Metallogeny: The rationale behind space (WHERE?) – time (WHEN?): distribution of ore deposits Whether there is any globally discernible unifying pattern about the space-time distribution pattern
The exclusivity in spatial-temporal distribution of ore deposits is a tangible proposition
Such distribution patterns are REAL and not apparent (not artifacts due to inadequate sampling)
Such patterns can be explained in the light of the known/emerging facts about the evolution of the planet Earth (⇒solid + fluid + living)
Salient Features of the global distribution pattern of ore Deposits A critical look at the spatial – temporal distributions of ore deposits on a global scale reveals the following salient features:
1. Chemical heterogeneity in the lithosphere – three distribution patterns:
a) Occurrence of metallogenic provinces characterized by specific metal(s) e.g. Arizona Cu (Archaean to Tertiary), Colorado plateau U-V ores (Triassic to Tertiary) b) Occurrence of barren tracts between regions of phenomenally rich mineralization (eg. Ecuador and New England states) c) S. African greenstone belt represents mantle heterogeneity of the extreme type as nearly 67% of the world’s chromite have been formed during 1.8 Ga
2 d) Extensive VMS mineralization (Cu-Zn-Au-Ag) throughout the Canadian Archaean shield e) Greater amount of Archaean Au in S. Africa compared to that in Canada, Western Australia and India.
2. Ore-lineament Association (eg. Rocky Mountain trench; Mother Lode, CA; Owen Rift, Tasmania, Singhbhum Shear Zone?) ⇒ received a new meaning with the advent of plate tectonics, leading to two interpretations: they were (i) paleo- subduction zones now represented by mega-
lineaments and (ii) transform faults which were either genetically related to oblique subduction zone of the ocean floor or extension of the transform fault related to the beginning of ocean floor spreading beneath the intracontinental rift (metal- rich brine in the Salton sea within the San Andreas fault system, CA or extension of the ridge-ridge transform faults into continental margin such as NW- trending fracture zone in Egypt and SArabia that extend into the axial zone of Read sea).
3. Centrifugal dispersion of ore bodies around Precambrian tonalitic batholiths in granite-granulite terrains and around the root zones of Mesozoic-Cenozoic Cordilleran fold belts ⇒ emphasize a consistent granitoid – hydrothermal ore relationship. THE CORDILLERAN SITUATION ≡ MODERN ANALOGUE OF THE ARCHAEAN GRANITE – GRANULITE BELT
4. Specificity of ORE – ROCK association: STRIKING in ultra mafics, PROMINENT in mafics, FREQUENTLY CLEAR in felsics and OCCASIONALLY CONSPICUOUS in sediments ⇒ imply common heritage of the ores that persisted through out geological time.
5. On the continental basis, two major distribution patterns are often seen: (i) syn-sedimentary ore deposits along the regional depositional and tectonic fabrics in orogenic belts (eg. The Aravallis in Rajasthan and Gujarat,
3 mineralized belt in western America) and (ii) those transverse to the regional tectonic fabrics (mostly epigenetic, genetically diverse, often lineament related and parallel to hot-spot tracts).
6. Matching of the tin belts (Bolivia & Nigeria) and BIF’s (India and Australia) in reconstructed mega continent are features eventually traced by global tectonics.
7. Temporal distribution pattern (as spatial) shows some distinctive features. Compilation of age vs abundance if various metals indicate that some metals (Au, U, Cu, Cr) formed ore deposits in rocks of all ages while others (Pb, Zn, Sn, W, Mo) occur in rocks mainly younger than 1 Ga → possibly indicating failure of evolutionary processes of concentration of younger metals did not evolve before 1 by. Again deposits such as Nisulfide, layered chromite, anorthosite- gabbro associated Fe-Ti- oxides largely occur in Precambrian rocks while porphyry Cu-Mo and Hg sulfide deposits were almost...