Jason Babcock

Thesis: A Comparative Study of Tourmaline Composition as a Potential Exploration Guide to Porphyry Copper Deposits; South American Cordillera, Chile and Peru

Abstract

Tourmaline is a complex borosilicate mineral, which is commonly found in association with the porphyry copper deposits of the South American Cordillera. The chemical composition of tourmaline is a direct result of the chemical environment in which it was formed. Detailed examination of the chemical composition of tourmaline can lead to discriminating between occurrences associated with porphyry systems with different copper producing potential.

Tourmaline occurs in all the porphyry belts of the South American Cordillera. The characteristics of tourmaline may be quite varied in hand specimens, ranging from microcrystalline breccia fillings, to radial aggregates of crystals replacing host rock. Tourmaline samples were classified according to their crystal habit, and morphological characteristics as well as whether they occurred in breccias, fractures, or as patches in the host rock. Such observable occurrence characteristics do not appear to be greatly influenced by the chemical composition. Tourmaline occurrences have also been classified by their apparent copper productivity. "P-type" occurrences are the giant producers, "s-types" are the small mines, and "n-types" are the occurrences with no copper productivity.

Electron microprobe analyses of tourmaline associated with porphyry systems can be classified as being a solid solution between schorl and dravite mineral end-members. Tourmalines of different productivity types have nearly the same compositional characteristics, with only subtle differences in molar abundance, hinting at a characteristic composition of productive deposits.

Discriminant function analysis of tourmaline has proven to be effective in discriminating between tourmaline from copper barren porphyry systems, small producers, and giant mines such as Rio Blanco/Los Bronces. A discriminant function with coefficients derived from relative molar abundance of K, Si(Z-site), Mn, and Ti in samples from locations in three productivity categories, plotted against similar discriminant function, only using Na, Al(Y-site), Mg(Y-site) and OH, defines three regions. When new sample data of known productivity is entered into the two discriminant functions and plotted, the sample falls within the correct region 80% of the time.

Jason gives poster at Society of Economic Geologists' Global Exploration 2002: Integrated Methods for Discovery,

Babcock, J.D. and Petersen E.U., 2002, Tourmaline Compositions in Breccias from Copper and Gold Systems; Milford, Utah, Arequipa, Peru, and La Joya, Bolivia: Global Exploration 2002: Integrated Methods for Discovery, Society of Economic Geologists Abstracts, Denver, [ABS] p. 55.

Babcock, J.D., 2005, A Comparative Study of Tourmaline Composition as a Potential Exploration Guide to Porphyry Copper Deposits; South American Cordillera, Chile and Peru. Unpubl. M.S. Thesis, University of Utah X. p.