Alteration haloes developed by VMS

Alteration haloes developed by VMS deposits are typically conical in shape, occur mostly stratigraphically below the original fluid flow location (not necessarily the ore itself), and are typically zoned.

The most intense alteration (containing the stringer sulfide zone) is generally located directly underneath the greatest concentration of massive sulfides, within the footwall volcanic sequence. If the stringer zone is displaced from the sulfides, it is often the product of tectonic deformation, or the formation of a hybrid SEDEX-like distal pool of sulfides.

The alteration assemblages of the footwall alteration zone is, from core outwards;

Silica alteration zone, found in the most intensely altered examples, resulting in complete silica replacement of the host rocks, and associated with chalcopyrite-pyrite stringer zones.
Chlorite zone, found in nearly all examples, consisting of chlorite +/- sericite +/- silica. Often the host rock is entirely replaced by chlorite, which may appear as a chlorite schist in deformed examples.
Sericite zone, found in nearly all examples, consisting of sericite +/- chlorite +/- silica,
Silicification zone, often gradational with background silica-albite metasomatism.
In all cases these alteration zones are metasomatism effects in the strictest sense, resulting in addition of potassium, silica, magnesium, and depletion of sodium. Chlorite minerals are usually more magnesian in composition within the footwall alteration zone of a VMS deposit than equivalent rocks within the same formation distally. The hangingwall to a VMS deposit is often weakly sodium depleted.

Alteration not associated with the ore forming process may also be omnipresent both above and below the massive sulfide deposit. Typical alteration textures associated with devitrification of submarine volcanic rocks such as rhyolitic glasses, notably formation of spherulites, of perlite, lithophysae, and low-temperature prehnite-pumpellyite facies sub-seafloor alteration is ubiquitous though often overprinted by later metamorphic events.

Metamorphic mineralogical, textural and structural changes within the host volcanic sequence may also further serve to disguise original metasomatic mineral assemblages

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Halo perubahan yang dikembangkan oleh deposito VMS biasanya mengerucut dalam bentuk, terjadi terutama stratigrafi bawah lokasi aliran fluida asli (tidak harus bijih itu sendiri), dan biasanya dikategorikan. Yang paling intens perubahan (mengandung zona stringer sulfida) umumnya terletak tepat di bawah konsentrasi terbesar sulfida masif, dalam urutan vulkanik footwall. Jika zona stringer dipindahkan dari sulfida, sering produk deformasi tektonik, atau pembentukan SEDEX seperti kolam renang distal hybrid sulfida. The kumpulan perubahan perubahan footwall zona, dari inti ke luar; Silica zona alterasi , ditemukan di paling intens diubah contoh, sehingga penggantian silika lengkap dari batuan host, dan terkait dengan zona stringer kalkopirit-pirit. zona Klorit, ditemukan di hampir semua contoh, terdiri dari klorit +/- serisit +/- silika. Seringkali batuan induk sepenuhnya digantikan oleh klorit, yang mungkin muncul sebagai sekis klorit di contoh cacat. Zona Serisit, ditemukan di hampir semua contoh, terdiri dari serisit +/- klorit +/- silika, zona silisifikasi, sering gradational dengan latar belakang silika metasomatisme -albite. Dalam semua kasus zona alterasi ini efek metasomatisme dalam arti ketat, sehingga penambahan kalium, silika, magnesium, dan penipisan natrium. Mineral klorit biasanya lebih magnesian dalam komposisi dalam zona ubahan footwall dari deposit VMS daripada batu setara dalam formasi yang sama distal. Hangingwall untuk deposit VMS sering lemah natrium habis. Perubahan tidak terkait dengan bijih proses pembentukan juga mungkin di mana-mana baik di atas dan di bawah deposit sulfida masif. Tekstur khas perubahan terkait dengan devitrifikasi dari batuan vulkanik bawah laut seperti kacamata rhyolitic, terutama pembentukan spherulites, dari perlit, lithophysae, dan suhu rendah prehnite-pumpellyite facies perubahan sub-dasar laut di mana-mana meskipun sering overprinted oleh peristiwa metamorf kemudian. Mineralogi metamorf, perubahan tekstur dan struktur dalam urutan tuan vulkanik mungkin juga lebih berfungsi untuk menyamarkan kumpulan mineral metasomatic asli