New Models in Old Trends
How are Discoveries be Made Where Others have Already Looked? As geologic models advance, their application becomes more broad. Let’s take a look at how new models for mineralization are helping make discoveries in old trends.
One of my favorite exploration models, and the focus of today’s blog is the lithocap. Lithocaps are a perfect example of a new exploration model that is pioneering new discoveries in the Western united states and along the major cordilleran regions around the pacific.
In reviewing some history, we can find that in 1985, Newmont made one of the most significant gold discoveries in the world at Yanacocha in Peru. The Yanacocha deposit is a high sulphidation epithermal deposit and was capped with tens of square miles of lithocap. Although the area was known to be mineralized since the 1970s, a deposit had not been found and the first exploration drill holes didn’t yield promising results. Only the lithocap had been drilled on Yanacocha, and in 1985, drilling below the cap started to delineate one of the largest gold resources on the planet. This drilling is what sparked a new understanding of intermediate and high-sulphidation deposits and how to target them.
Newmont’s Yanacocha has produced tens of millions of ounces and is both the pinnacle and pioneer to lithocap exploration. (Image: Wikimedia Commons)So what is a lithocap? Lithocap refers to a steam heated (in high sulphidation and intermediate sulphidation deposits) siliceous blanked that sat above a heat source like a porphyry. These deposits are oftentimes the product of an associated porphyry, Yanacocha being no exception. However, these porphyry’s are generally much deeper and inaccessible to economic mining. High and Intermediate sulphidation lithocaps have the distinction of being “steam-heated” referring to the area directly above a heat engine where the hot often highly acidic fluids are most prevalent. In essence, the lithocap is the ancient remnant of what was once the acid lake atop an active volcano. An excellent video describing epithermals from Sprott Global can be found here.
The acidic fluids moving through the host rock are what forms argillic or “advanced argillic” alteration. The destruction of the host rock into clays is the most distinguishing factor in argillic alteration. This argillic alteration occurs as a halo around high and intermediate-sulphidation deposits. This acidic environment also generates the high-sulphidation signature. Sometimes these areas can smell of rotten eggs with such high levels of sulphur present.
Intermediate-sulphidation deposits are also formed under the lithocap, however, they are usually farther away from the main volcanic processes that generate the high-sulphidation deposits. Intermediate sulphidation deposits are generally less acidic than their high sulphidation counterparts as fluids carrying mineralizing elements have lower acidity due to mixing with connate waters. Included in this class of deposits are the Comstock Lode and several other notable and significant scale deposits.
Although low sulphidation deposits can have caps as well, these are most generally quartz (like chalcedony) or adularia-sericite emplacements and were deposited in higher-ph environments where deposition is from clear water carrying the mineralizing elements. Low sulphidation deposits occur around the fringe of where the intrusive activity occurred. L-S deposits do not generally have a steam-heated lithocap, are known for their lack of sulphide ore, and the presence of physical gold crystals. Low sulphidation deposits are responsible for most of the famous placer deposits like Eldorado Creek in the Yukon or Gaines Creek in Alaska. Again, these deposits are formed through the same processes that bring about hot springs. A notable and exceptionally large example of a low-sulphidation deposit is Kinross’s Round Mountian mine.
As a cautionary note, these deposits often tend to grade into one another as a system develops over time.
This diagram shows the locations of high and low sulphidation deposits, intermediate sulphidation deposits occur in the permeable horizon between high and low sulphidation deposits. (Image: Greg Corbett)Now that we have covered some basics about epithermals, let’s jump back into why lithocaps are a great target.
Lithocaps above high and intermediate-sulphidation deposits are essentially barren. These caps have been leached, bleached, and are of no economic value. This property of lithocaps is what has hidden them from major discovery for the last millenia. Most commonly, artisanal and smaller-scale mining occured around the base of lithocaps where mineralization was accessible. However, without a model for exploration, our early counterparts were left with unappealing deposits compared to the flashy low-sulphidation workings of the same age. After Yanacocha, we now know that drilling will have to pass through this barren cap to reach mineralized zones, but in the early days of mining, these caps were essentially worthless. The worthless nature of these caps is what saved them for modern exploration and discovery.
Every lithocap hosts a deposit. The question is: how big is the deposit and is it economic to mine? This question can be answered in three parts: How big is the cap? As with all deposits, the longer the hot fluids were active and carrying mineralization upwards toward the surface, the larger the deposit will be. It is a matter of time and volume. One of the easiest ways to determine the potential viability of a lithocap is to evaluate its size both in aerial and vertical extent. Yanacocha, for example, has a cap that covers tens of square miles. It is the largest lithocap currently known and serves to show the length of time the deposit must have been forming. The larger the lithocap, the more fluids it took to form it. The more mineralizing fluids there were, the more metal deposited.
The second question is: What grades are in the cap? Although these caps are essentially barren, fire assays will reveal residual metals in them that can indicate the potential value of the deposit underneath. For a deposit like Yanacocha, the cap values were around a tenth of a gram Au per ton. These grades were substantially higher below the cap at the water table and paleo-water table. Generally, the higher the grades in the cap, the higher the grades of the deposit.
The third question is: How vuggy is the cap? Vuggy means the pore space left behind by the leaching of feldspars. In world-class epithermal deposits, caps can have zones that look like sponges where feldspars have left large holes in the caprock and produce very vuggy zones. The cap rock will not erode away as fast as other rock types as silica is very resistant to mechanical weathering, but the feldspars and subsequent clay minerals will.
Here is an example of vuggy silica. Image by Andrew Jackson Again, the first major discovery involving a lithocap was Yanacocha in 1985 and was validated for the second time at Barrick’s Pierina in Peru in 1996. This model is only 35 years old. In reality, it wasn’t applied to other cordilleran (mountainous) regions until will into the late ’90s and early 2000’s making this model one of the freshest in the industry. To put this into perspective, the model for Carlin Type deposits have been around since 1965, or almost twice as long.
A great example of a successful lithocap model is Fortitude Gold’s Isabella Pearl mine and East Camp Douglas project, which has recently seen successful drilling, along with Corvus’s North Bullfrog, Orogen’s Peral String and Convergent Mining’s HighTop. All are new discoveries in old districts using the application of this newly-validated model.
