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Magmatism and Tectonics in Southwest Japan

  • Magmatism related to diminished rates of subduction beneath southwest Japan.
  • Described changes in Pacific plate motion from north-northwest to west-northwest approximately 40 million years ago (Uyeda & Miyashiro, 1974; Ishihara, 1978).
  • Possible relation to the end of subduction and the beginning of dextral transform tectonics associated with the opening of the Japan Sea.

Granitoids of the Outer Zone

  • Granitoids in the Outer Zone of southwest Japan are predominantly more silicic compared to those in the Inner Zone.
  • Classified as part of the ilmenite series.
  • Initial isotopic values (87Sr/86Sr^{87}Sr/^{86}Sr and 18O^{18}O) although not uniformly high, suggest an origin in continental crust (Takahashi et al., 1980).

Groupings of Skarn Deposits

  • Broad classifications within a petrotectonic framework are helpful for geographic surveys and identifying similarities.
  • Importance of recognizing individual deposit characteristics.
  • Proposed classification based on predominant metals, per Knopf (1942):
    • Fe
    • W
    • Cu
    • Zn-Pb
    • Mo
    • Sn
  • Each type has its specific petrotectonic setting, mineralogy, and igneous association.

Iron Skarn Deposits

  • Definition: Skarn deposits with significant economic concentrations of magnetite.
  • Found in diverse geological environments including:
    • Oceanic island-arc terrains related to diorites and their extrusive equivalents
    • Andean-type continental margins with quartz monzonite stocks
    • Postorogenic terrains with quartz monzonite and granite
    • Rifted continental margins alongside diabase
  • Estimated sizes of deposits: Small (2-10 million tons of Fe), Large (40-300 million tons of Fe).
Specialized Environments for Iron Skarn
  • Two specialized environments producing skarns with magnetite as the sole ore mineral:
    1. Island-arc-type calcic skarn (class I-A-1, Table 2)
    2. Cordilleran-type magnesian skarn (class I-B-1, Table 2)
  • Examples referenced in Tables 3 and 4.
  • Additional deposits:
    • Massive magnetite bodies replacing calc-shale and tin-bearing skarn in West Malaysia (Hosking, 1973)
    • Inner garnet zones of zoned, zinc-bearing calcic skarns at Hanover, New Mexico (Hernon & Jones, 1968) and at Temperino, Italy (Bartholome & Evrard, 1970).
Island-arc Calcic Magnetite Skarns
  • Definition includes:
    • Magnetite calc-skarn and scapolite-albite-skarn magnetite deposits (Sokolov & Grigor'ev, 1977).
    • Iron-cobalt skarn type (Smirnov, 1976)
  • Noteworthy features:
    1. Association with gabbros and diorites in volcano-sedimentary sequences
    2. High skarn volume associated with igneous rocks
    3. Presence of widespread sodium metasomatism
    4. Anomalous concentrations of cobalt and sometimes nickel
  • Locations: island-arc terrains in the Urals, Philippines, Japan, Cuba, and western North America.
  • Typical volcanic rocks: basalt, andesite flows, tuffs interbedded with sedimentary rocks like sandstone and marl.
  • Intrusive rocks: ranged from gabbro to granodiorite; diorite most commonly associated with skarn formation.
  • Magnetite deposits: formed from diorite stocks or replacements of limestone xenoliths and diorite.
  • Example mines:
    • Empire mine, Vancouver Island (Haug, 1976)
    • Larap, Philippines (Frost, 1965)
  • Less skarn formation in thick carbonate horizons compared to thin limestone and volcanic sequences (Sangster, 1969).
Characteristics of Magnesian Magnetite Skarn Deposits
  • Table 4 provides metrics including tonnage, intrusive rocks, alteration, and morphology.
  • Typical minerals within deposits showing range of metamorphic conditions:
    • Opaque minerals (magnetite, hematite, etc.)
    • Prograde and retrograde mineral assemblages.
  • Mineral compositions often include high ratios of forsterite and pyroxenes in the skarn.

Retrograde and Morphological Features

  • Retrograde features commonly include actinolite, chlorite, calcite, and quartz.
  • Magnetite orebodies associated closely with garnet zones or limestone beyond skarn zones.
  • Depository sizes vary:
    • Smaller deposits: typically irregular, 5-20 million tons
    • Larger, stratiform bodies: tens of meters thick, several kilometers long in regions like the Urals and the Philippines.
  • Formation of hematite from surface oxidation processes.
  • Lesser amounts of copper recorded (typically <0.2 wt % in iron ores) along with other metals at various rates across specific deposits.
  • Cobalt content often noted in Larap and Goroblagodat deposits (0.03 wt % Co).
  • Additional metal occurrences: nickel, molybdenum, gold, and silver as trace components; intersected at specific localities like the Baguio district, Philippines.

Cordilleran Magnesian Magnetite Skarn Deposits

  • Comparison between dioritic plutons forming magnetite in limestone versus quartz monzonites in dolomite.
  • Formation ease of magnetite in magnesian skarns is due to lower iron absorption in solid solution during skarn-forming conditions.
  • These deposits often associated with hypabyssal stocks and dikes of granodiorite or quartz monzonite; infrequently with granite.
  • Characterized by limited endoskarn alteration in intrusives; primary alteration features include secondary feldspars and chlorite.

Tungsten Skarn Deposits

  • Found worldwide within Precambrian to Triassic limestone, typically linked to calc-alkaline intrusive formations.
  • Most tungsten reserves denoted as metasomatic in origin associated with calc-alkaline plutonism.
  • Tungsten skarns display reduced calc-silicate and opaque mineralogy.
  • Associated plutons characterized as coarse-grained porphyritic granodiorite to quartz monzonite, often containing aplite and pegmatite dikes.
  • Intrusives typically unaltered with limited megascopic alteration characteristics.
  • Host carbonate rocks are often intercalated with carbonaceous sequences, indicating low oxidation states in skarn-forming environments.
  • Tungsten skarns generally form in the lowest carbonate beds, mainly along with impure limestones of Cambrian age.