How to find hard rock gold

Gold in Quartz
Photo by Rob Lavinsky

Unlike placer gold hard rock or lode gold is much more expensive to locate, but under the right conditions isn’t impossible.  Most lode deposits are found by following placer gold deposits in a stream until the deposit suddenly chokes off then following float up hill until you finally find the Mother Lode.  Most lode deposits are always shedding float that goes downhill in an ever widening fan of the host rock.

Although gold can be found in virtually any rock type its usual home is in white, milky quartz that is readily distinguished from the host rock as termed in the mining industry “gangue.”  The white rock stands out so often pieces of it can be observed washing down mountainsides from its source with the naked eye or by using binoculars.

Although most hard rock gold is found in quartz veins that are usually readily visible, and the thinner the vein often the more gold it contains.  Another prime target for hard rock gold is the rock conglomerate where the biggest of all gold deposits are found in the Witwatersrand in South Africa.  These deposits have produced around 40% of all the gold that has ever been mined.  Deposits of this type also contain uranium and rare earth elements in payable amounts making it entirely possible to find deposits of this nature with a Geiger counter.  The gold/uranium deposits at Elliot Lake, Ontario are of this nature.

Quartz pebble conglomerate a likely place to find gold
Photo by Wiki-Schack

In many parts of the world it is extremely hard to see float because the area is covered with deep overburden so it is necessary to use other methods of finding hard rock gold; one of the most popular is using aeromagnetic maps or other geophysical methods like ground resistivity.  Aeromagnetic maps work on the principle that gold often occurs associated with the mineral magnetite or other iron bearing minerals like ilmenite causing the prospector to carefully examine the area around magnetic highs on an aeromagnetic map.

An analog multimeter that can be used for several different parameters in geophysical prospecting
Photo by KENPAI

Ground resistivity is one of many different geophysical techniques use to find ore deposits based upon the amount of underground corrosion that takes place in an orebody.  This is done using a multimeter that has high impedance and a strand of wire.  For convenience most of these rigs have 1,000 feet of wire mounted on a reel.  In use an area 1,000’ x 1,000’ is measured out on the land.  Parallel lines are drawn on its surface with readings taken from 100 feet apart to 10 feet apart along the lines.  The highest readings are connected with a line, and each successive lower reading until a map that resembles a topographic map is generated.

Any number of different parameters can be measured using this method, but the best explanation of their use is explained in a book about practical geophysics.  Parasinis authored a book on the subject that is still available today in the 1960s.

Gold Occurrences in Brazil

Coat of Arms of Brazil

Within its borders Brazil is the fifth largest country in the world; it touches upon all the countries in South America except Ecuador.  The country also has an astonishing variety of mineral deposits including iron, gold, gemstones, diamonds and oil.  There are literally hundreds of active mining companies found in Brazil.  Brazil also enjoys a rising economy and has a stable government with the future for mining being extremely bright.

The Amazonian shield covers much of the eastern portion of the South American continent and provides many of its gold mines.  Prospecting in the area is similar to prospecting in Canada’s Abitibi Gold Province in Ontario with a repeat of its greenstone belts with alternating gneiss’s.  There are also many deposits of turbidite hosted paleo-gold deposits similar to those found in the Witwatersrand in South Africa.  To the north of the Amazonian shield is the Guyana shield, to the south are the Rio Apa and Platian cratons and in the west is the Sao Francisco craton.  All of these areas are gold bearing.

Brazil covers 8.5 million square kilometers with 7,500 kilometers of coastline making it the largest country in South America, a size that dwarfs all its neighbors.  Its size is on continental size contains a vast resource base making it a prime target for attracting foreign investors and miners as well as Brazilian mining companies.  The country also has a long history of gold mining dating back into the 16^th century.

It is estimated that less then 30% of the country has been adequately mapped so there is plenty of room for prospecting in both placer and lode gold deposits. Placer deposits can be found throughout Brazil except in the depths of the Amazon Basin, and there are plenty of gold deposits found in the Precambrian Shield areas that dot the country.

One of the biggest problems facing the Brazilian mining industry are the number of artisanal mining operations that keep popping up; there was a gold rush in the 1990s that centered on the Guyana shield that reportedly attracted over 90,000 miners to the area.  These miners for the most part disregarded good environmental practices resulting in large areas that were contaminated with mercury and other contaminants.

It has been found that artisanal mining is today the largest source of mercury release into the environment not only in Brazil but other countries too.  One of the sites mentioned that is the largest is in the Tapajos River Basin in the Amazon region of Brazil.  The Global Mercury Project (GMP) under the auspices of the United Nations is aiming to reduce this mercury contamination.

