Key words: oxygen, isotope, exploration, carbonate, replacement
The analysis of oxygen isotopes in carbonates would appear to have many of the qualities of a useful exploration tool: the analyses are fast, cheap and produce absolute values (ie. are quantitative) and analytical laboratories that perform them are widely available. Several research groups claim to have identified isotopic halos surrounding well-known carbonate-hosted ore bodies and have proposed use of isotopic surveys in exploration for new deposits (Beaty and Landis, 1990; Kesler et al., 1995; Naito et al., 1995; Nesbitt, 1996). However, our experience in the Superior district of Arizona highlights some of the problems that should be considered when evaluating the utility of oxygen isotopes in exploration. There are geological factors, which are commonly overlooked, that may render it difficult or impossible to apply this technique in a given area. Here we present a systematic study of the important geological variations inherent in oxygen isotopic surveys and we propose some procedures both for assessing the applicability of oxygen isotope surveys as an exploration tool in a particular area and for minimizing potential errors in the interpretation of the data. This study is based on a tightly-controlled, systematic study of the oxygen isotope characteristics of carbonate rocks in the Superior district and the compilation of published data from similar deposits.
Epigenetic massive pyrite-chalcopyrite- hematite Cu-Au ores (13 Mt. averaging 5% Cu, 1 g/t Au and 45 g/t Ag) preferentially replaced dolomitic strata which are found interbedded with less receptive carbonates. The replacement mineralization occurs around steep bedding- transgressive veins (Paul and Knight, 1995).
A total of 125 core, underground and surface carbonate samples were selected for isotopic and chemical analysis from over 500 samples collected during three field seasons of mapping and core logging. Polished thin sections of all samples were studied in detail (including electron microprobe analyses and backscatter imaging) to characterize their petrography and to document the presence/absence of hydrothermal carbonate veins, cryptic alteration textures and overgrowths. All isotopic analyses were performed by the isotope laboratory at the University of Missouri-Columbia. Results are summarized on figures 1 and 2 .
Stratigraphic variation in isotopic values was determined by comparing analyses of matrix carbonates from several of the receptive dolomitic units. End-member isotopic values for hydrothermal carbonates were determined by analyzing hydrothermal veinlets separately. “Orebody-scale” variations in del18O values as a function of distance from ore within the district were determined by comparing matrix carbonate analyses within each host rock unit. Sample spacing was the approximate width of a typical economic orebody ( Figure 1 ). The stratigraphic location of these samples was known to within one metre, minimizing the potential effects of stratigraphically controlled isotopic variation. Reconnaissance scale (> 1 km) spatial variability of isotopic ratios ( Figure 2 ) was determined by comparing del18O values of matrix carbonate from the centre of the district with those from samples collected 300 m, 2500 m and 6500 m from the Magma Mine. Control samples were also collected far (50 km) from any known mineralization.
The underlying premise in the application of isotopic surveys to minerals exploration lies in the assumption that carbonate rocks with low del18O values (£ 15 ‰) represent the products of hydrothermal alteration and that these del18O values are distinguishable from the higher values (~20- 30‰) typical of sedimentary carbonates. The major difficulty in interpreting oxygen isotope results lies in the intrinsic natural variation of the isotopic composition of carbonate rocks. Reasons for this include:
(1) Several geological processes can lower the del18O values of carbonate rocks. Ore-related hydrothermal activity is responsible for only a small fraction of low del18O-carbonates. Isotopic analysis cannot discriminate between isotopic haloes formed from hot, circulating ore-bearing fluids and those formed in a barren contact metamorphic aureole or even by deposition of carbonate from cool meteoric groundwaters ( Figure 4 ).
(2) Individual carbonate units can undergo variable diagenetic modification and can display a range of isotopic values which may exceed the potential isotopic shifts due to hydrothermal alteration ( Figure 3 ). This is the case in the Superior, Leadville (Thompson and Beaty, 1990) and Gilman districts (Beaty and Landis, 1990).
