The Santo Tomas porphyry deposit that Oroco Resource Corp. (TSXV:OCO) is working to secure ownership over has an interesting history. It was "actively" explored from 1968-2003 with 106 diamond and RC drill holes comprising about 30,000 meters. That may sound like a lot, but it's really not. It's fair to say that Santo Tomas was incompletely drilled by past operators, who didn't chase mineralization along strike or to depth.
Just look at this cross section from 1994 pre-feasibility study done by Bateman Engineering that shows how the holes stopped at a set length, even when they were returning well-mineralized material with good grades.
Hindsight may be 20-20, but it's important to study the project's history in close detail to get handle on the scale of the opportunity as it stands and what kind of important questions may have remained unanswered for all these years. So far, I'm happy with what I am seeing.
The early days at Santo Tomas were led by ASARCO, a storied company originally named as the American Smelting and Refining Company. It is now a wholly-owned subsidiary of Grupo Mexico. ASARCO may have left a bunch of mining sites with nasty pollution, but they also left some great exploration stories that have been largely overlooked for one reason or another.
ASARCO drilled 43 core and 16 percussion holes at Santo Tomas, mostly in the North Zone. They drilled a few holes 2 kilometers away in the South Zone and found good copper mineralization, but the holes there were more widely spaced. All in all, ASARCO made a great initial push at Santo Tomas. Their work was foundational for everything that came after and their drilling program set a high-water mark that no-one else has surpassed yet.
Another legendary mining company made their mark on the project when Penoles optioned Santo Tomas in 1973 and drilled 6 holes. All the holes were drilled in the North Zone and were meant to to tighten the spacing to increase confidence as they calculated a new mineral resource. Almost 20 years went by before any more work was done.
In 1990 and 1991, some work was done by another group to "re-interpret the geology" and "confirm the reliability of the prior exploration programs". I am a big fan of reinterpreting the geological model for a particular deposit that isn't quite working, but I don't think that was necessary at Santo Tomas. I think it just needed more drilling -- step out and drill deeper!
Exall Resources started work in 1993 and drilled 33 RC holes for 4,000 meters, which was a step in the right direction.
For example, two holes in the oxide domain of the North Zone returned 1.14% copper over 160 meters near surface. These were great results that surprise me as copper is usually leached-out of the oxide domain of a porphyry deposit. It's just a footnote in a report now, but it could take on new life as Oroco Resource Corp. steps in to add a new chapter to the story of Santo Tomas.
As I discuss in this article, Exall eventually completed a resource estimate and PFS that used all the data on the project. Great news, but it focused mainly on the North Zone. What could be hiding in plain sight at the South Zone?
There is good potential to double the tonnage in M&I categories at the South Zone from Inferred with 10-15 holes, as in a 2011 report by John Thornton. However, there's even more potential for whoever is willing to step-out along strike as there is an untested area to the southwest of the South Zone. Between all this infill and step-out drilling, there is a lot of low-hanging fruit at the South Zone.
There is a lot of low-hanging fruit at the North Zone, too. There is still lots of room for infill drilling to convert indicated to measured and increasing the indicated resource itself by stepping out to the west or drilling deeper in the center of the high-grade ore zone. The 2011 report mentions this high-grade zone, but it can be a bit hard to see it in the reports.
We can see a bit of evidence of the high-grade in the past estimates of tonnage that show relatively large amounts above 0.4% copper. Note how almost half the total tonnage is above 0.4% copper equivalent, particularly at the North Zone.
Santo Tomas offers a full suite of options to increase confidence, extend the known envelope of mineralization, and better define the high-grade core. It all puts Oroco Resource Corp. in a position to come storming out of the gate as they bring the property back to the public markets. I will be watching for more from Oroco soon!
I was not compensated to write this article. This is not investment advice.