Brazil as a whole is mining friendly and enjoys a now stable government and a rapidly expanding economy that welcomes miners from around the world.  There are still vast amounts of the country that have not been explored, so one never knows what might be found just around the next bend of the river.

Recovering gold from old electronics scrap

Electronics work bench
Photo by James Bastow

Somebody made the observation that for every eight 250 ton haul trucks coming up out of a gold mine seven of then are hauling tailings and the eighth one is hauling gold ore that averages about two grams per tonne.  How would you like to find some gold ore that can run several times this much gold per tonne?  In many cases this is true, and the gold ore exists right on the surface, and in some cases can average more then 800 times more gold then conventional gold ore.

Where is this treasure trove of gold and other precious metals, well you can find it in your own backyard because it is electronic trash like old cell phones, computers both desktop and laptop and all sorts of other electronic goodies.  Practically all other electronics trash also contains a whole plethora of other metals ripe for the picking including gold, silver, palladium, REEs, copper, aluminum and steel.

What does it take to get into this lucrative business?

It can be done with off-the-shelf components used in the electroplating industry; in fact all you’ll need is available from a plating supply store.  A basic knowledge of the electroplating process business also comes in handy.  What this process is in reality a form of electroplating called electropolishing or deplating.  The process is like electroplating except you reverse the electrical charge causing the metal to be plated onto the cathode, and the metal itself that is being recovered acts as the anode.

Basically the process is just a plating process in reverse, so any equipment you’ll need can be bought off-the-shelf from a plating supply house.  You’ll need a source of low voltage DC electricity and some chemical resistant tanks.  It is important that you equip yourself with some heavy duty rubber gloves and a rubber coated apron because you will be working with some strong acids in the process.

The other thing needed is a separate building for this process because it sure isn’t anything you’d like to do in your living room!

Metasomatic Gold Deposits

Gold Nuggets

Geologists consider Metasomatism as the chemical alteration of rock by hydrothermal or other fluids that can come from either an igneous or metamorphic source.  

If it was caused by the action of magma intrusions or volcanic activity Metasomatism makes skarns, greisens and may also affect hornefels if there is a contact metamorphic aureole next to the intrusive magma.  In a metamorphic environment it takes the mass transfer of liquid from a mass of hot rock that has water driven from it by the action of metamorphism with the water or other fluids acting as a solvent.

This occurs whenever rocks buried in the deep crust are losing fluids (dewatered) as well as dissolved mineral components because of the breakup of hydrous minerals.  The fluid with its dissolved minerals percolate to the near surface and in some circumstances reach the surface as springs of juvenile water.  The action of this hot water can chemically alter and change the crustal rocks.

It is this mechanism that tells us that metasomatism is an open system that is different from the usual metamorphic process causing an in situ mineralogical change in a rock, but does not alter the chemistry of the rock.  In effect metamorphism and metasomatism go to gather like hand and glove.

In practice metasomatism is in reality a mass transfer process that is best shown by gold ore deposits that are the result of the focused concentration of gold bearing metasomatic fluids.  These fluids drop their load of dissolved gold in shear zones and lodes.  Often the first indicators of gold are altered rock.  It is this mechanism that removes gold from greenstone and deposits it in adjoining rock.

This fluid is extracted from many cubic kilometers of rock where the overall content of gold can be quite sparse yet it is concentrated in the host rock.  One common example of this is the so-called saddle reef where gold is concentrated at the nose of a vertical fold where sometimes a large quantity of gold occurs in a small deposit.  Another type of deposit is illustrated by the Carlin Trend in Nevada where the action of metasomatism has caused the depositing of gold throughout a massive formation of limestone.

In order to be affected by metasomatism it is necessary for the rocks to be permeable to water.  Some of the other minerals formed by this mechanism beside gold are: copper, iron, molybdenum lead, zinc and tin.  Some of the industrial minerals formed are: graphite, mica, talc and wollastonite.

Because the fluids are moving through permeable rock one of the best places to look for gold is where one rock type contacts another.  That area acts as a dam for holding the mineral laden water in place where it deposits its mineral load. 

Jade and Gold

A piece of uncut jade
photo by Immanuel Giel

According to John Sinkankas (1959) there is one small deposit of jade found in an undisclosed location in California that exhibits small flecks of gold making it a rarity.  Too say the least your author has seen one polished example of this jade, and it is quite handsome being green with specks of gold throughout. 

California contains both jadeite and nephrite two distinct minerals with jadeite being a pyroxene mineral while nephrite is an amphibole intermediate between actinolite and tremolite.  They are both found in a high pressure/low temperature environment like that found in the coastal ranges of California. 

