Distinctive composition of copper-ore-forming arcmagmas

Abstract

Magmatic fertility to generate porphyry- and high-sulfidation copper ore deposits generally develops, not at the scale of individual igneous complexes, but rather at the scale of petrochemical provinces spanning hundreds of kilometres of arc length, and containing many igneous complexes. The extent and duration of copper metallogenic provinces and epochs are apparently controlled by geodynamically imposed compressive stress that commonly lasts ca 5–20 Ma and fosters entrapment of mantle-derived basaltic magmas in chambers near the Moho where they cool very slowly and are likely to last long enough to experience intermittent replenishment during fractional crystallisation of layered ultramafic–mafic cumulate complexes on the chamber floor. Accumulation of dissolved H₂O through multiple cycles of replenishment and crystallisation in high-pressure magma chambers can explain the distinguishing chemical features of copper-ore-forming calc-alkalic arc magmas. The distinguishing features include unusually high contents of Sr and V, and unusually low contents of Sc and Y. The distinction from ordinary andesitic, dacitic and rhyolitic arc magmas can be enhanced by using ratios of enriched elements to depleted elements. The ratios Sr/Y and V/Sc are shown here to be effective in discriminating least-altered samples of ore-forming intrusions from ordinary, metallogenically unproductive arc magmas.

岩浆生成斑岩和高-硫化铜矿床的潜能，通常不是在个别火成岩复合体的规模上，而是在石油化学区域的规模上发展。这种石化区呈跨越数百公里的弧长，并含有许多火成岩复合体。铜成矿区域的程度和时代范围和持续时间显然是由通过地球动力施加的压缩应力所控制，通常持续约5-20Ma，使得幔源岩浆包封在近莫霍面处。他们冷却得很慢，可能持续足够长的时间以经受岩室底面上层状超镁铁质- 镁铁质堆积物在分离结晶过程中的间歇性补充。通过高压岩浆房内多个周期的补充和结晶导致的溶解水的积聚，可以解释形成铜矿石的钙碱性岛弧岩浆的显著化学特征。该显著特点包括锶和钒的含量非常高，钪和铱的含量异常较低。与普通安山岩、英安岩和流纹质岛弧岩浆的区别可以通过利用富集元素与耗尽元素之比率。锶/铱 和钒/钪的比率在这里都显示出可以有效地将成矿岩体的最小改变样品与一般的非生产性岛弧岩浆区别开来。

KEYWORDS:

• porphyry copper
• high sulfidation
• metallogeny
• arc magmatism
• exploration
• magma fertility
• adakite

ACKNOWLEDGEMENTS

I am grateful to journal reviewers Massimo Chiaradia, Anthony Crawford and David Cooke for substantially improving the clarity of presentation. The material has also been honed by discussions with exploration staff of many companies that sponsored much of this research and with academic audiences at many presentations.