#somepapers No. 7: An unusual epithermal system in Colombia

The paper

Rodriguez Madrid, A.L., Bissig, T., Hart, C.J.R., and Mantilla Figueroa, L.C., 2017, Late Pliocene high-sulfidation epithermal gold mineralization at the La Bodega and La Mascota deposits, northeastern Cordillera of Colombia. Economic Geology, v. 112, p. 347-374.

What it says

This paper is a pretty good example of a fairly common type of Master's thesis in economic geology. the authors describe the local geology, alteration, and mineralization stages and add some fluid inclusion and stable isotope work, for good measure. This sort of study is very useful to geologists like me who are working on a deposit and need to get up to speed on what is generally going on, but don't have the time to spend a summer figuring it out.

The La Bodega and La Mascota high sulfidation epithermal Au deposits are located in the Maricaibo tectonic block in northern Colombia. They are unusual because they are located more than 500 km from the nearest subduction zone. This type of deposit is generally coeval with shallow intrusions and volcanic rocks in volcanic arcs. There are Miocene (~10 Ma) porphyries in the district that did lead to some porphyry-style mineralization. However, Ar-Ar dates reported in this paper Au-Ag show the main stages of mineralization (there were 6 hydrothermal events here) are much younger, around 2 Ma. The highest grades are found in silicified breccias, which is pretty typical. Very little vuggy quartz has been found in this deposit, which is not so typical for this type of deposit (high sulfidation epithermal).

Why it matters

I don't know enough about epithermal deposits to say whether there's anything groundbreaking in these results, though I suppose high sulfidation deposits away from the volcanic arc is something new to think about.

These types of "typical" deposit work-ups are important to read because they help build geologists up a catalog of deposits to know what's normal. All deposits are different, but there's a lot of overlap. Understanding what parts of a deposit are normal and what are outliers can reduce the risks involved in drilling and mining. My job as a geologist in mining and exploration is to reduce risk; having a solid understanding of the deposit is key in reducing risk.

Why I read it

Epithermal deposits have become much more important to me than then were a couple months ago because I have a new job where we're looking for and mining these kinds of deposits. Although we don't have any mines or projects in this district, it's a good write up. It add to my my epithermal database. Another reason I read this paper is that this issue or Economic Geology was available, not packed up with all my office stuff.

Odds and Ends

One unusual aspect of this deposit is that the last stage of mineralization contained Zinc and Uranium. I was expecting some sphalerite, after all, it's hard to have a magmatic-hydrothermal deposit without some Zn floating around. But pitchblende?! Weird.

This is just the kind of thing that makes me happy to read a lot of papers and build my database. I'm still working on reading "a lot" of papers, but I'll get there.