This geological environment is considered by many geologists to be blue schist facies.  One area near Monterrey called Jade Cove is a place where many large boulders of jade have been found in the Pacific Ocean.  These boulders are usually found covered with a tannish crust caused by weathering.  The true nature of these boulders is not seen until this crust is broken through.

A nephrite desk set by Faberage
Photo by Shakko

Jadeite is a member of the pyroxene group of minerals with a composition of sodium aluminum silicate (NaAlSi₂O₆).  This variety of jade is found in California and other places in the world.  The most precious form of jade called Imperial Jade by the Chinese comes from Burma.  Jadeite deposits are relatively rare.

Nephrite jade sometimes called Wyoming Jade the state gemstone was first found in the Granite Mountains in central Wyoming with the most intensive exploration taking place from 1940 through 1960, but recent activity in the jade producing areas indicate a new interest in this gem.  Most of the high-grade nephrite jade has been found in alluvial deposits in and around the Granite Mountains in central Wyoming.

A Mayan facemask made from jadeite

Other large scale deposits of this mineral are also found in Alaska and British Colombia the finest jade in the world however is that found in Wyoming.

Nephrite is calcium magnesium silicate [Ca2(Fe, Mg) 5Si₈O₂₂(OH)₂. some of the nephrite found in Wyoming is colored emerald green by chromium atoms entrapped in its crystalline structure,  The normal green is caused by iron atoms and black nephrite is caused by an excess of iron.  When iron is absent the color is colorless, but appears cloudy white because of its fibrous structure.

Nephrite containing specks of gold is rare so far only known from one locality in California.  Further investigation will eventually locate further localities.

Gold Occurrences in the Guyanas

The Guyana Shield   CIA

The Guyana’s share the Guyana Shield with its larger neighbors Brazil, Colombia and Venezuela; this has been known to be a gold bearing region since its earliest days.  Throughout this area there have been many small scale miners that haven’t been known for their environmentally friendlyattitude.  At a high level conference between the governments of Brazil and the countries composing the Guyanas has brought forth a working agreement to rein in the small scale gold producers in the Amazon Basin.

During the 1990s a gold rush developed that engulfed the Guyanas that included Surinam, French Guinea and British Guinea as well as parts of Brazil, Colombia and Venezuela.  It has been estimated that over 100,000 people are earning their livelihood from the gold fields in these countries many of which are artisanal mining ventures that are damaging the rain forest.  These miners range from the national and Brazilian gold miners, operators and concession holders as well as heavy equipment owners. They're a rudimentary minds with small to medium scale with gold mining activities and often generate a legacy of extensive degradation as well as terrible social conditions both while gold was being mined and after the mining ceases.

The Guyana Shield is one of three cratons that are found on the South American plate. This craton is composed of 1.7 billion year old geological formation is found in northeast South America and forms part of the northern coast. The higher portions of this Shield are called the Guyana Highlands that is where the mysterious table-like mountains called

Tepuis are located.  This Shield is also the home of several very high waterfalls including the Angel Falls that is the highest in the world.

The rocks of the Guyana Shield correlate with another Shield found the northwest Africa with many of the same events traceable in both continents. Here are found rocks metasediments, metavolcanics (greenstones) and quasi-horizontal sediments causing horizontal layers of sandstone, Quartzsite, shale, and conglomerates. These rocks are heavily intruded by sills of younger mafic intrusives such as gabbro that were in place during the opening of the Atlantic Ocean during Jurassic times. This is very similar to the gold bearing rocks found in Portugal and Spain as well as North Africa.

The Precambrian rocks forming the shield are composed of three basic rock types including folded metasedimentary and metavolcanics rocks.  In some places there are coarse to fine grained sedimentary rocks that have been intruded volcanic rocks.  You can also find intrusive rocks of Mesozoic age in the folded rocks.

Much of the Guyana Shield consists of rocks like those found in the Canadian Shield with alternating belts of greenstone and granite with interleaving deposits of conglomerate.  A great deal of placer gold has been deposited in the rivers and streams of the region.  Most lode gold is found in fault zones of in the conglomerates like those found in the Abitibi Greenstone Belt of Canada.

Global Metallogeny and Gold Deposits

Map of the Kolyma Basin in the far east of Russia.
by Kmusser

The discovery of gold in the Kolyma Basin of the far Eastern Russia by the Soviet geologist Yuri Bilibin during the 1920s gave rise to a new science called Metallogeny and Global Tectonics. This gold producing area was later the subject of a book about the Soviet system of forced labor by Aleksandr Solzhenitsyn titled the Gulag Archipelago. 

