Deposits: Examples from Central and south America; the Ishiyara Symposium: Granites and Associated Metallogenesis; Geoscience Australia.
5 Kriz, 1990: Tectonic evolution and origin of the Golfo Dulce gold placers in southern Costa Rica; Revista Geologica de Centro America; v. 11, p. 27.
6 Berrangé, 1992: Gold from the Golfo Dulce Placer Province, Southern Costa Rica. Rev. Geol. Am Central, 14: 13-37
7 Tournon and Alvarado, 1995: Mapa Geologico de Costa Rica, escala 1:500,000; folleto explicativo, 79 paginas, Editorial Tecnologico de Cartago, Costa Rica.
See the original article here