Placer Gold Deposits
Placer gold refers to gold that has broken loose from the main lode due to weathering and erosion. It can be found close to or far from the original gold lode, depending on various factors. Placer gold deposits can be found in alluvial or placer deposits—areas where gold-bearing sands and gravel settle out from rapidly moving streams and rivers. The process begins with gold's high density, which causes it to sink more rapidly from moving water than lighter siliceous materials. Over time, gold accumulates in near-surface positions due to deep Tertiary weathering and supergene enrichment of Mississippian-Permian metasediments. Pleistocene glacial erosion then incorporates this gold into unconsolidated sediments. The term "placer" is thought to have originated from Catalan and Spanish, meaning a shoal or sandbar.
Main Placer Types:
- Residual Placer: These are gold deposits that have recently broken loose from the lode. As the host rock erodes, heavy gold accumulates in high concentrations at the same spot.
- Eluvial Placer: Once residual placers move a short distance from the original vein, they become eluvial placers (also called hillside placers). Eluvial gold has not yet been smoothened by rivers and gravels, resulting in a rougher, angular shape.
- Alluvial Placer: When gold migrates down hillsides and reaches waterways, it becomes alluvial gold. Alluvial deposits are common in rivers and streams.
Methods of Placer Gold Mining
Unlike “hard rock" gold mining, which extracts veins or minute particles (often not visible) of gold from solid rock, placer mining focuses on separating heavily eroded minerals like gold from sand or gravel.
- Panning: Panning is an ancient technique that miners used during the great gold strikes of the 19th century. Miners would place a few handfuls of gold-bearing soil or gravel along with a large amount of water into a pan. By swirling the contents of the pan, they washed away lighter material, leaving behind the gold and heavier materials.
- Rocker (Cradle): An improvement over panning was the rocker or cradle. Resembling a child's cradle, it sifted large quantities of ore. Gravel was shoveled onto a perforated iron plate, and water was poured over it. Finer material dropped through the perforations onto an apron that distributed it across riffles (wooden or iron pieces). As material moved through the cradle, gold got caught on the riffles for later removal.
- Sluicing: In sluicing or hydraulicking methods, miners used a slightly sloping wooden trough called a box sluice or a ditch cut in hard gravel or rock called a ground sluice as a channel. Gold-bearing gravel was carried by a stream of water along this channel. Riffles placed transversely along the bottom of the sluice caused water to eddy into small basins, slowing down the current so that gold could settle and be trapped.
History of Placer Gold Mining in the Cariboo, British Columbia
The Cariboo placer mining district in central British Columbia, centered around the communities of Wells and Barkerville, is a classic gold rush area. It gained prominence during the late 1850s when gold was discovered there. By 1861, Barkerville had become the largest town north of San Francisco and west of Chicago. Even today, this region accounts for almost 30% of the province's annual total placer gold output.
Placer Gold at Wingdam
Omineca (and its predecessor private company CVG Mining Ltd) has been actively exploring the auriferous Deep Lead Channel gravels at Wingdam since 2009. The gravels are part of a reworked or modified fluvial paleochannel basement that pre-dates the last Pacific Cordillera glacial period called the Fraser Period (95,000 to 10,000 ybp). The channel occupies the deepest portion of the bedrock floor along the Lightning Creek valley that is buried from top to bottom by a sequence of postglacial alluvium, glacial till, and interglacial lacustrine sediments totalling 48.8 m thick.
Gold concentrations along the Deep Lead Channel basement are made up of native placer particles averaging 90.9% pure (909 fineness) and are efficiently recoverable by gravity separation methods that require no crushing, milling or leaching. The gold particles were liberated from lode sources at unknown bedrock locations surrounding the Wingdam area by a long period of deep Tertiary weathering. The liberated gold was transported and concentrated along the bedrock floor beneath the present location of the Lightning Creek during the Pleistocene by a complex history of periodic interglacial streams. Stratigraphic evidence indicates that the auriferous gravel layer along the channel floor is a Sangamon (132,500-95,000 ybp) interglacial paleochannel remnant.
The Deep Lead Channel contains some of the highest placer gold concentrations historically reported in all of the Cariboo Mining District and perhaps throughout British Columbia that remains unmined. Parts of the channel were previously explored by drift methods and sampled along drilled fence lines during the 1910’s, 30’s and 60’s. Compilation of the historic drill results indicates that various areas along the channel contain a gold-enriched zone with grades averaging 33.65 g/m3 across a horizon that varies from 1.8 to 2.1 meters thick. This grade is equivalent to 13.74 g/tonne or 0.401 oz/ton. Historic and recent results from drilling and seismic surveying show that the channel floor width varies from 6 to 39 m wide and extends 2,430 m along the length of the Wingdam Property.
Six attempts were made to mine the Deep Lead Channel during the 1930’s and 60’s by utilizing the Australian deep-lead mining method. The method involved raises or breakthrough locations that were accessed from a parallel bedrock drift driven along the length of the auriferous gravel layer occupying the channel basement. The mining method proved to be unsuccessful through the unstable ground that directly overly the gold-enriched gravel layer. The unstable ground consists of water-saturated glaciolacustrine silt and sand layers (referred as slum) that easily cave and flow when undermined by a timber-supported raise.
Wingdam Property NI 43-101 Report;
Deep Lead Channel
Ground-Freeze Drift Sampling Program
Stephen Kocsis, P.Geo
June 15, 2012