In order to work three criteria have to be met, but in essence they say that mineral deposits can be found worldwide when these conditions are met.

1)      Mineral deposits are formed whenever energy is released at plate boundaries whether they are converging, diverging or transform plates.

2)      Plate tectonics are the controlling factor for the formation of mineral deposits.

3)      By reconstructing fragments of plates provide a useful tool in exploration for new mineral deposits.

In any tectonic setting for the production and accumulation of mineral deposits several requirements have to be fulfilled there has to be either a spreading center, a mountain building episode at a plate convergence or collision boundary, in craton rift centers, or cratonic basins.

The mid-Atlantic Ridge outlined by dotted lines.  This is a typical spreading center.

Deposits at Spreading Centers

In active spreading centers such as the mid-Atlantic Ridge or in the Red Sea where Africa is separating from Arabia are metallaiferous sediments on the flanks of the ridges that in many places are also marked by black smokers that contribute to the metal deposits.  Although these sediments primarily contain as sulfides iron, zinc, copper, lead, silver and gold; some deposits such as in the Red Sea containing iron, copper and zinc.

At some ridges important deposits of manganese oxide deposits are lain down.  This is especially important along the mid-Atlantic Ridge.

Another set of minerals found in ultramafic rocks that are called ophiolites are asbestos, chromite and nickel.  These minerals are found in Phanarozoic mountain belts where they are transported by tectonic movement.  The chromite is often found in Podiform deposits in ultramafic rocks most notably serpentines.  A different type of deposit of the same origin is massive sulfide deposits of iron and copper sulfides also associated with these ophiolite deposits.

A typical convergent plate margin aka a subduction zone.  

Deposits at Convergent Plate Margins

There are actually two types of convergent plate margins, one of them is where two continents are converging and the other is where two island arcs are converging metal deposits are commonly found at either type of plate margin.  The largest of these margins is the Circum Pacific Belt that contains major metallic deposits including over half the world’s production of copper.  All the metals can be found at convergent margins.

The zoning of mineral deposits in one of these zones is quite apparent with different zones being encountered the further you are away from the margin.  These varying deposits are liberated from the plate the further it descends beneath the mantle wedge with tin coming out of the slab at a depth of about 300 kilometers.  These metals come up with magmatic fluids and are concentrated in hydrothermal and magmatic fluids.  Epithermal deposits are commonly found in this regime.  Oil and gas are found associated with this type of convergent margin.  In some places hydrothermal fields are also found.

Collision Boundary Deposits

  

Collision boundary deposits are a wild mélange of differing types of mineral from a wildly differing variety of environments ranging from deposits associated with spreading centers to deposits found at plate margins.  Typical of these deposits are those found on the ocean floor that are spotted across the width of the plate that are subducted beneath the continental plate with the metal deposits being brought up into the margin by tectonic movements.  These ocean floor deposits are the primary deposits from which others are derived.

Death Vallet, California the yellow dots show the location of mines. This is a typical example of a cratonic rift system.
USGS

Cratonic Rift Systems

 Hot spots in the mantle of the earth cause a blister to form at the surface of the earth that causes three cracks to form on the surface two of them form an ocean and the third is called a failed arm of the sea or an aulacogen. A good example of this type of feature is the Ottawa Aulocogene in Canada.  Usually any granites emplaced during the early stages of this kind of development are rich in tin and fluorite.  Later in their development aulacogenes collect large accumulations of evaporates and other metallaiferous deposits.  In their late stages they are apt to develop deposits of fluorite, barite and carbonatites.  These can be characterized by deposits of niobium, phosphorus, rare earth elements (REE), uranium, thorium and in places tin bearing granites.  Geothermal fields also occur along rifts because of the upwelling of magma along the rifts. 

The Dead Sea from space.   NASA

Cratonic Basins

Cratonic basins are where the inflow of water from the sea causes an accumulation of organic debris from which petroleum products are derived.  The basin is also a place where evaporate deposits are laid down especially salt hence the association of salt and petroleum products.  The heat required for the transformation of organic matter is supplied by the burial of the debris under layers of sediment.  An example of a cratonic basin is Death Valley in eastern California.  One of the products of the ongoing evaporation is borax.  With continued rifting the basin usually becomes filled with water so that the circulation in the system goes from restricted to unrestricted with the depositation of organic matter ceases so does the depositation of evaporites.

Petroleum products aren’t the only thing deposited in cratonic basins like all the landforms described above gold and other metals are also deposited.  By understanding these features you have a pretty good idea of how metals are deposited so you can design an effective exploration plan.