Harmony Gold Mining (HMY) had the highest return in Q1 2016 by a mid-cap US stock, returning 246.7%.

ASSET	QUARTER	% RETURN
Harmony Gold Mining (HMY)	Q1 2016	246.68%
Centrus Energy (LEU)	Q1 2016	241.67%
Silvercorp Metals (SVM)	Q1 2016	193.31%
Skyline (SKY)	Q1 2016	170.76%
NexGen Energy (NXE)	Q1 2016	164.24%
Sibanye Gold Ltd ADR (SBSW)	Q1 2016	154.84%
DRDGOLD Limited ADR (DRD)	Q1 2016	142.49%
Aura Minerals Inc Common Shares (AUGO)	Q1 2016	136.47%
First Majestic Silver (AG)	Q1 2016	91.72%
Cleveland-Cliffs (CLF)	Q1 2016	91.08%
Equinox Gold (EQX)	Q1 2016	78.45%
Endeavour Silver (EXK)	Q1 2016	72.03%
Taseko Mines (TGB)	Q1 2016	70.69%
Compania de Minas Buenaventura SAA ADR (BVN)	Q1 2016	69.59%
Fortuna Silver Mines (FSM)	Q1 2016	68.83%
Century Aluminum (CENX)	Q1 2016	64.34%
B2Gold (BTG)	Q1 2016	61.16%
Companhia Energetica de Minas Gerais CEMIG Pref ADR (CIG)	Q1 2016	56.48%
Gerdau SA ADR (GGB)	Q1 2016	53.47%
IAMGold (IAG)	Q1 2016	51.37%
Skeena Resources (SKE)	Q1 2016	51.28%
Ternium SA ADR (TX)	Q1 2016	49.09%
Urban Outfitters (URBN)	Q1 2016	47.72%
Capri (CPRI)	Q1 2016	44.61%
Copa (CPA)	Q1 2016	44.53%

Mid-cap stocks are treated as having a market cap between $2B and $10B.