Woodhouseite and svanbergite in hydrothermal ore deposits; products of apatite destruction during advanced argillic alteration

Abstract

Woodhouseite and svaubergite, previously documented from roughly two dozen localities, have been found with advanced argillic alteration in three hydrothermal ore deposits. These aluminum phosphate-sllfate (APS) minerals are isostructural with alunite (R3m), with the generalized formula RAl3(POdl +r(SO/1-r(OH)6-r. (HzO)r, where R is Ca or Sr, and x is less than 0.5. At Summitville, Colorado, an epithermal gold*opper deposit, svanbergite occurs with hypogene kaolinite and alunite in the upper portions of the deposit, and woodhouseite is observed at deeper levels, where pyrophyllite is locally abundant. In the porphyry-copper deposit at La Granja @eru), woodhouseite occurs witl pyrophyllite and appears to have replaced apatite. The porphyry-copper deposit at La Escondida (Chile) contains woodhouseite-svanbergite solid solutions, some hypogene, others supergene, as judged from textual criteria. Alunite specimens from Summitville, La Escondida, and several other localities investigated in this study contain some APS component, lvith up to 2.41 wt. q0 P2O5 and l.l2wt.tlo SrO, reflecting tie coupled substitution K+ + SO42- = (Ca,Sr)z+ + POo3- io gt. t*t r*ture. The APS phases are considered to form by replacement of apatite in the acidic, sulfate-rich environment ttrat characterizes advauccd argillic alteration. Their occurrence probably has been overlooked in loany areas showing such alteration of apatite-bearing host rocks.