MAGMATIC-HYDROTHERMAL TRANSITION RECORDED BY MAFIC MINERALS IN LARAMIDE GRANODIORITE AND RELATED MINERAL DEPOSITS, PATAGONIA MOUNTAINS, AZ

The Patagonia Mountains are underlain by a NW-elongate granodiorite pluton (c.65 Ma) with a relatively constant modal composition.  Areas of strongly developed porphyritic texture are associated with hydrothermal alteration and base metal deposits.

            Regional dD patterns in magmatic biotites from the more deeply eroded south end of the pluton are uniform and vary from -78 to -83‰.  To the north dD is more erratic varying from -66 to -93‰ with more negative values found near the edge of the pluton.  This pattern suggests that meteoric water entered the pluton along its outer and upper contacts.  dD of hydrothermal biotites varies from -71 to -95‰ suggesting that mixing of magmatic and meteoric fluids continued into the early hydrothermal stage.

The textural transition from equigranular to porphyritic granodiorite in the Four Metals mine area is gradual and accompanied by increased MgO/MgO+FeO in magmatic biotite and hornblende and increased magnetite.  This relationship is consistent with experimental work of Wones and Eugster (1965) and Czmanske and Wones (1970) who demonstrated these changes in mineral chemistry correspond to increased fO₂ and fluid saturation in the magma.  Release of aqueous fluids during quench of the porphyritic granodiorite formed the Four Metals copper-bearing breccia pipe and related metal deposits. 

F and Cl increase in magmatic biotite and less consistently in hornblende as the granodiorite becomes more porphyritic.  F, Cl, and MgO/MgO+FeO in magmatic biotite are correlative with hydrothermal biotite compositions from the Four Metals mine, indicating a gradual transition from the magmatic to hydrothermal environment.  Multiple, parallel enrichment trends of F+Cl in biotite may identify multiple generations of aqueous fluid release from progressively more porphryitic phases of the porphyritic granodiorite.  Graybeal (1973) found that the copper content of biotite and hornblende decreases in the more porphyritic rocks, consistent with extraction of copper from these minerals into an aqueous fluid progressively enriched in F and Cl.  Distribution coefficients suggest that biotite exchanges copper in a coexisting F and Cl-enriched aqueous fluid more rapidly than hornblende, but that both minerals may be important sources of copper in associated metal deposits.