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Description
The La Prosperidad banded ferromanganese deposit, in central Baja California Norte, Mexico, is enclosed by Mesozoic marine arc volcanic-sedimentary rocks. A dome-like dacite porphyry footwall and a hanging wall of interbedded tuffs, tuffaceous shales and rhyodacite flows are contemporaneous with the deposit. Detailed geologic mapping indicates these rocks were deposited in a highly restricted marine or lacustrian basin which formed following the emplacement of a large rhyolite ash flow. The deposit occurs as a steep dipping 3 m thick, by 360 m long outcrop of thin and rhythmic laminated Fe and Mn oxides. Subaqueous chemical precipitation is suggested by the high continuity of these strikingly distinct lamelli. The deposit's trace element enrichments (Sb, As, W[?], Zn ± Ge, Co, Ba, Mo, V, Sr, Cu, Nb, Be) match those of present day Mn hot spring aprons and ancient syngenetic Mn deposits associated with silicic volcanism. Metal distribution within the deposit indicates adsorbtion on Mn and Fe oxides was an important mechanism of precipitation in a depositional basin that physiochemically was essentially homogeneous. This mode of precipitation was interrupted once by the rapid emplacement of an ash-rich unit and numerous times when hot spring activity decreased and thin travertine layers precipitated. Diagenesis and upper greenschist facies metamorphism caused complete recrystallization; however, their large scale effects on ore textures and compositions were minimal. More significant redistributions were produced by hydrothermal overprinting and supergene alteration. The La Prosperidad deposit belongs to the meta-volcanic mineral province of Baja California. However, the province is better characterized by deposits of Fe and Cu, formed mainly by contact metasomatic processes. Although it is possible that the La Prosperidad deposit was partially derived from surfacing Mn-rich residual solutions produced by the formation of a contact metasomatic Fe deposit at depth, most, if not all, of the Fe-Mn oxides probably were leached directly from the volcanic rocks underlying the La Prosperidad deposit by heated connate water and/or ground water.
