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Earth's copper resources estimated from tectonic diffusion of porphyry copper deposits

¹ Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA
² Department of Earth Sciences, Syracuse University, Syracuse, New York 13244-1070, USA

View larger version (45K): [in this window] [in a new window]   Figure 1. Time-space trajectories of 231 theoretical Phanerozoic porphyry copper deposits (light gray lines) emplaced at crustal depth of 2 km (arrow) relative to Earth's surface (straight heavy line), and tectonically moved (with the same probability) up, horizontally, or down by 468 m in age-depth space with each age step of 106 yr. Four paths (with ages and maximum burial depths) illustrate the range of burial histories. Numbered lines represent possible paths taken by: (1) Miami-Inspiration in Arizona (Livingston et al., 1968), (2) Gibraltar in British Columbia, (Bysouth et al., 1995), and (3) Chapada in Brazil (Richardson et al., 1986). Others remain in subsurface (e.g., 4) at various depths, including deposits that might enter mantle through crustal delamination. Inset to lower right shows age frequency distribution of 5000 such deposits (binned over 5 m.y.) that are now exposed at the theoretical Earth surface (straight heavy line). Note that numbers of deposits increase rapidly to modal age of 10 m.y., and then decrease gradually with increasing age.

View larger version (67K): [in this window] [in a new window]   Figure 2. Distribution of porphyry copper deposits in age (X axis)-depth (Y axis) space (upper panel) resulting from burial histories such as those taken by deposits in Figure 1. Colors are log scaled as number of deposits in a 1 m.y. x 1 km area. Lower panel compares age-frequency distributions for real deposits (light blue bars) and deposits at model surface (dark blue line). Good agreement between model and observed age frequencies indicates that, over the past 545 m.y., porphyry copper deposits emplaced at crustal depth of 1.9 ± 1 km formed at rate of 231 deposits/m.y. (Table 1). Histogram to left (yellow) shows vertical distribution of total copper endowments in porphyry copper deposits over 1 km crustal depth intervals; modal depth is 2.8 km (Table 1).