Porphyry deposits are very attractive exploration and mining targets due to their high tonnage and relatively easy open pit mining. They are commonly associated with porphyritic intrusive rocks, i.e., granite porphyry.
What exactly is porphyry?
The term “porphyry” refers to any igneous rock that has coarse-grained crystals in a relatively fine-grained matrix. Copper (Cu), molybdenum (Mo), silver (Ag), and gold (Au) are the common metals of economic value found in these deposits.
As I mentioned, an important feature of porphyry ore deposits is their large tonnage. For example, an individual porphyry copper deposit can contain several billion tons of Cu at 0.4-1%, making these deposits the largest source of copper ore. Ideal for open pit mining, they are commonly found close to the surface and can be mined on a large scale at a low cost. Although these deposits have low grades, the superior detection limits of certain portable x-ray fluorescence (XRF) analyzers allow easy measurement of the base metal concentrations.
Portable XRF analyzers can be used at any stage of exploration and mining of various metals in porphyry deposits, such as Cu, Au, Ag, Mo, lead (Pb), zinc (Zn), antimony (Sb), bismuth (Bi), etc. Detection limits for base metals in these instruments is low enough to allow even non-geologists to analyze any geological sample from outcrops to drill cores to soil specimen. In addition to base metals, other elements, such as potassium (K), calcium (Ca), and light elements [magnesium (Mg), aluminum (Al), silicon (Si), phosphorus (P), and sulfur (S)] can be assayed as well, which helps geologists in mapping the hydrothermal alteration of the exploration/mining area or 3D modeling of the alteration and mineralization. For example, high concentrations of potassium (K) indicate the presence of K-feldspar, which is the main mineral of potassic alteration. The potassic alteration zone commonly hosts a major part of the ore deposit.
To investigate the application of portable XRF in the porphyry Cu exploration, core samples were analyzed and a case study was carried out in the Miner Mountain porphyry deposit in British Columbia, Canada. This copper deposit is an “alkalic” type (Cu-Au ± Ag, PGE) porphyry deposit and is located in a belt of late Triassic to early Jurassic trachybasaltic/trachy-andesitic volcanic rocks and is associated with sub-volcanic microdioriticdioritici intrusions.
Because the rocks are undersaturated with silica and over-saturated with calcium, typically they do not have the quartz vein abundance, or acidic alteration (particularly the large pyrite halo and extensive sericite-argillic alteration) of the calc-alkaline systems. Instead, they are marked by progression from chlorite – epidote ± magnetite (propylitic) to an albiteepidote-chlorite dominated zone, and then to a K-feldspar, secondary magnetite ± diopside/biotite potassic core. Mineralization progresses from pyrite to chalcopyrite/pyrite to bornite-chalcopyrite with bornite/covelite/chalcocite locally present in the cores of the systems.
To investigate the application of portable XRF in the porphyry Cu exploration, core samples were analyzed by a handheld XRF analyzer. These samples were also analyzed by routine ICP analyses in a commercial lab. The study showed successful use of portable XRF analyzers in strip logging. Such real time analyses combined with having access to real time strip logs are very important for effective drilling and 3D modeling of the ore deposits.
Read the porphyry application note, including methodology, common applications, and results.