The paper
Fall, A. And Bodnar, R.J., How Precisely can the temperature of a fluid event be constrained using fluid inclusions? 2018, Economic Geology, v. 113, p. 1817-1844.What it says
As minerals crystallize from hydrothermal fluids, they trap tiny volumes of that fluid. As the system cools, so do these tiny trapped fluids, fluid inclusions. High T fluids separate into a liquid and gas phase at low T. Some precipitate solid phases. When these samples are heated back up on a special microscope stage, the temperature where the bubble disappears, is recorded (homogenization temperature) and is usually assumed to reflect the temperature of the fluids when trapped. This paper asks how precise these measurements are.They do this by considering fluid inclusion assemblages (FIAs). Each FIA represents an event - a stage of mineral precipitation, or maybe the healing of a fracture in a mineral. These FIAs each have many fluid inclusions with homogenization temperatures that usually span a range of T. The authors made some measurements and compiled many more from published studies to come up with a sort of precision you can expect from a fluid inclusion study from a number of common ore deposits.
Low-permeability sedimentary environments can expect FIAs to be in a range of about 2 degrees C (the median of the distribution of data from the compiled studies);
Mississippi Valley Type deposits have a median range of about 4 degrees C;
Epithermal deposits have a median range of about 9 degrees;
Porphyry deposits have a median range of about 15 degrees;
and progenitor Au deposits have a median range of about 9 degrees.
Trapping multiple fluid phases in one FIA event, variations in PT conditions of the fluids when they are trapped, and variations associated with sample preparation/measurement can all contribute to the ranges that are seen in the data. Overall, the median ranges are narrow enough that you can treat data from a good fluid inclusion study to represent the actual formation temperature of a hydrothermal system.
Why it matters
A lot of the details of what ore deposits look like, how they form, are temperature dependent. If you're looking for a huge copper deposit, but none of the temperatures in your quartz veins are above a couple hundred degrees, you're probably looking in the wrong place (or are way out on the edges of the deposit). Temperature can tell you a lot about the conditions at the time a deposit was formed. Knowing the PT conditions and a little high T chemistry can help a good exploration geologist know whether they might have the makings of an ore deposit.Why I read it
This was the article that jumped at me in the latest issue of Economic Geology. I enjoy learning about how far we can trust our data, and this paper deals with a fundamental type of data in the economic geology world. It's always good to know what kinds of assumptions and uncertainty you're dealing with.Odds and Ends
I've been having a bit of a hard time focusing lately. It is an affliction that has affected my whole family. For some reason the past couple of months have been kind of tough. I finished a big project in late January, which had seen me work pretty much every day from Jan 3 through Jan 25th or something. It was too much work. I think some of this stems back to a bit of burnout from that big push; it was too much. Might the weather have something to do with it? I don't know. We are in southern Arizona right now, so the winter hasn't been particularly harsh, by objective measures. It did snow that one day. It could be some changes that are coming our way in the next year or so. Who knows.During these past couple months I've thought quite a bit about how this rough patch is relative, and how I'm extraordinarily lucky to have this variety of trial. Things are good, overall. It's just inside my head that things seem to be falling apart. I think I see the light at the end of the tunnel, though. I'm not sure what it is, but life seems a little more hopeful lately.
