Abstract

In the El Hammam fluorite mining district (central Morocco, 50 km southwest of Meknes), interlayered pelite and limestone beds have been metamorphosed into banded calc-silicate rocks under the influence of a hidden granitic pluton of Variscan age. Various calc-silicate bands occur in the following zonal relationship: (1) pelitic hornfels (quartz, muscovite, biotite, and plagioclase), (2) pyroxene-feldspar band (salitic pyroxene, anorthite, K-feldspar, and titanite), (3) grossularite band (grossularite, accessory diopside, and idocrase), and (4) marble or wollastonite band. Field relationships suggest that at least part of the calc-silicate rocks predated the skarns and were not genetically linked to them. Mineral assemblages indicatethat the conditions of formation were 525 degrees < T < 580 degrees C and X (sub CO 2 ) < 0.15 at P total = 2 kbars (assumed).Skarns replaced the calc-silicate rocks but not the marble beds. Based on crosscutting and replacement relationships, a sequence of three successive skarn formation stages has been recognized.Scheelite-bearing skarns formed during the first stage. They occur as unzoned conformable bodies and consist of salite to ferrosalite, anorthite, scheelite, titanite, and K-feldspar.Malayaite-bearing skarns formed during the second stage. They occur conformably to the banding and display different zonal patterns along strike. Where the replaced calc-silicate band is a wollastonite band, the succession of zones is zone 1: tin-bearing andradite (variable tin content up to 1.6 wt % SnO 2 ) and wollastonite, zone 2: tin-bearing andradite (variable tin content up to 5.6% SnO 2 ) and malayaite (CaSnSiO 5 ), and zone 3: hedenbergite and malayaite. Where the replaced rock is a pyroxene-feldspar band or a scheelite-bearing skarn, malayaite-bearing skarns consist of hedenbergite and malayaite. Where the replaced rock is a grossularite band, the zonal pattern is disseminated malayaite, and disseminated tin-bearing andradite and malayaite.Axinite-bearing skarns formed during the third stage as veins and pods in calc-silicate rocks and scheelite- and malayaite-bearing skarns. They consist of ferroaxinite and datolite, and in places, tin-bearing manganaxinite (up to 1.7 SnO 2 wt %).Malayaite-bearing skarns formed at T < 480 degrees C, log f (sub O 2 ) < -25, and f (sub CO 2 ) < 100 bars assuming P total = 2 kbars. The temperature was probably higher for scheelite-bearing skarns and lower for axinite-bearing skarns. Late alteration of skarns involved partial silicification, sulfidation, recarbonation, and hydration.

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