(3) Large primary variations can occur in the del18O values of different stratigraphic units, even those deposited within a restricted interval of time (e.g. Devonian-Mississippian). Sampling of carbonate rocks without good stratigraphic control can therefore lead to the definition of false isotopic haloes.
(4) The formation of isotopic haloes by hydrothermal solutions relies on the exchange of oxygen isotopes between the rock and the hydrothermal fluid. Hydrothermal solutions below 300-400ºC do not exchange enough oxygen with the wall rocks to produce recognizable del18O shifts unless there is extensive replacement (>30% at Superior) of the carbonate wall rocks by hydrothermal carbonates or introduction of abundant hydrothermal carbonate veinlets (in which case hydrothermal alteration should be identifiable in the field). For example, our results from the Superior district failed to identify low del18O haloes in carbonate rocks beyond 1 m from the orebodies. These results are consistent with several studies in other districts (e.g. Thompson and Beaty, 1990; Beaty and Landis, 1990).
Before oxygen isotope surveys can be effectively used in exploration, methods will have to be developed to remove the isotopic effects of background geological variation. Until then, we propose that the following steps are taken to characterize a region prior to any large-scale use of isotopic surveys:
(1) Perform a detailed stratigraphic study characterizing stratigraphy down to the bed scale. Systematically sample the section to characterize the primary isotopic variation between stratigraphic units. Restrict any large-scale surveys to well-identified key beds.
(2) Check for the presence of carbonates with low del18O values which are unrelated to ore (e.g. diagenetic carbonates, contact metamorphosed carbonates).
(3) Analyze veinlets and pods separately from the matrix of carbonate rocks.
This methodology can help identify potential problems with the use of isotopic surveys as an exploration tool but can do little to remedy these problems. As the isotope technique is an indirect search method, data rarely have a unique interpretation and geological mapping of features such as carbonate veins (noting vein spacing, width and composition) and the presence of plutons and dikes will always be required.
Beaty, D.W., and Landis, G.P., 1990. Origin of the Ore Deposits at Gilman, Colorado – Part IV. Stable isotope geochemistry. in, Beaty, D.W., Landis, G.P., and Thompson, T.B., eds., Carbonate-hosted sulfide deposits of the Central Colorado Mineral Belt, Economic Geology Monograph 7, p. 228-245.
Kesler, S.E., Vennemann, T.W., and Vazquez, R., Stegner, D.P., and Frederickson, G.C., 1995, Application of large-scale oxygen isotope haloes to exploration for chimney-manto Pb-Zn-Cu-Ag deposits: in Coyner, Alan R., and Fahey, Patrick L., eds., Geology and Ore Deposits of the American Cordillera, Geological Society of Nevada, p. 1383-1396.
Naito, K., Fukahori, Y., Peiming, H., Sakurai, W., Shimazaki, H. and Matsuhisa, Y., 1995, Oxygen and carbon isotope zonations of wall rocks around the Kamioka Pb-Zn skarn deposits, central Japan – application to prospecting: Journal of Geochemical Exploration, v. 54, p. 199-211.
Nesbitt, B.E., 1996, Applications of oxygen and hydrogen isotopes to exploration for hydrothermal mineralization: Society of Economic Geologists Newsletter, v. 27, p. 1-13.
Paul, A.H., and Knight, M.J., 1995, Replacement ores in the Magma Mine, Superior, Arizona: in Pierce, Frances W., and Bolm, John G., eds., Bootprints along the Cordillera; Porphyry copper deposits from Alaska to Chile, Arizona Geological Society Digest, v. 20, p. 366-372.
Thompson, T.B., and Beaty, D.W., 1990, Geology and origin of ore deposits in the Leadville District, Colorado; Part II, Oxygen, hydrogen, carbon, sulfur, and lead isotope data and the development of a genetic model: in Beaty, D.W., Landis, G.P., and Thompson, T.B., eds., Economic Geology Monograph 7, p. 156-179.