Contrasting evolutions of hydrothermal alteration in quartz monzonite and quartz diorite wall rocks at the Sierrita porphyry copper deposit, Arizona

Abstract

Two wall-rock types of contrasting chemical composition host the causative Ruby Star quartz monzonite porphyry intrusion and hypogene mineralization at the Sierrita porphyry copper deposit. Vein-related hydrothermal alteration in the Harris Ranch quartz monzonite and biotite quartz diorite wall rocks consists of several mineralogically discrete assemblages. Temporal evolution of different alteration assemblages was established, in part, using petrographic relations within, and crosscutting relations among, individual veins. Temperature and salinity characteristics of hydrothermal fluids responsible for filling of individual veins were determined using primary fluid inclusions in vein-filling quartz. Each generation of primary vein filling introduced characteristic secondary fluid inclusions into earlier developed veins as well. Histograms of homogenization temperatures of primary and secondary fluid inclusions from different veins, and accompanying salinity data, permitted temporal correlations to be drawn between veins which either did not exhibit crosscutting relations in individual samples or which formed in different wall rocks and thus exhibited different alteration mineralogies.Evolution of hydrothermal activity in the area sampled commenced with potassic alteration in both quartz monzonite and quartz diorite wall rocks from 10 to 12 molal (37-41 wt %) NaCl equivalent fluids, inclusions of which homogenize by halite dissolution in the approximate temperature range 300 degrees to 370 degrees C. Salinities of later fluids were in the range 2 to 3 molal (10-15 wt %) NaCl equivalent with only minor salinity variations for the remaining time span monitored by this study. Homogenization temperatures of primary fluid inclusions in veins formed from these lower salinity fluids began near 400 degrees C and increased initially to approximately 430 degrees C where boiling occurred. The pressure defined by boiling of these fluids is about 330 bars. A continuous decrease in fluid inclusion homogenization temperatures followed down to about 300 degrees C, during which time deposition of quartz and K-feldspar with accessory biotite and/or hematite occurred in new and reopened veins in the quartz monzonite wall rock. Simultaneously in the quartz diorite, a sequence of veins and adjacent alteration halos formed, each consisting of an early assemblage of potassic affinity (quartz + biotite + K-feldspar + albite) which evolved to a propylitic assemblage (quartz + epidote + chlorite) as vein filling proceeded. With continued cooling of the solution below about 300 degrees C, muscovite took the place of K-feldspar as the stable potassium-bearing mineral in the quartz monzonite. An analogue to this late-stage quartz + muscovite veining in the quartz monzonite could not be established in the quartz diorite but may consist of zeolite (stilbite) + anhydrite.The bulk of hypogene copper mineralization in both wall-rock types was associated with approximately 2 molal (10 wt %) NaCl equivalent solutions. In the quartz diorite, significant chalcopyrite deposition is associated with fluid inclusions homogenizing from 370 degrees down to about 320 degrees C and always occurs with the later stage propylitic minerals in each vein. In the quartz monzonite wall rock, chalcopyrite was deposited during the transition from potassic and into phyllic alteration. Fluid inclusion homogenization temperatures for this mineralization range from about 330 degrees down to 200 degrees C. No primary chalcopyrite was seen to occur with earlier potassic veining formed from either high- or low-salinity fluids in the quartz monzonite. A very late stage of deposition of chalcopyrite, pyrite, and minor bornite filled the center of late phyllic veins in the quartz monzonite; correlated fluid inclusions have salinities from 1 to 5 molal (5-23 wt %) NaCl equivalent and homogenization temperatures in the range 140 degrees to 160 degrees C. The different alteration and sulfide mineral assemblages interpreted to have formed simultaneously in the two wall rocks of contrasting chemical character can be reasonably assigned to different chemical interactions of each rock type with similar, or equivalent, hydrothermal fluids.