Non-metallic and sedimentary mineral deposits associated with active continental margins
Harald G. Dill
Gottfried-Wilhelm-Leibniz University, Hannover, Germany
Active continental margins which are marked by subduction zones and can be mapped in modern fold belts around the Pacific Ocean and from the Pyrenees through to the SE Asian mountain ridges are favorable geodynamic settings for non-metallic and sedimentary mineral deposits. Sedimentary deposits in and at the edge of modern fold have been derived from the erosion of the rising mountain chains and from sedimentary processes, also known as chemical weathering which led to the supergene alteration of hypogene metal deposits whose origin and present position is accounted for by subduction. There is a gradual change from hypogene metallic deposits, mainly Cu and Au into hypogene non-metallic deposits enriched in phyllosilicates. To shed some light on these "mineral catenas" research into mineralogy, geochemistry, geology, geophysics, sedimentology and geography is mandatory. Which non-metallic commodities or industrials are mainly exploited along active continental margins? There are two different kinds of zonation at active continental margins, one oriented horizontally is related to the geodynamic setting and extends perpendicular to the active margin and a vertical one reflecting the morpho-climatic dynamic in this peculiar crustal section from the foothills to the top of the mountain.
Near the trench and most distal relative to the high-altitude mountain range, beach placers evolved taking up the clastic residue left over as obducted ophiolitic rocks undergo weathering and erosion (chromite, olivine, magnetite, PGM, Au). Being subjected to subaerial and submarine weathering and hypogene alteration, these basic and ultrabasic rocks get coated with ochre and umber deposits as well as penetrated by veinlets of magnesite and zeolite, which on one hand may be of economic grade and attractive to mining engineers but on the other hand could be detrimental to the construction raw materials so that it loses its rock strength. The construction raw material unaffected by such deleterious alteration is won in the upper basic volcanic and plutonic units of these ophiolitic series and prevalently quarried for aggregates and dimension stones.
Heading towards the continent, the magmatic suite sees the ultrabasic and basic magmatic rocks to turn into a series of intermediate and felsic magmatic rocks which are associated with a characteristic assemblage of deposits enriched in industrial minerals. Apart from sulfur produced by fumaroles, there is a wide range of phyllosilicates concentrated to economic grade in what is called the argillic alteration. Kaolinite, dickite, nacrite, halloysite, smectite, illite and pyrophyllite attain ore grade. Those minerals are often associated in caldera and maar lakes with diatomite, zeolites and different modifications of silica used, in places, as precious stones of jeweler´s quality.
Alunite-group minerals found in this environment deserve particular attention for their role these sulfate-phosphate minerals play as marker minerals and as industrial mineral forming deposits of their own. There are gradual transitions from high-sulfidation-type gold deposits to epithermal alunite-deposits barren in precious metals.
The felsic to intermediate volcanic, subvolcanic and pyroclastic rocks cater for a much wider spectrum of final use than their basic and ultrabasic counterparts on the seaward side of the catena. It is the group of the so-called "3-P deposits", harnessing the pumice, puzzolana and perlite deposits which are accumulated as air fall tuffs and ash flows forming a pyroclastic apron around the volcanic domes and subvolcanic necks. The various types of 3-P deposits and most pyroclastic rocks came into being either by the interaction of the hot felsic magma with cold ground water at shallow depth on the ascent or as the avalanches of lava and ash flows ran into a lacustrine environment. Phreatomagmatic eruptions and steam-heated epithermal deposition are only two different sides of the same coin.
The uplifted mountain ranges expose a morpho-climatic zonation that is in vertical equivalent to what is known from the equator through the polar region in horizontal. The chemical weathering operative along this horizontal transect from the lower to the higher latitude is also reflected in the vertical built-up of gossans of hypogene ore deposits from the lowland through the unvegetated mountain ridges above the tree-line.
As a consequence of strong uplift and erosion, placer deposits and red-bed deposits, mainly enriched in Au and Cu, come into existence at a distal position relative to the mountainous areas. If chemical weathering keeps pace with uplift and erosion, gossans and metalliferous duricrusts ("orecretes") will develop proximal to the mountain range and/or within the deposits, proper. This tremendous variation in the ratio erosion/ weathering has on one side positive effects on the supergene alteration of ore deposits in that gossans of different generation were telescoped into each other and by doing so upgrade the metal content and on the other hand may have negative effects on the non-metallic deposit mainly sand because of the limited time and transport distance given the debris to result in a quartzose sand qualifying as a good construction raw material depleted in labile constituents such as feldspar and lithoclasts.